method of constriction is not suited to tubes much over ¹⁄4 inch in diameter, since the mass of glass in the constricted part becomes so thick as to be difficult to handle when hot, and likely to crack on cooling. Larger tubes are therefore constricted by heating in a narrow flame, with constant rotation, and when soft, alternately gently pulling the ends apart and pushing them together, each motion being so regulated that the diameter of a short section of the tube is gradually reduced, while the thickness of the wall of the reduced portion remains the same as that of the rest of the tube, or increases only slightly. This pulling and pushing of the glass takes place in the flame, while the rotation is being continued regularly. The result may appear as indicated in c, Fig. 1. The strength of the work depends upon the thickness of the walls of the constricted portion, which should never be less than that in the main tube, and usually a little greater. This operation is most successful with tubing having a relatively thin wall.

Flanging a Tube.—This operation produces the characteristic flange seen on test-tubes, necks of flasks, etc., the object being twofold: to finish the end neatly and to strengthen it so that a cork may be inserted without breaking it. This flanging may be done in several ways. In any case the first operation is to cut the tube to a square end, and then heat this end so that the extreme sixteenth or eighth of an inch of it is soft and begins to shrink. The tube is of course rotated during this heating, which should take place in a flame of slightly greater diameter than the tube, if possible. The flange is now produced by expanding this softened part with some suitable tool. A cone of charcoal has been recommended for this purpose, and works fairly well, if made so its height is about equal to the diameter of its base. The tube is rotated and the cone, held in the other hand, is pressed into the open end until the flange is formed. A pyramid with eight or ten sides would probably be better than the cone.

Fig. 2.—Flanging tool.

A better flanging tool is made from a triangular piece of copper or brass, about ¹⁄16 inch thick, and mounted in a suitable handle. Such a tool is shown in Fig. 2, being cut from a sheet of copper and provided with a handle made by wrapping asbestos paper moistened with sodium silicate solution about the shank of the tool. It is well to have several sizes and shapes of these tools, for different sizes of tubing. The two sizes most used will be those having about the following dimensions: (1) a = 2 inches, b = 1 inch; (2) a = 1 inch, b = 1 inch. When the end of the tube is softened, the tool is inserted at an angle, as indicated in Fig. 3, and pressed against the soft part, while the tube is quickly rotated about its axis. If the flange is insufficient the operation may be repeated. The tool should always be warmed in the flame before use, and occasionally greased by touching it to a piece of wax or paraffin. After the flange is complete, the end must be heated again to the softening temperature and cooled slowly, to prevent it from cracking.

Fig. 3.—Flanging a tube with flanging tool.

Fig. 4.—Flanging a tube with carbon rod or wire.

Some glass-blowers use a small carbon rod, about ³⁄16 inch in diameter, as a flanging tool for tubes larger than about ³⁄8 inch diameter, and a small iron wire or similar piece of metal for smaller tubes. In this case the tube is heated as above described, and the rod or wire inserted in the end at an angle and pressed against the softened part, as indicated in Fig. 4, while the tube is rotated about its axis. For large heavy tubes a larger carbon would be used.

Rotation of the Tube.—This is the fundamental manipulation in glass-blowing, and upon it more than all else depends the uniformity and finish of the work, and often the possibility of accomplishing the work at all. Directions for it will be given on the assumption that the reader is right-handed; if otherwise, the position of the hands is of course reversed. The object of rotation is to insure even heating of the whole circumference of the tube at the point of attack, to equalize the effect of gravity on the hot glass and prevent it from falling out of shape when soft, and to keep the parts of the tube on each side of the heated portion in the same straight line.

In rotating the tube, both hands must be used, so that the two ends may revolve at the same rate and the glass in the hot part not be twisted. The rotation is performed by the thumb and first finger of each hand, the other fingers serving to support the tube. As it is almost always necessary to follow rotating and heating a tube by blowing it, the hands should be so placed that it will be easy to bring the right-hand end up to the mouth without shifting the hold on the glass. For this reason the left hand grasps the glass with the palm down, and the right hand with the palm turned toward the left. If there is any choice, the longer and heavier part of the tube is usually given to the left hand, and it is planned to blow into the shorter end. This is because it is easier to support the tube with the hand which has the palm down. This support is accomplished by bending the hand at the wrist so that it points slightly downward, and then curling the second, third and little fingers in under the tube, which is held between them and the palm. This support should be loose enough so that the thumb and first finger can easily cause the tube to rotate regularly on its axis, but firm enough to carry all the weight of the tube, leaving the thumb and first finger nothing to do but rotate it. The hand must be so turned, and the other fingers so bent, that the thumb and first finger stretch out nearly to their full length to grasp the tube comfortably.

The right hand is held with the palm toward the left, the fingers except the first slightly bent, and the tube held between the first finger and the thumb while it rests on the second finger and that portion of the hand between the base of the first finger and the thumb. Rotation of the tube is accomplished by rolling it between the thumbs and first fingers: the rotation being continued in the same direction regularly, and not reversed. It is better to roll slowly and evenly, with a series of light touches, each of which moves the tube a little, than to attempt to turn the tube a half a revolution or so with each motion of the hands. The hands must be held steady, and the tube must be under good control at all times, so that both ends may be rotated at the same angular velocity, even though they may be of different diameters, and the tube be neither drawn apart nor pushed together unless such a motion is expressly desired, as it sometimes is. The hot part of the glass must be constantly watched to see that it is uniformly rotated and not twisted, nor pulled out or pushed together