After some practice in preparing rods and freeing them of bubbles the operator will notice a distinct difference in the fusibility of the samples of quartz he investigates, though all may appear equally pure to the unaided eye. It should be mentioned, however, that high infusibility cannot always be taken as a test of purity, for the most infusible, or rather most viscous, sample examined by the writer contained more lithium than some less viscous samples.
Fig.
65.
During the process of freeing the quartz from bubbles the lithium and sodium will be found to burn away, or at all events to disappear.
A rod of quartz, say three inches long, one-sixteenth of an inch in diameter, and free from bubbles for half an inch of its length, even when examined by a strong lens, is suitable for drawing into threads. The rod is manipulated exactly in the manner described under GLASS-BLOWING, and is finally drawn down at the bubble free part into a needle, say 0.02 inch in diameter (No. 25 on the Birmingham wire gauge), and 2 inches long.
Fig.
66.
There is one peculiarity about fused quartz which renders its manipulation easier than that of glass — it is impossible to break fused quartz, however suddenly it be thrust into the blow-pipe flame. A rod having a diameter of three-sixteenths of an inch — and perhaps much more — may be brought right up to the tip of the inner cone of the oxy-gas flame and held there-till one side fuses, the other being comparatively cool, without the slightest fear of precipitating a smash. In seven years' experience I have never seen a bit of once fused quartz broken by sudden heating; whether it might be done if sufficient precautions were taken I do not know.