Laboratory Manual of Glass-Blowing

by Francis C. Frary

In order to insure a joint which is square and not ragged, it is essential that the angle between the inner and outer tubes at the joint be very nearly a right angle. For this reason the two tubes should not be of too near the same size, or if this cannot be avoided, a small bulb should be blown on the end where the joint is to be made. If this bulb be made with the same wall-thickness as the rest of the tube, and somewhat pear-shaped, it may be drawn out to the same size as the rest of the tube, if necessary, after the joint has been made.

This method is used wherever possible in preference to the second method (Exercise No. 9), as it is easier to get a good joint with it. It may also be used where it is desired to seal the tube through the side of a tube, or for a tube sealed through the wall of a bulb, as in a Geissler potash bulb or similar apparatus. Where there is not space to join the inner tube to the blowing tube by a rubber tube, this joint may be made with a small piece of gummed paper, which can readily be broken when desired.

EXERCISE NO. 9

Sealing a Tube Through Another Tube

Second Method—Making a Suction Pump

Select a piece of tubing ³⁄8 to ¹⁄2 inch in diameter, with walls about ¹⁄16 inch or a little less in thickness, heat a place about 4 inches from one end and draw it out so that when cut off at the proper point it will look like a, Fig. 12; the open end of the drawn out part being small enough to slip inside another piece of the original tube. A small thick-walled bulb is now blown as indicated by the dotted lines, and annealed. A piece of the original tubing is now prepared, 7 or 8 inches long, with one end cut square off and the other closed. A piece of ¹⁄4-inch tubing about 2 inches long, and drawn out at one end to a tail several inches long is also prepared, to form the inlet tube for the air. Another piece of the ³⁄8-inch tube is prepared, about 4 inches long, and provided with a tail drawn out as indicated in b, so that when cut off at about 2-¹⁄2 or 3 inches from the main tube its inner diameter may be slightly less than that of the narrowest point of the tube a. A small thick-walled bulb is blown at the point indicated by the dotted lines, and annealed. Care must be taken in drawing the capillary and blowing the bulb in both a and b that the capillary tubes are in the axis of the main tube, and in the same straight line with it.

Fig. 12.—Suction pump.

The open end of the 8-inch piece of tube and the bulb of the piece a are now warmed together, the end of the tube only moderately and the bulb to about its softening temperature. The tube a is now inserted in the open end of the large tube, and the bulb softened with a suitable flame and pressed into good contact with the tube. It is then reheated, including the joint, blown a little and pulled out to form a straight tube in line with the main tube. By warming the joint a little, and proper rotation, the capillary may be brought into the same straight line with the rest of the tube.

Keeping this joint hot, a place about an inch from it on the tube a is warmed, and the piece of ¹⁄4-inch tubing previously prepared is sealed on at that point. The joint is then well annealed and allowed to cool.