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depicting Christ in outline and dating from the 6th century. Initially only black and dark brown enamels were used to paint lines and shading details on panels of clear and coloured glass; these enamelswere composed ofhighlyfusible green lead glass ground to a powder and mixed with copper or iron oxide for colour, a binding medium such as plant gum or vegetable oil, and a flux to lower the melting point of the powdered glass (to about 620°C). This mixture was combined cold, and then painted on the glass and allowedtoharden. Whenthe painted glass was fired (in a small kiln called a muffle), the enamel melted and fused onto the glass surface.

Silver Stain
In the 14th century anew technique was developed for colouring the surface of glass. Silver suiphide or silver nitrate was ground and mixed with water to achieve a creamy consistency, and then the stain was appliedto the backof the glass panel. BeforefIring, the stained areawas brushed with a large, soft badger brush to spread the stain out evenly or move it into areas where a more concentrated colour was required. The firing temperatureforstain was much lower than that for enamel, ranging between 540 and 560°C, and for this reason a panel having both types of colour should have the enamel fired first and the stain second. Unlike enamel, silver stain actually bonds with the glass during firing, and thus is much less vu!~ nerable to deterioration.
The colour produced by the early silver stains varied according to the composition of the glass to which the stain was applied, the amount of stain used arid the number of applications, but ranged only between light yellow and an orange-red. The bestglass fortaking silver stain (on which a single application produced a deep golden orange) was kelp, a soft glass whose alkaliwas derived from seaweed. Numerous applications ofstainonthistypeofglasscouldproduce an almost true red, and it has been used extensively for this purpose since the 16th century. By the early 1830s, a new method for producing red involved the addition of sulphuric acid to the stain.
Silver stain was most often used for colouring hair, halos and decorative details in clothing, and it allowed for the use of two colours in a single pane of glass, thuseliniinatingtheneedforleading to surround the yellow colour. Panes of blueglasscouldalsobestainedtoproduce green. Where flashed glass was used, stain allowedforthe combinationofthree colours in a single pane, and it proved particularly useful in creating detailed heraldic designs inwhich golden crowns onions stood on a crest of blue and white
or red and white. Further line details could be done in black enamel. The use of silver staining reached its peak in the 15th century in Britain, when the perpendicular style of architecture dictated the needforenormous churchwindows.

Coloured enamels
The 16th century saw the development of a new range of enamels which were translucent and coloured by the addition of various metal oxides. These enamels added blue, green and purple to the existing palette of black and brown. The new technique consisted of melting the powdered lead glass base with the metal oxide first, then grinding the resulting coloured glass to a powder and mixing it with a medium to create a paint. When the painted glass panel was fired, the enamel regained its translucency and fused to the panel as a thin layer of coloured glass.This innovation created a gradual movement away from the use of leading to divide segments of pot-coloured glass, with artists choosing instead to exploit the new enamel’s painterly qualities. The Renaissance brought a new taste for and mastery of realism, and by the late 16th century much painted glass consisted of large, clear panels treated much like easel paintings. The leading, originally an integral part of the design as it joined the separate pieces of coloured glass, now served only to hold large squares or rectangles of clear painted glass together.
This change in taste coincided with a decline inthe availability of pot-coloured glass as a result of wars and political unrest in parts of Europe. By the middle of the 17th century, as the strife brought the traditional glassmaking centres of Lorraine andelsewhere to ruin, decorative glassbecame restricted almost exclusively to clear glass painted with enamel. It was in this way that the art of colouring glass in the pot was effectively lost in Europe until the revival of medieval-style stained glass in the first half of the 19th century.

19TH CENTURY COLOUR TECHNOLOGY
One of the most significant contributions of the first half of the 19th century to the applied arts was the revival and development of pot-coloured glass manufacture. The influence of the Reformation and emergingneb-classicaltastesbroughtthe production of pot-coloured glass virtually to a standstill, and in Britain, the prohibitive taxation on window glass manufacture from the 1690s to 1845 greatly discouraged investment and experimentation in the glass industry.
The Gothic revival of the late 18th century renewed demand for coloured window glass, but it was not until the
Excise- Duty on glass manufacture was repealed in 1845 that the industry could respond to the demand, and serious research and experimentation began. Although the knowledge of glass colouring agents had not been lost, the complex relationship between the metallic oxides, the furnace conditions and the ingredients of the glass was no longer understood. Just as the early craftsmen had developed their recipes through trial and error, the 19th-century glassmakers were forced to rediscover lost techniques through lengthy, painstaking experimentation.
It was soon discovered that the ‘improvements’ brought about by mechamsation did not produce glass of the same quality as was made by hand in earlier centuries. Impurities in medieval glass left flaws, small bubbles and some discoloration in the finished product, and lower furnace temperatures made the glass less transparent. As a result, medieval glass absorbed more light, and this made the colours appear rich and subdued. In contrast, advanced technology allowed 19th-century glassmakers to purify ingredients and achieve higher temperatures for fining (driving off air bubbles), thus producing a more transparent and virtually flawless glass. These results were far from satisfactory, however, since the fmal product was too bright and garish in colour. In an 1862 article entitled ‘On Stained Glass’, Apsley Pellatt wrote: “To succeed in making striated and bubbly- coloured glass. . . the fming process must be arrested during the latter part of the fusion, by reducing the heat of the metal to a sufficient consistency for working before the bubbles and striae are fully driven off: great attention is necessary on the part of the manufacturer to reduce the temperature of the furnace just at the righttimetopreventthe metal becoming too clear. This imitation of the ancients constitutes the chief improvement, since 1851, as regards the vitrified material... Pot metal blues, greens and rubies, etc., by this system of embodying in the mass the hindrances to the too free passage of the light, are far superior in effect to those of the ordinary, cheap, modern, clear, bright-coloured glass.” (The Builder, 11 October 1862, pp 735-736).
In addition to the re-creation of what wastermed ‘antiqne glass’, the technique of flashing glass had to be revived. This, too, was a technique that resisted mechanisation; to produce the thin layer ofcolouredglass on aclearbase, a cylinder had to be blown by hand. The method was rediscovered by Gustave Bontemps, a chemical engineer and director of the Choisy-le-Roi glassworks near Paris, who was recruited by a Birmingham firm,
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