Paint. - One of the most common methods of preserving the surface of stone is to paint it. This is effectual for a time, but the paint is destroyed by atmospheric influence in the course of a few years. "In London the time hardly amounts to three years even under favourable circumstances."1 Moreover, it cannot well be used in important buildings where appearance has to be considered.
Oil has also been used as a coating; it fills the pores of the stone and keeps out the air for a time, but it discolours the stone to which it is applied.
Paraffin is more lasting than oil, but is open to the same objection as regards discoloration of the stone.
Softsoap dissolved in water (3/4 lb. soap per gallon), followed by a solution of alum (^ lb. alum per gallon), has been frequently employed.1
"l1/2 lb. paraffin to a gallon of coal tar naphtha, and applied warm, is perhaps superior to both the former for this special purpose."
"There is, however, no evidence to show that any methods such as these are likely to be successful in affording permanent protection to stone."2
Beeswax dissolved in coal tar Naphtha has also been proposed,1 or, when the natural colour of the stone is to be preserved, white wax dissolved in double distilled Camphine.
The surface should be cleaned with water dashed with hydrochloric acid, but should be perfectly dry, the solution applied hot and thin.