was alleged to fall out easily! However, today a system exists using infra-red 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 discolouration or surface degradation due to exposure and too high an alkali content in the originalcomposition. 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 leadlights 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 colour.

CROWNGLASS:Agoodamountof 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 oftheprocessofmanufacture.Finallythe 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 handblown quality together with the slight bend. Alternatively, German and French Cylinder Blown type NR, can be used which are handblown glasses but without the curve. CYLINDER BLOWN SHEET: A large amount remains - English from mid 19th centuryand imported Frenchor German Cylinder prior to this. A fme handblown glass with a gentle wavy reflection. Replacement should be in German orFrench 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 It was more commonly used from the mid-i 9th centurywhen window and glass taxes were repealed and was notably used in mid-Victorian shopfronts. Replacementglass, 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 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 handblown Cylinder, not a great amount remains. It has a similar appearance to handblown 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 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 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 glass-makers art - a glass you can hardly see. A mirror like reflection from a perfectly flat surface, with nofaults, 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 mostoldglass canbe replacedormatched 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 London 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

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