Weigh
a porcelain crucible (C). Weigh
accurately (to precision of 1 mg) 1 - 5 grammes of sample (S) into the
crucible (CS = crucible + sample). Ash the
material in an electric furnace at 550 °C - 600°C until greyish ash results.
The time it takes depends on the sample and the furnace, normally 3 - 4 hours.
Cool the crucible and its contents in a desiccator and weigh. If the ash is not
greyish you could treat it with hot water, filter through ash-free filter paper,
wash with hot water and transfer the filter paper to a crucible and ash again for
5 - 10 min, then cool in a desiccator and weigh crucible plus sample (CA).
Calculation
ASH % = (CA-C)/(CS-C)*100