European white glass, although made beautifully transparent, free from coloring tints to a great degree, never can attain the brilliancy of flint glass. The Bohemians and Germans, who excel in the making of lime glass, treat it in its decoration precisely as the English, French, and Belgians do with flint. Light not being reflected with that brilliancy so strikingly noticeable in flint, this glass always has a bluish-gray tint, which detracts materially from its beauty.
We owe the invention of flint glass to the English. Wood was used as a fuel in the beginning of glass making, but it having become scarce in England coal was resorted to as a substitute. This fuel, however, producing a great quantity of smoke in the furnace, many impurities were mixed with the glass and gave it a coloring. To obviate this difficulty, pots, which had been made heretofore without covers, were now covered. This, however, reduced the temperature of the contents of the pots, and it was found that glass made with the ordinary mixtures was too hard to melt. Red lead was then introduced as a flux; this remedied the difficulty. To this day the English retain the supremacy in the manufacture of flint glass; in color, brilliancy, and superior purity of metal, it is not surpassed by any other country. France and Belgium are also making large quantities of flint-glass wares, and I particularly noticed that since 1867 French flint had been materially improved. The peculiar grayish tint which distinguished the French from the English then has now disappeared, and French flint is nearly equal to the English.
Belgium has somewhat departed from the manufacture of pure flint glass, and to meet the demand for cheap wares has been making ordinary white glass with a small proportion of lead added. The manufacture of flint glass in Belgium, at the present day, amounts to $1,600,000. In 1876 the exportation amounted to $340,000.
The materials used in flint-glass making are potash, red lead, sand, and small proportions of oxide of manganese as a corrector of color.
Potash is used in the form of carbonate, and should be as free as possible from sulphate, chlorate, soda salts, iron, and organic matters. Sometimes 10 to 20 per cent. of niter is substituted for soda, giving a very fine glass. Its use, however, is somewhat objectionable, as it destroys pots very rapidly; its high-cost price is another objection. The English use potash coming from America; it is refined by manufacturers, who sell the purified article direct to the glass-makers. France draws her potash from America, Tuscany,
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