There's a burnt spot on my mantle. I noticed it weeks ago and chalked it up to it having always been there without me ever having noticed, but I went back to the spot today and looked at it again. It looks like it's burnt worse than before and I was able to wipe away some blackened pieces with my hand. Is there a known cause for this? I can't recall having set something up there to have burned it and the wood around it doesn't appear damaged. Derek