Wood ash

Wood ash is the powdery residue remaining after the combustion of wood, such as burning wood in a fireplace, bonfire, or an industrial power plant. It is largely composed of calcium compounds along with other non-combustible trace elements present in the wood. It has been used for many purposes throughout history. A comprehensive set of analyses of wood ash composition from many tree species has been carried out by Emil Wolff, among others. Several factors have a major impact on the composition: Fine ash: Some studies include the solids escaping via the flue during combustion, while others do not. Temperature of combustion. Ash content yield decreases with increasing combustion temperature which produces two direct effects: Dissociation: Conversion of carbonates, sulfides, etc., to oxides results in no carbon, sulfur, carbonates, or sulfides. Some metallic oxides (e.g. mercuric oxide) even dissociate to their elemental state and/or vaporize completely at wood fire temperatures (.) Volatilization: In studies in which the escaped ash is not measured, some combustion products may not be present at all. Arsenic for example is not volatile, but arsenic trioxide is (boiling point: ). Experimental process: If the ashes are exposed to the environment between combustion and the analysis, oxides may convert back to carbonates by reacting with carbon dioxide in the air. Hygroscopic substances meanwhile may absorb atmospheric moisture. Type, age, and growing environment of the wood stock affect the composition of the wood (e.g. hardwood and softwood), and thus the ash. Hardwoods usually produce more ash than softwoods with bark and leaves producing more than internal parts of the trunk. According to one research on the average the burning of wood results in about 6–10% ashes. The residue ash of 0.43 and 1.82 percent of the original mass of burned wood (assuming dry basis, meaning that H2O is driven off) is produced for certain woods if it is pyrolized until all volatiles disappear and it is burned at for 8 hours.