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.
274. The outside of a building, when faced with pressed brick, should be cleaned down soon after completion. This is done by washing and scrubbing the walls with muriatic acid and water, in the proportion of from 15 to 20 parts of water to 1 of acid. This is applied with scrubbing brushes or corn brooms, and the operation should be continued until all stains are removed.
While the cleaning is in progress, the open joints under all window sills, as also the joints in stone and terra-cotta work, should be pointed, so that the wall will be left in perfect condition when the cleaning has been completed.
275. The face of brick buildings, after a heavy driving storm of rain or damp snow, are often covered with a white efflorescence, which is very unsightly. This efflorescence is due either to soda in the bricks, which is drawn out by capillary attraction; to pyrites in the clay, which when burned form sulphuric acid, which unites with the magnesia in the lime mortar; or to salt in the sand. The efflorescence seems to appear after the bricks have been thoroughly saturated with moisture, either when laid or when exposed to a driving storm. 2-11
Boiled linseed oil prevents this discoloration if applied every three to five years. Any of the waterproof preparations mentioned under "Dampness in Walls" will keep the efflorescence off. Washing the wall in diluted muriatic or nitric acid will remove it; when the brickwork is dry, the oil or waterproof material may be applied.