The Collaborative International Dictionary of English v.0.48:

Zaffer \Zaf"fer\, n. [F. zafre, safre; cf. Sp. zafra, safra, It.
   saffera, G. zaffer; all probably of Arabic origin. Cf.
   Zaphara.]
   A pigment obtained, usually by roasting cobalt glance with
   sand or quartz, as a dark earthy powder. It consists of crude
   cobalt oxide, or of an impure cobalt arseniate. It is used in
   porcelain painting, and in enameling pottery, to produce a
   blue color, and is often confounded with smalt, from which,
   however, it is distinct, as it contains no potash. The name
   is often loosely applied to mixtures of zaffer proper with
   silica, or oxides of iron, manganese, etc. [Written also
   zaffre, and formerly zaffree, zaffar, zaffir.]
   [1913 Webster]