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Ancient glass techniques: core-forming, slumping, mould-forming and blowing
Core-forming involved coating molten glass around a core of dung and clay mixed with a little water and adhered to a rod. Decoration was added by trailing soft glass of a different colour around the body of the vessel and combing it into patterns with a pointed instrument. After re-heating, the shoulder, neck and rim were formed and any handles added before placing the finished vessel in an annealing furnace to cool slowly. Finally the rod and core were removed. Beads and the like were formed by a similar process: a rod coated with release material (previously fired clay, pulverised and mixed with a little water) was used instead of a core.
In slumping, a flat piece of glass made by pouring plain colours or fusing together slices of multicoloured canes was shaped by heating. Gravity forced the glass downward over a positive or into a negative refractory form (heat-resistant mould).
Mould-forming describes a process of squeezing a mass of soft glass between two shaped refractory moulds to form a vessel with capacity, such as bowl or a dish. Mould-pressing is a similar, but distinct, squeezing process, whereby details of decoration are created in the surface of the vessel without changing the overall shape.
The technique of glass blowing completely transformed the glass industry. It enabled glassmakers to produce tableware and storage containers in many shapes and sizes, in a much wider variety, and more easily and quickly than ever before. It is still the foremost method for making glass by hand.