VIBRARY OF THE UNIVERSITY OF ILLINOIS U(ob AGRICULTURE CIRCULATING CHECK FOR UNBOUND CIRCULATING COPY SOAKING TREATMENT OF BASSWOOD At Low Temperatures As affected by temperature and viscosity of solution, wood moisture, and type of treatment By C. S. WALTERS OCTM Bulletin 633 / UNIVERSITY OF ILLINOIS AGRICULTURAL EXPERIMENT STATION CONTENTS Development and Present Status of the Cold-Soaking Method. ... 5 Procedures 8 Experimental materials 8 Design of experiment 11 Equipment 13 Obtaining absorption and penetration data 15 Obtaining moisture-solvent interchange data 20 Results and Discussion 23 Wood moisture-solvent interchange 24 Absorption of solution 30 Distribution of pentachlorophenol 40 Summary and Conclusions 48 Literature Cited . . .49 Urbana, Illinois August, 1958 Publications in the Bulletin series report the results of investigations made or sponsored by the Experiment Station ACKNOWLEDGMENTS A number of companies made contributions to the study reported here. The author wishes to thank the R. T. Vanderbilt Company, Inc., for the end-coating material used in kiln-drying the basswood lumber ; Shell Chemical Company and C. F. Brice, an employee of the company, for the solvents and solvent data; the Koppers Com- pany, Inc., for the adhesive used for end-coating the test blocks ; Delmhorst Instrument Company for the use of a Model RC-1 mois- ture detector; Monsanto Chemical Company for the purified penta- chlorophenol ; Casswood Industries, Inc., and T. A. Foley Lumber Company for the basswood lumber ; and Edward Hines Lumber Com- pany for the ponderosa pine used in pilot tests. The author expresses appreciation to his University of Illinois colleagues for assistance with various phases of the work: J. N. Spaeth for his counsel during the course of the study and for his review of the manuscript; H. W. Norton for suggestions regarding the statistical analyses and for his review of the manuscript; and K. R. Peterson for his help in the laboratory. Special acknowledgments are given George A. Garratt, Dean, F. F. Wangaard, and G. W. Furnival of the Yale University School of Forestry and Henry W. Hicock of the Connecticut Agricultural Experiment Station for their review of the manuscript and sugges- tions for improvement of the report. Appreciation also is expressed for the suggestions made by R. H. Baechler and Harold Tarkow, U. S. Forest Products Laboratory, for their helpful comments on the investigation, particularly those re- garding the solvent-moisture interchange phase of the study. This investigation was submitted to the faculty of the School of Forestry, Yale University, as partial fulfillment of the requirements for the degree of Doctor of Forestry. Soaking Treatment of Basswood At Low Temperatures By C. S. WALTERS, Professor of Forestry DEVELOPMENT OF THE "CLEAN TREATMENT" PROCESS, in which non- swelling, paintable (NSP) solutions of oil-soluble toxicants are com- monly used, has helped to encourage wide use of the dipping and soaking methods of treating wood with preservative chemicals. These two methods of treatment are often used for farm, home, and industrial applications where pressure methods are neither practicable nor eco- nomically possible. Current recommendations for the use of NSP solutions emphasize the desirability of treating only wood seasoned to moisture contents below 20 percent in solutions above 65 F. These recommendations are based on the results of a number of investigations, many of which in- volved pressure treatments. None of the investigations known to the author, however, were designed to study the interacting effects of the cold-soak treating variables under controlled conditions. The scope of this investigation has been limited to the study of the cold-soak method and variables other than anatomical structure that are believed to have the greatest effect on solution absorption and penetra- tion: solution temperature, wood moisture content at time of treatment, and type of treatment (intermittent or continuous). Although treating time is also an important variable (4) 1 it remained constant in this study. Development and Present Status of the Cold-Soaking Method A difference between soaking and dipping is commonly recognized. Dipping consists of submerging the wood in the preservative for a short time usually less than 15 minutes and then removing the product. Cold-soaking (hereafter referred to as soaking) consists of submerging the wood in a solution of an oil-soluble preservative for periods ranging up to several days at temperatures ranging from 65 to 80 F. (If the preservative is an aqueous solution, the process is called "steeping.") Immersing wood in chemicals to improve its service life undoubtedly is one of the oldest methods of preservative treatment. Hunt and Gar- 1 Numbers in parentheses refer to literature citations on page 49. 6 BULLETIN No. 633 [August, ratt (18) mentioned Homberg's work with mercuric chloride in Europe in 1705. The soaking (or steeping) method probably was in use much earlier than the beginning of the 18th century, and has probably changed little since it was first used. Earlier publications (17, 37, 38) recognized steeping, but the first recommendations for soaking did not appear in the literature until the late 1930's. Prior to the 1930's, the only commonly used oil preservative was creosote, including blends of it with petroleum and tar. The rela- tively high viscosity of these oils at room temperature tended to dis- courage interest in cold-soak treatments. Hubert (15, 16 1938) was probably the first to suggest the possibilities of applying pentachloro- phenol in "light" oils by soaking and dipping. In 1939 Carswell and Hatfield (7) reported on the merits of penta- chlorophenol as a wood preservative, but they stressed the dip an( pressure treatments and failed to mention the possibilities of soaking. Soaking was not used extensively until about 1940, when pentachlorc phenol appeared on the market. Since 1940 a number of reports have been published on dipping or soaking wood (14, 15, 21, 26, 35, 40). Many of these investigators have studied the relation between wooc moisture content and the absorption and penetration of preservative solutions. Wohletz and Ravenscroft (40) reported that cold-soak treating results were best when the wood had a moisture content of 15 to 20 percent. They cautioned against treating fence posts at below- freezing temperatures, however, because they believed ice crystals that formed in the frozen wood stopped the treatment. Moore and Ray (26) concluded that the length and character of the seasoning period for fence posts were more important in obtaining satisfactory results than the moisture content of the wood at time of treatment. They found that lodgepole pine posts seasoned below th< fiber-saturation point (about 30 percent moisture) were not satisfac- torily treated unless they had been seasoned for nine weeks or more. Hicock and his colleagues (14) found that the best treating results were secured by thoroughly seasoning the wood and treating only dur- ing warm weather. Marshall (21) concluded that the highly satisfactory results he obtained in his study of cold-soaking loblolly and shortleaf pine fence posts were due mainly to the method of seasoning. The retentions and penetrations obtained by this investigator decreased as seasoning time increased from 4 to 12 weeks and then increased as the time was in- creased to 16 weeks and longer. From these results, Marshall concluded 1958] SOAKING TREATMENT OF BASSWOOD 7 that glaze 1 or some other unknown factor inhibited treatment. Other investigators have also suggested the formation of glaze and surface hardening during seasoning as causes for unsatisfactory absorption and penetration of treating solution. Most of the literature concerning the soaking treatment describes the use of warm preservative solutions. Some of the early recommen- dations on use of the soaking method were not supported by research, but they undoubtedly were based on pressure-treating recommendations (25). Bateman (2) concluded, after his pressure-treating study, that the viscosity of the treating oil was definitely related to the depth of penetration, and that the change in temperature (warming) of the oil had no other effect than to change the oil's viscosity. He observed that depth of preservative penetration increased as a result of reduced viscosity. MacLean (23), in his study of the effects of intermediate temperatures (160-220 F) and viscosity of wood preservatives on penetration and absorption, found that during pressure treatment the viscosity of the preservative apparently had the most important single effect on penetration and absorption at any temperature, although the action of heat on the wood and the kind of oil used were also important. A year later (1927) MacLean (24) observed that the viscosity of creo- sote decreased with an increase in temperature, and the depth of pene- tration and absorption of the solution increased. Buckman ct al. (6) reported similar results for the soaking method. The only report known to the author of tests in which wood was treated with cold solutions was his study on winter treatment of white- pine fence posts published in 1948 (36). The results of treatment with solutions fluctuating in temperature between 14 and 26 F. were com- parable to those obtained in a "warm" room (perhaps 75 to 80 F.). No attempt was made in this study, however, to control such important variables as wood moisture content or solution temperatures, to test solution temperatures below 14 F., or to study the interacting effects of solution temperature and wood moisture content. There is a dearth of information on soaking wood in solutions below 65 F. for a number of reasons; among the most important are: 1. The poor absorption of solution and shallow depth of penetration usually obtained by soaking in comparison with pressure treatment. 2. The precipitation of some solutes when the solutions are cooled below a certain point. 1 Glaze was not identified, but it probably was the result of an accumulation of resins and other exudates on the surface during the early part of the season- ing period. 8 BULLETIN No. 633 [August, 3. The relatively poor protection the cold-soak method often gives wood that is to serve in contact with soil. In spite of these alleged shortcomings, however, there is a need for basic information on the cold-soak treatment, because the method has several desirable features. It is simple, economical, and practical. It requires only simple, inexpensive equipment, which is readily available. The sapwood of a few species can be treated as completely by soaking as by pressure methods. For these reasons a comprehensive investiga- tion of the effects of solution temperature, wood moisture content, and type of treatment was initiated in order to determine the limitations of the method. Procedures EXPERIMENTAL MATERIALS Pilot tests were made with ponderosa pine, but this species was abandoned because the treating solutions removed enough extractives to affect absorption weights. Basswood was chosen for the tests because a supply of sapwood was readily available; it has relatively low oil- soluble extractive content and a uniform texture; and it can be sea- soned free of defects. The test blocks for this study were obtained from one 10- foot bass- wood log about 17 inches in diameter. The log was selected from a number of freshly cut logs at a sawmill in Cass county, Illinois, for its good quality, uniform rate of growth, thick sapwood, and freedom from visible defects. Boards 1*4 inches thick and 6 to 9 inches wide were removed "around the log" so that most of them were practically flat sawn and contained only sapwood. All boards were identified in such a way that their original position in the log could be determined. The green boards were end-coated at the mill with a commercial end-coating material and wrapped to prevent loss of moisture during transit. In the laboratory the first eight boards removed from the outside of the log were judged to be free of heartwood and were then crosscut at mid-length. At the same time sections about 1 inch along the grain were removed from the center of each of three boards for moisture and specific gravity analyses. The 16 short boards were identified by num- ber, weighed to the nearest 0.01 pound, end-coated with a commercial end-coating product, and stacked in a kiln for seasoning. The three moisture-content samples were dried to constant weight at 221 F., and their moisture contents were calculated as a percent of oven-dry weight. The average moisture content, 97 percent, was used to calculate the oven-dry weight of each board. 1958} SOAKING TREATMENT OF BASSWOOD 9 The moisture-content samples also were used to calculate specific gravity. The average specific gravity of the three samples, based on oven-dry volume and oven-dry weight, was 0.48. The "predicted conditioned weight" of each board for the particular level of wood moisture content at which treatment was to be made, namely, 7, 12, 19, or 26 percent, was determined from the calculated oven-dry weight, and the boards were weighed periodically until they reached these weights. The predicted conditioned weight (PCW) in pounds was calculated as follows: PCW = COD + (COPHPMC) 1UU where COD was the calculated oven-dry weight in pounds and DMC was the desired moisture content in percent. The procedure was sim- ilar to that of using kiln sample boards to control the operation of a commercial dry kiln (11). The kiln schedule that was used (Table 1) was milder than the schedule recommended by the U. S. Forest Prod- ucts Laboratory (34) for commercially seasoning basswood. Table 1. Kiln Schedule Used for Drying Basswood Lumber for Test Blocks Moisture content change (percent) Dry-bulb temperature (F.) Wet-bulb depression (F.) From To 97 65 35 65 35 5 130 130 130 15 30 35 As the boards attained their predicted conditioned weights for each level of moisture content, they were removed from the kiln and sur- faced on both sides to a thickness of 1 inch. Thickness was checked with a micrometer to the nearest 0.001 inch. The boards were then ripped into sticks 1}4 inches wide. The sticks were dressed until they were 1X1 inch in cross-section. The cross-sectional dimensions were checked with a micrometer to the nearest 0.001 inch. Blocks. Four hundred eighty blocks 1 X 1 inch in cross-section and 6 inches long were cut, about 120 blocks each from boards of the same nominal moisture content. The blocks were cut to length with a planer saw. Variation in block length was controlled with a bar guide on the saw and a comparator gage measuring to the nearest 0.001 inch. Each block was inspected for freedom of defects, straightness of grain, 10 BULLETIN No. 633 [August, and smoothness of surface, and 400 were finally selected for treatment (100 of each nominal moisture content). Twelve of the rejected blocks (3 of each nominal moisture content) were subsequently used as mois- ture "check blocks" in the humidity chambers. Since longitudinal pene- tration (along the grain) is greater as much as 75 times for some species (4) than penetration across the grain, the ends of each block were given two coats of a phenolic resin adhesive, applied 24 hours apart, to prevent end penetration by the treating solutions. Test wafers. Two types of test wafers 1 X 1 X y\ inch along the grain were used in the investigation: a penetration wafer for measuring toxicant content at various depths in the test blocks and a moisture-solvent-interchange wafer. The preparation and use of these wafers are described on pages 17 and 20. Toxicant. Triple recrystallized pentachlorophenol was used to prepare the treating solutions. The purity of the chemical (99.9 per- cent) was at least the equivalent of an analytical reagent grade. Solvents. Diacetone alcohol (4-hydroxy-4-methyl-2-pentanone) and hexylene glycol (2-methyl-2,4-pentanediol) were chosen from a number of organic compounds that had the properties set as standards for the solvents. Both of these solvents had low vapor pressures and: high solubility for pentachlorophenol at temperatures as low as 5 F. Because they had different viscosities, the diacetone alcohol was classi- fied as a "light" solvent and the hexylene glycol as a "heavy" solvent. The properties and characteristics of the two compounds are shown in Table 2. The solvents were not reagent grade, although they met rigid man- ufacturing specifications. They were essentially anhydrous when man- ufactured, but they could have contained as much as 0.2 percent 1 of water by weight by the time they were used. Treating solutions. The pentachlorophenol w r as dissolved in each of the solvents at the rate of 5 gm. per 100 cc. of solvent. Ordi- narily pentachlorophenol solutions contain toxicant at a concentration of 5 percent of the solution weight; however, in this investigation dif- ferences in solution absorption and penetration were used to show the effects of treatment. Thus it was important that each unit volume of solution contain the same amount of toxicant regardless of the specific gravity of the solvent. Both solutions were analyzed for toxicant by the lime-ignition method (8) and the concentration of pentachlorophenol 1 Information contained in letter to author from C. F. Brice, Shell Chemics Corporation, dated November 3, 1955. 1958] SOAKING TREATMENT OF BASSWOOD 11 Table 2. Properties of Test Solvents for Pentachlorophenol vv Hexyleneglycol Property l 2 (2-methyl-2, 4- " pentanediol) Molecular weight ............................. 116.16 118.17 Specific gravity, 20/4 C ..................... .9382 .9216 Boiling point, C. at 760 mm. Hg .............. 169.1 198.3 Freezing point, C ........................... 44 . . . a Flash point, F .............................. 144 b 210 C Vapor pressure, mm. Hg. at 20 C .............. .81 .05 Viscosity, centistokes Cold condition ............................. 15.3 d 700. O e Warm condition ............................... f 32 Weight, Ib. per gal. at 20 C ................... 7.83 7.69 Solubility for pentachlorophenol at 5 F., percent by weight ................................. 31 32 Type of solvent .............................. Light Heavy a Becomes semi-solid at 40 C. without crystal formation. b Tag. Open Cup. c Cleveland Open Cup. d -15 C. (5 F.). e At approximately 10 F. f Data not available. f At approximately 80 F. balanced until Student's "t" test showed no significant differences be- tween the mean concentrations of the aliquot samples. Reagents. All reagents used in analyzing the wood samples for pentachlorophenol were analytical reagent grade and met the specifica- tions adopted by the American Wood- Preservers' Association in their Standard Method A5-54 (8). DESIGN OF EXPERIMENT The design of this experiment was 5X4X2X2 factorial relat- ing the effect of solution temperature, wood moisture content, solvent viscosity, and type of soaking treatment to preservative absorption and penetration. The 80 five-block groups were randomly assigned to treat- ments. Four groups formed a run. Each run was treated at one of the five levels of solution temperature, and all tests blocks in the run were of the same moisture content. No account was taken in the statistical analyses of the deviation of block moisture contents from the nominal values, although measured values were used in plotting the graphs. The results will be discussed in three sections. One section concerns wood moisture-solvent interchange. Another section concerns the ef- fects of the various treating variables on the absorption of preservative solution by the test blocks, and the final section concerns the effects of the variables on the concentration of toxicant at three mid-point depths in the blocks. Constant-humidity chambers were used to condition test blocks to con trolled moisture contents. Each chamber held about 125 blocks. The parts are: (1) floating bearing block, which supported stirring-rod shaft; (2) con- tainer for saturated salt solution, which also supported expanded meta shelf on which blocks were stacked for conditioning; (3) stirring rod (note propeller blade, which circulated air in chamber) ; (4) test blocks stackec for seasoning (note small stickers separating each course of blocks); anc (5) chamber lid (note two observation ports, which were sealed with clear plastic sheeting, and rod port, which was covered with sheet rubber gasket). Motor that powered stirring rod is not shown. (Fig. 1) Table 3. Environment and Saturated Salt Solutions Used to Obtain Controlled Moisture Contents of Basswood Test Blocks Wood moisture content (percent) Environment used to obtain moisture content Saturated salt solution Temperature (F.) Relative humidity (percent) Nominal Actual 5 12 19 26 7 13 20 28 75 75 75 41 20 66 88 95 KC-oHsC^ NaNO 2 KsCrO4 ZnSO 4 . 7H 2 O 1958] SOAKING TREATMENT OF BASSWOOU 13 EQUIPMENT Constant-humidity chambers. The blocks were stored in closed constant-humidity chambers (Fig. 1) over saturated salt solutions to obtain four levels of nominal moisture content: 5, 12, 19, and 26 per- cent. Table 3 shows the approximate environment and the salt solutions used to obtain the controlled moisture contents. The solutions of chem- ically pure salts contained an excess of solute so that a constant water- vapor pressure (relative humidity) would be maintained above their surfaces at a reasonably constant temperature. The chamber containing zinc sulfate was stored in a refrigerator at 41 F. to prevent mold and stain fungi from developing on the blocks during conditioning. An electrically driven fan constantly circulated the air in each chamber, and a stirring device circulated the saturated salt solutions. Constant-temperature chamber. Figs. 2 and 3 show the constant- Constant-temperature chamber for treating test blocks. Electric motor shown in lower-left corner powered the intermittent dipping system. To the right of the motor is the speed reducer, and immediately behind it is the counting device that tallied the number of dips given blocks in the treating baskets. The potentiometer on the small shelf in the background was used to measure solution temperature in the treating tanks. Both heating and cooling units were operated through the relay system shown on the instrument panel. (Fig. 2) 14 BULLETIN No. 633 [August, temperature chamber used to control solution temperatures during treatment. A food freezer was modified and equipped with a water-bath tank and temperature-regulating devices so that the blocks could be treated at various levels of solution temperature between 5 and 80 F. Adding a commercial antifreeze solution to the water prevented it from freezing, and an electric motor-driven stirrer constantly circulated the water. Four 4X5 inch sheet-steel treating tanks about 12 inches deep were set 9 inches deep in the water bath. The temperature of the water bath was controlled by a mercury thermoregulator that operated a 250-watt strip heater and the food Treating tanks. Shown from front to rear: (1) two plastic pitman arms that activated the dipping baskets by chains; (2) to the right of the mer- cury thermometer, the electric stirrer that circulated the water-bath solu- tion; (3) a 250-watt strip heater to the rear of the thermometer; (4) the two tanks in which blocks were continuously treated these had no wire baskets; (5) a single-tube mercury thermoregulator between the treating tanks; (6) the small vertical tube in one corner of each treating tank which served as a holder for a copper-constantan thermocouple. (Fig. 3) 1958] SOAKING TREATMENT OF BASSWOOD 15 freezer's compressor through a sensitive electronic relay. The thermo- regulator was sensitive to 0.1 F. temperature change. Copper-constantan thermocouples were located in each treating tank; however, only one thermocouple was used during treatment be- cause potentiometer readings made during the pilot tests showed that the range in solution temperatures was less than 0.1 F. A 75:1 speed reducer and a pulley-belt arrangement reduced the speed of a 1/3 HP electric motor to about 7 rpm. Fig. 3 shows the system of mechanical levers, eccentric, shafting, and pulleys that dipped two baskets into tanks of treating solutions at the rate of about seven times a minute, or 5,100 times in 12 hours, the length of all treatments. A counter automatically measured the number of dips given the blocks during intermittent treatment. OBTAINING ABSORPTION AND PENETRATION DATA It was difficult to obtain satisfactory moisture conditions in the 26-percent-moisture group. Fungal growth developed on one set of test blocks in a humidity chamber at room temperature (75 F.), and a substitute group of blocks was prepared from material that had been restored to green condition in distilled water after seasoning to about 24-percent moisture. The reconditioned blocks, containing about 45- percent moisture, were then stored in a humidity chamber at 41 F. until they were removed for treatment at 28-percent moisture. Blocks were stacked in spaced layers with 14-inch-square stickers separating each course (Fig. 1). The arrangement of blocks in the chamber permitted air to circulate freely among them. As a result of the humidity maintained in each chamber, the blocks attained an equilib- rium moisture content in two to six weeks. Periodic checks were made with an electrical resistance-type moisture meter on three "check blocks," which were placed in each chamber along with the test blocks, and on the samples cut from the check blocks and oven-dried to con- stant weight at 221 F. The test blocks were treated as soon as the measurements showed that they had reached the proper moisture content. Twenty test blocks were removed from a humidity chamber and stored in a polyethylene bag except for a short time when each block was being numbered, measured, and weighed until they were placed in the treating tanks at room temperature. 1 1 No tests were made to check the rate at which temperature of the blocks reached equilibrium with temperature of the solution; however, Boiler (3) re- ported that blocks of Douglas fir of similar size were cooled from room tem- perature to 300 F. in about 12 minutes. 16 BULLETIN No. 633 [August, A jig (Fig. 4) was used to mark the location of the penetration wafer and the point at which the test block was numbered. Thus the number remained on the wafer when it was sawn from the block fol- lowing treatment. Ten blocks were measured before and after they had been end- coated with adhesive, and the average thickness of the end-coatings was found to be 0.048 inch. This amount was subtracted from the gross length of each block before its cubic content was calculated. Jig for marking location of penetration wafers on test blocks. After block was marked, it was num- bered; this number also identified the penetration wafer. Block at left has been crosscut to remove a wafer V4 inch along the grain. (Fig. 4) Dimensional measurements were made on one block selected at random from the group stored in the 26-percent conditioning chamber to determine whether the conditioned blocks were below the fiber- saturation point. The three structural dimensions of the block were measured to the nearest 0.001 inch, the block was returned to green condition in distilled water by a vacuum treatment, and the dimensions were remeasured. The dimensions of the block increased by the follow ing percentages: tangential, 4; radial, 2; and longitudinal, less than 0.01 Although measurements were limited to one block, the swelling indi cated that the blocks represented by the random sample were treated a slightly below the fiber-saturation point. Each block was weighed to the nearest 0.01 gm. before and after it was treated. Each group of five blocks was placed in a wire basket for treatment. s : 1958] SOAKING TREATMENT OF BASSWOOD 17 Two five-block groups received continuous treatment for 12 hours, 1 one in the diacetone alcohol solution of pentachlorophenol and the other in the hexylene glycol solution. Two other groups were dipped 5,100 times in the 12-hour treating period, one group in each of the two solu- tions. Although all blocks were treated 12 hours, those treated by inter- mittent dipping were not constantly submerged in the treating solution during this period. In order to determine the percent of time the blocks were actually submerged in the solution, the length of time the blocks were completely submerged was measured with a stopwatch during a 7-minute sample treating period. Submerged time was calculated as a percent of total time. It was assumed that the proportion of the time the blocks were submerged during the sample period was the same as that for the 12-hour treating period. The treated blocks were wiped dry before they were reweighed. However, although the blocks were weighed promptly following treat- ment, some bleeding 2 was observed. Therefore, each block was wiped free of solution once again before it was crosscut at mid-point, and a wafer 14 mcn along the grain removed to analyze penetration. Each wafer was cut into three parts (Fig. 5): Shell A, the outer l/6-inch layer; Shell B, the second deepest layer, also i/g inch thick; and the core, the remaining innermost portion of the wafer. The wafers were cut into shells and cores within one hour after the blocks were removed from the treating tanks to minimize "creep" of the solution following treatment. The penetration analysis was simplified by grouping shells and cores. For example, all of the A shells cut from each group of five similarly treated blocks were ground together in a burr mill. The B shells and cores were similarly homogenized. Thus the 15 depth-of -penetration samples cut from each group of five blocks were reduced to three "average" samples. The grinding process reduced the samples to the fineness of sawdust, 97 percent of which passed a No. 10 screen (100 openings per square inch). About 53 percent of the sample was re- tained by a No. 20 screen; 32 percent was retained by a No. 40 screen; and 12 percent passed a No. 40 screen. The burr mill was thoroughly cleaned with ethyl alcohol between grindings. 1 Pilot tests showed that the treating solutions penetrated the blocks a "maximum allowable depth" in about 12 hours. The maximum allowable depth of penetration permitted treatment of a block's cross-section to a point just short of complete saturation ; thus the core received little, if any, treatment. Soaking periods longer than 12 hours often resulted in complete saturation of a few blocks; complete saturation obscured the effects of treatment. 2 Bleeding is the exudation of solution on the surface of the wood. 18 BULLETIN No. 633 [August, Jig for cutting shells and cores from penetration wafers. Wafer in jig shows location of three samples analyzed for toxicant content. At left is the group of shells and cores (three samples from each of five blocks) for one of the 80 treatments. The pile of "sawdust" shows how each group of shells or cores was ground to a single, homogenized sample by a burr mill. (Fig. 5) A sample of ground wood from each shell and core of each group was analyzed by the lime-ignition method adopted by the American Wood-Preservers' Association (8) as the standard method for calcu- lating the percent of pentachlorophenol in wood. The percent of penta- chlorophenol was determined on a weight basis for two reasons: (1) The moisture content of the ground wood was considered a con- stant following four months' storage, and (2) it was more accurate to weigh the sample than to determine its volume. Fig. 6 shows the mark-sense card on which thickness, width, length, and untreated and treated weights were recorded. The concentrations of pentachlorophenol found in Shells A and B and the core were re- corded on the back of the card. With minor exceptions, all data were organized and computed by business machines from the measurements coded on the mark-sense card. Cubic-inch volumes were converted to cubic centimeters by a conversion factor. Where absorption of solu- 1958] SOAKING TREATMENT OF BASSWOOD 19 tion has been shown in pounds per cubic foot, the values were obtained by multiplying metric data (gm./cc.) by 62.43. One 5-cc. and one-10 cc. sample of each treating solution were dried to constant weight at 80 to 85 F. three and a half months after the treatments were completed, and the amount of extractives removed from the basswood was calculated as a percentage of weight of solvent and extractive materials. 1 ? ^ * 5 I R 9 Q I XI n 12 13 I !4 15 16 17 I w 19 20 i I / 23 24 t% I Z 27 i i i i i i i i IP uun n?i no nBn)in CD !i)4i<'u .. ufcuKmui cl<.; M linn nlr M 9 9 '9 >~9-*:9'3c9? c 9 *"9 > 9 9 9 9 ;~9:>c9Dc9 jr.9_v:9-tgDc9bc9^c9-y_9:x:9:Dc93c9D l li4Tiii>jtiHalii) M r i?n *ii nMttx jt gu M a* IT ! *i u OM > M ttL u v HMU waliiftf;Mii nnMnk'n m*m Mark-sense cards used to record size, weight, and toxicant-concentration data. Identification data in the first 11 columns were interpreted near the top edge of the card for ease in reading. For example, the top card shows size and weight data for Block 474. Reading from left to right, block was in treating run 01, group 2, and was treated in solution having a tempera- ture of 05.0 F. at 26.0-percent moisture content. The solution was diacetone alcohol (coded "1") and the intermittent (coded "1") soaking method was used. The remaining numbers are the block numbers. The mark-sense codings were subsequently punched into eacli card. The bot- tom card is from a second deck, modified to record penetration data. The percent of pentachlorophenol found by laboratory analysis (1.575) was recorded in mark-sense position 6 to 9 with a special graphite pencil. (Fig. 6) 20 BULLETIN No. 633 [August, OBTAINING MOISTURE-SOLVENT INTERCHANGE DATA A test was made to determine whether there was an interchange of solvent and the moisture in the wood during treatment. This test was divided into two phases. The objective of the first phase was to determine whether the two solvents, diacetone alcohol and hexylene glycol, penetrated the cell walls during treatment. Hawley (12), DeBruyne (9), Stamm (29), Nayer (27), and a number of other investigators have shown that the wood swells if water or other liquids penetrate the capillary structure of the cell wall. A number of theories have been given to explain why the wood swells (30), but all that is of interest here is that swelling or dimensional changes do occur, and these changes in dimensions indicate cell-wall penetration. The objective of the second phase of the test was to determine whether the solvents absorbed moisture from the wood. A third possibility also existed, namely, that water absorbed from the wood would be replaced by solvent. Fifty-two wafers 1X1 inch in cross-section and }4 mcn along the grain were randomly selected from a group of 60 cut from two green basswood 1 squares about 15 inches long. The squares were ripped from a single board and dressed to size before 30 wafers were cut from each of them. The wafers were consecutively cut from the squares with a smooth-cutting planer saw; the i/^-inch longitudinal dimension of each wafer was controlled by a spacing jig on the table saw. The annual rings in all wafers were practically parallel to the tangential plane of growth. The wafers were numbered consecutively and assigned randomly to treatments. Thirteen wafers were exposed in each of four humidity chambers over saturated salt solutions (Fig. 7). The radial and tangential dimen- sions of each wafer were measured periodically to the nearest 0.001 inch (Fig. 8) until they reached a constant size after 93 hours of ex- posure. The salt solutions in the humidity chambers were selected to provide four levels of controlled equilibrium moisture content, namely, 5, 12, 19, and 26 percent (Table 4). At the end of the moisture-conditioning period, five wafers having a common moisture content were measured radially and tangentially and submerged in an 8-ounce, wide-mouthed jar containing one of the 1 Average moisture content of three samples of the freshly sawn basswood was 95 percent, based on oven-dry weight. Average specific gravity of the three wood samples was 0.37, based on oven-dry weight and oven-dry volume. 1958} SOAKING TREATMENT OF BASSWOOD 21 Constant-humidity chambers for conditioning test wafers in moisture- solvent interchange study. Bottom well contained a saturated salt solution that was stirred constantly by motor-driven stirring rods. Eacli stirring- rod shaft also turned a propeller blade that circulated the air inside the chamber. Jars of solvent are shown (front chamber on right) in the chambers; however, tests showed that it was impractical and unnecessary to condition the solvents, and the practice was discontinued. Test wafers were conditioned in a basket of hardware cloth supported on a hardware- cloth standard to prevent possible contamination by the salt solution. (Fig. 7) solvents. One 5-wafer group of each nominal moisture content was treated in each of the two solvents. The three untreated wafers remaining in each humidity chamber were weighed, dried to constant weight at 221 F., reweighed, and their average moisture content calculated as a percent of their oven-dry weight. The treated wafers were assumed to have the same moisture content as those analyzed for moisture content. 22 BULLETIN No. 633 [August, Table 4. Environment and Saturated Salt Solutions Used to Obtain Controlled Moisture Contents of Basswood Test Wafers Wood moisture content (percent) Environment used to obtain moisture content Saturated salt solution Tenjperature Sd (percent) Nominal Actual 5 12 19 26 7 10 16 26 70 70 70 43 32 52 81 95 CaCl 2 . 2H 2 O Na 2 Cr 2 O 7 (NH 4 ) 2 S0 4 ZnSO 4 . 7H 2 O The jars containing the solvents and blocks were quickly capped and stored for 12 hours in a water bath at 80 F. At the same time that the wafers were placed in the solvents for treatment, a sample of each solvent was similarly stored for subsequent moisture analyses. All jars were equipped with paper and aluminum-foil cap-liners to prevent the solvents from gaining or losing moisture to the atmosphere. At the end of the treating period the wafers were removed from the jars, wiped dry, and remeasured. The moisture content of each of the This comparator gage was used to measure basswood wafers. Each wafer was indexed for measurement by moving it to the rear and to the left side of the slot. To calibrate the gage with a 1-inch standard end meas- uring rod, a plastic filler strip was inserted in the slot to make gage readings fall between 1 and 2 inches. The thickness of the filler strip, 0.244 inch, was later subtracted from each meas- urement. (Fig. 8) 1958] SOAKING TREATMENT OF BASSWOOD 23 10 samples of solvents was determined by the Karl Fischer method (1), using commercially prepared reagent and alcohol-water solutions. The end-point of each titration was determined electrometrically. Du- plicate tests were made, but the analyses showed that the second meas- urements were consistently higher than the first. Since exposure of the solvents to the atmosphere apparently had some effect on the results, the original measurements were used and the duplicate tests abandoned. Results and Discussion No difficulty was experienced in bringing the blocks in the 5-, 12-,, and 19-percent moisture groups to a uniform moisture content at room temperature, although actual values exceeded nominal percentages by not more than 2 percentage points. The 26-percent group, however, required a temperature lower than those prevailing in the laboratory to prevent the development of fungi during the final conditioning treat- ment. Moisture content variations within a group of blocks were within a range of 2 percentage points. No complete or continuous records were obtained of the variation in solution temperatures. However, the equipment was sensitive to changes of 0.5 F. and the variations recorded were less than +2 F. The con- sistent overrun of temperatures was attributed to heat from the electric motor on the stirrer. Dimensional changes which developed after the blocks were end- coated and placed in the humidity chamber for conditioning apparently caused very fine cracks (crazing) to develop in the end-coatings on blocks in the 26-percent-moisture group. How much solution penetrated through the small cracks and along the grain is unknown. However, 10 treated blocks were crosscut at various distances from the ends and examined for evidence of solution penetration. None of the blocks examined showed solution penetrations deeper than 1 inch along the grain. Those blocks that were penetrated through the end-coating showed only an anastomosing network of fine lines corresponding to the pattern of cracks. The question, to what extent did longitudinal penetration affect liquid absorption in blocks of the 26-percent-moisture group, may be asked. Although no data are available on which an an- swer can be based, the effect is believed to be insignificant. If the longitudinal penetration did significantly increase absorption, the amount of change probably would be constant for all variables except moisture content. 24 BULLETIN No. 633 [August, Constant agitation of the hexylene glycol solution during intermit- tent treatment caused the liquid to froth at temperatures below 40 F. No froth was observed in the diacetone alcohol solution at any tem- perature. Bleeding of solution occurred on blocks treated at solution temper- atures below 40 F. The exudation probably resulted from the increase in temperature and expansion of air or liquid, or both, that took place in the blocks during the final weighing and sawing of penetration wafers. For example, blocks treated at 10 F. and 12-percent moisture content were weighed immediately after treatment (as were all blocks) and again 30 minutes later. The loss in weight from bleeding was about 0.5 percent. There was no difference in loss of weight over that period between intermittent and constant treatments, but the weight loss for blocks treated in the hexylene glycol solution was 2.4 times that for those treated in the diacetone alcohol solution. Some smearing of the solutions across the cross-section occurred during the sawing of the penetration wafers. Based upon color differ- ences, the smearing appeared to be limited to the outer }4-inch portion of the wafer. There was no practical means of controlling the smear- ing. It is believed, however, that the degree of contamination of shells and cores during sawing was constant for all treatments, and that contamination did not affect the significance of the results. The color of both treating solutions had darkened considerably by the time all blocks were treated, indicating that some extractive ma- terials had been removed during the treating process. An analysis of two samples of each solution showed that 1.63 percent of the weight of the diacetone alcohol solution and 1.36 percent of the weight of the hexylene glycol solution were extractives. The identification of the extractive materials was not within the scope of this investigation. Many factors controlled the movement of the treating solutions through the test blocks, but only solution temperature, wood moisture content, and type of treatment and solution were controlled. The extent to which the controlled factors affected solution absorption and pene- tration depended largely on the types of anatomical elements penetrated by the solutions. Thus before evaluating the absorption and penetration data, it is important to know whether the cell-wall capillaries were pene- trated by the treating solutions. This was the main reason for measur- ing the moisture-solvent interchange. WOOD MOISTURE-SOLVENT INTERCHANGE Tables 5, 6, and 8 and Figs. 9, 10, and 11 show the results of tests to determine whether there was an interchange of moisture in the wood 1958] SOAKING TREATMENT OF BASSWOOD 25 and diacetone alcohol or hexylene glycol during treatment. Table 7 shows the analysis of variance employed in testing the significance of treating effects on radial and tangential dimensions. The error correla- tion between the two dimensions was tested and found not significant; thus the effects of treatment on each dimension will be examined separately. The potential effects of the soaking treatment were these: 1. Solvent could have entered the capillary structure of the cell walls and caused swelling. 2. Hygroscopic moisture could have left the capillary structure of the cell walls and caused shrinkage. 3. A combination of the two previously listed possibilities could have occurred. In this event, all degrees of liquid interchange could have taken place, and the results would have been disclosed as dimen- sional changes in the wafers and as an increase in the moisture content of the solvents as a result of treatment. Table 5 shows that the average radial dimensions of the wafers shrank upon treatment at all wood moisture contents below 26 percent; treatment at 26 percent moisture content resulted in swelling. The average radial dimension was 0.986 inch before soaking in the organic solvents and 0.984 after treatment (Table 6). The amount of shrinkage measured is small, but the moisture content of the wood had a highly significant effect on the radial dimensional changes and so did solvent (Table 7). Changing solvents had the same effect on the radial dimen- Table 5. Average Dimensions of Basswood Wafers With Different Moisture Contents Before and After Soaking in Diacetone Alcohol and Hexylene Glycol for 12 Hours at 80 F. Average dimension (inches) Wood moisture content Radial Tangential (percent) Bef ^ e soaking After soaking Before soaking After soaking Treated with diacetone alcohol 7 974 .973 .922 .929 10 982 .977 .933 .940 16 989 .987 .942 .947 26 1.000 1.004 .955 .965 Treated with hexylene glycol 7 973 .970 .921 .919 10 983 .975 .932 .928 16 990 .987 .941 .940 26.. 1.001 1.002 .960 .968 26 BULLETIN No. 633 [August, 1.000 .975 .950 .925 - RADIAL DIMENSIONS BEFORE TREATMENT \ AFTER TREATMENT TANGENTIAL DIMENSIONS BEFORE TREATMENT AFTER TREATMENT 10 15 20 WOOD MOISTURE CONTENT- PERCENT 25 Effect of a 12-hour soak in hexylene glycol on the dimensions of basswood wafers treated at different moisture contents. (Fig. 9) 1.000 .975 .950 .925 RADIAL DIMENSIONS BEFORE TREATMENT AFTER TREATMENT TANGENTIAL DIMENSIONS AFTER TREATMENT 10 15 20 WOOD MOISTURE CONTENT - PERCENT 25 Effect of a 12-hour soak in diacetone alcohol on the dimensions of bass- wood wafers treated at different moisture contents. (Fig. 10) 1958] SOAKING TREATMENT OF BASSWOOD 27 Table 6. Dimensional Change in Basswood Wafers of Different Moisture Contents Before and After Soaking 12 Hours in Organic Solvents at 80 F. Wood moisture content (percent) Average dimension (inches) Dimensional change (percent) 8 Radial Tangential Before soaking After soaking Before soaking After soaking Radial Tangential 7 973 b .972 .976 .987 1.003 .984 .921 .932 .942 .957 .938 .924 .934 .^43 .966 .942 -.103 -.611 -.202 .300 -.203 .326 .215 .106 .940 .426 10 982 16 989 76 . 1 000 Ave rage. . .986 a Dimensional change expressed as percent of untreated dimension. b Each average based on a measurement of 10 wafers, half of which were soaked in diacetone alcohol and the remainder in hexylene glycol. sion at each level of moisture content as shown by the nonsignificant MS interaction in Table 7. In studies of the dimensional changes in wet wood following heat- ing, Koehler (19) and Wise and Jahn (39) have attributed the de- crease in the radial dimension to crooking of the rays or corrugation of the ray cells as a result of uneven tangential swelling. The crooking probably was caused by greater tangential than radial stresses which developed during the treating process. 1 The average tangential dimensions at each level of wood moisture content increased as a result of treatment (Table 5). The average tangential dimension of all wafers tested at all levels of wood moisture content swelled from 0.938 inch to 0.942 inch upon treatment (Table 6). The amount of swelling is small, but highly significant. Table 7 shows not only that the moisture content of the wood at time of treat- ment and solvent had significant effects upon the tangential dimensions, but also that the MS, moisture by solvent, interaction was significant at the 0.001 level of probability. Thus a change in solvents produced different effects on the tangential dimension at different levels of wood moisture content. The largest amount of swelling occurred in the wa- fers treated at 26-percent moisture content. The greatest swelling at the highest moisture content is attributed to the expansion of -the cell- wall capillaries which occurred with an increase in the moisture content of the wood. 1 No check was made to determine whether stresses were present in the wafers following the conditioning or treating processes; however, if stresses were present, the strain was not disclosed as a change in the square form of the wafers. 28 BULLETIN No. 633 [August, Table 7. Analysis of Variance for Wood-Moisture- Solvent Interchange Data Degrees Source of variation of freedom Radial dimensional change* (x) Tangential dimensional change" (y) Corrected sum of squares Mean square Variance Corrected ratio (F)b sum of squares Mean square Variance ratio (F)i> Moisture at time Solvent (! Moisture Error. . . . content of wood of treatment (M) 5) 3 1 3 32 39 413, 46. 9. 154. 624. 9750 2250 9750 8000 9750 137.9916 46.2250 3.3250 4.8375 28.5*** 376.0750 9.6** 540.2250 .7NS 120.4750 178.0000 1214.7750 125 540 40 5. .3582 2250 1583 5625 22.5*** ] 97.1*** < 7.2***| X solvent (MS) . Total Each measurement of dimensional change multiplied by 1,000. b ** Significant at .001 level of probability. ** Significant at .01 level of probability. NS, not significant. There is also the possibility that the size of the solvent molecule prevented it from readily penetrating the cell-wall capillaries at the lower moisture contents. Nayer (27) found that, as the size of the 1.2 1.0 .6 .4 .2 DIACETONE ALCOHOL HEXYLENE GLYCOL 10 15 20 WOOD MOISTURE CONTENT - PERCENT 25 Percent of moisture in solvents following treatment of basswood wafers of different moisture contents. (Fig. 11) 1958] SOAKING TREATMENT OF BASSWOOD 29 Table 8. Moisture Content of Diacetone Alcohol and Hexylene Glycol Before and After Soaking Basswood Wafers of Different Moisture Contents Moisture content of solvent (percent) Wood moisture content (percent) Diacetone Hexylene alcohol glycol Before soaking 7 After soaking 377 123 250 10 (197)" 451 (256) 185 (210) 318 16 (236) ... 568 (385) 288 (267) 428 26 (297) 1 003 (600) 591 (360) 797 Average (525) 600 (1231) 297 (670) (314) (618) * Numbers in parentheses show final moisture contents of solvents expressed as percent of their original moisture contents. molecule increased, there was an increase in the width of the molecules (due to thermal effects) and in resistance to diffusion. Nayer also reported that Hermans (13) found anhydrous glycerine did not diffuse into dry cellulose, but when a small amount of water was added to the glycerine, the water was selectively adsorbed by the cellulose and the resulting swelling created capillaries through which glycerine diffused. Thus at 28-percent moisture content the cell-wall capillaries would be swollen to maximum size and would permit mole- cules to enter that would be unable to penetrate at lower moistures. The moisture contents of the solvents in which the wafers were soaked were measured before and after the wafers were treated. Table 8 and Fig. 11 show the results of the measurements. At each level of wood moisture content, some of the moisture in the wood was absorbed by the diacetone alcohol and hexylene glycol. The amount of moisture absorbed by the solvents increased positively with an increase in wood moisture content. The diacetone alcohol absorbed a greater amount of moisture from the wood than the hexylene glycol. The compound is completely miscible with water, and it is used commercially as a solvent. The difference in moisture absorption, however, is attributed to the better penetrating characteristics of diacetone alcohol. Thus it appears that some moisture in the wood was absorbed by 30 BULLETIN No. 633 [August, the solvents and that some of the solvents penetrated the cell-wall capil- laries during treatment. The extent of the liquid interchange depended upon the amount of moisture in the cell walls at the time of treatment and the kind of solvent used to treat the wood. This means that the absorption figures given subsequently are prob- ably conservative, since the moisture lost from the wood during the 12-hour soaking period was replaced by treating solution. The moisture absorbed by the treating solution, however, reduced the concentration of toxicant so that the treating solutions contained less pentachlorophenol per unit volume at the end of the treating period than they did at the beginning. Whether the reduction in toxicant concentration was sig- nificant is not known. 1 ABSORPTION OF SOLUTION Tables 9 to 11 and Figs. 12 to 17 show the absorptions of solution in pounds per cubic foot 2 for various treatments. The average absorption for all blocks was 6.31 pounds, ranging from 4.74 pounds for blocks treated at 20-percent moisture in a 40 F. solution to 11.30 pounds for those treated at 28-percent moisture in the 80 solution (Table 9). Table 10 shows that the highest average absorption for the five levels of solution temperature (7.62 pounds) occurred at 80, and the lowest (5.62 pounds) at 40 F. The absorptions at 5, 10, and 20 F (6.12, 6.18, and 6.00 pounds, respectively) probably were not signifi cantly different (Fig. 12). The highest average absorption for the four levels of wood moisture content was 7.74 pounds for blocks treated at 28 percent (Table 10) The lowest absorption occurred at 20-percent moisture content. Thus absorption increased from 6.06 pounds to 6.43 pounds as moisture content increased from 7 to 13 percent, fell to 5.06 pounds at 20-percen level, and finally rose to 7.74 pounds for wood treated at 28-percen moisture (Fig. 13). The same trends are shown for continuous anc intermittent treatments (Fig. 16) and for both solutions (Fig. 17). The average absorption for the continuous treatment was 6.62 pounds, and the average for intermittent treatment was 6.00 pounds (Table 11). 1 No measurement was made of the toxicant content of the treating solu tions at the end of the tests. By the time the liquid interchange tests were completed and the data analyzed, approximately seven months had elapsed During this time the treating solutions were exposed to a range of relative humidities which made it inadvisable to make any comparisons. 2 Absorptions in terms of pounds of solution per cubic foot of wood wil be given without further reference to the common denominator. 1958] SOAKING TREATMENT OF BASSWOOD 31 Table 9. Absorption of Solution for Various Treatments of Basswood Test Blocks Average solution absorption (Ib. per cu. ft.) Solut'on Wood Continuous Intermittent (percent) Diacetone alcohol (light) solution Hexylene glycol (heavy) solution Diacetone alcohol solution Hexylene glycol solution Average 5 7 7.80 6.37 6.06 4.99 6.31 13 8.55 5.43 6.06 5.81 6.43 20 5.68 4.62 5.49 4.12 4.93 28 7.80 6.80 7.30 5.68 6.87 10 7 7.43 6.06 6.56 5.37 6.31 13 8.05 5.43 6.56 5.68 6.43 20 6.06 4.81 5.68 4.81 5.31 28 8.30 6.43 6.99 5.62 6.80 20 7 7.05 5.56 5.87 5.49 6.00 13 7.68 5.24 6.74 5.81 6.37 20 6.06 4.49 4.99 4.06 4.87 28 8.05 6.00 6.93 6.31 6.80 40 7 6.93 4.49 6.31 4.49 5.49 13 6.06 4.74 5.93 4.99 5.37 20 5.37 4.18 5.74 3.68 4.74 28 7.24 7.37 7.30 5.81 6.93 80 7 7.87 5.43 6.37 5.37 6.24 13 8.55 7.43 9.36 5.62 7.68 20 6.31 4.31 5.87 4.87 5.31 28 12.55 12.36 11.30 9.24 11.30 Average 7.43 5.87 6.62 5.37 6.31 An analysis of variance was used to determine whether absorption differences were significant (Table 12). Statistical analysis of absorption data. The analysis of variance was applied to data in terms of grams of liquid absorbed per cubic centimeter of wood, avoiding conversion of individual measurements to pound-per-cubic-foot units, and appears in Table 12. A suitable error term was not available for testing the wood- moisture variable, so a variance ratio is not shown in Table 12 for this source of variation. Table 10. Absorption of Solution by Test Blocks Treated at Different Moisture Contents in Solutions of Different Temperatures Absorption of solution (Ib. per cu. ft.) Wood moisture content c , . /0 r \ (Dei-rent^ Solution temperature ( F.) 5 10 20 40 80 Average 7.. 6 31 6 31 6 00 5 49 6.24 6.06 13 .... 6 43 6 43 6 37 5 37 7.68 6.43 20 4 93 5 31 4 87 4.74 5.31 5.06 28 6 87 6 80 6 80 6 93 11.30 7.74 Average . . 6.12 6.18 6.00 5.62 7.62 6.31 32 BULLETIN No. 633 [August, Solution temperature had a variance ratio of 2.8, not quite large enough to be significant at the 0.05 level, and thus did not have a sta- tistically significant effect on absorption. Although this result does not agree with those obtained by other investigators (6, 23, 24), the effect of temperature on solution viscosity and absorption is not likely to be as important within the range of temperatures tested (5 to 80 F.) as it is at temperatures above 100 F., the level studied by those investigators. Effect of solution temperature and wood moisture. Although a suit- able error term was not available for testing the TM interaction in the analysis of variance (Table 12), it is worth while to consider the possi- bilities of what appears to be an interacting effect of these two variables on solution absorption (Figs. 12 and 13 and Table 10). If these two variables do have an interacting effect on the absorp- tion of solution, it means that the amount of preservative solution ab- sorbed as a function of temperature was not the same for each level of wood moisture (and vice versa). It also means that the two variables 28% MOISTURE CONTENT 20% MOISTURE CONTENT 40 SOLUTION TEMPERATURE- F. 60 60 Effect of solution temperature on the absorption of solution by test blocks treated at different moisture contents. (Fig. 12) 1958] SOAKING TREATMENT OF BASSWOOD 33 o H 7 80 F.- 40 F. 10 15 20 WOOD MOISTURE CONTENT- PERCENT 25 30 Effect of wood moisture content on absorption of solution by test blocks treated in solutions of different temperatures. (Fig. 13) were interdependent in their effects on absorption, and their average independent effects on solution absorption become relatively meaning- less and unimportant. Such a relationship thus precludes the practice of discussing treating results in terms of adjusting only the tempera- ture of the treating solution or the moisture content of the wood. The results show, for example, that there was no significant change in the amount of solution absorbed by blocks treated at 28-percent moisture content and in solutions at temperatures of 40 F. or cooler (Fig. 12). Absorptions for blocks treated at 13-percent moisture, how- ever, were about the same for solution temperatures up to 20, but at 40 the absorption of solution by the 13-percent blocks was the lowest recorded for all treatments at that level of moisture. The greatest absorption for blocks treated at 13-percent moisture occurred at the 80 F. solution temperature. According to Stamm (31, 32), the movement of an aqueous or non- aqueous treating solution into softwoods treated by nonpressure meth- 34 BULLETIN No. 633 [August, ods involves a combination of capillary rise and liquid flow or, if a concentration gradient exists, diffusion. The movement of liquids through basswood may not be more complicated than the liquid flow through softwoods, where practically the total resistance to flow exists in the pit-membrane pores. The vessels and simple pits and perforation plates of basswood probably do not inhibit liquid flow any more than the corresponding anatomical feature of softwoods. For example, the diameters of the pit pores in basswood range from 5 to 8 microns (5), apparently about the same size of pit pores as in softwoods (20, 22, 31). It is likely that the openings in the pit membranes of basswood also are similar in size to those found in softwoods. Hawley (12) concluded that the effect of the viscosity of a liquid on its movement into wood was about what one would expect from the known facts of wood structure and the laws governing the flow of liquids into capillary tubes. The viscosities of the two treating solutions are curvilinear functions of temperature; thus as solution temperature was increased, the viscosities of the solutions were reduced. One might conclude, as other investigators have (2, 23, 24), that as a consequence of increased fluidity, the treating solutions penetrated the wood more readily and solution absorption increased. If absorption differences re- sulted only from a change in solution viscosity, the absorption curves in Fig. 12 would rise at a curvilinear rate corresponding roughly to the changes in solution viscosity. The lines show, however, that factors in addition to changes in solution viscosity must have influenced absorption. If we assume that all blocks were below the fiber-saturation point (they apparently were), then the cell lumina contained no free water and the treating solutions were free to move through the cell cavities by capillarity. Thus moisture in the form of free water in the lumina had no effect on the treating results. Since oil-borne preservatives ordinarily move from one wood cell to another through the lumina and pits in the cell walls, any factor affect- ing the size of the cell lumina or the diameter of the pits and pit- membrane pores affects liquid flow and ultimately the amount of liquid absorbed by the wood. The size of the pit apertures increases as the wood moisture content increases up to the fiber-saturation point (33). If the pit membranes, which are of lignin, have a hygroscopicity similar to that of wood, then the pit-membrane capillaries may also increase in size with an increase in moisture content. Thus it appears that the absorption of solution should have increased with increased moisture content; however, because this reasoning does not account for the 1958} SOAKING TREATMENT OF BASSWOOD 35 fluctuating absorption shown by the line for all blocks in Fig. 13, we must seek another explanation. Erickson et al. (10) concluded from their study of the flow of liquids through wood that there were other factors which might have greatly affected the permeability of a given wood to a given liquid. They found, for example, that benzene, a nonpolar compound, had little or no affinity for the polar wood substance and hence was more free to flow through the capillaries. Both of the solvents used in this investi- gation were polar compounds. Whether differences in polarity influ- enced absorption is unknown, but polarity in itself is believed to have an insignificant effect on absorption variation. Absorption curves show that some inhibiting influence caused ab- sorption to fall at 20-percent moisture content (Figs. 13, 16, 17). In measuring the air-pressure drops occurring through thin sections of wood exposed to air of different relative humidities, Stamm (31) found that the square root of the pressure-drop ratio increased linearly with a decrease in moisture content of the wood below 20 percent. Devia- tions in the linear regression at the 20-percent point were explained by the condensation of moisture in the pit-membrane pores which appar- ently took place when the relative humidity reached 90 percent. Wood attains a 20-percent equilibrium moisture content at such a relative humidity. The condensation of moisture in the pit-membrane pores ap- parently plugged them, thus eliminating them as a source of capillary flow. Therefore it is possible that an increase in moisture content of the bass wood from 7 to 13 percent caused the pit capillaries to expand and the absorption of solution to rise; but in the blocks treated at 20- percent moisture content, moisture condensed in the pit-membrane pores and restricted the flow of solution through them, thereby causing the drop in the absorption curve. At 28-percent moisture content, how- ever, the diameter of the pit capillaries increased, the inhibiting effect of the condensed moisture was overcome, and the relatively unrestricted flow of solution resulted in increased absorption. The relative positions of the curves in Fig. 12 also support the hypothesis that moisture con- densation in the pit-membrane pores reduced liquid absorption. The hypothesis is reasonable as long as the liquid moves into the wood by capillarity. However, Tarkow 1 points out that ". . . capillar- ity will generally occur only through the first, or perhaps the second, tier of cells," or until it is stopped by a constriction such as a pit- 1 Comment contained in letter to author by Dr. Harold Tarkow, U. S. Forest Products Laboratory, dated July 3, 1956. 36 BULLETIN No. 633 [August, membrane pore. Then hydrostatic pressure is required to overcome the resistance of the constrictions. The magnitude of the pressure in dynes per square centimeter can be determined by: where y is the surface tension of the liquid in dynes per centimeter, and r is the radius of the capillary. If the surface tension is 25 dynes per centimeter (hexylene glycol is about 33 and that for diacetone alcohol probably is greater than 25), and if the average radius of the pit-membrane pores is 3 microns (3 x 10~ 4 centimeter), the hydro- static pressure would be about 16.7x 10 4 dynes per square centimeter, or 2.5 pounds per square inch, a considerably greater pressure than that actually exerted by the 9-inch head of liquid in the treating tanks. Thus it seems reasonable that capillarity alone did not account for total absorption, but that the treating solutions moved into the wood as a result of a combination of capillarity rise and diffusion. Figs. 12 to 15 show that absorption often was lowest when blocks were treated in 40 F. solution. If the condensation of moisture men- tioned earlier took place at 40, then the strongest inhibiting effect should be shown for blocks treated at 20-percent moisture content in the 40 solution. Table 9 shows that the absorption for blocks treated under such conditions was 4.74 pounds, the lowest for all treatments. 10 CONTINUOUS TREATMENT INTERMITTENT TREATMENT 20 40 60 80 SOLUTION TEMPERATURE -F. Effect of solution temperature on absorption of solution for two methods of treatment. (Fig. 14) SOAKING TREATMENT OF BASSWOOD SOLUTION TEMPERATURE - F. Effect of solution temperature on absorption of two solutions of penta- chlorophenol. (Fig. 15) This explanation, however, becomes questionable when one observes in Fig. 13 and Table 9 that absorptions of 40 F. solution also were low at 13-percent and 7-percent moisture contents. It is doubtful that low absorptions at these moisture contents resulted from the condensation of moisture in the pit-membrane pores. Perhaps the cell-wall capil- laries in blocks treated at 7 percent and 13 percent were somewhat smaller than those in the 20-percent blocks and diffusion of liquid into the cell walls was restricted. Undoubtedly absorption was affected by a number of unknown factors, as well as by factors which were known but uncontrolled. For example, the effects of the crystallization of moisture in the cell-wall capillaries of blocks treated at temperatures below freezing, the change Table 11. Absorption of Solution by Test Blocks Treated in Two Solutions by Two Different Methods Type of treatment Absorption of solution (Ib. per cu. ft.) Diacetone alcohol Hexylene glycol A _,, (light) solution (heavy) solution Continuous 7.43 5.87 6.62 Intermittent. . 6.62 5.37 6.00 Average 7 . 05 5.62 6.31 38 BULLETIN No. 633 [August, 27 8s INTERMITTENT TREATMENT 10 15 20 WOOD MOISTURE CONTENT-PERCENT 25 30 Effect of wood moisture content on absorption of solution for two types of treatment. (Fig. 16) Table 12. Analysis of Variance for Solution Absorption Source of variation Degrees of freedom Mean square Variance ratio (F) Wood moisture content (M) 3 .032118 ( a ) Solution temperature (T) 4 .012143 2.8 NS b TM (error) 12 004256 Type of treatment (D) 1 .010598 32.7*** c Solvent type (S) 1 .052969 163.5*** SD.. 1 .000569 1.8 NS MS 3 .000319 1.0 NS MD 3 .000788 2.4NS TS 4 .000125 .4NS TD 4 .000390 1.2 NS MSB.. 3 .000519 1.6NS TSD 4 .000676 2.1 NS MTD 12 .000284 .9 NS MTS 12 .000210 .6 NS MSTD 12 .000542 1.7 NS Error 320 .000324 Total 399 tt A suitable error term not available for test. b NS, not significant at .05 level. c ** sig n i ncant at .001 level. 1958] SOAKING TREATMENT OF BASSWOOD 39 in liquid viscosity caused by the addition of pentachlorophenol to the solvents, and the temperature differentials resulting from treating warm blocks in cold solutions (and vice versa) were not measured. The effects of such influences on solution absorption were beyond the scope of this study. Effect of type of treatment. Table 11 and Figs. 14 and 16 show that continuous soaking was superior to intermittent treatment. The average solution absorption by blocks continuously treated was 6.62 pounds, whereas the average absorption for the intermittent type of treatment was 6.00 pounds. The difference is small, but statistically highly signifi- cant (Table 12). Since the head of liquid in each treating tank was approximately the same, hydrostatic pressure was constant as long as the blocks were completely submerged. The absorption differences are attributed primarily to difference in treating time. Although both groups were treated under the same conditions for 12 hours, the blocks treated intermittently were completely submerged in the treating solu- tions only 29 percent of the time. Effect of type of solvent. Table 1 1 shows that the average absorp- tion of the diacetone alcohol solution was 7.05 pounds and that for the hexylene glycol solution was only 5.62 pounds. Figs. 15 and 17 show that the absorption of the lighter or less viscous solution was greater at each level of moisture content and solution temperature. Table 12 shows that the difference in absorption was significant at the 0.001 level DIACETONE ALCOHOL SOLUTION HEXYLENE GLYCOL SOLUTION 10 15 20 WOOD MOISTURE CONTENT- PERCENT 25 30 Effect of wood moisture content on absorption of two solutions of penta- chlorophenol. (Fig. 17) 40 BULLETIN No. 633 [August, Table 13. Absorption of Solution by Test Blocks Treated in Solutions of Different Temperatures by Two Methods Absorption of solution (Ib. per cu. ft.) Type of treatment Solution temperature ( F.) 5 10 20 40 80 average Continuous 6 62 6 56 6 24 5 74 8 05 6.62 Intermittent 5 68 5 87 5 74 5 49 7 24 6 00 Average 612 6 18 6 00 5 62 7 62 6 31 of probability. Diacetone alcohol has a viscosity of 15.3 centistokes at 5 F. and hexylene glycol has a viscosity of 700.0 centistokes at approx- imately 10 F. (Table 2). Thus the difference in absorption is at- tributed to differences in viscosities. The "lighter" solution penetrated the blocks better. It is believed that the treatment in the frothy hexylene glycol had no significant effect on absorption. The absorption values shown in Table 13 for intermittent treatment are about 80 percent of those for continuous treatment at all levels of solution temperature. If treatment in the frothy solutions, which occurred at solution temperatures of 40 and less, had had a significant effect on absorption, the effect would have been shown by the above-mentioned comparison of absorption values. DISTRIBUTION OF PENTACHLOROPHENOL Tables 14 to 18 show the gross 1 concentration of pentachlorophenol in percent in the sample blocks. The average concentration was 0.696 percent, ranging from 0.507 percent for blocks treated at 7-percent 1 No corrections have been made in the data to show net concentrations or to convert the data to pentachlorophenol equivalencies, because the amount of chlorides in the basswood and the reagents and the specific gravity of the bass- wood were considered as constants in the analyses of the results. For those who wish to make the conversions, however, the correction for chlorides in the basswood and reagents was 0.055 percent, the average of seven analyses. The average specific gravity of the basswood was 0.48, based upon oven-dry weights and oven-dry volumes of three samples. Some of the percentages in the tables exceed 2.0. Although the AWPA standard method of analysis (8) indicates that the method ". . . is suitable for analysis (of wood samples containing) up to 2.0 percent pentachlorophenol . . ." the technique can be used as long as there is enough lime in the crucible to capture all the chlorides produced. When the wood sample is large enough to cause cavities or cracks to develop in the lime bed, the chlorides escape during the ignition period, and the results give percentages that are low. Such voids and cracks did not develop during the analysis of the test samples. SOAKING TREATMENT OF BASSWOOD 41 y O - EH G CO o O - rC O ^ rt U P O ^* ic O CN CN O CN in r^ ^ ir> in o fi oo *-* ^ vo T mcx^oo t^eNooi OMt^o o>^^t^ t^pi O^OO O'^O'H ^^O-H OCO >^5- <-> PO PO PC vfl 1PO vOCNrtPO O'ffSOO O'J'C^O, vo O*O <> ^* O Ov PC lr~ t'-'C^J'Ov >O PN tSt PCOvt^O lOfNH cs Tj ir> PC t^ ^^ Tf t^ PC ^H T^ ^ PCtNfNIO tSPCPC (SPCCSCS CS tS T(< \O O eN PC PC OOCSrfilO 00>O>OO OOOO >O-Ht^- PC PN-H 10 -H O OO CSO 00 2 PC t^ (^ PC oo PC vo cs PC ^ PN o ^ Ov PC fs r^ vo t-*- Ot^-^PC OOOO>O io>OCM^ O > O' V "*' >OO>^00 ID t^- ^^ \O IO t^ ^C 00 -^ IO CN PC 3" CN PN PC IO O Tt O 00 't "I (N fNOOO PN PCOOO t^PCOOO t>-PCOOO t^-PCOOO t^PCOOO tMCS (NPN PNtS PN(N i-l tS CS 42 BULLETIN No. 633 [August, Table 15. Concentration of Pentachlorophenol at Three Depths in Test Blocks Treated at Four Levels of Wood Moisture Wood moisture content Percent of pentachlorophenol (percent) Shell A Shell B Core Average 7 1.311 .339 .131 .594 13 1 627 449 131 736 20 1.442 .281 .088 .604 28 1.741 .582 .226 .850 Average 1 . 530 .413 .144 .696 moisture in a 40-degree solution to 1.273 percent for those treated at 28-percent mositure content in the 80-degree solution (Table 14). The distribution of toxicant, also referred to as "penetration," was about what one would expect with regard to the effect of depth of sample on concentration. The highest average concentration of penta- chlorophenol (1.530 percent) was found in Shell A, the outermost layer (Table 15). Somewhat less toxicant, 0.413 percent, was found in Shell B, the second deepest sample tested, and the core sample contained the least amount, 0.144 percent. These results suggest that 12 1.0 28% MOISTURE CONTENT 7% MOISTURE CONTENT 20 40 60 SOLUTION TEMPERATURE - F. 80 Effect of solution temperature on concentration of pentachlorophenol ir test blocks treated at different moisture contents. (Fig. 18) 1958} SOAKING TREATMENT OF BASSWOOD 43 Table 16. Concentration of Pentachlorophenol in Test Blocks Treated at Different Moisture Contents in Solutions of Different Temperatures Percent of pentachlorophenol Wood moisture content (percent) Solution temperature ( F.) 5 10 20 40 80 average 7.. 563 690 575 507 633 594 13 . 731 790 723 615 818 736 20 578 .637 607 .544 642 .604 28 836 .759 693 .688 1 273 850 Average 677 719 650 591 842 .696 a toxicant diffusion gradient existed in the blocks. It is believed, how- ever, that the differences in amount of toxicant were due to the pres- ence of greater amounts of solution at each successive depth in the blocks rather than to a concentration gradient. The following discussion of the effects of solution temperature, wood moisture content, type of treatment, and type of solvent on the distribution of pentachlorophenol will be brief; a more comprehensive discussion of the results is given later. Table 16 and Fig. 18 show that the average concentration of penta- chlorophenol increased from 0.677 percent to 0.719 percent as solution temperature increased from 5 to 10 F., then fell to a minimum of 0.591 percent for the 40 treatment. The maximum concentration, 0.842 percent, was found in blocks treated in the 80 solution. Table 16 and Fig. 19 show the pentachlorophenol concentrations found in blocks treated at four levels of wood moisture. The lowest concentration, 0.594 percent, was found in blocks treated at 7-percent moisture. The toxicant concentration rose to 0.736 percent when treat- ment was made at 13 percent, but fell to 0.604 percent, about the same level as that found in blocks treated at the 7-percent level, when treat- ment was made at 20-percent moisture content. The greatest amount of toxicant, 0.850 percent, was found in the blocks treated at 28-percent moisture. Table 17 and Fig. 20 show that blocks treated by continuous soaking contained more pentachlorophenol (0.714 percent) than those treated by intermittent soaking (0.677 percent). Table 18 and Fig. 20 show that the blocks treated in diacetone alcohol solution contained more pentachlorophenol (0.803 percent) than those treated in the hexylene glycol solution (0.588 percent). 44 BULLETIN No. 633 [August, 1.4 1.2 1.0 10 F 80 10 15 20 WOOD MOISTURE CONTENT- PERCENT 25 30 Effect of wood moisture content on concentration of pentachlorophenol in test blocks treated in solutions of different temperatures. (Fig. 19) 1.000 .800 .600 Si 1 .400 .200 DIACETONE ALCOHOL SOLUTION HEXYLENE GLYCOL SOLUTION TREATMENT SOLVENT Effect of type of treatment and solvent on concentration of pentachloro- phenol in test blocks. (Fig. 20) 1958] SOAKING TREATMENT OF BASSWOOD 45 Table 17. Concentration of Pentachlorophenol at Three Depths in Test Blocks Treated by Continuous and Intermittent Soaking Type of treatment Percent of pentachlorophenol Shell A Shell B Core Average Continuous 1.535 .439 .386 .413 .169 .119 .144 .714 .677 .696 Intermittent 1.526 Average 1 530 A multivariate analysis was used to determine whether penetration differences were significant (Table 19). Statistical analysis of penetration data. The multivariate analysis testing the significance of penetration results follows the method de- scribed by Rao (28), who refers to it as "analysis of dispersion." It is an extension of the univariate analysis of variance which was employed in the section on solution absorption. A suitable error term was not available for testing the wood mois- ture variable, so a variance ratio is not shown in Table 19 for this source of variation. Solution temperature was a nonsignificant source of variation. As mentioned earlier, this result does not substantiate results of earlier investigations of the effect of solution temperature on penetration (toxicant concentration in this study). However, the effect of temper- ature on solution viscosity and penetration into the wood is not likely to be as great within the range of solution temperatures tested (5 to 80 F.) as it is at higher temperatures, those above 100 F. for example. Although a suitable error term was not available for testing the TM interaction solution temperature and wood-moisture content it is worth while to review what appears to be an interacting effect of these two variables on toxicant penetration (Figs. 18 and 19). Effect of solution temperature and wood moisture. If we compare the concentration regressions in Figs. 18 and 19 with the absorption regressions in Figs. 12 and 13, we note a similarity in their patterns. It is reasonable to assume that the two sets of absorption and concen- tration curves should be similar, since a close relationship should exist between the amount of solution absorbed and the amount of toxicant found in the test blocks. There is no question but that the treatment of blocks at 28-percent moisture content in the 80 F. solution was superior to all other treat- ments as far as quantity of toxicant in the wood is concerned. 46 BULLETIN No. 633 [August, The position of the line for treatments made at 20-percent moisture content does not show the inhibiting effects of moisture condensation in the pit capillaries on toxicant concentration, discussed in the analysis of absorption results, as convincingly as the absorption data do (Fig. 12). The positions of the 20-percent and 7-percent curves in Fig. 18 are so arranged, however, that if we allow for experimental error, we may conclude that there was no significant difference between these curves at most levels of solution temperature. The family of curves in Fig. 19 shows about the same relationship between toxicant concentration and wood moisture content as is shown by the absorption data in Fig. 13. Here again, if we allow for experi- mental error, we may conclude that the cause-result relationship prob- ably was the same as that described for absorption data; namely, that with an increase in moisture content, the diameters of the pit pores increased, the absorption of solution increased, and higher concentra- tion of toxicant resulted. But in the blocks treated at 20-percent moisture content, moisture condensed in the pit capillaries and re- stricted the flow of solution through them, thereby causing a drop in the concentration of pentachlorophenol. At the 28-percent moisture level, however, the diameters of the pit and cell-wall capillaries were expanded to maximum size, the inhibiting effect of the condensed moisture was overcome, and the relatively unrestricted flow of solution resulted in maximum toxicant concentration. An increase in solution temperature decreased solution viscosity, and toxicant concentration should have increased positively with tem- perature. However, Table 16 shows the depressing effect that the treatment of blocks in the 40 F. solution had on the absorption of solution and the concentration of pentachlorophenol in the wood. Thus the greatest inhibiting effect of solution temperature combined with! wood-moisture content should have occurred when blocks were treated at 20-percent moisture content in the 40 F. solution. Table 16 shows that the concentration of pentachlorophenol for such a treatment was! 0.544 percent. Although the concentration obtained by treating blocks at the same solution temperature but at 7-percent moisture content was lower (0.507 percent), a "t" test showed that the mean concen- trations for the two levels of moisture were not significantly different. There is also the possibility that the size of the cell-wall capillaries, which were expanded to maximum size at 28-percent moisture content, had an effect on pentachlorophenol concentration. This possibility has been discussed in the section concerning the interchange of moisture 1958] SOAKING TREATMENT OF BASSWOOD 47 Table 18. Concentration of Pentachlorophenol at Three Depths in Test Blocks Treated in Solutions Containing Diacetone Alcohol or Hexylene Glycol Percent of pentachlorophenol solvent Shell A Shell B Core Average Diacetone alcohol 1 748 498 164 803 Hexylene glycol 1 313 327 124 588 Average , 1 530 413 144 696 and solvent during treatment and in the discussion of solution ab- sorption. Effect of type of treatment. Table 17 and Fig. 20 show that con- tinuous soaking resulted in a higher concentration of pentachlorophenol than intermittent treatment, 0.714 percent and 0.677 percent, respec- tively. Table 19 shows that the difference in chemical content was significant at the 0.01 level of probability. The difference is attributed to the greater length of time the blocks received treatment by the con- tinuous method. The same relationship between the two types of treat- ment was found in the analysis of absorption data. Effect of type of solvent. Table 18 and Fig. 20 show that the con- centration of pentachlorophenol was 0.803 percent in blocks treated in the diacetone alcohol solution and 0.588 percent in the blocks treated in hexylene glycol solution. The difference in chemical content was signifi- Table 19. Multivariate Analysis" for Penetration Data Source of variation Degrees of Sum of products matrix Level of ignificance b freedom A2 B* AB AC BC od moisture content (M) . . ution temperature (T) I (error) 3 4 12 1 1 4 4 3 3 1 43 79 2, 3 1. 9. 200 862 .660 002 790 258 038 075 .084 040 262 272 1.059 1.024 1.226 0.057 0.586 0.047 0.070 0.068 0.090 0.012 0.981 5.221 0.203 0.180 0.512 0.050 0.031 0.019 0.074 0.005 0.057 0.186 1.316 1 1 -0 0. 3. ,345 .803 .499 009 490 071 022 067 055 022 289 377 0.473 0.281 0.220 0.009 0.344 -0.039 -0.038 0.013 -0.020 -0.001 0.061 1.303 0.433 0.418 0.733 0.053 0.135 0.002 0.050 0.015 0.026 0.030 1.896 NS NS NS NS NS NS pe of treatment (D) vent type (S) . . . X. or " Called "analysis of dispersion" by Rao (28). ** Significant at .0001 level. ** Significant at .01 level. A suitable error term not available for test. d T, trace or less than 0.001. 48 BULLETIN No. 633 [August, cant at the 0.001 level of probability (Table 19). The same results were obtained in the analysis of absorption data, and they were at- tributed to the difference in viscosities of the treating solutions. The diacetone alcohol solution had the lower viscosity and was able to penetrate the blocks easier than the more viscous hexylene glycol. The greater concentration of pentachlorophenol probably was due to greater absorption of the lighter preservative solution. Summary and Conclusions The absorption of solution and the concentration of pentachloro- phenol were determined following constant and intermittent cold-soak treatments of basswood blocks of different moisture contents in solu- tions of different viscosities. Solution temperatures ranging between 5 and 80 F. had a statistically nonsignificant effect on the amount of solution and toxicant absorbed by the wood. No suitable error term was available for testing the effect of wood moisture content on absorption of solution and toxicant. No interaction of variables tested was found statistically significant, although an explanation is presented which ac- counts for what appears to be an interacting effect of solution tempera- ture and wood moisture content on solution absorption and toxicant concentration. The minimum absorption occurred when the blocks were treated at 20-percent moisture content in a 40 F. solution, an intermediate level for both variables. The result was attributed to the inhibiting effects of moisture which condensed in the pit capillaries under the moisture-temperature conditions. Approximately the same result was obtained when toxicant concentration was measured for such a treat- ment, and the same conclusions were reached. The highest absorption of solution and concentration of toxicant were found in blocks treated at 28-percent moisture content in an 80 F. solution. From these results it was concluded that increased solution temperature lowered viscosity and improved liquid flow, whereas an] increase in wood moisture content to 28 percent caused the pit andj cell-wall capillaries to expand to maximum diameter, allowing a maxi- mum amount of solution to flow into the wood. Continuous treatment was found to be superior to intermittent treatment during a 12-hour soaking period because of the shorter time the intermittently treated blocks were completely submerged in solution. A low-viscosity solution of pentachlorophenol containing diacetom alcohol solvent was absorbed in greater quantities than a more viscous 1958] SOAKING TREATMENT OF BASSWOOD 49 solution containing hexylene glycol solvent. The concentration of pentachlorophenol also was greater in blocks treated in the diacetone alcohol solution. It was concluded that the greater absorption and toxicant concentration were due to the lower viscosity of the diacetone alcohol solution. Dimensional changes in basswood wafers following a 12-hour con- tinuous treatment in the solvents showed that wafers shrank slightly radially and swelled in a tangential direction. The moisture content of the solvents increased during the treatment, even though the treatments were conducted in containers sealed to prevent exposure of the solvents to the atmosphere. It was concluded that an interchange of moisture in the cell walls of the wood and solvent took place. The radial shrink- age was attributed to crooking of the ray cells, whereas the tangential swelling was due to penetration of the cell- wall capillaries by the organic solvents. The interchange of moisture and solvent which took place in the cell-wall capillaries indicates the possibility of a moisture- treating solution interchange. Such an interchange probably affected the absorption measurements, making them conservative, and indicated that diffusion, as well as capillarity, caused liquid to move into the wood. 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