Heavy White Residue on a Brick Chimney
It’s called efflorescence, and it appears as a white residue on bricks. It’s caused by moisture that draws mineral salts from the center of the bricks to the surface. The residue itself isn’t harmful, and it’s commonly found in small quantities on many types of brick.
But heavy, thick, solid white efflorescence is an indication that moisture is condensing on the inside of the chimney and seeping outward. This could be caused by a cracked flue liner, which increases the risk of toxic fumes entering your home, or it could be the result of an interior plumbing leak, but whatever the cause, have a professional chimney sweep inspect the chimney.