Hand-blown glass: Manufacturing process

Broad sheet. Molten glass is gathered on a blowpipe, and blown to an elongated 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 blown, it was made into leadlights.

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 imported Cylinder glass until the mid 19th Century.

Blown plate. Produced from Broad Sheet, each sheet of glass was laboriously hand ground and polished on both surfaces. The plate was of a sufficient quality and size for mirrors or coach glasses.

Polish plate Produced by casting glass onto a table and then subsequently grinding and polishing the glass, originally by hand, later by machine. An expensive 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 quality result. Manufactured in the UK in the mid 19th Century,it had been manufactured in France and Germany (and imported to the UK) since the 18th Century.

The above methods of manufacture lasted at least until the end of the 19th Century. The early 20th Century marks the move away from handblown to machine manufactured glass.

1. 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 - 10ft cylinders, which were then cut lengthways, reheated and flattened. This process was used in the UK up to the end of the 1920's.

2. 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.

3. 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.

4. 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 today.

There are two main categories of period window glass London Crown Glass can supply and they are designed for different applications.

a. Genuine Handblown Cylinder glass (e.g. German Cylinder Blown Sheet Type NR). This is used for matching existing original Cylinder blown or Crown glass. It is glazed where existing original glass is broken or where replacement windows are required to match existing sashes in a period building. Wherever an authentic appearance is important, handblown Cylinder glass is the preferred choice.

b. Period Style window glass (e.g.3mm Georgian Sheet). This is a lower cost alternative for complete glazing of windows or a facade where no original glass remains and the fine quality of a handlblown glass is not required.
Handblown Cylinder Glass Period Style Glass

Handblown cylinder glass

As a genuine handblown glass, each sheet varies a little from the next. The attraction lies in the small blowing imperfections - a few small bubbles (seeds) or a trace of a ream (fold marks or a wave in the glass) found in this style of glass.

Looking through the glass, the view changes as the eye moves. From outside the reflection gently distorts trees, buildings and the surrounding landscape.

The effect which is entirely authentic is derived from the method of manufacture.

The glass is handblown into an elongated balloon. The ends are cut off to form a cylinder, which is then allowed to cool. The cylinder is split lengthways, reheated and then flattened in an oven.

Cylinder glass marked NR (no ream) is the most suitable for window sashes of the late 17th century up to early twentieth century.

Cylinder glass marked LR (low ream) is generally reserved for 16th century or earlier windows in lower grade housing or stables and for leaded lights. This is because the glass has a more heavily marked surface reflecting an earlier style and a lower grade of glass.

Cylinder glasses:

1. Vauxhall Glass
2. Cylinder Blown Type NR
3. Baltic Type LR

Because Crown glass is no longer made, a variant of Cylinder Blown glass called "Vauxhall Glass" is produced. This is similar to Cylinder Blown Sheet Type NR but during the final process, the glass is slightly curved to give the bulging appearance of Crown glass.

Period style glass

These glasses are designed to re-create the gently wavy reflection of Crown and Cylinder glass.

From inside the glass appears without the small imperfections found in handblown glass. However because of the appealing reflective qualities, it is used in place of standard window glass.

Ordinary window glass, which sadly is still sometimes specified for period buildings, is usually Float glass or Horticulture Sheet.

Float glass gives a bland mirror-like appearance when glazed in period window sashes. Horticultural Sheet additionally shows a machine wave spreading across the pane of glass, but only in one direction, giving an unattractive effect. Both Float glass (first used in 1959) and Horticultural Sheet (first used in 1919) spoil the appearance of a period building.

Our Period Style glasses are produced by carefully bending mechanically Drawn Sheet (i.e. not handblown) glass in special moulds to recreate the attractive wavy reflection of original Crown or Cylinder glass. Victorian Sheet is also manufactured by a Drawn Sheet process, but is not bent and can be used to make sealed double glazing units.

Period Style glass is generally glazed where no existing original glass remains (e.g. barn conversions) as they do not have the small flaws and imperfections which make cylinder glass so appealing. They appear plain (without reflection) when viewed from inside. However they give an attractive exterior appearance and are a cost effective improvement on the use of modern Float glass or Horticultural Sheet.

Period Style glasses:

1. Georgian Sheet
2. Victorian Sheet