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Treebeard's Stumper Answer
7 December 2001

Burned Again

I love rainy days and freezing nights, especially sitting in front of a cozy fire glowing in the fireplace or wood stove. But there's a danger when we fire up our stoves for winter. Soot and creosote in dirty chimneys can burn explosively in a chimney fire. The cure is to have a chimney sweep clean the smokestack. The stumper is that this "fuel" is just a build up of smoke particles that have already burned. Charcoal is another fuel made from burned wood. I understand that wood can burn. But how is it that chimney soot and charcoal can burn again?

Our great Vermont Castings wood stove is our only heat source at home. I
opened the doors for this photo, but we usually keep them closed. That
makes it more efficient, but it burns coolor and increases the chance of a...
Chimney fire! Fortunately this wasn't my
house. I found this photo on the Web at
the Chimney Safety Institute of America.


No smoke is good smoke when it comes from a wood fire. Visible smoke consists of soot that didn't get enough heat or oxygen to burn completely. It's still a fuel that can burn again in a chimney fire. Wood burns in two stages. Gases and tarry liquids first burn above the wood in flaming combustion with dancing flames. That's followed by the glowing combustion of the remaining charcoal after the gases are consumed. To be safe, burn only dry seasoned firewood, keep your damper open until the flames die down, and keep your chimney clean!

Notes:

After writing this, I realize the last sentence is pretty good advice for (middle-age) life!

I thought of this stumper driving to school one morning when the local news radio station had a chimney sweep on the air talking about chimney fires. We've never had our chimney cleaned for almost 20 years now. I feel up inside the stovepipe now and then and bang it with a stick to see what falls, and it seems pretty clean. We burn nothing but Coast Live Oak (Quercus agrifolia) and Madrone (Arbutus menziesii), which are clean-burning woods. I've never heard of a local chimney fire. That makes it easy to ignore the hazard, but we've scheduled a chimney sweep anyway. We're way overdue.

Creosote, soot, charcoal, paper pulp, pine tar, many essential oils, and (coal) coke are all produced by the same process of heating fuel without enough heat or oxygen for complete combustion. They can burn again because they never burned completely the first time.

You can learn a lot about fire by doing a few simple experiments with a candle. The classic source is Michael Faraday's great Chemical History of a Candle (1860) (also in print). These pictures are from Robert Brent's Golden Book of Chemistry Experiments. This classic now-suppressed childrens' book is trading for $100+ at Amazon. It's one of the great science books for smart kids if you can find it.

Michael Farady

Blow out the candle and quickly bring a lit match into to vapors to relight it. The flame will jump several inches back to the wick. This shows that the flame consists of burning paraffin vapors.
Take the glass tube from an eyedropper (or make your own) and hold one end in the dark part of the candle flame. You can (sometimes) light the white vapors at the other end. If you hold the tube in the bright part of the flame, you get black soot instead of fumes.
It's very interesting to hold a piece of wire screen (e.g. 1/8" hardware cloth) over the flame. The screen cools and stops the flame so you can look inside. Flames are glowing sacks of gas that are hollow inside! Try moving the screen into different parts of the flame and think about what happens.

It's worth spending some time with a candle and a piece of wire screen such as 1/8" hardware cloth. When you first put the screen into the candle flame, the flame stops completely. But after several seconds, the flame reappears above the screen. I figure the wire absorbs so much heat that the flame goes out until it warms the metal enough that it can burn through. If you heat the screen red hot with a propane torch before putting it in the flame, it burns right through it. Even though the flame can't rise above the (cold) wire screen, hot gases can. You can light them with a match above the screen to make a separate flame that floats above.

If you put the screen in the lower dark part of the flame, it is instantly covered with black soot. A spoon or even your wet finger tip is also covered with soot, and the candle gives off a plume of dark smoke. Move the screen or spoon up into the bright part of the flame and the soot quickly burns away. We used to use Aladdin kerosene lamps when we lived in the Kinevan Ranch cabin without electricity. They make bright white light with a fragile mantle like Coleman lanterns. If the wick was too high, the mantle would soon be covered with crusty black soot. Turn the flame back down and the soot burns away in glowing snakes of light. Soot definately burns.

This shows how candles and other flames work. They heat the fuel and break it down into volitile carbon compounds that can burn completely if they get enough oxygen. Different elements burn with different colors. Heat copper wire (or copper sulfate) over a flame and it burns bright green. Lithium burns brilliant red. Carbon powder from graphite or charcoal dust burns bright orange like a candle flame. It is these burning carbon compounds that make familiar visible fire. Heat in the lower part of a candle flame cracks the fuel into carbon soot particles that rise by convection and incandesce in the flame to make bright orange light.

Here's my favorite fire experiment. It's the first experiment in the IPS book by Science Curriculum Inc. This is a lab version of the traditional way to make charcoal. We will heat wood without oxygen and see what we get. Be sure to wear eye protection if you try this. That hot distilling tube might shatter!

Make wood shavings from a dry scrap of lumber with a pocket knife or a Stanley Surform "cheese grater" plane, and stuff them tight into a large test tube or flask. Use a one-hole stopper and glass tubing to connect it to another condensing tube or flask in a cold water bath with a two-hole stopper. Another piece of glass tubing allows gases to escape. Heat the wood with a couple of alcohol burners (or a propane torch if you're in a hurry). Heavy white gases soon appear and condense to a dark, oily liquid in the second tube. Hold a match to the escaping gases, and they burn with a flame. You can also collect the gases to a "pneumatic trough". Fill a jar with water and carefully invert it into a bucket of water. Run rubber tubing from the "gas out" tube up into the jar, and the gases will replace the water. This gas burns if you hold a match to the jar. The liquids that collect in the condensing tube also burn, and they can be further separated with distillation. When the action stops, let the apparatus cool and examine the wood shavings. They have become dry black charcoal. Light a piece and it will burn without flame with glowing combustion. By now, your room stinks, and you'll never get that glassware clean again. It's a unique smell. A BBQ, only more so. A burned house. That wood pulp mill on I-5 near Salem, Oregon. It's hard to get rid of that cloying smell.

This is basically the same industrial process of distructive distillation that is used to make charcoal and many useful chemicals. Ordinary distillation is used to separate alcohol from water to make whiskey, but the alcohol is already present in the fermented sour mash you start with. Destructive distillation is different because new compounds are formed by chemical changes in the process. Wood cellulose breaks down with heat to make a chemical soup of different volatile compunds, including familiar solvents such as wood alcohol (methanol), acetone, turpentine, and many more unfamiliar compounds, including several known carcinogens. These are all potential fuels that can burn again. They accumulate as the creosote coating in your chimney if you allow your fire to burn without enough oxygen for complete combustion.

Real fires burn with the same steps, but they're not so separated. Look close at the wood in your fireplace. The flames dance above the wood as escaping gases burn. You can see liquids boiling from the ends of your logs. If your fire smokes in places, there's not enough heat or draft there. In time, only glowing charcoal is left, and it burns clean without flame.

We use kindling to start a fire because it has more surface area. It heats the other wood enough to chemically change it to produce volatile gases. On the right there's enought heat and oxygen for these gases to burn in a flame, but on the left they're escaping unburned. A smoldering fire in your stove can release these unburned fumes to condense as they cool and produce the dangerous buildup in your chimney known as creosote, ready to burn again.
In time, all the volatile compounds from your firewood will burn off if it has enough heat and oxygen. Clean glowing combustion of the remaining carbon fuel takes over. This is the slow fire that will keep your house warm all night without creosote buildup. But don't close your damper too soon and cut off the oxygen flow or those volatile compounds might burn again in your chimney!

Here are some links for further research:

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Copyright © 2001 by Marc Kummel / mkummel@rain.org