On Laboratory Arts Part 3

The preliminary adjustment of air or oxygen supply will enable the change to a flame of maximum power to be made very quickly. The tube having been introduced with constant rotation, it will soon soften sufficiently to be worked. The beginner will find it best to decide the convenient degree of softness by trial.

With soda gla.s.s it does not much matter how soft the gla.s.s becomes, for it remains viscous, but with lead gla.s.s the viscosity persists for a longer time and then suddenly gives place to a much greater degree of fluidity. [Footnote: This is only drawn from my impressions acquired in gla.s.s-working. I have never explicitly tested the matter experimentally.]

It is just at this point that a beginner will probably meet with his first difficulty. As soon as the gla.s.s gets soft he will find that he no longer rotates the gla.s.s at the same speed by the right and left hand, and, moreover, he will probably unconsciously bend the tube, and even deform it, by pushing or pulling.

The second third of the art of the gla.s.s-blower consists in being able to move both hands about, rotating a tube with each thumb and finger, and keeping the distance between the hands, and also the speed of rotation, constant. Nothing but long practice can give this facility, but it is essential that it be acquired to some extent, or no progress can be made. Some people acquire a moderate proficiency very quickly, others, of whom the writer is one, only become reasonably proficient by months, or even years, of practice.

Supposing that the tube is now ready to be drawn down, the operator will remove it from the flame, and will gently pull the ends apart, interrupting his turning as little as possible. If the tube be pulled too hard, or if the area heated be too small (about three-eighths of an inch in length in the case given would be proper), it will be found that the ends of the two portions of the tube will be nearly closed at a very sharp angle (nearly a right angle to the length of the tube), that the ends will be thin, and that a long length of very fine tube will be produced. To heat a short length of tube and pull hard and suddenly is the proper way to make a very fine capillary tube, but, in general, this is what we want to avoid.

If the operation be successfully performed, the drawn-down tube will have the appearance exhibited, which is suitable either for subsequently closing or handling by means of the drawn-down portion.

The straightness of the point can be obtained by a little practice in "feeling" the gla.s.s when the tube is rotated as it cools just before it loses its viscous condition.

When the operation is carried out properly the shoulder of the "draw"

should be perfectly symmetrical and of even thickness, and its axis regarded as that of a cone should lie in the axis of the tube produced. The operation should be repeated till the student finds that he can produce this result with certainty, and he should not be discouraged if this takes several days, or even weeks. Of course, it is probable that within the first hour he will succeed in making a tolerable job, but it is his business to learn never to make anything else.

Fig. 9.

Fig. 10. Diagram of a folded end.

-- 20. Closing and blowing out the End of a Tube.

When it is desired to close the end of a particular bit of tube, this is easily done by heating the end, and at the same time heating the end of a waste bit of tube or rod; the ends, when placed in contact, stick together, and a point can be drawn down as before. [Footnote: "Point" is here used in the technical sense, i.e. it is a thin tail of gla.s.s produced by drawing down a tube.] Having got a point, it will be found that the thin gla.s.s cools enough to allow of the point being handled after a few moments.

The most convenient way of reducing the point to a suitable length (say 1.5 inch) is to fuse it off in the flame, but this must be done neatly; if a tail is left it may cause inconvenience by catching, or even piercing the finger and breaking off. The blow-pipe flame being turned down to a suitable size, and the shoulder of the "draw" having been kept warm meanwhile, let the tip of the flame impinge on a point where the diameter is about half that of the undrawn tube, and let the temperature be very high (Fig. 11). The tube is to be inclined to the flame so that the latter strikes the shoulder normally, or nearly so. Then, according to circ.u.mstances, little or much of the gla.s.s can be removed at will by drawing off the tail (Fig. 12), till, finally, a small drop of melted gla.s.s only, adheres to the end of the now closed tube (Fig. 13).

Fig. 11.

Fig. 12.

Fig. 13.

Fig. 14.

When this is satisfactorily accomplished, heat the extreme end of the tube most carefully and equally, holding it in such a position that the gla.s.s will tend to flow from the bead back on to the tube, i.e.

hold the closed end up to the flame, the tube being, say, at 45 degrees to the horizontal. Then when the temperature is such as to indicate complete softness lift the tube to the mouth, still holding the tube pointing with its closed end a little above the horizontal, and blow gently. A beginner almost always blows too hard.

What is wanted, of course, is a continued pressure, to give the viscous gla.s.s time to yield gradually, if it is uniform; or else intermittent puffs to enable the thinner parts, if there are any, to cool more, and hence become more resisting than the thicker ones. In any case a little practice will enable the operator to blow out a round and even end--neither thicker nor thinner than the rest of the tube.

-- 21. To make a Weld.

To begin with, try on two bits of gla.s.s of the same size, i.e. cut a seven-inch length of gla.s.s in half by scratching it with the knife, and pulling the ends apart with a slight inclination away from the scratch. In other words, combine a small bending moment with a considerable tensional stress. It is important to learn to do this properly. If the proportions are not well observed, the tube will break with difficulty, and the section will not be perpendicular to the main length. If the knife is in good order it will make a fine deep scratch--the feel of the gla.s.s under the knife will enable the operator to decide when the scratch is made. The operation of cutting large tubes will be treated further on. The two halves of the tube being held one in each hand, and one tube closed at one end, the extremities to be united will be warmed, and then put in the flame as before.

Fig. 16.

There are many ways of proceeding--perhaps the easiest is as follows.

As soon as the gla.s.s shows signs of melting at the ends--and care should be taken that much more is not heated--take both bits out of the flame. Stop rotating for a moment, and resting the arms carefully on the edge of the table, raise the tubes above the flame and bring the ends swiftly and accurately together. This is a case of "sudden death no second attempt at making the ends meet can be allowed; if the tubes join in any other than a perfectly exact manner a kink more or less objectionable will result. In practice the operator will learn to bring the ends together, commencing at one point; i.e. the axes of the tubes will be inclined at first, so as to cause adherence at one spot only. If this is not quite "fair", then less damage is done in moving one tube slightly up or down to get the contact exact.

The tubes will then be closed upon one another as if they were hinged at the joint. This must be done lightly, yet sufficiently, to ensure that the gla.s.s is actually in contact all round.

Having gone so far, replace the tubes--now one--in the flame, and carefully rotating the gla.s.s, raise the temperature higher than in the operation just described, in fact the higher the temperature, short of burning the gla.s.s, the better. Take the tube out of the flame and blow into the open end, turning constantly as before. One puff is enough. Then turn and pull the gla.s.s apart till it is of the same diameter and thickness throughout, and feel that it is straight as before.

Though it is in general of high importance that the joint should be well heated, the beginner will probably find that he "ties up" his gla.s.s as soon as it gets really soft.

If his object is to make one joint--at any cost--then let him be careful to use two bits of exactly the same kind of gla.s.s, and only get the temperature up to the viscous stage. If the joint be then pulled out till it is comparatively thin, it will probably stand (if of soda gla.s.s); certainly, if of lead gla.s.s, though in this case it may not be sound. In any case the joint should be annealed in the asbestos box if practicable, otherwise (unless between narrow tubes) with the asbestos rag. Care must be taken that the asbestos is dry.

-- 22. To weld two Tubes of different Sizes.

To do this, the diameter of the larger tube must be reduced to that of the smaller. The general procedure described in drawing down must be followed, with the following modification. In general, a greater length of the tube must be heated, and it must be made hotter. The tube is to be gradually drawn in the flame with constant turning till the proper diameter and thickness of gla.s.s are attained.

Fig. 16.

For this operation time must be allowed if the operator's hands are steady enough to permit of it; the shoulder should form partly by the gla.s.s sinking in and partly by the process of drawing the hot gla.s.s out. A shoulder properly prepared is shown in the sketch. Beginners generally make the neck too thin on large tubes, and too thick on smaller ones. There ought to be no great difference in thickness of gla.s.s between the neck on the larger tube, and the smaller tube. The diameters should be as nearly as possible alike.

Having drawn down the larger tube to a neck, take it out of the flame, and as it cools pull and turn till the neck is of the right thickness and is perfectly straight, i.e. make the final adjustment outside the flame, and to that end have the neck rather too thick (as to gla.s.s) before it is taken out. It is not necessary to wait till the neck gets cold before the end can be cut off. Make a scratch as before--this will probably slightly damage the temper of the file knife, but that must be put up with. Hold the tube against the edge of the table, so that the scratch is just above the level of the rim, and strike the upper part a smart blow with the handle of the gla.s.s knife rather in the direction of its length. [Footnote: A bit of hoop iron nailed against the side of the table is a very convenient arrangement, and it need not project appreciably above the general level of the rim.]

Of course this applies to a tube where economy has been exercised and the end is short. If the tail is long enough to form a handle, the tube may be pulled apart as before. As a rule a temporary joint between a tube and a rod is not strong enough to enable the shoulder to be broken at the scratch by mere pulling. The ends to be welded must be broken off very clean and true. Subsequent operations are to be carried out as already described.

-- 23. The above operations will be easily performed on tubes up to half an inch in diameter, if they are not too long. It is the length of tube, and consequent difficulty in giving ident.i.ty of motion with the two hands, which make the jointing of long tubes difficult. There are also difficulties if the tubes are very thin, have a very fine bore or a very large diameter.

All these difficulties merely amuse a good gla.s.s-blower, but to an experimenter who wants to get on to other things before sufficient skill is acquired (in the movement of the hands and arms) the following method is recommended. First, use flint gla.s.s. Then, a.s.suming that any drawing down has to be done, do it as well as possible, for on this the success of the method to be described especially depends. Be sure that the tubes to be welded are cut off clean and are as nearly as may be of the same size at the point of junction.

To fix the description, suppose it is desired to join two tubes (see Fig. 8), each about one inch in diameter and a yard long. Get four clip stands and place them on a level table. Be sure that the stands are firm and have not warped so as to rock. In each pair of clips place a tube, so that the two tubes are at the same height from the table, and, in fact, exactly abut, with axes in the same straight line. Close one tube by a cork and then fix the blowing apparatus as shown to the other.

In such an operation as this the drying apparatus may be dispensed with, and a rubber tube simply connected to one end of the system and brought to the mouth. Take the oxygen blow-pipe and turn the nozzle till the flame issues towards you, and see that the flame is in order.

Then turn down the oxygen till it only suffices to clear the smoky flame, and commence to heat the proposed joint by a current of hot air, moving the flame round the joint. Finally, bring to bear the most powerful flame you can get out of the blow-pipe, and carry it round the joint so quickly that you have the latter all hot at once.

Put down the blow-pipe, and, using both hands, press the tubes together (which wooden clips will readily allow), and after seeing that the gla.s.s has touched everywhere, pull the tubes a trifle apart.

Apply the blow-pipe again, pa.s.sing lightly over the thin parts, if any, and heating thicker ones; having the end of the rubber tube in his mouth, the operator will be able to blow out thick places. When all is hot, blow out slightly, and having taken the flame away, pull the tubes a little apart, and see that they are straight.

Throw an asbestos rag over the joint, loosen one pair of the clamps slightly, and leave the joint to anneal. It is important that the least possible amount of gla.s.s should be heated, hence the necessity of having the ends well prepared, and it is also important that the work should be done quickly; otherwise gla.s.s will flow from the upper side downwards and no strong joint will be obtained.

Fig. 17. Tube being opened at one end.

-- 24. To weld Tubes of very small Bore.

If the bore is not so small as to prevent the entrance of the point of the iron nail, get the ends of the tubes hot, and open the bore by inserting the end of the nail previously smeared over with a trace of vaseline. Work the nail round by holding the handle between the thumb and first finger of the right hand, the tube being similarly placed in the left. The tube and nail should be inclined as shown in the sketch.

Never try to force the operation; the nail soon cools the gla.s.s, so that only a very short time is available after each heat; during this the tube should be rotated against the nail rather than the nail against the tube. Be careful not to heat a greater length of tube than is necessary, or the nail will, by its component of pressure along the tube, cause the latter to "jump up" or thicken and bulge.

Both ends being prepared, and if possible, kept hot, the weld may be made as before, and the heating continued till the gla.s.s falls in to about its previous thickness, leaving a bore only slightly greater than before.

It is in operations such as this that the asbestos box will be found of great use. As soon as one end of the weld is ready cool it in the flame till soot deposits, and then plunge it into the asbestos. This will cause it to cool very slowly, and renders it less likely to crack when again brought into the flame. Turned-out ends, if the gla.s.s is at all thick, are very liable to crack off on reheating, so that they must be reintroduced (into the flame) with especial care. This liability to breakage is reduced, but not eliminated, by the asbestos annealing.

Figs. 18 and 19.

-- 25. When the bore is very fine, it is best to seal off the tubes, and blow an incipient bulb near one end of each tube. These bulbs may be cooled in asbestos, and cut across when cold by means of a scratch touched at one end (Figs. 18 and 19) by a fine point of highly incandescent gla.s.s. For details of this method see p. 46, Fig. 21.

Time is occasionally saved by blowing off the ends of the bulbs. The details of this process will be described when the operation of making thistle-headed tubes is dealt with.

-- 26. When the tubes are both of large diameter, long, and very thin (cylinder tubes), a considerable amount of difficulty will be experienced. On the whole, it is best to heat each end separately till the gla.s.s thickens a little, anneal in the flame and in asbestos, and then proceed as in -- 22. If the ends are not quite true, it will be found that quite a thickness of gla.s.s may be "jumped" together at one side of the tubes, while the edges are still apart at the other.

When this looks likely to happen, incline the tubes as if the joint were a hinge, and bend back quickly; do not simply continue to push the tubes together in a straight line, or an unmanageable lump of gla.s.s will be formed on one side.

If in spite of these precautions such a lump does form, proceed as follows. Take a rod of gla.s.s, at least one-eighth of an inch thick, and warm it in the flame at one end. Heat the imperfect joint till it softens all round, and then bring the flame right up to the thick part, and heat that as rapidly and locally as possible. The oxygas flame does this magnificently. Press the heated end of the gla.s.s rod against the thick part, and pull off as much of the lump as it is desired to remove, afterwards blowing the dint out by a judicious puff. Finish off as before.