What is the cause of this spot on the glass?

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PDutro

Member
Sep 26, 2021
81
Oregon
RSF Focus 3600. Great fireplace. This spot keeps appearing, any ideas?
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I get that on my gas fireplace so probably not the same as wood issue but it seems to be where there is a bit more air penetrating near the enclosure. If I push on the panel I can see the marking change a bit. I can only see it at certain angles and lighting. So I havent pursued how to resolve.
 
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