What associations appear at you at the words "burning firewood"? The kindled fireplace or the furnace, the inflamed fire; pleasant heat and heat from naked flame; the scattering sparks and, of course, crackling. Very few people think why firewood cracks. Depending on type of wood and extent of its drying, logs can almost or not publish acoustic noise, or to hiss, click, crack continuously... In silence the tree never burns down.
Perhaps, being child, you loved books and telecasts from the Fascinating Physics for Children series. The simple, found every day phenomena in them popularly spoke by practical consideration. Try to repeat similar elementary experiment from the childhood.
Open the burning furnace or sit down by fire, and then bring to flame metal ladle, ladle or spoon bottom top. The surface of metal will become covered by droplets soon. If even more to bring closer ware to fire, then it will turn black from soot.
At least lower the burning stick in deep vessel twice. The flame will die away, and each time quicker and quicker. The fact is that in capacity carbon dioxide accumulates. Water, coal and gas – here into what as a result will turn armful of firewood in your oven. It is considered that the good firewood (giving a lot of heat) have to dry before use until the humidity of wood is not about 15-16% (admissible maximum - 25%). However even the driest tree keeps moisture: it and so-called "capillary" water in cavities of cages, and "colloid" in cellular covers, and very small amount of chemically connected water. When the flame inflames, and firewood becomes covered by the first charred layer, all water evaporates. Continuous crackling of logs in the furnace is nothing but series of the most real microexplosions. Fibers of wood charcoal are broken off, and water vapors break loose. The charred fragments of wood recoil from each other, publishing loud clicks. Perhaps, during the fires you heard strong cottons. It were incidental emissions of hot gases of pyrolysis – process during which there is thermal decomposition of wood on various chemical elements. Experts even have learned to heat especially tree without air access (or with its limited access) for various production needs. So, the burning logs crack and that is interesting, everyone after the own fashion. If you put in the oven the dried-up firewood from beech, then can not hear familiar crackling almost. The aspen will peacefully crackle, the pine in flame just "zastrelyat" - from her cavities the accumulated pitch will begin to be released and will fill the room with pleasant air. The burning-down firewood not only "talks" in own way, but also has other distinctive features. For example, you should not use the same odorous pine in open fireplace – sparks will depart. It is better to burn branches of cherry or apple-tree there and to enjoy heat and pleasant aroma. If you use home or bath with the wood-fired furnace, surely consult to the expert before preparation of firewood.