A product of incomplete combustion: deposits of unburned, flammable tar vapors from wood smoke. Sometimes it is crusty or flaky in texture, but often sticky or hard, like slag. Creosote deposits are often hard to remove from chimneys, and pose a serious fire hazard.
One of the great misunderstandings in the world of woodstoves is how creosote fits into the picture. Contrary to popular belief, creosote is not an inevitable product of wood burning. Creosote forms when wood is burned incompletely, and is an indication of improper use, poor installation, or a poor wood stove design.
It is also extremely flammable, and is responsible for many chimney-related structural house fires each year.
The long and the short of it... If you find a buildup of creosote in your stove pipe or chimney, have the chimney cleaned right away, and determine what's causing it. There are four basic possibilities:
Operating the stove at a too-low burn rate
Especially in airtight stoves, if you damper the stove way down, for a long, low burn, you will create a smoky fire that emits lots of unburned tar vapors into the venting system. Since the temperature of the flue gasses will already be relatively low, these vapors will be particularly likely condense inside the pipe or chimney flue.
The solution is to keep the fire burning at a moderately-active rate. Go outside and check the flue. If lots of smoke is billowing from the chimney, you are burning it too low. Yes, this means you can't get as long a burn time from a load of wood, (unless you upgrade to a new, EPA-certified stove, which is designed to burn cleanly at a much lower burn rate) but you will actually get more heat from the same amount of wood, since creosote represents unburned fuel. You will also do your chimney and our environment a favor.
Using the wrong type of fuel
Burning green, wet, or excessively dry wood can cause creosote buildup.
Oversized flue or improper connection
If the chimney isn't quickly drawing the combustion products to the outdoors, due to an oversized flue, an excessively-long stove pipe, or too many elbows in the stove pipe – all of which tend to increase the amount of time the smoke stays in the venting system – then the smoke will tend to condense in the flue, forming creosote.
And if your stove pipe is over eight feet long, or contains more than two elbows, consider re- installing the stove for a shorter run with fewer elbows.
Poor woodstove design
Before the new EPA-certified stoves became available, "air- tight" wood stoves were considered the best type. Airflow into an airtight stove can be closely controlled, in some cases to the point that the user can literally put out the fire by closing the air controls.
In essence, the problem with air- tight stoves is that, while they offer the convenience of a long, low burn, they are not designed to burn the fuel efficiently during periods of low burn. Lots of fuel is wasted in the form of smoke, which condenses in the stove pipe and chimney as creosote.
The solution is the same as for Operating the stove at a too-low burn rate, above, although many older woodstove designs create a smoky burn no matter how you operate the stove.