Laboratory Manual of Glass-Blowing

by Francis C. Frary

Fig. 10.—Blowing a bulb on the end of a tube.

The tube is now reheated as before, taking care this time that the heating extends over all that part of the bulb to the right of the dotted line in the figure, as well as part of the main tube adjoining. If this heating has been properly placed, when the operation of blowing and pushing together is repeated the result will be to lengthen the bulb into a uniform cylinder, as shown in b, Fig. 10. Otherwise the result will be a series of bulbs, as in c, Fig. 10, separated by thickened ridges which will be almost impossible of removal later and will disfigure the final bulb. This operation of heating, blowing and pushing together is repeated several times, until the cylinder becomes as long as can be conveniently handled (about 1-¹⁄4 inches to 1-¹⁄2 inches). If more glass is needed than is then contained in the cylinder, the latter may now be heated as a whole, and blown and pushed gently into a shorter cylinder of a slightly greater diameter, and more glass then added as before.

When enough glass has been collected for the bulb, it is all well heated and blown gently a couple of times, pushing the mass together as required, until a thick bulb like d, Fig. 10, is obtained. The tail must now be removed at the point indicated by the dotted line. To do this, a very fine flame is directed on the point where the tail joins the bulb, and the tube well rotated as the glass softens at that point. When sufficiently soft, the work is raised a little, so that the flame instead of striking the glass squarely at the point indicated passes below and tangential to it. The tail is now drawn off slowly, continuing the rotation, raising the work just out of the flame whenever the thread of glass drawn off becomes too thin, and lowering it again to the point where the flame just touches it when the glass stiffens a little. By this means the tail may be drawn off without leaving an appreciable lump behind, as indicated in e and f, Fig. 10. When as much of the extra glass has been removed as is practicable, the flame is brought to play squarely upon the little lump left, the last of the tail removed, and the lump heated and gently blown to a small excrescence on the main bulb. The whole end of the latter is now heated until it begins to shrink a little, and gently blown to make it uniform in thickness. The whole bulb is then heated in a flame of the proper size, so that it all may shrink to about two-thirds of its diameter. The flame must be very carefully chosen and directed, so as to shrink all the bulb, right up to the main tube, but not soften the latter. As soon as this stage is reached, the bulb is removed from the flame, continuing the even rotation, and blown to the desired size, preferably by a series of gentle puffs following one another at very short intervals. During the blowing, the main tube is held in a horizontal position, and any tendency of the bulb to fall out of line is corrected by the rotation. If the shape of the bulb or its size are not satisfactory, it may be shrunk again and reblown. Such shrinking should begin in a large yellow flame, with just enough air to give it direction. The amount of air may be gradually increased as the bulb shrinks and the walls become thick enough to bear it without collapsing. If the bulb starts to collapse at any time, it must be immediately blown enough to regain its convex surface, before the shrinking proceeds further.

Discussion.—In collecting the glass for the bulb, enough must be gathered to give the walls the desired strength. Since the area of a sphere is proportional to the cube of its diameter, it is evident that doubling the size of a bulb diminishes the thickness of its walls to a very large extent. The limit of diameter for a strong bulb on ordinary ¹⁄4-inch tubing, collecting the glass as above, is about 1-¹⁄2 inches, and the beginner will do well not to blow his bulbs more than an inch in diameter.

The collection of the glass is one of the most important parts of the process. If the mass of glass be twisted, furrowed or ridged, or lop-sided, it is very difficult to get a good, even, spherical bulb, no matter how many times it is shrunk and blown. The greatest care should therefore be taken to get a uniform cylinder, on the same axis as the main tube; and to this end the rotation of the tube must be carried on very evenly. For method of holding the tube, see page 14.

If a very large bulb is required, it will often be economical to seal on the end of the tube a short piece of a large tube, provided with the proper tail, and use the glass in the large tube for the bulb instead of attempting to collect it from the small tube. In this case part of the small tube will usually be included in the bulb, so that the joint comes in the latter, and not where it joins the tube. As the amount of glass carried on the end of the tube increases in weight and size the difficulties of heating it uniformly, keeping it in the proper position and handling it increase rapidly.

In collecting glass, it is usually best not to leave the part of the cylinder next the tube with too thick walls. This is always the coolest part during the preparation for blowing the bulb, consequently it does not get blown out, and causes an ugly thickened appearance on that end of the bulb.

If the bulb grows too long or pear-shaped, it may be easily shortened by heating to the blowing temperature, and then blowing gently with the main tube in a vertical position, and the bulb at the top of it. Gravity will then shorten the bulb nicely.

The finished bulb should be a nearly perfect sphere, with the axis of the tube passing through its center, and the portion of the tube adjoining the bulb must not be distorted, twisted, or blown out. In order to prevent the distortion of the tube, care must be taken that it is never heated quite to its softening point during the process.