After a heavy driving storm of rain or damp snow the face of many brick buildings will often be seen to be covered with a sort of white efflorescence, which greatly mars the appearance of the brickwork. This efflorescence is due either to soda in the bricks, which is drawn out by capillary attraction to the surface, where it dries out, leaving a white deposit, or to pyrites in the clay, which when burned gives rise to sulphuric acid, which unites with the magnesia in the lime mortar. In either case the efflorescence only appears after the bricks have been thoroughly saturated with moisture, either when laid or by a driving storm, perhaps several years after. According to Mr. Samuel Cabot it is never due to the bricks alone, and seldom to the lime alone. It seems to be impossible to prevent its occurrence except by protecting the bricks by some waterproof or oily solution. After the white appears on the surface it may be washed off with clear water by vigorous scrubbing, and if, after the brickwork has become dry, a good coat of boiled linseed oil is applied, it will prevent the reappearance of the white until the life of the oil is destroyed, usually from three to five years, when another coat may be applied. Any other preparation which renders the bricks impervious to moisture will prevent the efflorescence.
All brick and stone walls absorb more or less moisture, and a wall 12 inches thick may sometimes be soaked through in a driving rainstorm. In the dry climates of Colorado, Arizona and New Mexico such storms rarely occur, and it is customary in those localities to plaster directly on the inside of the walls. In nearly all other portions of the country, however, it is desirable, for the sake of health and economy in heating, if not absolutely necessary, either to furr or strip the inside of solid walls with 1x2-inch strips, or to render the walls damp-proof, either by a coating of some kind applied to the outside of the wall, or by building the wall hollow. Furring the wall with wooden strips and then lathing on them prevents the moisture from coming through the plastering, but it does not prevent the wall itself from becoming soaked, thereby necessitating more heat to warm the building and gradually tending to the destruction of the wall. The hollow wall is probably the best device, when properly built, for preventing the passage of moisture and also of heat, but in most cases it is also the most expensive.
Brickwork may be rendered impervious to moisture either by painting the outside of the walls with white lead and oil or by coating the wall with a preparation of paraffine, or by some of the patented waterproofing processes. The preparations containing paraffine are usually applied hot, and the wall is also heated previous to the application by a portable heater. They give fairly good results, but are quite expensive, owing to the time and labor required for their application.
Sylvester's process, which consists in covering the surface of the wall with two washes or solutions - one composed of Castile soap and water and one of alum and water - has been used with much success for this purpose. A full description of the successful application of this process to the walls of the gate houses of the Croton Reservoir in Central Park, New York, is given in Baker's Treatise on Masonry Construction, pp. 178-180.
All of these preparations change somewhat the color and grain of the brick, and are generally considered as detracting from the appearance of the building.
Boiled linseed oil is often applied to brick walls, and two coats will prevent the absorption of moisture for from one to three years. The oil does not greatly change the color of the brick, and generally improves the appearance of a wall which has become stained or discolored in any way.
Common white lead and oil paint is probably the best material for damp-proofing external walls above ground, but it entirely changes the appearance of the building. Painting of new work should be deferred until the wall has been finished at least three months, and three coats should be given at first, after which one coat applied every four or five years will answer. A preparation known as Duresco and made in England has been used in New York and Chicago for damp-proofing with very satisfactory results. In Chicago it was used for coating the inside of the walls before the plastering was applied to prevent the moisture penetrating the plastering, which purpose it seems to have successfully accomplished.
Duresco, when applied to common or soft brick, not only renders them weatherproof, but the color gives the permanent appearance for which pressed brick are valued. It dries with a hard, uniform, impervious surface free from gloss, and does not flake off or change color. It is put up in 56-pound kegs, that quantity being sufficient for covering 1,000 square feet, two coats.
Cabot's Brick Preservative (made in Boston, Mass.). - It is claimed by the manufacturer that this preparation forms a thorough waterproofing for brickwork and sandstone, thus preventing the white efflorescence, the disintegration of chimneys by frost, and the growth of fungus.
It does not change the natural texture of the material to which it is applied and leaves no gloss. It has been found by actual experiment that one coat of this preservative makes as good a waterproofing as three coats of boiled linseed oil.
The preservative is manufactured in two forms : colorless, for use on any kind of brick to render them waterproof and to prevent the efflorescence, and, with red color added, to bring the bricks to an even shade without destroying the texture.
This material is applied with a brush in the same way as oil, no heat being necessary. To get the best effect the brickwork should first be washed down with acid (preferably nitric acid) to remove any efflorescence already formed. One gallon will cover about 200 square feet on the average rough brick and a little more on pressed brick. One coat is generally sufficient unless the bricks are extremely soft and porous.
To prevent moisture penetrating the top of brick vaults built underground a coating of asphalt, from ½ to ¾ of an inch thick and applied at a temperature of from 3600 to 518° F., seems to give the best results. Common coal tar pitch is often used for the purpose, but is not as good as asphalt. If the vault is to be covered with soil for vegetation the top course of brick should be laid in hot asphalt in addition to the coating.