Fireplace Help What is It

Some type of material inside a fireplace that looks like melted marshmellow foamy but has masonry material construction like concrete and crumbles to the touch. It is failing and breaking apart.

Photos would help

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moon pie exploded ???

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Hope this helps. Was having issues loading photos earlier.

Is your picture above the damper, inside the flue or smoke chamber?

It is at the flue and inside the smoke chamber but is not on all sides of the chimney lining. Was something burned in the fireplace that created this residue? Was it some half assed repair? Advised client for further inspection from chimney sweep.

An unknown object/substance was viewed in the smoke chamber that may present a safety or fire hazard.

I recommend a level 2 inspection on all wood burning appliance, even if it’s a new build.


anytime there was a fireplace or wood stove at a property I inspected I always recommended a cleaning and evaluation by a qualified chimney sweep before attempting to light a fire…


The substance is similar to the residue left behind from spraying wasp killer…but a few cans worth :smile:


Thanks for the responses. I did and always recommend further evaluation inspection of dirty or debris filled chimney and smoke chamber.

I did find a paper wasp nest in the basement, but what made you think it was wasp killer? Why would it foam up like that, it was also gray gritty and powdery like concrete powder when touched and crushed?

Was there a rain cap on the chimney? Almost looks like something leaching from the chimney.
It also looks like something is missing, is there a damper?

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No rain cap on chimney, there was a damper.

Just a thought really. I think we are all playing pin the tail on the donkey at this point :smile:


My 2nd guess was… :thinking:



That was my thought, it almost looks like major efflorescence/mineral buildup.
Was there a flue liner?

If they had a chimney fire, it might have been a chemical used by the fire department to put it out?


Some of those “creosote cleaning” logs and chemicals can make for odd residues. Not sure if that is the case here, but just another possibility.


Let’s just keep chipping away at it :smile: Did I win?


Thanks for all who played the guessing game what is it. Most seem like good guesses as to the mystery. Foaming Oven cleaner or foaming wasp spray sounds the most plausible. Bottom line is a qualified chimney contactor or chimney sweep will need further inspection and treatment.