EXERCISE NO. 7
Blowing a Bulb in a Tube
The tube is selected and one end closed as in the previous exercise, but it should be cut a little longer, say about twelve inches. Beginning at a point about four inches from the closed end, glass is collected and blown into a thick-walled bulb, exactly as in the previous exercise. Greater care must be taken, however, that the cylinder collected and this thick bulb are of uniform thickness and set squarely in the axis of the tube. Instead of removing the tail, the bulb must be blown in this case with both pieces of tubing attached, and care must be taken that they "line up" properly, i.e., are in the same straight line, and that this line passes as near as may be through the center of the bulb. The tube is held in approximately horizontal position during the blowing of the bulb, as in the previous case, and especial care taken with the rotation. Both pieces of tube must of course be rotated at the same rate, and their softened ends must be kept at exactly the proper distance from each other, so that the bulb may be spherical and not elongated. If the blowing of the bulb be quickly and accurately done, it may usually be completed before the glass is quite set, and the alignment of the two tubes may then be rectified while looking straight through the bore of the tube.
Discussion.—The two points of greatest importance are the collection of the glass, and the uniform rotation of the tube. A larger tube may be sealed in the middle of a small one when a large amount of glass is necessary. The piece of tubing used for the exercise must be long enough so that the fingers may be kept on a cool part of the glass without getting uncomfortably near the ends of the tube. It should not be any longer than necessary, however, as the extra weight and length make the manipulation of the hot glass more difficult.
When a string of bulbs are required on the same tube, a piece of glass 18 inches long may be used at the start, and the first bulb made near the closed end, as described. Each succeeding bulb will then be in plain view during the blowing, and when the open end becomes too short for comfort, it may be dried out, cut off, and another piece joined to it, starting as in the first method (Exercise No. 1), but instead of drawing out the thick bulb to a tube, it is made part of the glass collected for the next bulb. If the string of bulbs becomes awkward to handle on account of its length and weight, it may be made in several parts and these later sealed together by the second method, preferably blowing through a rubber tube attached to the open end, as described on page 22.
Very neat small bulbs may be made on tubing of a diameter of 3⁄16 inch or a little less, but the beginner is advised to start with tubing of about 1⁄4 inch diameter. The use of tubing with too thick walls usually produces bulbs which are thick-walled at the point where they leave the tube, but inclined to be too thin at the point of maximum diameter (perpendicular to the axis of the tube) where most of the strain comes and strength is particularly needed.