Laboratory Manual of Glass-Blowing

by Francis C. Frary

Great care must be taken in heating and cooling this, as in the case of the capillary tubing, and for the same reasons.

By joining pieces side by side, pressing with carbon plates or a plate and a rod, and other suitable manipulations, stirrers, spatulas, and other objects may easily be made from rod, and its manipulation is relatively easy on account of the fact that one does not have to worry about the bore of the tube. But the same general rule about not having thick and thin spots in contact, and making all changes in diameter on a taper if possible instead of abruptly, applies here. Thick pieces will cool and contract at different rates from thin ones, and cracks are likely to develop where they join. Work which has been formed with any tool must always be heated to the softening point afterward before allowing it to cool in order to remove the stresses caused by the contact of the tool with the hot glass.

When it is necessary to join a piece of rod to the side of a piece of tubing, the end of the rod is made very hot while the wall of the tube at the spot desired is heated to just below the softening temperature. The rod can then be pressed into firm union with the tube and drawn a little to remove the excess of glass without deforming the tube.

MENDING STOPCOCKS

Mending the Plug.—The plug of the stopcock occasionally falls out and is broken. If the break is in the main part of the plug, nothing can be done except to search for a spare plug of suitable size and grind it to fit, as described below. If only the little cross-piece at the end is broken off, it can easily be replaced. In most ordinary stopcocks the plug is solid, but the little handle is hollow. What has been said above regarding care in heating and cooling glass rod applies with especial force here. It is usually best to wind the whole of the plug with several thicknesses of asbestos cord, leaving bare only the end where the handle is to be joined. This diminishes the danger of cracking the plug by too rapid heating, and also makes it more comfortable to hold. A piece of rather thick-walled tubing of suitable diameter is chosen, drawn out so as to have a suitable taper (taking care to heat enough of the tube so that the capillary tail has good wall-thickness and strength), and then a corresponding taper is drawn to form the other side of the handle. The result is shown in Fig. 15, a. The capillary tail is now heated and bent back to form a handle which will be in the same straight line as the axis of the plug (b, Fig. 15) and the main part of the tube drawn off at the dotted line, making a neat seal at that point. The broken end of the plug is now slowly warmed in the smoky flame, the heat gradually increased by a gentle stream of air from the bellows, and the point at which this handle is to be attached finally brought to the temperature at which the glass flows freely. In the mean time, the little handle has been warmed almost to the softening point. It is now quickly pushed into place (c, Fig. 15), taking care that its axis is parallel to the hole in the plug, and then drawn away from the plug just enough to make a graceful neck instead of the bulging one indicated by the arrow in the figure. With a fine pointed flame the little tail is now drawn off at the point indicated by the dotted line (c, Fig. 15) and the whole carefully annealed. If necessary, the handle can be blown a little before the tail is removed. Local heating and blowing at the point where the handle joins the plug is often necessary in order to make a smooth job.

Fig. 15.—Stopcock plug.

Regrinding.—This is sometimes necessary to make stopcocks tight, when the grinding has not been properly done in the factory. For this, a very little fine flour of emery or carborundum is the best and quickest. If this is not at hand, some clean sand may be ground in an agate mortar, and if possible sieved. Only material which passes the 100-mesh sieve should be used. It will be ground still finer in the process. For the final polishing, a little infusorial earth or even kaolin will do.

The surface to be ground is moistened with water and dusted over with a little of the abrasive. The plug is now inserted in the stopcock, and turned with a gentle pressure. This turning should be in the same direction for several revolutions, then in the opposite direction for several more revolutions, etc. As the abrasive becomes finer during the grinding, a little more may be added if necessary. In general, only a little grinding will be required, and one small pinch of carborundum or emery will be ample. The beginner usually grinds too much, and with too coarse material. As the grinding surface becomes dry, water is added drop by drop, and the grinding continued until the abrasive seems to be reduced to an impalpable powder, most of which has been squeezed out of the stopcock. The two surfaces in the stopcock are usually grinding upon each other at this stage, and inspection will show whether the contact between them is uniformly good. If not, the grinding must be continued with a little fresh abrasive. If contact appears to be good, the surfaces are ground together for a little with practically no abrasive, so as to polish them, and the joint is then washed out and tested.

In grinding in a new plug to replace a broken one, the plug selected should have practically the same taper as the seat into which it is to be ground, and should be a very little too large. Care must be taken to so distribute the abrasive material as to grind mostly on the places where the plug fits tightly.