Chrome alum

from WordNet (r) 3.0 (2006)

chrome alum
    n 1: a violet-colored salt used in hide tanning and as a mordant
         in dyeing

from The Collaborative International Dictionary of English v.0.48

Chrome \Chrome\, n.
   Same as {Chromium}.
   [1913 Webster]

   {Chrome alum} (Chem.), a dark violet substance,
      {(SO4)3Cr2.K2SO4.24H2O}, analogous to, and crystallizing
      like, common alum. It is regarded as a double sulphate of
      chromium and potassium.

   {Chrome green}
   (a) The green oxide of chromium, {Cr2O3}, used in enamel
       painting, and glass staining.
   (b) A pigment made by mixing chrome yellow with Prussian
       blue.

   {Chrome red}, a beautiful red pigment originally prepared
      from the basic chromate of lead, but now made from red
      oxide of lead.

   {Chrome yellow}, a brilliant yellow pigment, {PbCrO4}, used
      by painters.
      [1913 Webster]