A.Contributing editor Michael Byrne, a tile-setter and consultant in Los Olivos, Calif., and moderator of JLC Online's tile forum, responds: The quality and density of slate can vary quite a bit. Honed slate tiles dense enough to be exposed to water don't absorb sealers readily, whereas some softer slates with cleft finishes will actually start disintegrating in water.

And while not all penetrating sealers perform well with slate, top-coating sealers can lead to disaster when they yellow and begin to chip off.

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