Period Flat Glass

See also Tube and Sheet Glass Making #SHEET
Back to Good Old Days Center
2009-06-19 Rev. 2011-02-04,

Period Window Glass
[All material is from site cited below, rearranged, although some images duplicate those used elsewhere on my site that were scanned directly. MF]
Institute of Historic Building Conservation (IHBC)
To introduce this feature, Christopher Salmond explains the types of glass used through history and suggests substitutes for those which are no longer available
Walk slowly past the facade of a fine old building and see how the reflections from the windows distort, shimmer and sparkle. The whole building comes alive and seems to flash with color and life. Compare this to some of the carefully restored buildings, which have been reglazed with modern window glass. A bland mirror-like reflection of float glass deadens the whole facade, robbing the building of its original charm. Even where a facade retains much of the original glass, new frames or even repairs to single broken panes that use modem glass spoil the overall effect and can be detected easily. Why is the difference so marked and why does old glass fit so perfectly into its surroundings? To answer these questions, we need to understand a little about the history of glassmaking in this country and the particular process of manufacture.
Window glass was not produced in significant quantities in this country [England] until:
1226 BROAD SHEET was first made in Sussex, but of poor quality, and fairly opaque. Manufacture slowly decreased and ceased by the early 16th century.
1330 The French glassmakers produced CROWN GLASS for the first time at Rouen. Some French Crown and Broad Sheet was imported into the UK.
1620 BLOWN PLATE was produced in London by grinding and polishing Broadsheet, and was used for mirrors and Coach plates.
1678 CROWN GLASS was first produced in London. This process predominated, because of its finer quality until the mid-19th century.
1688 The French produced POLISHED PLATE in larger sizes by casting and hand polishing.
1773 English POLISHED PLATE by the French process was produced at Ravenshead. By 1800 a steam engine was used to carry out the grinding and polishing of the cast glass.
1834 Robert Lucas Chance introduced IMPROVED CYLINDER SHEET, using a German process to produce finer quality and larger panes. This glass was used to glaze the Crystal Palace. The process was used extensively until early in the 20th century to make window glass. From this period onwards machines were developed to automate the production of obscured glass and later, window glass:
1847 James Hartley introduced a ROLLED PLATE glass with obscured ribbed finish, which is often found glazed in the roofs of railway termini.
1888 Chance Bros introduced MACHlNE ROLLED patterned glass.
1898 Pilkingtons introduced Hexagonal Rolled WIRED CAST.
1903 MACHINE DRAWN CYLINDER glass, invented in the USA, was manufactured in the UK by Pilkingtons from 1910 to 1933.
1913 Belgium produced the first machine FLAT DRAWN SHEET glass. It was first drawn in the UK in 1919 in Kent.
1923 First UK production of continuous POLISHED PLATE glass, using single grinding system.
1938. Pilkingtons developed the twin ground POLISHED PLATE system.
1959 FLOAT GLASS was launched on the UK market, invented by Sir Alistair Pilkington.

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 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 imported cylinder glass until the mid-l9th 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.
POLISHED 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 reheated and flattened. Larger panes and a much improved surface quality result. Manufactured in the UK in themid-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 7-10ft cylinders, which were then cut lengthways, reheated and flattened. This process 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 today.

Image of Crown Glass being spun (click for original site)

Picture of broad glass manufacture (click for original site)

CROWN GLASS: Fig. 1 shows the glassmaker having spun the table of Crown, about to drop it into a bed of sand (note the early version of virtual reality headgear 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

Plate Glas manufacture in France (click to show original site)

Polishing Broad Glass to make blown plate (click to original site)

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 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 jeweler's rouge is used to finish off sometimes before turning into mirrors


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 betaken 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 farmyard manure for 6-8weeks. The glass was alleged to fall out easily! However, today a system exists using infrared technology to soften the putty without heating 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 originally. Old glass may have discoloration or surface degradation due to exposure and too high an alkali content in the original composition. Unfortunately, such damage is irreversible.
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 matching. MR Cylinder glasses are used for leadlight repairs or specialist antique glasses, where the glass has a tint or color.
CROWN GLASS: 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 elegant panes should be in Vauxhall glass, which has the fine hand blown quality together with the slight bend. Alternatively, 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. Replacement should be in German or French Cylinder Blown type NR which are excellent matches.
PLATE GLASS: Blown Plate is comparatively rare, found in old mirrors and windows glazed before the mid-l7th century. Polished Plate, because it was made in large sizes and was expensive, was used in grand houses and palaces. Quality was generally good with little distortion. It was more commonly used from the mid-19th century when window and glass taxes were repealed and was notably used in mid-Victorian shop fronts. Replacement glass, because Polished Plateaus 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 extensively 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 hand blown Cylinder, not a great amount remains. It has a similar appearance to hand blown cylinder sheet, but in addition it may have vertical draw lines in one direction. As the process no longer exists, Cylinder Blown Sheet type NR is the nearest match.
FLAT DRAWN SHEET (including Horticultural Sheet): Used in the UK from1919 onwards (not before!). The glass does not have the quality of hand blown 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 giving a patchwork effect when glazed. Flat Drawn Sheet is still manufactured in Europe, although the quality is similar to Float standard in many cases.
FLOAT GLASS: The ultimate perfection of the glassmakers 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 replacement in a period sash sticks out like a sore thumb.
SUMMARY AND FINAL NOTES I have listed the chronology of the various manufacturing processes, together with process details, which explain the attractive appearance of old glass. Wherever possible preserve or re-use original glass. Where this is not possible, or additional 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/aluminum substitute.
Christopher R. Salmond is Director of The London Crown Glass Co Ltd.

 These may conflict with information gleaned by other researchers, as measures and quantities seem to have varied at different periods (and possibly in different places).
-  Crib: a hundredweight or 150 sq feet.   Case: 24 leaves, half cases also sold, of 12 leaves.
-  Neve a case ranged from 25 tables (French glass) to 45 of Newcastle. This dependent on diameter of the table: those 5 square foot were 45 to the case, at 6 square foot there were 35.
-  Table or leaf: each unit of glass, uncut, e.g. circles, or segmentally headed rec- tangles in broad glass. According to Neve, tables of crown were 3 ft 6 in or 3 ft 8 in, giving area of 9-10 ft2. The case then had a quantity of 220-240 ft2. Tables of crown became larger during the 18th century, measuring up to 5 ft and 6 ft in diameter.
-  Webb or way: this peculiar term to describe a quantity of glass seems to have been applied only in regard to Rhenish glass. Each webb consisted of bunches of six glass plates of about two square foot. At the end of the 17th century there were 100 bunches per webb, but later 60 was the usual amount.
-  Squares: the common term to describe a rectangular pane of glass in timber casements or sashes. Very often building accounts only mention the term without giving exact sizes. Sometimes this is elaborated upon the general sizes being from 6in by 8in upwards, with l9in by 12 in and 12 in by 16 in usual. Though most 244 panes were rectangular, square ones are also found.
-  Quarries: Neve gives details about calculating sizes of quarries, and the standards used, which would be of relevance to anyone needing to calculate exact measurements but are not necessary here. Suffice to mention that there were two types, ordinary and long, and that the most usual size was 12s (meaning that there are 12 of  l2s in a foot of glass); these are 6 in long by 4  in wide.
-  Measuring glass according to weight: Neve quotes a Mr Leybourn as stating that 225 ft of glass equals about 200 lb, i.e. 9 ft is about 8 lb weight. In an article which mentioned that the chapel in Dublin Castle bought 600 lb of glass in the late 14th century, the author quotes the then (1920s) clerk of works at York Minsteras estimating 3 lb per foot of glass. The discrepancy is interesting and may be explained by extreme thinness of good crown, which weighs less per foot.
 It is hoped that this short explanation of types and terms may be of some use to those who come into contact with this most aesthetic of materials. I am very interested in any other information on this subject, or others linked to it, if anyone has snippets or knows of obscure sources that I may have missed.
Nessa Roche is based in Dublin

Glass Types and Measures