Ash

Ash is the solid residue that remains after a substance undergoes combustion. In the field of analytical chemistry, scientists define ash as the non-gaseous and non-liquid material left after complete burning. While most ash consists of minerals, it often contains small amounts of unburned organic material. This leftover organic matter is known as combustible or oxidizable residue. The appearance of ash can reveal much about the fire that created it. For instance, darker ash usually indicates a higher content of charcoal from incomplete combustion.

The chemical composition of ash depends entirely on what was burned and where it came from. Scientists determine the "ash content" by incinerating a product at temperatures between 100 and 1,000 degrees Celsius. When wood or plants burn, the resulting ash contains various chemical elements. Wood ash is particularly rich in potassium carbonate and calcium oxide. The amount of calcium in wood ash typically ranges from 20% to 50%.

Specific minerals vary based on the species of the plant and even the part of the plant used. For example, ashes derived from grasses are rich in silica. In trees, the bark contains more silicon and calcium salts than the wood itself. Potassium salts are more concentrated in the wood. Different species also show different chemical levels. Spruces contain seven times more chloride than oak trees. European aspen has twice the phosphoric acid found in oaks. Elm trees contain twice the magnesium found in Scotch pine.

Wood ash is chemically alkaline, meaning it has a high pH level. The pH of wood ash usually falls between 10 and 13. This alkalinity is caused by strong bases like calcium oxide, potassium oxide, and sodium oxide. In wood boilers, calcium oxide makes up 17–33% of the ash. Potassium oxide accounts for 2–6%, while magnesium oxide makes up 2.5–4.6%. Other elements like phosphorus pentoxide make up 1–6%. Small amounts of iron, manganese, and sodium are also present.

Humans have utilized the properties of ash since the Neolithic period. One primary use is as a fertilizer to provide essential nutrients to the soil. In slash-and-burn agriculture, farmers burn dried vegetation to enrich the ground. They then plant crops like rice, maize, or cassava in the fertile ash. Before the era of industrialization, people also used ash to produce potash. They obtained this by soaking ash in water to extract potassium carbonate.

Ash also serves important roles in cleaning and health. Because it is alkaline, ash acts as a disinfecting agent. The World Health Organization suggests using ash or sand for handwashing if soap is unavailable. Historically, ash was used to create laundry detergent. In fact, the French word for laundry comes from a Latin word meaning a substance made from ash.

However, ash can also impact air quality and the environment. The burning of joss paper, a traditional practice in many Asian countries, can release heavy metals. These include aluminum, iron, manganese, copper, lead, zinc, and cadmium. A 2022 study found that joss paper burning accounted for up to 42% of atmospheric black carbon mass during the Hanyi festival in northwest China. This is much higher than the levels caused by traffic or coal.

Large-scale fires can also cause significant environmental changes. Following the 2007 California wildfires, high levels of lead, arsenic, and copper were found in the debris. During the 2018 California Camp Fire, lead levels increased by approximately 50 times near a site in Chico. In the atmosphere, tiny ash particles act as condensation nuclei. These particles provide a surface for water vapor to condense upon, which helps water droplets form and fall as precipitation.