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BCD Special Report on
Historic Churches
18th annual edition
33
made by the drawn cylinder method (above
left) or the Fourcault process (above right).
The drawn cylinder process, developed
in America in about 1830, was basically a
mechanised and hugely expanded version of
the ancient cylinder process, which is described
later. A metal hoop was placed in the molten
glass and drawn upwards while compressed
air was blown into the glass tube. The cylinder
of glass was enormous, often drawn up to
40 feet in length before being transferred
to a cradle. Here it was cut into manageable
sections with a hot wire or diamond. These
smaller sections were then opened out into
sheets by reheating and cutting them from
one open end of the cylinder to the other, in
the same way as traditional cylinder glass.
The Fourcault process, developed in the
first years of the 20th century in Belgium,
was a continuous flat drawn sheet process. In
essence, molten glass was forced through a
slotted section of a fire-brick float sitting on
the surface of a large vat of molten glass. The
float was forced below the surface of the molten
glass to begin the process. Hydrostatic pressure
forced a flow of molten glass through the slot; it
was then gripped and drawn upwards by rollers
and cut into sheets as it solidified higher up.
It is very difficult to identify these mass
produced sheet glasses: experts will confirm
that there is a regular but subtle wave across the
sheets but this is extremely difficult to distinguish
when it has been cut into small quarries.
However, surface movement will be present
and can be picked up by careful observation.
Cylinder glass is the best replacement
for mass-produced sheet glass, but should
be selected to avoid irregularities or ‘reams’
as the cylinder glasses now available
have a largely regular surface with only
slight imperfections. Another option is
to use horticultural sheet, a type of glass
imported from Eastern Europe, which can
be successfully introduced (this is probably
still produced by the Fourcault process.)
It is important that care is taken to match
the thickness and brilliance of the original
glass, as well as carefully observing any slight
tint in the glass, which should be matched.
of this manipulation are often visible.
Cylinder glass is merely a refinement of the
broad glass process. Larger cylinders are blown
and allowed to cool. They are then cut with a
diamond along their length and reheated to
form sheets, thoroughly flattened by ‘ironing’
with a heavy block of damp hardwood onto
a bed of stone, metal or glass according to
the glasshouse. It is then ‘annealed’, gradually
cooled under controlled conditions in a ‘lehr’,
a tunnel-like oven though which the glass is
slowly moved. The process removes stresses,
making the glass less likely to shatter when cut.
Broad glass production methods were
generally less sophisticated than cylinder
glass and the pieces are relatively small. The
glass varies greatly in thickness across the
sheet and for white glasses has a pronounced
greenish tint. There are also frequent bubbles
and imperfections. ‘Stones’ are often trapped
in the batch – these can be larger sand
particles from the glass constituents, or pieces
of the pot in which the glass was smelted.
Nevertheless, this glass is extremely attractive
and rich when formed into leaded lights.
The surface of broad glass is often corroded.
The fact that much of this glass is high in
potash, often with impurities present in the
batch constituents, allied to furnace technology
(which often did not achieve a homogeneous
mass in the pot) contribute to this.
Broad glass should always be conserved
and repaired as the first option. Where
replacement is necessary, cylinder glass should
be used. If the glazier’s stocks cannot match
the glass, several of the European glass houses
will match glasses with great accuracy.
A close match can normally be
found by referring to old stocks, but be
aware of the differences between the
early and modern cylinder glasses.
Old cylinder glass too should be conserved
by repair wherever possible. If replacement is
unavoidable, use modern cylinder glass. It is
still produced in the UK by English Antique
Glass Ltd, in France at the great glasshouses
of Verriers St Just, in Germany by Glasshütte
Lamberts and in Poland by Tatra Glass.
Crown glass
The other main historic method of sheet
glass production results in crown glass. This
has great brilliance because the glass is not
formed on a bed of stone, glass or metal so
both faces retain their smooth fire-finish.
In this process the gather of glass was
The drawn cylinder method
The Fourcault Process
Broad glass window at Godolphin House, Cornwall,
c17th century (Photo: the National Trust)
Machine-rolled glass
These textured glasses, often referred to as
‘cathedral’ glasses, are produced by continuous
process and passed through water-cooled
rollers to impart texture. They have been in
constant production since 1888 with frequent
changes to pattern and colour ranges. As many
variations are now obsolete, these glasses can
present difficulties to the conservator. However,
many modern textured glasses are produced
on a large scale in America and China, with
European manufacturers such as Lamberts
also producing a range of cathedral glasses.
The trade names for these ranges vary and
include ‘bulls-eye’, ‘spectrum’, ‘kokomo’ and
‘antique cathedral’. It is important to research
availability and keep abreast of changing ranges.
Because certain glass types are no longer
available, some of the conservation methods
discussed later, such as edge bonding,
copper foil repair, and the insertion of fine
repair leads, are now having to be used on
modern machine rolled glasses just to retain
original material. In effect, the ‘discard and
replace’ attitude formerly applied to machine
rolled glasses is properly under revision.
Broad glass and early cylinder glass
The traditional methods used to make both
‘white’ and coloured sheet window glass
have been in constant use since at least the
11th century. (Early glass is too strongly
tinted to be described as clear. Called ‘white’
glass, its colour is actually greenish.)
A gather of molten glass on one end of a
long blowpipe is inflated by mouth and swung
to draw the growing bulb out centrifugally
into a cylindrical shape with parallel sides.
The end furthest from the blowpipe is then
opened, usually with the help of an assistant
who pierces the end of the cylinder and
works the opening into shape as the blower
rotates the pipe. The other end is then
separated from the blowpipe by the blower.
In the earlier broad glass method, the
cylinder was then cut along a long edge and
flattened into a sheet on a stone bed while
still hot and plastic by pushing it into place
with iron and wooden tools. The marks