"Westminster" Secies THE PRECIOUS METALS The " Westminster " Series Crown 8-vo. Fully Illustrated. 6s. net each. The Manufacture of Paper. By R. W. SINDALL, F.C.S. Timber. By J. R. BATERDEN, A.M.I. C.E. Electric Lamps. By MAURICE SOLOMON, A. C.G.I., A.M.I.E.E. Textiles and their Manufacture. By ALDRED BARKER, M.Sc., Technical College, Bradford. The Precious Metals. By T. KIRKE ROSE, D.Sc., of the Royal Mint. Ornamental Glass Work. By A. L. DDTHIE. The Railway Locomotive. By VAUGHAN PENDRED, M.I.Mech.E., late Editor of" The Engineer." Iron and Steel. By J. H. STANSBIE, B.Sc. (Lond.), F.I.C. Town Gas for Lighting and Heating. By W. H. Y. WEBBER, C.E. Liquid and Gaseous Fuels, and the Part they play in Modern Power Production. By Professor VIVIAN B. LEWES, F.I.C., F.C.S., Prof, of Chemistry, Royal Naval College, Greenwich. Electric Power and Traction. By F. H. DAVIES, A.M.I.E.E. Coal. By JAMES TONGE, M.I.M.E., F.G.S., etc., Lecturer on Mining at Victoria University, Manchester. India-Rubber and its Manufacture, with Chapters on Gutta-Percha and Balata. By H. L. TERRY, F.I.C., Assoc. Inst. M.M. The Book: Its History and Development. By CYRIL DAVENPORT, F.S.A. Glass. By WALTER ROSENHAIN. Superintendent of the Department of Metallurgy in the National Physical Laboratory. Patents, Trade Marks and Designs. By KENNETH R. SWAN, B.A. (Oxon.), of the Inner Temple, Barrister-at- Law. Precious Stones. With a Chapter on Artificial Stones. By W. GOODCHILD, M.B., B.Ch. Electro-Metallurgy. By J. B. C. KERSHAW, F.I.C. Natural Sources of Power. By ROBERT S. BALL, B.Sc., A.M.I.C.E. Radio-Telegraphy. By C. C. F. MONCKTON, M.I.E.E. * further ^volumes 'will appear at short intervals. OF THE UNIVERSITY OF p j II h ^ ~. S -5 03 S S s o 5 1-3 " 8 I fl S rt a rF > THE PRECIOUS METALS COMPRISING GOLD SILVER AND PLATINUM BY T. KIRKE ROSE A.R.S.M. D.Sc. (i CHEMIST AND ASSAYER OF THE ROYAL MINT J VICE-PRESIDENT OF 'THE INSTITUTION OF MINING AND METALLURGY: HONORARY MEMBER OF THE CHEMICAL, METALLURGICAL AND MINING SOCIETY OF SOUTH AFRICA J FELLOW OF THE CHEMICAL SOCIETY, ETC. LONDON ARCHIBALD CONSTABLE & CO. LTD. 10 ORANGE STREET LEICESTER SQUARE W.C. 1909 SPRECKEt.3 BRADBURY, AGNEW, & CO. LD. , PRINTKK: LONDON AND TONBRIDOE. PREFACE THE term " precious metals " has usually been applied to gold and silver only, its use dating back to a time when no regard was paid to platinum. As platinum is now in common use and is more valuable, weight for weight, than gold, it may very well rank as a precious metal. Other metals of high price come under the heading of rare metals. In the preparation of this work, the aim has been to provide an introduction to the study of the, precious metals, and an elementary book of reference for those who do not wish to pursue the subject further. With these objects in view details have been generally omitted and a wide extent of ground has been covered, including the technical pro- cesses of minting and the manufacture of gold and silver wares. It need hardly be mentioned, however, that all questions of currency and finance have been left untouched. It is also perfectly obvious that the book is not intended to be a substitute for the larger treatises on metallurgy, some of which devote the whole of a bulky volume to the full consideration of a section of the subject dealt with here in the course of a few pages. The author desires to express his thanks to Mr. Edward I viii PREFACE Rigg, I.S.O., Superintendent of the Operative Department of the Royal Mint, to Mr. Arthur Westwood, Assay Master of the Birmingham Assay Office, and to others for kindly reading the proofs of various chapters and for their valuable suggestions. NORTHWOOD, February 1st, 1909. CONTENTS CHAPTER I. HISTORY OF GOLD. Gold in prehistoric times Its ancient names Early methods of extraction in Egypt Methods of refining gold practised by the Romans Alchemy Treatment of gold ores in the Middle Ages Use of mercury Later processes Parting gold from silver .pp. 1 10 CHAPTER II. PROPERTIES OF GOLD. Physical properties, colour, hardness, malleability, ductility, tenacity, crystallisation, density, melting point, volatilisation, conduc- tivity for electricity and heat Chemical properties Noble metals Chemical relations of gold Solvents for gold Identification of gold Reactions of gold Colloidal gold Purple of Cassius Preparation of pure gold ..... pp. 11 19 CHAPTER III. COMPOUNDS OF GOLD. Chlorides of gold Aurous chloride Auric chloride, its preparation, properties and decomposition Tests for auric chloride Chloro- auric acid Bromides of gold Iodides of gold Cyanides of gold Aurocyanides, their formation and decomposition Auricyanides Sulphocyanides of gold Oxides of gold Fulminating gold Thiosulphates of gold Silicates of gold Sulphides of gold Selenide, telluride, phosphide, arsenide, 'and antimonide of gold pp. 2036 x CONTENTS CHAPTEE IV. ALLOYS OF GOLD. Gold and silver Gold and copper Gold and mercury Gold and zinc Gold and cadmium Gold and tin Gold and antimony Gold and arsenic Gold and bismuth Gold and lead Gold and iron Gold and nickel Gold and cobalt Gold and manganese Gold and platinum Gold and palladium, rhodium, iridium, etc. Gold and aluminium Gold and tellurium Gold and thallium pp. 3763 CHAPTER V. THE OCCURRENCE OF GOLD IN NATURE : GOLD ORES. Gold dust and nuggets Placer deposits Crystals of gold -Ores of gold Sulphide ores Tellurides of gold Dissemination of gold in various rocks Gold in sea water Origin of gold ores Geological distribution of gold ores Geographical distribution of gold ores pp. 64 71 CHAPTEE VI. EXTRACTION OF GOLD FROM ITS ORES: GOLD WASHING. Classification of gold ores Classification of the methods of treatment Appliances used in washing gold ores The miner's pan The batea The cradle Sluices Hydraulic mining - Riffles Dredging for gold ....... pp. 72 84 CHAPTER VII. TREATMENT OF GOLD ORES BY CRUSHING AND AMALGAMATION. Stamp-mills Parts of a stamp Rock-breakers The method of feeding- ores to stamp-mills Mortars Screens Cams and cam shafts Method of running stamp-mills Water in stamp-mills Mercury in stamp-mills Method of catching gold on amalgamated plates The preparation and care of amalgamated plates Cleaning- up- Retorting of amalgam Melting the gold bullion Other machines CONTENTS xi for crushing gold ores Ball and roller mills Other amalgamating machines Loss of mercury in amalgamation Treatment of tailings from the stamp battery Concentrating machines- Classification of pulp from stamp-mills Treatment of concen- trates pp. 85 109 CHAPTER VIII. TREATMENT OF GOLD ORES BY WET METHODS. The cyanide process Collection of tailings from stamp batteries Dissolving the gold by cyanide solutions Washing out the dis- solved gold Precipitation of the gold by zinc Cleaning-up and recovery of the gold Treatment of slimed ore by decantation Electrical precipitation of gold Treatment of slimed ore by filter- pressing Re-grinding in tube-mills Treatment of telluride ores by cyanide solutions Dry crushing Roasting gold ores The chlorination process . ...... pp. 110 127 CHAPTER IX. SILVER AND ITS ALLOYS. Historical account of silver The physical and chemical properties of silver The preparation of pure silver The alloys of silver Silver and copper Silver and zinc Silver and cadmium Silver and lead Silver and mercury Silver and tin Silver and platinum Silver and aluminium Silver and bismuth Silver and nickel Silver and palladium Silver and thallium. . . pp. 128 143 CHAPTER X. COMPOUNDS AND ORES OF SILVER. Silver oxide Silver sulphide Silver chloride Silver bromide, iodide and cyanide Silver-potassium cyanide Silver sulphate The composition and mode of occurrence of the ores of silver >. 144153 xii CONTENTS CHAPTER XI. THE EXTRACTION OF SILVER FROM ITS ORES. Amalgamation processes Direct amalgamation in tubs and in the arrastra The method of working the patio process of amalgama- tion and the chemical reactions involved The Cazo process The Francke-tina process The Kronke process Pan amalgamation of roasted and unroasted ore Lixiviation processes Roasting to sulphate and lixiviation with water Roasting to chloride and lixiviation with brine Treatment of roasted ore by the Patera, Kiss and Russell thiosulphate solutions The cyanide process Smelting with lead ores in blast furnaces Desilverisation of lead The Pattinson process and its modifications Desilverisation by zinc Cupellation The German cupellation furnace The English cupellation furnace Separation of silver from metallic copper by electrolysis . pp. 154 182 CHAPTER XII. REFINING GOLD AND SILVER. Preliminary refining at ore-treatment mills Refining at the centres of industry The sulphuric acid process Gutzkow's modifications Refining by means of chlorine gas Electrolytic processes The Moebius process The Wohlwill process Advantages and dis- advantages of parting by electrolysis . . . pp. 183 196 CHAPTER XIII. THE ASSAY OF GOLD AND SILVER ORES. General description Sampling and crushing ores The assay ton Fusion of the ore sample with fluxes in crucibles and concentra- tion of the gold and silver in lead buttons The use of various fluxes The method of conducting the fusion Scorification of ores with lead Cupellation of the lead buttons Losses of gold and silver in cupellation Inquarting gold beads and parting gold from silver Proportions of silver to gold used in parting Weighing the parted gold ..... pp. 197 20o CONTENTS xiii CHAPTER XIV. THE ASSAY OF GOLD AND SILVER BULLION". Classification of methods Parting assay of gold bullion The ratio of silver to gold in the parting assay Cupellation in the muffle Boiling the assay pieces in nitric acid Use of check assays Parting assay of alloys of silver and gold Cupellation assay of silver bullion Gray Lussac process of silver bullion assaying Volhard method of silver bullion assaying India mint method of silver bullion assay by weighing the chloride . pp. 206 218 CHAPTER XV. MINTING. Historical introduction Valuation of bullion Manufacture of coin Melting of bullion Coke, oil and gas fuel Casting coinage bars Rolling Cutting-out and marking the blanks Annealing and blanching the blanks Striking and weighing the coins Telling machines Composition of gold and silver coins of the principal countries of the world f>p. 219 241 CHAPTER XVI. MANUFACTURE OF GOLD AND SILVER "WARES. Composition of gold wares Hall-marking Legal standards of gold plate in various countries Composition of silver wares Legal standards of silver plate in various countries Melting and rolling gold and silver for wares Wire rolling and drawing Manufac- ture of wares from plate Soldering Colouring gold wares Imitation gold and silver wares Electro-plating Gold and silver plating baths pp. 242 253 CHAPTER XVII. PLATINUM. Historical sketch Properties of platinum Platinum black Com- pounds of platinum Alloys of platinum The occurrence of CONTENTS platinum in Nature Composition of native platinum The extraction of platinum from its ores The Wollaston process Deville and Debray's process The preparation of pure platinum The uses of platinum Assay of platinum and its alloys M>. 254276 CHAPTEE XVIII. PRODUCTION AND CONSUMPTION OF THE PRECIOUS METALS. Production of gold in the world in ancient and modern times Present production in various countries Production of silver in the world Consumption of the precious metals in the industrial arts and in coinage World's stock of the 'precious metals Production and consumption of platinum ..... pp. 277 280 Index i>f>. 2S7295 LIST OF ILLUSTRATIONS KNIGHT'S DEEP AND SIMMER EAST JOINT PLANT. VIEW FROM SAND DUMP Frontispiece 1. ANCIENT EGYPTIAN FURNACE 3 2. GERMAN STAMP MILL OF THE SIXTEENTH CENTURY . . 8 3. CURVE SHOWING FREEZING POINTS OF THE ALLOYS OF GOLD AND SILVER 38 4. CURVES SHOWING FREEZING POINTS AND ELECTRICAL CON- DUCTIVITY OF THE ALLOYS OF GOLD AND COPPER . 43 5. GOLD, 25 PER CENT. ; 'COPPER, 75 PER CENT. X 90 . . -44 6. FREEZING POINTS OF THE ALLOYS OF GOLD AND ZINC . 47 7. GOLD, 31'5 PER CENT.; CADMIUM, 685 PER CENT. X 25 49 S. GOLD, 70 PER CENT. ; LEAD, 30 PER CENT. X 70 . . 53 9. FREEZING POINTS OF ALLOYS OF GOLD AND LEAD . . 54 10. FREEZING POINTS OF ALLOYS OF GOLD AND IRON . . 56 11. FREEZING POINTS OF ALLOYS OF GOLD AND NICKEL . . 57 12. FREEZING POINTS OF ALLOYS OF GOLD AND PLATINUM . 59 13. FREEZING POINTS OF ALLOYS OF GOLD AND PALLADIUM . 61 14. FREEZING POINTS OF MIXTURES OF GOLD AND TELLURIUM 63 15. 13ATEA 75 16. CHADLE 76 17. DIAGRAM SHOWING THE VARIOUS PARTS OF A STAMP . So 18. SECTION OF MORTAR . 86 19. SIMMER EAST STAMP MILL 100 20. TROMMEL AND SORTING TABLE, KNIGHT'S DEEP AND SIM- MER EAST JOINT PLANT .102 21. CYANIDE PLANT AND BATTERY HOUSE, KNIGHT'S DEEP AND SIMMER EAST JOINT PLANT 116 22. INTERIOR OF EXTRACTOR HOUSE, KNIGHT'S DEEP AND SIMMER EAST JOINT PLANT 118 xvi LIST OF ILLUSTRATIONS FIG. PAGR 23. SLIMES DAM, KNIGHT' S DEEP AND SIMMER EAST JOINT PLANT 119 24. TUBE MILL, KNIGHT'S DEEP AND SIMMER EAST JOINT PLANT . 121 25. FREEZING POINTS OF ALLOYS OF SILVER AND COPPER . 132 26. FREEZING POINTS OF ALLOYS OF SILVER AND ZINC . .134 27. FREEZING POINTS OF ALLOYS OF SILVER AND LEAD . . 136 28. FREEZING POINTS OF ALLOYS OF SILVER AND TIN . . 139 29. SILVER, 80 PER CENT. ; TIN, 20 PER CENT. X 24 . . 140 30. FREEZING POINTS OF ALLOYS OF SILVER AND PLATINUM . 141 31. SILVER, 94 PER CENT. ; ALUMINIUM, 6 PER CENT. X 30 . 142 32. AMALGAMATING PAN 162 33. ASSAY FURNACE 202 34. CUPEL AND MOULD 203 35. STAS PIPETTE 213 36. HAND PIPETTE FOR DECINORMAL SOLUTION, GAY LUSSAC PROCESS 213 37. SHAKING MACHINE USED IN GAY LUSSAC PROCESS . .214 38. SILVER MELTING HOUSE, ROYAL MINT .... 226 39. ROLLING ROOM, ROYAL MINT 228 40. CUTTING ROOM, ROYAL MINT 229 41. CUTTING MACHINES, ROYAL MINT 230 42. SCISSEL 231 43. ANNEALING FURNACES FOR SILVER BLANKS, ROYAL MINT . 232 44. PRESS ROOM, ROYAL MINT 234 45. WEIGHING ROOM, ROYAL MINT 236 46. TELLING MACHINE, ROYAL MINT 237 THE PRECIOUS METALS CHAPTEK I HISTORY OF GOLD IT is probable that gold was tbe first metal to attract the attention of primitive man, although no traces of it have been found in the dwellings and other relics of the Stone Age. It must be noted, however, that flint daggers with gilt handles have been found in Egypt. Whether gold was the first metal observed or not, however, it is certain that its lustre, bright colour, and toughness, and its wide distribution in a native state as pieces of all sizes in loose sands and gravels, must have led to the collection of gold in very early times. Professor Gowland considers 1 that the order in which the metals were discovered was not the same for every region, but that the metals which occur as such in Nature must have been the first known to the men inhabiting the localities in which they occurred. According to his view, although gold cannot have escaped the observation of the men of the Stone Age, it must generally have been found in the form of fine particles which could not have been applied to any useful purpose until after the invention of the art of melting. Even melted lumps or naturally occurring nuggets 1 Presidential Address, Anthropological Institute, 1906. r.M. B 2 THE PRECIOUS METALS of gold would have been too soft to be of value as weapons or implements, and must have been used only as personal ornaments. Hence gold could have played only a very small part, if any, in the development of culture in Neolithic times. It was well known in Egypt some 5,500 years ago, and had probably at that time already passed into use as a standard of value. Thus in the code of Menes, B.C. 3600, one part of gold was declared equal in value to two and a half parts of silver. The words originally used to denote gold seem to have been connected with its glittering appearance, and indicate the ideas associated with it in the minds of the early observers. Such words are the Egyptian nub, the Hebrew zaliab, and the Sanscrit, 7 ra/ita and liiranya. In later times the alchemists called gold sol, and denoted it by the same symbol as the sun, , not on account of its appearance, but because it was the most perfect of the noble metals, the King of Metals. Silver was lima, the moon, and the other known metals were named after the planets. The ancient methods of obtaining gold in Egypt are illustrated in rock carvings which date back to 2500 B.C. The auriferous gravels were washed with water in stone basins, and the gold melted in little furnaces with the aid of mouth blowpipes. The earliest furnaces being adapted from camp fires were urged no doubt by the wind only, and the introduction of artificial bellows marked a great advance in metallurgical skill. An ancient Egyptian furnace closely resembling a camp fire, but fitted with bellows, is shown in Fig. 1. This is from a wall painting at Thebes and is reproduced from the Journal of the Anthropological Institute, HISTORY OF GOLD ;i Vol. XXXVI., 1906, by kind permission of Professor Gowland and of the Koyal Anthropological Institute. Sloping tables of stone were also used, as well as basins, for washing gold ores, and sheepskins spread on the tables or on sloping rocks for entangling the particles of gold and helping in its collection were employed at an early date. The Golden Fleece seized by Jason in B.C. 1200 doubtless typified the gold obtained from the rivers by its use. FIG. 1. Ancient Egyptian Furnace. The introduction of ore-crushing to obtain gold contained in solid rock took place at some unknown time, but it was well established in Egypt in B.C. 59, when it was described by Diodorus the Sicilian as follows 1 : " The parts of Egypt near Ethiopia and Arabia are rich in gold mines. . . . The ore is a black mineral, marked with white veins and shining specks. The chiefs of the undertaking employ a great number of workmen who are all either condemned criminals or prisoners of war. Even 1 Diodorus Siculus, Bk.-IV., c. 11 and 12. B 2 4 THE PRECIOUS METALS the parents of the condemned are summoned when the number of workmen is insufficient. They work day and night without rest, and under the surveillance of barbarous soldiers, who speak languages different from those of the miners, so that they cannot be won over either by promises or by prayers. " The man who recognises the veins of gold is placed at the head of the workmen, and points out to them the place to excavate. The rocks are broken by iron wedges. The miners follow, in their labours, the direction of the metallic threads, and are assisted by the light of lamps in the subterranean darkness. The stones are carried outside, and are there crushed and reduced to small fragments. " The workmen never cease from their toil; they are forced incessantly to the work by bad treatment and by blows of the whip. Even children are not spared ; some are set to carry the blocks of stone, others to break them into fragments. The fragments are taken by older work- men, of over thirty years of age, and crushed in iron mortars. The fragments thus crushed are then ground in mills with arms, which are turned by women and aged men. Two or three people work at each mill. It is impossible to describe the sufferings of these unhappy ones ; exposed naked to cold and rain, they are allowed no repose ; there is no feeling of pity, either for a weakly woman or for an old man on the verge of the tomb ; no regard to the sick, who may be a prey to the shivering of fever ; they are all struck indiscriminately with repeated blows until they die of their sufferings on the very spot where they have worked." When the ore had been reduced to powder, it was spread on wide, slightly inclined tables, and a stream of water HISTORY OF GOLD 5 flowing over the tables carried off the earthy materials, and left the gold separated by its weight. This operation was repeated by the workmen several times ; they rubbed the powdered ore with their hands for some time, then they wiped it with little sponges in order to remove the impurities which water alone could not carry off. By this means the gold dust became clean and shining. Diodorus adds that these methods were very ancient and had been discovered by the first Kings of Egypt. Hollowed-out stone mortars and also stone grinding-mills have been found in many parts of the world besides Egypt, in proximity to ancient gold workings, and the methods described above were probably carried by the Komans to some of these countries. The use of sieves, which were originally made of hair and fibres, was also probably introduced by the Egyptians, but heavy pestles or stampers, requiring the efforts of several men to raise them and let them fall, had their origin in later times. The gold produced by these simple mechanical methods was naturally very impure, containing varying proportions of silver and other metals. When the silver was in sufficient proportion (20 to 35 per cent.) to affect the colour perceptibly, the alloy was called electrum. This natural mixture of gold and silver was used for some of the earliest gold coinages, but the removal of silver by cementation was begun in very ancient times. Gold, electrum and silver were all recognised and mentioned by Homer, and Dr. Schliemann found all three among the ruins of Troy. The purification of gold was effected, according to Agatharcides, by enclosing it in an earthenware vessel with 6 THE PKECIOUS METALS lead (proportionate in amount to the quantity of gold), salt, a little tin and some barley husks. The vessel was luted up and heated in a furnace for five days and five nights, and was then allowed to cool and opened. The gold, though it had lost a little of its weight, was very pure, and retained neither lead nor tin. Such gold was called aurum obryzum, and the process itself was called obrussa. 1 It seems to have been a combination of cupellation and cementation. The lead and the base metals in the gold would be eventually oxidised and absorbed by the pot, and the silver chloride, formed by the action of the common salt, would also fuse and pass into the pot. The use of the tin is obscure, but the bran would prevent the lead from being oxidised and absorbed too rapidly. This process was well known to the Romans and is mentioned by Cicero, Suetonius, and others. Strabo describes the use of cementation in Spain, and Pliny gives a still more exact description. The process differed from obrussa in the omission of lead and the addition of misy or sulphate of iron. The removal of silver from the gold was more perfect by this method. The Middle Ages were characterised more by the work of the alchemists than by any great progress in the metallurgy of gold. " Chemia " was defined by Suidas as the art of making gold and silver. It had its origin in Egypt at the beginning of our era, and great powers were attributed to the Egyptian priests, who carefully guarded their reputed secrets and made their experiments a part of the mystic rites of their religion. The history of alchemy affords very curious reading. 1 "Histoire de la Chiinie," F. Hoefer, Vol. L, p. 116. HISTORY OF GOLD 7 The alchemists were familiar with a number of chemical changes which seemed to them no less remarkable than the transmutation of mercury or copper into gold would be. For example, they knew that the addition of antimony to copper produced a beautiful violet-coloured substance, and that the incineration of lead yielded a bright yellow body. Why, they asked, should not the " projection " of some sub- stance into the base metals give them the yellow colour and the other properties of gold ? They sacrificed everything to experiments with this end in view and repeatedly announced that they had discovered the secret. Nevertheless no one seems to have prepared the transmuting agent for himself, but to have received it from a mysterious stranger. The credulity and desire for gold of the rich and powerful kept the alchemists supplied with funds to continue their researches, their patrons including many reigning mon- archs. Alchemy continued to flourish throughout Europe for over a thousand years, and the claims of the gold-makers were not generally discredited until about the end of the seventeenth century. Few improvements were introduced in the extraction of gold from its ores for several hundreds of years previous to the beginning of the sixteenth century. In 1519 stamp batteries for crushing ore were introduced at Joachimsthal, and screens set at the outlet of the mortars were described by Agricola in 1556 and were in use in the Hartz in 1767. l A stamp mill for dry crushing in use in the sixteenth century is shown in Fig. 2, which is taken from Agricola, "De Re Metallica," Lib. VIII. Here A is a screen frame, and B a screen which consists of iron wire and is fastened 1 " Voyages Metallurgiques, " Jars, Vol. II., p. 309. THE PKECIOUS METALS to the frame by iron rods D. The screen E is used for separating the fine material from the coarse ore which is FIG. 2. German Stamp Mill of the sixteenth century. fed to the stamps. The crushed material is sieved at F. The stamps are raised by water power. The use of mercury for separating gold from other materials was described by Pliny and was probably never HISTORY OF GOLD 9 forgotten. Thus Geber, who died in 777, was aware that mercury would dissolve gold and silver but not earthy materials, and Theophilus in the eleventh century described the extraction of gold from the sands of the Rhine by means of quicksilver. In the Tyrol the method of gold ore treat- ment included crushing in stamp batteries and the stirring of the crushed ore in circular bowls with large quantities of mercury. A stream of water passing through the bowl carried off the tailings, leaving the gold to sink to the bottom of the bowl and unite with the bath of mercury kept there. This process is of great antiquity, and its principle is still used in the modern amalgamating machines of Hungary. The catching of amalgam by means of copper plates is more modern, and was probably suggested by the Cazo process, which was introduced in Peru in 1609. l No mention is made of charging mercury into stamp batteries and catching the gold amalgam on copper plates before the year 1850. The chlorination process of gold ore treatment originated in 1848, and the cyanide process in 1887. As for the parting of gold from silver, the cementa- tion process continued in use by the side of some other furnace processes until it was displaced by the use of nitric acid, which dissolves silver and other metals and leaves gold unattacked. The process was described by Albertus Magnus in the thirteenth century, but was not used on a large scale until about 200 years later in Venice. It was introduced into Paris in 1514, and has become obsolete only within the last hundred years. The sulphuric acid method was established in 1802 in Paris, and has flourished 1 Barba, " Arte de los Metales," Lib. III., cap. xxi. 10 THE PEECIOUS METALS for about a century. It is now being superseded by the electrolytic process, which has already replaced it throughout America. In Australia the chlorine process of refining and parting, introduced in 1867, is the only one that has ever been used, owing to the high price of acids on that continent. CHAPTER II THE PROPERTIES OF GOLD PHYSICAL PROPERTIES. GOLD has a characteristic yellow colour, the so-called golden yellow. If it contains silver the colour is paler yellow, and if it contains copper the colour is a more reddish-yellow. The colour of pure gold is seldom seen, although it is closely imitated by certain mixtures of gold, silver, and copper, used for wares of 18- and 22-carat gold. Some specimens of gold-leaf consist of nearly pure gold. Finely divided gold is often red or purple, as may be seen in purple of Cassius (q.v.) and in some ruby glass, although most specimens of ruby glass are now coloured by oxide of copper instead of by metallic gold. Gold-leaf is green by transmitted light. Gold precipitated from solution varies in colour from black to bright red (see Faraday's gold). Volatilised gold condenses as a reddish-purple stain. Molten gold is green. Gold is softer than silver, but harder than pure tin. It is the most malleable and most ductile of metals. Leaf gold can be made only one three-hundred-thousandth of an inch thick by hammering, and an ounce of gold can be drawn out into fifty miles of wire. The tenacity of gold is seven tons per square inch, or less than that of silver or copper. A wire of O'l inch in diameter is broken by a 12 THE PEECIOUS METALS weight of 123 Ibs. Gold has an elongation of 30'8 per cent, before rupture. Gold crystallises in the cubic system, but large, well- formed crystals are only occasionally found in Nature, and are difficult to produce artificially. There are some large cubic crystals of pure gold preserved in the Mint, which remained in a crucible when the molten charge was poured out. The density of cast gold is about 19*3, and this can be raised by rolling to 19 '48, and by hammering to 19'65. The density of precipitated gold is usually somewhat higher, and one specimen was found to be as high as 20'72. The melting point of gold is 1,064 C., and it begins to volatilise at about the same temperature. The amount of volatilisa- tion, however, remains small at the temperatures attained in ordinary industrial furnaces. Gold boils readily in electric-arc furnaces, and its boiling point has been estimated by Krafft and Bergfeld to be about 2,530 C. under atmospheric pressure. Its electrical conductivity at ordinary temperatures is 76*7, that of silver being 100. Its conductivity for heat is also less than that of silver and of copper. It is non- magnetic. CHEMICAL PROPERTIES. Originally gold was called by the alchemists a noble metal, because it was not affected by fire. When put into the fire, and even melted, it. emerged unchanged, whereas the " base " metals were oxidised and "destroyed " (i.e., they lost their metallic character) under the same conditions. In the cupellation process the furnace was observed to THE PROPERTIES OF GOLD 13 remove the base metals from admixture with gold, leaving the noble metal unscathed. The other noble metal recog- nised by the alchemists was silver, but it was deemed less perfect in its nobility than gold, because it could be dissolved in aqua fortis, by which gold is unaffected. Hence gold was called the more noble and silver the less noble or less perfect metal. This identification of physical with moral qualities still has its effect on our nomenclature. The symbol of gold is Au (from aurum), and its atomic weight is 197'2, taking that of oxygen as 16. In its solution in mercury it is apparently monatomic. It bears a close relation to the heavy platinum metals, osmium, iridium, and platinum. Silver is similarly related to ruthenium, rhodium, and palladium, the light platinum metals. Thus the atomic weights of gold and the heavy platinum metals are not very different ; their densities are all very high, and they all form lower oxides which are feebly basic, and higher oxides which are both acidic and basic in a slight degree. At the same time gold is also related to silver and copper, compounds of the type RX existing in each case. The substances aurous chloride, AuCl, silver chloride, AgCl, and cuprous chloride, CuCl, resemble each other in several respects. They are insoluble in water (although AuCl is decomposed by it), but are soluble in potassium cyanide, hydrochloric acid, and ammonia. Gold differs from silver and copper in the extreme facility with which its compounds are decomposed and the metal isolated. At a gentle heat chlorine and oxygen are expelled from the chlorides and oxides of gold, and metallic gold alone remains. These compounds, therefore, act as oxidising agents. 14 THE PRECIOUS METALS Gold is not appreciably attacked at any temperature by water or by air, and consequently it does not become tarnished. Molten gold gradually absorbs oxygen from the atmosphere, and retains at least part of the oxygen after solidification. According to Neumann, finely divided gold absorbs from 32 to 48 volumes of oxygen at 450, or 0'24 to 0'36 per cent, by weight. Gold also occludes hydrogen and carbon monoxide under suitable conditions. Gold is not perceptibly attacked by alkalies or by hydro- chloric, nitric, or sulphuric acid. Finely divided gold is slightly soluble in boiling hydrochloric acid and in boiling nitric acid. It dissolves readily at the ordinary temperature in water containing chlorine, bromine, or a mixture of iodine and potassium iodide. It also dissolves readily in a boiling concentrated solution of ferric chloride, and in any mixture producing nascent chlorine, bromine, or iodine. It is slowly dissolved by a solution of potassium cyanide at the ordinary temperature. This action depends on the presence of oxygen or an oxidising agent. The most rapid solvent for gold is hot aqua regia (a mixture of three parts of hydrochloric acid with one part of nitric acid). Another convenient method of dissolving it is to pass a current of electricity through a solution of an alkaline cyanide, using an anode of gold. 1 The compounds of gold are not numerous and have not been fully studied. One of the chief chemical characteristics of gold is the difficulty with which its compounds are formed and the ease with which they are decomposed. 1 For further information on the solvents of gold see "The Metal- lurgy of Gold," T. K. Eose, 5th edition, 1906, pp. 9 and 12. THE PKOPERTIES OF GOLD 15 IDENTIFICATION OF GOLD. Gold in the metallic form can be readily identified by its colour, its great density, and by the fact that it is not attacked by hot nitric acid. For confirmation, the gold may be dissolved in aqua regia, and the yellow liquid evaporated to dryness with the occasional addition of hydrochloric acid. The residue, which must not be heated above 100, is redissolved in water, and sulphurous acid gas is passed through the liquid. A black or brown pre- cipitate insoluble in nitric acid denotes the presence of gold. Cupellation is also a good test of the presence of gold (see chapter on Assaying). The lead remaining on the cupel must be tested further, as it may consist of silver or the platinum metals. Gold in solution may be identified by acidifying with hydrochloric acid and passing sulphuretted hydrogen or sulphurous acid. If nitric acid or other oxidising agents are present they must be destroyed before SH 2 or S0 2 is passed through the solution. The precipitate with sul- phuretted hydrogen is black if the solution is cold, brown if it is boiling. The precipitate is soluble in ammonium sulphide, and if it is cupelled or merely ignited it yields metallic gold. Sulphurous acid gives a black precipitate of metallic gold, insoluble in nitric acid, and yielding a yellow bead when cupelled. In these points it differs from tellurium, which also gives a black precipitate with sul- phurous acid. Other reactions of gold in solution as chloride, in the absence of nitric acid, are as follows : A mixture of protochloride and perchloride of tin gives a precipitate of purple of Cassius. 16 THE PKEGTOUS METALS Ferrous salts yield a black or brown precipitate of metallic gold. In very dilute solutions, the liquid becomes blue by transmitted light and brown by reflected light. Oxalic acid and alkaline oxalates precipitate metallic gold, especially on the application of heat. Ammonia gives a yellow precipitate of fulminating gold. Potash and soda give a precipitate of oxide of gold soluble in excess. If a large quantity of hydrochloric acid is present the precipitate is not formed. In the absence of an excess of hydrochloric acid potassium cyanide gives a yellow precipitate soluble in excess. Solutions containing aurocyanides of the alkalies and thiosulphates of gold do not give the ordinary reactions of gold, although sulphuretted hydrogen precipitates sulphide of gold from thiosulphates. Solutions containing cyanides or thiosulphates may be tested for gold by heating them with aqua regia, evaporating to dryness, taking up with water and a little hydrochloric acid, and testing the solu- tions in the ordinary way. COLLOIDAL GOLD. Colloidal gold suspended in water may be obtained in various ways by precipitating dilute solutions of gold chloride. Such gold was first prepared by Faraday, and the ruby-coloured liquids containing it are known as " Faraday's gold." A good method of preparation is to raise to boiling a solution containing from O'Ol to O'OOl per cent, of gold chloride, made slightly alkaline by the THE PEOPEETIES OF GOLD 17 addition of a little magnesia, and then to add a few drops of formaldehyde or oil of turpentine, or a solution of acety- lene, or of phosphorus dissolved in carbon disulphide. The red liquid may also be prepared cold, and in that case the action is much more rapid if a drop of liquid containing colloidal gold is added to " inoculate " the solution. The gold is in the form of minute particles, which are far too small to settle in water under the influence of gravity, so that the liquids remain unchanged even if left undis- turbed for years. Nevertheless, the gold is not in solution, as is shown by the fact that it can be removed from the liquid by shaking with freshly precipitated barium sulphate and other precipitates. These particles of gold are rendered visible under the microscope by means of a powerful ray of light thrown from the side. The smallest particles that become visible are rather less than one hundred-thousandth of a millimeter in diameter, or about six times the diameter of a molecule, so that they are aggregates of about 200 atoms of gold. The gold is collected and the solutions decolorised by shaking with mercury. Larger aggregates of gold are formed and the liquid gradually changed to a blue colour by the action of acids, neutral salts, alcohol in excess, etc. The blue colour in time changes to violet and then to black, and the gold settles to the bottom. The same red colour is seen in ruby glass containing gold. The gold purples, such as the well-known purple of Cassius, also owe their colour to finely divided metallic gold or a " colloidal solution " of gold. Purple of Cassius is prepared by adding SnCl2 and SnCU to a solution of P.M. c 18 THE PKECIOUS METALS gold chloride. A purple precipitate of hydrated stannic oxide is formed, mixed with finely divided metallic gold. The same colour is seen when alloys of silver, tin and gold are attacked by nitric acid. Eecently it has been found by H. Moissan 1 that when a gold-tin alloy is raised to its boil- ing point in an electric furnace and the metallic vapours allowed to escape into the air, the tin burns and a substance is obtained with the properties of purple of Cassius, and containing tin dioxide, calcium oxide and gold. Similar purples are obtained by distilling mixtures of gold with lime, alumina, magnesia, silica, etc. Miiller also prepared in the wet way a number of purples,* containing magnesia, lime, baryta and alumina instead of oxide of tin. THE PEEPAKATION OF PURE GOLD. " Parted " gold, which contains little except a small propor- tion of silver, is dissolved in nitro-hydrochloric acid, and the excess of nitric acid is expelled by evaporation on a waterbath with additional HC1. The product consists chiefly of chloro-auric acid, HAuCl4. As soon as scales of gold begin to appear on the surface of the black fused liquid and a smell of chlorine is perceived, the. vessel is allowed to cool, when the contents solidify. The gold chloride is then dissolved in a little distilled water and poured into a larger volume of water. A yellow solution is produced with a white cloud of silver chloride suspended in it. This is allowed to settle and the solution is siphoned, off, diluted to about 150 c.c. for each gram of dissolved gold, and precipitated by sulphur dioxide or oxalic acid. The former 1 Comptea Bendus, Vol. CXLI. (1905), p. 977. UNIVERSITY THE PROPERTIES OF GOLD 19 is the more convenient reagent, as it acts at the ordi- nary temperature. The brown precipitate of gold (largely consisting of scales and plates, if oxalic acid is used) is allowed to settle, washed at first by decantation, and then, when the smell of 862 has been removed, by shaking with pure cold distilled water in a large flask. After shaking with many fresh additions of water, the flask is heated. As soon as it can be boiled for twenty-four hours with distilled water without showing a trace of hydrochloric acid in the liquid (tested by a considerable excess of silver nitrate), the gold is dried, melted in a plumbago or clay crucible and cast in a blackleaded mould. No scum should appear on the surface of the molten gold, but if a scum appears, it is advisable to clear it awa by adding a pinch of borax. The ingot is scratch-brushed and cleaned in acid. It is rolled down between carefully cleaned rolls. The gold may some- times be still not quite pure, and in that case it can be improved by passing it again through the whole process, redissolving and reprecipitating it. This method is used at the Eoyal Mint in the preparation of fine gold trial plates. The most convenient amount of gold to work on is from 10 to 20 oz. Larger amounts than these must be subdivided, and smaller amounts may give less satisfactory results owing to the introduction of impurities in melting and casting. c 2 CHAPTER III THE COMPOUNDS OF GOLD THERE are two series of simple compounds of gold, corresponding to the general formulae AuE and AuE 3 , in which E is a negative monovalent radicle. There are also some complex compounds which do not exhibit the ordinary reactions of gold. In these gold is regarded as forming part of certain complex ions (see below under " Cyanides of Gold," p. 28). GOLD CHLORIDES. There are two well-known chlorides, AuCl and AuCla. As their densities in the gaseous condition have not been determined their molecular formulas are not certainly known. The formulae given above are the simplest that can be assigned to these compounds. They are formed simultaneously by the action of chlorine on metallic gold at moderate temperatures, and are split up into gold and chlorine at slightly higher temperatures. Gold and chlorine unite at the ordinary temperature in the presence of moisture, and this property is taken advantage of in the extraction of gold from its ores in the Plattner and other chlorination processes. The rate of action increases as the temperature rises to about 225 C., when the rapidity of absorption of chlorine by gold in the presence of water vapour is at a maximum. At this temperature a mixture of about 20 per THE COMPOUNDS OF GOLD 21 cent, of AuCl and 80 per cent, of AuCl 3 is formed in an atmosphere of chlorine, approximately corresponding to the empirical formulae Au 2 Cl 5 . Meanwhile the opposing tendency of the chlorides of gold to decompose into gold and chlorine begins to be apparent at about 70 in an atmos- phere of chlorine, and the rate of decomposition increasing with the temperature becomes very rapid at about 300, so that above this temperature, even in an atmosphere of chlorine, gold chloride is not formed to any great extent. In the air, the decomposition of gold chloride takes place at far lower temperatures. At 100, 25 per cent, of AuCla is converted into AuCl in seven days. At 190, AuCla is completely converted into AuCl in ten hours, and into metallic gold in six days. The volatilisation of chloride of gold is of importance in connection with the loss of gold in the roasting of ores with salt. It was found by the author 1 that volatilisation begins at about 180 and rapidly increases to a maximum at about 290. It then falls off gradually until the temperature reaches 800, when it again begins to increase rapidly. The rate of volatilisation depends partly on the vapour tension of gold chloride and partly on its pressure of dissociation, which is at a maximum between 300 and 800. The volatilisation between 500 and 800 (very dull red to red heat) is small. The gold volatilises chiefly in the form of AuCl 3 , with which a little AuCl is mixed. Aurous chloride, AuCl, can be best prepared by heating AuCl 3 to 190 in air. It is a lemon-yellow hygroscopic powder, insoluble in water and ether, but soluble in potassium cyanide. It is decomposed by water at the 1 Chem. Soc. Jour., Vol. LXVII. (1895), p. 881, 22 THE PKECTOTJS METALS ordinary temperature into gold, which remains as a fine powder, and AuCl 3 , which dissolves in the water. The density of aurous chloride is 7'4. For its behaviour when heated see above. Auric chloride, AuCl 3 , can be prepared by acting on finely divided gold with chlorine, especially at about 200. or by dissolving gold in aqua regia. In the latter case the excess of acid is removed as far as possible by evaporation or a water bath, and the reddish-black liquid is allowed to crystallise. Some chloro-auric acid, HAuCLi, remains mixed with the auric chloride. Pure auric chloride is difficult to prepare. The product obtained by acting on gold with chlorine at 200 to 300 always contains less chlorine than corresponds to the formula AuCl 3 . If the product is dissolved in water, the metallic gold separated by decantation, and the solution evaporated to the crystallising point, large orange crystals of the hydrate AuCl 3 .2H 2 separate. These can be partly dehydrated by stand- ing on a porous tile over concentrated sulphuric acid for some days. Auric chloride is brilliant red to dark red in colour. It crystallises in needles and leaflets. Its melting point is 288 and its density is 4'3. It is deliquescent and dissolves in water readily with evolution of heat. It is also soluble in ether and alcohol. Water dissolves about five times its weight of AuCl 3 . The strongest solutions are dark red, but weaker solutions are of a bright lemon-yellow colour. These solutions are decomposed and gold precipitated by many reducing substances, such as sulphurous acid, oxalic acid, FeS0 4 , FeCl 2 , phosphorus, hypophosphites, nitrites and nitrous acid, NO, N 2 4? arsenious and antimonious THE COMPOUNDS OF GOLD 23 acids. The reaction in the case of precipitation by S(>2 may be expressed as follows : 2AuCl 3 + 3S0 2 + 6H 2 = 2Au + 6HC1 + 3H 2 S0 4 . Pure hydrogen, unless nascent, does not appear to act on chloride of gold, but most of the metals decompose it at once. Mercury forms a gold amalgam, lead gives dendritic gold, and tin often yields a purple-coloured precipitate resembling purple of Cassius as well as gold in the form of brown powder. Many organic compounds also precipitate gold, their action being assisted by light and heat. Charcoal acts best on boiling. Solutions of gold chloride leave a violet stain on the skin, which can be removed by cyanide solutions. Cyanide of potassium, if added to gold chloride in solution, produces at first a yellow precipitate soluble in excess ; if much free hydrochloric acid is present, no precipitate is formed. Sulphuretted hydrogen precipitates metallic gold at 100, but at lower temperatures mixtures of gold, sulphur and sulphides of gold are thrown down. Oxalic acid reduces gold chloride very slowly at ordinary temperatures, but rapidly at 100. The reaction may be expressed by the following equation : 2AuCl 3 + 3C 2 H 2 4 = 2Au + 6HC1 + 6C0 2 . The detection of minute quantities of gold chloride in solution is usually effected either by means of protochloride of tin or of ferrous sulphate. Protochloride of tin, SnCl 2 , gives a brown precipitate of ill-defined composition with concentrated solutions of auric chloride. A mixture of SnCl 2 and SnCl 4 , however, prepared by digesting a piece of metallic tin in a solution of SnCl 2 , with the addition of 24 THE PBECIOUS METALS a little hydrochloric acid, gives a purple precipitate of purple of Cassius. The test may be made a very delicate one. The best method of carrying it out is to heat the solution of gold chloride to boiling, and then to mix it suddenly with a few cubic centi- metres of a solution of stannous chloride. This is most easily effected by pouring the auric chloride solution into a large beaker containing the stannous chloride. A yellowish- white flocculent precipitate of tin hydrate is formed, and this settles rapidly, and can be separated and transferred, together with a few cubic centimetres of liquid, to a Nessler's tube. In this way the author has detected the presence of one part of gold in solution as chloride in 100,000,000 parts of water (or one grain of gold in six tons of water). The amount of solution used in this case was about three litres, and the colour of the precipitate was a faint greyish- violet, the difference between it and stannous chloride pre- cipitated by pure water being just visible. The presence of one part of gold per million is signalised by a purplish- red to blackish-purple colour. The ferrous sulphate test is less delicate, but more easily and quickly applied. In comparatively strong solutions a brown precipitate of metallic gold is formed, and in weaker solutions the colour of the solution becomes blue by trans- mitted light. By comparing two Nessler tubes, one con- taining distilled water and the other a solution of gold chloride, the presence of one part of gold in about 720,000 parts of water can be detected by the addition of ferrous sulphate. Chloro-auric acid, HAuC^. When gold is dissolved in aqua regia and evaporated with excess of hydrochloric acid THE COMPOUNDS OF GOLD 25 to the point of crystallisation, long yellow needles are formed on cooling, having the composition HAuCl 4 .4H 2 0. When heated these needles fuse and then decompose, yield- ing hydrochloric acid, chlorine, and a mixture of aurous and auric chlorides. The crystals are deliquescent and readily soluble in water. The solutions are in general reduced by the same agents as auric chloride. A number of double salts of the same type are known and are distinguished by the name chloro-aurates. They are prepared by adding a slight excess of metallic chloride to a solution of HAuCU, evaporating to dryness, taking up with pure water and crystallising out by evaporation in a vacuum. They may be purified by redissolving and recrystal- lising them. They are all soluble in water and alcohol, and may be represented by the general formula, AuCl 3 .RCl.nH 2 0. Potassium chloro-aurate crystallises in yellow needles, having the composition 2KAuCl4.5H 2 0. Sodium chloro- aurate, NaAuCl4.2H 2 0, which also forms yellow needles, is used in photography in the toning bath. When pure it contains 49*5 per cent, of gold. Chloro-aurates of ammonium, barium, strontium, lithium, magnesium (MgAu 2 Cl 8 .6H 2 0), calcium, manganese, nickel, cobalt, zinc, cadmium, rubidium and caesium are also known in crystalline form. They are all dehydrated at 100. Double chlorides of gold and phosphorus, gold and sulphur and gold and selenium have also been prepared. GOLD BROMIDES. Two bromides, AuBr and AuBr 3 , have been isolated cor- responding to the chlorides. Their vapour density has 26 THE PBECIOUS METALS not been determined, so that their molecular formulae are unknown. Aurous bromide, AuBr, is obtained by heating auric bromide, or better still bromo-auric acid, to about 115. If bromo-auric acid, HAuBr 4 .5H 2 0, is heated in a porcelain basin, it melts and gives off HBr and water. After being kept at 115 for some time, the colour becomes yellowish- grey, and nearly pure AuBr remains. Thus prepared it is a yellowish-grey amorphous powder, not deliquescent and insoluble in water, but decomposed by it on boiling into gold and auric bromide. Hydrobromic acid converts it into metallic gold and bromo-auric acid. At temperatures above 115 it is decomposed into gold and bromine. Auric bromide, AuBr 3 , is prepared by acting on finely divided gold with bromine in the presence of water. The action is slow and may be somewhat expedited by heat. Gold is more readily soluble in a mixture of nitric and hydrobromic acids, but in this case the nitric acid must be subsequently expelled by evaporating at a gentle heat, if auric bromide is to be prepared. In evaporating solutions containing either bromide or chloride of gold, the bottom only of the porcelain basin must be heated, while the sides are kept cool. If this precaution is not taken, the salt creeps up the sides and over the edge, and is found in part on the outside of the vessel. Auric bromide crystallises in black needles and in scarlet plates. It also occurs as a brownish-black powder. It is not deliquescent and is not very soluble in water, dissolving in about 100 parts of water at the ordinary temperature. Even highly dilute solutions are coloured red, and as the proportion of auric bromide increases the solution darkens, THE COMPOUNDS OF GOLD 27 and concentrated solutions are nearly black. Auric bromide is soluble in ether, and resembles auric cbloride in many of its properties. It is split up into gold and bromine at about 140. Its solutions are reducible in the same way as those of chloride of gold. Sulphur dioxide completely decolorises a solution of auric bromide before any gold is precipitated. The final result of the action may be repre- sented by the following equation : 3S0 2 + 2AuBr 3 + 6H 2 = 3H 2 S0 4 + 2Au + 6HBr. Auric bromide forms double bromides or bromo-aurates with bromides of some other metals. The bromo-aurates are of reddish-brown colour, and are similar in properties to the chlor-aurates. GOLD IODIDES. Iodine combines with gold less readily than either chlorine or bromine. Gold dissolves in a solution of potassium iodide containing iodine, forming a solution of potassium iodo-aurate, KAuI 4 . This salt crystallises in black, lustrous, quadrangular prisms which decompose on heating. Auric iodide, AuI 3 , is obtained as a dark green precipitate by adding AuCla little by little to a solution of potassium iodide. Auric iodide is very unstable, and breaks down into aurous iodide and iodine at the ordinary temperature. It is decomposed by potash with precipita- tion of metallic gold. Aurous iodide, Aul, is obtained as a pale yellow precipitate by adding potassium iodide little by little to a solution of auric chloride. It is decomposed into gold and iodine at a temperature of about 120. Some double iodides or iodo-aurates are also known, as, for example, KAuI 4 . 28 THE PRECIOUS METALS GOLD CYANIDES. The only simple cyanide known is aurous cyanide, AuCy. A number of double cyanides corresponding to a hypo- thetical auric cyanide, AuCy 3 , are also known, but no such compound as AuCy 3 has been isolated. Aurous cyanide is prepared by evaporating down a solu- tion of potassium aurocyanide, KAuCy2, with hydrochloric or nitric acid and washing it with water in the dark. The residue when dried consists of yellow crystalline grains of aurous cyanide. It is insoluble in water, alcohol and ether, and is unaffected by exposure to air and light provided that it has been well dried. It is not attacked by nitric, hydro- chloric or sulphuric acid, but dissolves in aqua regia. When heated, cyanogen is given off and metallic gold remains. It is soluble in ammonia, sodium thiosulphate, yellow ammonium sulphide, and in alkaline cyanides. Aurocyanides of ammonium, potassium, sodium, barium, strontium, calcium, cadmium, zinc and cobalt have been isolated. These compounds when in solution contain the 'ion AuCy2, " aurocyanidion," and not the simple ion Au. They are colourless and do not show the ordinary reactions of gold. When a solution of potassium auro- cyanide, KAuCy 2 , is electrolysed, it is believed that potassium is set free at the cathode and the ion AuCyg at the anode. The potassium acts on water, liberating hydrogen and forming potash, and also perhaps in part replaces the gold in solution directly, precipitating the gold at the cathode and forming potassium cyanide. The AuCy2 set free at the anode immediately breaks down into AuCy and Cy. The AuCy is either precipitated in the anode THE COMPOUNDS OF GOLD 29 sludge or is redissolved in excess of KCy if that is present, and the nascent cyanogen generally attacks the anode, dissolving or oxidising it. Aurocyanicle of potassium, KAuCy2, is formed by dissolving AuCy, fulminating gold or Au 2 in a solution of cyanide of potassium. It is also formed by passing a current of elec- tricity through a solution of cyanide of potassium, using a gold anode. Soon after this was discovered it was found by Bagration * that plates of gold are slowly dissolved by cyanide solutions without the aid of an electric battery. This is due to the presence of oxygen, as was pointed out by Eisner. 2 The amount of oxygen required was shown by Maclaurin 3 to be in accordance with the equation 4Au + 8KCy + 2 + 2H 2 = 4KAuCy 2 + 4KOH. According to Bodlander 4 the action takes place in two stages, the first stage being accompanied by the production of hydrogen peroxide, which then acts as an oxidiser and enables the cyanide to dissolve more gold, thus : (1) 2Au + 4KCy + 2H 2 + 2 = 2KAuCy 2 + 2KOH + H 2 2 . (2) H 2 2 + 4KCy + 2Au = 2KAuCy 2 + 2KOH. Oxygen can be replaced by many other oxidisers in this reaction. Among these may be mentioned sodium dioxide, barium dioxide, hydrogen peroxide, potassium permanga- nate, potassium ferricyanide, chlorine, bromine, etc. The 1 Bull, de VAcard. des Sciences de St. Petersburg, Vol. II. (1843), p. 136. 2 Erdm. Jour. Prak. Chem., Vol. XXXVII. (1846), p. 441. 3 Jour. Chem. Soc., Vol. LXIII. (1893), p. 724. * Zeit. fur Angewandte Chemie, 1896, p. 583. 30 THE PEECIOUS METALS dissolution of gold is assisted in all cases by heat, if the oxidiser is not destroyed by it too rapidly. Aurocyanide of potassium is also formed without the presence of an oxidiser by the action of the double cyanide of mercury and potassium on metallic gold. In this case the gold directly replaces the mercury in solution according to the equation HgCy 2 .2KCy + 2Au = 2KAuCy 2 + Hg. The action is greatly expedited by boiling the solution. Aurocyanide of potassium crystallises in colourless rhom- boidal octahedra, which remain unchanged in air at the ordinary temperature, but are decomposed by heat, cyanogen being given off and a mixture of gold and potassium cyanide remaining. The aurocyanide is soluble in seven parts of cold water and in half a part of boiling water. When it is heated with nitric, hydrochloric or sulphuric acid, hydro- cyanic acid is given off and aurous cyanide is precipitated. On boiling with aqua regia all the cyanogen is removed and gold chloride remains. Ferrous salts are without action on solutions of KAuCy2, but according to Lindbom oxalic acid and sulphur dioxide precipitate aurous cyanide from hot solutions. Gold is precipitated from solutions of potassium auro- cyanide by the passage of a current of electricity or by the action of certain metals, notably zinc, copper and aluminium. The reaction is probably mainly one of simple displace- ment, thus : 2KAuCy 2 + Zn = K 2 ZnCy 4 + 2Au. When cyanide of potassium is not present in excess, the action stops almost at once, owing to the formation of a THE COMPOUNDS OF GOLD 31 layer of zinc hydrate or zinc cyanide on the zinc, or perhaps to the " polarising " action of a layer of hydrogen. When KCy is present in excess the zinc dissolves in it, hydrogen is freely evolved, and potash is formed. In the cyanide process, the zinc shavings used to precipitate the gold tend to become coated with a " white precipitate," which consists of a mixture consisting mainly of zinc hydrate, zinc cyanide, and ferrocyanide of zinc and potassium. 1 Gold is also precipitated from solutions of KAuCy2 by charcoal, and AuCy is precipitated by the addition of copper sulphate after acidification with SCV Aurous cyanide is also precipitated by neutralising a solution of KAuCy2 by means of sulphuric acid and adding cuprous chloride. Aurocyanide of potassium is used in gold baths for electro-plating. The baths may be prepared by passing an electric current through a solution of potassium cyanide, using a large anode of gold and a small cathode. Aurocyanides of ammonium, NH 4 AuCy 2 , and sodium, NaAuCys, are similar in their properties to the potassium salt. The ammonium salt is easily soluble in water, but the sodium salt is only slightly soluble. All these aurocyanides, when acted on by chlorine, bromine, or iodine, yield compounds, of which KAuCy2.Cl2 + 2H 2 0, may be taken as a type. These compounds are very soluble in water, and are decomposed by heat. Auncyanides are salts of auricyanhydric acid, HAuCy4, which can be prepared as follows : A solution of AgNOa is added to a solution of auricyanide of potassium. A 1 Prister and Bay, Jour. Chem. Met. and Mmj. Soc. of S. A., Vol. V. (1904), p. 75. 32 THE PRECIOUS METALS precipitate of auricyanide of silver is obtained, and this is washed and then treated with a small quantity of HC1. The liquid is then filtered, to remove the silver chloride, and evaporated in a vacuum, when colourless tabular crystals of HAuCy 4 + 1JH 2 result. Auricyanide of potassium, KAuCy 4 + 1JH 2 0, is formed by adding a perfectly neutral solution of AuCl 3 to a warm concentrated solution of KCy. On evaporating down, large tabular colourless crystals of the auricyanide are formed. They are easily soluble in hot water, and are less soluble in cold water. Auricyanide of potassium begins to decom- pose when heated to 200, but fuses at a red heat long before its decomposition is complete. Auricyanides of ammonium, silver and cobalt are also known. Sulpl 10 cyanides of gold are known only as double sulpho- cyanides,such as aurous potassium sulphocyanide, KAuS2Cy2, formed by adding AuCl 3 to KSCy at 80. This salt forms insoluble precipitates with many metallic salts. Seleno- cyanates of gold are also known, but cyanate of gold has not been prepared. OXIDES OF GOLD. Three oxides have been isolated, Au 2 0, AuO and Au 2 3 . These are obtained by precipitating the haloid compounds. They are not readily formed by the action of oxidising agents on metallic gold, and are all reduced at a moderate temperature by heat alone, oxygen being given off and metallic gold left as a residue. Auric oxide, Au 2 03, is obtained by the action of potash or magnesia on a boiling solution of gold chloride. The orange-coloured precipitate THE COMPOUNDS OF GOLD 33 of hydrate of gold, Au0 3 H 3 , is dehydrated by drying at about 100. At about 110 oxygen is given off, and at 160 the dark chestnut auro-auric oxide AuO is produced, soluble in hydrochloric acid. Aurous oxide, Au 2 0, is produced by the action of potash on AuBr. It is a violet-coloured powder, soluble in cold water and in alkalies, and decom- posed by hydrochloric acid. It breaks down into gold and oxygen at about 250. Nevertheless, at 450 gold absorbs and retains from 0*24 to 0'36 per cent, of its weight of oxygen, an amount which corresponds to from 6 to 9 per cent, of Au 2 0. In cupellation (q.v.) it is possible that some aurous oxide is produced and absorbed as such by the cupel. Auric oxide dissolves in hydrochloric acid, forming HAuCLi, and it is also soluble in sulphuric and nitric acids. On dilution, hydrated auric oxide is reprecipitated, but by cooling the solution in nitric acid by means of a freezing mixture a nitrate of gold, Au(N03)3.HN0 3 .3H 2 may be crystallised out. Schottlander also obtained sulphate of gold from the nitrate. When metallic gold dissolves in nitric acid the nitrate is probably directly produced. Several crystallisable double nitrates have also been pre- pared. Auric oxide also dissolves in alkalies, forming aurates, whence auric oxide is sometimes called auric acid. Potassium aurate, KAu02.3H 2 0, has been obtained in yellow crystals, and from its aqueous solutions various insol ble aurates have been obtained by precipitation with metallic salts. The aurates are readily reduced by heat, or by organic compounds with the production of metallic gold. When a solution of gold chloride is precipitated by P.M. 34 THE PRECIOUS METALS ia,, fulminating gold is produced, but this is perhaps better regarded as an amine than an aurate. Easchig states that if prepared in this way it is a mixture of auric diamine, (AuN.NH 3 ) 2 .3H 2 0, with auric imidochloride, NH.AuCl. It is also produced by the action of ammonia on moist auric oxide. It is of a yellow colour, is soluble in potassium cyanide and in hydrochloric acid, and detonates when struck or heated to 143. Sometimes it explodes spontaneously at ordinary temperatures. The products of its decomposition are gold, ammonia, nitrogen and water. Other fulminates are obtained by the action of ammonia on aurous oxide, Au 2 0. The aurates also give rise to double sulphites of gold and the alkalies (or auro sulphites), when alkaline aurates are acted on by means of alkaline sulphites. The ammonium, sodium, potassium, and barium salts are known. Thiosulphate of gold and sodium, Au2S203.3Na2S 2 03.4H 2 0, crystallises in fine colourless needles, readily soluble in water. Its solutions are sweet to the taste. It is pre- pared by adding neutral chloride of gold to thiosulphate of soda, and precipitating the salt with alcohol, in which it is very slightly soluble. It was formerly used, under the name " Sel de Fordos et Gelis," for intensifying the images of daguerreotype pictures, gold being deposited on the silver amalgam. The salt still plays an important part in photography in " toning " silver prints, as it exists in combined toning and fixing baths. The solution of the salt exhibits neither the characteristic reactions of gold nor those of thiosulphates. Neither ferrous sulphate, stannous chloride nor oxalic acid precipitates gold from thiosulphate solutions, and sulphuric or hydrochloric acid does not at THE COMPOUNDS OF GOLD 35 once precipitate sulphur or disengage sulphurous acid. On heating for some time with these acids, or on passing sulphuretted hydrogen through the solution it yields sulphide of gold. Silicates of gold are prepared by fusion of an aurate or of gold chloride mixed with an oxidiser, such as antimony oxide or nitre, with a large excess of a glass rich in silica. The result is a clear yellow glass containing about 0'2 per cent, of gold. SULPHIDES OF GOLD. Two sulphides are known, Au 2 S and AuS, corresponding to the lower oxides, and a number of double sulphides or thio-salts corresponding to the aurates have also been pre- pared. When H 2 S is passed into a cold solution of AuCla a black precipitate of AuS mixed with sulphur is obtained, the action being partly represented by the following equation : 8AuCl 3 + 9H 2 S + 4H 2 = 8AuS + 24HC1 + H 2 S0 4 . If the solution is heated variable mixtures of sulphur, metallic gold and gold sulphides are obtained. Sulphide of gold begins to decompose at 140, and loses all its sul- phur at about 270. It is unattacked by mineral acids except aqua regia, but is soluble in alkaline sulphides, form- ing thio-salts which are decomposed by mineral acids. Gold sclcnide and gold telhiride are similarly obtained by precipitation from a solution of AuCla by means of H 2 Se and H 2 Te. Phosphide of gold is a grey, readily fusible substance formed by gently heating gold in phosphorus vapour or by passing phosphoretted hydrogen into a solution of gold chloride. It burns when heated in air to about 115. D 2 36 THE PKECIOUS METALS Arsenide of gold is a similar grey, brittle body obtained by heating together gold and arsenic or by acting on gold with the vapour of arsenic. On being heated in air most of the arsenic is expelled. On grinding arsenide of gold with mercury, the gold is amalgamated and the arsenic expelled, but the mercury and amalgam become subdivided into a number of little globules coated with a greyish powder, and it is difficult to collect them again. Antiinonide of gold is similar. See also pp. 50 and 51. CHAPTER IV THE ALLOYS OF GOLD GOLD AND SILVER. GOLD and silver unite in all proportions when melted together, forming homogeneous alloys. On solidification no separation of the constituents takes place, and the microscopic structure resembles that of pure gold or pure silver. If the mixture is kept melted for some time without stirring, a partial separation of the two metals takes place under the action of gravity, gold settling to the bottom. The colour of gold becomes paler when small quantities of silver are added to it, and is white with a scarcely perceptible tinge of yellow when 50 per cent, of silver is present. Alloys containing more than 60 per cent, of silver are silver-white. Gold containing small quantities of silver (10 to 20 per cent.) are stated by some observers to be of a green colour (" green gold "). The melting point of alloys containing less than 5 per cent, of silver is the same as that of pure gold, and further moderate additions of silver lower the melting point only slightly. The alloy containing 35 per cent, of silver melts at 1,061, or only three degrees below the melting point of gold. As the percentage of silver increases the melting point falls more rapidly, but if there is only a trace of gold in silver, its melting point is still above that of pure silver, 1 see Fig. 3. 1 Koberts- Austen and Eose, Proc. Roy. Sac., Vol. LXXI. (1903), p. 161 . 38 THE PRECIOUS METALS All the alloys are soft, malleable and ductile, being intermediate in these properties between pure gold and pure silver. The densities of the alloys are also intermediate between those of the metals, and show no marked expansion or contraction due to the mixing of the metals, although Hoitsema 1 found evidence of slight contraction of about 1100 950 900 10 20 30 4-0 50 GO 70 Atoms oF Silver per cent 80 100 FIG. 3. Curve showing Freezing Points of the Alloys of Gold and Silver. 0-4 per cent, in some of the alloys. At 15 the densities of unworked cast alloys are as shown in the table on p. 39. The electric conductivity of alloys of gold and silver was found by Matthiessen 2 to be below that of either gold or silver. The electric conductivity of silver being taken at 100, that of gold is 78, and that of the 50 per cent, alloy only 15. There is no chemical compound of gold and silver known to be formed, but they appear to form isomorphous mixtures, 1 Zeitschrift fur Anvrgan. Cliem., Vol. XLI. (1904), p. 66. 2 Chem. Soc. Jour., Vol. IV. (1867), p. 201. THE ALLOYS OF GOLD 39 the crystalline structure of all the alloys being precisely similar. The behaviour of these alloys when treated with nitric or sulphuric acid is of interest, as upon it are based the usual methods of refining gold. Alloys containing at least 60 per cent, of silver are completely " parted" by boiling in nitric acid, the silver being dissolved and the gold left behind in a brown porous state. The gold obstinately retains about TABLE OF DENSITIES OF THE ALLOYS OF GOLD AND SILVER. Gold. Density. 100 per cent 19*31 91-6 .... 18-04 79-8 .... 17'54 78-4 .... 16-35 75-0 .... 16-03 66-7 .... 15-07 50-0 .... 13-60 33-3 .... 12-88 25-0 .... 11-78 16-7 .... 11-28 .... 10-47 O'l per .cent, of its weight in silver, and if boiling is continued in strong acid, gold dissolves whilst the amount of silver is not appreciably reduced. If the alloy originally contains less than 60 per cent, of silver, the parting is not complete, but the alloy of gold 1 to silver 1*25, containing 55*5 per cent, of silver, yields gold containing only 0'3 per cent, of silver. With equal parts of gold and silver, however, nitric acid leaves half the silver in the gold, and with smaller proportions of silver the acid has less and less 40 THE PEECIOUS METALS effect. Copper or other base metals can replace silver without materially affecting the results. Alloys of gold and silver containing over 50 per cent, of gold are difficult to dissolve in acid, and should be melted with more silver if it is required to part them. Nitric acid has little effect on them, and nitro-hydrochloric acid, which dissolves the gold, converts the silver into insoluble chloride, which protects the alloy from further attack. The alternate action of nitro-hydrochloric acid and ammonia eventually results in the dissolution of all the gold, but the method is a tedious one. A mixture of aqua regia and common salt is also sometimes used in the dissolution of these alloys, the silver chloride being dissolved by the salt. A large volume of liquid must be used, asonlyO'127 gram AgCl can be held in solution by 100 c.c. of a saturated solution of common salt. Alloys of gold and silver have been used from very early times in jewellery and for coins. Phidon, King of Argos, struck coins of electrum in ^gina about the year 720 B.C. Electrum includes pale coloured alloys, containing from 15 to 35 per cent, of silver. A little later pure gold and electrum were being coined side by side in Lydia, with different money values, the electrum being taken as worth 25 per cent, less than pure gold. 1 The addition of copper to harden both gold and electrum was made in Eoman coins some time before the Christian era. GOLD AND COPPEB. 2 These metals are miscible in all proportions when molten, and on solidification, separate only to a slight degree. The 1 Lenormant, "La Monnaie dans 1'Antiquite," Vol. I., p. 194. 2 Eoberts-Austen and Eose, Proc. Roy. Soc., Yol. LXVII. (1900), p. 105. THE ALLOYS OF GOLD 41 first additions of copper to gold cause a rapid lowering of the melting point. Standard gold, containing gold 91*6 per cent., copper 8*3 per cent., solidifies at 951, or 103 below the melting point of pure gold, and gold of the French standard coinage alloy containing 90 per cent, of gold, solidifies at 946. The alloy containing 82 per cent, of gold and 18 per cent, of copper has the lowest melting point, viz., 905, and this alloy may be a true eutectic. The melting point curve rises continuously with the increase of copper from the eutectic alloy to pure copper, which melts at 1,083. The densities of the alloys are as follows l : Gold. Density. 100 per cent 19*31 91*7 . 17-35 90-0 ,. .... 17-17 83-3 .... 15-86 75-0 ., .... 14-74 58-3 ... 12-69 25-0 .... 10-03 ... 8-7 These densities refer, however, to cast specimens. When hardened by rolling or hammering the densities of the alloys become greater. Thus the usual density of an English sovereign (containing 91*66 per cent, of gold) is about 17 '48, but these coins for the most part contain a small and variable percentage of silver. From the densities it is evident that gold and copper contract slightly (from 0'5 to 1*0 per cent.) on being alloyed. 1 Eoberts- Austen, Ann. Chim. Plnjs., [5] XIII. (1878), p. 118; Hoitsema, Zeit. An. Chem., Vol. XLI. (1904), p. Go. 42 THE PKECIOUS METALS The colour of gold becomes redder when copper is added to it, and if silver is added at the same time, the two metals counteract the effect of each other, so that triple alloys can be formed having a colour closely resembling that of pure gold. The electric conductivity of alloys of gold and copper is less than that of either gold or copper, 1 and is even less than that of the silver-gold alloys (see above). The chemical compound of gold and copper is known to exist, and the alloys in general show no marked change in properties when the proportions of the metals are slightly varied. Copper hardens gold and decreases its malleability, these effects continually increasing until the eutectic alloy is reached. This alloy (containing 18 per cent, of copper) is distinctly brittle, breaking under the hammer with a conchoidal fracture, and showing an elongation before rupture of 3'3 per cent., that of pure gold being 3O8 per cent., and that of pure gold alloyed with 0*24 per cent, of lead being 4*9 per cent. The tensile strength of pure gold is 7 tons per square inch, that of standard gold (containing 8*3 per cent, of copper) 16 tons, and that of the eutectic alloy 7*87 tons per square inch. According to Kurnakoff and Schemtchuschny 2 the two metals are isomorphous, and form a continuous series of mixed crystals. In Fig. 4, which gives the results of their work, the curve of electric conductivity of the alloys is shown in the line PQE. The line ALMLiB is the liquidus curve connecting the points at which solidification begins, and ASMSiB is the solidus curve, denoting the completion of solidification. The minimum M, corresponding to the 1 Matthiessen, Chem, Soc. Jour., Vol. IV. (1867), p. 201. 2 Jour. Euss. Phys. Chem. /Soc., Vol. XXXIX. (1907), p. 211. THE ALLOYS OF GOLD 43 alloy containing about 40*5 atoms per cent, of copper, is coincident with the eutectic observed by Eoberts-Austen and Rose. At this point the two curves meet, and there is no pasty stage. When copper is in excess, that is, when 700 < 10 20 30 40 50 60 70 80 90 100% Percentage of Gold in Atoms. FIG. 4. Curves showing Freezing Points and Electric Conductivity of the Alloys of Gold and Copper. more than 18 per cent, by weight or 40*5 atoms per cent, of copper is present, the part first solidified is rich in copper, forming crystallites, which are darkened by a mixture of chloride of iron and hydrochloric acid. The peripheral layers of the crystallites and the matrix are richer in gold THE PRECIOUS METALS and remain unattacked : see Fig. 5, which is a photo- micrograph, after Kurnakoff, of the alloy containing 25 per cent, gold magnified 90 diameters. The gold-copper alloys are used extensively for coinage and jewellery, pure gold and the gold-silver alloys being too soft, so that they soon become defaced by wear. In jewellery some silver is usually added to the copper and gold (see Chapter XVI.). When alloys of gold and copper are heated in the air they blacken from the formation of oxide of copper, but the black scale can be removed by dissolving in hot dilute sulphuric acid. GOLD AND MERCURY. These alloys are FIG. 5. Gold, 25 per cent. ; Copper, known as amalgams, and 75 per cent. X 90. -, ,,, , are formed, although with difficulty, by the direct union of the two metals at the ordinary temperature. According to Kasentsof, 1 mercury dissolves O'll per cent, of gold at 0, 0*126 per cent, at 20 and 0'65 per cent, at 100. The mixtures con- taining less than these quantities of gold are the alloys, liquid at ordinary temperatures, silver-white like mercury, and passing through cloth or other filtering material unchanged. On the other hand, gold will absorb about six times its 1 Bull. Soc. Chem., [2] Vol. XXV., p. 20. THE ALLOYS OF GOLD 45 weight of mercury, forming a silver- white solid alloy. If this solid amalgam, which contains about 13*5 per cent, of gold, is mixed with more mercury, the result is not a homogeneous body, but a saturated solution of gold in mercury, in which solid particles of gold amalgam are suspended. These solid particles are heavier than mercury and settle to the bottom. They can be separated from mercury by filtration under pressure, but usually retain a small excess of mercury. The amalgams recovered in gold-mills, in which ores are treated by the amalgamation process, are not true alloys. Before straining they consist of mercury containing a number of little "nuggets" of gold into which mercury has penetrated to some extent. On straining, these nuggets coated with mercury are separated, and the gold miner's amalgam is consequently of variable composition, generally containing from 25 to 50 per cent, of gold, the percentage of gold being highest when the average size of the gold particles is greatest. When mercury is applied to the clean surface of a piece of solid gold, it " wets " it and at once penetrates through it, passing between the crystalline faces, making it brittle and of silver-white colour. A gold coin or ring is imme- diately whitened by mercury, and can be broken in the fingers, the fractured surfaces (crystalline faces) being as white as the outside. The amount of mercury so taken up is, however, small, and most of it can be removed by careful heating below a red heat, when the mercury volatilises and, unless air is excluded, the coin blackens. At 440, or about 150 below red heat, a large part of the mercury is removed from amalgam by distillation, and a 46 THE PRECIOUS METALS mixture containing about 75 per cent, of gold remains. At a bright red heat almost all the remainder of the mercury is expelled, but about 0*1 per cent, is retained and cannot be driven off below the melting point of gold. Amalgams are all attacked by hot nitric acid, the mercury being dissolved and the gold left as a spongy mass. If the action of the acid is kept very slow by dilution and cooling, the gold is left behind, nearly pure, in the form of crystalline needles. No evidence has been adduced of the existence of any true compounds of gold and mercury. GOLD AND ZINC. The alloys containing less than 14 per cent, of zinc are pale yellow and of about the same hardness as gold. They increase gradually in brittleness with the increase in per- centage of zinc. The first additions of zinc rapidly reduce the melting point and give rise to a long pasty stage during solidification, so that gold containing one or two per cent, of zinc can be readily welded. The alloy, gold 85*6, zinc 14*4 (approximately corresponding to the formula Au 2 Zn, that is, containing gold 66*6 atoms, Zn 33*4 atoms, see Fig. 6) , is a eutectic, solidifying at 648. As zinc increases from 14 to 25 per cent. (33'4 to 50 atoms per cent.), the colour of the alloy gradually changes from pale yellow to a beautiful reddish-lilac tint, and the melting point rises from 648 to a maximum of about 750. This is the melting point of a distinct compound, AuZn, containing 25 per cent. of zinc. It forms lustrous crystals and is brittle. With further additions of zinc the lilac colour fades, the alloys containing from 25 to 30 per cent, of zinc consisting of THE ALLOYS OF GOLD 47 lilac polygonal crystals of AuZn set in a white matrix. The compound AuZn 2 , containing 39'8 per cent, of zinc (66*6 atoms per cent.) is a hard, white, homogeneous alloy, and will scratch steel, but is as brittle as glass and breaks with a conchoidal fracture, which has a brilliant lustre. It solidifies at 650. According to Vogel 1 this compound is 1000 300 800 700 600 500 400 Au-In Auln 1064 1000 900 800 500 \ 3 fo 500 fc I 400^- 10 20 30 4-0 50 60 7O 80 Percentage of Gold in Atoms 90 100 FIG. 6. Freezing Point of the Alloys of Gold and Zinc. Au 3 Zn 5 and contains 35'5 per cent, of zinc. The alloys con- taining more than 40 per cent, of zinc (67 atoms per cent.) become less brittle and less lustrous as the proportion of zinc increases. They are not homogeneous, and present a dull bluish-white appearance resembling zinc. The com- pound AuZn 8 , containing 72*6 per cent, of zinc (88*9 atoms per cent.), makes its appearance in these alloys. It is a 1 Zeit. Anory. Chem., Vol. XLVIII. (1906), p. 319. 48 THE PRECIOUS METALS hard white substance solidifying with surfusion at about 470 (along the lineXY, Fig. 6), and probably occurs in the zinc crusts which are formed in Parke's process. The alloys containing more than 72 per cent, of zinc melt at temperatures between 419 and 470 (see Fig. 6). All the alloys of gold and zinc are brittle, but those con- taining from 30 to 80 per cent, of gold (13 to 58 atoms per cent.) are more brittle than the others. The alloys poor in gold are best prepared by adding solid gold to molten zinc below a red heat. The gold is taken up by the zinc quietly. In preparing the alloys containing over 50 per cent, of gold which solidify above a red heat, the metals ignite with momentary incandescence, and a small part of the zinc is volatilised and lost. On heating these alloys to about 1,000 the zinc can be boiled off for the most part. GOLD AND CADMIUM. These alloys present a general resemblance to the alloys of gold and zinc. Heycock and Neville, 1 by distilling off the excess of cadmium from an alloy of the two metals, obtained residues which corresponded approximately in composition with the formula AuCd, but Vogel 2 believes that the compound is Au 4 Cd 3 , containing 30 per cent, of cadmium. The compound is greyish or silver-white, some- what like cadmium, and is very brittle. It solidifies at 623. Another compound, AuCd 3 , containing 63 per cent, of cadmium, melts at 493 and is extremely hard and brittle. There is a eutectic alloy containing 87 per cent. 1 Chem. Soc. Jour., Vol. LXI. (1892), p. 914. 2 Zeit. Anorg. Chem., Vol. XLVIII. (1906), p. 333. THE ALLOYS OF GOLD 49 of cadmium, and freezing at 303, the melting point of cadmium being 320. The alloy containing 31'5 per cent, gold and 68'5 per cent, cadmium, etched with concentrated nitric acid, is shown under a magnification of 25 diameters in Fig. 7. It consists of white crystallites of AuCd 3 set in a black eutectic mixture. The photomicrograph is after Vogel. The hardness of this series attains to a maximum in the alloys containing 18 to 30 per cent, and 51 to 63 per cent, of cadmium respectively. The re- maining alloys have about the same degree of hardness as the metals themselves. The metals when melted together unite with in- candescence to form the alloys, some of the cadmium being volati- . & FIG. 7. Gold, 31-o per cent.; lised and oxidised. Cadmium, 68-5 per cent, x 25. Like the alloys of gold and copper and of gold and zinc, these alloys can be "parted" by nitric acid and are completely dissolved by aqua regia. The zinc-gold and cadmium-gold alloys are also readily parted by dilute sulphuric acid. GOLD AND TIN. Three definite compounds of gold and tin have been found to exist. 1 They are AuSn, containing 62*4 per cent. 1 E. Vogel, Zeit. Anorg. Chem., Vol. XL VI. (1905), p. 60. P.M. E 50 THE PEECIOUS METALS of gold, AuSn 2 , containing 45*5 per cent, of gold, and AuSn 4 , containing 29*4 per cent, of gold. AuSn is a metallic silver- grey alloy, harder and more brittle than the other alloys, and having a higher electrical conductivity than other gold-tin alloys except those containing over 95 per cent, of gold. AuSn 2 is also silver-grey, but consists of large crystals, which are often 10 millimetres long. AuSn 4 is coloured gold-brown by nitric acid, while AuSn 2 is unchanged. AuSn is as resistant as pure gold to the action of acids. The curve of fusion falls sharply from the melting point of gold to a eutectic point at 280 and 20 per cent, of tin. It then rises less rapidly to a maximum at 418, the melting point of AuSn, and again falls to a second eutectic point at 217 and 90 per cent, of tin, finally rising to the melting point of tin, 226. When alloys of gold and tin are treated with aqua regia the colour of purple of Cassius is seen, and when a gold-tin alloy is heated in an electric arc furnace and the metallic vapours allowed to escape into the air, the tin burns and a substance is obtained having the properties of the purple of Cassius and containing tin oxide, calcium oxide and gold. 1 The residual ingot of gold and tin in the furnace is richer in gold than the original alloy. The alloy containing 8 per cent, of tin is brittle and has a pale whitish-yellow colour externally, with a yellowish-grey earthy fracture. Gold containing only 1 or 2 per cent, of tin is, however, perfectly ductile. GOLD AND ANTIMONY. About 1 per cent, of antimony makes gold brittle when it is not annealed, and none of the alloys are malleable. 1 H. Moissan, Ccmptes Rmdus, Vol. CXLI. (1905), p. 977. THE ALLOYS OF GOLD 51 One definite compound, AuSb 2 , exists, containing 45 per cent, of gold. 1 This substance is of the same colour as antimony and is extremely brittle and harder than its components. It melts at 460. The lowest freezing point of the series of alloys is at 360, corresponding to a eutectic which contains 76 per cent, of gold. A small quantity of antimony greatly reduces the melting point of gold and gives rise to a long pasty stage. Hatchett showed that molten gold absorbs the vapours of antimony and is made brittle thereby. All the alloys are formed with con- traction. They are not easily attacked except by aqua regia. According to Cosmo Newbery, when they are finely powdered and triturated with mercury they are slowly decomposed, yielding gold-amalgam and black metallic antimony. GOLD AND ARSENIC. These alloys have been little studied, but appear to resemble the antimony alloys. They are easily fusible, brittle, pale yellow or grey alloys. Owing to the volatility of arsenic they are not readily formed above a dull red heat. See also p. 36. GOLD AND BISMUTH. Bismuth alloys with gold, forming pale yellow or greyish alloys which are readily fusible and are all very brittle, although none are hard. Of all metals bismuth has the greatest effect in decreasing the ductility of gold. Even 0*25 part of bismuth in 1,000 parts of fine gold is enough to make the metal fragile, 2 so that it breaks under the hammer. This is due to the fact that bismuth is not 1 E. Vogel, Zeit. Anory. Chem., Vol. L. (1906), p. 151. 2 33rd Annual Report of the Mint, 1902, p. 72. 2 52 THE PEECIOUS METALS dissolved by gold, and no definite compound of gold and bismuth exists. The freezing-point curve of the series of alloys consists of two straight branches which meet in a eutectic point at 240 . 1 The eutectic mixture contains 82 per cent, of bismuth. Molten gold absorbs the vapours of bismuth and becomes brittle. All the alloys have a strong tendency to segregate. Small quantities of bismuth do not lower the melting point of gold rapidly as in the cases of antimony and cadmium, but when the bismuth exceeds about 5 per cent, there is a long pasty stage terminating at 240. GOLD AND LEAD. These alloys are readily fusible, and very brittle, ranging in colour from pale yellow by yellowish-grey to bluish- white. About 0*15 per cent, of lead makes pure gold some- what brittle. There are two definite compounds of gold and lead, AuPb 2 and Au2Pb, containing 32 and 65*5 per cent, of gold respectively. 2 The former forms long white needle-shaped crystals, which are readily distinguished under the microscope from the large well-shaped crystals of Au 2 Pb. Large white crystals of Au 2 Pb set in the eutectic are shown in Fig. 8, which is a photomicrograph after Vogel of the alloy containing 70 per cent, of gold, slowly cooled and magnified 70 diameters. There are three eutectic alloys, containing mixtures of lead and AuPb 2 , AuPb 2 and Au 2 Pb, and Au 2 Pb and gold respectively. The first of these (D, Fig. 9) has the 1 E. Yogel, Zeit. Anorg. Chem., Vol. L. (1906), p. 145. 2 E. Vogel, Zeit. Anorg. Chem., Vol. XLV. (1905), p. 11. THE ALLOYS OF GOLD 53 composition gold 14*8 per cent., lead 85*2 per cent., and solidifies at 215 or 111 below the melting point of lead. The second (C, Fig. 9) solidifies at 254 and the third (B, Fig. 9) at 418. All the alloys containing no more than 70 per cent, of gold solidify below a red heat. The freez- ing-point curve is shown in Fig. 9, which is after Vogel. Lead is separated from gold by solution in nitric acid, but is still more readily removable by exposure of the molten alloys to a current of air at a red heat (cupellation, q.v.). Under these conditions the lead is oxidised, and the litharge, PbO, so formed can be removed by absorption by a porous substance, such as bone-ash or compacted magnesia, or may be Fl0 ' 8 - Id ' 70 f r "*% Lead - J 30 per cent, x 70. allowed to flow away in shallow channels in the furnace bed, the molten metal being held together by its higher surface tension. Almost the last traces of lead are removable in this way, although it is oxidised only when at the surface of the molten gold. Moreover, the molten litharge dissolves and carries away the oxides of other metals which are not readily fusible alone, and consequently a convenient method of removing base metals from gold is to melt it with lead and subject the whole to cupellation. The method is used in refining and 54 THE PEECIOUS METALS in assaying. Silver, platinum and other metals remain with the gold, and from this cause, as already stated, silver 1100 Au 1000' 900< 800' 700' BOO 1 500' 400 ( 300 200 100' 10 20 30 40 50 60 70 80 90 AuPb. noo [ I000 1 900< 800 ' 700 600 500' 400' Pb 300' 200' too' 10 20 30 40 50 60 70 80 90 100 Percentage weight of lead . EIG. 9. Freezing Points of Alloys of Gold and Lead. and gold were known as " noble metals," because they would resist the fire, whilst the " base metals " were destroyed as THE ALLOYS OF GOLD 55 such. The art of cupellation is of great antiquity, and was known to the ancient Hebrews. GOLD AND IRON. Gold and iron unite without much difficulty and in all proportions, forming alloys which are hard but malleable and ductile, so long as the proportion of iron does not exceed 80 per cent. The only difficulty encountered in preparing the alloys is caused by the high temperature at which iron melts, but it is gradually taken up by molten gold at a temperature of 1,100 to 1,200. If cast iron is used, melt- ing at about 1,130, this difficulty disappears. The freezing- point curve of gold and iron has been determined by Isaac and Tammann. 1 It is shown in Fig. 10. The first additions of iron to gold cause a fall in the melting point, the lowest point reached being 1,040, when 5 per cent, of iron is present. With a further increase in the proportion of iron the melting point rises again and reaches 1,168, when 26 per cent, of iron is present. This tempera- ture marks the completion of solidification of all mixtures containing between 26 and 74 per cent, of gold. The gold does not affect the temperatures of the critical points (changes in allotropic form) of iron. The alloys containing 8 to 10 per cent, of iron are pale yellow, very ductile and susceptible of receiving a high polish. Alloys containing between 15 and 20 per cent, of iron are used in jewellery in France under the name or gris. They are greyish-yellow and very hard, but are easily worked. The alloy with 25 per cent, of iron is also employed by jewellers under the name or bleu. Those 1 Zei't. Anorg. Chem., Vol. LHI. (1907), p. 291. 56 THE PKECIOUS METALS containing from 75 to 80 per cent, of iron are silver- white, extremely hard> and are attracted by the magnet. The gold 1600 1500 1400 1300 1200 /I00 '000 900 800 Au d \ \ \ \ \,, ^ \ \ \ \ \ N \ \ Xy \ \ S XT a b s x e 10 20 30 40 50 60 70 80 30 100 Percentage weight of Go/a. FIG. 10. Freezing Points of Alloys of Gold and Iron. and iron in these alloys are not easily separated by cupella- tion, but they are readily soluble in aqua regia. If the per- centage of iron is not too small it is readily removed by THE ALLOYS OF GOLD 57 dissolution in hot dilute sulphuric acid, or less conveniently by hydrochloric acid or nitric acid. The alloys of gold and iron are formed with expansion. J484- GOLD AND NICKEL. Small quantities of nickel alloyed with gold produce pale yellow, hard, veryductile alloys, which are magnetic and take a good polish. Hatchett l found that 8 per cent, of nickel made gold brittle with a coarse-grained earthy fracture. The two metals form a eutectic, 2 which contains about 22 per cent, of nickel and melts at 950. The freezing-point curve of the alloys is shown in Fig. 11, which is after Levin. too Percentage weight of Nickel. FIG. 11. Freezing Points of Alloys of Gold and Nickel. GOLD AND COBALT. Cobalt has a far greater effect than nickel in making gold brittle. According to Hatchett, standard gold becomes brittle if only one-eightieth part of cobalt is melted with it. 1 Phil. Trans., 1803, p. 43. 2 Levin, Zeit. Anorg. Cham., Vol. XLV. (1905), p. 238. 58 THE PEECIOUS METALS GOLD AND MANGANESE. These alloys are very hard and melt at a higher tempera- ture than gold. The alloy containing about 12 per cent, of manganese is of a pale greyish-yellow colour with a lustre like that of polished steel. It is somewhat malleable, and shows a coarse-grained fracture. The manganese can be removed by cupellation with lead. The alloy containing 67 per cent, of manganese is grey, hard, very slightly ductile, and has a coarsely granulated fracture. The alloy of gold 10 per cent., manganese 90 per cent., is pale grey in colour, and is very ductile with a fine-grained fracture. GOLD AND PLATINUM. These metals are not known to form any definite com- pounds, but can be alloyed in all proportions. The first additions of platinum at once raise the melting point of the alloy above that of gold, and the alloys formed are malleable and ductile. When the proportion of platinum exceeds 50 per cent, the alloys are brittle and greyish in colour. A thermal study of the alloys containing up to 60 per cent, platinum has been made by Doerinckel 1 with the results shown in Fig. 12. The curve ABC marks the beginning and the curve ADC the end of solidification. The two metals appear to form a continuous series of mixed crystals. When examined under high powers of the micro- scope the alloys are seen to consist of crystals of which the cores, richer in platinum than the edges, appear in relief. By annealing at the temperatures indicated by the curve ADC, the alloys become completely homogeneous. 1 Zeit. Anorg. Chem., Yol. LIV. (1907), p. 345. THE ALLOYS OF GOLD 59 The yellow colour of gold disappears rapidly on the addition of platinum, and only a slight yellow colour remains when 30 per cent, of platinum is present. If the platinum is above 40 per cent, the colour is that of platinum. The alloy of gold 10, platinum 90, has a lauu" ^ ,C \ ,' / x s' 1 /* S ~ 7 7' f_ / 1400 ~*~~ - ~~~ ~~ -/> lb~ * /300 / ./ / r ,^ sx* tioo A\ - / 2 3 4 5 6 7 8 9 1000 AL f Prrr 'fnffi nF PlaH Pt FIG. 12. Freezing Points of Alloys of Gold and Platinum. brilliant white crystalline structure, and the alloy gold 25, platinum 75, is hard and brittle, resembling grey cast iron. (E. Matthey). E. Matthey has shown 1 that the alloys containing from 5 to 20 per cent, of platinum are not uniform in composition, 1 Phil. Trans., Vol. CLXXXIIlA. (1892), p. 629. 60 THE PKECIOUS METALS the platinum moving towards the centre of the ingot on solidification. These alloys cannot be separated into their constituents by cupellation and parting in acid in the ordinary way. They are not attacked by single acids, but are dissolved by aqua regia. GOLD AND PALLADIUM, EHODIUM, IRIDIUM, ETC. These alloys have been very little studied. The gold- palladium alloys form a continuous series of mixed crystals (see Fig. 18, which gives the curves of fusibility after Kuer). Gold containing from 10 to 20 per cent, of palladium is nearly white and is hard but ductile. The alloy of gold 50 per cent., palladium 50 per cent., is said to be iron-grey in colour and deficient in ductility. Gold forms difficultly fusible but ductile alloys with osmium and rhodium. It is doubtful whether any real alloy with iridium is formed. When gold containing iridium is melted, the iridium sinks to the bottom. On treatment with aqua regia the gold is dissolved and the iridium left in the form of a black powder which is mainly if not entirely oxide of iridium. GOLD AND ALUMINIUM. These metals form at least two definite compounds. One of them, Au 2 Al, containing 6'4 per cent, of aluminium, is a hard white substance melting at 622 . 1 The eutectic alloy between this compound and pure gold contains only 4 per cent, of aluminium but melts at 525, over 500 below the melting point of pure gold. The other compound, AuAl 2 , i Phil. Trans., Vol. CXCIVA. (1900), p. 201, THE ALLOYS OF GOLD 61 containing 21*5 per cent, of aluminium, solidifies at a higher temperature than the melting point of pure gold itself. 1 This compound is of a beautiful purple colour which resembles but is deeper in tint than the compound 1600 1500 1400 - 1600 - 1200 - 1100 1000 woo 90 100 10 20 30 40 50 60 10 80 Percentage weight of Palladium, FIG. 13. Freezing Points of Alloys of Gold and Palladium. AuZn. It is formed whenever gold and aluminium are fused together, provided that at least 10 per cent, of aluminium is present. The purple compound is readily seen in any of these alloys, occurring in patches in a white 1 Proc. Roy. Soc., Yol. L. (1892), p. 367. 62 THE PRECIOUS METALS matrix consisting of the excess of aluminium or of the white compound of aluminium. There is little evidence of the existence of any other compounds of gold and aluminium except these two. GOLD AND TELLURIUM. 1 The curve of fusibility is shown in Fig. 14. A compound, AuTe 2 , containing 43'6 per cent, of gold exists, freezing at 452. This corresponds in composition to the mineral species sylvanite and calaverite. There are two eutectics, one containing 60 per cent, of gold and fusing at 432, and the other containing 20 per cent, of gold and fusing at 397. The presence of 0*05 per cent, of tellurium is enough to cause the formation of a small quantity of the gold-rich eutectic, and gold containing this amount of tellurium is accordingly deficient in ductility. When more tellurium is present, the alloys are very brittle. The alloys rich in gold consist of crystals of gold set in the greyish-white eutectic. Gold containing 2'4 per cent, of tellurium is of a pale brass-yellow colour, but even when 40 per cent, of tellurium is present, a faint yellowish tinge is still perceptible, which disappears before the pure com- pound AuTe2 is reached. The behaviour of tellurides of gold on roasting is partly explained by the remarkably low temperatures of fusion of these alloys, all mixtures con- taining less than 60 per cent, of gold being completely fused before a red heat is reached. The production of spherical globules of gold must therefore accompany the roasting of gold ores containing tellurides, however carefully the operation may be carried out. These globules will not be free from tellurium. 1 T. K. Eose, Trans. InsL o/Mng. and Met., Vol. XVII. (1908), p. 285. THE ALLOYS OF GOLD GOLD AND THALLIUM. 1 These metals form neither compounds nor homogeneous solid solutions. There is a eutectic alloy containing 27 per cent, of gold and melting at 131. This eutectic occurs in 30 40 50 60 70 BO 90 100 (000 - Au - 900 10 20 30 40 50 60 70 80 90 Percentage weight of Gold FIG. 14. Freezing Points of Mixtures of Gold and Tellurium. all the alloys, so that the addition of a small amount of thallium to gold gives rise to a long pasty stage, terminating at 131. The alloys are all brittle. 1 Levin, Zeit. Anorg. Chem., Vol. XLV. (1905), p. 31. CHAPTEE V OCCURRENCE OF GOLD IN NATURE I GOLD ORES GOLD generally occurs in Nature in the metallic state alloyed with a varying proportion of silver and smaller quantities of copper and other metals. Native gold is never found pure, but sometimes contains less than 1 per cent, of impurities. According to Furman, 1 the finest native gold which has been met with is that from the Pike's Peak mine, at Cripple Creek, Colorado, which contained 99 '9 per cent, of pure gold. Gold occurs in the form of nuggets, grains and " dust " in many river gravels. In this form it was known to the ancients, and, as already pointed out, was probably the earliest metal known to man, although it is unlikely that any use was made of it until long after its first discovery. In some gravels masses of gold of considerable size occur. The largest piece recorded was the "Welcome" nugget which was found in Victoria. It weighed 2,195 oz., and yielded gold to the value of 8,376 10s. 6d. Large nuggets have also been found in other parts of Australia, in Kussia, and in California. Auriferous deposits consisting of sands and gravels formed by the erosion of older rocks are known as placer deposits or placers. The gold in these deposits is rounded by attrition, all its sharp angles having been worn off. The nuggets of gold are often larger and purer 1 Colliery Manager and Metal Miner, October, 1896, p. 89. THE OCCUEEENCE OF GOLD IN NATURE : GOLD OEES 65 than the masses found in lodes in the same localities ; but there is now little doubt that the gold in placer deposits previously existed in ores in veins and has been derived from them by the action of running water, or in rare cases by the waves of the sea. A large proportion of the gold production of the world was formerly obtained from placers by washing, but placer gold is now of secondary importance, most of the present enormous output of gold coming from gold ores which require to be crushed before the values can be extracted. The occurrence of well-defined crystals of gold is rare, but has been noted in lodes in Transylvania, California, Australia and Brazil. The usual forms are cubes and octahedra, with rod- and plate-like forms. The crystal faces are frequently striated, and the crystals are generally rounded, uneven, or even curved. Twinning gives rise to dendritic and reticulated groups, but individual crystal faces and edges are usually very small. Moss-gold, leaf- gold and wire-gold are forms occurring sometimes. The interior of nuggets when polished and etche'd shows a crystalline structure similar to that of fused masses. No traces have been observed of a laminated, concentric structure such as might be expected if the nuggets had grown by accretion by means of deposition from solution. It is of course arguable that the deposited gold may have been rearranged after deposition by the slow effects of solid diffusion. Gold ores are usually found in veins or lodes in rock formations. The greater part of the material (" gangue") forming the lodes is usually quartz, but various silicates also occur, and sulphides, arsenides or antimonides of base P.M. F 66 THE PRECIOUS METALS metals are almost invariably present. The upper portions of these veins near the surface of the ground are often found to have undergone oxidation from the influence of the atmosphere, so that the sulphides of iron, etc., have been converted into oxides. These gossans, or oxidised portions, may extend downwards to a considerable depth, and the gold contained in them, associated as it is with a loose mixture of silica and oxide of iron, is often readily separated. When the oxides pass into sulphides with increase of depth, the extraction of gold may become more difficult. The great deposit of ferruginous sinter, mainly composed of silica, found at Mount Morgan in Queensland, from which gold to the value of several millions of pounds has been extracted, is the richest gossan known. Generally, below the water level in the rocks of the district gold is found associated with metallic sulphides. According to Dr. Don 1 gold is always present in lodes, in the districts of Australia which he examined, when pyrites occurs, and is absent when no pyrites is observable. According to L. Wagoner, however, gold sometimes occurs in Western America in rocks free from sulphides. The gold in lodes is in the form of minute scales, threads or grains disseminated through the ore. It is sometimes visible to the eye without magnification, but is more often in too fine a state of division to be seen. It also occurs in copper and iron pyrites, gilding the faces of the crystals or otherwise associated with them, probably in the metallic state, not as a sulphide. If sulphides of gold are rare or unknown, on the other hand tellurides of gold, containing the compound AuTe2, 1 Trans. A. /, M. E., Vol. XXVII. (1897), p. 564. THE OOOUBEBNOB OF GOLD IN NATURE : GOLD ORES 67 are very common. They occur in considerable quantities in Western Australia, Colorado and Transylvania, and have been reported from many other localities. Various rniner- alogical names have been assigned to the tellurides from different localities, the best known being calaverite, which has the composition AuTe 2 , sylvanite or graphic tellurium, which appears to be a variable mixture of AuTe 2 and Ag 3 Te 4 , petzite (Ag 2 Te, in which some silver is replaced by gold) and nagyagite, or foliated tellurium, which contains a considerable percentage of lead. The tellurides are for the most part dark grey or black in colour, rarely silver-grey. They are often mixed with metallic gold, which sometimes gives them a brassy-yellow colour. When heated in air they oxidise, fuming and giving off Te02, and fuse below a red heat. The removal of most of the tellurium leaves the gold in the form of round pellets which have solidified from fusion. Calaverite has a density of about 9, and contains about 44 per cent, of gold, part of which is usually replaced by silver. Sylvanite has a density of about 8*1. Analyses of two specimens from South Dakota and Cripple Creek respec- tively gave the following results 1 : (1) Gold . 7*64 per cent. (2) Gold . 5'61 per cent. Silver . 32'39 Silver . 34*23 Tellurium 59'96 Tellurium 60*16 99-99 100-00 Petzite, or Ag 2 Te, has a density of 8'86 (Miers). A specimen from California contained 25 '6 per cent, of gold 1 F. C. Smith, Trans. A. I. M. E., Vol. XXVI. (1896), p. 485. F 2 68 THE PRECIOUS METALS and 41*86 per cent, of silver (Genth). Nagyagite is foliated like graphite, but has a density of about 7. It contains only a few per cent, of gold. The proportion of gold present in ores or gravels which can be treated profitably for its extraction is very small. The mean return from the Rand ores is less than half an ounce per ton, or about one part of gold in 70,000 parts of worthless material. Auriferous gravels which do not require to be crushed are sometimes treated at a profit when they contain only two or three grains of gold per ton, or say one part of gold in five millions. Beyond the limits of profitable extraction gold is very widely disseminated. Minute quantities of gold appear to occur in all ores of silver, copper, lead, antimony and bismuth. 1 It has also been detected in igneous rocks in almost every case in which a diligent search for it has been made. It is perhaps even more generally distributed throughout metamorphic rocks, and sedimentary rocks are seldom quite free from it. L. Wagoner found 2 an average of 0'37 part of gold per million, or 5 grains per ton in granites, 0*03 part per million in sandstones, and 0'007 part per million in limestones. It also occurs in many clays and shales. For example, the bed of clay on which the city of Philadelphia is built is an auriferous deposit. It has been suggested that the gold in sedimentary deposits has been derived from the sea, but it seems at least equally probable that the gold previously existed in the older rocks from which these deposits were formed. The comparatively small quantity of gold in limestones which are formed in clear 1 " Metallurgy of Gold," T. K, Rose, 5th edition, p. 36. a Trans. A. I. M. E., November, 1901, THE OCCURRENCE OF GOLD IN NATURE : GOLD ORES 69 water far from land points rather to the land as the place of origin of the gold. / Gold was first detected in sea water by Sonstadt, 1 who states that the amount in British waters is far less than one grain per ton. Liversedge found 2 between one-half and one grain per ton in various samples of water taken from the sea off the coast of Australia. L. Wagoner 3 found from 0'6 to 3'7 grains of gold per ton of water from the depths of the Atlantic, and only 0*2 grains per ton in Chesapeake Bay. The total quantity of gold in the sea is evidently enormous, amounting, on the figures given by Liversedge, to about 10,000,000 to each inhabitant of the globe, but there appears to be no prospect of its successful extraction on a large scale. It is not known how gold came to be present in sea water. / The origin of gold ores is not yet certainly determined. Gold is usually contained in quartz veins which pass through rocks of many different kinds. The effects of segregation, by which solid crystals of definite composition grow in a fluid containing a number of different constituents, seem to be quite enough to account for the existence of quartz veins in igneous and metamorphic rocks and of gold in quartz veins. Given a molten mass of rock, there would be nothing surprising in nearly pure silica beginning to solidify at certain points and receiving additions of more silica by the operations of diffusion. When the whole mass had become solid, the segregated silica would readily become converted into quartz and also receive still further 1 Chem. News, Vol. XXVI. (1872), p. 159. 2 Roy. Soc. N.S.W., October 2, 1895. 3 Trans.'A. L M. #.,1907. 70 THE PBECIOUS METALS additions by the agency of heated water, by which silica would be dissolved, transferred and re-precipitated. The concentration of gold in quartz veins and other ores may have been effected in a similar manner. 1 It is of course well known that when a crystalline nucleus of any mineral, such as gold, has been formed, there is a tendency for all particles of the same mineral which are brought near the nucleus by diffusion or in other ways to be fixed to it, so that the crystal grows. The almost universal dis- semination of gold throughout rocks of all sorts and in the waters passing through them would thus afford an unfailing source for the creation of gold ores by the enrichment of certain zones. The length of time required for the formation of the gold ores which have been discovered is not known. The veins containing gold ores mainly traverse meta- morphic rocks, especially slates or schistose rocks, such as hydromica and chlorite schists. Valuable gold ores are sometimes met with in basalt, rhyolite, and other igneous rocks, and also occur " in many different forms, as replace- ment-deposits in limestones, as disseminations in igneous and sedimentary rocks, and as contact-deposits near intrusive masses . . ." 2 Gold-bearing gravels are, generally speaking, valuable when they occur in river-beds in districts containing gold ores, and consequently they are for the most part confined 1 For a full discussion of the origin of gold ores, see papers by Posepny, Spurr, Bickard, Weed, Wagoner, Don, and others, in the Transactions of the American Institute of Mining Engineers, and else- where. For a short resume with references, see "Metallurgy of Gold," T. K. Eose, 5th edition, pp. 4244. 2 " Igneous Bocks as related to Occurrence of Ores," J. E. Spurr, Trans. A. I. M. E., 1902. , THE OCCURRENCE OF GOLD IN NATURE : GOLD OEES 71 to rivers flowing through metamorphic rocks, especially slates and schists. The beds of ancient rivers which have long ceased to flow are sometimes very rich. Such river gravels have been covered by thick masses of eruptive basalt and other deposits in California and Victoria. In the latter country they are known as " deep leads." The geographical distribution of gold ores corresponds roughly with what has been said as to the geological distribution. Mountainous districts and the streams flow- ing from them are frequently auriferous. Wide, flat, alluvial plains generally contain no gold. In Europe the chief gold mines are in Hungary and Transylvania. Some gold is produced in North Italy, Norway, Wales, etc. The Eussian gold production is mainly from the placers of Siberia. In India gold occurs at Colar, in Mysore, and in small quantities in Madras. The other goldfields of Asia are in Borneo, Celebes, Sumatra, and some other East Indian islands, in China, Korea, Japan, and in the Malay Peninsula. In Africa the greater part of the gold produced is from the Transvaal and Khodesia, but deposits of less importance occur in West Africa, in Egypt, and in Abyssinia. Gold is widely distributed in the western moun- tains of North America, alike in Canada, the United States, and Mexico. Minor goldfields exist in the- Appalachians and in Eastern Canada. The goldfields of Central and South America were formerly more important than at present. They occur both on the Pacific and the Atlantic slopes. Gold occurs in many parts of Australia and in several districts in New Zealand. The production of gold for different countries is given in Chapter XVIII. CHAPTER VI EXTKACTION OF GOLD FEOM ITS ORES : GOLD WASHING WHEN the treatment of gold ores is under consideration they fall naturally into two divisions (a) loose aggregations, from which the gold can be removed by concentration in virtue of its high specific gravity, and (b} relatively hard ore, which must be crushed before the gold can be extracted. It is convenient to include in the first division gravels which have been cemented by infiltration of silica, oxide of iron, lime, etc., so that they have become too coherent to be treated without crushing. Many examples of such " cement gravels " occur in California. The second division includes some quartzose ores, in which the gold is mainly in the form of grains of moderate size, so that after crushing most of the gold can be collected by simple concentration. Ores of this character have been met with chiefly in Australia. Gold in a finer state of division escapes under such treatment, but is caught by mixing the crushed ore with mercury. The gold then becomes " amalgamated," and the small particles of gold-amalgam are recoverable by taking advantage of their tendency to coalesce with one another, or with fresh mercury, so as to form large globules, which readily sink in pulp diluted with water. Gold- amalgam may also be separated from the crushed ore or pulp by bringing it into contact with a metal plate, the surface of which has been previously amalgamated by GOLD WASHING 73 rubbing with mercury. In the latter case the gold-amal- gam adheres to the amalgamated plate, and is removed at intervals by scraping. Mercury is also largely used in the treatment of auriferous gravels. Very finely divided gold, however, may escape amalga- mation, and the resistance to amalgamation is still more marked in the case of compounds of gold, such as tellurides, which show little or no tendency to yield up their gold when brought into contact with mercury. Many ores, too, contain materials which exercise a deleterious effect on mercury, preventing its globules from coalescing, so that they are lost in the tailings, together with the gold which they have picked up. All such ores which do not yield a satisfactory percentage of their values on treatment with mercury are known as "refractory" ores, as distinguished from " free-milling " ores. In a sense all ores are partly refractory, inasmuch as a part of the gold contained in them is not in a condition to be recovered by means of mercury, however carefully the treatment is carried out. Accordingly it is now customary to treat the tailings from the amalgamation process by mixing them with a solution of cyanide of potassium. Very finely divided gold is dissolved by cyanide solutions, which are then separated from the ore. The gold is obtained from them by precipitation. Sometimes tailings containing rich pyrites or other sulphides are treated by concentration, and the concentrates are smelted with lead, or roasted and subjected to the action of a solution of chlorine in water by which the gold is dissolved, with a view to subsequent precipitation. Refractory ores are treated by smelting, cyaniding, or 74 THE PRECIOUS METALS chlorinating, according to their nature and to various other considerations. When the gold has been obtained, whatever methods have been employed, it is melted and cast into bars and afterwards refined to fit it for use in the arts. The methods of treatment of gold ores may accordingly be tabulated as follows : 1. Simple "washing " or concentration, with or without the aid of mercury. This is suitable for auriferous sands and gravels (placer deposits). 2. Crushing and concentration, usually with the aid of mercury : suitable for free-milling quartzose ores. 3. Subjecting crushed ores, usually without roasting, to the action of cyanide of potassium : suitable for the tailings from free-milling ores and for certain refractory ores. 4. Boasting and chlorination : suitable for concentrates from free-milling ores which have already been amal- gamated, and for certain refractory ores. 5. Smelting in blast furnaces to obtain the gold as an alloy of lead : suitable for certain refractory ores and con- centrates. This is described under the treatment of silver ores, Chapter XL The methods given above will now be briefly described, and some of their chief modifications touched upon. 1. GOLD WASHING. Small quantities of rich sands are washed in the miner's pan, which was originally the cooking-pan of the prospector. It is a flat-bottomed pan of sheet-iron, about 16 inches in diameter, with sloping sides 4 or 5 inches wide. It is often made with one or more " riffles " or shallow trenches in the GOLD WASHING side or on the bottom near the angle to catch mercury and amalgam. When used, it is nearly filled with sand and water, and is first well shaken to enable the gold to settle to the bottom, and then dipped in and out of water in an inclined position to enable the sand to escape over the edge by degrees. The final operation of separating the last remaining sand from the gold requires a con- siderable degree of skill. This implement was much used in early days in California and Aus- tralia, when a single panful of dirt sometimes yielded several ounces of gold. A somewhat similar wooden (usually mahogany) bowl, the batea, is especially favoured by the negro race. It is a conical vessel about 18 inches in diameter, and 2 inches FIG. 15. Batea. deep in the centre (see Fig. 15). The gold clings to the wood when it would slide over iron. In the East a wooden trough is used. For concerted work by a small party of diggers, the cradle, or rocker, is more advantageous. It consists of a wooden box, resting on two rockers DD, like a baby's cradle (see Fig. 16, which is a sectional elevation). One man shovels gravel into the box A, the bottom of which 76 THE PEECIOUS METALS consists of a sieve, while another pours on water with one hand and rocks the cradle with the other. Mercury is sprinkled on at intervals. The fine sands and gold pass through the sieve and over some inclined plates B, to which cross-bars of wood, C, are nailed. The heavy materials gold, mercury, amalgam, pyrites, etc. accumulate behind the cross-bars, or " riffles," and every now and then are scraped out and panned. The amalgam is heated on a shovel and the mercury driven off, leaving the gold behind. PIG. 16. Cradle. When larger quantities of auriferous gravel are available, a sluice is generally used. A sluice-box is a slightly inclined trough made of wood, through which the gravel is carried by a stream of water. Mercury is sprinkled on at short intervals of time. A large number of boxes are sometimes fitted together, making a sluice which may be some hundreds of yards long. The methods of conveying the auriferous material to the sluices vary with the scale of the operations and the other conditions. When small sluices or rocking cradles are used the gravel is piled near at hand and shovelled into them. In Siberia, where the valleys are shallow and the GOLD WASHING 77 inclination of the ground small, the gravel is carried in carts up an inclined plane to an elevated wooden platform whence the sluice starts. In California, where the " gulches " are deep, the fall of the ground rapid, water plentiful, and the auriferous deposits of great thickness, the banks of gravel are some- times attacked by jets of water under high pressure, and the earth washed down and carried through the sluices without being touched by hand. This is called hydraulic mining, or " hydraulicking." When the gravel beds are below the general level of the country they may be raised by the " hydraulic elevator," a jet of water under a head of as much as 400 or 500 feet, carrying water, sand and boulders alike up a pipe inclined at some 60 degrees to the horizon, so as to deliver them all at the head of the sluice, the vertical lift being sometimes over 50 feet. When gravels are buried beneath great thicknesses of lava or other materials, as in the case of the " deep leads " of Bendigo, Ballarat and other places in Victoria, the gravel is mined and raised through shafts. If the gold-bearing material is hard and compacted, it is coarsely crushed in a stamp mill resembling those described in Chapter VII., but sometimes called a " cement mill." The sluice is paved in various ways to take up the wear and to catch the gold and amalgam. An irregular bottom is required with plenty of depressions, chinks and crannies into which the gold can settle. The obstructions are known as riffles. The simplest contrivances are fir poles about four inches in diameter, left rough with the bark on. These poles are cut to the required length and nailed or wedged in the bottom of the sluice either transverse to the 78 THE PRECIOUS METAL direction of the current or longitudinally. The latter is generally preferred. Other riffles in use consist of cut slats of wood nailed to the bottom of the trough or sluice. In Siberia square " pigeon-hole " depressions have been con- tinuously used for more than fifty years. In California the sluice is sometimes paved with 6-inch square blocks of wood placed about two inches apart, or with large rounded stones, or ordinary iron rails placed longitudinally. All these afford plenty of crevices where the amalgam can lodge. In later years the use of perforated iron plates and of " expanded metal " has largely increased. To catch light spangles of gold, blankets are spread, the loose fibres of which become charged with pyrites and gold, and in New Zealand plush is a favourite gold-catcher. Amalgamated plates, both stationary and shaking, are also used. On dredges, where the sluice is necessarily short, a number of different kinds of riffles are often used in the same sluice. The view has been expressed 1 that this is advantageous because " the ripple in the water as it flows down the box is altered, and consequently the material being treated is tossed about, its course altered, and the gold which may be adhering to the stones or grit has more chance of being liberated." There are, of course, two some- what opposed operations to be carried out in a sluice. The process of disintegration of the gravel, if not previously completed, must be finished in the sluice by " tossing about " the auriferous material, but the main function of the sluice must be to save gold. This is done by affording opportunities to the denser particles of matter to settle to 1 E. S. Marks and Gr. N, Marks, Trans. Itwt. of Mng. and Met., Vol. XV. (1906), p. 479. UNIVERSITY CF GOLD WASHING 79 the bottom, and any eddies in the water, " boiling-up " or disturbances of any kind interfere with the process. The flakes of gold, instead of continuing to settle towards the bottom of the sluice, are in such cases carried up again towards the surface of the stream. Consequently, where further disintegration is not required the water is allowed to flow as smoothly as possible. Sometimes disintegration in the sluice is necessary, and this is effected partly by the boulders occurring in the gravel and partly by arranging drops of a few feet in the sluice. When boulders are allowed to work down the sluice, the wear of the pavement is increased. At the La Grange hydraulic mine in North California, 1 the sluice-boxes are 6 feet wide and 4 feet deep, and are paved with steel rails weighing 40 Ib. per yard, but the number of boulders passing through the sluice is large, and the rails are worn out in about six months. In some sluices the larger stones and lumps of clay are removed by passing the stream of gravel and water through a set of parallel iron bars (a "grizzly "), or in small operations by means of a fork or by hand. The dimensions of the sluice and its angle of slope vary with the quantity and nature of the material to be treated and the amount of water available. The art of sluicing consists in arranging that the heavy materials gold, mercury, amalgam, and "black sand" shall sink to the bottom of the stream and lodge in the irregularities in the sluice-bed, but that the lighter worthless sand shall be carried away by the water. If sand accumulates in the sluice, the object is defeated and the amount of water must and Scientific Press, October 10, 1908, p. 492. 80 THE PKECIOUS METALS be increased or the angle of inclination raised. The diffi- culty lies chiefly in catching finely divided gold, which is liable to be swept away with the sand. With this object the stream is kept as shallow as possible, and various special devices have been introduced, such as the undercurrent. In this, a part (say one-fourth to one-third) of the fine material and water are drawn off through a sieve placed in the bed of the sluice and spread in a thin, even stream over the surface of a wide, inclined table, sometimes supplied with transverse riffles. After flowing over the table the material is returned to the main sluice. The fine gold settles on the table and is caught by the " burlap " or other material with which it is covered. At intervals the work is stopped, and a " clean up " takes place to recover the gold caught in the sluice. The water is allowed to run until it passes through clear, and it is then deflected from the sluice, and the material lodged in the riffles, etc., is scooped out and panned or ground in a revolving barrel with more mercury and a few cannon balls. The amalgam and quicksilver collected in this way are squeezed through chamois leather or canvas, as in the days of Pliny. Nearly pure mercury passes through, and pasty amalgam containing 30 or 40 per cent, of gold remains in the bag. Pliny describes the process in the following words : " ut et ipsum [i.e., argentum vivum] ab auro discedat, in pellis subactas effunditur, per quas sudoris vice defluens purum relinquit aurum." The pasty amalgam is retorted, the mercury driven off and condensed by cold water, and the residue melted and cast into bars. The tailings, or refuse material from which the gold has been extracted, are carried off by the water and discharged GOLD WASHING 81 into the nearest river or collected and impounded in brushwood dams. One of the main difficulties in hydraulic mining is in the disposal of the tailings, which may amount to millions of cubic yards in a year from a single mine. Vast quantities of debris were discharged by the hydraulic miners into the rivers of California between 1862 and 1884, with the result that the river-beds were gradually filled up and floods became more frequent and extensive. The farmers' lands were being covered more and more by deposits of barren gravel and sand and their industry was threatened with destruction. They accordingly obtained an injunction from the Courts which put an end to hydraulic mining in a great part of California in 1884. 1 Within the last few years the use of dredges has been rapidly extending. A dredge is a flat-bottomed boat, with machinery for raising gravel from the bottom of a stream or pond, hoisting it on board, washing it over inclined tables to save the gold, and then throwing or " dumping " the tailings overboard at the stern. A dredge may thus work its way up-stream, or may proceed across a flat plain floating in a pond, cutting out the bank in front of it and piling up the tailings behind. The latter method is known as "paddock dredging." A continuous supply of clear water is necessary for washing in the latter case, as very muddy water is found to interfere with the gold-saving. Dredges were first successfully used on the rivers of New Zealand, but are now at work in many countries. Some- times suction pumps or grab-bucket dredges are used to lift 1 See Production of Precious Metals in the United States, 1884, pp. 524530. P.M. G 82 THE PEEOIOUS METALS the gravel, but ladder-bucket dredges similar to those used in ordinary dredging operations are much more common. The following is a description of a typical New Zealand ladder-bucket dredge 1 : The pontoon is 119 feet long and 50 feet wide at the stern, and its greatest draught is 9 feet 6 inches. The engine is of 25 h.p. nominal. The capacity of the buckets is 7 cubic feet, and they are worked at a speed of ten buckets per minute. The ladder carrying the buckets can be raised and lowered by means of a winch. The pontoon is moored to the bank and moved from place to place by a head-line and four side-lines, each of which is supplied with a separate winch. The buckets empty the gravel into a trommel, or revolving screen, which is 31 feet long and 4 feet 6 inches in diameter. The coarse material is delivered into the buckets of the main tailings elevator, and the fine material falling through the screen passes to the washing tables, where the gold is saved. The tailings are deposited in a settling tank, whence they are lifted out and delivered to the main tailings elevator, which is 145 feet long and capable of stacking tailings at a height of 80 feet above the water level. The capacity of the dredge is reckoned at about 120 cubic yards per hour. Some dredges are not supplied with a revolving screen, and the material is tipped from the buckets direct into a sluice. In either case the length of the gold-saving tables is strictly limited, and the gravel must be thoroughly dis- integrated and an opportunity afforded for the gold to settle in a short distance. The tables are supplied with expanded metal or perforated plates placed on cocoanut 1 " Eeport of the Department of Mines, New Zealand, for 1899 1900," p. 43. GOLD WASHING 83 matting or other material. Angle-irons and Hungarian riffles are also used, but the losses of finely divided gold are frequently heavy. Tailings in river dredging are often discharged into the water over the stern of the pontoon. In paddock-dredging it is of course necessary to stack them on the bank. One of the drawbacks is that stacked tailings do not usually form ground capable of cultivation, and arable land is often laid waste by dredging operations. Cleaning-up usually takes place weekly. The expanded metal or other plates are lifted and the mats washed in tubs until free from all gold, grit, etc. The concentrates may be again washed on a " streaming-down " table, which is covered with plush or green baize, and the final concentrates are amalgamated and panned. Dredges are very economical in work, some large ones treating from 3,000 to 6,000 cubic yards or more per day, at a total cost of 2d. or 3d. per cubic yard, as in the subjoined table. No. 1 Dredge. No. 2 Dredge. Actual time worked . 6,161 hours 5,572 hours Time lost . 17'7 per cent. 25'6 per cent. Capacity of dredge . Material treated 130 cub. yds. per hour 325,896 cub. yds. 112-5 cub. yds. per hour 303,360 cub. yds. Gold recovered . 1,198-6 oz. 1,393-9 oz. Net value . 1,816 5,104 Working expenses . 3,322 3,150 Profit 1,494 1,954 Value per cubic yard 1-76 grains or 3-