Laboratory Manual of Glass-Blowing

by Francis C. Frary

CAPILLARY TUBING

This is commonly used in many forms of apparatus for gas analysis, and one is often called upon to join two pieces or to make a tee on it. The methods are nearly the same as with other tubing, except that more care and patience are required. The work must be done much more slowly on account of the thickness of the walls, and open ends of the tube must always be enlarged before joining them to anything. This is best done by carefully sealing the end and then blowing, with several suitable reheatings, to form a pear-shaped bulb as in a, Fig. 14. The end of this is then heated and blown off, and the piece is ready to be joined to another similar end, or to a piece of ordinary tubing if desired. The joints are best not blown too much, as thick walls shrink very slowly. Much may be done by gently pushing the tube together or pulling it apart in the flame, to remove lumps and irregularities. It is necessary that the bore of the joint be approximately that of the main tube, and care must be taken that the latter is not constricted at the point where the joint begins.

Fig. 14.—Capillary tubing.

Especial care must be taken to warm the tube slowly when starting and cool it slowly when through, as the thick walls frequently crack if not carefully handled. For this reason the whole neighborhood of the joint must be heated somewhat so that there may not be stresses set up between the heated and unheated portions.

In making the tee (b, Fig. 14) the inability to blow the joint makes itself decidedly felt, but if the side tube is properly enlarged as previously described, a good joint can be made by alternately pulling and pushing on the end of the side tube, and shrinking well.

Very fine capillary tubing should be blown with a rubber bulb instead of the mouth, so as not to get moisture into the tube. The rubber bulb may also be used to advantage on some of the coarser capillary tubing.

When a bulb is to be joined to a piece of capillary tubing, the joint is preferably made before blowing the bulb, and will then be taken up a little way on the bulb during the process. Care must of course be taken not to constrict the capillary; the pear-shaped bulb blown on the end (a, Fig. 14) may well extend back a little further than usual into the tube so as to prevent this. If a bulb is required in the middle of a capillary tube, the latter is usually best cut and a piece of ordinary tubing of suitable size sealed in to provide material for the bulb.

GLASS ROD

Joints, tees, etc., in glass rod are made on the same principle as in tubing, except that of course they cannot be blown, and regularity must be obtained by accumulating a small mass of uniformly heated glass, and then drawing it to a suitable rod, on the same principle as Exercise No. 1.