作者：Frary, Francis C.
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Laboratory manual of glass-blowing试读：
The purpose of this little book is to provide a clear and detailed discussion of the elements of glass-blowing. Many laboratories in this country, especially in the west, are located a long way from any professional glass-blower, and the time and money spent in shipping broken apparatus several hundred miles to be mended could often be saved if some of the laboratory force could seal on a new stopcock, replace a broken tube, or make some temporary repairs. Many men in physical or chemical laboratories have occasion to modify some piece of apparatus designed perhaps for other uses, or to design new apparatus. To such also, the ability to perform some of the operations herein described may be very valuable.
No originality is claimed for the methods here described. They are those which the author has found most suitable and convenient in his own work, and most easily learned by students. The aim has been to describe each operation in such detail that a beginner can follow the process without help and, with practice, attain satisfactory results. It is, however, much easier to perform any of the operations described, after seeing some one else perform it correctly; since the temperature, the exact time to begin blowing the glass, and many other little details are very difficult to obtain from a description.
It has not been thought worth while to describe the process of making stopcocks, thermometers, vacuum tubes, etc., as such things can be purchased more cheaply and of much better quality than any amateur can make unless he is willing to spend a very large amount of time in practice. For similar reasons the manipulation of quartz glass has been omitted.
The author will be grateful for all suggestions and criticisms tending to improve the methods presented. If some of them appear to be given in excessive detail, the reader will remember that many things which are obvious to the experienced worker are not so to the beginner, and that it is the little details in the manipulation which often spell success or failure in glass-blowing.F. C. F.Minneapolis, Minn.,January, 1914.
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.
In the arrangement of the exercises here presented, several factors have been considered. It is important that the first exercises be simple, although not necessarily the simplest, and they should teach the fundamental operations which will be used and amplified later. They should in themselves be things which are of importance and commonly used in glass-work, and they should be so arranged that the fundamental points, such as the rotation of glass, the proper temperature, blowing and shrinking the glass may be learned with a minimum expenditure of time, glass and gas. It is therefore recommended that the beginner take them up in the order given, at least as far as No. 7, and that each be mastered before attempting the next. The beginner should not leave the first exercise, for example, until he can join together two pieces of tubing so that they form one piece of substantially uniform inner and outer diameter, and without thick or thin spots. From two to four practice periods of two hours each should suffice for this. This chapter and the following one should also be frequently read over, as many of the points discussed will not be understood at first and many of the manipulations described will not be necessary in the simpler exercises.
Cutting the Glass.—For this purpose a "glass-knife" is preferred to a file, if the glass is cold: if it is hot a file must always be used, and its edge slightly moistened to prevent drawing the temper. The glass-knife is simply a flat piece of hard steel, with the edges ground sharp on an emery wheel. The bevel of the edge should be from 30 to 60 degrees. An old flat file can easily be ground into a suitable knife. The glass-knife makes a narrower scratch than the file but appears more likely to start the minute crack which is to cause the tube to break at that point, and the break is more likely to give a good square end. The scratch should be made by passing part of the knife or file once across the glass, never by "sawing" the tool back and forth. This latter procedure dulls the tool very quickly.
In breaking a piece of glass tubing, many persons forget that it is necessary to pull the ends apart, as well as to bend the tube very slightly in such a direction as to open up the minute crack started in the scratch. Care in breaking the tube is essential, as it is impossible to do as good work with uneven ends as with square ones.
When tubing of large diameter or thin wall is to be cut, it is often better not to attempt to break it in the usual way, but to heat a very 11small globule of glass (⁄16 to ⁄8 inch diameter) to red heat, and touch it to the scratch. This will usually start the crack around the tube; if it has not proceeded far enough, or has not gone in the desired direction, it may be led along with a hot point of glass. This is put a little beyond the end of the crack, and as the latter grows out toward it, moved along the path where the crack is desired. This point of glass is also very useful in breaking off very short ends of tubes, where there is not room to get a firm enough hold and sufficient leverage to break the tube in the ordinary way, and for breaking tubes attached to large or heavy objects, which would be likely to make trouble if treated in the ordinary way.
Another way of cutting large tubing, especially if it has rather thick walls, is to make a scratch in the usual way, and then turn on the smallest and sharpest possible flame of the blast lamp. The tube is next taken in both hands and held horizontally above the flame so that the scratch is exactly over it. The tubing is now rotated rapidly about its axis, and lowered so that the flame is just tangent to its lower side. After about ten seconds of heating, it is removed from the flame and the hot portion quickly breathed upon, when it will generally crack apart very nicely. Care must be taken to hold the tube at right angles to the flame during the heating, and to rotate it so that only a narrow strip of the circumference is heated, and the scratch should be in the center of this heated strip. By this means tubing as large as two inches in diameter is readily broken.
Griffin's glass cutter, which contains a hardened steel wheel, like that on any ordinary window-glass cutter, and a device by which this can be made to make a true cut clear around the tube, is a very handy article, especially for large tubing, and may be obtained from any dealers in chemical apparatus.
Bending Glass.—Inasmuch as this is one of the commonest operations in the laboratory, it is assumed that the reader knows how to perform it. However, it should be noted that in order to obtain the best results a broad (fish-tail burner) flame should generally be used, and the tube rotated on its axis during the heating, and allowed to bend mostly by its own weight. If large tubing is to be bent, one end must be stoppered and great care used. Whenever the tube shows signs of collapsing or becoming deformed, it must be gently blown out into shape, heating the desired spot locally if necessary. A blast-lamp is likely to be more useful here than the fish-tail burner.
Drawing Out a Tube.—Most students learn this the first day of their laboratory work in chemistry, but few take pains to do it well. The tube should be heated in the flame of a Bunsen burner, or blast lamp (preferably the latter) until it is very soft. During this time it must be continuously rotated about its axis, and so held that the edges of the heated zone are sharply defined; i.e., it should not be allowed to move back and forth along its own axis. When so hot that it cannot longer be held in shape, the tube is removed from the flame, and the ends slowly and regularly drawn apart, continuing the rotation of the tube about its axis. By regulating the rate of drawing and the length of tube heated, the desired length and diameter of capillary may be obtained. The tube should always be rotated and kept in a straight line until the glass has set, so that the capillary may have the same axis as the main tube. This capillary or "tail" is often a very necessary handle in glass-blowing, and if it is not straight and true, will continually make trouble.
In drawing out very large tubing, say from one to two inches in diameter, it is often necessary to draw the tube in the flame, proceeding very slowly and at a lower temperature than would be used with small tubing. This is partly on account of the difficulty of heating large tubing uniformly to a high temperature, and partly in order to prevent making the conical part of the tube too thin for subsequent operations.
Constricting a Tube.—Where a constriction is to be made in a tube, the above method must be modified, as the strength of the tube must be maintained, and the constricted portion is usually short. Small tubes are often constricted without materially changing their outside diameter, by a process of thickening the walls. The tube is heated before the blast lamp, rotating it about its axis as later described, and as it softens is gradually pushed together so as to thicken the walls at the heated point, as in a, Fig. 1. When this operation has proceeded far enough, the tube is removed from the flame, and the ends cautiously and gently drawn apart, continuing the rotation of the tube about its axis and taking care not to draw too rapidly at first. The resulting tube should have a uniform exterior diameter, as shown in b, Fig. 1.Fig. 1.—Constricting a tube.1
This method of constriction is not suited to tubes much over ⁄4 inch in diameter, since the mass of glass in the constricted part becomes so thick as to be difficult to handle when hot, and likely to crack on cooling. Larger tubes are therefore constricted by heating in a narrow flame, with constant rotation, and when soft, alternately gently pulling the ends apart and pushing them together, each motion being so regulated that the diameter of a short section of the tube is gradually reduced, while the thickness of the wall of the reduced portion remains the same as that of the rest of the tube, or increases only slightly. This pulling and pushing of the glass takes place in the flame, while the rotation is being continued regularly. The result may appear as indicated in c, Fig. 1. The strength of the work depends upon the thickness of the walls of the constricted portion, which should never be less than that in the main tube, and usually a little greater. This operation is most successful with tubing having a relatively thin wall.
Flanging a Tube.—This operation produces the characteristic flange seen on test-tubes, necks of flasks, etc., the object being twofold: to finish the end neatly and to strengthen it so that a cork may be inserted without breaking it. This flanging may be done in several ways. In any case the first operation is to cut the tube to a square end, and then heat this end so that the extreme sixteenth or eighth of an inch of it is soft and begins to shrink. The tube is of course rotated during this heating, which should take place in a flame of slightly greater diameter than the tube, if possible. The flange is now produced by expanding this softened part with some suitable tool. A cone of charcoal has been recommended for this purpose, and works fairly well, if made so its height is about equal to the diameter of its base. The tube is rotated and the cone, held in the other hand, is pressed into the open end until the flange is formed. A pyramid with eight or ten sides would probably be better than the cone.