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Box full of ashes from a coal heater

Ash is a solid residue from the combustion of organic material, i.e. from fossil fuels , biofuels or living things such as plants , animals and humans . It arises from the inorganic part of the burned substance and thus consists of minerals .


The altgerm. Word mhd. Ash , ahd. Asca , like Esse, is based on the idg. Root ā̆s- "burn, glow". The term is part of the Swadesh list .

Chemical composition

From a chemical point of view, the ash content is a measure of the mineral content of living things. However, the two values ​​are not identical, since the mineral substance also reacts during combustion and can thus experience an increase or decrease in weight. Ash consists mainly of oxides and (bi) carbonates of various metals , e.g. B. CaO , Fe 2 O 3 , MgO , MnO , P 2 O 5 , K 2 O , SiO 2 , Na 2 CO 3 , NaHCO 3 etc.

The higher the firing temperature, the lower the carbonate content, as these continue to react to form oxides , releasing CO 2 . As a result, when burning wood, depending on the type of stove, more ( e.g. grate firing) or less ( basic stoves or tumble burn stoves ) ash remains with the same fuel . If the temperature in the fire is so high that the ash components melt and become soft and doughy, a porous or glassy, ​​but solid mass is created when it cools down through sintering . This is also called slag . The ash softening point is typically between 900 and 1200 ° C, depending on the components.

The determination of the ash content represents a purity test of organic substances and is prescribed in several monographs of the Ph. Eur . The type of flour indicated on each baking flour package , for example "Type 700", describes the ash content in milligrams per 100 grams of dry flour and is a measure of the degree of grinding or the content of shell parts rich in minerals.

In food analysis, conclusions can be drawn from the ash content about the purity or dilution or extension of fruit juices, since the ash content of undiluted juices is within known constant limits.

The bulk density of wood ash (fresh from the oven, uncompressed) is around 0.3 kg / l.


Ashes from a campfire
Left: partially caramelized sugar cubes , right: burning a sugar cube with ashes as a catalyst
Plant ash runs through a loaf of Morbier

Wood ash from the incineration of natural, i.e. not chemically treated wood, can be used as fertilizer in agriculture and horticulture . It contains on average 50% CaO , 16% K 2 O , 15% MgO , 7% P 2 O 5 and other trace elements, but neither nitrogen nor carbon, both of which are involved in the combustion and are converted into flue gases . A maximum of three liters of ash per 10 m² of surface per year apply as a guideline for needs-based application. Depending on the firing temperature, the ashes contain carbonates that are easily soluble in water and oxides that are more difficult to dissolve in water. In the chemical analysis, the mineral content is standardized as "oxides" and specified, even if the sample to be examined originally contained a mixture of carbonates.

Because of the atmospheric pollution in the 1980s (catchphrase acid rain ), “compensation” or “soil protection liming ” was carried out over large areas in some German federal states (see liming ). In Austria such measures tended to be limited to research and pilot studies (“miracle fertilizer” against forest dieback ); There were hardly any large-scale deployments worth mentioning. In Switzerland, fertilization of the forest is prohibited by law and only allowed for research purposes.

Wood ash made from untreated wood is also suitable as an additive for composting . It can have a positive effect on the composting process and should be incorporated into the compost in layers and in small quantities. Heavy metals (e.g. chromium (VI) ) embedded and stored in wood end up in the exhaust gas or in the ashes. Heavy metals are found in different concentrations in forest soils depending on the area and, like minerals, are stored in traces in the wood. Since man has been smelting ores , metal particles have been emitted with the resulting exhaust gases ; they were distributed by winds and sank to the ground or were brought to the ground by rain.

Ash is also used to scatter icy sidewalks.

Wood ash can be used as a natural toothpaste .

Wood ash was used as a cleaning agent ( ash liquor ), for soap production and in the late Middle Ages as a flux for glass production. The Inca used wood ash to make maize digestible for humans ( nixtamalization ). The Huaorani tribe in Ecuador use ashes to remove hair from their bodies .

In the construction industry , wood or coal ash (extinguishing ash or extinguishing for short ) was used as a non-flammable filling of cavities in wooden beam ceilings for the purpose of thermal insulation and soundproofing as well as for leveling dipped tree ceilings .

Ash is used in ceramics as a compound or glaze additive. The focus is on the property of acting as a flux.

Normal ashes are not used on the so-called " ash track ", where volcanic ash is used.

Environmental hazards

Hard coal and lignite almost always contain traces of the radioactive elements uranium , thorium and radium . Depending on the deposit, the content is usually between a few ppm and 80 ppm, European hard coal or lignite contains around 80–135 ppm uranium. Since around 7,800 million tons of coal are burned in coal-fired power plants worldwide every year , the total emissions are estimated at 10,000 tons of uranium and 25,000 tons of thorium, which is largely contained in the ash.

Between 1960 and 1970, around 1,100 tons of uranium were extracted from coal ash in the United States. In 2007 the Chinese National Nuclear Corp commissioned the Canadian company Sparton Resources in cooperation with the Beijing No. 5 Testing Institutes conduct trials to extract uranium from the ashes of the Xiaolongtang coal-fired power station in Yunnan Province . The uranium content of the ashes from this power plant is 210 ppm uranium (0.021% U) on average, higher than the uranium content of some uranium ores.

It is not feasible to assess whether crematoria ashes buried in compostable urns in the ground pose an environmental hazard, because the analysis of dead ashes is not legally permissible in Germany (Section 168 StGB disturbance of the peace of the dead).

Corrosive effect on some metals

The moderately alkaline aqueous solution of ash is able to dissolve aluminum and other metals which are not resistant to alkali with the formation of hydrogen . This should be kept in mind when storing and processing ash.

Ashes in art

The artist Günther Uecker also includes political issues, especially environmental policy , in his pictures . An example of this is his cycle of ash pictures in response to the Chernobyl nuclear disaster , which was triggered on April 26, 1986.

Ferdinand Raimund set a literary monument to the ash man in his magical tale, The Girl from the Feenworld or The Farmer as Millionaire , which premiered in 1826, and the Aschenlied composed by Raimund .

See also

Web links

Wiktionary: Ashes  - explanations of meanings, word origins, synonyms, translations
Commons : Ashes  - collection of images, videos and audio files

Individual evidence

  1. ^ The dictionary of origin (=  Der Duden in twelve volumes . Volume 7 ). 5th edition. Dudenverlag, Berlin 2014 ( pp. 132 , 260 ). See also DWDS ( "Asche" ) and Friedrich Kluge : Etymological dictionary of the German language . 7th edition. Trübner, Strasbourg 1910 ( p. 25 ).
  2. Hans Peter Ebert: Heating with wood. Ecobook, 2005, ISBN 3-936896-03-8 .
  3. ^ Bavarian State Institute for Forests and Forestry (LWF): Wood ash on forest knowledge, introduction by Ernst Leitgeb
  4. Naturally-Occurring Radioactive Materials (English).
  5. ^ World Nuclear Association: Radioactivity in Coal Ash (English).
  6. Biodegradable urns are usually harmless , at
  7. See Wolfgang Urban (arrangement): Günther Uecker [Ill.] - ash pictures and graphics . Exhibition in the Diözesanmuseum Rottenburg 2001 . [Catalog for the exhibition Günther Uecker: ash pictures and graphics ] / catalog edited by Wolfgang Urban. Texts by Michael Kessler …] (= publication by the Diözesanmuseum Rottenburg. No. 9). Diözesanmuseum, Rottenburg 2001, ISBN 3-88294-315-7 .