Improvements in or relating to the production of glass

To produce relatively large pieces of arsenic trisulphide glass, the crucible in which it is prepared is suddenly chilled during the slow cooling of the melt when the latter has become rigid, to cause separation without fracture. About 5 kg. of liquid arsenic trisulphide is collected in a conical silica crucible at 570 DEG to 700 DEG C. The crucible is then heated and contents stirred for 20 minutes at 625 DEG C. to homogenize the melt and thereafter the temperature is maintained at 300 DEG C. for seven hours. The temperature is then slowly reduced to 220 DEG C. and maintained for 4 1/2 hours after which the crucible is plunged in water at room temperature for twenty seconds, causing the glass to separate from it. The crucible and contents are then returned to the furnace and maintained at 220 DEG C. for one hour before slow cooling. This glass is transparent to infra-red light and contains 58.4% arsenic and 41.6% sulphur but reference is made to the addition of modifying agents such as arsenic, sulphur, selenium and tellurium.