LITMIR.BIZ

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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)

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