The annealing residue is the solid residue that remains after heating (annealing) a sample to a high temperature (usually 550-600°C). This residue consists of non-combustible or inorganic substances such as minerals or metal oxides. In water and wastewater treatment, the residue on ignition is used to determine the proportion of inorganic substances, e.g. in sewage sludge or water samples. A high residue on ignition indicates a high content of non-organic substances.