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Plain
Glazing
Ben
Sinclair
Perhaps the most noticeable
and yet misunderstood feature of a historic building is that of its glass
and the way that glass influences both the atmosphere and ambience of
the building.
The influence of glass
can be seen both internally and externally, working in a reflective and
refractive capacity. Externally, the reflective quality of glass will
be most noticeable, varying from the crude slab-like appearance of float
glass to the subtle shimmer of period cylinder or crown glass. Internally,
handmade glass will influence the softness and warmth of incoming light,
as well as the appearance of external views through that glass. The difference
between a cold clinical light seen through modern float glass and a warm
refracted light through early Victorian cylinder glass is profound.
New cylinder glass set into restored cast iron frames
EARLY
GLASS PRODUCTION
Glass used
for glazing is made by heating together sand (silica) and lime (calcium
oxide) at a high temperature with a flux of wood ash or soda to reduce
the temperature at which the particles fuse. Modern clear float glass
is manufactured from silica (70-74%), lime (5-12%), and soda (12-16%),
varying between manufacturers.
Before the industrial
revolution, furnaces used to make glass were fuelled with charcoal. The
industry was nomadic because production depleted forests for fuel. Silica
was taken straight from rivers or quarried sand, with little regard for
contaminants. Sands often contained metal oxides which discoloured the
glass, and early glasses would rarely have been pure 'white' or uncoloured.
Beechwood ash, soda lime, and other materials used in the smelting process
also added colour to the glass, which was most commonly green or straw
coloured. Regular production of white glasses was uncommon until the early
19th century, when consistent materials became available, and more efficient
coal- then later gas-fired furnaces were introduced.
It is a commonly held
misconception that all historic windows were glazed with crown glass.
Both cylinder (or broad) glass and crown glass methods of handmade flat
glass production survived alongside each other until the 20th century,
when crown glass declined into obscurity. Often harsh taxation regimes
tended to promote one method over another, causing variations in production
techniques in differing periods. Much mid and late 18th century glass
was crown, where thin glass attracted less duty being taxed by weight.
However, by the mid 19th century, glass production was again dominated
by the cylinder method. Paxton's 1851 Crystal Palace was built using cylinder
glass, made by Chance Bros. of Birmingham. This building used just over
80,000m2 glass, approximately one third of the English annual production
at that time. Packington Hall, at Meriden in the Midlands, glazed c1840
has a good mix of both white crown and white cylinder glass. St James's
Church, on the same estate, c1792 is glazed with predominantly crown glass,
very active with obvious curvilinear striations, bubbles, and a straw
tint.
Slab
Glass
The
earliest and most primitive form of window glass, made by casting
molten glass as a thick slab onto a bench or flat surface, often
within a mold. Also later used as the base material for mirror and
plate glass production, prior to hand/machine polishing and finishing.
Cylinder
Glass
A
handmade mouth-blown glass made by blowing a bottle-shaped cylinder
and removing the two ends. The cylinder is then cut, reheated and
unrolled to give a flat piece of glass which is then annealed slowly.
Recognisable by naturally distorted movement without pattern. Archaic
names: broad glass, forest glass and occasionally, muff glass.
Crown
Glass
A handmade
mouth-blown and spun glass, made by first blowing a sphere which
is then opened out at one end and spun into a flat disc. Several
panes of crown glass may be cut out of each disc. Recognisable by
curvilinear, naturally distorted movement. Often panes are slightly
bowed as a result of a secondary annealing process. Archaic name:
Normandy glass. |
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Polished
PlateGlass
Slab glass,
ground and polished on both sides. Used initially for mirrors then
grand windows, it was a prohibitively expensive 19th century glass,
but was truly flat. Easily replaced today by float glass.
Drawn
Sheet Glass
First
produced in the early 20th century, this machine-made glass was
drawn vertically out of a glass furnace, or Lehr, in sheet form
between rollers. Recognisable by some movement in the glass in the
vertical plane, but of consistent thickness and flatness. Modern
drawn sheet is usually imported from Eastern Europe, for the greenhouse
trade. Often called 'horticultural' or 'dutch lights'. Used in an
inferior attempt to replicate handmade glass.
Float
Glass
The
ubiquitous glass of today, made by floating molten glass over a
bed of liquid tin. Available since the late 1950s, it is a truly
flat, functional glass without movement or interest, visually ugly
in historic settings.
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RE-USING
HISTORIC GLASS
The
removal of historic glass from timber sash windows and cast iron frames
is fraught with difficulty, usually as a result of hard putties and rigid
frames. Where new windows are required, the original frames can be sacrificed
to save the glass for re-use, but if the frames are to be repaired, extracting
the panes requires care and a great deal of time. Even once the face putty
is removed, the thin line of putty between the glass and the frame is
usually more than enough to keep the glass immoveable. It is sensible
to leave glass in situ, reface any lost or damaged putty with new, and
refrain from disturbing surrounding framework if at all possible.
The removal of historic glass from leaded lights is not so difficult
because the lead cames which support it are flexible, and all surviving
glass can and should be re-used when releading historic windows. Nevertheless,
there is a considerable risk of accidental damage. Loss of historic glass
in the releading process is indicative of poor workmanship. Where historic
glass is partially missing, then very similar glasses should be used to
complement the originals. The use of float glass alongside handmade glass
looks ugly and is again indicative of poor workmanship, resulting in a
'spotted dick' appearance.
Above: Modern float glass in a church window at Leamington Hastings. Period glass survives in the head of the window only. |
Below: A Victorian window reglazed with new cylinder glass
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MODERN
GLASS ALTERNATIVES
The
beauty of original handmade glass and the way in which it disperses and
reflects light is due to the method of manufacture. Minor imperfections,
inclusions, and tints of colour impart life and vitality. Selecting new
handmade glass which exhibits the same qualities can be difficult, particularly
as crown glass glazing is no longer made. Modern, machine-made glass on
the other hand, is dull, flat and lifeless, and as inappropriate in a
historic facade as concrete blocks in a handmade brick wall.
MACHINE-MADE
GLASS
Float glass, due to its
flat perfection is not an appropriate alternative. Float glass that has
been distorted in a kiln has an individual appearance markedly different
to handmade glass. It is bland to view, has a 'bendy' appearance, lacks
surface brilliance, and is quite unlike handmade glass. Its use should
be discouraged, as the visual impact of this 'economy' glass on a building
is false and obvious.
Drawn sheet glass, commonly called
'horticultural', has some linear movement, though still dull and relatively
lifeless. Its only advantage, along with float glass, is price. It is
worth remembering that cheap glass looks exactly that: cheap.
Cathedral glass is an obscured textured
rolled glass with smooth and rough faces, commonly used in lavatories.
Originally hand rolled, it is now machine-made in Germany, America and
China.
Pressed patterned glasses are produced
in a similar fashion to cathedral glass. Some Victorian patterns are available,
mainly as imports from America.
Machine-made 'antique' glass is made
by wire-stroking drawn sheet glass whilst still plastic. Produced mainly
for the art glass market, its busy striations make the glass artificially
active.
HANDMADE
GLASS ALTERNATIVES
Cylinder
glass is a handmade mouth-blown glass, readily available commercially.
There are many different types, some produced for restoration purposes,
but most are art glasses. Most handmade glass for the stained glass market
is produced by this method.
Crown glass is a handmade mouth-blown spun glass which is only available
today in the form of small, heavy bullions. True crown glass replicating
the fine thin crown glasses typical of the mid 19th century is unavailable,
and any modern glass sold today as 'crown' glass is likely to be cylinder
glass of one type or another. Fine clear handmade cylinder glass is the
only compatible glass alternative today.
Care must be involved in the selection of handmade glass: there are many
types of cylinder glass, and specifying that term alone is not enough.
The exaggerated appearance of many contemporary types of 'antique', 'reamy'
or 'seedy' cylinder glasses produced for the art glass market make them
poor substitutes for historic window glass.
There are good imported glasses from Germany, France and Poland that
are extremely close to old glasses that still survive, and it is these
that must be sought out. There is now no handmade window glass made commercially
in the UK.
CASE
STUDY: LODGE PARK
Lodge
Park, near Aldworth in Gloucestershire, is an ornate Jacobean pavilion
built c1634 to view the sport of deer coursing. Over the next 300
years it was converted into a small, two-storey house, and substantially
altered several times. By the time the National Trust acquired the
building in 1982 little of its original interior was to be seen
and many of its original fittings had been lost, including all early
glazing.
The building has been recently restored under the direction of
the architect Michael Reardon, in collaboration with Jeffrey Haworth
of the Trust's Severn Region office, and the archaeologist Warwick
Rodwell.
The Glass used to glaze the 13 principal windows was purpose-made
as a traditional cylinder, or broad glass. This was a deliberate
attempt to replicate English cylinder glass typical of the mid 17th
century, with new glass manufactured in France. Primitive surviving
glasses were copied from this period, based largely on plain broad
glass dated 1667 in the private church at Compton Wynyates, Tysoe,
Warwickshire. Although deliberately active, straw green tinted,
and with numerous bubbles, inclusions and imperfections, this glass
was made as it was 350 years ago. It is not excessively distressed
or over active, as a modern art glasses might appear; neither is
it as perfect as a 19th century handmade cylinder, or crown glass.
The colouring is typical of a glass manufactured from contaminated
sands and potash within a charcoal furnace; the oxides and impurities
creating a sparkling, lightly coloured and imperfect glass with
tremendous life and movement.
The resultant light in Lodge Park is much warmer than the clinically
clear white light one might expect of a more modern building, emphasising
the natural colour of fine stone, timber and lime plasterwork now
found within the building. |
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© Cathedral
Communications Limited 2009 |
