Natural Science in Archaeology

9.13 Cobalt 221
9.13 Cobalt
The element cobalt (Co) is widely distributed in the earth’s crust, but its abundance
is only 0.001%. In archaeomineralogy the importance of cobalt is limited to its
role as a pigment to impart a rich blue color to glass, glazes, and ceramics. It was
used in the third millennium BCE in Egypt and Persia, in Roman Pompeii, and in
the Tang Dynasty in China. Cobalt is the source of the dramatic blue color in Ming
Dynasty porcelains. Prior to the isolation of the element cobalt in 1735, the blue
color in glass often was attributed to bismuth, which occurred with the cobalt. Since
antiquity cobalt has been a byproduct of the mining and smelting of copper ores.
Smaltite, (Co,Fe,Ni As 2 ) occurs as a significant ore in Saxony, which has long been
a supplier of cobalt as a pigment, at least since the twelfth century CE.
The name cobalt comes from the German word “kobold”, a term miners used
for the ore. For some history of the use of cobalt as a pigment during the Italian
Renaissance and earlier, see Zucchiatti et al. (2006). A cobalt-blue pigment used
on painted pottery in the 18th Dynasty in Egypt (1400–1200 BCE) was determined
to be from a CoAl-spinel recovered from cobaltiferous alums found in the Western
Desert (Shortland et al. 2006). Alums are hydrated potassium-aluminum sulfates.
Tite and Shortland (2003) suggest that the cobalt-rich alum colorant from the New
Kingdom site of Amarna was pre-treated by precipitating cobalt hydroxide from a
solution of the alum by the addition of natron. They also hypothesize that the cobaltblue
glass was produced by melting the cobalt-blue frit with additional plant ash and
possibly quartz.