This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol5", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
Painting the face of the stone either with boiled oil or oil colour. This method is in use in some of our large towns, and when properly done is certainly effective while it lasts. The objections to it are: Firstly, on the score of expense, as at least four coats are required to give a thoroughly sound skin to the material to be protected, the first and often the second coat also merely sinking in and filling up the pores. Secondly, on the score of durability it is not economical, as a renewal of the paint must be made every three or four years at the outside.
Paraffin in a liquid condition, or in solution in naphtha, has been tried in place of boiled oil and is a more lasting material, but it also has the objections of producing an unsightly greasy or glazed surface, likely to pick up dirt, etc. Soft soap and alum laid on one after another have their advocates, but are not permanent.
The best processes are undoubtedly those in which the applied material enters into chemical combination with the constituents of the stone. These materials are mostly based on the principle of digesting silica in some powerful solvent, such as caustic soda. The carbonates, if a limestone, enter into chemical combination with the silica, forming silicates of lime and magnesia, which have great resisting properties to weather.
To sum up the pros and cons with reference to this much vexed question, - it resolves itself into a repetition of the advice to the architect to avoid all such methods, and rely on a personal examination of the stone intended for the proposed building, and to ensure by direct knowledge and due care, together with a series of tests, that the stone is the best for the purpose, and from the very best bed in the quarry selected.
Finally, many of the tests, weights, etc., herein contained have been obtained from the quarry owners, and the writer has also personally inspected many of the quarries, and verified, where possible, the characteristics of the stones commented on.
The Theory of Arches, Vaults, and Buttresses 117