Laboratory Manual of Glass-Blowing

by Francis C. Frary

It is sometimes required to join two pieces of tubing end to end, by means of a ground joint. Whenever possible, a regular sealed joint should be used instead of this ground joint, as it is quicker to make, and more certain to be tight. Where a ground joint is necessary, however, it is best made in the conical form shown in c, Fig. 17. If the wall of the tube to be used is not very thick, it is thickened by collecting glass as for a bulb on the ends of two tubes (Exercise No. 6), and drawing to form cones of suitable shape (a and b, Fig. 17) and of such relative sizes that a will slip about half way into b. In order to make a straight and give it the proper angle, it may be rolled when hot, upon a hot plate of carbon. Blowing during this rolling is often helpful to remove depressions. After b has been drawn to nearly the proper size and shape, it may be smoothed by the use of a small carbon rod, held inside it at a slight angle, or better by the use of a truncated hexagonal pyramid of carbon, whose edges have the proper slant to make the inside of the cone right. The proper taper for both these cones is the same as that used in stopcocks of similar size. The hexagonal carbon can easily be made by carefully filing down an electric light carbon, and finally impregnating it with paraffin or beeswax, and is extremely useful wherever a conical surface has to be formed from the inside of a tube.

Fig. 17.—Ground joint.

The tail is allowed to remain on piece a, as a sort of guide in grinding, and should therefore be in the axis of the tube and have rather thick walls. Grind with emery or carborundum, as described under a previous head. (Regrinding plug for stopcock.) If many such joints are to be made, it will pay to have a little sleeve of brass made with the proper taper, and rough down the plug a in it to about the proper size, while b is roughed down by means of a brass or iron plug having the same taper. This prevents excessive grinding of one-half of the joint in order to remove a defect in the other half, and is the method commercially used in making stopcocks.

SEALING IN PLATINUM WIRE

Very often it is necessary to seal platinum wire into the wall of a tube. Professional glass-blowers usually use a special sort of glass ("Einschmelzglas") which is usually a lead glass, and is made of such composition that it has the same or practically the same coefficient of expansion as platinum. A little globule of this glass is sealed into the tube in such a way that it joins the platinum to the glass of the tube. To do this, the small globule of special glass is fused on the platinum wire at the proper point and the tube into which the wire is to be sealed is heated and a small tail drawn out at the point where the wire is to be inserted. The lump of the special glass should be from ³⁄32 to ¹⁄8 inch in diameter, and the tail drawn on the tube should have a slightly less diameter at the point (about ¹⁄8 inch or less from the tube) where it is cut off. There are now two ways of sealing in the wire. (1) The wire with the globule of glass is placed inside the tube and the latter revolved until the end of the wire sticks out of the cut tail (a, Fig. 18). The latter is now gently heated, and the two glass surfaces fused together, taking care to use only the end of the hissing flame, if the special glass contains lead. (See Chapter I, page 1.) The whole circumference of the tube is then heated and annealed carefully. (2) The end of the wire which is to be outside the tube is attached to the end of a thin scrap of glass, by heating the glass and thrusting the wire into it a very little way. Using this piece of glass as a handle, the wire is inserted in the cut tail (b, Fig. 18) and the globule brought near to the end of the tail. (If the main tube is cold, it must of course first be warmed.) With the end of the hissing flame, as in the first method, the globule of glass is melted and the end of the tail softened. The wire is now pushed into place, the handle removed by heating the end and withdrawing it, and the tail reheated a little if necessary to make it shrink back into line with the walls of the tube. The whole circumference of the tube is heated at that point and annealed as usual.

Fig. 18.

The use of this special glass is not absolutely necessary if the platinum wire is small (¹⁄4 millimeter or less in diameter), and in fact it is often better in such cases not to use it, unless the apparatus is to be subjected to a very high vacuum. On small tubes, especially, it is undesirable to use the special glass, as a lump of it will usually cause the tube to crack on cooling. When such glass is not at hand or is not to be used, the procedure is altered somewhat. The tail which is drawn out is very fine, having only a sufficient diameter so that when it is cut off the wire can be inserted in it. Such a fine tail is readily made by heating a small spot on the tube, touching it with a warm platinum wire, removing from the flame and drawing out the tail with the wire. After cutting off the tail the wire is inserted in it, being held on a scrap of glass as in the previous case, and the wire and tail heated until the latter shrinks back into line with the walls of the tube. If too great shrinkage occurs, the place may be blown out gently after reheating. Thus the wire is sealed through the wall of the tube without changing the thickness of the latter, and consequently without developing undue stresses at that point. Such a joint must of course be carefully reheated and annealed. With fine platinum wire there is very little risk of the tube cracking if care is taken to avoid formation of any lump and to reheat the whole circumference of the tube at that point.

Any glass adhering to the end of the platinum wire, where the scrap of glass was sealed on for a handle, may be removed when the glass has cooled by crushing it carefully with a pair of pliers.