This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
Damp cellar walls are due either to water soaking through from the outside, or to wet bottoms, from which water rises in the walls by capillary action. The decay of vegetable matter contained in dirty water generates gases injurious to health, so that the prevention of dampness in walls is a point of great importance. Provision must be made to convey the water away from the walls, as well as to make the latter damp-proof. No earth should be placed against the wall, a 12" or 18" space next to it being filled with broken stone or gravel, with, if practicable, an open-jointed tile drain laid at the bottom, as shown in Fig. 13. The outside of the walls and footings should be plastered thickly with 1-to-l cement mortar; or, preferably, with asphalt and coal tar, mixed in the proportion of 9 to 1, and applied, while hot, about | in. thick to the dry walls. The asphalt course should also extend through the walls, as shown at the under side of the concrete floor, which should be 3 or 4 in. thick, and laid on a 6" or 8" bed of broken stone. In place of concrete, a very durable composition, made of 60 parts of hot asphalt, 10 of coal tar, and 30 of sand, may be used.
The ascent of moisture in walls may be prevented by inserting on the footing course two courses of roofing slate, or very hard brick, laid with broken joints in cement mortar, or a few layers of tarred felt may be used.
Efflorescence, or the white coating frequently seen on the outside of stonework and brickwork, is caused by the deposition of soluble matter in the mortar, which, being dissolved in the water used in mixing, is deposited upon the surface of the wall, as the water evaporates. This efflorescence may be removed by washing the walls with a dilution of muriatic acid in 20 parts of water. When clean and dry the walls may be coated with a preservative; one of the commonest being boiled Unseed oil, which, applied in 2 coats, and, when dry, washed over with weak ammonia, will be effectual for 2 or 3 years before needing renewal. Lead and oil paint is sometimes used, but is objectionable, as it changes the color of the masonry, and also flakes off. Cabot's brick preservative is used to waterproof brick and sandstone, and is effectual where the heart of the wall is kept dry.
Sylvester's process, which has proved quite successful, and is simple in preparation and application, consists of two washes, the first being made of Castile soap and water, in the proportion of 3/4 lb. of soap per gallon of water, and the second of 1/8 lb. of alum per gallon. The soap wash is applied, boiling hot, with a brush, to the clean and dry walls, and allowed to dry 24 hours before the alum wash - which need not be hot - is put on in the same manner. The coats are applied alternately 2 or 3 times, making the wall practically impervious. All ordinary cements will cause efflorescence, and when such are used in stonework, the face stones should be set entirely with Vicat, Lafarge, or some other non-staining cement mortar, mixed with sand in the proportion of 1 to 2. If this method be too expensive, the joints of the stonework may be raked out to a depth of 1 in., and pointed with a mortar made with one of these cements. Good lime mortar is often used in setting ashlar, and by some is claimed to be practically as good for the purpose, and considerably cheaper than the cements named.