I have a solid fuel central heating system with a gorgeous real fire in my living room. The fire has a door with glass, which, over the last thirteen years, has gone a bit weird. It's as though it's melted. The black marks won't come off no matter what I use to clean it, and it's not clear glass any more - more like frosted (where it's not black). I'm getting a new piece of glass to fit in the summer (when the fire will be out and I can take the door off to get the glass out). Does anyone know why this has happened, and if I can prevent it happening on the next one? Thanks.