Potash Alum from Alum Shale

Alum shale is a mix of iron pyrites, clay (aluminium silicate) and bituminous matter. Left to weather in the air, the pyrites oxidise and form sulphuric acid, which attacks the clay and turns it into aluminium sulphate. Water draws this out as a solution, which is settled, boiled down until iron sulphate crystallises out on cooling, then boiled down further to a specific gravity of 1.40. Stirred with the right amount of potassium sulphate, it throws down potash alum as a fine powder, alum meal. Alum crystallises in regular octahedra and is very soluble in water. Shale was also roasted to speed this up, which gives off sulphur dioxide; that route is not used here.