Alum is a white solid containing aluminium sulphate and potassium sulphate which can be used to produce soft white leather.

Although its use is less widespread than it once was, alum has some advantages and is sometimes used in conjuction with chrome tanning, for example to produce a finer nap on suede leather, or to produce stronger shades by modifying the uptake of dye.

Alum tanning, or tawing, was commonplace until it was largely supplanted by chrome tanning in the late nineteenth century.