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Preservative Treatment of Wood by Pressure Methods

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Preservative Treatment of Wood by Pressure Methods PRESERVATIVE TREATMENT OF WOOD BY PRESSURE METHODS Agriculture Handbook No. 40 * lllv-v ^t.,^.. UNITED STATES DEPARTMENT OF AGRICULTURE Forest Service ERRATA SHEET for Preservative Treatment of Wood by Pressure Methods Agriculture Handbook No. 40 Page 9 -^ Line 2, "older and heartwood" should read "older the heartwood". 17 ^ Group 4, second col. "(read heartwood)" should read "(red heartwood)". 38 t. Line 21, reference italic "(50)" should be italic "(51)". 47i^Table 6, col. 4, change 200-200 to 200-210. 48 ^ Table 7, col. 4, first line, cha«ge 01.0 to 11.0. 72*^ Table 9, col. 5, beginning with second line, change 235 y to 135, 255 to 155, 276 to 176. SO'-^Table 13, col. 9, last line, change 37.7 to 3.7. 105'^Table 17, col. 2, last line, change 2.067 to 2.167. 105 Table 17, col. 7 heading, change "per" to "both". 106 ^ 'Table 17, col. 3, 7th line, change 1.090 to 1.091. 106' Table 17, col. 7, 8th line, change 6.808 to 6.818. 108 i^Equation bottom of page should read "0.531W" not , ".0531W". 110 ¿^ Example, last term in denominator should read "0.55" not ".055". 121¿XTable 20, col. 1, 4th line under "Steamed Green Lumber .and Timbers", insert figure "1". 135 ^Wd definition should read "oven-dry wood per unit" etc. 139 ^Table 22, Footnote 1, a\ should read a,, /^ should read /^ 150 '-'First equation, delete " = (A)." insert "(A)" flush right. 150 ¿/line 17, "pp. 149-150" should read "pp. 150-151". 150 '-''Line 7 from bottom, "p. 150" should read "p.l49". Forest Service U. S. Department of Agricultuije March 1953 6P0 »»-SM PRESERVATIVE TREATMENT OF WOOD BY PRESSURE METHODS! By J. D. MACLEAN, 2 Engineer Forest Products Laboratory, ^ Forest Service Agriculture Handbook No. 40 U. S. Department of Agriculture December, 1952 For sale by the Superintendent of Documents, U. S. Government Printing Office Washington 25, D. C.—Price 40 cents 1 This handbook supersedes the MANUAL ON PRESERVATIVE TREATMENT OF WOOD BY PRESSURE, U. S. Dept. Agr. Misc. Pub. 224. 2 Acknowledgment is made to various members of the Forest Products Laboratory who have been consulted on subjects relating to their fields of investigation and from whom much helpful data and information have been obtained. 2 Maintained at Madison, Wis., in cooperation with the University of Wisconsin. CONTENTS Page Introduction __ _ i Pressure processes 2 Full-cell processes o Empty-cell processes __ _ 2 Wood preservatives ^_ _ _ 3 Coal-tar creosote _ _ _ _ 4 Water-gas tar and water-gas-tar creosote 4 W^ood-tar creosote ^ Coal tars _ 4 Petroleum oils 4 Creosote, coal-tar solutions 5 Coal-tar creosote and petroleum solutions ___ 5 Chemicals dissolved in solvents other than water 5 Water-borne preservatives 5 Proprietary preservatives G Effect of wood structure on treatment 6 Differences in structure of hardwoods and softwood 6 Heartwood and sapwood g Effect of tyloses on penetration 10 Bordered pits and simple pits 11 Resin passages 13 Density 13 Influence of structure on direction of penetration 14 Classification of species with respect to penetrability 16 Moisture content, fiber saturation point, shrinkage and swelling, specific gravity, weight of wood, and air space in wood 18 Moisture content 18 Fiber saturation point 20 Shrinkage and swelling 25 Specific gravity of wood 26 Specific gravity of wood substance 28 Relation of specific gravity, moisture content, and air space in wood 28 Preparation of timber for treatment 30 Air-seasoning 32 Mechanical preparation 38 Conditioning processes 39 Rate of temperature change in wood and various factors affecting the results. _ 53 Heating medium 53 Moisture content 54 Rate of temperature change in transverse and longitudinal directions 54 Effect of specific gravity on the rat«' of temperature change in wood 55 Effect of snow or ice on rate of heating 55 Effect of vacuum on rate of heating 55 III Page Difference in temperatures obtained at tops and butts of long poles and piles 56 Compensating for the time needed to bring the heating medium up to the required temperature 56 Temperature computations 56 Temperatures required to sterilize wood 67 Inj ecting preservatives 67 Influence of viscosity and temperature of preservative 67 Relative penetration of preservative oils and water solutions 81 The use of vacuum 81 Preliminary air pressure 86 Preservative pressure 88 Pressure period 89 Kick-back 90 Final heating and vacuum, using steam or expansion bath 93 Absorption and penetration 94 Full-cell and empty-cell absorptions 95 Relation of dimensions of timber to absorption and penetration 96 Measurement of absorption 107 Effect of treatment on the strength and properties of the wood 110 Effect of preservatives 110 Variables affecting the strength properties HI Effect of steaming on strength and physical condition of the wood 112 Effect of Boulton process on strength 113 Bleeding of treated wood 114 Treating conditions used in commercial practice 115 Specifications for treatment 123 General considerations 123 Avoiding injurious treating conditions 123 Selection of treating process 125 Unnecessary requirements 126 Retentions 127 Penetration 133 Framing and boring 134 Appendix 135 Symbols used in formulas relating to physical properties of wood 135 Formulas relating to physical properties of wood 136 Method of using figures when finding temperatures to be expected in tim- bers heated under any given conditions 138 Adjusting for different heating-medium temperatures 142 Temperatures in round timbers 143 Temperatures in sawed timbers 147 Symbols and formulas relating to temperature change in wood 149 Literature cited 152 Index 156 IV INTRODUCTION The principal wood-destroying agencies are the decay fungi, insects, marine borers, and fire. Preservative treatment, which is used to retard or prevent the operation of these agencies, has made wood an economical material to use in many fields, whereas without such treatment its short life and consequent high cost make its use prohibitive. Pressure treatment with preservatives suitable for the particular service requirements is now widely employed for material such as ties, poles, piling, posts, bridge timbers, mine timbers, cross arms, conduits, and lumber and timbers used in buildings where conditions are favorable for attack by decay or insects. It is also employed in the impregnation of wood with fire-retarding chemicals. Pressure treatment when intelligently applied results in lower annual cost, more satisfactory service, and a decreased drain upon the forest resources. The effectiveness and economy of preservative treatment depend on the qualities of the preservative and the thoroughness of the treatment. Good treatment is economical, but poor treatment, whether resulting from improper choice of preservative, improper specifications, low preservative retentions, or insufficient penetration, is expensive and may prove disastrous. The problem of adequate treatment is complicated by many factors, including the species, size, form, condition, and proposed use of the timber; kind and amount of preservative to be injected; details of the treating process employed; and the skill of the treating-plant operator. The methods now in use for the injection of preservatives by pressure ^ are the result partly of technical research and partly of the accumula- tion of more than a century of world experience in wood preservation. By the proper use of these methods it is practicable for purchasers to obtain adequately treated timber that can be depended on for long life under the most severe conditions of exposure. The Forest Products Laboratory has done much work on the many technical problems involved in the pressure treatment of wood, including numerous experiments and observations at commercial treating plants. The purpose of this publication is to discuss the application of the re- sults of these experiments and observations, and to present general information that will be of value to engineers, treating-plant operators, inspectors, and others interested in pressure-treating processes and in the preparation of specifications. ^ The Treatment of timber by open-tank ar^d other nonpressure methods is dis- cussed in U. S. Department of Agriculture Farmers' Bulletin 2049, {5a). Italic numbers in parentheses refer to Literature Cited, p. 152, 2 AGRICULTURE HANDBOOK 40, U. S. DEPT. OF AGRICULTURE PRESSURE PROCESSES The most effecti\T method of treating wood with preservatives is by means of pressure.^ There are a number of pressure processes, all of \\'hich employ the same general principle but differ in the details of application. The timber to be treated is loaded on tramcars, which are run into a large steel cylinder. After the cylinder door is closed and bolted, preservative is admitted and pressure applied until the required absorption has been obtained. Two principal types of pressure treat- ment, the full-cell (Bethell) and empty-cell (Lowry and Rueping), are in common use. The essential characteristics of these processes are de- scribed below; their fields of usefulness are discussed on pages 95, 125, and 126. FULL-CELL PROCESSES In making treatment« with the so-called '^full-cell'' or Bethell process, a preliminary vacuum is first applied to remove as much air as practicable from the wood cells. The preservative is then admitted into the treating cylinder without admitting air. After the cylinder is filled with pre- servative, pressure is applied until the required absorption is obtained. A final vacuum is commonly applied immediately after the cylinder has been emptied of preservative to free the charge of dripping preservative. When the timber is given a preliminary steaming-and-vacuum treat- ment (p. 39), the preservative is admitted at the end of the vacuum period following steaming. In case the charge has received a preliminary conditioning treatment by the Boulton or boiling-under-vacuum process (p. 44), the unfilled space at the top of the cylinder is filled with preserva- tive and pressure is applied as soon as this conditioning process has been completed.
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