Glass casting is the process in which glass objects are Casting by directing molten glass into a mould where it solidifies. The technique has been used since the 15th century BCE in both Ancient Egypt and Mesopotamia. Modern cast glass is formed by a variety of processes such as kiln casting or casting into sand, graphite or metal moulds.
History
Roman period
During the
Roman Empire, 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.
[Stern, E.M., Roman Mould-blown Glass, Rome, Italy: L'Erma di Fretshneidur in association with the Toledo Museum of Art.] Evidence from
Pompeii suggests that molten hot glass may have been introduced as early as the mid-1st century
Common Era.
Blank vessels were then annealed, fixed to
and cut and polished on all surfaces to achieve their final shape.
[Grose, D.F., Early Imperial Roman cast glass: The translucent coloured and colourless fine wares, in Roman Glass: two centuries of art and invention, M. Newby and K. Painter, Editors. 1991, Society of Antiquaries of London: London.] 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 Roman Empire polychrome cast glass, whereas monochrome cast glasses are more predominant elsewhere in the Mediterranean.[Price, J., A survey of the Hellenistic and early Roman vessel glass found on the Unexplored Mansion Site at Knossos in Crete, in Annales du 11e Congres. 1990: Amsterdam.] 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.[Allen, D., Roman Glass in Britain, ed. J. Dyer. 1998, Princes Risborough, Buckinghamshire: Shire Publications.] 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.
Modern techniques
Sand casting
Sand casting involves the use of hot molten glass poured directly into a preformed mould.
[Henry Halem (1996) Glass Notes (3rd Edition). Franklin Mills Press] 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.
The surface of the mould can be covered in coloured glass powders or frits to give a surface colour to the sand cast glass object. When the mould preparation is complete hot glass is ladled from the furnace at temperatures of about to allow it to freely pour. The hot glass is poured directly into the mould. During the pouring process, glass or compatible objects may be placed to later give the appearance of floating in the solid glass object.[ Linda R Fraser Sculpture – Sandcast Glass Design Process and Art] This very immediate and dynamic method was pioneered and perfected in the 1980s by the Swedish people artist Bertil Vallien.
Kiln casting
Kiln casting involves the preparation of a mould which is often made of a mixture of plaster and
refractory materials such as silica.
[Peter Layton (1996) Glass Art. Craftsman House] A model can be made from any solid material, such as wax,
[Knitting#Glass.2FWax] wood, or metal, and after taking a cast of the model (a process called investment) the model is removed from the mould. One method of forming a mould is by the
Cire perdue or "
lost wax" method. Using this method, a model can be made from wax and after investment the wax can be steamed or burned away in a kiln, forming a cavity. The mould is equipped with a funnel-like reservoir filled with solid glass granules or lumps. The heat resistant mould is then placed in a kiln and heated to between and to melt the glass. As the glass melts it runs into and fills the mould.
Such kiln cast work can be of very large dimensions, as in the work of Czech artists Stanislav Libenský and Jaroslava Brychtová.[Dan Klein (1989) Glass A Contemporary Art. William Collins Sons and Co] Kiln cast glass has become an important material for contemporary artists such as Clifford Rainey,
Pâte de verre
Pâte de verre is a form of kiln casting and literally translated means glass paste.
In this process, finely crushed glass is mixed with a binding material, such as a mixture of
gum arabic and water, and often with colourants and
Vitreous enamel. The resultant paste is applied to the inner surface of a negative mould forming a coating. When the coated mould is fired at the appropriate temperature the glass is fused creating a hollow object that can have thick or thin walls depending on the thickness of the pate de verre layers. Daum, a French commercial crystal manufacturer, produce highly sculptural pieces in pate de verre.
Graphite casting
Graphite is also used in the hot forming of glass. Graphite moulds are prepared by carving into them, machining them into curved forms, or stacking them into shapes. Molten glass is poured into a mould where it is cooled until hard enough to be removed and placed into an annealing kiln to cool slowly.
See also
Further reading
