Articles

Stained Glass Production

From the 10th or 11th century, when stained glass began to flourish as an art, glass factories were set up where there was a ready supply of silica, the essential product of glass manufacture. Glass was usually coloured by adding metallic oxides to the glass while in a molten state in a clay pot over a furnace. Glass coloured in this way was known as pot metal. Copper oxides were added to produce green, cobalt for blue, and gold was added to produce red glass. Much modern red glass is produced using ingredients less expensive than gold and giving a brighter red of a more vermilion shade.

Cylinder glass This glass was collected from the pot into a molten ball and blown, while being continually manipulated until it formed a large cylindrical bottle shape of even diameter and wall-thickness. It was then cut open, laid flat and annealed to make it stable. This is the type of glass most commonly used for ancient stained glass windows.

Crown glass This glass was partly blown into a hollow vessel, then put onto a revolving table which could be rapidly spun like a potter’s wheel. The centrifugal force caused the molten material to flatten and spread outwards. It could then be cut into small sheets. This glass could be made coloured and used for stained glass windows, but is typically associated with small paned windows of 16th and 17th century houses. The concentric, curving ripples are characteristic of this process. The center of each piece of glass received less force during the spinning, and thus produced was a thicker piece. These centres were for the special effect created by their lumpy, refractive quality. They are known as bull’s eyes and are feature of late 19th century domestic leadlight and are sometimes also used with cathedral glass or quarry glass in non-pictorial church windows of that date.

Table glass This glass was produced by pouring molten glass onto a metal table and sometimes rolling it with a large metal cylinder. The glass thus produced is heavily textured by the reaction of the glass with the cold metal. Glass of this appearance is commercially produced and widely used today, under the name of cathedral glass, although it was not the type of glass favoured for stained glass in ancient cathedrals. It has been much used for lead lighting in churches in the 20th century. Modern glass made by this technique is often heavily patterned by the use of an engrave metal roller.

Flashed glass Red pot metal glass was often undesirably dark in colour and prohibitively expensive. The method developed to produce red glass was called flashing. In this procedure, a semi-molten cylinder of clear glass was dipped into a pot of red glass so that the red glass formed a thin coating. The laminated glass thus formed was cut, flattened and heat annealed.

There are a number of advantages to this technique. It allows a variety in the depth of red ranging from very dark and almost opaque, through ruby red to pale and sometimes streaky red that was often used for thin border pieces. The other advantage was that the red of double-layered glass could be engraved or abraded to allow light to shine through the clear glass underneath. In the late Medieval period, this method was often employed to add rich patterns to the robes of Saints. The other advantage, much exploited by late Victorian and early 20th century artists, was that sheets could be flashed in which the depth of colour varied across the sheet. This was applied to a range of colours. Some stained glass studios, notably Lavers, Barraud and Westlake in England, made extensive use of large segments of irregularly flashed glass in robes and draperies.

Modern production of traditional glass There are a number of glass factories, notably in Germany, USA, England, France, Poland and Russia, which produce high quality glass by traditional methods. Such glass is produced primarily for the restoration of older windows from 1920s and before. The production of new windows in traditional Victorian, Arts and Crafts and early 20th century styles often uses traditional glass. Modern stained glass windows also often use a variety of these different types of glass, or employ commercially made glass.