The Methods of Glass Blowing and of Working Silica in the Oxy-Gas Flame / For the use of chemical and physical students

by W. A. Shenstone

A good joint may be recognised by its freedom from lumps of glass, its regularity of curve, and by a sensibly circular line at H, where the two tubes are united.

When the joint after annealing has become quite cold, the pin-hole at C on the inner tube may be closed, after removing the paper support, by warming the outer tube, and then directing a fine pointed flame through D on to C. And the end D of the outer tube may be closed in the ordinary manner, or a narrow tube may be sealed to it. As the end of glass at D will be too short to be held by the fingers when hot, another piece of tube of similar diameter must be attached to it to serve as a handle (see [p. 35], [Fig. 9]).

Blowing a Bulb or Globe of Glass.—For this purpose it is very important that the glass tube employed shall be of uniform substance. The size and thickness of the tube to be employed depends partly on the dimensions of the bulb desired, and partly on the size of neck that is required for the bulb. It is easier to blow large bulbs on large-sized tubes than on those of smaller size. When it is necessary to make a large globe on a small tube, it can be done, however, if great care be taken to avoid overheating that part of the small tube which is nearest to the mass of viscous glass from which the bulb is to be formed. For the purpose of blowing a very large bulb on a small tube, it is best to unite a wide tube to that which is to serve as the neck, as it will save some time in collecting the necessary mass of glass from which to form the globe.

Fig. 20.

To blow a Bulb at the End of a Tube.—Select a good piece of tube, say 1·5 cm. in diameter, and about 30 cm. long; draw out one end to a light tail (a, [Fig. 20]) about 3 inches in length. Then heat up a short length of the tube at b, with a small brush flame, by rotating the glass in the flame, and gently press it together when soft to thicken it; blow into it if necessary to preserve the regularity of its figure. Repeat this process on the portion of tube nearest to that which has been first thickened, and so on, till as much glass has been heated and thickened as you judge will serve to make a bulb of the size desired. You should have a mass of glass somewhat resembling that shown at B ([Fig. 20]), but probably consisting of the results of more successive operations than are suggested in that diagram. Apply the flame as before to the narrower parts cc of B, gently compress and blow until all the small bulbs first made are brought together into a mass still somewhat resembling the enlarged end of B, but more nearly cylindrical, with the glass as regularly distributed as possible, and of such length from d to the contracted part that the whole of it may easily be heated simultaneously with the large brush flame of your blow-pipe. Take great care in the foregoing operations not to allow the sides of the mass of glass to fall in and run together, and, on the other hand, do not reduce the thickness of the glass needlessly by blowing it more than is necessary to give the glass as regular a form as possible. When you are satisfied with the mass of glass you have collected, melt off the tail a, and remove the pointed end of glass that remains, as directed on page 33. Turn on as large a brush flame as is necessary to envelop the whole mass of glass that you have collected, and heat it with constant rotation, so that it may gradually run together to the form seen at C ([Fig. 20]), taking care that it does not get overheated near d, or the tube which is to form the neck will soften and give way.

The position in which the mass of heated glass is to be held will depend upon circumstances; if the mass of glass be not too great, it is best to keep it in a nearly horizontal position. If the mass of glass be very large, it may be necessary to incline the end B downwards; but as that is apt to result in an excess of glass accumulating towards d, avoid doing so if possible by rotating the glass steadily and rapidly. If at any time the glass shows indications of collapsing, it must be removed from the flame and gently blown into shape, during which operation it may be rotated in the perpendicular position; indeed, to promote a regular distribution of the glass by allowing it plenty of time to collect, it is well from time to time to remove the heated mass of glass from the flame, and slightly expand it by blowing. Finally, when a regular mass of glass, such as is shown at C ([Fig. 20]) has been obtained, remove it from the flame, and blow it to its final dimensions. A succession of gentle puffs quickly succeeding each other should be employed, in order that the progress of the bulb may be more easily watched and arrested at the right moment. During the process of blowing, the hot glass must be steadily rotated.

To collect the glass for blowing a bulb of lead glass, employ the flame described on [pp. 17]-[22] for heating lead glass.

If the tube be held horizontally whilst the globe is blown, its form will most nearly approach that of a true globe. If it be held in the perpendicular position, with the mass of glass depending from it, the form of the bulb will usually be somewhat elongated. If it be held perpendicularly, with the mass of glass upwards, the resulting bulb will be flattened.