I Googled creosote to learn a little about it. This is what I found:
http://www.daviddarling.info/encyclopedia/C/AE_creosote.html
Creosote is a liquid byproduct of wood combustion (or distillation) that condenses on the internal surfaces of vents and chimneys. If not removed regularly, it can corrode the surfaces and fuel a chimney fire. Creosote condenses when stack temperatures drop below 250°F, so the type of chimney is very important. An insulated one is desirable. A large, airtight heater in a small home in a choked position will produce creosote regardless of the type of wood burned or its moisture content. Minimize creosote formation by:
http://www.mastersweep.com/wood.htm
Properly seasoned wood produces the most heat, and produces the least creosote. . . . 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 smolder. 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 hardwood's, like oak and madrone, 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 hardwood's tend to smolder 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!
http://www.bucustommasonry.com/id72.html
Seasoned firewood by definition contains 20 percent moisture or less by weight.
So as I understand it, creosote build up is caused by condensation, which can be caused by a fire that is not hot enough, fuel that is too wet and a cooker top that is too cold. In my earlier post I showed the difference between by first three cooks when I had no build-up and the fourth when there was build up. Here is some more info.
First three cooks, air temp was 68-91º , humidity between 24 and 41%
Last cook, air temp was 59º, humidity 82% and raining, however no rain hit the WSM.
I just put some rib tips and trimmings on. Start air temp was 68 º, humidity 49% and overcast. I’ll see what happens.