Indirect (external) heating allows more precise control but to transmit the heat to the charge is difficult and inefficient and metal retorts are almost essential. By-products can be recovered free of contamination from the products of combustion. A hybrid method heats the charge of wood by passing hot gas through it. The hot gas is obtained by burning a fuel which can be wood, oil or gas. Precise control is needed to ensure that the hot gas is free of oxygen, otherwise some of the wood will be burned instead of being merely carbonised. Heat transfer from the hot gas to the wood is quite efficient and where the gases are recirculated under proper control, it is feasible to condense and collect by-products and the combustible wood gas.