was alleged to fall out easily! However,

today a system exists using infra-red tech-

nology to soften the putty without heat-

ing the glass - it is also relatively less

odorous! It is sometimes possible for a

good glass cutter to cut old glass to a new

size, provided it was well made origi-

nally. Old glass may have discolouration

or surface degradation due to exposure

and too high an alkali content in the

original composition. Unfortunately, such

damage is irreversible.

WHICH ORIGINAL GLASSES REMAIN

AND HOW CAN THEY BE REPLACED

IF BROKEN?

BROAD SHEET (also muff glass): This

early glass still exists in old lead lights or

iron window frames of the 17th century

or earlier. It has a rough surface and is not

always transparent. Replacement glasses

are available but may need careful match-

ing. MR Cylinder glasses are used for

leadlight repairs or specialist antique

glasses, where the glass has a tint or

colour.

CROWNGLASS: A good amount of Crown

still exists - mainly in Georgian sashes.

There are three distinguishing features

of Crown - at least one of these is usually

evident. Because the pane was cut from

a circular disc or ‘table’ there may be

traces of circular ream or faults (small

bubbles) which lie in a circular pattern.

On larger panes there may be a slight

bulging or convex/concave effect, part

of the process of manufacture. Finally the

glass (when clean!) has a bright fire finish

which flashes in the sunlight. Crown is

no longer made, so replacement for el-

egant panes should be in Vauxhall glass,

which has the fine hand blown quality

together with the slight bend. Alterna-

tively, German and French Cylinder

Blown type NR, can be used which are

hand blown glasses but without the curve.

CYLINDER BLOWN SHEET: A large

amount remains - English from mid 19th

century and imported French or German

Cylinder prior to this. A fine hand blown

glass with a gentle wavy reflection. Re-

placement should be in German or French

Cylinder Blown type NR which are ex-

cellent matches.

PLATE GLASS: Blown Plate is compara-

tively rare, found in old mirrors and win-

dows glazed before the mid-l7th cen-

tury. Polished Plate, because it was made

in large sizes and was expensive, was

used in grand houses and palaces. Qual-

ity was generally good with little distor-

tion. It was more commonly used from

the mid-19th century when window and

glass taxes were repealed and was nota-

bly used in mid-Victorian shopfronts.

Replacement glass, because Polished Plate

is no longer manufactured, has to be in

Float, although it will not have the patina

and occasional faults of the original.

OBSCURED GLASS: Machine and Table

Rolled glasses were introduced in the

mid-l9th century and in many patterns.

Hartley’s Rolled Glass had a fine ribbed

pattern on one surface and was often

used in industrial buildings and exten-

sively in the roofs of Victorian railway

termini. Replacement glasses generally

do not exist. Even where the original

rollers carrying the pattern can be found,

the cost of setting up a modem glass-

rolling machine to make a comparatively

short run is prohibitive. Salvaging similar

old obscured glass from the same or

another building is often the only choice

- apart from choosing one of the blander

modem rolled obscured glasses.

MACHINE DRAWN CYLINDER SHEET:

As this was only produced for a short

time in the UK and competed with

handblown Cylinder, not a great amount

remains. It has a similar appearance to

handblown cylinder sheet, but in addi-

tion it may have vertical draw lines in one

direction. As the process no longer ex-

ists, Cylinder Blown Sheet type NR is the

nearest match.

FLAT DRAWN SHEET (including Horti-

cultural Sheet): Used in the UK from

1919 onwards (not before!). The glass

does not have the quality of handblown

cylinder sheet. Any distortion is in the

form of a wave but only in one direction.

This is caused as the ribbon of molten

glass is drawn vertically between rollers.

Horticultural glass made by this method

has often been specified as replacement

glazing. However it is not authentic for

pre-1919 buildings and can look very

mechanical with the machine wave giv-

ing a patchwork effect when glazed. Flat

Drawn Sheet is still manufactured in Eu-

rope, although the quality is similar to

Float standard in many cases.

FLOAT GLASS: The ultimate perfection

of the glass-makers art - a glass you can

hardly see. A mirror like reflection from a

perfectly flat surface, with no faults, flaws

or distortion and designed for modern

buildings. Any attempt to use this glass in

period windows is doomed to failure.

Even a single float glass pane replace-

ment in a period sash sticks out like a sore

thumb.

SUMMARY AND FINAL NOTES

I have listed the chronology of the vari-

ous manufacturing processes, together

with process details, which explain the

attractive appearance of old glass. Wher-

ever possible preserve or re-use original

glass. Where this is not possible, or addi-

tional glass is required, remember that

most old glass can be replaced or matched

and the cost is not exorbitant - although

naturally more than the undemanding

option of Float glass or Horticultural Sheet.

For example, to replace all the glass in a

6 over 6 Georgian sash (3 ft 6 in wide by

5 ft 3 in high) in German Cylinder Blown

Sheet would cost around £170 + VAT.

The effect gained is out of all proportion

to the increased cost over Float or Sheet

glass.

Finally, if a draughty window frame

leaks, rattles and appears to be

disintegrating call in a good joiner or

specialist window repair company. The

window can often be properly repaired,

weather-stripped and upgraded at less

cost than a upvc/aluminium substitute.

Christopher R. Salmond is Director of The Lon-

don Crown Glass Co Ltd.

BROAD GLASS: Fig. 4 The title is misleading as this shows the manual polishing of Broad Glass

to turn it into Blown Plate. Wooden blocks mounted on a willow spring, have felt pads

underneath and fine polish such as jeweller’s rouge is used to finish off sometimes before

turning into mirrors

8

CONTEXT 48