PRESERVATION OF WOOD. A wood pavement fails through wear and decay. A number of methods have been in vented for increasing• the durability of timber against decay; and although these methods have been employed to a considerable extent for preserving piles, railroad ties, and bridge timbers, they have been used only to a limited degree in this country for pre serving paving blocks.
Experiments in wood preserving date back some centuries, and the list of substances experimented with seems nearly endless; but there are only a few antiseptics that have stood the test of time or have been worked commercially. According to the antiseptic employed, the principal methods of preserving timber may be grouped into four classes: The only one of these methods that is suitable for preserving wood paving-blocks is the last, since the mineral salts used by the others wash out. There are various methods of using the above anti septics, as applying externally with a brush, steeping or immers ing, exposing to vapors, injecting in closed tanks under pressure, etc.; but the only method employed for paving blocks is that of injecting the creosote under pressure.
The timber to be preserved should be thoroughly seasoned. It is then put into a closed cylinder, and the air is exhausted. Hot creosote is allowed to flow in; and when the cylinder is full, a force-pump is applied and the pressure raised to 150 or 200 pounds per square inch. The wood remains under pressure until it has absorbed the requisite quantity of oil, as indicated by a gage on the tank. For protection against decay, it is necessary to inject from 8 to 12 pounds per cubic foot. The woods which are best adapted to this treatment are those which are most absorbent, and therefore the easiest and quickest destroyed, as the gums and cottonwoods. Cypress, cedar, pine, and porous oaks are
absorbent and can be successfully treated. Creosoting is quite expensive owing to the cost of the creosote and to the expense of injecting it, the cost being from $12 to $18 per 1,000 feet, board measure.
Creosoted paving blocks have been laid during the last five or six years in several Western and Southern cities, notably Indian apolis, Terre Haute, Galveston, and New Orleans. In Indian apolis the experience of six years' service has been highly satis factory, the pavement showing increased popularity over all other forms of roadway. Creosoted wood blocks are exclusively used in Paris and much used in London.
Creosote is a good preservative, but evaporates and is washed out by the rains. To remedy these defects, two modifi cations of the above process have been recently introduced. One is called the kreodone-creosote and the other the creo-resinate process.
Samples of this pavement were laid in 1901 in Chicago on Michigan Avenue in front of the Auditorium Hotel, and in Indian apolis on North Delaware Street.