Period character guide: Glass

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One of the things that helps define an old house for me is its glass. It flashes and sparkles – and that precise moment when the sun projects the shadow play of bubbles and waves of an aged pane across a wall is magical. By comparison, modern glass appears lifeless and bland because it lacks the ‘imperfections’ that traditionally resulted during manufacture.

Glass appears to have first been made in Egypt. A 16th-century encyclopedia in Pilkington Glass Museum explains that glass ‘tis a concrete of salt and sand or stones’ – produced by melting together silica (sand), sodium carbonate (soda), an alkaline mineral evaporated from brine deposits, and calcium carbonate (limestone).

The Romans are thought to have initiated the use of glass architecturally, and throughout the Middle Ages the best types of glass tended to be imported from France or Germany; it was not until the 13th century that glassmaking began in England, notably at Chiddingfold in Surrey.

Molten glass was blown to form a cylinder that was then manipulated into a broad sheet and, as it was only possible to make small panes from this, each piece was ‘leaded’ together to create a whole window.

The quality of glass improved with the introduction of crown glass (see ‘Word for Word’), which was first produced in London in 1678 and predominated in the Georgian period.

The manufacture of larger panes gradually became easier and, in 1834, Robert Lucas Chance introduced improved cylinder sheet glass, 299,655 panes of which were used to glaze the Crystal Palace that housed the Great Exhibition of 1851.

From Victorian times, patterned glass, textured on one side during the manufacturing process, was employed to provide privacy and decoration. In 1959 the invention of float glass by Sir Alistair Pilkington revolutionised glass making because of its high quality and cheaper production costs.

ABOVE: In leaded lights ‘cames’, thin ‘H’-shaped strips of lead are used to hold in place ‘quarries’, small panes of clear or coloured glass.

Skills alive today
Mouth blown cylinder glass is still made in the traditional way by Saint- Just, in France, at a glassworks that dates from 1826 and was sited because of the proximity of sand from the River Loire and coal from the mines at Saint-Étienne. Jean-François Outin, Saint-Just’s general manager, explained to me that, with the glass at a temperature of 1,100°C, the glass makers work in teams of two: a gatherer and a blower.

The gatherer collects the molten glass and creates a rounded ‘parison’ at the end of a blowing iron using a bronze mould to give it its shape. The blower quickly blows the parison until it takes on the appearance of a balloon.

This is then reheated in the ‘glory hole’ at a temperature of 1,320°C. The dexterity of the industrious glass makers at Saint-Just really shows when the blower begins the task of elongating the cylinder in the swinging pit. The cylinder is stretched by its own weight and by the glass maker, who continues to blow while swinging the cylinder to increase the stretching. Jean-François pointed out that the total weight of the blowing iron and parison is nine to 10 kilograms.

When the cylinder has been sufficiently stretched, the blower pierces the end while bringing it horizontally into contact with the flame of a blowtorch. Then, once again, the cylinder goes back into the swinging pit to round off the opening and give it balance before it is ‘sheared off’ or separated from the blowing iron by using a thermal shock, after which it is split lengthways.

The split cylinder is then placed on a flattening table in an oven. Here it is brought to a softening point of between 650°C and 700°C and a polisher is used carefully to open the cylinder and flatten the glass so it becomes a sheet; it is then annealed, a slow and progressive cooling that lasts about an hour. Finally the sheet of glass arrives with the ‘squarer’ who cuts the four sides of the sheet square.

TOP RIGHT Small cracks in the corner of a pane need not be a reason to replace the pane ABOVE AND ABOVE RIGHT The bubbles and waves in old glass give it a sense of life, distorting reflections and making the window glint in the light.

Caring for antique glass
If you have windows containing original glass, it is worth treating it with special care, retaining it wherever possible and being particularly careful when erecting scaffolding. With my own windows I have retained panes that have suffered minor damage like a crack across the corner and have even been able to save the glass when making repairs to the frame. Old stained glass windows should be regarded just as highly as works of art so their repair and conservation is best left to experts.

One of the most frequent problems with leaded light panels is that they often buckle or bulge due to the deterioration of the leading which, in turn, causes the glass to crack and water to seep through. Some panels will only need careful flattening, localised soldering and cleaning. Others will require a complete rebuild. If a leaded light does need to be dismantled, a useful way of making an exact record of it is to take a ‘rubbing’. Luckily, there are many specialist firms able to undertake repairs.

In the case of sash and casement windows, it is often only the putty that will need attention. Where the putty is missing or cracked, re-puttying is an important maintenance job because otherwise the window may leak and rot. If putty needs to be removed for repairs to a window, proceed with care as it is easy to crack the glass. As I know to my cost, it is never wise to use a heat gun to soften putty as this will almost certainly end in disaster even if you are careful. Special ‘putty lamps’ are available to soften the putty without damaging the glass so it is well worth enquiring whether your joiner has access to one.

Where new glass has to be used, opt for glass made in traditional ways such as cylinder glass. Crown glass is no longer produced; a good alternative is Cordele (pronounced ‘cor-delay’), sometimes known as P1, which is available from stained glass suppliers. Avoid horticultural or art glass as it tends to have a rather fake appearance, particularly in Georgian windows.

Glass is best ordered ready cut but remember to deduct 3mm from the height and width to allow a fitting tolerance. If the pane is irregular, cut a template out of thick paper or card and give this to your glass supplier.

1 & 2 The Saint-Just glassworks in France still makes traditional, mouth-blown cylinder glass. Here, the gatherer collects the molten glass and creates a rounded ‘parison’ at the end of a blowing iron.

3 The blower blows the parison until it takes on the appearance of a balloon.
4 The blower undertakes the shaping of the cylinder.

5 The finished cylinder.
6 The cylinder is split lengthways.

7 Part of the stock of glass held by Saint-Just.

Word for word: A glass glossary
Crown glass: molten glass was blown into a balloon shape, a ‘punty’ rod was attached and the glass was then spun rapidly until a disc was formed causing characteristic circular lines. The outer portion was cut into small panes while the central portion, marred by a knob left by the punty, was traditionally used for less important windows.
Cylinder, muff or broad glass: more common than crown glass, it is characterised by straight ripples and by occasional ‘seeds’ or bubbles in the glass.
Float glass: made by floating molten glass on a bed of molten metal. The resulting sheet has a very smooth surface and uniform thickness.
Leaded lights: windows where ‘cames’, thin ‘H’-shaped strips of lead are used to hold in place ‘quarries’, small panes of clear or coloured glass. The cames are soldered at the joints and the gaps between the lead and glass are packed with special ‘cement’.
Reams: subtle waves that you notice in period glass. Seeds: tiny bubbles seen in period glass.
Stained glass: takes its name from a technique discovered early in the 14th century where, by the application of silver nitrate on to white glass, a palette of yellows is produced.

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WORDS ROGER HUNT PHOTOGRAPHS ROGER HUNT; APOLLO STAINED GLASS; SAINT-JUST
Featured in the May 2009 issue of Period Living