Chapter I (2)

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 circumstances, little or much of the glass can be removed at will by drawing off the tail (Fig. 12), till, finally, a small drop of melted glass 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 glass 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 glass 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 glass of the same size, i.e. cut a seven-inch length of glass 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 glass 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 glass 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 glass is actually in contact all round.

Having gone so far, replace the tubes--now one--in the flame, and carefully rotating the glass, raise the temperature higher than in the operation just described, in fact the higher the temperature, short of burning the glass, 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 glass 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 glass 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 glass, 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 glass); certainly, if of lead glass, 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 glass 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 glass sinking in and partly by the process of drawing the hot glass 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 glass 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 glass) 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 glass 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 identity 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 glass-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 glass. Then, assuming 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 glass has touched everywhere, pull the tubes a trifle apart. Apply the blow-pipe again, passing 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 glass 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 glass 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 glass, 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 glass 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 glass 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 glass. 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 glass 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 glass 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 glass will be formed on one side.

If in spite of these precautions such a lump does form, proceed as follows. Take a rod of glass, 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 glass 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.

§ 27. Occasionally, when it is seen that in order to produce a joint closed all round, one side of the tube would be too much thickened, it is better to patch the open side. For this purpose take a glass rod about one-sixteenth inch in diameter, and turn the flame to give its greatest effect, still keeping rather an excess of air or oxygen. See that the side of the joint already made is kept fairly hot--it need not be soft; interrupt any other work often enough to ensure this. Then, directing the flame chiefly on the thin rod, begin to melt and pull the glass over the edges of the gap. When the gap is closed get the lump very hot, so that all the glass is well melted together, and then, if necessary, pull the excess of glass off, as before described.

It must be remembered that this and the method of the previous section are emergency methods, and never give such nice joints as a manipulation which avoids them, i.e. when the ends of the tubes are perfectly straight and true to begin with. Also note that, as the tubes cannot be kept in rotation while being patched, it is as well to work at as low a temperature as possible, consistently with the other conditions, or the glass will tend to run down and form a drop, leaving a correspondingly thin place behind.

Fig. 20.

§ 28. A very common fault in cutting a tube of about an inch in diameter is to leave it with a projecting point, as shown. This can be slowly chipped off by the pliers, using the jaws to crush and grind away the edge of the projection; it is fatal to attempt to break off large pieces of glass all at once.

§ 29. It will be convenient here to mention some methods of cutting large tubes. With tubes up to an inch and a half in diameter, and even over this--provided that the glass is not very thick--we may proceed as follows: Make a good scratch about half an inch long, and pretty deep, i.e. pass the knife backwards and forwards two or three times. Press a point of melted glass exactly on one end of the scratch; the glass point even when pressed out of shape should not be as large as a button one-twelfth of an inch in diameter. If this fails at first, repeat the operation two or three times.

Fig. 21.

If a crack does not form, touch the hot place with the cold end of the nail. If no success is obtained, try the other end of the scratch. If failure still pursues the operator, let him make another cut on the opposite side of the tube and try again. In general, the tube will yield the first or second time the hot drop of glass is applied. Never apply the drop at the centre of the scratch, or a ragged crack, which may run in any direction, will result. Very often, with a large tube, the crack formed by a successful operation will only extend a short distance. In this case it is desirable to entice the crack round the tube, and not trust to its running straight when the tube is pulled apart.

On the whole, the best method in this case is to employ a flame pencil, which should be kept ready for use. This merely consists of a bit of glass tube of about the same dimensions as an ordinary lead pencil, drawn down to a very fine jet at one end. The jet must not be very long or thin, or the glass will soon fuse up. A few trials will enable the operator to get the proper proportions, which are such that the tube has the general appearance of a pencil normally sharpened (say with a cone of 60'). This tube is best made of hard glass. Connect it to a gas supply by light flexible tubing, and turn down the gas till the flame from the end of the jet is not more than one-tenth of an inch long. Then apply the jet, beginning from the end of the crack, and gradually draw it (the crack) round the tube. The operation will be assisted if a rubber ring is slipped on the tube to begin with, so that the eye has some guide as to whether the flame is being drawn round properly or not. The ring must, of course, be far enough away to escape the effect of the flame. The crack will be found to follow the flame in the most docile manner, unless the tube is thick or badly annealed. Some operators recommend a pencil of glowing charcoal, but the flame is undoubtedly better.

§ 30. To cut very thick Tubes.

A large number of methods have been proposed, and nearly everybody has his favourite. The following has always succeeded with me. First mark on the tube, by means of a little dead black spirit paint, exactly where the cut is to be. Then sharpen the glass knife and scratch a quite deep cut all round: there is no difficulty in making the cut one-twentieth of an inch deep. It will be proper to lubricate the knife with kerosene after the first mark is made. [Footnote: The edge of the knife may be advantageously saved by using an old file moistened with kerosene for the purpose. I find kerosene is not worse, but, if anything, better than the solution of camphor in turpentine recommended by Mr. Shenstone.]

If the glass is about one-eighth of an inch thick, the scratch maybe conveniently about one-twentieth of an inch deep, but if the glass is anything like one-quarter of an inch thick, the scratch must be much deeper, in fact, the glass may be half cut through. To make a very deep scratch, a wheel armed with diamond dust, which will be described later on, may be used. However, it is not essential to use a diamond wheel, though it saves time.

When the cut is made to a sufficient depth proceed thus: Obtain two strips of bibulous paper or bits of tape and twist them round the tube on each side of the scratch, allowing not more than one-eighth of an inch between them. Then add a few drops of water to each, till it is thoroughly soaked, but not allowing water to run away. Dry out the scratch by a shred of blotting paper.

Turn down the oxygas flame to the smallest dimensions, and then boldly apply it with its hottest part playing right into the nick and at a single point. Probably in about two seconds, or less, the tube will break. If it does not, rotate the tube, but still so that the flame plays in the nick. After making the tube very hot all round--if it has not broken--apply the flame again steadily at one point for a few seconds and then apply a bit of cold iron. If the tube does not break at once during these processes, let it cool, and cut the groove deeper; then try again. [Footnote: This method is continually being reinvented and published in the various journals. It is of unknown antiquity.]

Fig. 22.

If the tube breaks after great heating and long efforts, it will probably leave incipient cracks running away from the break, or may even break irregularly. A good break is nearly always one that was easily made. If a number of rings have to be cut, or a number of cuts made on glass tubes of about the same size, it will be found economical in the end to mount a glazier's diamond for the purpose. A simple but suitable apparatus is figured (Fig. 23).

Fig. 23.

The only difficulty is to regulate the position of the diamond so that it cuts. In order to do this, carefully note its cutting angle by preliminary trials on sheet glass, and then adjust the diamond by clamps, or by wriggling it in a fork, as shown. Weight the board very slightly, so as to give the small necessary pressure, and produce the cut by rotating the tube by hand. When a cut is nearly completed take great care that the two ends join, or irregularity will result. This is not always easy to do unless the tube happens to be straight. Having got a cut, start a crack by means of a fine light watchmaker's hammer, or even a bit of fused glass, and entice the crack round the cut by tapping with the hammer or by means of the flame pencil.

If the cut is a true "cut" the tube will break at once. As a supply of electrical current for lighting will, in the near future, be as much a matter of course for laboratory purposes as a gas supply, I add the following note. To heat a tube round a scratch, nothing--not even the oxygas blow-pipe--is so good as a bit of platinum or iron wire electrically heated. If the crack does not start by considerable heating of the glass, stop the current, unwind the wire, and touch the glass on the crack either with a bit of cold copper wire or a wet match stem. I prefer the copper wire, for in my experience the water will occasionally produce an explosion of cracks. On the other hand, the cold wire frequently fails to start a crack.

Judging from the appearance of thick tubes as supplied by the dealers, the factory method of cutting off appears to be to grind a nick almost through the tube, and right round; and for really thick glass this is the safest but slowest way; a thin emery wheel kept wet will do this perfectly. Suitable wheels may be purchased from the "Norton" Emery Wheel Co. of Bedford, Mass, U.S.A.--in England through Messrs. Churchill and Co. of London, importers.

§ 31. To blow a Bulb at the End of a Tube.

I must admit at once that this is a difficult operation--at all events, if a large bulb is required. However, all there is to be said can be said in few words. In general, when a bulb is required at the end of a tube it will be necessary to thicken up the glass. A professional glass-worker will generally accomplish this by "jumping up" the tube, i.e. by heating it where the bulb is required, and compressing it little by little until a sufficient amount of glass is collected. The amateur will probably find that he gets a very irregular mass in this way, and will be tempted to begin by welding on a short bit of wide and thick tubing preparatory to blowing out the bulb.

However, supposing that enough glass is assembled by-either of these methods, and that it is quite uniform in thickness, let the thickened part be heated along a circle till it becomes moderately soft, and let it then be expanded about one-fifth, say by gently blowing. It is perhaps more important to keep turning the glass during bulb-blowing than in any other operation, and this both when the glass is in the flame and while the bulb is being blown. It is also very important to avoid draughts. In general, a bulb is best blown with the tube in a nearly horizontal position, but sloping slightly upwards from the mouth. If it be noticed that a bulb tends to blow out more at one side than another, let the side of greatest protuberance be turned down, so that it is at the lowest point, reduce the pressure for an instant, and then blow again. It will be observed that the bulb will now expand at the top.

The reason of this is chiefly that the under side cools most rapidly (according to Faraday, Chemical Manipulation, § 1194), and consequently can expand no further; but also it is not unlikely that the glass tends to flow somewhat from the upper side, which remains hot, and consequently the bulb, when the next puff reaches it, will tend to yield at this point. By heating several zones the tube will become gradually expanded.

Fig. 24.

Fig. 25.

Fig. 26.

When the length of the thickened part of the tube only slightly exceeds its diameter (Fig. 25), let the whole be brought to a temperature which, with flint glass, should be just short of that of perfect fluidity; and then, holding the tube horizontally and constantly turning it, let the bulb be blown out to its full size, noting the appearances and correcting too great protuberance on any side by the means above mentioned. If the bulb appears pear-shaped turn the tube so that the melted mass is directed upwards; if the bulb have the contrary fault, correct in the corresponding manner.

The bulb when finished may be lightly tapped on the table, when, if there is any weak place owing to inequality of thickness, the bulb will break, and the operation may be started afresh. "A good bulb is round, set truly on the tube, and free from lumps of thick glass or places of excessive thinness." When the amateur has succeeded in blowing a bulb two inches in diameter on the end of a strong bit of thermometer tube--say for an air thermometer--he may well seek the congratulations of his friends.

In case the bulb is not satisfactory on a first attempt, it may be melted down again, if the following precautions are taken. Directly creases begin to appear in the bulb let it be withdrawn from the flame, and gently blown till the creases come out. By alternate heating and blowing the glass can be got back to its original form, or nearly so, but unless the operator shows great skill and judgment, the probability is that the glass will be uneven. By heating and keeping the thicker parts in the higher position, and blowing a little now and again, the glass may be got even, and a new attempt may be made. It must not be supposed that this process can be carried on indefinitely, for the glass tends to lose its viscous properties after a time, or, at all events, it "perishes" in some way, especially if it has been allowed to get very thin; consequently too frequent attempts on the same glass are unprofitable. Two or three trials are as many as it generally pays to make. As a rule the largest possible flame may be used with advantage in this operation.

§ 32. To blow a bulb in the middle of a tube, the procedure is much like that already treated, but the manipulation is, if anything, more difficult, for the further end of the tube must be carried and turned as well as the end which is held to the lips.

§ 33. To make a side Weld.

This is by no means difficult, but is easier with lead glass than with soda glass. The tube to which it is desired to make a side connection having been selected, it is closed at one end by rubber tube stops, or in any other suitable manner. The zone of the proposed connection is noted, and the tube is brought to near softness round that circle (if the tube is made actually soft, inconvenience will arise from the bending, which is sure to occur). Two courses are then open to the operator, one suitable to a thick tube, the other to a tube of moderate thickness.

Taking the former first. Provide a piece of glass rod and warm its end. Direct a small flame against the spot on the thick tube where the proposed joint is to be. When the glass becomes almost incandescent at this spot, put the end of the rod against it and draw out a thread of glass till sufficient "metal" has been removed. Then fuse off the thread close to the tube.

Fig. 27.

The subsequent procedure is the same as for thin tubes. In this case heat the spot by the smallest flame available, and get the spot very hot. Blow it out gently into a bubble, perhaps extending to a height equal to its diameter. Then heat the top of the bubble till it is incandescent and blow violently. This will produce an opening fringed by glass so thin as to exhibit interference colours. Remove the filmy part, and heat the frayed edges till they cohere and form an incipient tube. If the flame has been of a correct size, the tube will now be of the same diameter as the tube to be welded on, and will project perhaps one-sixteenth of an inch from the surface of the main tube (Fig. 28).

Fig. 28.

Fig. 29.

When this stage is reached, again heat the tube all round till it nearly softens, and by means of the other hand heat the end of the other tube which it is proposed to weld. Just before the main tube actually softens, turn it so as to heat the edges of the aperture, and at the same time get the end of the side tube very hot. Take both out of the flame for an instant, and press the parts together, instantly slightly withdrawing the side tube. If the operation is well performed, it will be found that the point of maximum thickness of glass is now clear of the main tube. The joint is then to be heated all round and blown out--a rather awkward operation, and one requiring some practice, but it can be done.

Fig. 30.

If great strength is wanted, heat the main tube all round the joint bit by bit, and blow each section slightly outwards. If the operator is confident in his skill, he should then heat the whole joint to the softening point, blow it out slightly, and then adjust by pulling and pushing. Cool first in the gas flame, and then plunge the joint into the asbestos and cover it up--or if too large, throw the asbestos cloth round it.

In the case of soda glass this final "general heat" is almost essential, but it is not so with flint glass, and as the general heat is the most difficult part of the job, it will be found easier to use lead glass and omit the general heating. With soda glass a very small irregularity will cause the joint to break when cold, but flint glass is much more long-suffering. It is easy to perform the above operation on small tubes. For large ones it will be found best to employ flint glass and use the clip stands as in the case of direct welds, treated above, but, of course, with suitable modifications. Never let the main tube cool after the hole is made until the work is done.

§ 34. Inserted Joints.

In many instances the performance of apparatus is much improved by joints of this kind, even when their use is not absolutely essential.

There are two ways in which inserted joints may be made. The first method is the easier, and works well with flint glass; but when one comes to apply it to soda glass there is a danger of the glass becoming too thick near the joint, and this often leads to a cracking of the joint as the glass cools.

Fig. 31.

Suppose it is desired to insert the tube B into the tube A (Fig. 31). Begin by reducing the size of the end of tube A till B will just slip in quite easily. With B about one-quarter inch in diameter, a clearance of about one-twentieth of an inch, or less, in all (i.e. one-fortieth of an inch on each side) will be proper.

Heat B by itself at the proposed zone of junction, and blow out a very narrow ring; then compress this slightly so that it forms an almost closed ring of glass. The figure refers to the close of this operation (Fig. 31, B). It does not matter much whether the ring remains a mere flattened bulb, or whether it is a solid ring, but it must be one or the other. Some judgment must be exercised in preparing the ring. In general, the beginner will collect too much glass in the ring, and consequently the joint, when made, will either be thick and liable to crack easily, or it will be blown out into an erratic shape in endeavours to reduce this thickness. Accordingly, the operator will, if necessary, thin the tube B by drawing slightly, if he considers it desirable, before the little enlargement is blown out. In general, two heats must be used for this operation.

Fig. 32.

Get the approximating parts of both A and B up to a temperature just below that at which they will adhere, and having closed the other end of A, place B carefully within it up to the ring, and if it can be arranged, have a mica wad in A, with a central hole through which the end of B can project. This will very much facilitate the operation, especially if B is long, but may be dispensed with by the exercise of care and skill.

The operation is now simple. Fuse the junction and press the tubes lightly together, being careful not to collect more glass than can be helped; finally, blow out the joint and reduce the thickness by mild drawing (Fig. 33). In order to make a really good joint, two points must be particularly attended to--the rim must be thin and its plane perfectly perpendicular to the axis of tube B; the end of tube A must be cut off quite clean and perpendicular to its axis before B is inserted. So important are these conditions--especially the latter that the writer has even occasionally used the grindstone to get the end of A into a proper condition, an admission which will probably earn the contempt of the expert glass-worker.

Fig. 33.

Now for the second method, which is often practised in Germany, where soda glass is chiefly used. With this glass the chief point is to get a very even and not too thick ring at the junction, and consequently the extra thickening produced by making a rim on B is rather a drawback. The method consists in cutting off from B the length which it is desired to insert, slipping this into A (which may be an otherwise closed bulb, for instance), and then gradually melting up the open end of A till the piece of B inside will no longer fall out. By holding the joint downwards so that the inserted portion of B rests on the edges of the opening, a joint may be made with the minimum thickening.

The external part of B, previously heated, is then applied, and the joint subjected to a "general" heat and blown out. Very nice joints may be made by this method, and it is perhaps the better one where the external part of B is to be less in diameter than the inserted part. It was in this manner that the writer was taught to make glass velocity pumps, one of which, of a good design, is figured as an example.

In all cases good annealing should follow this operation. If the inserted part of the inner tube (B) is anything like an inch in diameter, and especially if it is of any length, as in some forms of ozone apparatus, or in a large Bunsen's ice calorimeter, the arrangements for supporting the inner part must be very good. A convenient way of proceeding when the inner tube is well supported is to make the mouth of A only very little larger than the diameter of B, so that B will only just slip in. Then the mouth of A and the zone of B may be heated together, and B blown out upon A. This, of course, must be arranged for, if necessary, by temporarily stopping the inner end of B.

The inner support of B should be removed as soon as practicable after the joint is made, or, at all events, should not be perfectly rigid; a tightly-fitting cork, for instance, is too rigid. The reason is, of course, that in cooling there may be a tendency to set B a little to one side or the other, and if it is not free to take such a set, the joint most probably will give way. Good annealing both with flame and asbestos is a sine qua non in all inserted work.

Fig. 34.

§ 35. Bending Tubes.

I have hitherto said nothing about bending tubes, for to bend a tube of a quarter of an inch in diameter, and of ordinary thickness, is about the first thing one learns in any laboratory, while to bend large tubes nicely is as difficult an operation as the practice of glass-blowing affords. However, even in bending a narrow tube it is possible to proceed in the wrong way. The wrong way is to heat a short length of the tube and then bend it rapidly, holding the plane of the bend horizontal. The right way, per contra, is to use a batswing burner to heat, say, two inches of the tube with constant turning till it is very soft, and then, holding the glass so that the bend will be in a vertical plane passing through one eye (the other being shut), to make the bend rather slowly.

If an exact angle is required, it is as well to have it drawn out on a sheet of asbestos board. In this case bend the glass as described till it is approximately right, and finish by laying it on the asbestos board and bringing it up to the marks. A suitable bit of wood may be substituted for the asbestos on occasion.

N.B: The laboratory table is not a suitable piece of wood. A right-angled bend is often wanted. In this case the corner of a table will serve as a good guide to the eye, the glass being finished by being held just above it. If great accuracy is wanted, make a wooden template and suspend it by a screw from the side of the table, so that the vertex of the gauge for the interior angle projects downwards, then finish by bending the tube round it. The wood may be about half an inch thick.

If a sharp bend is required, heat the tube in the blow-pipe, and bend it rapidly, blowing out the glass meanwhile. The reason why a long bend should be held in a vertical plane is that the hot part tends to droop out of the plane of the bend if the latter be made in a horizontal position. To bend a tube above half an inch in diameter is a more or less difficult operation, and one which increases in difficulty as the diameter of the tube increases.

A U-tube, for instance, may be made as follows: Use the four blow-pipe arrangement so as to heat a fair length of tube, and get, say, two inches of tube very hot--almost fluid, in fact--by means of the carbon block supported from a stand. Remove the tube rapidly from the flame and draw the hot part out to, say, three inches. Then, holding the tube so as to make the bend in a vertical plane, bend it and blow it out together to its proper size.

This operation seems to present no difficulties to experienced glass-workers, even with tubes of about one inch in diameter, but to the amateur it is very difficult. I always look on a large U-tube with feelings of envy and admiration, which the complex trick work of an elaborate vacuum tube does not excite in the least. It will be noted that this method may, and often does, involve a preliminary thickening of the glass.

With tubes over an inch in diameter I have no idea as to what is the best mode of procedure--whether, for instance, a quantity of sand or gas coke might not be used to stuff out the tube during bending, but in this case there would be the difficulty of removing the fragments, which would be sure to stick to the glass.

Of course, if the bend need not be short, the tube could be softened in a tube furnace and bent in a kind of way. I must admit that with tubes of even less than one inch in diameter I have generally managed best by proceeding little by little. I heat as much of the glass as I can by means of a gigantic blow-pipe, having a nozzle of about an inch in diameter, and driven by a machine-blower.

When I find that, in spite of blowing, the tube begins to collapse, I suspend operations, reheat the tube a little farther on, and so proceed. If by any chance any reader knows a good laboratory method of performing this operation, I hope he will communicate it to me. After all, the difficulty chiefly arises from laboratory heating appliances being as a rule too limited in scope for such work.

The bending of very thin tubes also is a difficulty. I have only succeeded here by making very wide bends, but of course the blowing method is quite applicable to this case, and the effect may be obtained by welding in a rather thicker bit of tube, and drawing and blowing it till it is of the necessary thinness. This is, however, a mere evasion of the difficulty.

§ 36. Spiral Tubes.

These are easily made where good heating apparatus is available. As, however, one constantly requires to bend tubes of about one-eighth inch in diameter into spirals in order to make spring connections for continuous glass apparatus, I will describe a method by which this is easily done. Provide a bit of iron pipe about an inch and a quarter in outside diameter. Cover this with a thick sheath of asbestos cloth, and sew the edges with iron wire. Hammer the wire down so that a good cylindrical surface is obtained. Make two wooden plugs for the ends of the iron pipe. Bore one to fit a nail, which may be held in a small retort clip, and fasten a stout wire crank handle into the other one. Support the neck of the handle by means of a second clip. In this way we easily get a sort of windlass quite strong enough for our purpose.

Fig. 35.

Provide a large blow-pipe, such as the blow-pipe of a Fletcher crucible furnace, Select a length of tubing and clean it. Lash one end to the cylinder by means of a bit of wire, and hold the other end out nearly horizontally. Then start the blow-pipe to play on the tube just where it runs on to the asbestos cylinder, and at first right up to the lashing. Get an attendant to assist in turning the handle of the windlass, always keeping his eye on the tube, and never turning so fast as to tilt the tube upwards. By means of the blow-pipe, which may be moved round the tubing, heat the latter continuously as it is drawn through the flame, and lay it on the cylinder in even spirals.

If the tubing is thin, a good deal of care will have to be exercised in order to prevent a collapse. A better arrangement, which, however, I have not yet tried, would, I think, be to replace the blow-pipe by two bats-wing burners, permanently fastened to a stand, one of them playing its flame downwards on to the top of the flame of the other. The angle between the directions of the jets might be, say, 130°, or whatever is found convenient. In this way the glass would not be so likely to get overheated in spots, and better work would doubtless result. However, I have made numbers of perfectly satisfactory spirals as described. Three or four turns only make a sufficiently springy connection for nearly all purposes.

§ 37. On Auxiliary Operations on Glass:-

Boring Holes through Glass:- This is much more easily done than is generally supposed. The best mode of procedure depends on the circumstances. The following three cases will be considered:-

1. Boring holes up to one-quarter inch diameter through thick glass (say over one-eighth inch), or rather larger holes through thin glass.

2. Boring holes of any size through thick glass.

3. Boring round holes through ordinary window glass.

§ 38. Boring small Holes.

Take a three-cornered file of appropriate dimensions, and snip the point off by means of a hammer; grind out most of the file marks to get sharp corners. Dip the file in kerosene, and have plenty of kerosene at hand in a small pot. Place the broken end of the file against the glass, and with considerable pressure begin to rotate it (the file) backwards and forwards with the fingers, very much as one would operate a bradawl against a hard piece of wood. The surface of the glass will shortly be ground away, and then the file bradawl will make much quicker progress than might be expected. Two or three minutes should suffice to bore a bit of sheet window-glass.

The following points require attention:

(1) Use any quantity of oil.

(2) After getting through the skin reduce the pressure on the file.

(3) Be sure to turn the file backwards and forwards through a complete revolution at least.

(4) When the hole is nearly through reduce the pressure.

(5) When the hole is through the glass be exceedingly careful not to force the file through too rapidly, otherwise it will simply act as a wedge and cause a complete fracture.

(6) In many cases it is better to harden the file in mercury before commencing operations; both files and glass differ so much in hardness that this point can only be decided by a trial. If it is found necessary to harden the file, use either a large blow-pipe and a coke or charcoal bed, or else a small forge. A small blowpipe, such as is generally found in laboratories, does more harm than good, either by burning the end of the file or raising it to an insufficient temperature.

(7) To sharpen the file, which is often necessary after passing through the "skin" of the glass, put it in a vice so that the point just protrudes clear of the jaws. Then, using a bit of waste iron as an intermediary anvil or punch, knock off the least bit from the point, so as to expose a fresh natural surface. The same result may be brought about by the use of a pair of pliers.

If several holes have to be bored, it is convenient to mount the file in the lathe and use a bit of flat hard wood to press up the glass by means of the back rest. A drilling machine, if not too heavy, does very well, and has the advantage of allowing the glass to remain horizontal so that plenty of oil can be kept in the hole.

Use a very slow speed in either case--much slower than would be used for drilling wrought iron. It is essential that the lubricant should flow on to the end of the file very freely, either from a pipette or from the regular oil-feed. If a little chipping where the file pierces the back surface is inadmissible, it is better, on the whole, to finish the bore by hand, using a very taper file. It is not necessary to use a special file for the lathe, for a well-handled file can be chucked very conveniently in a three-jaw chuck by means of the handle.

Mr. Shenstone recommends a lubricant composed of camphor dissolved in turpentine for general purposes. With the object of obtaining some decisive information as to the use of this lubricant, and to settle other points, I made the following experiments. Using an old three-cornered French file, I chipped off the point and adjusted the handle carefully. I also ground out the file marks near the point, without hardening the file in mercury. Using kerosene and turpentine and camphor, I began to bore holes in a hard bit of 3/32 inch window glass.

Each hole was bored to about one-eighth inch in diameter in four minutes with either lubricant. After hardening the file in mercury and using kerosene, I also required four minutes per hole. After mounting the file in a lathe which had been speeded to turn up brass rods of about one-half-inch diameter, and therefore ran too fast, I required one and a half minutes per hole, and bored them right through, using kerosene. On the whole, I think kerosene does as well as anything, and for filing is, I think, better than the camphor solution. However, I ought to say that the camphor-turpentine compound has probably a good deal to recommend it, for it has survived from long ago. My assistant tells me he has seen his grandfather use it when filing glass.

I beg to acknowledge my indebtedness to Mr. Pye, of the Cambridge Scientific Instrument Company, for showing me in 1886 (by the courtesy of the Company) the file method of glass-boring; it is also described by Faraday in Chemical Manipulation, 1228.

It is not necessary, however, to use a file at all, for the twist drills made by the Morse Drill Company are quite hard enough in their natural state to bore glass. The circumferential speed of the drill should not much exceed 10 feet per minute. In this way the author has bored holes through glass an inch thick without any trouble except that of keeping the lubricant sufficiently supplied. For boring very small holes watchmaker's drills may be used perfectly well, especially those tempered for boring hardened steel. The only difficulty is in obtaining a sufficient supply of the lubricant, and to secure this the drill must be frequently withdrawn.