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The chimney sweep just left, but not before telling you, “You have a very serious third stage creosote problem”
What on earth is that ? Well seeing is believing so first grab a flashlight, go to your fireplace or chimney inspection hole, and take a look. Your flue's insides probably look like a black mirror, with shines, slick deposits on the wall that look almost painted on, giving the appearance of black ice or black bathroom tiles. That's third degree creosote.

To understand why it brings a grim look to your sweeps face, a little more explanation is in order. Creosote, as you know, is the black sticky substance that's produced whenever wood is burned. It is a natural by product, and its flammable, which is why you called your sweep to remove it in the first place.

But when it condenses on the inside of your chimney, it can take a number of forms, most of which fall into three categories.
First stage: The most common, people usually call it soot. It's a light powder, if you look on your chimney sweeps face you'll probably see some.
Second stage: Looks like pumice stone from a volcano or black potatoes chips or a burnt marshmallow, usually light and airy. Third stage: Similar to a black bathroom tile, black ice or an asphalted road. A very hard shiny glaze. The most dangerous and difficult to clean. Your chimney sweep can do an excellent job on first and second stage creosote by using a series of wire brushes designed to fit your chimney and stove pipes.
Third stage creosote is a totally different animal. It is easiest to understand why your sweep's wire brushes are not effective on it, when you realise that not only does it look like a black mirror, but it has many of the same qualities. The wire brush rubbed across a mirror would only slightly scratch the surface, basically the mirror would be unchanged. What makes third degree creosote so dangerous? First when its heated in a chimney fire it mushrooms far beyond its original size. As it expands across your chimney it can choke the flue, causing the fire to find a new way to exhaust it's fumes. This new avenue of exhaust often is through your attic or roof.On occasion, pieces of a chimney have been blown out by the tremendous pressure exerted from this expansion. And a third stage driven fire is so hot that it easily exceeds the temperature limits for which the chimney was designed, thereby damaging the chimney. Such chimney fires routinely exceed the temperatures of a blow torch.
So how do we get rid of it? First don't blame your chimney sweep for the situation! (Yes indeed some people do. His burning habits didn't form the creosote that way, yours or the unique physical characteristics of your chimney did. The sweep has more then done his or her job by informing that you have a serous problem and outlining the possible choices you face. He is entitled to be compensated for his time and expense, much as a doctor is paid for an office visit. wether he can cure your problem or not.

For many years sweeps couldn't cure third stage chimneys. Recently, however several technological advances have occurred that allow the sweep to deal with this problem much more effectively. These new cures range from the use of special chemical treatments, to sandblasting, to scouring with motorized tools. The specific one used will depend on the construction of your chimney, its age and the material used within the chimney. While these methods aren't as inexpensive as a regular cleaning they do save your chimney without the expense of replacing it entirely.
Third stage creosote can't be ignored. It must be removed, or the chimney must be abandoned or replaced. Fortunately removal of this sort of creosote now is possible. Once it's removed your sweep can guide you from keeping it from happening again.

A word of caution: Who sweeps your chimney is often just as important as getting it swept

Mirror Mirror on the wall I am the deadliest of  them all