Methods of Treatment.—Bef ore treatment all bark should be removed and the ends of the timber cleansed. It is important to note that the inner bark either stops or retards lateral penetration, which is one of the most difficult obstacles to be overcome. All processes, with the exception of the Boucherie and Powell pro cesses, require that the timber should be seasoned. The degree of seasoning required will vary with the process, the class of tim ber to be treated and the use to which it is to be put. The moisture in wood occurs in the pores and cavities between the fibres and in the cell walls; the former being termed free-water and the latter sorbed water. As the timber seasons, the free-water is first given off until there remains from 25% to 3o% by weight of the sorbed water. The moisture content should be reduced to below 2o%, so that all the free, and part of the sorbed, moisture disappears, leaving the cavities free to take up the antiseptic.
The methods by which timber may be antiseptically treated fall under three heads, namely, the brush method, treatment in open tanks and treatment in closed cylinders under pressure.
By the open tank method the timber is immersed either in coal-tar creosote or a salt solution, in a tank which is heated by placing a fire beneath it, or preferably by means of steam coils. The antiseptic is gradually raised to a temperature of 9o° C to 95° C, maintained at that temperature for a period dependent on the thickness of the material under treatment, and then allowed to cool down to atmospheric temperature. The heating period should, generally speaking, be maintained for an hour for every inch in thickness of the material. When the timber is heated in the solution it expands, and air is expelled from the cellular and intercellular spaces ; as the timber cools that air is replaced by antiseptic. Fair penetration may be obtained by this process, which in pine-wood sleepers containing a moderate percentage of sapwood, amounts to approximately 5 lb. per cubic foot. This method of treating timber is applicable to cases where moderate quantities of timber have to be dealt with. A process by which wood is treated with saccharine solutions, known as the Powell process, is carried out in open tanks; the timber is boiled in a diluted solution of sugar, to which a small percentage of arsenic is added for protection, in tropical countries, against the attacks of white ants. The process consists in boiling green timber for several hours, the time varying according to the dimensions and species of wood under treatment. It is then allowed to cool down and is placed in a drying chamber. This process has been success fully employed in Australia, while extens've tests carried out with Powellized sleepers on Indian railways give good results.
inder or storage tank is generally provided, which is generally placed above the pressure cylinder. Surrounding the treating plant is the timber yard, which is conveniently laid out with feeder lines to and from the treating cylinder. The Full-cell or Bethell process, which employs coal-tar creosote, consists in subjecting the seasoned timber to a pressure of too to i8o lb. for one or more hours, after which the oil is run off and the timber allowed to drip. In some cases a vacuum is drawn for a short period, to hasten the removal of the coal-tar creosote left on the surface of the timber. Some operators when dealing with green timber, first subject it to a steaming period of several hours, and then draw a vacuum for one or two hours, in order partially to remove the free moisture in the wood, but this does not result in such satisfactory penetration as does the treatment of seasoned timber. Treatment by a full-cell process results in an absorption of from 7 to 15 lb. of coal-tar creosote per cubic foot.
The open-cell process may be carried out according to two different methods; one method was patented by C. B. Lowry, by which the coal-tar creosote is run into the cylinder at a tempera ture of 200° F, and a pressure of from 180 to zoo lb. is applied, until no more oil can be forced into the timber, i.e., until the tim ber has been treated "to refusal." The pressure is then released, the oil run off, and a vacuum drawn, until the oil in the inter cellular spaces has been removed ; this oil may amount to 40% of the original absorption, leaving some 6 to 7 lb. per cubic foot behind in the timber. The other open-cell process, discovered by Riiping, aims at as deep a penetration as possible and at the same time economizes, recovering, as does the Lowry process, a portion of the oil originally introduced. To carry out this process a second cylinder, capable of withstanding too lb. air pressure, is placed at a higher level than the treating cylinder. The air-dried timber is placed in the treating cylinder and subjected to an air pressure of 6o to 8o lb. per square inch. The high level cylinder is filled with the preservative to within a few inches of the top, and an equal air-pressure is applied to its upper surface. The antiseptic is then allowed to flow by gravity from the upper to the lower cylinder, the air displaced in the latter being led into the former by a by-pass. The two cylinders are then disconnected and the pressure in the treating cylinder is gradually raised to 15o lb. per square inch and held there until to to 12 lb. per cubic foot have been absorbed. The pressure is then released and the oil run off; this results in a partial vacuum being set up, and allows the com pressed air in the wood to expand and to force out the surplus oil.