Crown Glass

For optical purposes it is made of sand, chalk, pearl ash, and soda ash with a little arsenious oxide. It is melted in pots, cooled, and broken horizontally. The clear pieces are selected, softened by heat, molded in form, cooled and polished for lenses. For window gla , made only in England by this method, crown gla contains no pearl ash and is blown in the form of globes, which are welded by gla to a rod, opened opposite to the rod, and, by revolving rapidly, are expanded to a flat disk by centrifugal force. It is then annealed. This gla is more brilliant than plate or cylinder gla , but not as cheaply made as the latter.