180. There are a great many preparations that have been used for preventing the decay of building stones, but all are expensive, and none have proved very satisfactory.

Paint. - The substance most generally used for preserving stonework is lead and oil paint. This is effectual for a time, but the paint is destroyed by the atmospheric influences, and must be renewed every three or four years. The paint also spoils the beauty of the stone.

The White House at Washington is built of a porous red sandstone, which has been painted white for many years.

Oil. - Boiled linseed oil is sometimes used on stonework, but it always discolors a light-colored stone, and renders a dark-colored one still darker. "The oil is applied as follows : The surface of the stone is washed clean, and, after drying, is painted with one or more coats of boiled linseed oil, and finally with a weak solution of ammonia in warm water. This renders the tint more uniform. This method has been tried on several houses in New York City, and the waterproof coating thus produced found to last some four or five years, when it must be renewed. The preparation used in coating the Egyptian obelisk in Central Park is said to have consisted of paraffine containing creosote dissolved in turpentine, the creosote being considered efficacious in preventing organic growth upon the stone. The melting point of the compound is about 1400 F. In applying, the surface to be coated is first heated by means of especially designed lamps and charcoal stoves, and the melted compound applied with a brush. On cooling it is absorbed to a depth dependent upon the degree of penetration of the heat. In the case of the obelisk about inch." *

A soap and alum solution has also been used for rendering stone waterproof, with moderate success.

Ransome's Process. - This consists in applying-a solution of silicate of soda or potash (water glass) to the surface of the stone, after it has been cleaned, with a whitewash brush until the surface of the stone has become saturated. After the stone has become dry a solution of chloride of calcium is applied freely so as to be absorbed with the silicate into the structure of the stone. The two solutions produce by double decomposition an insoluble silicate of lime, which fills the pores of the stone and binds its particles together, thus increasing both its strength and weathering qualities. This process has been used to a considerable extent in England, and is perhaps the most successful of all applications. The process of applying the solutions is more fully described in "Notes on Building Construction," Part III., p. 78.

*"Stones for Building and Decoration," pp. 399-400.