When a bulb is not of a sufficiently regular form, it may sometimes be re-made by re-collecting the glass, and re-blowing it. The greatest care is needed at the earlier stages of re-heating to prevent the glass from collapsing into a formless and unworkable mass. This is to be prevented in all such cases by gently blowing it into shape from time to time whilst gathering the glass.
Fig. 21.
To blow a Bulb between two Points ([Fig 21]).—Select a piece of suitable tube, seal or cork one end, gather together a mass of glass at the desired part, as directed for blowing a bulb at the end of a tube; when a mass of glass has been collected of sufficient thickness, blow it into shape from the open end of the tube by a rapid succession of short blasts of air, till the expanding glass attains the desired dimensions. The tube must be held horizontally, and must be rotated steadily during the process. By slightly pressing together the glass while blowing, the bulb will be flattened; by slightly drawing apart the two ends of the tube, it will be elongated.
A pear-shaped bulb may be obtained by gently re-heating an elongated bulb, say from a to a, and drawing it out. It is easiest to perform this operation on a bulb which is rather thick in the glass.
If the tubes bb are to be small, and a globe of considerable size is wanted, contract a tube as shown in [Fig. 22], taking care that the narrow portions of the tube are about the same axis as the wider portions, for if this be not the case, the mouths of the bulb will not be symmetrically placed; seal at C, cut off the wider tube at B, and make the bulb, as previously described, from the glass between AA. If, as probably will be the case, the contracted portions of the tube be not very regular, they may be cut off, one at a time, near the bulb, and replaced by pieces of tube of the size desired.
Fig. 22.