Electric welding of glass

ALTHOUGH glass can be ground, sawed, and drilled in the solid state, its hardness is such that many operations are performed most easily on glass which has been reheated to a plastic state. From early antiquity, fire and heat have been used for sealing, moulding, and joining glass parts together. Flame heating applications are limited by inherent disadvantages — flame heating is surface heating; steep temperature gradients occur which sometimes boil away volatile constituents; and flame heating is difficult to restrict into sharply localized paths without excessive sacrifice in heat.