Page 34 - Curriculum Visions Dynamic Book

with another material, for example, lead. Thus a difficulty in coping with the science of glassmaking resulted in some of the most beautiful of all works
of coloured glass art – glass mosaics in Roman times and the stained glass windows in churches and other buildings by the 12th century. (The reason that it is called stained glass is that stains were fused to the surface of the glass; the whole glass was not coloured.)
In later centuries people continued to experiment with getting reliable and rich colours. Johann Kunckel in Germany found that ruby-red glass could be made using gold chloride. To get the chloride, gold was dissolved in aqua regia (a mixture of concentrated sulphuric and hydrochloric acids).
(Right) The beauty of stained glass depends on the colours produced by the glassmaker. Medieval stained glass of the 12th to 14th centuries was made by cutting out tiny pieces of coloured glass and then attaching them to one another with lead. It was a time-consuming and delicate art. The colours were obtained by using molten metal oxides such as cobalt to produce blue and antimony for yellow. Even the sheets of glass did not begin
as sheets at all but were made as bubbles of coloured glass that were elongated into cylinders and then cut down their length and flattened out while the glass was still hot and therefore plastic. In this way small sheets about 30cm across were produced.
Although this technique of stained glassmaking has been
much copied since, it has never been equaled because of the extraordinary amount of labor involved in both making the glass and then assembling the pieces. “Modern” stained glass uses special “antique” glass made in traditional ways, although quite often the colours as well as the images are painted onto clear glass rather than made up as a mosaic.
(Left) The Romans made a wide range
of coloured glasses. This jar is about
12 cm tall. It was made by applying random fragments of coloured glass to a premade jar. The jar was probably rolled in the coloured glass fragments and then reheated until the glass softened so that the fragments flattened out. The jar could then be blown bigger so that the flattened fragments of glass were stretched into this attractive pattern.
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