If your wood is not dry enough, the combustion temperature will be too low. The wood will then smolder instead of burn, and this results in soot on the glass. You can clean the glass with ordinary window cleaner or a special glass cleaner, which you can buy from your local HETA dealer.
If a bigger piece has chipped off, you can glue it back on with water glass, which you can buy from your local HETA dealer.
If a piece is missing, or the impact mark is deep, you can repair the damage with a mixture of soapstone powder and water glass, which you can buy from your local dealer.
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