,<r *1, 
 
 
 
 / j 
 
 RESISTANCE TO EEACFiING AND DECAY PROTECTION OE 
 VARIOUS PRECIPITATES FORMED IN WOOD BY 
 DOUDEE DIFFUSION 
 1941 
 
 
 
 UNIV OF FL LIB 
 DOCUMENTS DEPT 
 
 
 1 
 
 
 
 1 
 
 U.S. DEPOSITORY 
 
 
 UNITED STATES DEPARTMENT OF AGRICULTURE 
 
 FOREST SERVICE 
 
 FOREST PRODUCTS LABORATORY 
 
 Madison, Wisconsin 
 
 In Cooperation with the University of Wisconsin 
 
Digitized by the Internet Archive 
 
 in 2013 
 
 http://archive.org/details/resistaOOfore 
 
RESISTANCE TO LEACHING AND DECAY PROTECTION OF 
 
 VARIOUS PRECIPITATES FORl-iED IN WOOD BY DOUBLE DIFFUSION * 
 
 By R. H. BAECHLER, Chemist 
 
 It has long "been recognized that a water-soluble material may be 
 introduced into green or water-logged wood by merely steeping the wood in an 
 aqueous solution of the material which then tends to diffuse from the solu- 
 tion into the w'ater contained in the wood. This phenomenon has been utilized 
 in the chemical seasoning of wood— but for various reasons its application in 
 the field of wood preservation has been limited.- A disadvantage of the 
 process is the fact that it is confined to materials that are water soluble 
 and, consequently, subject to leaching from wood exposed to damp surroundings, 
 
 The experiments here described were undertaken to investigate (l) 
 the possibility of precipitating materials of very low solubility in wood by 
 a two-stage diffusion treatment, and (2) the resistance to leaching and the 
 toxicity to wood-destroying fungi of certain materials so formed. 
 
 Only a small number of the compounds that may be precipitated from 
 an aqueous system are relatively insoluble, and at the same time toxic to wood- 
 destroying .fungi. Of this small number a few are very expensive. The ex- 
 periments reported here were confined to the following precipitates: copper' 
 arsenate, copper chromate, nickel arsenate,' nickel chromate, and magnesium 
 ammonium arsenate. Parallel tests were made on the soluble salts— used in 
 forming these precipitates, as well as on zinc chloride, a preservative of 
 established .value. 
 
 Penetration of Sal ts 
 
 It was assumed at the start that if wet wood containing one soluble 
 salt were immersed in a solution of a second salt which reacted with the 
 first to form an insoluble precipitate, the ingress of the second salt would 
 
 ^Presented at 37th Annual Meeting of the Ainer. Wood-Pres. Assn. , Louisville, 
 . Y.y., Feb. 4-6, 1941; and published in the Proc. , 1941, p. 23. 
 
 -Chemical Seasoning of Wood, by W.K.Loughborough. South. Lbrman. , Sept. 15,1936, 
 
 2 
 
 —The osmose process, although not a steeping process, depends upon diffusion 
 
 as does the process of filling alternate holes with copper sulfate and 
 sodium arsenite, as described by J. F.Harkom in the Froc. of the Amer. 
 Wood-Pres. Assn., 1939, p. 306. 
 
 - No tests were made on magnesium chloride, which is known to be practically 
 nontoxic to fungi. 
 
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be retarded "by the precipitate as it formed in the outer parts of the wood. 
 This would necessitate a longer steeping period for the second salt, other 
 conditions being equal. This problematical degree of retardation, rather 
 than known differences in the rates of diffusion of the various salts, made 
 the selection of treating periods a matter of guesswork. 
 
 Preliminary tests were made in which 15-inch lengths- of peeled 
 green aspen fence posts 4 inches in diameter were steeped in a 10 percent 
 solution of copper sulfate for five days and then transferred to a 20 percent 
 solution of sodium chromate. (A saturated solution of sodium chromate has a 
 concentration about twice that of a saturated solution of copper sulfate at 
 the same temperature; the exact ratio depends upon the temperature at which 
 the comparison is made.) 
 
 Pieces were removed at the end of the second five days and kiln 
 dried, after which discs were cut from the middle and ground up for analysis.— 
 It was found that the absorption of each salt amounted to roughly 3 percent 
 of the dry weight of the wood. This was considered a satisfactory ratio of 
 the two chemicals since their molecular weights are nearly the same 
 (CuS0 /:i = 159.5, Na ? Cro = 162) and the copper and chromate ions are both 
 diva.lv ni". When the order of treatment was reversed, that is, the wood 
 steeped first in the sodium chromate solution and then in the copper sulfate 
 solution, a marked deficiency of copper sulfate was found. These tests were 
 repeated with other combinations of salts and similar results were obtained. 
 When the wood was first treated with the salt of the heavy metal, magnesium, 
 copper or nickel, subsequent penetration by sodium arsenate or sodium 
 chromate was comparatively rapid. However, when the wood was first treated 
 with the sodium salt, penetration by the salt of the heavy metal was much 
 slowe.r* This difference may be oxplained on the basis of difference in pH. 
 Due to hydrolysis, the solutions of the salts of heavy metals are acidic, 
 whereas solutions of di sodium arsenate and sodium chromate are alkaline.. 
 All chromates and arsenates are soluble in dilute mineral acids; consequently, 
 when wood is first treated with the salt of the heavy metal no precipitate 
 would be expected to form until a sufficient amount of the alkaline sodium 
 salt 1 s present. 
 
 More extensive tests have been started to study the gross absorp- 
 tion and distribution of different salts from varied concentrations and at 
 varied periods. Results of these tests ere not yet available. Up to the 
 present, other factors which arc known to affect penetration, especially 
 the initial moisture content of the wood and the temperature of the solution, 
 have been kept fairly constant but their effects have not been investigated. 
 
 The question of the distribution of two salts introduced by succes- 
 sive steeping was then studied briefly. It would be possible for a cross 
 
 — - , — — 
 
 All pieces used in subscauent treatments were peeled and were 15 inches 
 long. Diameters varied from 3-l/2 to 6 inches. 
 5 
 Acknowledgment is made of the valuable aid of Philip Servais, student 
 
 assistant, in the conductance of these analyses. 
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section of a treated piece to show two salts present in an ideal proportion, 
 but if one were concentrated near the outside to a greater extent than the 
 other, it would be in excess in that portion and the excess would be leach- 
 able, as would a corresponding excess of the other salt in the interior of 
 the piece. 
 
 One piece 4 inches in diameter was treated for five days in a 7.45 
 percent copper sulfate solution and then for 30 days in a 14.95 percent 
 soditxra chromate solution. A matched piece was treated for five days in a 
 16.75 percent sodium chromate solution and then for 30 days in an 8.38 percent 
 copper sulfate solution. Discs were then cut from the middle of the pieces 
 after which each disc was cut into three circumferential sections of approxi- 
 mately equal width, leaving a center core with a radius equal to the width 
 of the sections. 
 
 Table 1 shows the copj^er and chromium content of 'the different 
 sections after a total of 35 days' steeping. The total amounts are, naturally , 
 very different from that obtained after a total of 10 days' steeping. The 
 term "ring" is used as suggestive of the shape of the sections and has no 
 reference to annual rings. 
 
 It may be seen that the gradation in concentration from the surface 
 to the center was of more or less parallel order for both salts. 
 
 Table 1. Distribution of copper and chromium in treated pieces after 
 35 days' steeping 
 
 
 Treated first with 
 
 Treated first with 
 
 
 copper sulfate 
 
 : sodium 
 
 chromate 
 
 Section 
 
 
 
 
 
 Copper 
 
 Chromium 
 
 Copper 
 
 Chromium 
 
 
 : calcu- 
 
 calcu- 
 
 calcu- 
 
 calcu- 
 
 
 lated as 
 
 lated as 
 
 lated as 
 
 lated as 
 
 
 CuS0 4 
 
 Na 2 Cr0 4 
 
 CuS0 7 
 
 4 
 
 TS& CrO 
 
 Outside ring. 
 Second ring. , 
 Third ring. . , 
 Center core., 
 
 3.62 
 1.25 
 
 .44 
 .006 
 
 Percent 
 
 16.13 
 
 6.94 
 
 3.25 
 
 .26 
 
 6.21 
 1.36 
 1.37 
 1.79 
 
 5.37 
 
 1.65 
 
 .89 
 
 .96 
 
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Leaching Tests 
 
 To study the leaching characteristics of the various chemicals, 
 a preliminary series of tests was first made. Blocks 5 cm. long by 2,5 cm. 
 wide by 1,5 cm, thick (approximately 2 by 1 by 0.6 inches) were cut from 
 near the outside of treated and dried basswood pieces. About 3 inches of 
 each end of the piece were discarded. Some of the blocks were ground and 
 analyzed for their content of chemical before leaching. Fourteen blocks 
 were placed in a vacuum bottle, a vacuum of about 28' inches of mercury was 
 drawn, and', with vacuum pump still running, 500 ml. of distilled water was 
 drawn into the bottle. The vacuum was then released, causing a rapid ab- 
 sorption of water by the blocks so that some of them sank while the rest 
 floated very low in the water. The flask was then shaken occasionally and 
 at the end of an hour the leaching water was poured off and saved for anal- 
 ysis. Another 500 ml. of distilled water was poured over the blocks (omitting 
 the vacuum) and, with occasional shaking, left for 2 hours after which it was 
 poured off and. saved for analysis. In a similar manner the blocks were 
 leached for a period of 4 hours and then for 31 periods of 8 hours each. Only 
 one 8-hour leaching was made in any 24-hour period and between leachings the 
 blocks were left in the stoppered bottles without water. Analyses were made 
 of the leaching water from each of the first four learnings, after which 
 leachings were combined as follows: fifth and sixth period, seventh to tenth 
 period, eleventh to eighteenth period, nineteenth to thirty-fourth period, 
 
 Because it was believed that the number of blocks available from 
 each treatment was too small to correct for variability in the blocks, the 
 results of the analyses were considered only indicative and will not be re- 
 ported in detail. It was found that the depletion of metallic ions was 
 much slower than from wood given a single treatment. For example, under the 
 conditions described, 77 percent of the copper was leached from wood treated 
 with copper sulfate alone. Only 2.8 percent of the copper was leached from 
 wood treated with about the same amount of copper sulfate, followed by an 
 excess of sodium arsenate, and 3.2 percent of the copper was leached from 
 copper sulfate-sodium chromate treated wood with the latter chemical in ex- 
 cess. In other treatments in which a deficiency of either second chemical 
 was found, the' leaching of copper, as would be expected, was much faster at 
 first because of the presence of some uncombined copper sulfate. The leaching 
 of nickel and magnesium in the presence of an excess of either of the sodium 
 salts was likewise very slow, 
 
 Witli the acid ion', the leaching of chromate was very slow, as has 
 been found by various investigators. The leaching of arsenate was' always 
 considerable, .For example, 26 percent of the arsenic leached from blocks 
 containing sodium arsenate and an excess of nickel sulfate' as compared with 
 51 percent from blocks treated with sodium arsenate alone. The true picture 
 of the leaching of arsenic from an arsenate formed under favorable conditions 
 is difficult to discern. An excess of the metallic s : alt creates an acidic 
 condition which increases the solubility of the arsenate. An excess of 
 sodium arsenate furnishes an alkaline condition which is desirable for its 
 depressant effect on the solubility of the precipitated arsenate, but the 
 leaching of this excess sodium arsenate masks the leaching of the precipitated 
 
 R1290 -4- 
 
arsenate. When slightly soluble salts of weak bases arc exposed to the action 
 of water, some hydrolysis takes place; the solution contains relatively more 
 of the acid ion than the basic ion and the remaining undissolved material 
 becomes progressively more basic. Numerous tests have shown this change to 
 take place in treated wood. For this reason onu would expect the arsenic 
 to be leached more rapidly than the metal from wood treated with an insoluble 
 arsenate and also that the remaining material would, in time, become too in- 
 soluble to be toxic. This rate of leaching might still be very slow com- 
 pared with that of a highly soluble salt. Much work would be required to 
 determine the most desirable ratio of any two salts from the standpoint of 
 toxicity, permanence, corrosion, and cost. At present it is believed that 
 a slight excess of the alkaline reacting salt is preferable to stoichio- 
 metric proportions. 
 
 A second series of treatments was made, using sections of green 
 mrple fence posts; these posts were about 6 inches in diameter with a 2-inch 
 band of sapwood and had an average moisture content of 92 percent. As a 
 result of experience gained in the first treatments, a more satisfactory 
 ratio of the reacting salts was introduced into the wood. Weaker treating 
 solutions wore used than in the first series in an effort to arrive at ab- 
 sorptions of salt in the region of those employed in commercial pressure 
 treatments. The absorptions actually obtained were higher than desired. On 
 the basis of average specific gravity, the absorptions of totrl chemical, when 
 calculated to pounds per cubic foot, ranged from 0.58 magnesium chloride + 
 sodium ammonium arsenate to 1.78 nickel sulfate + sodium chromrte. A larger 
 amount of material wrs treated so that several hundred blocks were obtained 
 from each treatment. One-third of the blocks from each treatment were 
 ground together rnd analyzed to show the concentration of chemicals before 
 leaching. The remaining blocks were leached by first water logging them, 
 then placing them in wire baskets and keeping them submerged in a bath 
 which was constantly being changed by a stream of water. The block? were 
 leeched continuously, day end night, for one month so th~t conditions of 
 le^chin^ were more severe than in the first series. One-half of the leached 
 blocks, comprising one-third of the original treated blocks, were dried .and 
 ground for analysis. The remaining blocks were set aside for decay tests. 
 
 The absorptions and losses of chemicals by leaching are given in 
 table 2. Absorptions, as well as content of chemical after leaching, are 
 calculated on the basis of the original chemicals used in the treatment, no 
 specific chemical reaction being assumed. The data on the absorptions ob- 
 tained with different combinations of chemicals are not strictly comparable. 
 Since these experiments were exploratory in nature, the increased time and 
 expense necessary to control clo?ely all the conditions affecting absorp- 
 tion were not considered justified. Solutions of about 6 percent were used. 
 The concentrations were checked by analysis but were not adjusted to any 
 exact figure. Because of checks which developed in magnesium-ammonium- 
 arsenate-treated pieces, it was necessary to cut blocks farther from the sur- 
 face than was otherwise done. 
 
 With allowances made for differences in conditions, the results 
 obtained in the second series of leaching tests are in general agreement 
 with those obtained in the first series. The rrther high loss of nickel 
 
 R1290 -5- 
 
from the nickel sulfate-sodium arsenate treated wood is difficult to explain, 
 ps is the unusually high loss of chromate from the copper sulfate-sodium 
 chromate treated wood. The loss of copper from the latter is not surprising 
 if the excess of copper sulfate is considered. 
 
 Decay Resistance of Treated Blocks After Leaching; 
 
 The leached blocks were tested for their resistance to decay by a 
 method used extensively in the Division of Plant Pathology, U. S. Bureau of 
 Plant Industry. It is a modificrtion of the Kolle flask method, square 
 bottles with metal screw tops being used in place of the flasks. After a 
 vigorous growth of the fungus has covered the agar, a small gl?ss rod bent 
 into an angular shape is laid upon the mycelium to support the test block. 
 Only one block is placed in a bottle. The screw top is closed loosely to 
 permit access of air and, in order to avoid drying of the agar, the bottles 
 are kept in a room maintained at a relative humidity of 80 percent and a 
 temperature of 80° P. 
 
 The leached blocks were first conditioned to constant weight in a 
 65 percent relative humidity chamber. Extrr blocks were used to determine 
 the average moisture content by drying at 105°. The oven-dry weight of each 
 block at the start of the test was obtained by calculation to avoid any 
 chemical changes that night be produced through oven drying. The test 
 blocks were immersed in boiling water for about 10 seconds before they were 
 placed over the fungus mat. After four months' exposure they were removed, 
 inspected for signs of fungus attack, dried, and weighed. 
 
 Pour test fungi were used, namely, Madison 517, Con ie-phora ce ro- 
 be 11a 658, Poria inc rassrta 563, and Lenzites trabea 617, The cultures 
 were obtained originally from the Division of Forest Pathology, U. S. Bureau 
 of Plant Industry. Ten blocks from each treatment were tested against each 
 fungus. Th^, loss in weight reported is the average for ten blocks except 
 that an occasional block was discarded for contamination of the fungus mat. 
 
 As has been stated., the number of blocks obtained in the first 
 series of treatments was inadequate in view of the differences between 
 individual blocks. In some decay tests made with Lenzites trrbea only, the 
 leached blocks that had been treated with the insoluble nickel and copper 
 compounds showed practically no loss due to decay, whereas those treated with 
 single soluble salts showed considerable loss in weight. The absorptions 
 were above those commonly used in commercial practice. 
 
 The results of the second series of decay tests are given in table 
 2. The fungi are placed in the order of the intensity of attack on untreated 
 wood, but it is apparent that this bears no relation to their resistance to 
 different chemicals. As has been found by others. Lenzites trabea is shown 
 to be arsenic tolerant; it is susceptible to copper but does not seem to be 
 highly susceptible to nickel. In the first series of tests both nickel 
 chromate and nickel arsenate protected the wood against L. trabea but they 
 were present in about double the final 'concentration of the second series. 
 
 R1290 -6- 
 
Table 2. —Lea ching and decay tests on treated maple blocks 
 
 Treatment 
 
 Concentration 
 of chemical 
 (Based on 
 
 oven-dry wood) 
 
 Before: After: 
 leach-: leach- 
 ing : ing 
 
 Loss of 
 chemical 
 
 by 
 leaching: 
 
 Loss of weight due to decay 
 during four months' exposure 
 to pure cultures of 
 
 :Lenzi tes : Madison: Conio-: Poria 
 trabea : 517 :phora : in- 
 
 : : cere- rcrassata 
 : : lsella : 
 
 Percent 
 
 (Copper sulfate, 5 days 
 
 ( + 
 
 (Sodium arsenate, 20 days... 
 
 (Nickel sulfate, 5 days 
 
 ( + 
 
 (Sodium arsenate, 20 days... 
 
 (Magnesium chloride, 5 days. 
 ( + 
 
 (Sodium ammonium arsenate-^-. . 
 ( 20 days 
 
 (Copper sulfate, 5 days.. 
 
 ( + 
 
 (Sodium chr ornate, 20 days 
 
 (Nickel sulfate, 5 days.. 
 
 ( + 
 
 (Sodium chr ornate, 20 days 
 
 Copper sulfate, 5 days... 
 
 Nickel sulfate, 5 days... 
 
 Sodium arsenate, 5 days.. 
 
 Sodium chromate, 5 days.. 
 
 Zinc chloride, 5 days. . . . 
 
 Untreated controls 
 
 1.43 
 3,59 
 1.61 
 2.47 
 .54 
 1.26 
 
 2.00 
 1.59 
 1.99 
 3.52 
 1.03 
 1.63 
 1.77 
 2.16 
 2.14 
 
 1.36 
 1.09 
 1.33 
 1.34 
 .51 
 .50 
 
 1.65 
 
 1.30 
 
 1.85 
 
 3.45 
 
 .089 
 
 .24 
 
 .092 
 
 1.98 
 
 .40 
 
 4.9 
 69.7 
 17.4 
 45.8 
 
 5.6 
 60.3 
 
 17.5 
 
 18.2 
 
 7.0 
 
 2.0 
 
 91.3 
 
 85.7 
 
 94.3 
 
 8.3 
 
 81.3 
 
 ) 
 
 ) 0.06 
 
 ) 
 
 ) 
 
 ) 14.2 
 
 ) 
 
 ) 
 
 ) 66.4 
 
 ) 
 
 ) 
 
 ) 5.24 
 ) 
 
 ) 
 
 ) 23.5 
 ) 
 
 .71 
 
 53.0 
 
 51.2 
 
 42.1 
 
 64.0 
 
 49.2 
 
 0.37 
 
 1.27 
 
 1.65 
 
 3.55 
 
 8.35 
 
 6.67 
 13.0 
 26.3 
 31.8 
 24.8 
 32.2 
 
 : 0. 
 
 481 
 
 : 1. 
 
 14: 
 
 5 , 
 
 5 : 
 
 : 11. 
 
 5 : 
 
 : 16. 
 
 9 : 
 
 : 32. 
 
 5 : 
 
 : 39. 
 
 3 : 
 
 : 2. 
 
 3 : 
 
 : 35, 
 
 > " • 
 
 : 39, 
 
 4 • 
 
 : 30 
 
 .9 : 
 
 16*8 
 
 10.6 
 
 24.4 
 
 27.3 
 
 19,0 
 
 19*4 
 19,7 
 
 34. 7 
 25.4 
 
 24.0 
 
 'Solution contained 5 percent ammonium hydroxide. 
 
 B.1290 
 
Coniophora cerebella , on the other hand, is susceptible to arsenic but rather 
 tolerant to copper. None of the- chemicals in the amounts present and under 
 the conditions of the test gave decisive protection against Poria incrassata . 
 In any comparison between a single treatment and a double treatment, allow- 
 ance should be made for the fact that higher absorptions of total chemical 
 were present in the latter. 
 
 The lack of protection afforded by zinc chloride might seem to 
 indicate that these tests were extremely severe. It must be remembered that, 
 of the factors affecting permanence, only leaching and, to some extent, 
 hydrolysis were accelerated. The absence of gradual deterioration by chemi- 
 cal changes which might take place in service remains to be proven. However, 
 it appears that the insoluble materials investigated are sufficiently prom- 
 ising to merit further study. Losses in weight combined with appearances 
 of the blocks at the end of the test point to copper arsenate as the most 
 effective of the materials studied. However, it may be suspected of present- 
 ing a greater corrosion hazard than any of the other materials. In this 
 respect magnesium ammonium arsenate seems less likely to attack metal fasten- 
 ings than the others; moreover, either of the solutions used in this treatment 
 may be held in an iron container, which is not true of copper sulfate or 
 nickel sulfate. 
 
 Shorter treating periods than were employed in these experiments 
 might be combined with higher concentration of solution to yield a. desired 
 absorption of chemical. This would result in a steeper concentration 
 gradient in the treated wood which, while undesirable in the preparation of 
 atcrial for experimental purposes, might be desirable in practice. 
 
 Conclusions 
 
 1. Compounds of low solubility may be deposited in green wood oy 
 successively steeping it in solutions of different salts. 
 
 2. The chroma.tes and arsenates of copper and nickel and magnesium 
 ammonium \r senate, when formed in wood by this method, are leached more 
 slowly than are soluble salts. 
 
 3. The leached blocks show considerable resistance to attack by 
 three wood-destroying fungi and some resistance to a fourth. 
 
 m 
 
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UNIVERSITY OF FLORIDA 
 
 3 1262 08927 3238