KILN SCHEDULE fCR BLACK WALNUT GUNSTCCK BLANKS Information Reviewed and Reaffirmed January 1951 No. LQ1433 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison 5, Wisconsin In Cooperation with the University of Wisconsin KILN SCHEDULE FOR BLACK WALNUT GUH STOCK BLANKS 2y H. H. SMITH, Assistant Technologist and 0. W. TQRGESON, Senior Engineer The rapid mobilization of a large army lias greatly increased the demand for dry black walnut gunstock "blanks. Holdover supplies of dried blanks are depleted and most gunetocks are now manufactured from material direct from the kiln, thus further taxing the already overloaded kiln capacity of the lumber- producing and wood-using industries. Poring World War I, black walnut gunstock blanks green from the saw were dried successfully in the water-spray type of kiln according to methods worked out at the Forest Products Laboratory, and with varying degrees of success in natural-circulation kilns. Since then the natural-circulation kiln has been used rather extensively for drying both green and air-dried gunstock material. Drying in such kilns, however, requires considerable time, often more than 3 months for fully green stock. Meanwhile, the efficiency of the lumber dry kiln has been improved by the application of mechanical refinements and better design. The modern type of dry kiln with forced circulation and automatic control of temperature and relative humidity has made possible considerable reduction in the drying time of gunstock blanks. To determine what further reduction in drying time might be obtained by the use of accelerated kiln schedules, and to study the feasibility of such accelerated schedules, the Fpi'est Products Laboratory during the past year or juore has undertaken a series of experimental kiln runs designed to study several factors of drying. Among these are (l) the effect and desirability of preliminary steaming; (<0 the efficiency of various end coatings in retarding end drying and thus in reducing end checking and end honeycombing; and (3) drying conditions, including initial conditions of temperature and relative humidity conducive to rapid drying without degrade, intermediate changes of temperature and relative humidity, final temperatures and relative humidities, and final conditioning treatment for relief of drying stresses. The results of these drying experiments indicate that faster drying is possible than has been customary in the past and that certain definite limits of drying conditions exist beyond which drying cannot be accomplished without prohibitive degrade. Report No, E1433 -1- rreliminary Stealing In the manufactvre of "black: walnut gunstocks, green flitches or bla: should not be sttaued before kiln drying. The steaming of green black walnut lumber to darken the sapwood is standard practice in industry, and users of "black walnut lumber prefer the steamed stock. Steaming of 4/4 stock does darken the sapwood materially, making a :.iore uniform color throughout tiie piece. The color change ir test at or near the surface, however, and gunstocks that are carved from thick blanks either steamed or unsteamed require the application of stain to the sapwood during the finishing opera- tion to obtain a sufficiently uniform color. Results of expt ri...ents indicate that preliminary steaming does not reduc, Pinal kiln dr tine. ,tched steamed and -nstenmed material indicated that 6teai„- for 3 days at 160°i 180°, and 200° F. under saturated conditions produced a significant decrease in toughness. The decrease ranged from about 6 percent for Bt earning at 160° ?. to 18 percent in the wood steamed at 200° . ■ indicating that toughness decreases with increases in the preliminary st- temperature. Preliminary steaming of green blanks also contributes to the develop- ment of end and surface checks that are often sufficiently severe to warr; rejection. f feet ire 2nd C oa ting P-egulred e need of a suitable end coating to retard end drying and resultant end checking, especially for a refractory species such as black walnut, has long been recognized. Considerable attention wan given to this problem when black walnul n tock blanks were dried during World War I. The end coatings of rosin and lampblack and mixtures of rosin and coal-tar rttch used at that time were effective within the temperature limits used then, but, when higher temperatures were considered, additional experimental work on end coating was necessary to determine the ability of various coatings to adhere readily, to withstand handling, and to maintain an effective moisture barrier during drying. The results of such experiments indicate that hot coatings are more desirable for green blanks than are coatings applied cold. Hot coatings OS be applied successfully by holding the end of the green blank against a power-driven roller partly submerged in a vat of molten end-co; . terlal. e roller of this equipment should revolve at approximately 40 revolutions per minute, and the temperature of the coating when applied should be between i and 4^0° ., depending upon tne particular costing used. Various coal- tar pitches i rosins, and asphalts were tried, separately and in different combinations, end-coating mixture was developed that has proved effective and has been used commercially. The material consists of: ; ort ho. P0.433 -2- 60 parts 213° F. coal-tar pitch 25 parts 155° F. coal-tar pitch 15 parts 210°-220° F. asphalt Mixtures of either rosin or high-melting-point asphalts and high- melting-point coal-tar pitches are also satisfactory, and some asphaltic enamels have shown promise. The addition of rosin has been advocated by some operators to increase the adhesiveness of pitch coatings. Ordinary roofing pitches and asphalts and 155° F. coal-tar pitch, used alone, are not satisfactory. An adequate end costing is necessary because of its ability to protect stock against degrade due to end checking. There is an upper temperature limit for every coating beyond which complete protection will not be afforded. Drying Conditions The initial conditions of temperature and relative humidity are gen- erally limited to those ranges which do not cause surface checking. Black walnut is not particularly susceptible to surface checking; thus the initial conditions are limited by the danger of end checking, which in turn is con- trolled by the effectiveness of the end coating. Initial Temperature Initial temperatures of 125° or 130° F. are believed to be safe. Initial temperatures above 120° F. , however, have been used only to a ver;y limited extent and cannot be recommended with assurance until experience indicates further that their use is desirable. Furthermore, in drying mate- rial green from the saw, such low initial temperatures are not particularly detrimental to total drying time as increases in temperatures, especially towards the end of the run, can be made as the drying progresses. Relative Humidity Initial relative humidity is not particularly critical. Early experi- ments designed to establish the minimum initial relative humidity that could be used without producing surface checking failed because no checking occurred even in initial relative humidities as low as 67 percent. Appar- ently, an initial relative humidity of 75 to 78 percent is entirely safe, and is, moreover, conducive to rapid drying. Initial humidities as low as 70 percent are believed to be safe, but it is conservative to keep above 70 percent because of the variations of drying conditions that are more or less common in any kiln. Report ITo. R1433 In several experimental kiln rone the relative humidity was lowered from the initial condition to 25 percent during the first 6 to 9 days without producing surface checks or any apparent end checks. This result further indicated the ability of blank walnut to withstand relatively severe ; it;: conditions provided it has adequate end protection. 2oth experimental and commercial d; ta Lndic ite that the Initial relative humidity can be gradually lowered to 30 or ercent durin, * £2 or 14 days of drying. Lowering ti:. relative huuidity by reducing the wet-bulb temperature on a time basis aids rapid drying without subjecting the material to conditions t iroduce drying degrade. Under this method, most or all of the relstiv humidity c >e are Dade before the temperature is increased. In 1 1 r mediate Temper a t ures Intermediate temperature changes should be based or. the moisture con- tent of ti.' wood. The use of high tempi raturos while the wood is above I fiber- ation point (ap; roximately 30 percent moisture content) is apt to ul't in serious degrade. IPhis danger was made apparent by excessive degrade that resulted when a commercial kiln los of green blanks was dried by a schedule wherein the desired decreases in relative humidity were obtained by maintaining a con- stant wet-bulb temperature and raising the dry-bulb temperature from 130° to 165° 7. before all the wetter stocks had dried to the fiber- saturation point. Past drying without degrade was obtained in a similar experimental sohed-ile employing even higher initial and final temperatures. The method is dangerous, however, and may cause excessive degrade when used for commercial drying because of the greater variability of drying characteristics encountered in wood obtained from various localities having a wide range of growth conditions. A temperature much above 130° F. is not recom.. ended before the wood has dried below the fiber-saturation point. The first temperature changes should be conservative, allowing for any slow-drying or particularly wet blanks that are not as dry as the kiln samples indicate. After the blanks have dried further, the temperature can be increased more rapidly. At the time of intermediate dry-bulb temperature changes, the wet-bulb temperature should also be changed to maintain the desired equilibrium moisture content (the moisture content that wood '-ill eventually attain at any given condi- tion of temperature and relative humidity). linal Temperatures ar.d Belative Humidities temperatures and relative humidities to be used in the kiln drying of black walnut gunstock blanks are governed largely by the effect of b temperatures on the strength of the wood and the effect of low final relative humilities on the final moisture content. The Laboratory considers ort No, 31*33 -4- 165° F. to be the maximum temperature deniable from a strength standpoint , and higher temperatures are not recommended, even though they do .ncrease the drying rate and tend to reduce the occurrence of wet cores. The use of 165° I. after the blanks have been dried to an average cross-sectional moisture cqntent of 15 percent is believed safe. A final relative humidity of 25 or 30 percent (equivalent to an equilibrium moisture content of 3.5 or 4.0 percent) is considered der.irable. Higher humidities will retard the final drying period, and lower relative humidities will produce excessive shrinkage and dryness* Final Conditioning A final conditioning treatment is necessary to relieve casehardening stresses that develop during the usual kiln drying procedure. Final condi- tioning distributes moisture more nearly uniformly within the wood. Com- binations of temperature, relative humidity, and time were tried experi- mentally to determine the best conditioning treatment for 2-1/2- inch blanks. The experiments indicated that drying stresses can be satisfactorily relieved at a relative humidity of 65 percent and a temperature of 165° F. for 24 hours. Th~y further showed that a final conditioning treatment at a given temperature and relative humidity will relieve the drying stresses only of blanks that are within a limited range of moisture content. As an example, the recommended conditioning temperature of 165° F. and relative humidity of 65 percent (equivalent to an equilibrium moisture content of about 8.5 percent) will not produce the desired stress relief in blanks having a moisture concent higher than approximately 8 percent, and will cause reverse casehardening in blanks having a very low moisture content The best relief of drying stress will be obtained if the blanks have been dried to a uniform moisture content throughout the kiln load. The degree of uniformity of moisture content increases as the blanks continue to dry. A kiln load dried below 8 percent moisture content will be more nearly uniformly conditioned than a similar kiln load having a higher moisture con- tent. A uniform moisture content of 6 to 7 percent in the dried blanks is desirable to prevent shrinking and end checking in cores exposed by end trimming to the dry air of heated shops during the winter months. Tests show that blanks having a core moisture content above 10 percent are apt to check in the coru area when exposed to dry shop conditions after being trimmed to length. Storage of the kiln-dried blanks before manufacture into gunstocks also helps to equalize residual drying stresses and moisture variations. Hep or t ITo. R1433 -5- Recommendations and Precautions For the most effective protection against end checking, green blarucB should to end coated immediately after sawing. An end coating should be used that will adhere to green end grain and will form a continuous moisture barrier capable of withstanding the abuse of repeated nendling of the blanks. She end coating should be able to withstand the temperatures encountered in tne lciln during dryi/ig. Recommendations regarding a schedule for drying black walnut gunstock blanks are subject to modifications in order to obtain the best drying in the many different types and kinds of lumber dry kilns. The schedule in table 1 is designed for a modern forced-circulation coi.partment kiln with automatic control of temperature and relative humidity. Table 1.— P roposed cor.mercinl kiln schedule for drying black walnut gunstock blanks Temperature scnedul< >.oi<:ture content at which c .--nge is to be made I^ry-bulb teooerature Percent •jr. Aoove 35 At 35 i At 30 At 25 At 20 , At 15 and to final 120 125 130 140 150 I65 Relative humidity schedule Duration of &ryir Relative ..idity Da-": 1st and 2nd. . . 3rd and 4th. . . 5th srid 6th . . 7 th and gtb . . 9th 10th 11th 12th to final. Percent 77 7U 70 65 60 50 40 30 or less In general, slightly lower initial - humilities can b» used in natural-circulation kilna. Lower initial relative humidities are de- sirable to prevent mold in the slcv drying areas of natural-circulation kilns. Because of the slower drying r^te, however, the drying time for each relative humidity step should be extennod, possibly an additional day. Report No. IU33 -6- In using this schedule the temperature is governed by the moisture content of the kiln samples. The relative humidities to be used with these temperatures are shown on a time basis. Thus, blanks in the kiln for six days may have dried from 70 to 50 percent moisture content and the temperature should then bo maintained at 120°F. until tnc blanks have dried to a moisture content of 35 percent. Tne relative humidity, however, -/ill have been lowered on the tima basis from 77 to 65 percent after 6 days of drying. This schedule is considered safe for drying sound, well end-coated and preferably unsteamed block v/alnut gunstock blanks. More severe drying conditions should not be used unless experience indicates the practicabil- ity of such a change. Higher temperatures increase the danger of degrade, especially 77'hen applied before the average cross-sectional moisture content has reached the fioer saturation point. Even after the moisture content of the samples iriicates a moisture' content beloT? J>0 percent, only moderate temperature changes are recommended because of occasional high moisture content cores cr ^ret spots. As the core fibers begin to dry and, in drying, become stronger, the . rperatures can be used with gree.ter safety. In goner--'.! , filial conoi ioning to eliminate easohrrdening should be carried on for ?-' hours at a i •; : .peraturo equal to the final dry-bulb tc.Tpera.ture and at a relative humidity squivalent to an equilibrium moisture content 2 percent higher than the avoraa-c cross-sectional uois- ture content of the material- lor example, ii the final temperature is I65 * 1 ' ana the blanks have dried to a moisture content of 6 to 7 percent, they should be ecu/lit cnec fer _ ;i hours at a ^eirperature of lo5°^« and a relative hujaidit~y ei 65 r rceni. The unrc nearly uniform the moisture content ; eonp individual blanks, the more nearlj uniform will be the re- lief of the c as ©hardening stresses. Report Ko< 1^33 -7- UNIVERSITY OF FLORIDA i'iimiiiii minimi urn mi ii 3 1262 08925 4311