White Smoke? Blue Smoke? The Age Old Debate

Discussion in 'Woods for Smoking' started by hog warden, Nov 2, 2009.

  1. hog warden

    hog warden Smoking Fanatic

    Don't know if this will morph into the definitive solution to this age old debate, but there are some smart folks on this forum, so maybe it can be done.

    What most Q-heads all seem to agree on is that white smoke is bad, blue smoke is good. But how do you get one while avoiding the other? I've done a bit of Internet research, along with some field testing on my patio, and here is what I've come up with. Feel free to join in to set me straight.

    To start with, in the beginning.......there was wood. From the Q-head's standpoint, the smoke we are talking about comes from wood. Take a normal piece of wood, apply heat and things start happening. The catch all phrase for this is pyrolysis. A couple quick references to what happens when you heat wood:

    Basic pyrolysis:


    The process of making charcoal:


    But if you read these, what you find is that as heat is applied to wood, solids and liquids trapped in the wood fibers, either by themselves or as components of other compounds) break free, turn into gases and start to evaporate off. One of the first is water. The guys making charcoal attribute the white smoke to being mostly water vapor. Mostly water, yes, but there are other things too. (This is the basic refining process used to make moonshine, ethanol and turning crude oil into the various petroleum products. All these compounds have different boiling points....apply heat....and keep catching the various compounds as they boil off).

    Next comes blue smoke, which includes alcohols and other compounds evaporating, and lastly comes yellow smoke, which include tars. As this progresses, the residual wood chars (turns black). This is the basic charcoal making process. Apply enough heat to drive off the volatile compounds but not ignite the residual charred wood. How long is a function of how big are the chunks of wood you start with. How long does it take to heat the wood chunk to a certain degree to drive that stuff off? Small chips? Maybe minutes. Big chunks? Maybe hours or days even.

    When all these volatile compounds are removed, what you are left with is the residual carbon. Shut down the process now (cut the heat and starve it for oxygen) and you get charcoal. Lump charcoal to be exact. Apply heat and turn the air up and oxygen in the air can now react with the carbon compounds to burn (orange glow....not a flame), which gives off heat and reduces the carbon compounds (charcoal) to it's mineral ash.

    So that is the basic process, but there are subtle sub plots. Some of those gases that evaporate (like alcohol) will burn. For a long time now folks have been able to run internal combustion engines by putting what amounts to a charcoal kiln in the back of a vehicle and piping the smoke/gases that are being driven off to the engine. In those indirect methods, part of the heat to fuel the process comes from venting the gases and smoke driven off to the fire to help raise the temp to complete the process. It will also burn inside the smoker if given enough time and air (oxygen) to complete the process.

    So when you put wood into your smoker, all this is going on. The speed at which it happens is also linked to how big your wood chunks are. The pyrolysis process starts at the surface, and works in. It's going to happen faster with small chunks (chips) than larger chunks. So if you want a lot of white smoke, toss on a large pile of wet chips. Starve it for air and those volatile compounds don't burn. They might escape through the vent, or left to mill around inside the smoker, might condense back to liquids when they come into contact with a cold piece of meat or the side of an uninsulated smoker. Creosote and the various tars are heavy and will be some of the first to condense....even on what appears to be a hot surface. Hot being relative to the burn temp of the compound. The longer it's in there, the more likely it is to condense. UDS comes to mind.

    Taken to a different extreme, guys smoking with splits of hardwood alone are generally throwing larger chunks of DRY wood onto an already hot fire, the pyrolysis process works on the surface and combines the smoke and gases with heat already coming from previously burnt wood (charcoal). The offset fire box allows allows a mixing process and time for all this to happen, where the heat from the charcoal and burning gases combine to produce indirect heat with a little smoke thrown in. This is also why a lot of UDS guys use lump charcoal (briquettes have sawdust and other fillers that smoke) and just a few large chunks. Again, if you want a lot of white smoke in your UDS, put in a lot of wood chips or chunks, and starve it for air.

    Lastly comes the wood itself. Wood density varies by species (hickory, pecan and oaks being some of the hardest), so you get more BTU's, but also the mix of volatile compounds vary. I have never seen a list of what those are, but we all know they exist. Cherry, apple and maple smell different than pecan or hickory. They also taste different. Walnut has compounds too, but they taste bitter. Softwoods like pine and fir have all kinds of stuff in them to generate heavy doses of creosote. Bad for smokers but also bad for wood stoves as the creosote condenses in the chimney and causes fire hazards.

    OK.....for starters, those are my thoughts. Yours?
  2. hog warden

    hog warden Smoking Fanatic

    BTW, if you have ideas on why hickory sawdust or small pieces of wood are better inside some devices like low temp sausage smokers, I'd like to hear it.

    Pecan shells for example. Can I use these in an open pan over a heat source to generate desirable smoke? Our local shellers are running big time and I can get literally thousands of pounds of pecan shells......probably for free.
  3. the dude abides

    the dude abides Master of the Pit OTBS Member

    Great information, and a lot to try to wrap my head around. But I think it does explain why, when using my WSM, using lump and chunks of wood via the Minion Method, I seem to get thicker white smoke right at the beginning then after 15-20 minutes it backs off to the thin blue type of smoke. For that reason, I usually wait to put the meat on until that initial burn is over. And seeing that this is when the meat is the coldest, it sounds like that's a good thing since the info says that the nastys will stick to the cold meat.

    Based on your info above I'd say this is because a couple of the chunks "light up" right away and burn pretty fast and release all the stuff you mentioned. That's because there will be a couple of these chunks that are right against the heat in the middle.

    Then there are a few other chunks that are not in direct contact with the lit lump. My best guess would be that those are slowly heated and release their stuff at a slower pace. Then by the time the lump burns out to these chunks they've released all the stuff that creates the "white" and leaves them with the "blue".

    Does that sound like it makes sense. I really don't know. Just trying to relate your information to what I've noticed in my cooks.
  4. beer-b-q

    beer-b-q Smoking Guru OTBS Member

    So much information, My Brain Hurts...[​IMG]
    cryptkicker likes this.
  5. eman

    eman Smoking Guru Staff Member Moderator OTBS Member

    Pecan shells work great , I have used them in a pan w/ foil covering it w/ just a few small holes in the foil. Same w/ chips.
    small foil pans hold around 4 hand fulls of chips, Pack the shells or chips in the pan and cover w/ 2 layers of heavy foil. set on the side of your coals or element where the pan is just touching the coals.
    BTW , The pecan shelling plafce here now sells all their shells to one of the companys that make smoke discs .
  6. oldschoolbbq

    oldschoolbbq Smoking Guru OTBS Member

  7. hog warden

    hog warden Smoking Fanatic

    Very good information. I think you summed up exactly what to do to get it right for cooking/smoking butts, ribs, etc. You get a good burn going and all the wood pyrolyzed and essentially down to huge chunks of charcoal before you start the smoke.....then add smaller pieces of "smoke" wood, along the way.

    Comments about starting wood off to the side and adding them to the smoker as embers dovetails this concept. You are essentially making your own charcoal on the fly out of larger sticks of wood.

    To mimic this, a UDS guy would need to get his charcoal going, temps up, smoke cleared and then add one or two "smoking wood" chunks, or have them buried in the basket. BTW, if you go to the WIKI site on pyrolysis, and read the part about cooking, you see what happens when grease burns. It creates some nasty stuff. A lot of the white smoke you see coming out of a UDS is the grease dripping on the charcoal, burning and that smoke from the burning grease circulates around inside, condensing on the meat and sides of the drum. I put a drip pan in mine to keep the grease out of the fire, which also disperses the heat a little better. I don't think my Q from that gets as black and I think tastes better.
  8. hog warden

    hog warden Smoking Fanatic

    The process you describe with the shells or chips sealed up pan is essentially the same process used to make charcoal. As an interesting side note, if you seal up a pan or a can and crank up the heat and it gets going good, if you put a match near the smoke and the gas will light.

    Similar to this, if a piece of wood is smoking heavy, it is probably on the verge of the evaporating gases igniting into flame. Put a match to it and it may bust out into flame, at which point the smoke stops.

    I know it's common practice to seal dust and chips up into a covered pan (heat inside the pan raises to "bake" the wood, vs. hot metal to chip contact in an open pan. But I when I cover it, I get more of the white smoke. It won't flame up as there isn't enough air to burn the escaping gases, but it's not the sweet smelling blue smoke either. The residual is usually a foul smelling charcoal.

    You mention putting the pan off to the side. That suggests to me that you can get your metal too hot, which is probably what I'm doing. See above. Better to have a slow charing process going over a wide area, vs. a red hot metal touching wood. In other words, there is an optimum metal temp to get this going and you can get it too hot. Not hot enough and it doesn't do anything.

    I also find I get blue smoke with hickory, pecan and oak and white smoke with apple and cherry. The fruit woods seem to stay "white" the whole way through.

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