Green & black stains on ceramic surface of gas fireplace (C) InspectAPedia SH Causes & Cures for Black & Green Stains on Gas Fireplaces

  • GAS FIREPLACE CERAMIC STAINS - CONTENTS: Are these black & green stains caused by mold or something else? Are they dangerous? What are the likely causes of the fluffy-looking dark or black & green deposits on the gas fireplace wall insert shown here? How should the stains be removed & prevented?
  • POST a QUESTION or READ FAQs about stains, soot, mold or other deposits on gas fireplace surfaces & how they are cleaned or prevented
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Gas fireplace stains on ceramic surfaces

This article describes black & green stains found on the ceramic surfaces of a wall-mounted gas fireplace.

We discuss possible causes of these deposits or stains, the safety hazards that might be implied by their presence, and suggest products that might help in removing soot, stains, or other deposits from the gas fireplace ceramic surfaces.

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Mold on Gas Fireplace Ceramic Surfaces

Green & black stains on ceramic surface of gas fireplace (C) InspectAPedia SH Question: what are these black & green deposits, are they toxic mold harmful to building occupants, & how do I get rid of them?

This fireplace panel where the flames go is green and black, it is not fuzzy and cannot scrape with fingernail. It does not fade with bleach or mold remover - just curious, do you know if this is a color from flames burning over time.

The apartment does not have any mildew or dampness under rug (replaced it) and the walls are not moldy behind wallpaper (removed and painted) and it is dry.

Do you know if this is a discoloration that old fireplace inserts get? I could not find this addressed on your website.

I called an antiques dealer in Wash. DC as he had photos of similar fireplace inserts in his store for sale and says its wear. - Anonymous by private email 2016/07/29

[Click to enlarge any image]


It's an odd pattern that doesn't look flame-related; but if the stains were from algae or mold on a ceramic surface I'd think they'd at least lighten with use of a cleaner or bleach solution. The photograph itself is insufficient, and your description of inability to remove any of the green and black stain areas by cleanign or wiping argues against it, but the pattern and colors do look a bit like mold growth.

A laboratory analysis by a qualified mycologist or forensic microscopist can usually identify the genera/species of mold on building surfaces, something that can't be done from your photo alone - if this is indeed mold. To the naked eye and from a photo only the stain pattern and colors do look like two or more mold genera/species perhaps including a green Trichoderma.

Reader follow-up:

Here is one closer up. The unit is not functional, as gas is no longer connected, and I think it has beauty, but not if it could become a health hazard.

Guess even you encounter something new every day, huh?

Green & black stains on ceramic surface of gas fireplace (C) InspectAPedia SH Moderator reply:

If the fireplace surface has been wet or damp that could be mold growth of several species. I'm quite surprised that you could not clean any of that off; can you lift off some of the green or black material on clear adhesive tape? See

This question and discussion appeared originally at MOLD on GAS FIREPLACE CERAMIC in an article series of the appearance of mold growth on various building surfaces.

Reader follow-up:

I believe we can. I can send it to you, for evaluation ...

This is in a 1800s home in Western New York. I have a tenant who has had that apartment for three years and never complained. But I asked for her to start paying the $10 fee I am being assessed monthly because she refused to take her recycling out every other week, and now she is saying “it’s mold, so what are you going to do about it?” — looking for money, I suppose. Ugh.

It’s somewhat retaliatory, but I also want to be proactive if it is a health concern. And, as I mentioned, I did the renovation in there with my hubby and there was no surface mold or mildew anywhere, not even in the window pains, etc.

I hate to remove it, because aesthetically it looks nice but I am willing to just take it out and board it up. I should check to see if the vent chimney is capped - possibly the chimney is damp.

Moderator reply:

Watch out: if this gas fireplace requires venting of combustion products to the outdoors as well as a supply of combustion air, as most do, then a chimney cap that closes off the chimney top completely, a chimney that is oversized for the fireplace, or other defects could cause the production of both soot and fatal carbon monoxide gas.

At MOLD TEST KITSI describe the method I recommended to you for collecting tape samples suitable for settled dust, mold or other particle examination tests. You can use this procedure for surface dust or debris analysis byt any qualified mold or forensic laboratory.

This is a gas fueled fireplace, right?

Combustion product deposits: Burning natural gas or LP gas in perfect combustion produces CO2 and water vapor, leaving no deposits on surfaces. Imperfect combustion produces potentially fatal carbon monoxide CO.

The patterns of black and green on your heater do not look like flame / soot deposition but I cannot be certain as I'm not expert on this specific fireplace and how it burns. (It'd be instructive to see it lit to see what the surface looks like when in use).

Wear deposits: Your antiques dealer claiming the marks are "wear" is IMO arm-waving in the absence of factual knowledge or objective data. Even if the cause of these marks relates to use and flame patterns, "wear" is almost certainly a carelessly-chosen and inaccurate word to explain what is actually causing the depositions.

One might as well say, as a neurologist once told me when he and his team could not explain nor diagnose a probem, "Well, we have a diagnosis from our senior physician. He says 'It's just one of those things'. Come back in a few months if the problem remains and we'll try again." In other words, they didn't know.

Moisture, condensation, mold deposits: If there has been moisture in the chimney, fireplace or at other nearby sources and considering that the gas heater has been off for two years or more, it is possible that there has been mold growth on the ceramic surface.

While there are mold genera/species that will grow on most surfaces and materials, in general the mold needs something organic to support itself. There might be organics in dust or other deposits on the surface of your heater.




Other surface deposits where moisture is not currently present: Absence of moisture in the building is a helpful but incomplete clue, since when the fire is burning it can certainly produce its own.

Heat, fire, flame would tend to dry out and burn off mold on the ceramic surface. But if mold in damp conditions or anything else formed on the ceramic surface and then was exposed to flame, that might result in colored impurities that penetrate into the ceramic and would be difficult to remove.

Deposits that penetrate and stain the ceramic surface of a gas fireplace

Speculating: if there were depositions of something on the ceramic surface following a period of dis-use, and then the flame were ignited, it might oxidize and color the ceramic from that combination.

From researching "stains on ceramic in gas fireplaces" I find a general opinion that burned-in stains are pretty-much there to stay; scrubbing with various oxidizing cleansers such as Bar Keeper's Friend might lighten them but probably won't remove them as the stain consists of mineral or other contaminants that have been absorbed into the ceramic and then heated or further oxidized.

Companies selling gas-log fireplace offer spray-on soot removers intended for use on ceramic-fiber gas logs; I'm not sure how effective thie product would be on your fireplace but is an example.

Typical MSDS for gas log soot removers

Each of these gas fireplace log cleaner MSDSs gives the ingredients in those products. You could contact the manufacturer of one of these products to ask about its efficacy on a ceramic surface such as your gas fireplace back.

Reader follow-up:

Yes, it is a gas fueled fireplace, but it has not be “in use” for several years, but left in place purely for aesthetics. As you know, it can be very humid in NY. This seems to have appeared over the last there months. I am going to try to investigate further as to any other contributing moisture than humidity.

Please feel free to use the photos and descriptives on your website. Certainly, someone else has encountered this at one time or another. The web is a great place to cross pollinate ideas and share solutions; your site has surely helped many.

If I get a sample evaluated, I will share results

Moderator reply:

I should have added that the total area and volume of the material on the ceramic surface of the gas fireplace in your photos is trivial and further, is may be behind glass and further is below a working chimney (presumably).

Also you indicate that wiping the surface does not produce any visible debris on the wiping material. So the probable health risk of airborne mold from this source is likely be be below the limits of detection. There is no justification for removing this heater because of a health worry from its stains.

However if there are damp or wet conditions in the building then there could be other indoor air quality or mold concerns.

Research on Gas Fireplace Safety & Gas Fireplace Stains & Stain Removal


Continue reading at STAINS on & in BUILDINGS, CAUSES & CURES - home,or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see MOLD on GAS FIREPLACE CERAMIC - appearance of mold on various building & material surfaces

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