作者：Bolas, Bernard D.
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A handbook of laboratory glass-blowing试读：
PREFACE
To cover the whole field of glass-blowing in a small handbook would be impossible. To attempt even a complete outline of the methods used in making commercial apparatus would involve more than could be undertaken without omitting the essential details of manipulation that a novice needs. I have, therefore, confined myself as far as possible to such work as will find practical application in the laboratory and will, I hope, prove of value to those whose interests lie therein.
The method of treatment and somewhat disjointed style of writing have been chosen solely with the view to economy of space without the undue sacrifice of clearness.
BERNARD D. BOLAS.
CHAPTER I
Introduction and Preliminary Remarks—General Principles to be observed in Glass Working—Choice of Apparatus—Tools and Appliances—Glass.
Glass-blowing is neither very easy nor very difficult; there are operations so easy that the youngest laboratory boy should be able to repeat them successfully after once having been shown the way, there are operations so difficult that years are needed to train eye and hand and judgment to carry them out; but the greater number of scientific needs lie between these two extremes. Yet a surprisingly large number of scientific workers fail even to join a glass tube or make a T piece that will not crack spontaneously, and the fault is rather one of understanding than of lack of ability to carry out the necessary manipulation.
In following the scheme of instruction adopted in this handbook, it will be well for the student to pay particular attention to the reason given for each detail of the desirable procedure, and, as far as may be, to memorise it. Once having mastered the underlying reason, he can evolve schemes of manipulation to suit his own particular needs, although, as a rule, those given in the following pages will be found to embody the result of many years' experience.
There is a wide choice of apparatus, from a simple mouth-blowpipe and a candle flame to a power-driven blower and a multiple-jet heating device. All are useful, and all have their special applications, but, for the present, we will consider the ordinary types of bellows and blowpipes, such as one usually finds in a chemical or physical laboratory.
The usual, or Herepath, type of gas blowpipe consists of an outer tube through which coal gas can be passed and an inner tube through which a stream of air may be blown. Such a blowpipe is shown in section by Fig. 1. It is desirable to have the three centring screws as shown, in order to adjust the position of the air jet and obtain a well-shaped flame, but these screws are sometimes omitted. Fig. 1, a and b show the effects of defective centring of the air jet, c shows the effect of dirt or roughness in the inside of the air jet, d shows a satisfactory flame.Fig 1
For many purposes, it is an advantage to have what is sometimes known as a "quick-change" blowpipe; that is one in which jets of varying size may be brought into position without stopping the work for more than a fraction of a second. Such a device is made by Messrs. Letcher, and is shown by e, and in section by f Fig. 1. It is only necessary to rotate the desired jet into position in order to connect it with both gas and air supplies. A small bye-pass ignites the gas, and adjustment of gas and air may be made by a partial rotation of the cylinder which carries the jets.
For specially heavy work, where it is needed to heat a large mass of glass, a multiple blowpipe jet of the pattern invented by my father, Thomas Bolas, as the result of a suggestion derived from a study of the jet used in Griffin's gas furnace, is of considerable value. This jet consists of a block of metal in which are drilled seven holes, one being central and the other six arranged in a close circle around the central hole. To each of these holes is a communication way leading to the gas supply, and an air jet is arranged centrally in each. Each hole has also an extension tube fitted into it, the whole effect being that of seven blowpipes. In order to provide a final adjustment for the flame, a perforated plate having seven holes which correspond in size and position to the outer tubes is arranged to slide on parallel guides in front of these outer tubes.Fig. 2
The next piece of apparatus for consideration is the bellows, of which there are three or more types on the market, although all consist of two essential parts, the blower or bellows proper and the wind chamber or reservoir. Two patterns are shown in Fig. 2; a, is the form which is commonly used by jewellers and metal workers to supply the air blast necessary for heating small furnaces. Such a bellows may be obtained at almost any jewellers' supply dealer in Clerkenwell, but it not infrequently happens that the spring in the wind chamber is too strong for glass-blowing, and hence the air supply tends to vary in pressure. This can be improved by fitting a weaker spring, but an easier way and one that usually gives fairly satisfactory results, is to place an ordinary screw-clip on the rubber tube leading from the bellows to the blowpipe, and to tighten this until an even blast is obtained.
Another form of bellows, made by Messrs. Fletcher and Co., and common in most laboratories, is shown by b; the wind chamber consists of a disc of india-rubber clamped under a circular frame or tied on to a circular rim. This form is shown by Fig. 2, b.
The third form, and one which my own experience has caused me to prefer to any other, is cylindrical, and stands inside the pedestal of the blowpipe-table. A blowpipe-table of this description is made by Enfer of Paris.
There is no need, however, to purchase an expensive table for laboratory use. All the work described in this book can quite well be done with a simple foot bellows and a quick-change blowpipe. Nearly all of it can be done with a single jet blowpipe, such as that described first, or even with the still simpler apparatus mentioned on page 84, but I do not advise the beginner to practise with quite so simple a form at first, and for that reason have postponed a description of it until the last chapter.
Glass-blowers' tools and appliances are many and various, quite a number of them are better rejected than used, but there are a few essentials. These are,—file, glass-knife, small turn-pin, large turn-pin, carbon cones, carbon plate, rubber tube of small diameter, various sizes of corks, and an asbestos heat reflector. For ordinary work, an annealing oven is not necessary, but one is described on page 60 in connection with the special cases where annealing is desirable.
Fig. 3 illustrates the tools and appliances. a is an end view of the desirable form of file, and shows the best method of grinding the edges in order to obtain a highly satisfactory tool. b is a glass knife, shown both in perspective and end view, it is made of glass-hard steel and should be sharpened on a rough stone, such as a scythe-stone, in order to give a slightly irregular edge. c is a small turn-pin which may be made by flattening and filing the end of a six-inch nail. d is the large turn-pin and consists of a polished iron spike, about five inches long and a quarter of an inch diameter at its largest part. This should be mounted in a wooden handle. e and f are carbon cones. A thin rubber tube is also useful; it may be attached to the work and serve as a blowing tube, thus obviating the necessity of moving the work to the mouth when internal air pressure is to be applied. In order to avoid undue repetition, the uses of these tools and appliances will be described as they occur.Fig. 3
Glass, as usually supplied by chemical apparatus dealers is of the composition known as "soda-glass." They also supply "hard" or "combustion" glass, but this is only used for special purposes, as it is too infusible for convenient working in the ordinary blowpipe flame.
Soda-glass consists primarily of silicate of sodium with smaller quantities of silicate of aluminum and potassium. Its exact composition varies. It is not blackened, as lead glass is, by exposure to the reducing gases which are present in the blue cone of a blowpipe flame, and hence is easier for a beginner to work without producing discolouration.
Further notes on glasses will be found on page 55, but for ordinary purposes soda-glass will probably be used.
CHAPTER II
Easy Examples of Laboratory Glass-Blowing—Cutting and Sealing Tubes for Various Purposes; Test-Tubes, Pressure-Tubes, Tubes for High Temperature Experiments—Thermometer-Bulbs, Bulbs of Special Glass, Pipettes, Absorption-Bulbs or Washing-Bulbs—Joining Tubes; Branches, Exhaustion-Branches, Branches of Dissimilar Glass—Blowing Bulbs; A Thistle Funnel; Cracking and Breaking Glass; Leading and Direction of Cracks—Use of Glass Rod or Strips of Window-Glass; Joining Rod, Feet and Supports—Gripping Devices for use in Corrosive Solutions—The Building Up of Special Forms from Solid Glass.
Perhaps the most common need of the glass-blower whose work is connected with that of the laboratory is for a sealed tube; and the sealing of a tube is an excellent preliminary exercise in glass-blowing.
We will assume that the student has adjusted the blowpipe to give a flame similar to that shown in d, Fig. 1, and that he has learned to maintain a steady blast of air with the bellows; further, we will assume that the tube he wishes to seal is of moderate size, say not more than half an inch in diameter and with walls of from one-tenth to one-fifth of an inch thick.Fig. 4
A convenient length of tube for the first trial is about one foot; this should be cut off from the longer piece, in which it is usually supplied, as follows:—lay the tube on a flat surface and make a deep cut with the edge of a file. Do not "saw" the file to and fro over the glass. If the file edge has been ground as shown in a, Fig. 3, such a procedure will be quite unnecessary and only involve undue wear; one movement with sufficient pressure to make the file "bite" will give a deep cut. Now rotate the tube through about one-eighth of a turn and make another cut in continuation of the first. Take the tube in the hands, as shown in a, Fig. 4, and apply pressure with the thumbs, at the same time straining at the ends. The tube should break easily. If it does not, do not strain too hard, as it may shatter and cause serious injuries to the hands, but repeat the operation with the file and so deepen the original cuts. In holding a tube for breaking, it is important to place the hands as shown in sketch, as this method is least likely to cause shattering and also minimises the risk of injury even if the tube should shatter. To cut a large tube, or one having very thick walls, it is better to avoid straining altogether and to break by applying a small bead of intensely heated glass to the file cut. If the walls are very thin, a glass-blower's knife should be used instead of a file. The tube and glass-blower's knife should be held in the hand, and the tube rotated against the edge of the knife; this will not produce a deep cut, but is less likely to break the tube. A bead of hot glass should be used to complete the work.
The next operation is to heat the glass tube in the middle; this must be done gradually and evenly; that is to say the tube must be rotated during heating and held some considerable distance in front of the flame at first; otherwise the outer surface of the glass will expand before the interior is affected and the tube will break. From two to five minutes, heating at a distance of about eight inches in front of the flame will be found sufficient in most cases, and another minute should be taken in bringing the tube into the flame. Gradual heating is important, but even heating is still more important and this can only be obtained by uniform and steady rotation. Until the student can rotate a tube steadily without thinking about it, real progress in glass-blowing is impossible.
When the tube is in the flame it must be held just in front of the blue cone and rotated until the glass is soft enough to permit the ends to be drawn apart. Continue to separate the ends and, at the same time, move the tube very slightly along its own axis, so that the flame tends to play a little more on the thicker part than on the drawn-out portion. If this is done carefully, the drawn-out portion can be separated off, leaving only a slight "bleb" on the portion it is desired to seal. This is illustrated by b, Fig. 4.
To convert the seal at b, Fig. 4., into the ordinary form of test-tube seal, it is only necessary to heat the "bleb" a little more strongly, blow gently into the tube until the thick portion is slightly expanded, re-heat the whole of the rounded end until it is beginning to collapse, and give a final shaping by careful blowing after it has commenced to cool. In each case the glass must be removed from the flame before blowing. The finished seal is shown by c, Fig. 4. If desired, the open end may now be finished by heating and rotating the soft glass against the large turn-pin, as illustrated in d, but the turn-pin must not be allowed to become too hot, as if this happens it will stick to the glass. After turning out the end, the lip of glass must be heated to redness and allowed to cool without coming in contact with anything; otherwise it will be in a condition of strain and liable to crack spontaneously. The finished test-tube is shown by e.
When it is necessary to seal a substance inside a glass tube, the bottom of the tube is first closed, as explained above, and allowed to cool; the substance, if a solid, is now introduced, but should not come to within less than two inches of the point where the second seal is to be made. If the substance is a liquid it can more conveniently be introduced at a later stage.
Now bring the tube into the blowpipe flame gradually, and rotate it, while heating, at the place where it is to be closed. Allow the glass to soften and commence to run together until the diameter of the tube is reduced to about half its original size. Remove from the flame and draw the ends apart, this should give a long, thick extension as shown by f, Fig. 4. If any liquid is to be introduced, it may now be done by inserting a thin rubber or other tube through the opening and running the liquid in. A glass tube should be used with caution for introducing the liquid, as any hard substance will tend to scratch the inside of the glass and cause cracking. The final closure is made by melting the drawn-out extension in the blowpipe flame; the finished seal being shown by g, Fig. 4.
If the sealed tube has to stand internal pressure, it is desirable to allow the glass to thicken somewhat more before drawing out, and the bottom seal should also be made thicker. For such a tube, and especially when it has to stand heating, as in a Carius determination of chlorine, each seal should be cooled very slowly by rotating it in a gas flame until the surface is covered with a thick layer of soot, and it should then be placed aside in a position where the hot glass will not come in contact with anything, and where it will be screened from all draughts.
Joining Tube.—We will now consider the various forms of join in glass tubing which are met with in the laboratory. First, as being easiest, we will deal with the end-to-end joining of two tubes of similar glass. a, b, and c, Fig. 5, illustrate this. One end of one of the tubes should be closed, a lip should be turned out on each of the ends to be joined, and both lips heated simultaneously until the glass is thoroughly soft. Now bring the lips together gently, until they are in contact at all points and there are no places at which air can escape; remove from the flame, and blow slowly and very cautiously until the joint is expanded as shown in b, Fig. 5. Reheat in the flame until the glass has run down to rather less than the original diameter of the tube, and give a final shaping by re-blowing. The chief factors of success in making such a join are, thorough heating of the glass before bringing the two tubes together, and avoidance of hard or sudden blowing when expanding the joint. The finished work is shown by c, Fig. 5.Fig. 5
To join a small glass tube to the end of a large one, the large tube should first be sealed, a small spot on the extreme end of the seal heated, and air pressure used to expand the heated spot as shown in d. This expanded spot is then re-heated and blown out until it bursts as shown in e, the thin fragments of glass are removed and the end of the small tube turned out as shown in f. After this the procedure is similar to that used in jointing two tubes of equal size.
When these two forms of joint have been mastered, a T piece will present but little difficulty. It is made in three stages as shown in Fig. 5, and the procedure is similar to that used in joining a large and small tube. Care should be taken to avoid softening the top of the "T" too much, or the glass will bend and distort the finished work; although a slight bend can be rectified by re-heating and bending back. Local re-heating is often useful in giving the joint its final shape.
An exhaustion branch is often made by a totally different method. This method is shown by g, h, and i, Fig. 5; g is the tube on which the branch is to be made. The end of a rod of similar glass should be heated until a mass of thoroughly liquid glass has collected, as shown, and at the same time a spot should be heated on that part of the tube where it is desired to make the branch. The mass of hot glass on the rod is now brought in contact with the heated spot on the tube and expanded by blowing as shown by h. The air pressure in the tube is still maintained while the rod is drawn away as shown by i. This will give a hollow branch which may be cut off at any desired point, and is then ready for connection to the vacuum pump.
If the rod used is of a dissimilar glass, the branch should be blown much thinner. Such a branch will often serve as a useful basis for joining two tubes of different composition, as the ordinary type of branch is more liable to crack when made with two glasses having different coefficients of expansion.
Blowing Bulbs.—A bulb may be blown on a closed tube such as that shown by c, Fig. 5, by rotating it in the blowpipe flame until the end is softened, removing it from the flame and blowing cautiously. It is desirable to continue the rotation during blowing. In the case of a very small tube, it is sufficient to melt the end without previous sealing, rotate it in the flame until enough glass has collected, remove from the flame and blow while keeping the tube in rotation.
Thermometer Bulbs.—If the thermometer is to be filled with mercury, it is desirable to use a rubber bulb for blowing, as moisture is liable to condense inside the tube when the mouth is used, and this moisture will cause the mercury thread to break. In any case, a slight pressure should be maintained inside the thermometer tube while it is in the flame; otherwise the fine capillary tube will close and it will be very difficult to expand the heated glass into a bulb.
Large Bulbs.—When a large bulb is needed on a small or medium sized tube, it is often necessary to provide more glass than would be obtained if the bulb were blown in the ordinary way. One method is to expand the tube in successive stages along its axis, as shown by a, Fig. 6. These expanded portions are then re-heated, so that they run together into one hollow mass from which the bulb is blown; b and c, illustrate this. Another method, and one which is useful for very large bulbs, is to fuse on a length of large, thick-walled, tubing. The heat reflector, g, Fig. 3, should be used, if necessary, when making large bulbs. It consists of a sheet of asbestos mounted in a foot, and is used by being placed close to the mass of glass on the side away from the blowpipe flame while the glass is being heated.
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