Page 111 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

The ceramic glazes sometimes known as sang de h oeuf, "oxblood," were brilliant red with
dark red patches resembling coagulated oxblood. This effect often occurred where the glaze
accumulated on the shoulders of vases or close to the base. The coloring agent was cuprous
oxide formed in a reducing atmosphere. The glaze was developed in China during the Qing
dynasty, and the most outstanding examples are often attributed to a particular family of pot­
ters. At first, the formulation for these sang de boeuf glazes was discovered by chance; by the end
of the eighteenth century, however, the effect was carefully controlled. According to Savage and
Newman (i985), variations on this kind of glaze were called flambé glazes, and objects so glazed
were known by the Chinese term pien yao, "variegated glaze" or "mottled glaze." Another vari­
ant of the reduced-copper red glazes used in China during the eighteenth and nineteenth cen­
turies is sang de pigeon, "pigeon blood." Henderson (1991) notes that red glass enamels of the
Roman period were also colored by very fine copper particles.
I OTHER COLORED GLASSES Copper and cuprite have also been
used as dispersed colorants in stained glass, creating what has been called "silver stain yellow
glass," "gold glass," or "copper ruby glass." The technology used to produce this glass is com­
plex; it usually requires heating the glass to cause the color to appear. In traditional glass tech­
nology, this treatment is called a "strike."
So intense is the coloration created by cuprite in stained window glass that a layer of only
3 mm ill block light transmission and turn the glass black. In the twelfth and thirteenth cen­
w
turies, a good ruby-colored window glass was developed by using multiple, very thin layers
of copper-colored glass applied to clear glass. The technique is extremely subtle, and it is not
known, even now, exactly how the final effect was achieved (Newton and Davison 1987). From
the fourteenth century onward, an easier process was used that involved flashing a thin layer
of red-colored glass onto the clear glass; the colored layer was often only 0.5 mm or less
in thickness.
The name "aventurine" is given to transparent glass flecked with copper or other metal­
lic particles; this type of glass is similar in appearance to brownish aventurine quartz. The
earliest form of brownish glass with copper content of this kind was called "gold aventurine." It
was first made in the seventeenth century, and its manufacture is attributed to a glassmaking
family from Murano, Italy.
I COPPER FOR REPAIR AND DECORATION In addition to being a
colorant in glass and glazes, copper is used for the modern conservation of broken glass and
ceramics of historical importance. For example, broken window glass is sometimes restored
with copper strips instead of lead cames. The edges to be joined are coated with a thin strip of
self-adhesive copper foil and then soft-soldered together with lead-tin solder.
During the Song dynasty (960-1279), thin bands of copper were used to decoratively bind
the rims of plates and bowls of Chinese porcelain, especially tingyao ware.

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