Laboratory Manual of Glass-Blowing

CHAPTER I. Materials and Apparatus

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. The glass seems to work all right, but an ugly crystallized ring is left at the edge of the portion heated. This kind appears most frequently in old glass which was originally of good quality, but has in time been superficially altered, probably by the loss of alkalies. The other variety of devitrification does not appear when the glass is first heated; but after it has been maintained at or above its working temperature for a longer or shorter time, it will be noticed that the outer surface has lost its smoothness, and appears to be covered with minute wrinkles. It will also be found that the glass has become harder, so that it becomes impossible to work it easily. Further heating only makes the matter worse, as does the use of a higher temperature from the start. In fact it will often be found that a piece of comparatively soft glass which devitrifies almost at once in a "hissing" flame can be worked without serious difficulty if care be taken to use a flame still decidedly tinged with yellow. Even good glass will begin to devitrify in this way if heated too long at the highest temperature of the flame, so care should always be taken (1) to reduce the time of heating of any spot of glass to a minimum; i.e., get the desired result at the first attempt, if possible, or at least with the minimum of reheating and "doctoring," and (2) avoid keeping the glass at the highest temperature of the flame any longer than necessary. This may be accomplished by doing all heating, shrinking, etc., of the glass in a flame more or less tinged with yellow, and only raising the temperature to the highest point when ready to blow the glass. This kind of devitrification is apparently due to volatilization of the alkalies from the glass in the flame, and it is said that it can be partly remedied or prevented by holding a swab of cotton saturated with a strong solution of common salt in the flame from time to time as the glass is heated.

The toughness of glass, i.e., its ability to withstand variations of temperature, depends on its composition and the care taken in its annealing. In general, large pieces of glass should be heated very slowly in the smoky flame, and the larger the diameter of the tube the greater the length which must be kept warm to prevent cracking. All large pieces should be carefully heated over their whole circumference to the point where the soot deposit burns off, before being finally cooled. After being thus heated they are cooled in a large smoky flame until well coated with soot, then the flame is gradually reduced in size and the object finally cooled in the hot air above it until it will not set fire to cotton. If thought necessary, it may then be well wrapped in cotton and allowed to cool in the air. If not properly annealed the place heated may crack spontaneously when cold, and it is quite certain to crack if it is reheated later.

Next in importance to the glass are the blow-pipe and the bellows. Any good blast lamp, such as is ordinarily used in a chemical laboratory for the ignition of precipitates, will be satisfactory; provided it gives a smooth regular flame of sufficient size for the work in hand, and when turned down will give a sharp-pointed flame with well-defined parts. Where gas is not available, an ordinary gasoline blow-torch does very well for all operations requiring a large flame, and a mouth blow-pipe arranged to blow through a kerosene flame does well for a small flame. Several dealers make blow-torches for oil or alcohol which are arranged to give a small well-defined flame, and they would doubtless be very satisfactory for glass-work. Any good bellows will be satisfactory if it does not leak and will give a steady supply of air under sufficient pressure for the maximum size of flame given by the lamp used. A bellows with a leaky valve will give a pulsating flame which is very annoying and makes good work very difficult. When compressed air is available it can be used, but if possible it should be arranged so that the supply can be controlled by the foot, as both hands are usually needed to hold the work. For the same reason the supply of air is usually regulated by varying the rate of operation of the bellows, rather than by adjusting the valve of the blast-lamp. On the other hand, it will be found best to always adjust the flow of the gas by means of the cock on the lamp, rather than that at the supply pipe. The operator must have complete control over the flame, and be able to change its size and character at short notice without giving the work a chance to cool, and often without ceasing to support it with both hands.

Glass-blowing should be done in a good light, but preferably not in direct sunlight. The operator should be seated in a chair or on a stool of such a height that when working he may comfortably rest one or both elbows on the table. The comfort of the operator has a decided influence on the character of his work; especially in the case of a beginner, who often defeats his purpose by assuming uncomfortable and strained positions. Steadiness and exact control of both hands are essential in most operations; any uncomfortable or strained position tires the muscles and weakens the control of the operator over them.

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