At high temperatures the silica acts as a fairly strong acid, and decomposes the fused carbonates of sodium and potassium with evolution of gas. This is the rationale of the fluxing action of such alkaline substances of rather low melting point. Other mixtures act somewhat analogously but in a fashion commonly too complex to follow.
The final result is a thick solution, and the chief concern of the optical glass maker is to keep it homogeneous, free from bubbles, and as nearly colorless as practicable. To the first two ends the temperature is pushed up to gain fluidity, and frequently substances are added (e.g., arsenic) which by volatility or chemical effect tend to form large bubbles from the entrained gases, capable of clearing themselves from the fluid where fine bubbles would remain. For the same purpose is the stirring process.
The stirrer is a hard baked cylinder of fire clay fastened to an iron bar. First heated in the mouth of the pot, the stirrer is plunged in the molten glass and given a steady rotating motion, the long bar being swivelled and furnished with a wooden handle for the workman. This stirring is kept up pretty steadily while the heat is very slowly reduced until the mass is too thick to manage, the process taking, for various mixtures and conditions, from three or four hours to the better part of a day.
Fig. 37.—Testing Optical Glass in the Rough.
Then begins the careful and tedious process of cooling. Fairly rapid until the mass is solid enough to prevent the formation of fresh striæ, the cooling is continued more slowly, in the furnace or after removal to the annealing oven, until the crucible is cool enough for handling, the whole process generally taking a week or more.
Then the real trouble begins. The crucible is broken away and there is found a more or less cracked mass of glass, sometimes badly broken up, again furnishing a clear lump weighing some hundreds of pounds. This glass is then carefully picked over and examined for flaws, striæ and other imperfections.
These can sometimes be chipped away with more or less breaking up of the mass. The inspection of the glass in the raw is facilitated by the scheme shown in elevation Fig. 37. Here A is a tank with parallel sides of plate glass. In it is placed B the rough block of glass, and the tank is then filled with a liquid which can be brought to the same refractive power as the glass, as in Newton’s disastrous experiment. When equality is reached for, say, yellow light, one can see directly through the block, the rays no longer being refracted at its surface, and any interior striæ are readily seen even in a mass a foot or more thick. Before adding the liquid a ray would be skewed, as C, D, E, F, afterwards it would go straight through; C, D, G, H.
The fraction that passes inspection may be found to be from much less than a quarter to a half of the whole. This good glass is then ready for the next operation, forming and fine annealing. The final form to be reached is a disc or block, and the chunks of perfect glass are heated in a kiln until plastic, and then moulded into the required shapes, sometimes concave or convex discs suitable for small lenses.
Then the blocks are transferred to a kiln and allowed to cool off very gradually, for several days or weeks according to the size of the blocks and the severity of the requirements they must meet. In the highest class of work the annealing oven has thermostatic control and close watch is kept by the pyrometer.