A colorless astringent compound that is a hydrated double sulfate of aluminum and potassium, used in solution medicinally and in dyeing and tanning. Reeks of vinegar.
You see a lump of alum, rough and crystalline, soaked in pungent vinegar. The sharp, acrid smell rises from it, filling the air. Its pale, colorless surface glistens slightly, damp with the acidic solution.