THE PROCESS OF GLASS CASTING TECHNIQUE
Sand casting – This involves the use of hot molten glass poured directly into a mould & is similar to casting metal into a sand 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 or pattern is prepared (made out of wood, a found object terracotta or even a body part such as a hand or foot etc.) which is tightly pressed into the sand to make a clean impression. This impression then forms the void in the mould in which the molten glass would settle. 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 1200 C to & poured directly into the mould. In the pouring process, glass or compatible objects may be placed to later give the appearance of floating within glass.
Kiln casting involves the preparation of a mould which is often made of plaster or plaster mixtures i.e Gypsum, quartz, ceramic fibre particles and sometimes a coat of graphite. A model can be made of any solid material such as wax, 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 Purdue or lost wax process. Using this, a model can be made from wax and after investment (melting and burning) the wax can be steamed or burned away in a kiln. The heat resistant mould is then placed in a kiln with a funnel like refractory opening which is filled with solid glass granules or lumps. The kiln is heated to a high temperature, normally between 800 and 1000 degrees Celsius, and as the glass melts it runs, settling into and filling the mould. Runners and air vents are an important part of the mould making so that the glass flows properly and there are no air lock pockets.
Pâte de verre
Pâte de verre is another 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 or a binder that evaporates completely with heat. Colourants and or enamels may be introduced with this. The resultant paste is applied to the inner surface of a negative mould forming a coating. After 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.
Graphite is also used in the hot forming of glass. Graphite moulds are prepared by carving into them, machining them into curved forms or by stacking them into shapes. Molten glass is poured into the moulds then cooled until hard enough to be removed and placed into an annealing kiln to cool slowly.