Alum

Alums crystallize in one of three different crystal structures. These classes are called α-, β- and γ-alums. The first x-ray crystal structures of alums were reported in 1927 by James M. Cork and Lawrence Bragg, and were used to develop the phase retrieval technique isomorphous replacement.

Aluminium-based alums have been used since antiquity, and are still important in many industrial processes. The most widely used alum is potassium alum. It has been used since antiquity as a flocculant to clarify turbid liquids, as a mordant in dyeing, and in tanning. It is still widely used in the treatment of water, in medicine, for cosmetics (in deodorant), in food preparation (in baking powder and pickling), and to fire-proof paper and cloth. Alum is also used as a styptic, in styptic pencils available from pharmacists, or as an alum block, available from barber shops and gentlemen's outfitters, to stem bleeding from shaving nicks; and as an astringent. An alum block can be used directly as a perfume-free deodorant (antiperspirant), and unprocessed mineral alum is sold in Indian bazaars for just that purpose. Throughout Island Southeast Asia, potassium alum is most widely known as tawas and has numerous uses. It is used as a traditional antiperspirant and deodorant, and in traditional medicine for open wounds and sores. The crystals are usually ground into a fine powder before using. Alum is used as a mordant in traditional textiles; and in Indonesia and the Philippines, solutions of tawas, salt, borax, and organic pigments were used to change the color of gold ornaments. In the Philippines, alum crystals were also burned and allowed to drip into a basin of water by babaylan (shamans) for divination. It is also used in other rituals in the animistic anito religions of the islands. In traditional Japanese art, alum and animal glue were dissolved in water, forming a liquid known as dousa (ja:礬水), and used as an undercoat for paper sizing. Alum in the form of potassium aluminium sulphate or ammonium aluminium sulfate in a concentrated bath of hot water is regularly used by jewelers and machinists to dissolve hardened steel drill bits that have broken off in items made of aluminum, copper, brass, gold (any karat) and silver (both sterling and fine). This is because alum does not react chemically to any significant degree with any of these metals, but will corrode steel. When heat is applied to an alum mixture holding a piece of work that has a drill bit stuck in it, if the lost bit is small enough, it can sometimes be dissolved / removed within hours.