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THE WORKING CRISIS.
as a mass of fused Flint Glass: it will detect the presence of metallic colouring matter, especially iron, although the most carefully conducted analysis may fail in discovering the slightest trace of it. Mr. Josiah Wedgwood found that 1/20,000 part of gold would give a rose-coloured tint to Flint Glass. With the best possible recipe, and the purest materials, good results depend upon an intense and continuous fusion: too little caloric will fail to refine it, and drive off air-bubbles, and the colouring matter of the manganese; and too long continuance of intense heat will destroy the manganese, cause the Glass to attack the pot, and become striated, gelatinous, and greenish. Extra time, at a lower rate of temperature, will not make up for want of continuous intensity. The most intense heat can scarcely be considered too great; but the moment the metal is fully fused, and refined by continuous rapid fusion, the high temperature of the furnace should be reduced from its maximum heat to a working temperature: this period being considered the crisis. Achromatic plate should be made while the metal is in this quiescent state: it is then most free from striæ; but, should intense heat be subsequently renewed, it tends to reproduce them. There is, therefore, a period of comparative perfection and purity, at which, if Flint Glass be not worked for optic use, the opportunity is for ever lost.
When Flint Glass is kept in fusion beyond the crisis, it not only assumes a greenish tint, by acting upon the iron of the Stourbridge clay pot, but takes up a small portion of its alumina; which, by its inferior density, rises to the surface, frequently with detached portions of the pot, causing striæ, and other impurities, which render it entirely unfit for optical purposes.