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, and has been used for many purposes throughout history.
Composition
Variability in assessment
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.
-
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 (.)
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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: ).
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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.
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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
Measurements
The burning of wood results in about 6–10% ashes on average.
The residue ash of 0.43 and 1.82 percent of the original mass of burned wood (assuming
dry basis, meaning that
Water is driven off) is produced for certain woods if it is pyrolized until all volatiles disappear and it is burned at for 8 hours. Also the conditions of the combustion affect the composition and amount of the residue ash, thus higher temperature will reduce the ash yield.
Elemental analysis
Typically, wood ash contains the following major elements:
Chemical compounds
As the wood burns, it produces different compounds depending on the temperature used. Some studies cite calcium carbonate () as the major constituent, others find no carbonate at all but
calcium oxide () instead. The latter is produced at higher temperatures (see
calcination).
The equilibrium reaction CaCO
3 → CO
2 + CaO has its equilibrium shifted leftward at and high
partial pressure (such as in a wood fire) but shifted rightward at or when partial pressure is reduced.
Much of wood ash contains calcium carbonate (CaCO3) as its major component, representing 25%
Trace elements
There are trace elements of
iron (Fe),
manganese (Mn),
zinc (Zn),
copper (Cu) and some
heavy metals.
Their concentrations in ash vary due to combustion temperature.
Decomposition of
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
at higher temperature of combustion.
Some elements in wood ash (all fractions given in mass of elements per mass of ash) include:
Fuels
One study has determined that a slowly burning wood ( ) emissions typically include 16 alkenes, 5
Diene, 5
and several
and arenes in proportions.
,
acetylene and
benzene were a major part at efficient combustion.
Proportion of C
3-C
7 were found to be higher for smouldering.
Benzene and 1,3-butadiene constituted ~10–20% and ~1–2% by mass of total non-methane hydrocarbons.
Uses
Fertilizers
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
Soil pH.
[
]
Wood ash can also be used as an amendment for organic hydroponic solutions, generally replacing inorganic compounds containing calcium, potassium, magnesium and phosphorus.
Composts
Wood ash is commonly disposed of in , but with rising disposal costs, ecologically friendly alternatives, such as serving as compost for agricultural and forestry applications, are becoming more popular.
Pottery
Wood ash has a very long history of being used in Ash glaze, 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.
Soaps
For thousands of years, plant or wood ash was leached with water, to yield an impure solution of potassium carbonate. This product could be mixed with oils or fats to produce a soft "soap" or soap like-product, as was done in ancient Sumeria, Europe, and Egypt.
Bio-leaching
The ectomycorrhizal fungi Suillus granulatus and Paxillus involutus can release elements from wood ash.
Food preparation
Wood ash is sometimes used in the process of nixtamalization, where certain types of corn (typically maize or sorghum)[Odukoya, Julianah Olayemi; De Saeger, Sarah et al.: ' Effect of Selected Cooking Ingredients for Nixtamalization on the Reduction of Fusarium Mycotoxins in Maize and Sorghum': National Center for Biotechnology Information][Cabrera-Ramírez, A.H.; Luzardo-Ocampo I.; Ramírez-Jiménez, A.K.; Morales-Sánchez, E.; Campos-Vega, R.; Gaytán-Martínez, M.: ' Effect of the nixtamalization process on the protein bioaccessibility of white and red sorghum flours during in vitro gastrointestinal digestion': Food Research International
Volume 134, August 2020] are 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.
Nixtamalization was originally practiced in Mesoamerica, from which it spread northwards through various indigenous tribes of North America. In eastern North America, nixtamalized corn was traditionally eaten in porridges and stews, a dish that Europeans would call hominy.
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.[Arzani A.: Emmer (Triticum turgidum spp. dicoccum) flour and breads. In Preedy V.R., Watson R.R., Patel V.B. (Eds. 2011), Flour and Breads and their Fortification in Health and Disease Prevention, Academic Press, California, pp. 69-78.] In present day, the amount of wood ash content in bread flour, as measured by the Chopin alveograph,[Li Vigni, M.: Monitoring Flour Performance in Bread Making. In Preedy V.R., Watson R.R., Patel V.B. (Eds. 2011), Flour and Breads and their Fortification in Health and Disease Prevention, Academic Press, California, pp. 69-78.] is strictly regulated by France.
See also
Notes
