A handbook of laboratory glass-blowing

by Bernard D. Bolas

Thermometer Bulbs.—If the thermometer is to be filled with mercury, it is desirable to use a rubber bulb for blowing, as moisture is liable to condense inside the tube when the mouth is used, and this moisture will cause the mercury thread to break. In any case, a slight pressure should be maintained inside the thermometer tube while it is in the flame; otherwise the fine capillary tube will close and it will be very difficult to expand the heated glass into a bulb.

Large Bulbs.—When a large bulb is needed on a small or medium sized tube, it is often necessary to provide more glass than would be obtained if the bulb were blown in the ordinary way. One method is to expand the tube in successive stages along its axis, as shown by a, Fig. 6. These expanded portions are then re-heated, so that they run together into one hollow mass from which the bulb is blown; b and c, illustrate this. Another method, and one which is useful for very large bulbs, is to fuse on a length of large, thick-walled, tubing. The heat reflector, g, Fig. 3, should be used, if necessary, when making large bulbs. It consists of a sheet of asbestos mounted in a foot, and is used by being placed close to the mass of glass on the side away from the blowpipe flame while the glass is being heated.

Fig. 6

Bulbs of Dissimilar Glass.—These may be made by the second method given under "Large Bulbs," but the joint should be blown as thin as possible. Further instructions in the use of unlike glasses are given on page 94.

A Bulb in the Middle of a Tube.—Unless the bulb is to be quite small, it will be necessary to join in a piece of thick glass tubing, or to draw the thin tube out from a larger piece, thus leaving a thick mass in the middle as shown by d, Fig. 6. This mass of glass should now be rotated in the blowpipe flame until it is quite soft and on the point of running together. Considerable practice will be necessary before the two ends of the tube can be rotated at the same speed and without "wobbling," but this power must be acquired. When the glass is thoroughly hot, remove from the flame, hold in a horizontal position, and expand by blowing. It is essential to continue the rotation while this is done. Should one part of the bulb tend to expand more than the other, turn the expanded part to the bottom, pause for about a second, both in rotating and blowing, in order that the lower portion may be cooled by ascending air-currents; then continue blowing and turning as before.

Absorption Bulbs or Washing Bulbs.—These are made by an elaboration of the processes given in the last paragraph, g, h, and i, Fig. 6, illustrate this.

A Thistle Funnel.—This is made by blowing a fairly thick-walled bulb on a glass tube, bursting a hole by heating and blowing, and enlarging the burst-out part by heating and rotating against a turn-pin.

Bending Glass Tube.—Small tubing may be bent in a flat flame gas burner and offers no special difficulty. Large or thin-walled tubing should be heated in the blowpipe flame and a slight bend made; another zone of the tube, just touching the first bend, should now be heated and another slight bend made. In this way it is possible to avoid flattening and a bend having any required angle can gradually be produced. A final shaping of the bend may be made by heating in a large blowpipe flame and expanding slightly by air pressure.

Glass Spirals.—If a tube is heated by means of a long, flat-flame burner, the softened tube may be wound on to an iron mandrel which has previously been covered with asbestos. The mandrel should be made slightly conical in order to facilitate withdrawal. It is desirable to heat the surface of the asbestos almost to redness by means of a second burner, and thus avoid undue chilling of the glass and the consequent production of internal strain.