METHODS OF PRESERVING. Vitruvius, the Roman architect, two thousand years ago recommended that stone should be quarried in summer when driest, and that it should be seasoned by being allowed to lie two years before being used, so as to allow the quarry sap to evaporate. It is a notable fact that in the erection of St. Paul's Cathedral in London, England, Sir Christopher Wren required that the stone, after being quarried, should be exposed for three years on the sea-beach before its introduction into the building.
The methods of dressing a stone have an important bearing upon its durability. If the surface is finished with a tool similar to the bush hammer (Fig. 44, page 270) or the patent hammer (Fig. 46, page 271), the heavy blows deaden the face of the stone, i.e., break the grains and produce minute fissures, and render it much more suscept ible to the action of frost. Granite and other compact crystalline rocks are most durable with a rock-face finish, i.e., a surface untouched by chisel or hammer; while the softer and more absorbent stones are usually most durable when finished with a sawed or rubbed surface.
It has already been stated that, in order to resist the effects of both pressure and weathering, a stone should he placed on its natural bed. This simple precaution adds considerably to the durability of any laminated stone.
Many methods have been devised for preventing or checking the action of the weather upon building stones; but none of them are satisfactory or very efficient. These preservatives consist of some liquid into which the stone may be clipped or which may be applied with a brush to its outer surface, to fill the pores and prevent the access of moisture. Paint, coal tar, linseed oil, paraffin, and nu merous chemical preparations have been used.* As an example of a simple and comparatively efficient preparation used for this purpose, see 1l 379 and 642.
Another method of treatment consists in bathing the stone in successive solutions, the chemical actions bringing about the forma tion of insoluble silicates in the pores of the stone. For example, if a stone front is first washed with an alkaline fluid to remove dirt, and this followed by a succession of baths of silicate of soda or potash, and the surface is then bathed in a solution of chloride of lime, an insoluble lime silicate is formed. The soluble salt is then washed
away, and the insoluble silicate forms a durable cement and checks disintegration. If lime-water is substituted for chloride of lime, there is no soluble chloride to wash away.
A large number of tests have been applied to the building stones of the United States. For the results and details of some of the more important of these tests see: Report on Strength of Building Stone, Gen. Q. A. Gillmore, Appen. II, Report of Chief of Engineers, U. S. A., for 1875; Tenth Census of the U. S., Vol. X, Report on the Quarry Industry, p. 330-35; the several annual reports of tests made with the U. S. Government testing machine at the Watertown (Mass.) Arsenal, published by the U. S. War Department under the title Report on Tests of Metals and Other Materials; Transactions of the American Society of Civil Engineers, Vol. II, p. 145-51 and p. 187-92; ibid., Vol. XXXIII, p. 233-56; Jour nal of the Association of Engineering Societies, Vol. V, p. 176-79, Vol. IX, p. 33-43; Engineering News, Vol. XXXI, p. 135 (Feb. 15, 1884); The Materials of Construction, J. B. Johnson, John Wiley & Sons, New York, 1897, p. 630-51; Notes on the Compressive Resistance of Freestones, Brick Piers, and Hydraulic Cement Mortars and Con cretes, Gen. Q. A. Gillmore, John Wiley & Sons, New York, 1888; and the reports of the various State geological Surveys, and the commissioners of the various State capitols and of other public buildings.
By way of comparison the following reports of tests of building stones of Great Britain may be interesting: Proceedings of the Institute of Civil Engineers, Vol. CVII (1891-92), p. 341-69; abstract of the above, Engineering News, Vol. XXVIII, p. 279-82 (Sept. 22, 1892).
In consulting the above references or in using the results, the details of the manner of making the experiments should' be kept clearly in mind.