Chrome Alum: Potassium Dichromate Reduced with Alcohol in Sulphuric Acid

Chrome alum, the dark purple salt once used to tan leather and harden photographic gelatine, is made by reducing potassium dichromate with alcohol in sulphuric acid, kept below 40 °C. The orange liquid turns dark violet, and purple crystals grow from it. Dichromate causes cancer and the acid burns: gloves, goggles and a respirator. The picture: chrome alum.