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Q.Some of the brickwork on a house I built has developed a white stain that resembles efflorescence but is limited to only two areas: a mailbox and a window. The gutter above the window overflows occasionally, but the water doesn't appear to actually fall against the brick. What's causing the stain, and how do I get rid of it?

A.Mike DeBlasio, a masonry contractor in Littleton, Mass., responds: Efflorescence occurs when soluble salts in the masonry are dissolved by moisture being driven through the material via vapor transmission or hydrostatic pressure. When the solution reaches the surface and evaporates, a salt deposit — efflorescence — is left behind.

However, with efflorescence you'd see more widespread staining at grade and above. The problem you describe is more likely caused by contact between aluminum and the brick and mortar. Mortar is highly alkaline; it will react with an aluminum mailbox or window frame, breaking down the metal and causing staining.

To prevent direct contact and reduce the staining, you should cut the aluminum back, away from the masonry, and install a proper sealant joint. Unfortunately, I don't know of any stain remover that will fix this problem. (Don't use muriatic acid; it will attack and damage the mortar.)