Glass casting

Glass casting is the process in which glass objects are cast by directing molten glass into a mould where it solidifies. The technique has been used since the Egyptian period. Modern cast glass is formed by a variety of processes such as kiln casting, or casting into sand, graphite or metal moulds.

During the Roman period moulds consisting of two or more interlocking parts were used to create blank glass dishes. Glass could be added to the mould either by frit casting, where the mould was filled with chips of glass (called frit) and then heated to melt the glass, or by pouring molten glass into the mould. Evidence from Pompeii suggests that molten hot glass may have been introduced as early as the mid-1st century CE. Blank vessels were then annealed, fixed to lathes and cut and polished on all surfaces to achieve their final shape.Pliny the Elder indicates in his Natural History (36.193) that lathes were used in the production of most glass of the mid-1st century.

Italy is believed to have been the source of the majority of early Imperial polychrome cast glass, whereas monochrome cast glasses are more predominant elsewhere in the Mediterranean. Forms produced show clear inspiration from the Roman bronze and silver industries, and in the case of carinated bowls and dishes, from the ceramic industry. Cast vessel forms became more limited during the late 1st century, but continued in production into the second or third decade of the 2nd century. Colourless cast bowls were widespread throughout the Roman world in the late 1st and early 2nd century CE, and may have been produced at more than one centre. Some revival of the casting technique appears in the 3rd or 4th century, but appears to have produced relatively small numbers of vessels

Sand casting involves the use of hot molten glass poured directly into a preformed mould. It is a process similar to casting metal into a mould. The sand mould is typically prepared by using a mixture of clean sand and a small proportion of the water-absorbing clay bentonite. Bentonite acts as a binding material. In the process, a small amount of water is added to the sand-bentonite mixture and this is well mixed and sifted before addition to an open topped container. A template is prepared (typically made of wood, or a found object or even a body part such as a hand or fist) which is tightly pressed into the sand to make a clean impression. This impression then forms the mould.

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