1938. Pilkingtons developed the twin

ground POLISHED PLATE system.

1959 FLOAT GLASS was launched on the

UK market, invented by Sir Alistair

Pilkington.

blown, it was made into lead lights.

CROWN GLASS: Molten glass is gathered

on a blowpipe, and a balloon shape is

blown. The blowpipe is removed, a solid

‘punty’ rod is attached and the glass is

spun rapidly until a disc is formed. The

outer portion beyond the central knob is

then cut into panes. By the 18th century

quality was often very good with an

almost unmarked fire-finished surface.

Crown was the preferred choice for

window glass, together with some im-

ported cylinder glass until the mid-l9th

cntury.

BLOWN PLATE: Produced from Broad

Sheet, each sheet of glass was laboriously

hand ground and polished on both sur-

faces. The plate was of a sufficient quality

and size for mirrors or coach glasses.

POLISHED PLATE: Produced by casting

glass onto a table and then subsequently

grinding and polishing the glass, origi-

nally by hand, later by machine. An ex-

pensive process requiring a large capital

investment.

CYLINDER BLOWN SHEET: A similar

process to Broad Sheet, except that larger

cylinders are produced by swinging the

cylinder in a trench. The glass is allowed

to cool before cutting the cylinder, which

is then re-heated and flattened. Larger

panes and a much improved surface qua!-

ity result. Manufactured in the UK in the

mid-l9th century, it had been made in

France and Germany (and imported to

the UK) since the 18th century.

The above methods lasted at least

until the end of the 19th century. The

early 20th century marks the move away

from hand blown to machine

manufactured glass.

MACHINE DRAWN CYLINDER SHEET:

The first mechanical method of drawing

glass; 40 ft high cylinders of glass were

drawn vertically from a circular tank. The

glass was annealed and then cut into 7-10

ft cylinders, which were then cut length-

ways, reheated and flattened. This proc-

ess was used in the UK up to the end of

the 1920s.

FLAT DRAWN SHEET: The glass was

drawn vertically in a flat sheet until it

cooled sufficiently to allow the glass to be

cut. The Belgians invented the original

process but it did not reach the UK until

1919. Horticultural Sheet is produced by

a later variation of this process. The glass

was noted for having a wave in one

direction only.

SINGLE AND TWIN GROUND POLISHED

PLATE: Here the glass is cast and then

subsequently ground and polished on a

conveyor belt, to a fine quality without

distortion.

FLOAT GLASS: A layer of molten glass is

‘floated’ on to a bath of molten tin and

produces a fine quality of glass, but with

a mirror like reflection, without any wave

or distortion. It is the standard modern

method of producing window glass to-

day.

PRESERVING AND RE-USING

ORIGINAL GLASS

Where old glass exists in good condition,

it should, if at all possible, be retained.

When this is not possible, care should be

taken to select the correct glass.

Old glass can be fragile and care needs to

be taken when removing it from a frame.

The 19th century method of removing

valuable old glass from its Georgian sash,

was to dump frame and glass in a heap of

farmyardmanurefor6-8weeks. The glass

BROAD GLASS: Fig. 2 shows part of the process of Broad Sheet manufacture. ‘d’ shows the muff

about to be sheared open, ‘h’ shows the sheet after flattening

MANUFACTURING PROCESS

BROAD SHEET: Molten glass is gathered

on a blowpipe, and blown to an elon-

gated balloon shape. The ends are cut off

and the resulting cylinder is split with

shears while still hot, then flattened on

an iron plate. This is the forerunner of the

cylinder process. The quality of the glass

was not good, with many imperfections.

Because of the relatively small sizes

PLATE GLASS: Fig. 3 This shows the French process of Cast Plate manufacture. The box of molten

glass on the jib has been turned over and the molten glass is poured onto the casting table and

the large iron roller will be passed over the glass to make a uniform thickness prior to polishing

and grinding

CONTEXT 48

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