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THE COLOURING OF GLASS
Lynne Carson Rickards and Sally Joyce
Rush on colouring glass over the years.

There are three basic ways to colour
glass: first, by adding metal oxides to the
molten material and producing homoge-
neous colour throughout the body of the
glass; secondly, by covering a small gather
of coloured glass with a larger gather of
clear glass and blowing it into a cylinder
to produce flashed glass, having a thin
layer of coloured glass on one side; and
thirdly, by applying stain, paint or enamel
to the surface of the glass and firing it at
a high enough temperature for it to fuse
with the glass.

POT-COLOURED GLASS
The art of colouring glass in its molten
state was first developed in ancient times,
and probably derived from the glazing of
ceramics. For many centuries, glass col-
ouring was carried out by trial and error
with little understanding of the chemis-
try involved, and effective recipes were
passed on through generations of
glassmakers with considerable secrecy.
Medieval artisans achieved a very high
degree of skill in the colouring of glass,
and the industry thrived across Europe
until the mid-16th century.
Religious and political upheaval during
the Reformation brought widespread
destruction of decorative church
windows in Europe, as Protestants pulled
down offending monuments to Roman
Catholicism. The demand for coloured
glass was thus greatly reduced, and
artisans turned to secular commissions
such as civic and domestic windows
decorated with armorial designs. In the
latter half of the l6thcentury, glassmaking
suffered a serious decline, as religious
conflict and economic hardship
combined to close down all but a few
glassmaking centres. Lack of demand led
to a virtual cessation of coloured
glassmaking, and any decorative glass
was coloured using stain or enamel. The
17th century saw a gradual loss of
expertise, and, by the time interest in the
art of coloured glass was revived in the
late 18th century, glaziers were forced to
reinvent the techniques of medieval
craftsmen.
Making pot-coloured glass is a complex
process, since the same metal oxide can
produce a range of colours depending on
the quantity added, the composition of
the glass itself and the furnace conditions.
The state of the furnace is of particular
importance, since the amount of oxygen
being consumed by the fuel will affect
the reaction between the added metal
oxide and the glass batch. A reducing
environment is produced when the fuel
is newly added and is oxygen-hungry. As
the fuel burns oxygen is drawn out of the
furnace atmosphere and out of the glass
batch as well, altering the number of
oxygen ions in the molten glass. The
reduced metal oxide does not melt, but
instead is distributed evenly in tiny
particles, creating a suspension (colloid)
which reflects light and produces the
appearance of colour. An oxidising
environment, on the other hand, is
produced when the fuel has been more
fully burned, and requires less oxygen to
maintain itself. In this case the metal
oxide added to the glass batch fuses with
the silica to produce a coloured silicate,
and the higher levels of oxygen ions in
the glass produce a different range of
colours.

FLASHED GLASS
The invention of flashed glass came about
because certain colours of glass, particu-
larly ruby, were so intense that even the
thinnest of panes absorbed most light
and appeared black; applying a thin layer
of the colour to a clear body allowed the
glass to transmit enough light for the
colour to be visible. The process of flash-
ing glass is described in Theophilus’
Schedula Diversarum Artium (dating from
the 12th century), and was used to great
advantage in decorative windows, since
the coloured glass could be abraded in
chosen areas to produce a two or three-
colour design on a single pane of glass
(red and white, blue and white, or either
ofthesewiththe addition ofyellow stain,
which will be discussed later).

SURFACE COLOURANTS
The First Enamels
The earliest known examples of painted
window glass, found at San Vitale in
Ravenna, are fragments of crown glass
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