Book 27, Question 66
Why do white fluffy crystals grow on brick walls?
Bricks and concrete hold tiny amounts of salts.
When water seeps through the wall, it dissolves some salt. At the surface, the water evaporates and leaves the salt behind as a white, fluffy coating of crystals. This is called efflorescence. It's harmless but tells builders that moisture is moving through the wall. It can be brushed off when dry.