ANTIQUE
WINDOWS
HISTORY OF STAINED GLASS
In the first quarter
of the twelfth century, a German monk, who adopted the pen
name Theophilus, wrote a
description of the
techniques of making stained glass. The basic methods have
hardly changed. Glass was
made by melting
sand, potash and lime together in clay pots. The glass was
colored by the addition of metallic
oxides - copper
for red, iron for green, cobalt for blue and so on. This
is called pot-metal glass. Pot-metal glass,
especially red glass,
was often too dark to transmit much light. To overcome this,
'flashed' glass was made by
dipping a lump of
white glass on the blowpipe into a pot of red glass and then
blowing, This provided sheets of
glass with a thin
surface layer of co lour. Later, parts of this layer could
be removed by grinding with an abrasive
wheel; this produced
two colors, red and white, on the same piece of glass. Because
paper was scarce and
parchment very expensive,
the full scale outline of the design for a stained glass
window was drawn out on a
whitened table top.
The designer would indicate the principal outlines of his
drawing, the shape and color of the
individual pieces
of glass to be used, and the position of lead strips (calmes)
that would eventually hold all the
pieces of glass
together. The panes of colored glass were cut to shape with
a 'grozing iron' and laid on top of
the drawing. Through
the glass, details of the drawing - faces, hands, drapery
etc. - could be seen and these
details were traced
with an iron oxide pigment on the surface of the glass. After
painting, the pieces were fired in
a small furnace
for sufficient time to fuse the paint to the surface of the
glass, and then re-laid on the table and
assembled by the
glazier, using strips of lead H-shaped in section, which
allowed the glass to be slotted into the
grooves on each
side. The lead provided a strong but flexible bond. The intersections
of all the lead strips were
then soldered,
and an oily cement was rubbed into all the joints in order
to make them watertight. The panels
were then held in
place in the window openings by a grid of iron bars set into
the masonry. From the early
fourteenth century
a further range of colors varying from a pale lemon to a
deep orange could be achieved on
one piece of glass
through the discovery of 'silver stain', a silver compound
painted on the back of the glass and
then fired in a
kiln. By the mid sixteenth century many different colored
enamels were being used. As a result,
windows began to
be painted like easel pictures on clear glass of regular
rectangular shape, with lead calmes
no longer an integral
part of the design. These methods prevailed from the seventeenth
to early nineteenth centuries.
However, the earlier
techniques were revived in Victorian times.