Here's a link that may help. Echoes Chris' comments for the most part.
Originally Posted by CajunSmoker
In my limited experience it has always been caused by poor exhaust airflow. Trying to cook by controling the exhaust instead of controlling the intake air. Caused the smoke to build up inside the smoker and get stale before it could exhaust out.
I think the inlet/outlet throttling debate is somewhat mistaken. Restricting inlet or outlet air restricts combustion air to the fire - it makes no difference from a combustion standpoint.
The error comes from associating the throttling of exhaust with nastyQ. This mainly comes from pits where the exhaust inlet is at the top of the cooking chamber (common to many pits, and wrong imho). With the damper wide open, even nasty smoke from the firebox flows in to the cooking chamber, rises directly to the top, runs along the top of the cooking chamber and out the stack without ever touching the meat. Throttling the outlet damper lowers the smoke level, basically submerging the meat in smoke. This is a good thing with clean smoke but a bad thing with uncombusted smoke - it basically amplifies the effect of smoke quality.
Think of the smoker as an upside down bathtub and you'll get the idea
It is creosote for sure. Entrained particulates give it a black color.
Originally Posted by jcfontario
Bigabyte, I concur that you have provided a great explanation. I have been puzzled why everyone it seems is able to set and forget temperatures for 10 hours on their cooker and I have been fiddling with my offset's firebox every 30 minutes. It sounds like if I follow your approach I can at least stretch it out to 90 minutes or so. For a small offset, how many lit coals would you start with?
The big difference here is the fuel, not the smoker type. You can't get creosote from a charcoal-only fire.