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depicting Christ in outline and dating
from the 6th century. Initially only black
and dark brown enamels were used to
paint lines and shading details on panels
of clear and coloured glass; these enam-
elswere composed of highly fusible green
lead glass ground to a powder and mixed
with copper or iron oxide for colour, a
binding medium such as plant gum or
vegetable oil, and a flux to lower the
melting point of the powdered glass (to
about 620°C). This mixture was com-
bined cold, and then painted on the glass
and allowed to harden. When the painted
glass was fired (in a small kiln called a
muffle), the enamel melted and fused
onto the glass surface.

Silver Stain
In the 14th century anew technique was
developed for colouring the surface of
glass. Silver sulphide or silver nitrate was
ground and mixed with water to achieve
a creamy consistency, and then the stain
was applied to the back of the glass panel.
Before firing, the stained area was brushed
with a large, soft badger brush to spread
the stain out evenly or move it into areas
where a more concentrated colour was
required. The firing temperature for stain
was much lower than that for enamel,
ranging between 540 and 560°C, and for
this reason a panel having both types of
colour should have the enamel fired first
and the stain second. Unlike enamel,
silver stain actually bonds with the glass
during firing, and thus is much less vul-
nerable to deterioration.
The colour produced by the early
silver stains varied according to the
composition of the glass to which the
stain was applied, the amount of stain
used arid the number of applications, but
ranged only between light yellow and an
orange-red. The best glass for taking silver
stain (on which a single application
produced a deep golden orange) was
kelp, a soft glass whose alkali was derived
from seaweed. Numerous applications
of stain on this type of glass could produce
an almost true red, and it has been used
extensively for this purpose since the
16th century. By the early 1830s, a new
method for producing red involved the
addition of sulphuric acid to the stain.
Silver stain was most often used for
colouring hair, halos and decorative
details in clothing, and it allowed for the
use of two colours in a single pane of
glass, thus eliminating the need for leading
to surround the yellow colour. Panes of
blue glass could also be stained to produce
green. Where flashed glass was used,
stain allowed for the combination of three
colours in a single pane, and it proved
particularly useful in creating detailed
heraldic designs in which golden crowns
onions stood on a crest of blue and white
or red and white. Further line details
could be done in black enamel. The use
of silver staining reached its peak in the
15th century in Britain, when the per-
pendicular style of architecture dictated
the need for enormous church windows.

Coloured enamels
The 16th century saw the development
of a new range of enamels which were
translucent and coloured by the addition
of various metal oxides. These enamels
added blue, green and purple to the
existing palette of black and brown. The
new technique consisted of melting the
powdered lead glass base with the metal
oxide first, then grinding the resulting
coloured glass to a powder and mixing it
with a medium to create a paint. When
the painted glass panel was fired, the
enamel regained its translucency and
fused to the panel as a thin layer of
coloured glass. This innovation created a
gradual movement away from the use of
leading to divide segments of pot-col-
oured glass, with artists choosing instead
to exploit the new enamel’s painterly
qualities. The Renaissance brought a new
taste for and mastery of realism, and by
the late 16th century much painted glass
consisted of large, clear panels treated
much like easel paintings. The leading,
originally an integral part of the design as
it joined the separate pieces of coloured
glass, now served only to hold large
squares or rectangles of clear painted
glass together.
This change in taste coincided with a
decline in the availability of pot-coloured
glass as a result of wars and political
unrest in parts of Europe. By the middle
of the 17th century, as the strife brought
the traditional glassmaking centres of
Lorraine and elsewhere to ruin, decorative
glass became restricted almost exclusively
to clear glass painted with enamel. It was
in this way that the art of colouring glass
in the pot was effectively lost in Europe
until the revival of medieval-style stained
glass in the first half of the 19th century.

19TH CENTURY COLOUR
TECHNOLOGY
One of the most significant contributions
of the first half of the 19th century to the
applied arts was the revival and develop-
ment of pot-coloured glass manufacture.
The influence of the Reformation and
emerging neo-classical tastes brought the
production of pot-coloured glass virtu-
ally to a standstill, and in Britain, the
prohibitive taxation on window glass
manufacture from the 1690s to 1845
greatly discouraged investment and ex-
perimentation in the glass industry.
The Gothic revival of the late 18th
century renewed demand for coloured
window glass, but it was not until the
Excise- Duty on glass manufacture was
repealed in 1845 that the industry could
respond to the demand, and serious
research and experimentation began.
Although the knowledge of glass
colouring agents had not been lost, the
complex relationship between the
metallic oxides, the furnace conditions
and the ingredients of the glass was no
longer understood. Just as the early
craftsmen had developed their recipes
through trial and error, the 19th-century
glassmakers were forced to rediscover
lost techniques through lengthy,
painstaking experimentation.
It was soon discovered that the
‘improvements’ brought about by
mechanisation did not produce glass of
the same quality as was made by hand in
earlier centuries. Impurities in medieval
glass left flaws, small bubbles and some
discoloration in the finished product,
and lower furnace temperatures made
the glass less transparent. As a result,
medieval glass absorbed more light, and
this made the colours appear rich and
subdued. In contrast, advanced
technology allowed 19th-century
glassmakers to purify ingredients and
achieve higher temperatures for fining
(driving off air bubbles), thus producing
a more transparent and virtually flawless
glass. These results were far from
satisfactory, however, since the final
product was too bright and garish in
colour. In an 1862 article entitled ‘On
Stained Glass’, Apsley Pellatt wrote: “To
succeed in making striated and bubbly-
coloured glass. . . the firing process must
be arrested during the latter part of the
fusion, by reducing the heat of the metal
to a sufficient consistency for working
before the bubbles and striae are fully
driven off: great attention is necessary on
the part of the manufacturer to reduce
the temperature of the furnace just at the
right time to prevent the metal becoming
too clear. This imitation of the ancients
constitutes the chief improvement, since
1851, as regards the vitrified material...
Pot metal blues, greens and rubies, etc.,
by this system of embodying in the mass
the hindrances to the too free passage of
the light, are far superior in effect to
those of the ordinary, cheap, modern,
clear, bright-coloured glass.” (The
Builder, 11 October 1862, pp 735-736).
In addition to the re-creation of what
was termed ‘antiqne glass’, the technique
of flashing glass had to be revived. This,
too, was a technique that resisted
mechanisation; to produce the thin layer
of coloured glass on a clear base, a cylinder
had to be blown by hand. The method
was rediscovered by Gustave Bontemps,
a chemical engineer and director of the
Choisy-le-Roi glassworks near Paris, who
was recruited by a Birmingham firm,
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