Romancing the stone
Restoring discoloured slate around a fireplace isn’t easy,
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 discoloured light grey. The fireplace installer said it’s normal for the slate to get slightly hot and to use dishwashing liquid to restore its colour and finish when it cools down. We did this, but when we turned on the fireplace, the slate turned a more pronounced light grey, almost white, and it retained this colour after it cooled off. What can we do to restore the discoloured slate?
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.”
Slate is a metamorphic rock consisting of numerous minerals. Depending on the mix, it can be black, grey, blue-grey, red, purple or even green. Some slate quickly fades to softer colours after it’s installed on roofs or pavement, while other kinds of slates, which the industry classifies as “unfading,” retain their original colour. The natural fading of some slates isn’t likely to be the cause of your problem because that colour change is triggered by ultraviolet light, Muehlbauer said.
Your slate was probably coated with a sealer or colour enhancer, and that’s what’s turning white or grey in response to heat. Sealers make stone less porous. Colour enhancers, which often double as penetrating sealers, darken stone and make the colour 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.
Installers are typically the ones who apply sealers and colour 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 been aware of whether the stone was already sealed or treated with a colour 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 slight 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 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.
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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