Q: We have an attractive free-standing gas fireplace that is four years old. The top is ringed in black slate. When the fireplace is turned on, the slate turns a discolored light gray. The fireplace installer said it's normal for the slate to get slightly hot and to use dishwashing liquid to restore its color and finish when it cools down. We did this, but when we turned on the fireplace, the slate turned a more pronounced light gray, almost white, and it retained this color after it cooled off. What can we do to restore the discolored slate?
Lewes, Del.
A: “There is such a thing as fading slate,” said Chuck Muehlbauer, technical director of the Natural Stone Institute, a trade association that offers training and technical advice about the use of stone in many building applications. “But this doesn’t sound like that.”
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Slate is a metamorphic rock consisting of numerous minerals. Depending on the mix, it can be black, gray, blue-gray, red, purple or even green. Some slates quickly fade to softer colors after they’re installed on roofs or pavement, while other kinds of slates, which the industry classifies as “unfading,” retain their original color. The natural fading of some slates isn’t likely to be the cause of your problem because that color change is triggered by ultraviolet light, Muehlbauer said.
Your slate was probably coated with a sealer or color enhancer, and that’s what’s turning white or gray in response to heat. Sealers make stone less porous. Color enhancers, which often double as penetrating sealers, darken stone and make the color look more vibrant, especially if the stone has dulled from weathering or wear.
“Sealers have been correctly marketed and also over-marketed,” Muehlbauer said. “There are a lot of applications where it was applied where it didn’t need to be.” The only reason to seal stone, he said, is to make porous stone more stain-resistant. Slate varies in how porous it is, so for a slate countertop — as for countertops made of many other types of stone — sealing may make sense. But for slate surrounding a fireplace, there is no reason to add a sealer, Muehlbauer said.
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Installers are typically the ones who apply sealers and color enhancers after the stone is installed. But distributors sometimes do this before the slabs are sold to fabricators or homeowners. Because the slate pieces on your fireplace are small, it’s possible the installer used stone left over from another job and may not have known whether the stone was already sealed or treated with a color enhancer.
Unfortunately, there is no easy way to determine what type of product may have been used on your stone or what will take it off. You would need to test — and even then, it can be difficult to tell whether a stripper is working. “You’re not going to see the residual sealer come off on your rag,” Muehlbauer said. So first wipe water across the stone, then see how much it beads up. Then apply the stripper, following instructions on the label. When the stone is dry, wipe water across it again. If the stripper worked, more of the water should soak in and less should bead up. If there is only a little change, you might need to repeat the stripping process — or switch to a different stripper.
Water-based products are easiest and safest to use, but they aren’t effective against all sealers. StoneTech Epoxy Grout Haze & Coating Stripper ($35.10 per quart at the manufacturer’s website, laticrete.com) is a gel formula, which would be easier to control on a vertical surface like you have, and it’s listed as suitable to use on slate. It’s acidic, so wear rubber gloves and goggles.
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If a water-based stripper doesn’t work, you might need to use a solvent such as acetone or toluene, Muehlbauer said. Acetone is considered less toxic than many other industrial solvents, but it and toluene are both highly flammable, and breathing even moderate amounts of the fumes can cause health problems. Extinguish all flames first, open windows and doors, wear goggles and gloves, and apply only a small amount at a time.
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