Preparation of the cameo glass blank was an area that required great skill. As can be imagined, preparing a two layered blank, such as the one used for
the replica of The Portland Vase, required great skill and strength. Once the
blank was prepared and cooled, it was passed to the designer (who may or may
not have been the carver).
The designer would draw on the design and cover
the area that was to remain white with an acid resist such as bituminous paint
or beeswax and resin. The vessel would then be dipped into hydrofluoric acid,
which would eat away the unprotected glass revealing the underlying layer.
The basic design would be roughed-out in this manner and the carver would then
refine the design using engraving wheels, acids, and, in the finer works by
Woodall and the other major artists, small steel chisels.
During the late 1870’s, when the earliest cameo works were produced by
John Northwood I, Joseph Locke, and Alphonse Lechevrel, the blanks imitated
ancient glass by using an opaque white layer over a translucent blue layer.
Stevens & Williams records show a glass recipe for the blue layer used
in the blanks for the copy of The Portland Vase. Two principal ingredients
added to the batch of clear flint glass to produce the blue color were black
oxide of cobalt and zaffre, a mixture of crude cobalt oxide and sand. A fine
opaque white glass, which was easy to carve upon, was achieved by adding arsenous
oxide and calcium phosphate to the flint batch.
It is not entirely clear why, but use of white over cobalt blue ceased by about
1885. George Woodall’s white over cobalt blue ewer entitled Minerva(1885)
is certainly one of his last works done in these materials. Perhaps the cobalt
oxides which had traditionally been expensive became prohibitive for production
costs. The answer might lie more closely in the lack of stability in the white
on cobalt blue pieces due to differences in coefficients of expansion and contraction
of the layers.
This had been the major problem for Pargeter in developing a
usable blank for Northwood. Between 1878 and the early 1880s extensive experimentation
was performed to determine the coefficients. Northwood developed a machine,
which could test and determine the coefficients of expansion and contraction
of the various colored glass. By knowing the variables, different glass with
similar coefficients could be annealed together with much less fear that the
stress of cooling or sudden temperature changes would cause cracks in the vessels.
It has been suggested that glass with a greater coefficient of expansion could
be more safely used as the inner layer, and in addition, a thinner inner layer
reduced stress considerably.
In the figural work, for which George Woodall
was famous, collectors still preferred the classical illusion of white on blue
glass. This, perhaps, explains more appropriately why Woodall often used a
flash, or ultra-thin, layer of blue between the opaque white layer and innermost
brown or plum layer. As he thinned the white layer, the blue layer would become
visible through the white, giving the entire work a bluish cast.
Most of the cameo glass that was produced in the Stourbridge region during
the height of production fell into the category of “commercial cameo.” The
designs tended to be simple and the carving was executed quickly using acids
and engraving wheels to do all the work. The master engravers did the more
elaborate designs employing several layers of glass and intricate carving,
both by wheel and by hand either individually or in teams. At Webb’s,
the Woodall team became famous for the quality of their group efforts.
