When I send my sweeps into a home, not only do they check the integrity of the chimney to ensure that it is structurally sound, they also sweep the inside of the flue system to remove any creosote or other debris that may have built up since the last cleaning. Creosote is usually puffy and loose and relatively easy to remove from the walls of the flue system. Creosote like this is caused by the natural byproducts of burning wood. Sometimes, however, the creosote is not in this loose, puffy form. Glazed creosote starts out as a substance that is a lot like tar in texture and viscosity. The best word to describe the way that this stage of glazed creosote feels and looks like is gooey. After some time, this gooey creosote will harden so that it is very shiny and quite hard. You will not be able to merely scrape off creosote that has been glazed onto the walls of your flue system.
When the layers of glaze stack up on each other at a rapid enough pace that the layer below the new layer is not given a chance to dry out, then the gooey substance is formed which turns into glazed creosote. If you are venting an appliance through a flue system which is larger than the size of the appliance’s ventilation opening the chimney will not be able to draft quickly enough which aids in the formation of glazed creosote. Another catalyst for heavy glazing is burning wood that has not had time to season properly. Wood that has not had sufficient time to dry out will allow more creosote to build up than dry wood because it burns at a slower and cooler rate.
Removing glazed creosote is no easy task. Glazed creosote must be chemically altered before it can be properly removed. Any blunt force to the glazing that is powerful enough to remove it will more than likely damage the flue system as well, and that is certainly not a problem that you want to have to tackle. There are various chemical treatments available to chemically alter glazed creosote.
One example is Anti-Creo-Soot, or ACS which you spray on the logs prior to burning them. The spray will cause gases that slowly break down the glazing as it evaporates up into the flue system. Similarly, there are powders, such as Cre-Away, which are sprayed directly up into the firebox. As the heat of the fire causes the chemicals to react, it will loosen up the glazed creosote to make it sweepable. When these products and others like them react with the glazed creosote, it “etches” it, which makes it more like the loose, puffy creosote that can be swept away using standard chimney cleaning tools and methods.
Missing mortar joints like these are very dangerous and can be missed by a chimney professional if glazing is prominent in the chimney.
Do not take creosote glazing lightly. If your chimney professional recommends using a creosote glazing removal product, you should follow their instruction because that heavy glazing could be hiding a cracked flue tile or missing mortar joint that can potentially cause a chimney fire.