Laboratory Manual of Glass-Blowing Part 2

EXERCISE NO. 6

BULB AT THE END OF A TUBE

For this exercise tubing of 1/4 inch diameter and moderately strong walls is selected. A tail is drawn out on one end of the tube, and a piece of tubing about nine or ten inches long is cut off. The tail should be carefully drawn in the axis of the tube, and in the same straight line with it, as it is to be used as a handle in a.s.sembling the gla.s.s for the bulb. This tail must be long enough so that it can be conveniently held in the left hand, as described on page 13, and rotated about the same axis as the main tube. Holding the main tube in the right hand and the tail in the left, the tube is rotated in a large flame so that a piece of it, beginning where the tail stops and extending about an inch to the right, may be uniformly heated to the highest temperature at which it can be kept in shape. As soon as this temperature is reached, the tube is removed from the flame, continuing the rotation and taking care not to draw out the heated part, and gently blown. The rotation is carefully continued during the blowing, holding the tube in approximately a horizontal position. As soon as the tube has expanded a little the tail is pushed gently toward the main tube, continuing the gentle blowing. If this is properly done, the heated piece of tube will become a short bulb of about double its original diameter, and about the same wall thickness as the original tube. It will have somewhat the appearance of _a_, Fig. 10, when properly manipulated.

[Ill.u.s.tration: FIG. 10.--Blowing a bulb on the end of a tube.]

The tube is now reheated as before, taking care this time that the heating extends over all that part of the bulb to the right of the dotted line in the figure, as well as part of the main tube adjoining.

If this heating has been properly placed, when the operation of blowing and pus.h.i.+ng together is repeated the result will be to lengthen the bulb into a uniform cylinder, as shown in _b_, Fig. 10. Otherwise the result will be a series of bulbs, as in _c_, Fig. 10, separated by thickened ridges which will be almost impossible of removal later and will disfigure the final bulb. This operation of heating, blowing and pus.h.i.+ng together is repeated several times, until the cylinder becomes as long as can be conveniently handled (about 1-1/4 inches to 1-1/2 inches). If more gla.s.s is needed than is then contained in the cylinder, the latter may now be heated as a whole, and blown and pushed gently into a shorter cylinder of a slightly greater diameter, and more gla.s.s then added as before.

When enough gla.s.s has been collected for the bulb, it is all well heated and blown gently a couple of times, pus.h.i.+ng the ma.s.s together as required, until a thick bulb like _d_, Fig. 10, is obtained. The tail must now be removed at the point indicated by the dotted line. To do this, a very fine flame is directed on the point where the tail joins the bulb, and the tube well rotated as the gla.s.s softens at that point.

When sufficiently soft, the work is raised a little, so that the flame instead of striking the gla.s.s squarely at the point indicated pa.s.ses below and tangential to it. The tail is now drawn off slowly, continuing the rotation, raising the work just out of the flame whenever the thread of gla.s.s drawn off becomes too thin, and lowering it again to the point where the flame just touches it when the gla.s.s stiffens a little. By this means the tail may be drawn off without leaving an appreciable lump behind, as indicated in _e_ and _f_, Fig. 10. When as much of the extra gla.s.s has been removed as is practicable, the flame is brought to play squarely upon the little lump left, the last of the tail removed, and the lump heated and gently blown to a small excrescence on the main bulb. The whole end of the latter is now heated until it begins to shrink a little, and gently blown to make it uniform in thickness. The whole bulb is then heated in a flame of the proper size, so that it all may shrink to about two-thirds of its diameter. The flame must be very carefully chosen and directed, so as to shrink all the bulb, right up to the main tube, but not soften the latter. As soon as this stage is reached, the bulb is removed from the flame, continuing the even rotation, and blown to the desired size, preferably by a series of gentle puffs following one another at very short intervals. During the blowing, the main tube is held in a horizontal position, and any tendency of the bulb to fall out of line is corrected by the rotation.

If the shape of the bulb or its size are not satisfactory, it may be shrunk again and reblown. Such shrinking should begin in a large yellow flame, with just enough air to give it direction. The amount of air may be gradually increased as the bulb shrinks and the walls become thick enough to bear it without collapsing. If the bulb starts to collapse at any time, it must be immediately blown enough to regain its convex surface, before the shrinking proceeds further.

=Discussion.=--In collecting the gla.s.s for the bulb, enough must be gathered to give the walls the desired strength. Since the area of a sphere is proportional to the cube of its diameter, it is evident that doubling the size of a bulb diminishes the thickness of its walls to a very large extent. The limit of diameter for a strong bulb on ordinary 1/4-inch tubing, collecting the gla.s.s as above, is about 1-1/2 inches, and the beginner will do well not to blow his bulbs more than an inch in diameter.

The collection of the gla.s.s is one of the most important parts of the process. If the ma.s.s of gla.s.s be twisted, furrowed or ridged, or lop-sided, it is very difficult to get a good, even, spherical bulb, no matter how many times it is shrunk and blown. The greatest care should therefore be taken to get a uniform cylinder, on the same axis as the main tube; and to this end the rotation of the tube must be carried on very evenly. For method of holding the tube, see page 14.

If a very large bulb is required, it will often be economical to seal on the end of the tube a short piece of a large tube, provided with the proper tail, and use the gla.s.s in the large tube for the bulb instead of attempting to collect it from the small tube. In this case part of the small tube will usually be included in the bulb, so that the joint comes in the latter, and not where it joins the tube. As the amount of gla.s.s carried on the end of the tube increases in weight and size the difficulties of heating it uniformly, keeping it in the proper position and handling it increase rapidly.

In collecting gla.s.s, it is usually best not to leave the part of the cylinder next the tube with too thick walls. This is always the coolest part during the preparation for blowing the bulb, consequently it does not get blown out, and causes an ugly thickened appearance on that end of the bulb.

If the bulb grows too long or pear-shaped, it may be easily shortened by heating to the blowing temperature, and then blowing gently with the main tube in a vertical position, and the bulb at the top of it. Gravity will then shorten the bulb nicely.

The finished bulb should be a nearly perfect sphere, with the axis of the tube pa.s.sing through its center, and the portion of the tube adjoining the bulb must not be distorted, twisted, or blown out. In order to prevent the distortion of the tube, care must be taken that it is never heated quite to its softening point during the process.

EXERCISE NO. 7

BLOWING A BULB IN A TUBE

The tube is selected and one end closed as in the previous exercise, but it should be cut a little longer, say about twelve inches. Beginning at a point about four inches from the closed end, gla.s.s is collected and blown into a thick-walled bulb, exactly as in the previous exercise.

Greater care must be taken, however, that the cylinder collected and this thick bulb are of uniform thickness and set squarely in the axis of the tube. Instead of removing the tail, the bulb must be blown in this case with both pieces of tubing attached, and care must be taken that they "line up" properly, _i.e._, are in the same straight line, and that this line pa.s.ses as near as may be through the center of the bulb. The tube is held in approximately horizontal position during the blowing of the bulb, as in the previous case, and especial care taken with the rotation. Both pieces of tube must of course be rotated at the same rate, and their softened ends must be kept at exactly the proper distance from each other, so that the bulb may be spherical and not elongated. If the blowing of the bulb be quickly and accurately done, it may usually be completed before the gla.s.s is quite set, and the alignment of the two tubes may then be rectified while looking straight through the bore of the tube.

=Discussion.=--The two points of greatest importance are the collection of the gla.s.s, and the uniform rotation of the tube. A larger tube may be sealed in the middle of a small one when a large amount of gla.s.s is necessary. The piece of tubing used for the exercise must be long enough so that the fingers may be kept on a cool part of the gla.s.s without getting uncomfortably near the ends of the tube. It should not be any longer than necessary, however, as the extra weight and length make the manipulation of the hot gla.s.s more difficult.

When a string of bulbs are required on the same tube, a piece of gla.s.s 18 inches long may be used at the start, and the first bulb made near the closed end, as described. Each succeeding bulb will then be in plain view during the blowing, and when the open end becomes too short for comfort, it may be dried out, cut off, and another piece joined to it, starting as in the first method (Exercise No. 1), but instead of drawing out the thick bulb to a tube, it is made part of the gla.s.s collected for the next bulb. If the string of bulbs becomes awkward to handle on account of its length and weight, it may be made in several parts and these later sealed together by the second method, preferably blowing through a rubber tube attached to the open end, as described on page 22.

Very neat small bulbs may be made on tubing of a diameter of 3/16 inch or a little less, but the beginner is advised to start with tubing of about 1/4 inch diameter. The use of tubing with too thick walls usually produces bulbs which are thick-walled at the point where they leave the tube, but inclined to be too thin at the point of maximum diameter (perpendicular to the axis of the tube) where most of the strain comes and strength is particularly needed.

CHAPTER IV

ADVANCED EXERCISES

EXERCISE NO. 8

SEALING A TUBE THROUGH ANOTHER TUBE

_First Method--Making a Gas-was.h.i.+ng Tube_

This first method can be used whenever one can work through an open end opposite to the end of the tube where the joint is to be made. To ill.u.s.trate it, take a piece of rather thin-walled tubing, about 3/4 inch in diameter, and some pieces of rather strong tubing a little less than 1/4 inch in diameter. Draw off the large tube in a short cone, then draw off the tail as in the making of the bulb on the end of the tube, blow out the little lump slightly, shrink the whole cone a little and blow gently to form a rounded end like that on a test-tube, with walls about the thickness of those of the rest of the tube. Cut this tube to a suitable length, say about six inches, and provide two corks which will fit the open end of it. Now cut a piece of the small tubing of the proper length to form the piece which is to be inside the large tube.

For practice purposes, this piece should be about an inch shorter than the large tube. f.l.a.n.g.e one end of this tube a little, and anneal the f.l.a.n.g.e well in the smoky flame. Bore one of the corks so that a piece of the small tubing will fit it, and cut a couple of notches in the side of this cork so that air can pa.s.s between it and the gla.s.s. Pa.s.s a short piece of the small tubing through this cork, and attach the f.l.a.n.g.ed piece of small tube to this by means of a short piece of rubber tubing, so that when the whole is inserted in the large tube it is arranged as in _a_, Fig. 11. The piece of gla.s.s tubing projecting out through the cork is now cut off so as to leave an end about 1/2 inch long when the cork is firmly seated and the inner tube pushed into contact with the center of the end of the large tube, as shown in the drawing. Care should be taken that the little rubber tube which joins the two pieces is arranged as in the figure; _i.e._, most of it on the piece of tubing which pa.s.ses through the cork, and very little on the other piece, so that when the cork is removed after the small tube has been sealed through the large one, the rubber tube may easily come with it. Select a short piece of the small tubing of suitable length for the piece which is to be on the outside of the large tube as a continuation of the piece inside, and another piece for the delivery tube. A small bulb may be blown in the latter at a point about 2-1/2 inches from the closed end, and the open end cut off about 1-1/2 inches from the bulb. A cork or cork-boring of suitable size to stopper the small tube is prepared, and laid ready with the other (unbored) cork for the large tube.

[Ill.u.s.tration: FIG. 11.--Gas-was.h.i.+ng tube.]

When everything is in readiness, the rounded end of the large tube is slowly heated until it softens and joins firmly to the small tube inside. After it has shrunk down well, it is blown out to its original size, placing the whole end of the large tube, cork and all, in the mouth. Now with a fine-pointed flame the gla.s.s covering the end of the small tube is heated to the softening temperature, and then is blown out to an excrescence by blowing on the end of the small tube which pa.s.ses through the cork. The end of this excrescence is heated and blown off in the usual way, so as to leave the small tube sealed on the inside of the large one and opening through it into this short tube which has been blown out. The end of the small tube which pa.s.ses through the cork is now closed with the cork prepared for it, and the short outer tube is joined to the tube that has just been blown out, so that the joint appears like _b_, Fig. 11. Use the first method (Exercise No. 1) for this joint. Reheat the whole of the end of the tube nearly to the softening temperature, anneal it a little, and allow to cool a few seconds until well set. Now remove the cork, short gla.s.s tube and rubber tube from the open end of the large tube and insert the solid cork in their place. Warm the joint and the whole of that end of the tube again carefully up to about the softening point, then seal on the side tube for the delivery of the gas in the usual way, taking care that the whole of the end and the joint are kept warm meanwhile. When thoroughly sealed, the delivery tube is bent up parallel to the tube through which the gas enters, and then out at right angles to it, as shown in _c_. The whole of the end of the tube is now cautiously reheated and then cooled slowly to anneal it.

The cork may now be removed from the open end of the large tube, this end heated in a large flame, caught together with a sc.r.a.p of gla.s.s tubing and drawn off into a cone so that the base of the cone is about opposite the end of the inner tube. The lump of gla.s.s is drawn off the point of this cone and it is reblown to form a rounded end, as previously described.

After this cools, the tube through which the gas enters may be heated at the proper point and bent at right angles to form the finished apparatus as shown in _d_. The ends of the small tube are cut off square and fire-polished.

=Discussion.=--After the joint has once been made, great care must be taken that it is kept hot during all the subsequent manipulations, and if it becomes somewhat cool at any time it must be reheated very slowly.

It is obvious that the rate of heating and cooling of the inner tube will be slower than that of the outer tube, and this will readily produce stresses which tend to crack the tube at the joint. The amount of heating and cooling which such a joint will stand depends upon its form. The beginner should examine such a joint on regular factory-made apparatus, and note the uniformity of wall-thickness and the "clean-cut"

appearance of the joint, as a model for his imitation. A ragged joint, where the line of joining of the inner and outer tubes wavers instead of going squarely around the tube, is almost sure to crack during the cooling and heating unless extra precautions are taken with it. The presence of a small lump of gla.s.s at any point on the joint affords an excellent starting place for a crack, as do also the points on a ragged joint where the inner tube comes farther down on the outer tube than at other points.

In order to insure a joint which is square and not ragged, it is essential that the angle between the inner and outer tubes at the joint be very nearly a right angle. For this reason the two tubes should not be of too near the same size, or if this cannot be avoided, a small bulb should be blown on the end where the joint is to be made. If this bulb be made with the same wall-thickness as the rest of the tube, and somewhat pear-shaped, it may be drawn out to the same size as the rest of the tube, if necessary, after the joint has been made.

This method is used wherever possible in preference to the second method (Exercise No. 9), as it is easier to get a good joint with it. It may also be used where it is desired to seal the tube through the side of a tube, or for a tube sealed through the wall of a bulb, as in a Geissler potash bulb or similar apparatus. Where there is not s.p.a.ce to join the inner tube to the blowing tube by a rubber tube, this joint may be made with a small piece of gummed paper, which can readily be broken when desired.

EXERCISE NO. 9

SEALING A TUBE THROUGH ANOTHER TUBE

_Second Method--Making a Suction Pump_

Select a piece of tubing 3/8 to 1/2 inch in diameter, with walls about 1/16 inch or a little less in thickness, heat a place about 4 inches from one end and draw it out so that when cut off at the proper point it will look like _a_, Fig. 12; the open end of the drawn out part being small enough to slip inside another piece of the original tube. A small thick-walled bulb is now blown as indicated by the dotted lines, and annealed. A piece of the original tubing is now prepared, 7 or 8 inches long, with one end cut square off and the other closed. A piece of 1/4-in tubing about 2 inches long, and drawn out at one end to a tail several inches long is also prepared, to form the inlet tube for the air. Another piece of the 3/8-inch tube is prepared, about 4 inches long, and provided with a tail drawn out as indicated in _b_, so that when cut off at about 2-1/2 or 3 inches from the main tube its inner diameter may be slightly less than that of the narrowest point of the tube _a_. A small thick-walled bulb is blown at the point indicated by the dotted lines, and annealed. Care must be taken in drawing the capillary and blowing the bulb in both _a_ and _b_ that the capillary tubes are in the axis of the main tube, and in the same straight line with it.

[Ill.u.s.tration: FIG. 12.--Suction pump.]

The open end of the 8-inch piece of tube and the bulb of the piece _a_ are now warmed together, the end of the tube only moderately and the bulb to about its softening temperature. The tube _a_ is now inserted in the open end of the large tube, and the bulb softened with a suitable flame and pressed into good contact with the tube. It is then reheated, including the joint, blown a little and pulled out to form a straight tube in line with the main tube. By warming the joint a little, and proper rotation, the capillary may be brought into the same straight line with the rest of the tube.

Keeping this joint hot, a place about an inch from it on the tube _a_ is warmed, and the piece of 1/4-inch tubing previously prepared is sealed on at that point. The joint is then well annealed and allowed to cool.

The tube _a_ is now cut at such a place that when _b_ is inserted in the open end the point will come near the end of the constriction of _a_, as shown in _c_. Care is taken to get a clean square cut. The side tube is now cut off about an inch from the main tube and corked. Tube _b_ is sealed into the open end of _a_, in the same way as _a_ was sealed into the large tube, and the joint carefully annealed.

=Discussion.=--As in the first method, the secret of success lies in getting a square joint, and having the inner tube leave the outer one at nearly right angles. All the remarks about annealing, lumps, etc., made under the previous method apply here.

This method may be applied in sealing a small tube into the end of a large one, the latter being either drawn to a cone and cut off at the desired diameter, or else given a rounded end like a test-tube and a hole the proper size blown in the center of it. A suitable thick-walled bulb is to be blown on the small tube, as in the case described above.

This method is also used in making the Kjeldahl trap (_a_, Fig. 13), the small tube to be inserted being first drawn, the thick bulb blown at its point of union with the main tube, and then the small tube bent and cut.

The large bulb is best made with rather heavy wall, being either blown in the middle of a tube, and one piece of the tube drawn or cut off, or else made on the end of a tube. In the latter case a drop of gla.s.s must be put on the point where the joint is to be, so as to get a hole of the proper size with enough gla.s.s around it to prevent it from growing larger when it is heated. The author prefers to blow the bulb in the middle of the tube, draw off one end of the bulb, and blow out the desired hole where the tube was drawn off. The whole bulb must generally be reheated and blown a little at the end of the process, and well annealed.

[Ill.u.s.tration: FIG. 13.--_a_, Kjeldahl trap; _b_, suction pump on smaller tubing.]

The suction pump can also be made on 1/4-inch tubing, and one joint saved if desired, by constricting the tube to form the raceway for the water and air, as shown in _b_, Fig. 13. (See page 10 for method.) But it is more difficult to make a square joint on such small tubing.