CHAPTER XIII.

OPTICAL GLASS.

The manufacture of optical glass--Raw materials-- Mixing--Furnaces and crucibles--Kilns for heating pots--Transfer of pots from kiln to melting furnace-- Introduction of cullet and raw materials--The fining process, difficulties and limitations--The stirring process--The final cooling of the glass--Rough sorting of the glass fragments--Moulding and final annealing of the moulded glass--Grinding and polishing of plates and discs for examination; smallness of yield obtained-- Difficulty of obtaining large blocks of perfect glass _p._ 223