Efflorescence

In this case, it turned out to be a sign of a serious water intrusion problem. But sometimes it's harmless.

Efflorescence is caused by water migrating through the concrete, mortar, or in the case of a home in Piedmont, California, through the brick. As the water travels, it picks up mineral salts, which then dry on the surface. It's often found on newly built floors or walls, and sometimes on older outside surfaces exposed to rain or water. In those cases, it's not usually a sign of trouble, and will probably weather away.

But if you find persistent efflorescence on an interior surface, it's probably caused by a leak or water penetration through the floor or wall. And over time, the leak can destroy brick, mortar or even concrete. In that case:

• Find and stop any source of moisture. Check downspouts, flashing, and grading. Then, try removing the efflorescence with just a stiff brush and water.

• If that doesn't work, try a solution of water and a solvent called muriatic acid, available at any hardware store.

But if you suspect that you've got problem efflorescence, it's a good idea to get a professional evaluation first, to help you track down its source.