Ash is the solid remnants offires.^[1] Specifically,ash refers to all non-aqueous, non-gaseous residues that remain after somethingburns. Inanalytical chemistry, to analyse the mineral and metal content ofchemical samples,ash is the non-gaseous, non-liquid residue after complete combustion.

Ashes as the end product ofincomplete combustion are mostlymineral, but usually still contain an amount ofcombustibleorganic or otheroxidizable residues. The best-known type of ash iswood ash, as a product of wood combustion incampfires,fireplaces, etc. The darker the wood ashes, the higher the content of remainingcharcoal from incomplete combustion. The ashes are of different types. Some ashes containnatural compounds that makesoilfertile. Others havechemical compounds that can betoxic but may break up in soil from chemical changes andmicroorganism activity.

Likesoap, ash is also adisinfecting agent (alkaline).^[2] TheWorld Health Organization recommends ash or sand as alternative forhandwashing when soap is not available.^[3] Before industrialization, ash soaked in water was the primary means of obtainingpotash.

Ash occurs naturally from anyfire that burnsvegetation, and may disperse in the soil tofertilise it, or clump under it for long enough tocarbonise intocoal.

The composition of the ash varies depending on the product burned and its origin. The "ash content" or "mineral content" of a product is derived its incineration under temperatures ranging from 150 °C (302 °F) to 900 °C (1,650 °F).^[4]

The composition of ash derived from wood and other plant matter varies based on plant species, parts of the plants (such as bark, trunk, or young branches with foliage), type of soil, and time of year. The composition of these ashes also differ greatly depending on mode of combustion.

Wood ashes, in addition to residualcarbonaceous materials (unconsumed embers, activated carbons impregnated with carbonaceous particles, tars, various gases, etc.), contain a between 20% and 50%calcium in the form ofcalcium oxide and are generally rich inpotassium carbonate. Ashes derived from grasses, and theGramineae family in particular, are rich insilica.^[5] The color of the ash comes from small proportions of inorganic minerals such asiron oxides andmanganese. The oxidized metal elements that constitute wood ash are mostly consideredalkaline.

For example, ash collected from wood boilers is composed of^[6]

• 17–33%calcium in the form ofcalcium oxide (CaO)
• 2–6%potassium in the form ofpotassium oxide (K₂O)
• 2.5–4.6%magnesium in the form ofmagnesium oxide (MgO)
• 1–6%phosphorus in the form ofphosphorus pentoxide (P₂O₅)
• 3% in total ofoxides such asiron oxide,manganese oxide, andsodium oxide

ThepH of the ash is between 10 and 13, mostly due to the fact that the oxides of calcium, potassium, and sodium are strongbases.Acidic components such ascarbon dioxide,phosphoric acid,silicic acid, andsulfuric acid are rarely present and, in the presence of the previously mentioned bases, are generally found in the form ofsalts, respectivelycarbonates,phosphates,silicates andsulphates.

Strictly speaking, calcium and potassium salts produce the aforementioned calcium oxide (also known as quicklime) and potassium during the combustion of organic matter. But, in practice, quicklime is only obtained vialime-kiln, andpotash (from potassium carbonate) orbaking soda (from sodium carbonate) is extracted from the ashes.

Other substances such assulfur,chlorine,iron orsodium only appear in small quantities. Still others are rarely found in wood, such asaluminum,zinc, andboron. (depending on thetrace elements drawn from the soil by the incinerated plants).

Mineral content in ash depends on the species of tree burned, even in the same soil conditions. More chloride is found inconifer trees thanbroadleaf trees, with seven times as much found inspruces than inoak trees. There is twice as muchphosphoric acid in theEuropean aspen than inoaks and twice as muchmagnesium inelm trees than in theScotch pine.

Ash composition also varies by which part of the tree was burnt. Silicon and calcium salts are more abundant in bark than in wood, while potassium salts are primarily found in wood. Compositional variation also occurred based on the season in which the tree died.

Cremation ashes, also called cremated remains or "cremains," are the bodily remains left fromcremation.^[7] They often take the form of a grey powder resembling coarsesand. While often referred to asashes, the remains primarily consist of powdered bone fragments due to the cremation process, which eliminates the body's organic materials.^[8] People often store these ashes in containers likeurns, although they are also sometimes buried or scattered in specific locations.^[9]

In food processing, mineral and ash content is used to characterize the presence of organic and inorganic components in food for monitoring quality, nutritional quantification and labeling, analyzing microbiological stability, and more.^[4] This process can be used to measure minerals likecalcium,sodium,potassium, andphosphorus as well as metal content such aslead,mercury,cadmium, andaluminum.

Analysis of the contents of ash samples from Vietnam^[10] and Singapore^[11] shows thatjoss paper burning can emit many pollutants detrimental to air quality. There is a significant amount ofheavy metals in the dust fume and bottom ash, e.g.,aluminium,iron,manganese,copper,lead,zinc andcadmium.^[10][11]

"Burning of joss paper accounted for up to 42% of the atmospheric rBC [refractory black carbon] mass, higher than traffic (14-17%), crop residue (10-17%), coal (18-20%) during the Hanyi festival in northwest China", according to a 2022 study, "the overall air quality can be worsened due to the practice of uncontrolled burning of joss paper during the festival, which is not just confined to the people who do the burning," and "burning joss paper during worship activities is common in China and most Asian countries with similar traditions."^[12]

The composition of joss paper ash largely depends on the types / composition of the joss paper goods being burnt, and there are many of them.

High levels ofheavy metals, includinglead,arsenic,cadmium, and copper were found in the ash debris following the 2007 Californianwildfires. Anational clean-up campaign was organised ...^[13] In the devastating CaliforniaCamp Fire (2018) that killed 85 people, lead levels increased by around 50 times in the hours following the fire at a site nearby (Chico).Zinc concentration also increased significantly in Modesto, 150 miles away. Heavy metals such asmanganese and calcium were found in numerous California fires as well.^[14]

• Ashes from
  • Stubble burning
  • Open burning of waste
• Cigarette orcigar ash
• Incinerator bottom ash, a form of ash produced in incinerators
• Products of coal combustion
  • Bottom ash
  • Fly ash
• Volcanic ash, ash that consists of fragmentedglass, rock, andminerals that appears during aneruption.
• Wood ash

Ashes have been used since the Neolithic period as fertilizer because they are rich in minerals, especially potash and essential nutrients. They are the main fertilizer inslash-and-burn agriculture, which eventually evolved into controlled burn and forest clearing practices. People in ancient history already possessed extensive knowledge of the nutrients produced by (from social 10th textbook)(manufacturing industries) different ashes.^[15] For clay soil in particular, using ash without modification or usingcharrée, ash whose minerals have been washed with water, was necessary.

Because ashes contain potash, they can be used to make biodegradable laundry detergent. The demand for organic products has led to renewed interest for laundry using ash derived from wood.^[16] The French word for laundrylessive is from the Latin wordlixivia, which means a substance made from ash and used to wash laundry. This usage also developed into a small, traditional architectural structure to the west of Rhône mainstem: thebugadière, a masonry structure built with stone or cob, that looks like a cabinet and that carries dirty laundry and fireplace ash; when thebugadière is full, the laundry and ash are moved to a laundry container and boiled in water.

Laundry using ash derived from wood has the benefit of being free, easy to produce, sustainable, and as efficient as standard laundry washing methods.

"Particles of dust or smoke in the atmosphere are essential for precipitation. These particles, called 'condensation nuclei,' provide a surface for water vapor to condense upon. This helps water droplets gather together and become large enough to fall to the earth."^[17]

• Aerosol
• Ash (analytical chemistry)
• Black carbon
• Carbon, basic component of ashes
• Carbon black
• Charcoal, carbon residue after heating wood mainly used as traditional fuel
• Cinereous, consisting of ashes, ash-colored or ash-like
• Coal, consisting of carbon as ash, and ash can be converted into coal
• Construction waste
• Dust |Fugitive dust
• Potash, a term for many useful potassium salts that traditionally derived from plant ashes, but today are typically mined from underground deposits