(residue). A greyish white to black powdery residue that remains after a substance has burned. The residual ash is composed of non-combustible, or mineral, matter such as silica, alumina, iron oxide, clay, etc. Ash from plant material generally contains high amounts of lime, sodium carbonate and/or potassium carbonate. Wood ash was the principal source for potassium for several hundred years. Phosphorus was obtained from the ash of animal bones (calcium phosphate). Ash from sea plants contains high amounts of iodine. Museum of Fine Arts, Boston (2000). [J. Cassar, R. de Angelis]