If the glass turns white, this is an indication that the glass has not been cleaned regularly and/or that one has been burning:

• Newspaper adverts, milk cartons, etc.
• Pressure-treated wood, sawdust, etc.
• Non-approved fuel sources, such as fuel pellets, coal, etc.

This means that at very high temperatures, chemicals will be released that unfortunately settle on the glass in form of a white film that cannot be removed.