Why does a popping sound come from burning pieces of wood

Wood is the dried branches or trunk of trees and all varieties of wood are aggregates of plant fibres. These aggregates resemble fine tubes running along the length of the wood.

This capillary structure is known to be useful for the trees in transporting nutrient solutions from the roots upwards to the different parts of the tree.

The structure of wood accommodates several other components; notable among them are special function channels or vessels like the latex vessels as in banyan and rubber tree wood, oil glands in almost all wood, resin containing resin ducts as in pine varieties.

The trees also store several chemicals inside the cells and in the interstitial cavities of the fibres of the wood and most of these chemicals act as binding agents of the fibres. In live trees these chemicals are seen as saps, gums and oils. Also in the plant cells, often the trees store species specific alkaloids, raphides, and other mineral crystals and composites.

These chemicals undergo physical and chemical changes when the temperature is raised during burning of the wood. One common effect is physical expansion of the chemicals or the thermal products. Often, it is observed that melted or liquefied chemicals ooze out from the cold end of wood pieces burnt at the other end.

In case the chemical gets transformed into gaseous components, it can expand suddenly and escape by breaking the enclosure structure inside the wood. This is much like busting a miniaturized cracker making popping sounds every time the above process takes place.

This is evident from the frequent observation that the flame gets pushed by a jet of gases emanating from the burning wood. If it does not find a free escape path, it would make one by popping out the covering material of the wood and thereby producing the sound.