Myrna Rode, age 12, of St. Catharines,
What makes a wood fire crackle?
Smoke and blazing flames stream up from the burning wood. Sparks shoot out and sometimes the merry fire crackles like miniature gunfire. The tree took many yours to grow the log. Yet the fire is soon over and nothing is left but a handful of ashes. Many of Andy's readers have ask‑.whether fire is animal, mineral or vegetable. Actually it is none of these things. Fire is a chemical reaction Find the smoke, sparks, flames and crackles are all part of this process.
The same chemical reaction takes place in all burning processes. Substances in the fuel combine with oxygen. Hence we call the burning process oxidation. Oxidation may be rapid, =~s in the burning fire. It may be slow, as in the rusting of iron. In any case, fuel substances are combining with oxygen. Oxidation takes place in your body. The oxygen you breathy combines with fuel from the food you eat. There Pro no flames, sparks or crackles, however, for this is a slow process of oxidation. blazing fire is a very fast chemical reaction. The fuel must be very burnable and there must be plenty of oxygen around. The nature of the flames, the sparks and crackles depends on the kind of fuel being burned.
In a wood fire the fuel is mostly cellulose. And, of course, cellulose is composed of cellulose molecules. Each molecule is composed of atoms of carbon, hydrogen and oxygen. In order to burn, the wood must be hot enough for these atoms to break apart. The solid cellulose breaks up into atoms of gases. Soma of the carbon becomes soot and some combines with the oxygen in the air to form carbon dioxide. The oxygen in the air is absolutely necessary to keep the fire going.
Wood, however, contains other substances besides cellulose. Pine logs contain resin and soma resins are very volatile. When hot they tend to become gaseous and expand. These gases get trapped in pockets within the wood. When hooted in the fire they must expand, but there is no place to go, no room. So a tiny packet of gas bursts its prison walls.
It breaks out of the little pocket with a pop and a crackle.
Most of the crackling of a wood. fire comas from plant resins. This is why pine logs crackle the most. However, most wood will crackle some as it burns. For volatile substances are present in all woods to a degree. Certain coals also contain those substances.
A machine gun crackle from the burning grate is actually a series of little explosions. Wood and even coal contain tiny pores too smell for our eyes to see. These are the little pockets where the gases get trapped. When they become hot, they explode and give us the pleasant crackle from our pretty fire