The Washington Post - 19.09.2019

the washington post

.

thursday, september

19

,

2019

DC

2

BY JEANNE HUBER

Q: We have a 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,” s aid Chuck

Muehlbauer, technical director

of the Natural Stone Institute.

“But this doesn’t sound like that.”

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,” r etain

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.

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. StoneTe ch Epoxy Grout

Haze & Coating Stripper ($35.10

per quart at laticrete.com) is a gel

formula, which would be easier

to control on a vertical surface

like you have. 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, open windows and doors,

wear goggles and gloves, and

apply a small amount at a time.

 Have a problem in your home?

Send questions to

[email protected]. Put “How

To” in the subject line, tell us where

you live and try to include a photo.

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Restoring discolored slate around a fireplace

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A reader wants to know why the slate on this fireplace turns a lighter color when the fireplace is in use.

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