The following is the composition of the finest flint glass : Fine white sand, 300 parts ; red lead, or litharge, 200 ; refined pearl ashes, 80 ; nitre, 20 ; arsenic and manganese, a small quantity.
These, all mixed together with the utmost nicety, are termed a flint batch.
The following results were obtained by M. Zeiher of St Petersburgh, respecting the optical effects of valying the proportions of the ingredients of flint glass.
From this Table, it is obvious that a greater quantity of lead not only increases the refractive, hut also the disper sive powers of the glass. When M. Zeiher mixed alka line salts with his glass, he found that they diminished the other refraction, without making any change in the dis persion. He then obtained a kind of glass which had three times the dispersive power of crown glass, and a refractive power of only 1,61. See Mena. Acad. Berl. p. 150.
Cazalct of Boutdeaux, has lately announced a me thod of making flint glass for telescopes free of all veins and imperfections. It consists of The sand must be calcined and pounded in an iron mor tar, and afterwards washed by ebullition with sulphuric acid, and then purified with muriatic acid. These ingre dients are put into a platina crucible, capable of holding 12 ounces of flint glass, and then carried to a bottle glass furnace. After 36 hours, it will be melted, and is then poured into water, dried, and reduced to a fine powder. It is then washed and purified in the same manner as sand ; again melted and thrown into water ; and after being again pulverised and purified with acids, it is melted a third time, and at the end of 48 hours it is poured upon a warm plate of copper, upon which it is allowed to cool gradually. It will then be found free of all imperfections.
A number of interesting experiments on the manufac ture of flint glass for optical purposes, has been made by M. D'Artigues. He always found that the excellence of the flint glass depended on the purification of the red lead ; that the middle part of a large mass of glass was always the freest from veins ; and that it. is only good when it is manufactured on a great scale. The middle portion of the pot of glass is blown into cylinders, and afterwards opened into plates. See Bulletin de la Societe d'Encouragement,
N°. 83.
THE materials for making plate glass must be of the finest quality. The principal ingredient is fine white sand, which is caused to vitrify by adding alkali and nitre, or salt, and sometimes other fluxes. The desirable qualities in plate glass are, that it shall be perfectly transparent and colourless, free from bubbles or specks in the casting. To attain this it must melt with a moderate degree of heat ; for, without this quality, it is scarcely possible to have it cast so quickly, that it will not cool in some degree dur ing the operation, and thus have specks. The best alka line substance for the flux is soda extracted from the ashes of barilla by lixiviation. if pearl ashes are used, they should be purified by washing or dissolving them in water, in which the impurities will subside ; and the clear liquor which is drawn off must be boiled down in iron pans, till, by the evaporation of the water, the clean ashes are obtained. Borax is very useful to facilitate the fusion, and also the running into plates. Lime has the same quality ; but the quantity must be small, otherwise it will affect the transparency.
The composition recommended by the author of the Handmaid to the Arts, is 60 parts of white sand washed clean, 25 parts of pearl ashes, 25 parts of nitre, and 7 of' borax. To these are sometimes added a small quantity of lime and manganese, according to circumstances.
The sand and alkali, and also the lime and manganese, if any are used, are first well mixed together, and thrown into a reverberatory furnace, represented in section by Fig. 11, where A is the fire-place, situated between two circular ovens or domes B, upon the flat floors of which the mate rials are laid ; and the flame rising from the grate being reflected down by the arched roofs, strikes upon the floors of the ovens with sufficient heat to nearly fuse the mate rials, and cause the sand to become very white by a semi vitrification. This calcination is continued for five or six hours, and the materials are constantly stirred and turned over with an iron poker, to expose every part to the heat; until they cease to afford any vapour, or to undergo any farther change.