A handbook of laboratory glass-blowing

by Bernard D. Bolas

Fig. 7

A Thermo-Regulator for Gas.—Fig. 7, a-e, shows an easily constructed thermo-regulator. The mercury reservoir, a, and the upper part, b, are made by joining two larger pieces of tubing on to the capillary. The gas inlet passes through a rubber stopper, in order to allow of adjustment for depth of insertion, and the bye-pass branches, d and e, are connected by a piece of rubber tubing which can be compressed by means of a screw clip, thus providing a means of regulating the bye-pass.

Use of Glass Rod.—Apart from its most common laboratory use for stirring; glass rod may be used in building up such articles as insulating feet for electrical apparatus or acid-resisting cages for chemical purposes. Such a cage is shown by f, g and h, Fig. 7. Further, by an elaboration of the method of making an exhaustion branch, given on page 18, blown articles may also be constructed from rod. Note the added parts of e, Fig. 9.

A Simple Foot.—The form of foot shown by Fig. 7, k, is easy to make and has many uses. First join a glass rod to a length of glass tubing as shown (the joint should be expanded slightly by blowing), cut off the tube and heat the piece remaining on the rod until it can be turned out as shown by i. This should be done with the large turn-pin, and care should be taken not to heat the supporting rod too strongly, otherwise the piece of tube will become bent and distorted; it is better to commence by heating the edge of the piece of tube and turn out a lip, then extend the heating by degrees and turn out more and more until the foot looks like that shown by i.

We now need to make three projections of glass rod. These are produced as follows:—Heat the end of the glass rod until a thoroughly melted mass of glass has accumulated (the rod must be rotated while this is being done, otherwise the glass will drop off); when sufficient melted glass has been obtained, the edge of the turned-out foot should be heated to dull redness over about one-third of its circumference, and the melted glass on the rod should be drawn along the heated portion until both are so completely in contact as to form one mass of semi-fluid glass. The rod should now be drawn away slowly, and, finally, separated by melting off, thus producing a flat projection. A repetition of the process will give the other two projections, and the finished foot may be adjusted to stand upright by heating the projections slightly and standing it on the carbon plate mentioned on page 7. After the foot is adjusted it should be annealed slightly by heating to just below the softening point of the glass and then rotating in a smoky gas flame until it is covered with a deposit of carbon, after which it should be allowed to cool in a place free from draughts and where the hot glass will not come in contact with anything. The finished foot is shown by k, Fig. 7.

Building up from Glass Rod.—A glass skeleton-work can be constructed from rod without much difficulty, and is sometimes useful as a container for a substance which has to be treated with acid, or for similar purposes. The method is almost sufficiently explained by the illustration in Fig. 7; f shows the initial stage, g the method of construction of the net-work, and h the finished container. It is convenient to introduce the substance at the stage indicated by g. The important points to observe in making this contrivance are that the glass rod must be kept hot by working while it is actually in the flame, and that the skeleton must be made as thin as possible with the avoidance of heavy masses of glass at any place. If these details are neglected it will be almost certain to crack.

Stirrers.—These are usually made from glass rod, and no special instructions are necessary for their construction, except that the glass should be in a thoroughly fused condition before making any joins and the finished join should be annealed slightly by covering with a deposit of soot, as explained on page 16. The flat ends shown in a, Fig. 8, are made by squeezing the soft glass rod between two pieces of carbon, and should be re-heated to dull redness after shaping. Fig. 8 also shows various forms of stirrer.

In order to carry out stirring operations in the presence of a gas or mixture of gases other than air, some form of gland or seal may be necessary where the stirrer passes through the bearing in which it runs. A flask to which is fitted a stirrer and gas seal is shown in section by b, Fig. 8. The liquid used in this seal may be mercury, petroleum, or any other that the experimental conditions indicate.