Laboratory Manual of Gla.s.s-Blowing.

by Francis C. Frary.

PREFACE

The purpose of this little book is to provide a clear and detailed discussion of the elements of gla.s.s-blowing. Many laboratories in this country, especially in the west, are located a long way from any professional gla.s.s-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 stopc.o.c.k, 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 gla.s.s, 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 stopc.o.c.ks, 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 gla.s.s 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 gla.s.s-blowing.

F. C. F.

MINNEAPOLIS, MINN.

_January, 1914.

CHAPTER I

MATERIALS AND APPARATUS

One of the most important factors in the success of any piece of gla.s.s-blowing is the gla.s.s employed. As is well known, there are two general varieties of gla.s.s: Lead gla.s.s and soda gla.s.s. Formerly much apparatus was made of lead gla.s.s, but at present it is very seldom met with, except in the little drops of special gla.s.s used to seal platinum wires into the larger sizes of tubes. Lead gla.s.s is softer and more readily fusible than soda gla.s.s, 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 gla.s.s restored to its original clearness.

Almost all the soft gla.s.s on the market is a soda gla.s.s, although sometimes part of the soda is replaced by potash. Most of the hard gla.s.s appears to be a potash gla.s.s. The following qualities are desirable in a gla.s.s for ordinary working: (1) moderately low working temperature, (2) freedom from air bubbles, striations and irregularities, (3) proper composition, so that the gla.s.s 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 gla.s.s"

varies a good deal, and is best determined by trial. The gla.s.s 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 gla.s.s is too hard, the shrinking of the gla.s.s, collection of material for a bulb, and in fact most of the working processes will be slower, and the gla.s.s 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 gla.s.s 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 gla.s.s. Crookedness and non-uniformity of diameter are troublesome only when long pieces must be used.

Devitrification is one of the worst faults gla.s.s can possibly have. It is especially common in old gla.s.s, and in gla.s.s which has contained acids. It seems to be of two sorts. One variety manifests itself on the surface of the gla.s.s before it reaches its working temperature, but if the gla.s.s be heated to the highest temperature of the flame it will disappear except in the portion at the edge of the heated part. The gla.s.s 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 gla.s.s 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 gla.s.s 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 gla.s.s 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 gla.s.s 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 gla.s.s 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 gla.s.s 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 gla.s.s at the highest temperature of the flame any longer than necessary_. This may be accomplished by doing all heating, shrinking, etc., of the gla.s.s in a flame more or less tinged with yellow, and only raising the temperature to the highest point when ready to blow the gla.s.s. This kind of devitrification is apparently due to volatilization of the alkalies from the gla.s.s 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 gla.s.s is heated.

The toughness of gla.s.s, _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 gla.s.s 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 circ.u.mference 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 gla.s.s 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 gla.s.s-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 c.o.c.k 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.

Gla.s.s-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 a.s.suming 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 gla.s.s-work, and they should be so arranged that the fundamental points, such as the rotation of gla.s.s, the proper temperature, blowing and shrinking the gla.s.s may be learned with a minimum expenditure of time, gla.s.s 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.

CHAPTER II

GENERAL OPERATIONS

=Cutting the Gla.s.s.=--For this purpose a "gla.s.s-knife" is preferred to a file, if the gla.s.s is cold: if it is hot a file must always be used, and its edge slightly moistened to prevent drawing the temper. The gla.s.s-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 gla.s.s-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 pa.s.sing part of the knife or file once across the gla.s.s, never by "sawing" the tool back and forth. This latter procedure dulls the tool very quickly.

In breaking a piece of gla.s.s 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 small globule of gla.s.s (1/16 to 1/8 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 gla.s.s. 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 gla.s.s 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 circ.u.mference 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 gla.s.s cutter, which contains a hardened steel wheel, like that on any ordinary window-gla.s.s 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 Gla.s.s.=--Inasmuch as this is one of the commonest operations in the laboratory, it is a.s.sumed 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 gla.s.s 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 gla.s.s-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.

[Ill.u.s.tration: FIG. 1.--Constricting a tube.]

This method of constriction is not suited to tubes much over 1/4 inch in diameter, since the ma.s.s of gla.s.s 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 gla.s.s 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 f.l.a.n.g.e 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 f.l.a.n.g.e 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 f.l.a.n.g.e is formed. A pyramid with eight or ten sides would probably be better than the cone.

[Ill.u.s.tration: FIG. 2.--Flanging tool.]

A better flanging tool is made from a triangular piece of copper or bra.s.s, about 1/16 inch thick, and mounted in a suitable handle. Such a tool is shown in Fig. 2, being cut from a sheet of copper and provided with a handle made by wrapping asbestos paper moistened with sodium silicate solution about the shank of the tool. It is well to have several sizes and shapes of these tools, for different sizes of tubing.

The two sizes most used will be those having about the following dimensions: (1) _a_ = 2 inches, _b_ = 1 inch; (2) _a_ = 1 inch, _b_ = 1 inch. When the end of the tube is softened, the tool is inserted at an angle, as indicated in Fig. 3, and pressed against the soft part, while the tube is quickly rotated about its axis. If the f.l.a.n.g.e is insufficient the operation may be repeated. The tool should always be warmed in the flame before use, and occasionally greased by touching it to a piece of wax or paraffin. After the f.l.a.n.g.e is complete, the end must be heated again to the softening temperature and cooled slowly, to prevent it from cracking.

[Ill.u.s.tration: FIG. 3.--Flanging a tube with flanging tool.]

[Ill.u.s.tration: FIG. 4.--Flanging a tube with carbon rod or wire.]

Some gla.s.s-blowers use a small carbon rod, about 3/16 inch in diameter, as a flanging tool for tubes larger than about 3/8 inch diameter, and a small iron wire or similar piece of metal for smaller tubes. In this case the tube is heated as above described, and the rod or wire inserted in the end at an angle and pressed against the softened part, as indicated in Fig. 4, while the tube is rotated about its axis. For large heavy tubes a larger carbon would be used.

=Rotation of the Tube.=--This is the fundamental manipulation in gla.s.s-blowing, and upon it more than all else depends the uniformity and finish of the work, and often the possibility of accomplishing the work at all. Directions for it will be given on the a.s.sumption that the reader is right-handed; if otherwise, the position of the hands is of course reversed. The object of rotation is to insure even heating of the whole circ.u.mference of the tube at the point of attack, to equalize the effect of gravity on the hot gla.s.s and prevent it from falling out of shape when soft, and to keep the parts of the tube on each side of the heated portion in the same straight line.

In rotating the tube, both hands must be used, so that the two ends may revolve at the same rate and the gla.s.s in the hot part not be twisted.

The rotation is performed by the thumb and first finger of each hand, the other fingers serving to support the tube. As it is almost always necessary to follow rotating and heating a tube by blowing it, the hands should be so placed that it will be easy to bring the right-hand end up to the mouth without shifting the hold on the gla.s.s. For this reason the left hand grasps the gla.s.s with the palm down, and the right hand with the palm turned toward the left. If there is any choice, the longer and heavier part of the tube is usually given to the left hand, and it is planned to blow into the shorter end. This is because it is easier to support the tube with the hand which has the palm down. This support is accomplished by bending the hand at the wrist so that it points slightly downward, and then curling the second, third and little fingers in under the tube, which is held between them and the palm. This support should be loose enough so that the thumb and first finger can easily cause the tube to rotate regularly on its axis, but firm enough to carry all the weight of the tube, leaving the thumb and first finger nothing to do but rotate it. The hand must be so turned, and the other fingers so bent, that the thumb and first finger stretch out nearly to their full length to grasp the tube comfortably.

The right hand is held with the palm toward the left, the fingers except the first slightly bent, and the tube held between the first finger and the thumb while it rests on the second finger and that portion of the hand between the base of the first finger and the thumb. Rotation of the tube is accomplished by rolling it between the thumbs and first fingers: the rotation being continued in the same direction regularly, and not reversed. It is better to roll slowly and evenly, with a series of light touches, each of which moves the tube a little, than to attempt to turn the tube a half a revolution or so with each motion of the hands. The hands must be held steady, and the tube must be under good control at all times, so that both ends may be rotated at the same angular velocity, even though they may be of different diameters, and the tube be neither drawn apart nor pushed together unless such a motion is expressly desired, as it sometimes is. The hot part of the gla.s.s must be constantly watched to see that it is uniformly rotated and not twisted, nor pulled out or pushed together more than is desired. Care must also be taken to keep the parts of the tube in the same straight line, or as near it as possible, during the heating and all other manipulations.