ricultural Scien 
 
 UNIVERSITY OF CALIFORN 
 
 kJ ^ 4*nT*<J 
 
 hi 
 
 ELERATING 
 
 THE 
 ^DRYING OF 
 
 REDWOOD 
 LUMBER 
 
 '*-■% 
 
 
 
 ife 
 
 f*' 
 
 H. RESCH and B. A. ECKLUND 
 
 CALIFORNIA AGRICULTURAL 
 EXPERIMENT STATION 
 
 BULLETIN 803 
 
Annually, some 980 million board feet of California's redwood sawtimber are converted into 
 lumber (USDA Forest Service, 1958). The largest portion of this product must be dried to 
 8-10 per cent moisture content. The long drying time required, the longest for any com- 
 mercial softwood species, represents a major problem in the manufacturing process. Millions 
 of board feet of lumber are tied up in air drying yards, representing a large capital invest- 
 ment. 
 
 Speeding up the drying process would make the lumber available for the market much 
 sooner after sawing than is now possible and reduce the amount of lumber that needs to be 
 stored. 
 
 This bulletin 
 
 • briefly reviews the literature of redwood drying, 
 
 • presents results on experimental kiln drying of 4/4" heavy segregation lumber, and 
 
 • discusses various aspects of the acceleration of drying. 
 
 JULY, 1964 
 
 THE AUTHORS: H. Resch is Lecturer and Assistant Wood Technologist, Forest Products Labo- 
 ratory, Richmond. B. A. Ecklund is Assistant Specialist, Forest Products Laboratory, Richmond. 
 
 [2] 
 
ACCELERATING THE DRYING OF REDWOOD LUMBER 1 
 
 Acceleration in the kiln drying of red- 
 wood can be accomplished effectively by 
 the use of low temperatures, not to exceed 
 120°F, combined with a stepwise reduc- 
 tion in relative humidity over about two 
 thirds of the total drying time required. 
 
 As long as the wood is above the fiber 
 saturation point, increases in temperature 
 above 120°F will cause collapse in a 
 certain portion of lumber. Thus, the slow- 
 est-drying pieces should determine the 
 drying progress. 
 
 Under the segregation procedure pres- 
 ently used, the shortest feasible kiln dry- 
 ing time for 4/4" heavy segregation 
 lumber appears to be 26 to 27 days. Better 
 and faster drying can be accomplished 
 only if lumber containing clusters of 
 birdseye, dark birdseye, dark colored 
 extractive streaks, and abnormal wood 
 formation is segregated and air seasoned 
 before kiln drying. Because of the great 
 variation in the drying rate, moisture 
 gradient, and thus in the stress pattern of 
 boards during drying a better and, ulti- 
 mately perhaps, automated segregration 
 is desirable. 
 
 An even greater saving in air season- 
 ing as well as kiln-drying time results 
 from a presteaming treatment. However, 
 in order to prevent checking of birdseye, 
 lumber containing this feature has to 
 be segregated. 
 
 An increase in the air speed normally 
 used in commercially available kilns 
 (around 400 to 500 ft per minute) does 
 not aid appreciably in decreasing the 
 over-all drying time because the drying 
 rate largely depends on the water and 
 water vapor diffusion through wood 
 below the fiber saturation point. 
 
 Kiln drying redwood green from the 
 saw does not eliminate sticker stain. A 
 more intensive study would have to verify 
 
 1 Submitted for publication December 16, 
 1963. 
 
 the impression that staining is generally 
 reduced by this drying method. 
 
 A plot of drying rate against the cur- 
 rent moisture content of boards on a 
 log-log scale yields a straight line. This 
 relationship can be used to estimate dry- 
 ing time from previously taken records 
 and is accurate mainly during the second 
 drying stage after capillary water move- 
 ment has come to a halt. 
 
 Kiln drying heavy segregation lumber 
 green from the saw is very costly. How- 
 ever, since temperatures should not ex- 
 ceed 120°F for about two thirds of the 
 total drying time, "predriers" or low- 
 temperature kilns could be used in com- 
 bination with conventional dry kilns. This 
 combination may compare favorably with 
 the present procedure of air seasoning 
 followed by kiln drying, and bring the 
 market closer to the saw. 
 
 LITERATURE 
 
 As early as 1925, a comprehensive study 
 was published (Green, 1925) which re- 
 ported on the problems of air seasoning 
 and commented on piling methods, sticker 
 spacing, yard layout, and lumber segre- 
 gation, and also gave information on 
 proper kiln construction. The immediate 
 objective of this study was to find a 
 profitable balance between drying time 
 required and losses occurring from de- 
 grade of the stock due to drying defects. 
 
 The large number of investigations 
 since that time was more or less focussed 
 on this basic objective. 
 
 An increase in the drying rate of 
 lumber stacked for air seasoning by 
 proper yard layout and the prevention of 
 end checking were of main interest 
 (Anon., 1935). The principal questions 
 to be answered in kiln drying were 
 originally connected with the drying 
 
 [3] 
 
behavior of the wood, the wide variation 
 encountered in the drying rate of boards 
 as related to visible characteristics, and 
 the construction and use of kilns and kiln 
 equipment. (Rankin, 1929; Utschig, 
 1935; Stegeman, 1938 and 1943; Davis, 
 19436) . In kiln drying redwood lumber, 
 the main concern was with two defects, 
 collapse and checking of birdseye, al- 
 though the occurrence of stain was also 
 undesirable (Davis, 1943a) . Of course, 
 other defects like checking and warping 
 would develop whenever improper drying 
 methods were used or abnormal wood 
 formations were encountered. In general, 
 such degrade was not frequent. Kiln 
 operators always encountered losses due 
 to collapse which seemed to range be- 
 tween 2 and 10 per cent of the total 
 amount of lumber being dried. Air sea- 
 soning followed by kiln drying reduced, 
 but hardly eliminated, the occurrence of 
 collapse. Checking or "popping" of birds- 
 eye was believed to be caused by the 
 more rapid drying of birdseye than of 
 surrounding wood. A birdseye is actually 
 an adventitious branch bud and behaves 
 similar to a knot upon drying. Two types 
 of birdseye could be differentiated — light 
 and dark. It is found singly, in groups of 
 a few, or in big clusters. Checking oc- 
 curred more frequently in dark birdseye. 
 
 To segregate redwood lumber in classes 
 having approximately the same drying 
 characteristics, Stegeman (1940) recom- 
 mended first sorting of the mill-run prod- 
 uct into three weight classes according to 
 moisture content. Stegeman later pointed 
 out that a great difference in the drying 
 rate existed between lumber having 
 coarse grain and lumber with dense grain 
 and a high percentage of summerwood. 
 
 To compare experimental results and 
 exchange personal experiences in the dry- 
 ing of redwood at different mills, the Red- 
 wood Seasoning Committee of the Cali- 
 fornia Redwood Association was formed 
 and a continuing series of minutes of 
 meetings and discussions was published 
 and new research initiated (Calif. Red- 
 
 wood Ass'n, 1944) . A series of publica- I 
 tions produced after the war gave specific 
 recommendations for economical and 
 high-quality air and kiln drying of red- 
 wood lumber (Manson and Clayton, 
 1948; Manson, 1949). It was again sug- 
 gested that green lumber be sorted into x 
 three weight segregations in case the lum- 
 ber is to be thoroughly pre-air seasoned. 
 When the largest portion of the lumber 
 was to be kiln dried green from the saw, 
 a fourth segregation including all boards 
 containing massed or black birdseye, 
 water streaks or streaks of yellowish or \ 
 brown color, bands of compression wood 
 or abnormal grain formation or of ex- 
 tremely fine grain was recommended. \ 
 Under all circumstances this fourth segre- 
 gation was to be air dried before being 
 brought to the kilns. Generally for all red- j 
 wood lumber, air drying before kiln dry- 
 ing rendered the best results as far as the 
 quality of the lumber was concerned. 
 Finally, kiln schedules for pre-air sea- 
 soned as well as for green lumber were 
 derived which safeguarded against great 
 losses from defects. 
 
 An attempt to market the lumber closer . 
 to the saw was made by using predriers 
 to bring the lumber down to the fiber 
 saturation point in a much shorter time 
 than could be done by air seasoning 
 (Sherman, 1937; Clausen, 1953; Anon., 
 1955) . The advantages of a predrier were 
 the following: 1) a constant, planned 
 production of partially dried lumber, 2) 
 a reduction in the volume of inventory, 
 3) a reduction in losses from end check- 
 ing and a decrease in the occurrence of 
 sticker stain. Also a fan- drying system 
 was designed to reduce the shipping 
 weight of redwood 2x4 studs (Tarason, 
 1957). 
 
 Following the very extensive California 
 Redwood Association reports, mentioned 
 before, quality-control studies were un- 
 dertaken making use of statistical tech- 
 niques in order to assure either uniform 1 
 final moisture contents of the end product . 
 or to develop time schedules for kiln dry- 
 
 [4] 
 
ing of pre-air seasoned stock (Manson, 
 1950; Pratt, 1953a; Lane et al, 1956; 
 Johnson, 1958) . 
 
 Again the increase in the drying rate 
 of lumber in the air seasoning yard and 
 the prevention of end checking became 
 the two major areas of investigation. The 
 performance of end sealers was studied 
 (Clausen, 1953a; Lambert and Pratt, 
 1955; Dost, 19566; Brubaker, 1959). 
 An increase in the drying rate of lumber 
 was found possible by proper spacing be- 
 tween lumber piles. For this reason, the 
 effect of pile spacing on the drying rate 
 and the cost of air seasoning were investi- 
 gated (Pratt, 1951; Clausen, 19536; 
 Dost, 1956a) . Increased drying rates of 
 units in air seasoning yards and a reduc- 
 tion in stain usually occurring in lumber 
 were attained by the use of cover roofs 
 (Benjamin, 1962). 
 
 The presence and distribution of ex- 
 tractives in redwood had been studied 
 (Shearrard and Kurth, 1933) and 
 groundwork for the recognition of the 
 chemical nature of these extractives was 
 laid (Buchanan et al, 1944; Anon., 
 1945). The distribution pattern of these 
 extraneous materials within boards was 
 investigated in green, air dried, kiln dried 
 and solvent seasoned lumber (Pratt, 
 19536; Anderson et al, 1960; Anderson 
 and Fearing, 1961) . Research results sug- 
 gested that chemical stain is associated 
 with the movement of water which carries 
 water-soluble extractives to the surface of 
 the lumber. The process underlying the 
 discoloration of redwood was also investi- 
 gated by determining various physical 
 and chemical factors on which the dark- 
 ening of the aqueous extract of redwood 
 depended (Zavarin and Smith, 1962). 
 Finally, a new phenolic compound re- 
 sponsible for the discoloration was iso- 
 lated (Balogh and Anderson, 1964). 
 
 Many attempts were made to prevent 
 the development of stain in redwood lum- 
 ber. Treatments with various aqueous 
 solutions of chemicals had a negligible 
 effect (Johnson, 1956). Some believed 
 
 that drying lumber immediately after it 
 is sawn reduces the occurrence of stain 
 to a large degree (Manson, 1949) . On the 
 other hand, the results of laboratory in- 
 vestigations seemed to indicate in one 
 case that variations in the drying methods 
 or drying schedules will not provide 
 means of controlling stain (Ellwood et al, 
 1960) . In another case, air drying fol- 
 lowed by kiln drying resulted in less stain 
 than kiln drying from the green condition 
 (Ellwood and Erickson, 1962) . Presteam- 
 ing of redwood lumber at 212°F before 
 drying proved to be successful in reduc- 
 ing, to a certain degree, the development 
 of stain and extractive bleeding (Ellwood 
 et al, 1960; Ellwood and Erickson, 1962 ; 
 Benjamin and Tobey, 1961; Ecklund 
 et al, 1962). 
 
 The problem of accelerating the drying 
 of redwood had once been approached by 
 using various chemical dip treatments be- 
 fore conventional drying; however, this 
 method did not appear very promising. 2 
 Again in recent years, the interest in ac- 
 celerating lumber drying had led to a 
 number of experimental studies. 
 
 Solvent seasoning, using acetone as an 
 extractant, proved in pilot test runs to be 
 a rapid method of removing water and 
 some extractives from redwood, thus 
 minimizing stain in the dried material 
 and yielding the extractive as chemical 
 by-product (Anderson and Fearing. 
 1961 ; Anderson et al, 1962) . 
 
 The presteaming treatment was also 
 used successfully to increase the drying 
 rate of lumber in air and kiln drying 
 (Ellwood et al., 1960; Ellwood and 
 Erickson, 1962; Benjamin and Tobev. 
 1961; Ecklund et al, 1962). The reduc- 
 tion in drying time was explained partly 
 by direct expulsion of water, rapid va- 
 porization of moisture during the cooling 
 period after steaming and presumably an 
 increase in the permeability of wood 
 (Ellwood and Ecklund. 1961). 
 
 2 Espenas, L. D. Accelerated drying of 3-inch 
 redwood. Unpublished report, USDA, Forest 
 Prod. Lab. 
 
 [5] 
 
PRESENT PRACTICE 
 
 Today the drying practice of all large 
 redwood mills with dry kilns is to segre- 
 gate first the green redwood lumber on the 
 basis of drying requirements or charac- 
 teristics — which are recognized by weight 
 and appearance. Usually the lumber is 
 segregated into light and heavy classes. 
 However, some mills practice three-way 
 segregation including a medium-weight 
 class. The light segregation is kiln dried 
 from their green condition to the desired 
 moisture content in a comparatively short 
 time. Heavy (and usually medium) segre- 
 gation lumber, however, is first air sea- 
 soned to between 30 and 50 per cent 
 moisture content before being kiln dried. 
 A survey showed that the amount of 
 lumber first air dried and then kiln dried 
 represents between 51 and 75 per cent 
 of a mills production. The air drying 
 period extends over 6 to 12 months de- 
 pending on the local climate, the policy 
 of the mill, and the current market situa- 
 tion. Kiln drying that follows air season- 
 ing requires an additional 6 to 20 days. 
 The wide range in kiln drying time is 
 explained by the range in moisture con- 
 tent of the lumber going into the kilns 
 and by the fact that almost every mill uses 
 a different kiln schedule. 
 
 Air drying followed by kiln drying has 
 the advantage that most of the free water 
 in the wood is evaporated during the air- 
 seasoning period and the kiln capacity 
 required is kept to a minimum. Drying 
 defects are largely prevented. On the 
 other hand, a large air-drying yard and 
 a high capital investment in stored lum- 
 ber is required. Even mills with a large 
 inventory cannot always be in the posi- 
 tion to fill an order immediately. From 
 all the other methods of drying mentioned 
 in the literature, only predrying is used 
 to a very limited degree. Kiln drying 
 heavy segregation lumber from its green 
 condition is very seldom practiced and 
 the schedules suggested for it (Manson, 
 1960; Rasmussen, 1961) are almost 
 
 never used because 30 to 35 days are re- 
 quired to dry 4/4" green stock to about 
 10 per cent moisture content. 
 
 EXPERIMENTAL OBJECTIVES 
 
 The experimental objectives of this 
 study were to establish to what degree 
 and by which conventional means the kiln 
 drying of heavy segregation redwood 
 lumber can be accelerated without dimin- 
 ishing product quality ; and to investigate 
 the influence of the three parameters of 
 drying — air temperature, relative hu- 
 midity, and air velocity, as well as the 
 effect of presteaming. 
 
 MATERIAL AND 
 EQUIPMENT 
 
 "Clear All Heart" and "A" grade lum- 
 ber was broken down into two main 
 groups with three subgroups. The two 
 main groups were flatsawn and quarter- 
 sawn boards. The subgrouping was ac- 
 complished by segregating into: 1) lum- 
 ber exhibiting a large number of birds- 
 eye, single and/or in clusters, but free 
 of water streaks; 2) lumber having a 
 large amount of water streaks but virtu- 
 ally no birdseye; and 3) lumber free of 
 characteristics mentioned in 1) and 2). 
 Experimental drying of lumber segre- 
 gated in this way made it possible to note 
 under which drying conditions and at 
 what particular stage of drying the dif- 
 ferent drying defects may occur. 
 
 To assure that only green material was 
 used, the boards were cut from freshly 
 sawed logs, segregated by category on the 
 green chain, and wrapped and shipped 
 to the University Forest Products Labo- 
 ratory at Richmond as quickly as possible. 
 The material was received in small 
 batches with delivery coinciding with 
 kiln scheduling. The boards used for the 
 first kiln runs were received as 4/4" 
 lumber. The major portion of boards, 
 however, was shipped as 8/4" lumber 
 
 [«] 
 
and resawn at the Laboratory. In this way 
 matched material was obtained and water 
 streaks could be marked on the freshly 
 sawn surface. 
 
 For the preliminary study, the kiln 
 charges were constructed as follows: Ex- 
 perimental kilns with variable air veloci- 
 ties were baffled extensively and 10 
 boards (1" x 6" x 8') were dried in each 
 test run. In addition a microkiln was 
 constructed in which intensive studies on 
 still smaller quantities (10 samples 22" 
 in length) were dried. 
 
 After establishing a knowledge of dry- 
 ing rates and occurrence of defects under 
 various conditions, three kiln runs were 
 conducted using mill-run heavy segrega- 
 tion lumber. One kiln run was carried out 
 on 100 M bd ft in the kilns of the Union 
 Lumber Company in Fort Bragg by the 
 company personnel, one on about 2 M 
 bd ft in an experimental kiln of the Pa- 
 cific Lumber Company, and the third on 
 5M bd ft in a kiln at the University 
 Forest Products Laboratory. 
 
 PROCEDURE 
 
 Air velocity and drying behavior 
 
 As A first step the influence of various 
 constant drying conditions on the occur- 
 rence of defects in boards was investi- 
 gated (table 1). In addition, 2 ft long, 
 end-coated samples were dried in condi- 
 tioning rooms with constant climate. 
 They were end matched with a number 
 of boards dried in some kiln runs. 
 
 A duplication of certain kiln runs were 
 made with only the air velocity changed. 
 Air speeds of 380 and 980 ft per minute 
 were chosen. It was also investigated 
 whether a difference existed in the drying 
 rate of closely matched 22" sample boards 
 dried at 110°F with a 20°F wet bulb de- 
 pression and air velocities of 214 and 107 
 ft per minute. 
 
 The current moisture content for each 
 individual board was calculated on the 
 basis of oven dry weight from weight 
 recordings made once and sometimes 
 twice a day in connection with the visual 
 
 Table 1. DRYING CONDITIONS FOR PRELIMINARY INVESTIGATIONS OF DRYING 
 CHARACTERISTICS AND THE INFLUENCE OF AIR VELOCITY 
 
 Temperature °F 
 
 Air velocity 
 ft/mi n 
 
 Kiln run 
 number 
 
 Lumber characteristic 
 
 Grain direction 
 
 
 Dry bulb 
 
 Wet bulb 
 
 Selection of material 
 
 150 
 
 110 
 
 380 
 
 3 
 
 plain 
 
 flat and vertical 
 
 randomly 
 
 150 
 
 110 
 
 980 
 
 4 
 
 plain 
 
 flat and vertical 
 
 randomly 
 
 130 
 
 95 
 
 380 
 
 5 
 
 waterstreak 
 
 flat and vertical 
 
 matched by resawing 
 
 130 
 
 95 
 
 980 
 
 6 
 
 waterstreak 
 
 flat and vertical 
 
 matched by resawing 
 
 130 
 
 95 
 
 380 
 
 7 
 
 birdseye 
 
 flat and vertical 
 
 matched with kiln run 8 
 
 120 
 
 90 
 
 380 
 
 1 
 
 waterstreak and birdseye 
 
 flat 
 
 randomly 
 
 120 
 
 90 
 
 380 
 
 2 
 
 waterstreak 
 
 vertical 
 
 randomly 
 
 110 
 
 90 
 
 380 
 
 8 
 
 birdseye 
 
 flat and vertical 
 
 matched with kiln run 7 
 
 73 
 
 55 
 
 5 
 
 condition- 
 ing rooms 
 
 waterstreak and birdseye 
 
 flat 
 
 end matched with boards in 
 kiln run 1 
 
 70 
 
 62 
 
 5 
 
 condition- 
 ing rooms 
 
 waterstreak and birdseye 
 
 flat 
 
 end matched with boards in 
 kiln run 1 
 
 [7 
 
inspection for drying defects. The 
 relationship between drying rate and 
 moisture concentration in lumber was 
 analyzed statistically by a regression 
 analysis using the method of least squares. 
 The steepness of moisture gradients es- 
 tablished in green boards when dried 
 under relatively mild kiln conditions was 
 determined to show approximately when 
 the evaporation of free water from the 
 surface was ended and the evaporation 
 zone had moved into the interior of 
 boards. Also observations were made 
 whether interior checking of birdseye 
 would occur. This was accomplished by 
 periodically cutting samples from all 
 boards in two kiln runs. They were cut 
 into 5 thin slices about %" thick. The 
 slices were closely inspected and their 
 moisture content determined. 
 
 Relative humidity 
 
 Gradual lowering of the relative hu- 
 midity early in a kiln run was investi- 
 gated as one of the means to increase 
 the drying rate. Such a procedure, al- 
 though suggested by the U. S. Forest 
 Products Laboratory (Rasmussen, 1961), 
 is not followed in any one of the com- 
 mercial schedules. The first kiln run [KR 
 
 13] was a modified version of the US 
 FPL schedule (table 2) . The only change 
 was an initial lower wet-bulb depression, 
 which seemed to be justified by results of 
 previous tests. The other two runs [KR 
 13A and 14] introduced further modifi- 
 cations in temperature during the inter- 
 mediate stages of drying. The material in 
 all three runs consisted of lumber selected 
 to contain birdseye and waterstreaks. The 
 air velocity was 380 ft per minute for all 
 
 runs. 
 
 Presteaming 
 
 Presteaming of lumber was included in 
 this study. Two- and four-hour steaming 
 periods were employed. Presteamed 
 boards and controls were always matched. 
 In the first test, six 2" x 6" x 8' bastard- 
 sawn boards containing a large amount of 
 birdseye in clusters were resawn and sur- 
 faced green. Half the number of resulting 
 4/4" boards were presteamed for two 
 hours at 212°F. After removal from the 
 steam chamber they were allowed to cool 
 for half an hour after which all the boards 
 were dried in the same charge under 
 equal conditions. 
 
 The second test was carried out on 22" 
 long samples which were dried together 
 
 Table 2. EXPERIMENTAL KILN RUNS WITH DECREASING WET BULB TEMPERATURES 
 DURING THE EARLY STAGES OF DRYING 
 
 
 KR 13 
 
 KR 13A 
 
 KR 14 
 
 Moisture content 
 
 Dry bulb 
 
 Wet bulb 
 
 Dry bulb 
 
 Wet bulb 
 
 Dry bulb 
 
 Wet bulb 
 
 Per cent 
 
 Temperature °F 
 
 Temperature °F 
 
 Temperature °F 
 
 Initial - 2 days 
 
 110 
 110 
 110 
 110 
 110 
 110 
 110 
 120 
 130 
 140 
 180 
 180 
 180 
 
 95 
 90 
 90 
 90 
 90 
 85 
 80 
 85 
 95 
 105 
 130 
 152 
 170 
 
 110 
 110 
 110 
 110 
 110 
 110 
 120 
 120 
 125 
 130 
 150 
 170 
 180 
 
 95 
 90 
 90 
 90 
 85 
 80 
 88 
 85 
 90 
 95 
 115 
 150 
 170 
 
 110 
 110 
 110 
 110 
 115 
 120 
 120 
 125 
 130 
 150 
 170 
 170 
 180 
 
 95 
 
 - 90 
 
 90 
 
 90-70 
 
 70-60 
 
 85 
 80 
 
 60-50 
 
 50-40 
 
 85 
 88 
 
 40-30 
 
 30 25 
 
 25-20 . 
 
 85 
 90 
 95 
 115 
 135 
 150 
 
 20-15... 
 
 15 -end 
 
 Equalizing 
 
 Conditioning 
 
 170 
 
 [3] 
 
with matched controls in the microkiln. 
 In this instance, the presteaming period 
 lasted for four hours with one hour cool- 
 ing. The control set was started drying 
 while one half of the samples underwent 
 the steaming treatment. 
 
 Extractive concentration 
 
 The influence of presteaming and rapid 
 drying on the extractive concentration in 
 the surface layers of boards was investi- 
 gated. 
 
 In the first test, the concentration of 
 water soluble extractives was determined 
 on matched 1" x 6" x 22" samples boards 
 in the 2 mm surface layer and in the 2 
 mm layer taken from the very center of 
 the boards. The amount of extractives was 
 determined, when the wood was green 
 after kiln drying, from the green condi- 
 tion, and after presteaming followed by 
 kiln drying. Extraction was done with hot 
 water for 24 hours in a Soxhlet extractor. 
 
 A second test was carried out on an- 
 other board in almost the same manner 
 as described above. However, only the 
 extractive content in the surface layer 
 was determined. The remaining samples 
 now surfaced were placed in a condition- 
 ing room and exposed to high relative 
 humidity (70°F and 95 per cent RH) 
 for a four-month period. This exposure is 
 a proven method to accelerate or initiate 
 staining. 
 
 Average mill-run lumber 
 
 Also investigated was the effect of dry- 
 ing on the behavior of average mill-run 
 heavy segregation lumber grown in dif- 
 ferent areas of the redwood region. 
 
 First, a kiln charge consisting of ap- 
 proximately 100 M bd ft 4/4" lumber 
 was dried by the Union Lumber Com- 
 pany at Fort Bragg, using a schedule 
 quite similar to the one used in kiln run 
 14. Drying time was equally divided be- 
 tween a predrier and a conventional kiln. 
 The progress of drying was observed on 
 30 kiln samples. After completion of dry- 
 
 ing, the rough lumber was inspected on 
 the dry sorter by kiln personnel and com- 
 pany graders. 
 
 The schedule used in kiln run 13A was 
 evaluated on mill-run lumber dried at 
 the Pacific Lumber Company, Scotia. 
 One unit of 208 4/4" boards ranging 
 from 14 to 20 ft in length was dried in 
 the small experimental kiln of the com- 
 pany. The progress of drying was fol- 
 lowed by using 10 kiln samples which 
 had been selected for their original heavy 
 weight and density. After drying, each 
 board was inspected for drying defects by 
 a company grader. 
 
 A final kiln run was carried out on 5 M 
 bd ft of 4/4" lumber which had been 
 cut at the Samoa mill of Georgia Pacific 
 Corporation. The charge was dried in a 
 kiln of the Richmond Laboratory. Boards 
 in only two out of four units were end 
 coated on one side to make possible a 
 comparison whether end coating would 
 prevent checking. The air velocity 
 through the courses averaged 295 ft per 
 minute. The kiln conditions were changed 
 according to the average moisture con- 
 tent of ten samples, which were prepared 
 from heavy and dense boards, and the 
 schedule used in kiln run 13A. In addi- 
 tion, the amount of strain developed and 
 the moisture content at different depths 
 at each stage of drying were determined 
 on two 6" wide, flatsawn boards with 
 growth rings of wide curvature. Observa- 
 tions were made on wafers cut from the 
 boards 1 inch along the grain. Each wafer 
 was cut into five strips 4 mm thick, with 
 strips 1 and 5 taken from the surface, 
 strips 2 and 4 from the intermediate zone, 
 and strip 3 representing the center zone. 
 The strain pattern was determined by 
 measuring the change in width of each 
 strip before and after sawing it from the 
 wafer (McMillan, 1958). After comple- 
 tion of the kiln run, all boards were in- 
 spected by graders of the Redwood In- 
 spection Bureau before and after remanu- 
 facturing them to S4S boards and bevel 
 siding. 
 
 [9] 
 
RESULTS AND DISCUSSION 
 
 Air velocity 
 
 The increase in air velocity from 380 
 to 980 ft per minute had a relatively 
 minor effect on the over-all drying rate 
 (figures 1 and 2). A comparison of the 
 
 drying rates of matched boards in kiln 
 runs 5 and 6 showed that faster drying 
 was accomplished only during the very 
 first portion of drying (table 3). After 
 that, the moisture concentration in the 
 wood appeared to be the main factor in 
 determining the drying rate. A similar re- 
 
 Figure 1. 
 INFLUENCE OF AIR VELOCITY ON DRYING TIME 
 
 4/4 REDWOOD 
 HEAVY SEGREGATION 
 AIR VELOCITY 378V MIN. 
 9787 « 
 
 40 80 120 160 200 240 280 320 
 
 DRYING TIME, HOURS 
 
 Table 3. DRYING OF HEAVY-SEGREGATION 4x4" REDWOOD LUMBER 
 AT TWO DIFFERENT AIR VELOCITIES 
 Conditions: 130° F dry bulb temperature Air Velocities: KR 5:380 ft/min 
 
 95° F wet bulb temperature KR 6:980 ft/min 
 
 Drying time 
 
 Average moisture content 
 [per cent of oven-dry weight) 
 
 Average drying rate 
 [per cent moisture content per hour] 
 
 hours 
 
 KR 5 
 
 KR 6 
 
 KR 5 
 
 KR 6 
 
 
 
 3 
 
 8 
 
 141 
 120.4 
 102.2 
 75 
 66 8 
 53.0 
 47.6 
 38.6 
 35 
 28 6 
 16 8 
 15 4 
 
 138 
 
 114.2 
 97.4 
 71.0 
 62.9 
 49.7 
 44.3 
 35.7 
 32.5 
 26.2 
 15.8 
 14.4 
 
 6.867 
 3.640 
 • 1.700 
 1.025 
 0.863 
 0.675 
 0.563 
 0.450 
 0.355 
 0.256 
 0.175 
 
 7.933 
 3.360 
 1.650 
 
 24 
 
 32 
 
 1.013 
 825 
 
 48... 
 
 675 
 
 56 
 
 537 
 
 72 
 
 400 
 
 80 
 
 350 
 
 98 
 
 226 
 
 144 
 
 175 
 
 152 
 
 
 
 
 [10 
 
Table 4. SLOPES OF REGRESSION LINES CALCULATED FOR THREE SETS OF DATA 
 
 Drying conditions 
 
 Number of 
 
 Avg. 
 initial 
 M.C. 
 
 Regression-line 
 
 DBT 
 
 WBT 
 
 Air veloc. 
 
 Boards 
 
 Observa- 
 tions 
 
 Regr. coeff. 
 n-slope 
 
 Drying rate at 100% MC 
 % MC loss 
 
 Correlation 
 coeff. 
 
 °F 
 
 °F 
 
 ft/min 
 
 Per cent 
 
 per hr 
 
 per day 
 
 
 150 
 130 
 110 
 
 110 
 95 
 90 
 
 380 
 380 
 380 
 
 10 
 10 
 10 
 
 100 
 110 
 104 
 
 180 
 142 
 115 
 
 1 271 
 1.360 
 1.544 
 
 2.01 
 2.18 
 1.22 
 
 48.2 
 52.3 
 29.3 
 
 .961 
 .916 
 .895 
 
 Figure 2. 
 INFLUENCE OF AIR VELOCITY ON DRYING TIME 
 
 9on 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 IDU 
 
 
 
 
 
 
 
 
 
 
 
 D.B.T. 
 
 
 
 
 
 
 
 
 
 120 
 
 
 
 
 
 
 W.B.T. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 80 
 
 40 
 
 O 
 
 
 4/4 REDWOOD 
 HEAVY SEGREGATION 
 AIR VELOCITY 378'/ MIN. 
 
 
 *» 
 
 ^Q&# 
 
 
 
 
 
 ■ ^SQj 
 
 ^^ 
 
 ***oo m 
 
 ■■•••O'MQ 
 
 
 
 
 40 80 120 160 200 240 280 320 
 
 DRYING TIME, HOURS 
 
 suit was obtained by observing the drying 
 rate of end- and side-matched 22" long 
 sample boards dried in a microkiln at air 
 speeds of 214 and 107 ft per minute. 
 
 The explanation for these results lies 
 in the fact that the surface of redwood 
 lumber dries rapidly and the capillary 
 moisture movement to the surface comes 
 to an early halt (figure 3) . Once the sur- 
 face layers are depleted of free water, the 
 moisture has to move through them by 
 diffusion. 
 
 Although higher air velocities had little 
 influence on the over-all drying time, it 
 
 should not be concluded in the commer- 
 cial kiln drying practice for redwood that 
 an extremely slow air circulation would 
 be just as effective as the generally recom- 
 mended higher velocities. 
 
 For good uniformity in drying 
 throughout the kiln, a fairly high air 
 velocity is usually necessary to prevent 
 too great a temperature drop and humidi- 
 fication of the air moving across the load. 
 (Most of the double track redwood kilns 
 having an air speed of 350 ft per minute 
 and more on the leaving side of a charge 
 seem to perform satisfactorily) . If higher 
 
 [ii] 
 
Figure 3. 
 MOISTURE DISTRIBUTION AT DIFFERENT DEPTHS 
 
 200 
 
 4/4 REDWOOD 
 —4mm. SHELL 
 
 4mm. INTERMEDIATE LAYER 
 REMAINING CORE 
 
 60 90 120 
 
 DRYING TIME, HOURS 
 
 180 210 
 
 240 
 
 air velocities are contemplated, the eco- 
 nomic aspect of power consumption will 
 have to be considered. The energy re- 
 quired to turn fans varies with the 2.4 
 to 3.0 power of the increase in air speed 
 (Kollmann and Schneider, 1960) . 
 
 Drying rate 
 
 The dependency of drying rate on the 
 current moisture content of the wood was 
 quite apparent under constant drying 
 conditions. A straight-line relationship 
 between the logarithmic transformation 
 of their values could be used to estimate 
 the progress of drying quite closely (cor- 
 relation coefficients of 0.9 and above) . 
 In other words, when the moisture con- 
 tent loss per unit time was plotted against 
 the average moisture content on log-log 
 paper, a straight line relationship was ob- 
 tained. 
 
 This result is in close agreement with 
 findings by Peck et al. (1952). They 
 started with the general diffusion equa- 
 
 tion for one-directional diffusion of mois- 
 ture through sides of a rectangular slab 
 and developed the following drying rate 
 equation: 
 
 dC 
 
 A0 
 
 = -MC n 
 
 where d C = change in average moisture 
 dO — change in time 
 C = average moisture content 
 The exponent n is equal to the slope of 
 this line. The value M, read on the ordi- 
 nate, is equal to the drying rate d C at the 
 
 d¥ 
 average moisture content C = 1. The time 
 required for the drying of a board from 
 one given moisture content to another 
 can be calculated by integration of this 
 rate equation. 
 
 The slopes of regression lines calculated 
 for three sets of data obtained in this 
 study are presented in table 4. 
 
 In respect to the method of evaluating 
 drying rates demonstrated here, the fol- 
 
 [12 
 
lowing must be considered. The sample 
 size was relatively small, and the results 
 may not be representative of all heavy 
 segregation redwood lumber. However, 
 they were quite useful for this study. The 
 method has the limitation that the initial 
 moisture content of boards will influence 
 the regression line to a certain extent. 
 Thus the above formula does not describe 
 the initial stage of drying during which 
 free water is brought to the surface by 
 capillary action and evaporated there. 
 Further, one has to keep in mind that 
 the regression lines are based on drying 
 rates observed on boards dried in an ex- 
 perimental kiln. They will not apply to a 
 commercial kiln where cooling and hu- 
 midification of the air takes place while 
 it moves across the load. Therefore, dry- 
 ing rates in commercial kilns are much 
 slower. 
 
 Temperature 
 
 Using higher temperatures to acceler- 
 ate drying is not advisable for the first 
 and second stage of drying because col- 
 lapse is likely to occur. 
 
 During the first portion of the study, 
 when the development of defects was ob- 
 served under constant and changing kiln 
 conditions (kiln runs 1 through 14), col- 
 lapse appeared closely related to temper- 
 ature and the moisture content of the 
 lumber. The higher the temperature to 
 which the lumber with a moisture con- 
 tent above the fiber saturation point was 
 subjected, the larger was the amount of 
 collapse. Lumber without birdseye had 
 a critical temperature of approximately 
 130°F. Above this temperature collapse 
 occurred more frequently. Lumber with 
 a large amount of birdseye, especially 
 when present in clusters, and lumber con- 
 taining dark streaks indicating a high ex- 
 tractive content, showed areas of collapse 
 at lower temperatures. The critical limit 
 appeared to be around 120°F. It was not 
 noticed that collapse occurred more fre- 
 quently in areas of waterstreak than in 
 the adjacent wood. 
 
 Under constant drying conditions, the 
 presence of collapsed cell formation usu- 
 ally became noticeable when the lumber 
 contained from 30 to 80 per cent mois- 
 ture. When the kiln conditions were 
 changed stepwise (kiln runs 13, 13 A, and 
 14), collapse did not occur until the tem- 
 perature was raised to 130°F and higher. 
 Then, a few sample boards in either run 
 showed slight depressions which always 
 surfaced off when the lumber was planed 
 to 25/32 of an inch. It was interesting 
 that at the time these slightly collapsed 
 areas became noticeable, the average 
 moisture content of such boards was be- 
 tween 35 and 20 per cent. Because of the 
 fairly steep moisture gradiant, however, 
 the core of these boards still contained up 
 to 50 per cent moisture content. 
 
 Toward the end of the kiln runs, when 
 higher temperatures could be used safely, 
 it was not possible to shorten the drying 
 time. Because of the ununiformity in dry- 
 ing rates between heavy segregation 
 boards, some of them reached a moisture 
 that commanded the start of an equaliz- 
 ing treatment while others had moisture 
 contents around the fiber saturation point. 
 
 Relative humidity 
 
 Gradual lowering of the wet-bulb de- 
 pression at constant low dry-bulb temper- 
 atures increased the drying rate appreci- 
 ably during the second stage of drying. 
 The drying time for kiln runs 13, 13A, 
 and 14 were respectively 9, 11 and 15 per 
 cent shorter than the drying time re- 
 quired to bring lumber from an initial 
 average moisture content of 165 per cent 
 to 40 per cent under a constant kiln set- 
 ting of 110°F with a 20°F wet-bulb de- 
 pression. 
 
 Birdseye proved to be quite sensitive 
 to high temperatures and/or low humidi- 
 ties. This was true mainly during the first 
 days of drying when the surface layer of 
 heavy segregation material dried down 
 very rapidly. The checking of light birds- 
 eye could always be prevented by using 
 a starting temperature of 110°F with a 
 
 13 
 

 
 Table 6. KILN RUN AT UNION LUMBER COMPANY 
 
 
 
 Day 
 
 DBT 
 
 WBT 
 
 Average 
 M. C. 
 
 Average 
 
 M. C. loss 
 
 per day 
 
 Day 
 
 DBT 
 
 WBT 
 
 Average 
 M. C. 
 
 Average 
 
 M. C. loss 
 
 per day 
 
 
 °F 
 
 °F 
 
 per cent 
 
 per cent 
 
 14 
 
 °F 
 
 °F 
 
 per cent 
 
 per cent 
 
 
 
 110 
 
 90 
 
 177.3 
 
 
 120 
 
 90 
 
 
 
 1 
 
 110 
 
 90 
 
 152.0 
 
 25.3 
 
 15 
 
 120 
 
 90 
 
 29.6 
 
 4.6 
 
 2 
 
 110 
 
 90 
 
 
 
 16 
 
 125 
 
 90 
 
 
 
 3 
 
 110 
 
 90 
 
 113.6 
 
 19.2 
 
 17 
 
 125 
 
 89 
 
 
 
 4 
 
 110 
 
 90 
 
 
 
 18 
 
 130 
 
 95 
 
 
 
 5 
 
 110 
 
 87 
 
 91.1 
 
 11.3 
 
 19 
 
 135 
 
 100 
 
 
 
 6 
 
 110 
 
 84 
 
 
 
 20 
 
 140 
 
 105 
 
 15.5 
 
 2.8 
 
 7 
 
 110 
 
 81 
 
 
 
 21 
 
 145 
 
 110 
 
 
 
 8 
 
 112 
 
 83 
 
 
 
 22 
 
 150 
 
 115 
 
 
 
 9 
 
 115 
 
 86 
 
 
 
 23 
 
 155 
 
 120 
 
 
 
 10 
 
 118 
 
 86 
 
 52.7 
 
 7.7 
 
 24 
 
 160 
 
 120 
 
 9.3 
 
 1.6 
 
 11 
 
 120 
 
 87 
 
 
 
 25 
 
 170 
 
 120 
 
 
 
 12 
 
 120 
 
 88 
 
 
 
 26 
 
 160 
 
 135 
 
 6.7 
 
 1.3 
 
 13 
 
 120 
 
 88 
 
 
 
 27 
 
 170 
 
 160 
 
 8 6 
 
 
 juice was pressed out of closely matched 
 green redwood samples of which half had 
 been steamed at 212°F for four hours, a 
 colorimetric comparison in alkaline me- 
 dium showed no difference between the 
 sampled liquid. 8 
 
 Kiln runs on mill-run lumber 
 
 In the first commercial kiln run at 
 Union Lumber Company the time re- 
 quired to reduce the average moisture 
 content of the kiln samples from 177.3 per 
 cent to 6.7 per cent was 26 days. During 
 one additional day, the lumber was con- 
 ditioned and the moisture content of sam- 
 ples raised to 8.6 per cent (table 6) . The 
 time required to complete this kiln run 
 was much longer than for the experimen- 
 tal drying in the laboratory kiln. Again, 
 this is explained by the temperature drop 
 and humidification of air which takes 
 place across two 8 ft-wide loads in a 
 double track predrier or kiln. From the 
 seventh day on the lowest obtainable wet- 
 bulb depressions were used. 
 
 The inspection of the lumber on the 
 dry sorter showed that the number of 
 boards in the entire charge which showed 
 seasoning defects amounted to less than 4 
 per cent. The defect most often encoun- 
 tered was collapse. All of the boards with 
 
 3 Analysis by B. Balogh, Assistant Specialist, 
 Forest Products Laboratory, Richmond. 
 
 collapse were of dark color indicating a 
 high extractive content. The amount of 
 degrade was not exceptional, and the kiln 
 run was considered successful. 
 
 The progress of drying in the experi- 
 mental kiln of the Pacific Lumber Com- 
 pany is diagrammed in figure 5. The 
 original average moisture content of 
 167.5 per cent in the 10 samples was re- 
 duced to 7.0 per cent in 22 days. At the 
 completion of the kiln run, a "shell and 
 core" test showed in a few samples still 
 some casehardening, indicating the need 
 for additional equalizing and condition- 
 ing treatment. 
 
 The main defect encountered was end 
 checking of 20 ft long boards which had 
 their ends overhanging while drying. It 
 was felt that box piling of the lumber 
 would have prevented this defect. Four 
 per cent of the boards showed areas of 
 collapse and 1 per cent surface checks. 
 The general feeling of the grader was that 
 the unit appeared similar to the normal 
 mill run as far as drying defects were con- 
 cerned. 
 
 The last kiln run carried out on stock 
 provided by the Georgia-Pacific Corpora- 
 tion's mill in Samoa was concluded in 23 
 days. In this period, the moisture content 
 of samples was reduced from 160.5 to 7.8 
 per cent (figure 6) . During the first half 
 of the kiln run, low dry-bulb temperatures 
 
 [16] 
 
Figure 5. 
 EXPERIMENTAL DRYING AT THE PACIFIC LUMBER CO. 
 
 200 
 
 180 
 
 4/4" REDWOOD CLEAR 
 HEAVY SEGREGATION 
 m AVERAGE "1 
 
 •• MAXIMUM } MC% 
 
 15 
 
 DRYING TIME, DAYS 
 Figure 6. 
 
 EXPERIMENTAL DRYING AT RICHMOND LABORATORY 
 
 25 
 
 \ 
 I80M, 
 
 4/4 REDWOOD CLEAR 
 HEAVY SEGREGATION 
 — - MAXIMUM ") 
 
 AVERAGE Y M.C.(%) 
 
 D.B.T. 
 
 10 15 
 
 DRYING TIME, DAYS 
 
 [17] 
 
 20 
 
 25 
 
with stepwise increasing wet-bulb depres- 
 sions gave good drying rates. When the 
 average moisture content of the samples 
 had reached 29 per cent moisture content 
 on the 13th day, a stepwise increase in 
 dry-bulb temperature above 120°F was 
 started. 
 
 The kiln run, which was intended to 
 be marginal in respect to the time- defect 
 relationships, was only partially positive 
 in its results. Surface and end checking 
 was fully prevented (only one board out 
 of the 5 M bd ft charge had a surface 
 check and one an end check) . Light birds- 
 eye was intact on the surface, and only 
 a small percentage of the dark birdseye 
 had fine separations. However, interior 
 checking of birdseyes could be found and 
 some cases of honeycomb were associated 
 with birdseye clusters. Also about 6 to 7 
 per cent of the inspected boards showed 
 areas of collapse which either had to be 
 trimmed off or degraded the boards. The 
 lumber generally contained more of these 
 two defects than is usual in mill-run lum- 
 ber first air seasoned and then kiln dried. 
 It appeared somewhat brighter in color; 
 however, some boards had sticker marks. 
 
 The moisture-content distribution and 
 the strain patterns in the outside, inter- 
 mediate, and center layer of two boards 
 are diagrammed in figures 7 and 8. Inso- 
 far as strains are representative of stresses 
 in boards, it can be seen that compression 
 and tension stresses built up equally rapid 
 in the interior and surface zones of both 
 boards and remained at a moderate level 
 throughout the first half of the drying 
 time although the relative humidity was 
 gradually reduced. Thus, checking did 
 not occur. Stress reversal in Board 1 
 started quite early on the 9th day. In the 
 second board however, stress reversal did 
 not take place until the 16th day, yet at 
 almost the same moisture content as in 
 Board 1, that is, at 26 per cent. In both 
 boards, the tensile strain reached the 
 maximum value in the center zone shortly 
 after stress reversal. It was believed that 
 the interior checking of birdseye which 
 
 occurred in other boards happened dur- 
 ing such a peak period of tensile strain. 
 
 Just as the strain pattern was quite dif- 
 ferent for the two boards so was the cau- 
 sative moisture gradiant. All layers of 
 Board 1 dried readily. In contrast, the 
 intermediate layer of Board 2 reached 
 fiber saturation point quite late on the 
 16th day when stresses reversed while the 
 core was still at 54 per cent moisture con- 
 tent. At the same time, the kiln tempera- 
 ture was raised to 150°F. Although Board 
 2 did not collapse, others seemingly did 
 in this stage of drying. These data, of 
 course, are not sufficient to answer the 
 question of whether collapse was due to 
 either liquid tension, a result of water- 
 filled cells at a late drying stage, or cell 
 failure due to compressive stresses that 
 mounted prior to stress reversal. 
 
 Looking at the entire kiln run, it ap- 
 peared that the first portion of the sched- 
 ule performed extremely well, but an 
 extension of the period at 120°F would 
 have been necessary until all boards in 
 the charge reached fiber saturation point. 
 From the recorded drying rates, a mini- 
 mum drying time of 26 to 27 days was 
 estimated for a commercial run that 
 would reduce defects below an economi- 
 cally acceptable level. The period during 
 which the lumber had to be dried under 
 "low temperature drying conditions" 
 would extend over two thirds of the total 
 kiln-drying time. 
 
 According to information provided by 
 personnel of the Georgia-Pacific Corpora- 
 tion, the schedule performs well without 
 modification for medium segregation 
 lumber. It is commercially used now for 
 drying of 4/4" lumber of this segrega- 
 tion from its green condition to 10 per 
 cent moisture content in about 17 days. 
 
 ECONOMIC ASPECTS 
 
 The supreme test of an industrial oper- 
 ation lies in the field of economics. Yet 
 the separation of individual cost factors 
 of drying often appears to be difficult in 
 
 [18] 
 
STRESS SECTIONS 4x4 REDWOOD, BOARD NO. 2 
 
 TEMP. 
 
 E.M.C. 
 
 Figure 7. 
 
 8 12 16 
 
 DRYING TIME, DAYS 
 
 an integrated lumber operation. Factors heavy segregation lumber, the cost of dry- 
 like steam and power consumption or ing may vary considerably from mill to 
 even drying time for a given sortiment mill, and it is difficult to evaluate an aver- 
 are seldom evaluated accurately. For age for the whole industry. Different 
 
 [19] 
 
STRESS SECTIONS 4x4 REDWOOD, BOARD NO. 1 
 
 "" .0015 
 
 TEMP. DEGREES F 
 E.M.C., PER CENT 
 
 Figure 8. 
 
 8 12 16 
 
 DRYING TIME, DAYS 
 
 operations install different types and 
 brands of kilns, and these require dif- 
 ferent investment and maintenance ex- 
 penditures. Also, different policies and 
 
 drying schedules are followed. In addi- 
 tion, an evaluation of losses due to drying 
 defects is hard to come by since the 
 amount of degrade and trim varies with 
 
 [20] 
 
Table 7. KILN DRYING COSTS USING A 100 M BD FT CAPACITY KILN 
 
 Per year Per day 
 
 Indirect costs 
 
 Amortization of the kiln 
 
 (over 20 years at 6 per cent interest) $8,718.00 $23. 89 
 
 Taxes 
 
 (6 per cent of 1/3 of the installation cost) 2, 000 . 00 
 
 Insurance 1,000.00 
 
 Maintenance 
 
 (3 per cent of the installation cost) 3,000.00 
 
 Interest on lumber 
 
 (6 per cent on $125/M bd ft) 
 
 Direct costs 
 
 Electric power 3,528.00 
 
 Steam consumption 7, 519 . 00 
 
 Labor 2, 555 .00 
 
 Total cost 
 
 Dollar/day/100 M bd ft $80.24 
 
 or 
 Dollar/day/M bd ft .80 
 
 5.48 
 2.74 
 
 8.22 
 
 2.64 
 
 9.67 
 
 20.60 
 
 7.00 
 
 the type of timber, drying season, kiln 
 performance, drying schedule, and many 
 other factors. 
 
 The total cost for drying redwood lum- 
 ber was investigated when the effect of 
 pile spacing in the air drying yard on the 
 drying rate of lumber was determined 
 (Pratt, 1951; Clausen, 19536). Also an 
 unpublished report 4 based on informa- 
 tion supplied by various parts of the red- 
 wood industry compared costs of different 
 drying methods. All these reports provide 
 excellent information on basic cost fac- 
 tors, like capital investment, taxes, insur- 
 ance, etc. 
 
 Our study did not intend to arrive at 
 either a minimum or an average cost 
 value; this would have required an exact 
 knowledge of the monetary loss from dry- 
 ing defects. It was our aim to bring the 
 experimental results into focus from an 
 economic point of view. 
 
 In estimating kiln-drying costs, the fol- 
 lowing basic assumptions were made : the 
 initial investment in kiln construction and 
 kiln equipment amounts to $100,000 for 
 a 100 M bd ft capacity kiln ; the kiln oper- 
 ates 365 days per year. The factual amor- 
 tization (not the book amortization) for 
 such a structure is 20 years, and the value 
 of M bd ft 4/4" redwood lumber in the 
 
 * Dost, W. A. Seasoning costs. Manuscript at 
 the California Redwood Association. 
 
 rough is $125.00. With the remaining 
 values for insurance, taxes, and direct 
 costs quoted from information furnished 
 by the California Redwood Association, 
 a cost anlaysis for kiln drying may resem- 
 ble the one presented in table 7. Accord- 
 ing to the analysis of table 7 the cost of 
 kiln drying would be $0.80 per day per 
 M bd ft. For air seasoning, a cost of 
 $0.03 per day per M bd ft can be quoted 
 from the study on air seasoning by Pratt. 
 1951. The largest part of this later figure 
 represents the interest on the value of 
 lumber stored in the yard. The cost of 
 lumber transportation for both ways, to 
 and from the yard, may be close to $0.60 
 perMbdft. 
 
 In common mill practice, the combina- 
 tion process of air seasoning followed by 
 kiln drying is employed. A cost estimate 
 for this process depends on the time re- 
 quirement for the two part procedures. 
 Here a factual drying scheme of one 
 company may be used. It has been derived 
 in conjunction with a quality control 
 study of the California Redwood Associ- 
 ation on about 90 mill bd ft and gives 
 excellent results in respect to grade re- 
 covery. Table 8 lists the number of days 
 required for kiln drying 4/4" heavy seg- 
 regation lumber after a given number of 
 months of air seasoning. The scheme 
 accounts for the seasonal differences in 
 
 [21] 
 
Table 8. TIME REQUIREMENT FOR AIR SEASONING FOLLOWED BY KILN DRYING 
 (based on CRA Research Report No. 3.222120) 
 
 
 Number of months in the air seasoning yard before kiln drying 
 
 Lumber to the 
 yard in 
 
 2 
 
 3 
 
 4 
 
 5 6 
 
 7 
 
 8 
 
 9 
 
 10 
 
 11 
 
 12 
 
 
 Number of days required for kiln drying 
 
 January 
 
 27 
 
 24 
 
 22 
 
 19 
 
 16 
 
 14 
 
 12 
 
 10 
 
 7 
 
 5 
 
 
 February 
 
 27 
 
 24 
 
 22 
 
 19 
 
 16 
 
 13 
 
 10 
 
 8 
 
 6 
 
 5 
 
 
 March 
 
 26 
 
 23 
 
 20 
 
 18 
 
 15 
 
 13 
 
 11 
 
 9 
 
 7 
 
 5 
 
 
 April 
 
 25 
 
 22 
 
 18 
 
 15 
 
 13 
 
 12 
 
 11 
 
 9 
 
 8 
 
 6 
 
 5 
 
 May 
 
 25 
 
 21 
 
 18 
 
 14 
 
 12 
 
 10 
 
 9 
 
 8 
 
 6 
 
 5 
 
 
 June 
 
 25 
 
 22 
 
 18 
 
 16 
 
 14 
 
 12 
 
 10 
 
 8 
 
 6 
 
 5 
 
 
 July 
 
 26 
 
 23 
 
 21 
 
 18 
 
 16 
 
 14 
 
 13 
 
 11 
 
 9 
 
 7 
 
 5 
 
 August 
 
 27 
 
 25 
 
 23 
 
 22 
 
 20 
 
 18 
 
 16 
 
 14 
 
 10 
 
 5 
 
 
 September 
 
 28 
 
 27 
 
 25 
 
 24 
 
 22 
 
 19 
 
 16 
 
 13 
 
 9 
 
 5 
 
 
 October 
 
 29 
 
 27 
 
 25 
 
 23 
 
 20 
 
 18 
 
 13 
 
 9 
 
 5 
 
 5 
 
 
 November 
 
 30 
 
 28 
 
 26 
 
 22 
 
 19 
 
 16 
 
 12 
 
 10 
 
 5 
 
 5 
 
 
 December 
 
 30 
 
 27 
 
 24 
 
 20 
 
 18 
 
 14 
 
 11 
 
 7 
 
 5 
 
 5 
 
 
 weather conditions by considering the 
 time of the year when the lumber was 
 stacked for air seasoning. 
 
 The total cost of air seasoning followed 
 by kiln drying can be calculated from the 
 information on drying time presented in 
 table 9 and the cost factors listed in table 
 7. Table 9 lists costs for various time com- 
 binations and from it the optimum com- 
 binations were found (minimum cost 
 figures are underlined) . The average 
 minimum cost for the year is $14.58 per 
 M bd ft. 
 
 Very often shorter than optimum air 
 seasoning periods are used whenever 
 certain assortments are needed. 
 
 From an economical point of view, the 
 practice of kiln drying green heavy seg- 
 regation lumber cannot compare with air 
 drying followed by kiln drying, not even 
 when an accelerated schedule requiring 
 only 26 or 27 days is used. Kiln drying 
 green lumber in 27 days to about 8 per 
 
 cent average moisture content would cost 
 an estimated $21.65. To come close to the 
 cost of air drying plus kiln drying, the 
 kiln drying time for green lumber would 
 have to be reduced to about 18 to 22 days. 
 This is virtually impossible. Kiln drying 
 heavy segregation lumber directly from 
 the saw should be practiced only when 
 unexpected demand has to be met that 
 justifies an extra expenditure. 
 
 The use of predriers or low-temperature 
 kilns should be considered as a possible 
 means of evaporating free water from 
 lumber at a lower cost than in a kiln. For 
 redwood predrying, temperatures up to 
 110°F to 120°F may be used until the 
 lumber reaches a moisture content of 30 
 to 25 per cent. Below this state, optimum 
 drying rates cannot be obtained any 
 more, and a transfer to the kiln is neces- 
 sary. 
 
 Unlike dry kilns, predriers can be made 
 of lighter construction, do not require 
 
 [22 
 

 
 Table 9. COST OF AIR SEASONING FOLLOWED BY KILN DRYING 
 
 
 
 Lumber 
 
 
 
 Number of months in the 
 
 air seasoning yard before kiln drying 
 
 
 
 to the 
 yard in 
 
 2 
 
 3 
 
 4 
 
 5 
 
 6 
 
 7 
 
 8 
 
 9 
 
 10 
 
 11 
 
 12 
 
 
 Dollars per M bd ft 
 
 Jan. 
 
 24.08 
 
 22.58 
 
 21.90 
 
 20.40 
 
 18.92 
 
 18 22 
 
 17.54 
 
 16.84 
 
 15.42 
 
 14.66 
 
 
 Feb. 
 
 24.08 
 
 22.58 
 
 21 90 
 
 20.40 
 
 18.92 
 
 17 42 
 
 15.94 
 
 15.24 
 
 14 62 
 
 14.66 
 
 
 March 
 
 23.28 
 
 21.78 
 
 20.30 
 
 19.60 
 
 18.12 
 
 17.42 
 
 16.74 
 
 16 04 
 
 15 42 
 
 14.66 
 
 
 April 
 
 22.48 
 
 20.97 
 
 18.70 
 
 17.19 
 
 16.52 
 
 16 61 
 
 16.74 
 
 16 04 
 
 16 23 
 
 15 46 
 
 15 59 
 
 May 
 
 22.48 
 
 20.17 
 
 18.70 
 
 16.39 
 
 15.71 
 
 15.01 
 
 15.14 
 
 15 24 
 
 14 62 
 
 14 66 
 
 
 June 
 
 22.48 
 
 20.97 
 
 18.70 
 
 17.99 
 
 17.32 
 
 16.61 
 
 15.94 
 
 14 24 
 
 14 62 
 
 14 66 
 
 
 July 
 
 23.28 
 
 21.78 
 
 21.10 
 
 19.60 
 
 18.92 
 
 18.22 
 
 18.35 
 
 17.64 
 
 17.03 
 
 16.26 
 
 15 59 
 
 Aug. 
 
 24.08 
 
 23.38 
 
 22.71 
 
 22.80 
 
 22.13 
 
 21.43 
 
 20.75 
 
 20.05 
 
 17 83 
 
 14 66 
 
 
 Sept. 
 
 24.89 
 
 24.89 
 
 24.31 
 
 24.41 
 
 23.73 
 
 22.23 
 
 20 75 
 
 19.25 
 
 17.03 
 
 14 66 
 
 
 Oct. 
 
 25.69 
 
 24.89 
 
 24.31 
 
 23.61 
 
 22.13 
 
 21.43 
 
 18.35 
 
 16.04 
 
 13 82 
 
 14 66 
 
 
 Nov. 
 
 26.49 
 
 25.79 
 
 25 11 
 
 22.80 
 
 21.33 
 
 18.82 
 
 17.54 
 
 16.84 
 
 13 82 
 
 14.66 
 
 
 Dec. 
 
 26.49 
 
 24.89 
 
 23 51 
 
 21.20 
 
 20.53 
 
 18 22 
 
 16.74 
 
 14.43 
 
 13 82 
 
 14 66 
 
 
 steam spray lines, and can operate with 
 a smaller coil surface area. Therefore, 
 capital investment is less based on charge 
 capacity. 
 
 For the theoretical cost analysis a figure 
 of $65,000 was assumed for capital invest- 
 ment in a 100 M bd ft capacity predrier. 
 Other assumptions were the following: the 
 steam consumption of a predrier is not 
 more than two thirds the steam consump- 
 tion of a conventional kiln and the main- 
 tenance charge is 2 per cent of the 
 installation cost. Charges for lumber 
 transfer system could vary considerably. 
 A figure of $0.10 per M bd ft was used in 
 the estimation in table 10. 
 
 The estimate above gives a predrying 
 cost of $0,575 per day per M bd ft. If 
 a 27-day schedule is employed, with 18 
 days of predrying and 9 days of kiln 
 drying, the total charge for the combina- 
 tion process, inclusive of transfer costs, 
 would amount to $17.67. Accepting the 
 
 accuracy of the above assumptions, it 
 would appear that the cost for the com- 
 bination process of predrying followed 
 by kiln drying is within the economical 
 range. 
 
 An exact cost analysis comparing the 
 different methods of drying can only be 
 carried out when it is based directly on 
 a case study of a particular operation. 
 When predrying is primarily employed, 
 the savings due to reduced or eliminated 
 checking and the value of marketing the 
 lumber closer to the saw may have to be 
 determined as well as possible losses due 
 to other defects not encountered in air 
 seasoning (losses due to endchecking 
 which developed during air seasoning 
 were found in one study (Brubaker. 
 1959) to range between $3.90 and $10.50 
 per M bd ft) . The possible need for an 
 increased dry storage area should also be 
 considered. 
 
 [23 
 
Table 10. ASSUMED PREDRYING COSTS USING A 100 M BD FT CAPACITY PREDRIER 
 
 Per year Per day 
 
 Indirect cost 
 
 Amortization of the predrier 
 
 (over 20 years at 6 per cent interest) $5,666.70 $15.52 
 
 Taxes 
 
 (6 per cent of 1/3 of the installation) 1,300 00 3.56 
 
 I nsurance 650 . 00 1 78 
 
 Maintenance 
 
 (2 per cent of the installation cost) 1,300.00 3.56 
 
 Interest on lumber 
 
 (6 per cent on $125/M bd ft 2 64 
 
 Direct costs 
 
 Electric power 3,528.00 9.67 
 
 Steam consumption 
 
 (2/3 of steam consumption in kiln drying) 5,013.00 13 73 
 
 Labor 2, 555 . 00 7 00 
 
 Total cost 
 
 Dollar/day/100 M bd ft 57.46 
 
 or 
 Dollar/day/M bd ft 0.58 
 
 [24] 
 
LITERATURE CITED 
 
 Anderson, A. B., E. L. Ellwood, E. Zavarin and R. W. Erickson 
 
 1960. Seasoning stain of redwood lumber. Forest Prod. J. 10(4) :212-18. 
 Anderson, A. B. and W. B. Fearing 
 
 1961. Distribution of extractives in solvent seasoned redwood lumber. Forest Prod. J. 11(5) : 
 240-42. 
 
 Anderson, A. B., W. B. Fearing, and C. R. Wilke 
 
 1962. Solvent drying of California redwood. Forest Prod. J. 12 ( 10) :493-96. 
 Anon. 
 
 1935. Air seasoning redwood. West Coast Lumberman 62(6) :8-9. 
 1945. Redwood. The Inst, of Paper Chem., Appleton, Wis. Res. Bui. 
 
 1955. Predriers pay, perform well. Timberman 57(2) :92, 94. 
 Balogh, B. and A. B. Anderson 
 
 1964. Chemistry of the Genus Sequoia II. The isolation of a new phenolic compound from the 
 coast redwood. J. Organ. Chem. (in press). 
 Benjamin, W. L. and G. R. Tobey 
 
 1961. Presteaming redwood and its effect on stain control and drying rate. Ann. Mtg. Western 
 Dry Kiln Clubs Proc. 
 
 Benjamin, W. L. 
 
 1962. Roof covers in redwood air drying yards; their advantages over loose cover boards. Ann. 
 Mtg. Western Dry Kiln Clubs Proc. 
 
 Brubaker, W. 
 
 1959. End checking prevention methods. Calif. Redwood Ass'n Res. Rept. 3.22412C. 
 Buchanan, M. A., H. F. Lewis, and E. F. Kurth 
 
 1944. Chemical nature of redwood tannin and phlobaphene. Ind. Eng. Chem. 36(9) :907-10. 
 California Redwood Association 
 
 1944. Redwood Seasoning Com. Minutes, San Francisco. 
 Clausen, V. H. 
 
 1953a. End checking prevention methods. Calif. Redwood Ass'n Res. Rept. 3.22412A. 
 
 19536. Air seasoning of California redwood. J. Forest Prod. Res. Soc. 3:183-88, also Calif. Red- 
 wood Ass'n Res. Rept. 3.2141 1A-F. 
 
 1953c. Predriers. Calif. Redwood Ass'n Res. Rept. 3.22100A. 
 Davis, S. 
 
 1943a. This much we know — we think. West Coast Lumberman 69(10) :44, 46. 
 
 19436. One theory with restrictions. West Coast Lumberman 70(5) :88. 
 Dost, W. A. 
 
 1956a. Air drying of California redwood. Calif. Redwood Ass'n. Res. Rept. 3.21411H. 
 
 19566. End checking prevention methods. Calif. Redwood Ass'n. Res. Rept. 3.22412B. 
 Ecklund, B. A., W. A. Dost, and W. L. Benjamin 
 
 1962. The effect of presteaming on stain control and drying rate of redwood. Ann. Mtg., West- 
 ern Dry Kiln Clubs. Also Calif. Redwood Ass'n Res. Rept. 3.2420. 
 Ellwood, E. L., A. B. Anderson, E. Zavarin, and R. W. Erickson 
 
 1960. The effect of drying conditions and certain pre-treatments on seasoning stain in California 
 redwood. Forest Sci. 6(4) :315-30. 
 
 Ellwood, E. L. and B. A. Ecklund 
 
 1961. Treatments to improve wood permeability, as an approach to the drying of redwood. Proc. 
 Western Dry Kiln Clubs 13th Ann. Mtg. 
 
 Ellwood, E. L. and R. W. Erickson 
 
 1962. Effect of presteaming on seasoning stain and drying rate of redwood. Forest Prod J. 
 12(7):328-32. 
 
 Green, C. B. 
 
 1925. The air seasoning of lumber in the California pine and redwood regions. Timberman 
 26(12) :49-52. 
 Johnson, P. 
 
 1956. Seasoning stain. Calif. Redwood Ass'n Res. Rept. 3.22421E,F. 
 
 1958. Time schedules for kiln drying heavy segregation redwood lumber. Calif. Redwood Ass'n 
 Res. Rept. 3.222120A. 
 Kollmann, F. and A. Schneider 
 
 1960. Der Einfluss der Beliiftungsgeschwindigkeit auf die Trocknung von Schnittholz mit 
 Heissluft-Dampf-Gemischen. Holz Roh-u. Werkstoff 18(3) :81-94. 
 
 [25] 
 
Lambert, G. M. and W. E. Pratt 
 
 1955. End checking preventatives. Western Dry Kiln Clubs Proc. 7 :50. 
 Lane, L. W., H. E. Menkens, A. F. Peterson and W. E. Pratt 
 
 1956. Quality control of moisture content in lumber drying. Proc. Western Dry Kiln Uubs 
 8th Ann. Mtg. 
 
 McMillan, J. M. 
 
 1958. Stresses in wood during drying. U. S. Forest Prod. Lab. Kept. 1652 (rev.) 
 Manson, B. C, and D. W. Clayton 
 
 1948. Experimental data. Calif. Redwood Ass'n, Redwood Seasoning Com., San Francisco. 
 Manson, B. C. and D. W. Clayton 
 
 1949. Drying of California redwood. Calif. Redwood Ass'n Res. Rept. 3.20000A. 
 
 1950. Quality control and improved seasoning processes of the redwood industry. Forest Pro. 
 Res. Soc. Proc. 4:29-35. 
 
 Peck, R. E., R. T. Griffith, and K. N. Rao 
 
 1952. Relative magnitudes of surface and internal resistance in drying. Ind. Eng. Chem. 
 44(3):664-69. 
 Pratt, W. E. 
 
 1951. Drying cost study. Calif. Redwood Ass'n Res. Rept. 3.20300A. 
 
 1953a. Some applications and statistical quality control to the drying of lumber. Forest Prod. 
 
 Res. Soc. 7th Ann. Mtg. 
 19536. Seasoning stain and bleeding study. Calif. Redwood Ass'n Res. Rept. 3.22421 C. 
 Rankin, J. H. 
 
 1929. New method evolved for kiln drying redwood on a large scale. West Coast Lumberman 
 58(11) :18-19. 
 Rasmussen, E. F. 
 
 1961. Dry kiln operator's manual. USDA Agric. Handbook 188. 
 Shearrard, E. C. and E. F. Kurth 
 
 1933. Distribution of extractives in redwood. Its relation to durability. Ind. Eng. Chem. 25(3) : 
 300-02. 
 Sherman, H. 
 
 1937. Yard dryers for redwood. Timberman 38(9) :51-52. 
 Stegeman, J. D. 
 
 1938. Kiln drying redwood. West Coast Lumberman 65(9) :38, 66. 
 
 1940. Density segregation. West Coast Lumberman 67(6) :30, 74. 
 
 1941. Some things to know about redwood. West Coast Lumberman 68(12) : 66-67. 
 1943. Drying lumber at Hammond Lumber Company. Timberman 44(9) : 17-18. 
 
 Tarason, J. 
 
 1957. Predrying of redwood 2x4 studs. Calif. Redwood Ass'n Proc. Redwood Seasoning Com. 
 Min. 114th Mtg. 
 
 USDA Forest Service 
 
 1958. Timber resources for America's future. Resource Rept. 14. 
 Utschig, I. G. 
 
 1935. Redwood kiln drying in California. Timberman 36(11) : 16-17, 19-20. 
 Zavarin, E. and L. V. Smith 
 
 1962. Processes underlying discoloration of redwood (Sequoia sempervirens) during drying and 
 in use. Holzforschung 16(1) : 11-17. 
 
 ACKNOWLEDGMENTS 
 
 The authors express their appreciation for the cooperation received from the Califor- 
 nia Redwood Association whose financial support made this study possible and all of 
 the individuals and companies concerned. Lumber was selected and supplied through 
 the courtesy of A. F. Peterson and D. Clayton of Georgia-Pacific Corporation. A 
 trial kiln run was conducted by L. Andreani and D. Lazzarini of Union Lumber 
 Company and the other test by C. E. Ashburn and H. Byker, and graded by K. 
 Covington, of The Pacific Lumber Company. Further lumber inspection was carried 
 out by R. Talvola, head grader, Redwood Inspection Service. Acknowledgment is 
 also made to E. F. McIntyre for laboratory assistance. 
 
 [26] 
 
In order that the information in our publications may be more intelligible it is sometimes necessary 
 to use trade names of products or equipment rather than complicated descriptive or chemical iden- 
 tifications. In so doing it is unavoidable in some cases that similar products which are on the 
 market under other trade names may not be cited. No endorsement of named products is intended 
 nor is criticism implied of similar products which are not mentioned. 
 
 7|m-7'64(E5831)J.F. C^K) 
 
 [27] 
 
KNOWLEDGE GAINED BY RESEARCH CAN HELP 
 CONSERVE CALIFORNIA'S WILDLAND RESOURCES 
 
 THE 
 
 CALIFORNIA WILDLANDS... 
 
 • 65 million acres of mountains, foothills, canyons, rivers, lakes, and sea coasts. 
 
 • a giant "farm" for timber and forage. 
 
 • a vital source of California's water supply. 
 
 • an "outdoor playground" for millions of vacationers. 
 
 THREAT: the onslaught of... 
 
 population growth. 
 
 urban and industrial expansion. 
 
 increasing demand for water, lumber, forage. 
 
 wildfires. 
 
 • insects and plant and animal diseases. 
 
 • waste. 
 
 THE SOLUTION: coordinated research on using wildland resources tr 
 realize their full potential . . . 
 
 present rate of timber growth could be doubled. 
 
 usefulness of timber cut could be doubled by new products made from current 
 
 waste. 
 
 forage production for livestock and game could be tripled. 
 
 watersheds could be made to yield more usable water and cause fewer floods. 
 
 tens of millions of dollars lost to fire, insects, diseases could be saved. 
 
 timber, forage, and recreation uses need not exclude each other. 
 
 THE WILDLAND RESEARCH CENTER at the University of California was established to help 
 conserve California wildland resources through research. It operates within the University's state-wide 
 Agricultural Experiment Station, with administrative headquarters on the Berkeley Campus. 
 
 THE CENTER... 
 
 • coordinates and supports research in more than a dozen fields. 
 
 • integrates studies of complex wildland problems. 
 
 • strengthens cooperation between University and other research workers. 
 
 • promotes the exchange of information between research workers and wildland managers and 
 policy makers. 
 
 • collects and disseminates scientific data on wildland studies. 
 
 TO KNOW IS TO LIVE IN ABUNDANCE