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.

Wood ash from a campfire


Variability in assessment

A comprehensive set of analyses of wood ash composition from many tree species has been carried out by Emil Wolff,[1] among others. Several factors have a major impact on the composition:[2]

  1. Fine ash: Some studies include the solids escaping via the flue during combustion, while others do not.
  2. Temperature of combustion.[3] Ash content yield decreases with increasing combustion temperature which produces two direct effects:[2]
    • 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 (600 °C (1,112 °F).)
    • 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: 465 °C (869 °F)).
  3. 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.
  4. 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[2] with bark and leaves producing more than internal parts of the trunk.[2]


According to one research on the average the burning of wood results in about 6–10% ashes.[2] 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.[lower-alpha 1] Also the conditions of the combustion affect the composition and amount of the residue ash, thus higher temperature will reduce the ash yield.[4]

Elemental analysis

Typically, wood ash contains the following major elements:[2]

Chemical compounds

As the wood burns, it produces different compounds depending on a temperature used. Some studies cite 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).[3] 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.[5]

Much of wood ash contains calcium carbonate (CaCO3) as its major component, representing 25%[6] or even 45% of total ash weight.[7] At 600 °C (1,112 °F) CaCO3 and K2CO3 were identified in one case.[lower-alpha 2] Less than 10% (ten percent) is potash, and less than 1% (one percent) is phosphate.[6]

Trace elements

There are trace elements of iron (Fe), manganese (Mn), zinc (Zn), copper (Cu) and some heavy metals.[6] Their concentrations in ash vary due combustion temperature.[3] 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.[3] 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.[3] Some elements in wood ash (all fractions given in mass/mass) include:[2]: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.[lower-alpha 3][8] Ethene, acetylene and benzene were a major part at efficient combustion.[8] Proportion of C3-C7 alkenes were found to be higher for smouldering.[8] Benzene and 1,3-butadiene constituted ~10–20% and ~1–2% by weight of total non-methane hydrocarbons.[8]



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.[6]

Wood ash can also be used as an amendment for organic hydroponic solutions, generally replacing inorganic compounds containing calcium, potassium, magnesium and phosphorus.[9]


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.[10] Because wood ash has a high char content, it can be used as an odor control agent, especially in composting operations.[11]


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.[12]


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.[13] However only certain types of plants could produce a soap that actually lathered.[14] Later, medieval European soapmakers treated the wood ash solution with slaked lime, which contains calcium hydroxide, to get a hydroxide-rich solution for soapmaking.[15] However it was not until the invention of the Leblanc process that high quality sodium hydroxide could be mass produced, rendering obsolete the earlier forms of soap using crude wood or plant ash.[16] This was a revolutionary discovery that facilitated the modern soapmaking industry.[17]


The ectomycorrhizal fungi Suillus granulatus and Paxillus involutus can release elements from wood ash.[18]

Food preparation

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.[19] In present day, the amount of wood ash content in bread flour, as measured by the Chopin alveograph,[20] is strictly regulated by France.[21]

See also

  • Ash burner (traditional occupation)
  • Bottom ash
  • Charcoal
  • Fly ash
  • Ashery  A location or factory producing lye from wood ash


  1. Woodchips of different wood species (Aspen, Yellow poplar, White oak, White oak bark, Douglas-fir bark) were pyrolyzed in a closed container in a furnace at 500 °C (932 °F).[3]
  2. Woodchips of different wood species (Aspen, Yellow poplar, White oak, White oak bark, Douglas-fir bark) were pyrolyzed in a closed container in a furnace at 500 °C (932 °F).[3]
  3. By using gas chromatograpy analytical method.


  1. Wolff, Emil (1871). Aschen-Analysen. Berlin: Wiegandt und Hempel.
  2. Siddique, Rafat (2008), "Wood Ash", Waste Materials and By-Products in Concrete, Berlin, Heidelberg: Springer Berlin Heidelberg, pp. 303–321, doi:10.1007/978-3-540-74294-4_9, ISBN 978-3-540-74293-7, retrieved 24 July 2022
  3. Misra MK, Ragland KW, Baker AJ (1993). "Wood Ash Composition as a Function of Furnace Temperature" (PDF). Biomass and Bioenergy. 4 (2): 103–116. doi:10.1016/0961-9534(93)90032-Y.
  4. Etiegni L, Campbell AG (1991). "Physical and chemical characteristics of wood ash". Bioresource Technology. 37 (2): 173–178. doi:10.1016/0960-8524(91)90207-Z.
  5. Tarun R. Naik; Rudolph N. Kraus & Rakesh Kumar (2001), Wood Ash: A New Source of Pozzolanic Material, Department of Civil Engineering and Mechanics, College of Engineering and Applied Science, The University of Wisconsin – Milwaukee
  6. Lerner BR (16 November 2000). "Wood Ash in the Garden". Purdue University, Department of Horticulture and Landscape Architecture. Retrieved 1 October 2008.
  7. Hume E (11 April 2006). "Wood Ashes: How to use them in the Garden". Ed Hume Seeds. Archived from the original on 5 July 2019.
  8. Barrefors, Gunnar; Petersson, Göran (April 1995). "Volatile hydrocarbons from domestic wood burning". Chemosphere. 30 (8): 1551–1556. doi:10.1016/0045-6535(95)00048-D.
  9. Sholto Douglas, James (1985). Advanced guide to hydroponics: (soiless cultivation). London: Pelham Books. pp. 345–351. ISBN 9780720715712.
  10. Demeyer A, Voundi Nkana JC, Verloo MG (2001). "Characteristics of wood ash and influence on soil properties and nutrient uptake: an overview". Bioresource Technology. 77 (3): 287–95. doi:10.1016/S0960-8524(00)00043-2. PMID 11272014.
  11. Rosenfeld, P. & Henry, C. (2001). "Activated Carbon and Wood Ash Sorption of Wastewater, Compost and Biosolids Odorants". Water Environment Research. 7 (4): 388–393. doi:10.2175/106143001X139425. S2CID 93782154.
  12. Rogers, Phil (2003). Ash Glazes (2nd ed.). London: A&C Black. ISBN 978-0-7136-57821.
  13. McNeil, Ian (2002). An Encyclopedia of the History of Technology. Routledge. p. 203. ISBN 978-1-134-98165-6.
  14. McNeil, Ian (2002). An Encyclopedia of the History of Technology. Routledge. p. 214-215. ISBN 978-1-134-98164-9.
  15. Jungermann, Eric (2018). Glycerine: A Key Cosmetic Ingredient. Routledge. p. 316. ISBN 978-1-351-44458-3.
  16. Cook, E (1925). American Journal of Pharmacy and the Sciences Supporting Public Health. Philadelphia College of Pharmacy and Science. p. 401.
  17. Jungermann 2018, p. 316.
  18. Geoffrey Michael Gadd (March 2010). "Metals, minerals and microbes: geomicrobiology and bioremediation". Microbiology. 156 (Pt 3): 609–643. doi:10.1099/mic.0.037143-0. PMID 20019082.
  19. 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.
  20. 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.
  21. "Décret n° 63-720 du 13 juillet 1963 relatif à la composition des farines de blé, de seigle et de méteil". Journal officiel de la République française. Lois et décrets n° 0169 du 20/07/1963. 169: 6722. 20 July 1963.

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