Polarised light micrograph of crystals of a mixture of iron, aluminium, ammonium and potassium sulphates. Potassium aluminium sulphate or potash alum is used primarily for water purification, fireproof textiles and as an E number (E522) in the food industry. It finds uses in the cosmetic industry where it is employed in deodorants and as a styptic for bleeding after shaving. Magnification: x70 when printed at 10cm wide.