We have a fireplace with glass doors. "How beautiful!" we thought when we bought the place. It didn't take long to figure out it was a curse rather than a blessing. They fog over with only one fire and within a week can be black and opaque. We tried many different techniques and even resorted to scraping it with a razor blade at times. I think I read this tip in a book full of similar tips. I didn't really believe that it would work but was pleasantly surprised! Cleanup has gone from a several hour affair to about 15 minutes. You can disregard the amounts... the recipe was rejected for lack of quantities originally so I had to add them to get it accepted.