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:
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 (dry basis) is produced for certain woods if it is pyrolized until all volatiles disappear and it is burned at 350 °C (662 °F) for 8 hours.[a] Also the conditions of the combustion affect the composition and amount of the residue ash, thus higher temperature will reduce ash yield.
Typically, wood ash contains the following major elements:
As the wood burns, it produces different compounds depending on temperature used. Some studies quote calcium carbonate (CaCO3) as the major constituent, others find no carbonate at all but calcium oxide (CaO) instead. The latter is produced at higher temperatures (see calcination). The equilibrium reaction CaCO3 → CO2 + CaO has its equilibrium shifted leftward at 750 °C (1,380 °F) and high CO2 partial pressure (such as in a wood fire) but shifted rightward at 900 °C (1,650 °F) or when CO2 partial pressure is reduced.
Much of wood ash contains calcium carbonate (CaCO3) as its major component, representing 25% or even 45% of total ash weight. At 600 °C (1,112 °F) CaCO3 and K2CaCO3 were identified in one case.[b] Less than 10% (ten percent) is potash, and less than 1% (one percent) is phosphate.
There are trace elements of iron (Fe), manganese (Mn), zinc (Zn), copper (Cu) and some heavy metals. Their concentrations in ash vary due combustion temperature. Decomposition of carbonates and the volatilization of potassium (K), sulfur (S), and trace amounts of copper (Cu) and boron (B) may result from increased temperature. The study has found that at raised temperature K, S, B, sodium (Na) and copper (Cu) decreased, whereas Mg, P, Mn, Al, Fe, and Si did not change relative to calcium (Ca). All of these trace elements are, however, present in the form of oxides at higher temperature of combustion. Some elements in wood ash (all fractions given in mass/mass) include:: 304
One study has determined that a slowly burning wood (100–200 °C (212–392 °F) ) emissions typically include 16 alkenes, 5 alkadienes, 5 alkynes and several alkanes and arenes in proportions.[c] Ethene, acetylene and benzene were a major part at efficient combustion. Proportion of C3-C7 alkenes were found to be higher for smouldering. Benzene and 1,3-butadiene constituted ~10–20% and ~1–2% by weight of total non-methane hydrocarbons.
Wood ash can be used as a fertilizer used to enrich agricultural soil nutrition. In this role, wood ash serves as a source of potassium and calcium carbonate, the latter acting as a liming agent to neutralize acidic soils.
Wood ash can also be used as an amendment for organic hydroponic solutions, generally replacing inorganic compounds containing calcium, potassium, magnesium and phosphorus.
Wood ash is commonly disposed of in landfills, but with rising disposal costs, ecologically friendly alternatives, such as serving as compost for agricultural and forestry applications, are becoming more popular. Because wood ash has a high char content, it can be used as an odor control agent, especially in composting operations.
Wood ash has a very long history of being used in ceramic glazes, particularly in the Chinese, Japanese and Korean traditions, though now used by many craft potters. It acts as a flux, reducing the melting point of the glaze.
Potassium hydroxide can be made directly from wood ash by leaching and filtering it by using water. The technique for the lye production described as following: the hardwood ash is loaded into a waterproof vessel with gravel and straw laid at the bottom, with a small hole for the water to drip out. The drippings are usually collected by using a separate vessel underneath of the primary one. The apparatus is usually left for several hours (overnight) to collect leachate. The water and ashes may be continuously added to make more yield. In this form, it is known as caustic potash or lye. Because of this property, wood ash has also traditionally been used to make wood-ash soap.
The ectomycorrhizal fungi Suillus granulatus and Paxillus involutus can release elements from wood ash.
Wood ash is sometimes used in the process of nixtamalization, where corn is soaked and cooked in an alkali solution to improve nutritional content and decrease risk of mycotoxins. The alkali solution has historically been made from wood ash lye.
An early leavened bread was baked as early as 6000 BC by the Sumerians by placing the bread on heated stones and covering it with hot ash. The minerals in the wood ash could have supplemented the nutritional content of the dough as it was baked. In present day, the amount of wood ash content in bread flour, as measured by the Chopin alveograph, is strictly regulated by France.
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