This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
115. After pointing, it is usually necessary to remove the mortar stains, etc. from the face of the wall. This may be done by washing the stonework with a brush dipped in water containing muriatic acid, in the proportion of about 20 parts of water to 1 part of acid. For cleaning granite-, marble, and limestone, wire brushes are used; for sandstones and other soft stones, stiff bristle brushes will serve the purpose.
The stonework should be scrubbed until all mortar stains are removed.
The sand blast, worked either by steam or compressed air, does the work of cleaning walls very effectively and rapidly; it removes from 1/64 to 1/32 inch of the discolored stone, leaving a fresh, bright surface. Even fine carvings have been very successfully cleaned by this method.
116. The durability of masonry may be somewhat increased by covering the exposed surfaces with a preservative, but none of the numerous preparations for protecting stonework are cheap or satisfactory.
Lead-and-oil-paint is the most generally used for this purpose, but, while it may be temporarily effective, it spoils the appearance of the stonework and requires frequent renewals, owing to the action of rain and other atmospheric influences.
Boiled linseed oil is also sometimes used, but darkens the color of the stone. To apply the oil, the surface of the stone is first washed clean and dried; the wall is then covered with one or more coats of oil, and finally washed with weak ammonia, which makes the coloring more even. Oil thus applied will last 4 or 5 years.
Another preparation is paraffin containing creosote, dissolved in turpentine. The purpose of the creosote is to prevent vegetable growths on the stone. Before applying the preparation, the stone should be heated in some manner; the melted compound is then applied with a brush. It will penetrate some kinds of stone to a depth of 1/2 inch.
117. Sylvester's process consists in the application of two washes, the first composed of a hot solution of Castile soap in water, and the second of alum water, which is applied about 24 hours after the soap solution. This process has been found more or less successful, when the stone is not subjected to great variations in temperature.
A method, known as Ransome's process, has been used in England with good results. It consists in the application of a solution of silicate of soda or potash to the clean surface of the stone, until it has been thoroughly saturated. When this has dried, a solution of chloride of calcium is applied, the effect of which is to produce an insoluble silicate of lime, thus forming a waterproof coating. (See also Arts. 277 and 378, Masonry, §7.)