But the most eager search of Arabian chemistry was the transmutation of metals, and the elixir of immortal health: the reason and the fortunes of thousands were evaporated in the crucibles of alchemy, and the consummation of the great work was promoted by the worthy aid of mystery, fable and superstition."
The crucibles or pots used for the production of optical glass very closely resemble those used in the manufacture of flint glass for other purposes; they are " covered " and the molten materials are thus protected from the action of the furnace gases by the interposition of a wall of fireclay, but as crucibles for optical glass are used for only one fusion and are then broken up, they are not made so thick and heavy as those used in flint-glass making, since the latter remain in the furnace for many weeks.
On the other hand, the chemical and physical nature of the fireclays used in the manufacture of such crucibles requires careful attention in order to secure the best results.
For melting the leadless glasses, open, bowl-shaped crucibles are used, ranging from 12 to 40 in.
Glass mixtures containing lead are melted in covered, beehive-shaped crucibles holding from 12 to 18 cwt.