This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The surfaces of walls are often covered with an efflorescence of an unsightly character, formed by a process known as "saltpetreing." It shows itself chiefly in the case of newly built walls, but also in those parts of older walls which are exposed to damp. It varies somewhat in appearance and chemical composition, and is most apparent in dry weather. It is generally white in colour and crystalline in structure: the crystals presenting the appearance of very fine fibres or needles, or looking like a thin coating of snow or white sugar. Chemical analysis has shown that these crystals vary considerably in composition. They often consist of magnesia sulphate, also of lime sulphate; of soda carbonate, sulphate, or nitrate; of soda and potash chlorides, and potash carbonate. Efflorescence is attributable sometimes to the bricks or stones of a wall, sometimes to the mortar. Dampness is favourable to its formation. Cold as low as the freezing point stops it. In bricks burnt with coal fires, or made from clay containing iron pyrites (iron bisulphide), the sulphur from the fuel converts the lime or magnesia in the clay into sulphates. When the bricks are wet, these dissolve; when dry, they evaporate, leaving crystals on the surface.
The magnesia sulphate is generally found in much greater quantity than the lime sulphate, as it is far more soluble in water. Many limestones contain magnesia; these are acted upon during calcination by the sulphur in the fuel; sulphates are formed, which find their way into the mortar and produce effects similar to those above mentioned. Again, the sulphur acids evolved from ordinary house fires attack the magnesia and lime in the mortar joints of the chimney; these dissolve and evaporate on the surface. The formation of chlorides is nearly sure to take place, if sea sand or sea water be used, or in bricks made from clay which has been covered by salt water. In some situations the formation of the nitrates has been attributed to the absorption of ammonia from the air. The potassium and sodium salts are supposed in many cases to be derived partly from the limestone used for the mortar, and partly from the fuel employed in burning the lime.
Not only does the efflorescence present a disagreeable appearance, but it causes damp patches on the surface of the wall; it will eat through any coat of paint that has been applied after the efflorescence has once commenced, and will even detach small fragments of the materials composing the wall. Prevention in this case is better than any attempt at cure. The best plan is to avoid all the materials above mentioned as likely to give rise to efflorescence. In the case of bricks, clay containing pyrites or much magnesia should not be used; special bricks may be burnt with coke or wood. As regards mortar, the use of limestones containing magnesia to any great extent may generally be avoided. If, however, it doe3 occur in spite of all precautions, the following remedies may be tried, (a) In the case of ashlar work : (1) The surface may be covered with a wash of powdered stone, sand, and water, which is afterwards cleaned off; this fills up the pores of the stone, and temporarily stops the efforescence; when the wash is removed, the saltpetreing will recommence, but in a weaker degree than before.
(2) Painting the surface is sometimes efficacious if it is done before the efflorescence commences. (b) The mortar before use may be treated to prevent it from causing efflorescence: (1) By mixing with it any animal fatty matter; Gillmore recommends 8-12 lb. of fatty matter, 100 lb. quicklime, and 300 lb. cement powder.
(2) Potash salts may be rendered harmless by adding hydrofluosilicic acid.