TREATMENT OF WOOD BLOCKS.
The most serious objection commonly raised to the older type of wood pavement is that wood, being porous, absorbs moisture readily, and is thus liable both to destruction through decay and to become injurious to health. Various methods were therefore proposed for rendering the blocks less pervious and more durable by impregnating them with various substances which fill the pores and act as preservatives. The earlier attempts in this direction were not in the main success ful and little seemed to be gained in durability by the treatment. Solutions of mineral salts were tried but were found unsuitable for the purpose. Creosoting the blocks, which consists in impregnating the wood with the oil of tar, or creosote, was more successful, but with the type of construction in use seemed of doubt ful economic value. In this process the wood is first thoroughly dried, usually by heating it in a kiln, and the hot creosote is then forced in under pressure. The method of accomplishing this varies in different places. In order to be effective the process must be thoroughly carried out and the pores well filled. It has been commonly recommended that from 8 to 12 pounds of creosote per cubic foot of timber should be forced in, as a minimum requirement for the softer woods, such as are commonly used in pavements. Creosote has the property of destroying the lower forms of animal life, and is therefore an effective preservative against destruction through these agencies where they exist. This method is therefore often employed for the preservation of timber for subaqueous construction in sea-water.
This process, when properly applied, is effective in preventing decay, and therefore in lengthening the natural life of the wood. It also renders the wood practically impermeable, and thus removes the objec tion to the pavement based upon its absorbent nature. It does not, in general, appear to increase the resistance of the wood to the wear of the traffic, and in most cases the advantage to be gained seems so small as to render its economic value for this purpose at least doubtful.
In more recent practice, the treatment has been directed to the hardening of the block as well as the prevention of decay. This has been accomplished by modifying the character of the material with which the wood is impregnated through the introduction into the creosote of some substance which hardens in the pores of the wood, thus waterproofing the timber and hardening its fibres. Experience seems to indicate that treatment by the improved methods considerably increases the life of the timber and its resistance to wear, and this has resulted in a considerable revival of the use of wood-block pavements, which had for the most part ceased.
The methods most commonly employed in the United States for the treatment of blocks are the Kreodone process and the Creo-resinate process. The Kreodone process consists in impregnating the blocks with an oil derived from creosote oil, which possesses the original preservative properties and forms a water proof coating on the exterior of the wood. In the Creo-resinate treatment, the wood is impregnated with a substance consisting essentially of a mixture of the oil of tar with resin, the resin acting as the water proofing and hardening material. The amount of resin required varies from about 25 per cent to 50 per cent of the mixture, and depends upon the character of the oil used, a heavy dense oil requiring less resin than a lighter and more volatile oil. The method of treat ment is thus described by Mr. F. A. Kummer:* "Blocks, after being cut to size, are placed in circular cages made of band steel of approximately the di ameter of the cylinders in which the treatment takes place, and are then, while in these cages, run into the cylinders on cars. The cylinders themselves are usually about 6 feet in diameter and somewhat over Too feet long, and are provided with steam coils along the bottom and sides to provide heat for . drying and preparing the lumber for treatment. The blocks are heated in this way, some works employing live steam instead of steam coils, and others a combination of the two. After several hours both by the use of heat and by the use of a vacuum pump, a large portion of the moisture and light volatile oils in the wood, if the latter contain any such, are driven off. The preservative material is run into the cylinder under a vacuum and hydraulic pressure of 200 lbs. per square inch, applied from two to three hours, or for such longer period of time as may be necessary to thoroughly treat the charge, the result being accomplished when the gauges show that no more material is entering the wood. " The specifications for treated blocks used by Mr. Tillson in New York City in 1908 are as follows: "The blocks are to be treated throughout with an antiseptic and waterproof mixture, not more than 75 per cent of which shall be creosote or heavy oil of coal tar conforming to the specifications hereinafter set forth. All parts of each individual block shall be thoroughly treated, and not less than twenty (20) pounds of the mixture per cubic foot shall be injected.