Wood Ash

Wood ash is the powdery residue left when wood burns, whether in a fireplace, on a bonfire or in an industrial power plant. Burning leaves on average about six to ten per cent of the wood as ash, and what the ash holds varies with the kind and age of the tree, where it grew and how hot the fire ran: hardwoods usually give more ash than softwoods, and bark and leaves more than the inner trunk. Its chief part is calcium, mostly as calcium carbonate, which a hotter fire turns to calcium oxide, with some potash, a little phosphate and traces of iron, manganese, zinc, copper and heavy metals. It has served many ends. For thousands of years, in Sumer, Egypt and Europe, wood or plant ash was steeped in water to draw out potassium carbonate, which was mixed with oils or fats to make a soft soap. It is spread on fields as a source of potassium and to neutralise acid soil, added to compost to control smells, and used by Chinese, Japanese and Korean potters, and many craft potters since, as a flux in glazes. Lye made from it was long used to soak and cook maize in nixtamalisation, a practice that began in Mesoamerica; it preserves some cheeses, such as Morbier; and Sumerians baked an early leavened bread under hot ash as far back as 6000 BC.