The Telescope

by Louis Bell

One cannot deduce accurately the percentage mixture of the raw materials from an analysis of the glass, and it is notorious that the product even of the best manufacturers not infrequently fails to run quite true to type. Therefore the optical properties of each melting have carefully to be ascertained, and the product listed either as a very slight variant from its standard type, or as an odd lot, useful, but quite special in properties. Some of these odd meltings in fact have optical peculiarities the regular reproduction of which would be very desirable.

The purity of the materials is of the utmost importance in producing high grade glass for optical or other purposes. The silica is usually introduced in the form of the purest of white sand carrying only a few hundredths of one per cent of impurities in the way of iron, alumina and alkali. The ordinary alkalis go in preferably as carbonates, which can be obtained of great purity; although in most commercial glass the soda is used in the form of “salt-cake,” crude sodium sulphate.

Calcium, magnesium, and barium generally enter the melt as carbonates, zinc and lead as oxides. Alumina, like iron, is generally an impurity derived from felspar in the sand, but occasionally enters intentionally as pure natural felspar, or as chemically prepared hydrate. A few glasses contain a minute amount of arsenic, generally used in the form of arsenious acid, and still more rarely other elements enter, ordinarily as oxides.

Whatever the materials, they are commonly rather fine ground and very thoroughly mixed, preferably by machinery, before going into the furnaces. Glass furnaces are in these days commonly gas fired, and fall into two general classes, those in which the charge is melted in a huge tank above which the gas flames play, and those in which the charge is placed in crucibles or pots open or nearly closed, directly heated by the gas. In the tank furnaces the production is substantially continuous, the active melting taking place at one end, where the materials are introduced, while the clear molten glass flows to the cooler end of the tank or to a cooler compartment, whence it is withdrawn for working.

The ordinary method of making optical glass is by a modification of the pot process, each pot being fired separately to permit better regulation of the temperature.

The pots themselves are of the purest of fire clay, of moderate capacity, half a ton or so, and arched over to protect the contents from the direct play of the gases, leaving a side opening sufficient for charging and stirring.

The fundamental difference between the making of optical glass and the ordinary commercial varieties lies in the individual treatment of each charge necessary to secure uniformity and regularity, carried even to the extent of cooling each melting very slowly in its own pot, which is finally broken up to recover the contents. The tank furnaces are under heat week in and week out, may hold several hundred tons, and on this account cannot so readily be held to exactness of composition and quality.

The optical glass works, too, is provided with a particularly efficient set of preheating and annealing kilns, for the heat treatment of pots and glass must be of the most careful and thorough kind.

The production of a melting of optical glass begins with a very gradual heating of the pot to a bright red heat in one of the kilns. It is then transferred to its furnace which has been brought to a similar temperature, sealed in by slabs of firebrick, leaving its mouth easy of access, and then the heat is pushed up to near the melting temperature of the mixture in production, which varies over a rather wide range, from a moderate white heat to the utmost that a regenerative gas furnace can conveniently produce. After the heating comes the rather careful process of charging.

The mixture is added a portion at a time, since the fused material tends to foam, and the raw material as a solid is more bulky than the fluid. The chemical reactions as the mass fuses are somewhat complex. In their simplest form they represent the formation of silicates.