The method may involve pyrolysis or thermolysis, or it may not (for instance, a simple mixture of ice and glass could be separated without breaking any chemical bonds, but organic matter contains a greater diversity of molecules, some of which are likely to break).
If there are no chemical changes, just phase changes, it resembles classical distillation, although it will generally need higher temperatures.
When substances of vegetable origin, e.g. coal, oil shale, peat or wood, are heated in the absence of air (dry distillation), they decompose into gas, liquid products and coke/charcoal.
If air is absent, the final product (since there is no oxygen present to react with the wood) is charcoal.
The solid residue, charcoal, is mainly carbon (about 70%) and small amounts of tarry substances which can be driven off or decomposed completely only by raising the temperature to above about 600 °C.
The by-products can be recovered by passing the off-gases through a series of water to yield so-called wood vinegar (pyroligneous acid) and the non-condensible wood gas passes on through the condenser and may be burned to provide heat.
The wood is split into dimensions of a finger, stacked densely, and finally covered tight with dirt and moss.