Laboratory Manual of Glass-Blowing

Excerpt

Materials and Apparatus

One of the most important factors in the success of any piece of glass-blowing is the glass employed. As is well known, there are two general varieties of glass: Lead glass and soda glass. Formerly much apparatus was made of lead glass, but at present it is very seldom met with, except in the little drops of special glass used to seal platinum wires into the larger sizes of tubes. Lead glass is softer and more readily fusible than soda glass, but has the disagreeable property of growing black in a few seconds unless worked in a strong oxidizing flame. This may be prevented by using a "hissing" flame, with a large excess of air, and working in the extreme end of the flame; or the black lead formed may thus be reoxidized, and the glass restored to its original clearness.

Almost all the soft glass on the market is a soda glass, although sometimes part of the soda is replaced by potash. Most of the hard glass appears to be a potash glass. The following qualities are desirable in a glass for ordinary working: (1) moderately low working temperature, (2) freedom from air bubbles, striations and irregularities, (3) proper composition, so that the glass will not devitrify or crystallize while being handled at its working temperature, (4) ability to withstand rapid heating without cracking.

The working temperature of different samples of so-called "soft glass" varies a good deal, and is best determined by trial. The glass should become almost soft enough for blowing in a flame that still shows a little yellow near the tip, so that at the highest temperature of the flame it may flow fairly freely and thus easily eliminate irregularities in thickness. If the glass is too hard, the shrinking of the glass, collection of material for a bulb, and in fact most of the working processes will be slower, and the glass will not stay at its working temperature long enough after its removal from the flame to permit it to be properly blown.

Air bubbles in the original batch of glass are drawn out into long hair-like tubes during the process of manufacture. When such tubing is worked, the walls of these microscopic tubes collapse in spots, and the air thus enclosed will often collect as a small bubble in the wall, thus weakening it. Irregularities are of various kinds. Some of the larger sizes of thin-walled tubing often have one half of their walls much thicker than the other, and such tubing should only be used for the simplest work. Some tubing has occasional knots or lumps of unfused material. The rest of the tube is usually all right, but often the defective part must be cut out. The presence of striations running along the tube is generally an indication of hard, inferior glass. Crookedness and non-uniformity of diameter are troublesome only when long pieces must be used.

Devitrification is one of the worst faults glass can possibly have. It is especially common in old glass, and in glass which has contained acids. It seems to be of two sorts. One variety manifests itself on the surface of the glass before it reaches its working temperature, but if the glass be heated to the highest temperature of the flame it will disappear except in the portion at the edge of the heated part....