Creosote is the condensation of unburned, flammable particulates present in the exhausting flue gas (smoke). The actual cause of creosote condensation is the surface temperature of the flue in which the flue gas comes in contact. Like hot breath on a cold mirror, if the surface temperature of the flue is cool, it will cause the vaporized carbon particles in the flue gas (smoke) to solidify. This condensation is creosote build-up. If the wood you are using is rain logged, or green, the fire will tend to smoulder. Wet wood causes the whole system to be cool, and inefficient. But, dry wood means a hot fire! A hot fire means a hot flue, and a hot flue means much less creosote.
Back in the early 1980’s, tests were conducted to discover which kind of wood created the most creosote in a regular “open” fireplace. The results were surprising. Contrary to popular opinion, the hardwoods, like oak, created MORE creosote than the softwoods, like fir and pine. The reason for this is that if the softwoods are dry, they create a hotter, more intense fire. The draft created by the hotter fire moves the air up the chimney faster! Because it is moving faster, the flue gas does not have as much time to condense as creosote inside the chimney. Also, because the flue gas is hotter: it does not cool down to the condensation point as quickly. On the contrary, the dense hardwoods tend to smoulder more, so their flue gas temperature is cooler. Thus, more creosote is able to condense on the surface of the flue. So, saying that “fir builds up more creosote than oak” just isn’t true! It is a misunderstanding to think that it’s the pitch in wood which causes creosote. It’s not the pitch that is the problem; it’s the water IN the pitch. Once the water in the wood has evaporated, that pitch becomes high octane fuel! When dry, softwoods burn extremely hot!