Question: White efflorescence stains are appearing at random locations on a brick wall. I know efflorescence is salt drawn to the surface by water. But how do I determine the source of the salts, and why are the salts deposited randomly?

Answer: To determine the source of the salt, you should have the efflorescence salt deposits chemically analyzed by a testing laboratory, You also can do simple tests in the field. For example, if a drop of acid placed on the salt fizzles, it's probably calcium carbonate. Once you know what the salt is, you can determine its probable source using the table shown here from the Brick Institute of America (BIA). But you can't ever be totally certain. Water entering the wall also can carry salt with it. Why is the efflorescence at random spots? It's probably not because the masonry contains random salt deposits. It's more likely the water drawing the salts to the surface is entering or exiting the masonry at random locations. This could be because at random spots mortar joints were partially filled, mortar beds were furrowed too deep, flashing or caulking was not properly installed, brick was picked up and relaid (thus breaking its bond), or the wall was not protected from weathering.
Common Sources of Efflorescence
Principal Efflorescing Salt Chemical Formula Most Probable Source
Calcium sulfate CaSO4·2H20 Brick
Sodium sulfate Na2SO4·10H20 Cement-brick reactions
Potassium sulfate K2SO4 Cement-brick reactions
Calcium carbonate CaCO3 Mortar or
concrete backing
Calcium hydroxide Ca(OH)2 Cement
Sodium carbonate Na2CO3 Mortar
Potassium carbonate K2CO3 Mortar
Potassium chloride KCl Acid cleaning
Sodium chloride NaCl Seawater
Vanadyl sulfate VOSO4 Brick
Vanadyl chloride VOCl2 Acid cleaning
Manganese oxide Mn3O4 Brick
Iron oxide Fe2O3 or Fe(OH)3 Iron in wall
Source: "Efflorescence Prevention and Control, Part II," Technical Notes on Brick Construction, 23A, Brick Institute of America, 11490 Commerce Park Dr., Reston, VA 22091.