Technically a liquid, though it has all the appearance of a solid, glass is a magical material, capable of being shaped, coloured and decorated in all manner of fascinating ways. The beauty of glass, clear or coloured, can be enhanced by a variety of decorative techniques.
Although the basic technology of glass-making is ancient, many of the techniques used to shape and decorate it have been developed and refined over the past two or three centuries. Knowing about these can be a help in dating an identifying a piece of glass - a notoriously difficult problem when the great majority of glass objects are not marked or signed in an way.
Glass is generally shaped in its molten state, allowed to cool, then finished by a number of different techniques. The most basic of these is cutting. Cutting gives glass an added brilliance by providing angled facets to caich and scatter the light. The heavier and thicker the glass, the greater the sparkle. Cutting is always done with hand-operated drills and other machinery, making it a rather lengthy and expensive process.
The earliest form of cuts were flutes, prisms, shallow diamonds and stars which had six or twelve rays. Diamonds were cut to stand proud or hollowed out. Those in relief are sharply pointed, and tended to be either contained within a larger, hollow diamond, or set in panels arranged in a checked pattern.
Cut glass should have a good ring when flicked with a fingernail and feel sharp to the touch. This distinguishes it from pressed or moulded glass, which can be superficially similar in appearance. Pressed glass first made its appearance in Britain in the 1840s. It was made by pressing molten glass between two moulds. The force of pressing gave a clean, crisp finish to the cooled glass, and proved an ideal method of making fairly cheap decorative plates, bowls and vases. Pieces of hollow ware were made in two halves which were then fused together. The line made by the seam was fire-polished out.
This process permitted the mass production of decorative glass and also allowed makers to add their trademarks or other mark to the piece, making it easy to date and identify. Clear pressed glass is readily available, but coloured examples are generally more collectable.
Glass is given colour by adding various metals to the molten glass mixture. Many types of coloured glass are collectable in their own right. Cranberry glass is a good example. It's pale pink, the colour of runny raspberry jam; it shouldn't be confused with ruby glass, which is a rich, deep red. Cranberry glass was made in the mid-19th century and was always blown, never press-moulded.
It was used to make a variety of small items, including frilled baskets, sweetmeat dishes, tumblers, jugs and table ornaments. It was sometimes decorated with white enamelling, sometimes cased in white glass and cut, and often combined with Vaseline glass.
Vaseline glass is well named. A jar of petroleum jelly held up to the light has the same oily yellow opalescence. True Vaseline glass was made by adding uranium salts to the cooling glass mix, then reheating it; the depth of colour varies according to the length of the reheating process. Opalescent glass in various other colours is sometimes incorrectly described and sold as Vaseline glass, particularly the dense, acid yellow pearline glass and oily green glass.
CASING AND CAMEOS
Sophisticated decorative effects can be achieved by fusing together glass of two or more different colours. In cased glass, the base layer is always clear, and the top layer fused over it is later cut away to leave a pattern of clear glass showing through. Cased glass was used for decanters, drinking glasses, bowls and vases. A similar effect could be achieved by 'flashing' clear glass items, that is, dipping them in a dye, before cutting a pattern.
Cameo glass is made by carving two or more layers of glass to leave a relief pattern in cameo style. It wasn't made in any great quantity until about 1870 in England, and the late 1880s in France, where it was eagerly adopted by art nouveau glass-makers.
At first, the glass was always carved by hand, but this was a time-consuming process. By 1880, glass cutters were using hydrochloric acid to remove large areas, and a copper wheel for more detailed work. After 1890, acid was used more frequently, leading to a flatter, more shallow cut.
The smooth surface of blown glass provides a rich field for decoration. Enamel paints have been used but engraving is a more common method.
There are three techniques for engraving a pattern onto glass. In diamond point, the design is scratched into the glass with a hand-held tool. Stipple engraving is where tiny dots are 'hammered' into the glass to form the pattern. Wheel engraving is done by holding the glass against an abrasive, fast-spinning wheel.
Etched glass is sometimes confused with engraved glass. The results may be similar, but the method is very different. The glass is coated with wax, and the design scraped out of this. The whole thing is then dipped into acid, which eats away at the uncovered glass. The resulting designs tend to be flatter and more shallow than those achieved by engraving.
Engraved patterns are often seen on pieces that need to be light in weight, such as drinking glasses. Cut glass is always thick and is often used in decanters, unlike pressed pieces such as commemorative plates.