This historic masonry term refers to the process where moisture migrating through a brick wall brings dissolved soluble salts to the surface. As the water evaporates, it leaves a crystalline white deposit behind. Trades refer to this condition as efflorescence, which requires specific chemical cleaners to remove safely without damaging the mortar joints.
Heard on the job
The whole north wall started to whitewash after that wet spring, so we had to scrub it down before sealing.