I've been burning in a Lopi Freedom insert for about a year now and feel like I've started to get the hang of it. Last night I loaded it up before heading to bed and after it took off shut down the air like I do most nights. The slow burn often leaves the glass a little dirty but when I got up this morning the glass was completely black. I took the door off and out to the garage to clean it and the deposit on the glass was black and hardened; needed a razor blade to remove it. I've never had this happen before and wonder if any of you have any idea what might have happened happened. BTW, the wood I'm burning is dry and seasoned elm, walnut and ash, all between 15-19% MC. Thanks.