Sometimes it's so bad, the chemical doesn't work and the glaze has to be removed in a process call Ro-kleening. A tool is fitted on the end of special flexible rods and spun by an electric drill which then breaks away the glaze in chunks. This process is expensive. An added problem is that if the glaze is too hard the chimney tiles could break in the process.

This is why it is important to clean the chimney for wood burning appliances every year, regardless of how little wood is burned. If you only burn one fire on Christmas Eve and one fire on New Year's Eve, and that's the extent of your chimney use, every other year is sufficient for cleaning.