Laboratory Manual of Glass-Blowing Part 3

CHAPTER V

MODIFIED METHODS AND SPECIAL OPERATIONS

CAPILLARY TUBING

This is commonly used in many forms of apparatus for gas a.n.a.lysis, 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 pus.h.i.+ng 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.

[Ill.u.s.tration: 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 pus.h.i.+ng 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 coa.r.s.er 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.

GLa.s.s ROD

Joints, tees, etc., in gla.s.s rod are made on the same principle as in tubing, except that of course they cannot be blown, and regularity must be obtained by acc.u.mulating a small ma.s.s of uniformly heated gla.s.s, and then drawing it to a suitable rod, on the same principle as Exercise No.

1.

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 gla.s.s.

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 gla.s.s without deforming the tube.

MENDING STOPc.o.c.kS

=Mending the Plug.=--The plug of the stopc.o.c.k 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 stopc.o.c.ks the plug is solid, but the little handle is hollow. What has been said above regarding care in heating and cooling gla.s.s 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 gla.s.s 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.

[Ill.u.s.tration: FIG. 15.--Stopc.o.c.k plug.]

=Regrinding.=--This is sometimes necessary to make stopc.o.c.ks 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 pa.s.ses the 100-mesh sieve should be used. It will be ground still finer in the process. For the final polis.h.i.+ng, 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 stopc.o.c.k, 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 coa.r.s.e 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 stopc.o.c.k. The two surfaces in the stopc.o.c.k 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.

=Sealing on a New Tube.=--It frequently happens that one of the tubes of the stopc.o.c.k is broken off close to the c.o.c.k itself, and a new one must be joined to the stub of the old one. With care, this may often be successfully done even where the break is within 1/4 inch of the stopc.o.c.k. The first step is to clean and dry the stopc.o.c.k, remove the plug, cork the open ends of the stopc.o.c.k sleeve and the other tube, and wind a couple of layers of asbestos cord carefully over the sleeve and the most of the corks which close it. A suitable tube, having as near as possible the same diameter and wall strength as the one broken off, is selected and a piece the desired length cut off. The broken end of the tube on the stopc.o.c.k is now squared off as well as possible, by cutting or by heating and drawing off the projections, and the new tube sealed on, usually with the first method (Exercise No. 1). If the break is very close to the stopc.o.c.k, very little reheating and blowing can be done, on account of the danger of getting the stopc.o.c.k sleeve out of shape, and the work must be heated very slowly to prevent cracking. The main reliance is then placed on making a good joint when the tubes are brought together, and then drawing out this joint a little, at once, to get an even wall.

CLOSED CIRCUITS OF TUBING.

In some pieces of apparatus closed circuits of circular or rectangular shape are required. A similar problem is involved in apparatus like the ordinary Soxhlet extractor, where a small tube is joined to the side of a large one, bent to form a siphon, and attached again to a continuation of the original large tube. The difficulty in all such cases is to provide for the contraction taking place as the last joint cools. If part of the circuit has the shape of the letter S, or is a spiral, the natural springiness of the gla.s.s will take care of this. If not, the side of the circuit opposite to the joint and parallel to it must be heated also, the two being finally heated together to the softening point after the joint is completed, and then allowed to cool together.

To make the last joint, the rest of the tube is made in approximately the desired form, the two pieces which are to be joined to make the last joint being just enough out of the desired position to allow them to pa.s.s one another. The final joint is preferably made in the middle of a straight piece of tube, not at a tee. The two pieces which are to be joined are bent so as to just pa.s.s each other, marked at the right point with the gla.s.s-knife, and cut there, preferably with a small bead of hot gla.s.s. One or both of these tubes are now warmed to the softening point in such a place that the tubes can be made to meet properly, and the two cut ends pressed together. They are now warmed in the flame, and joined together, either by simultaneously warming the opposite side of the circuit or some other suitable part, so as to allow the two ends to be pushed together again after they are softened, or by gently touching the places that do not unite with a hot bead of gla.s.s, and using the gla.s.s to fill up the crack where the ends do not quite meet. Care must be taken not to leave knots or lumps of gla.s.s in the finished joint, and the latter should be well reblown, and if necessary left as a small bulb or enlargement, rather than have it have too thick walls.

SPIRALS

Spirals of gla.s.s tubing are probably best made free-hand before the blow-pipe, unless one has a great many of them to make, and extreme accuracy is desired. To begin with, a piece of tubing of the desired size (say 3/16 inch in diameter) and a convenient length (about two feet) is selected, one end closed, and a right-angle bend made about six inches from the closed end. Holding the closed end in the left hand and the long open one in the right, the spiral is begun. The short closed end is to be parallel to the axis of the spiral, and preferably in that axis. Using a moderate-sized flame, of somewhat yellow color, and taking care to heat the whole circ.u.mference of the tube, the long open end is wound little by little into a spiral having the short end _a_ (Fig. 16) as an axis. The bend at _b_, where the tube changes from the radius to the circ.u.mference of the circle, must be rather short, but the tube must not be flattened or constricted here. Especial pains is to be taken with the first turn of the spiral (_b_ to _c_, Fig. 16), as the shape of this determines the diameter of the whole spiral, and serves as a guide for the rest of the turns. The winding of the tube is best accomplished, after a portion has been softened, by slowly turning the short end _a_ a little about its own axis, while the long open end remains where it was.

This winds the tube into a spiral, just as if there were a solid cylinder in the center of it, and this cylinder was being turned about its axis, and was winding up the soft gla.s.s upon its circ.u.mference. As the cylinder is not actually there, the curve of the turns must be carefully estimated by the eye, so that the spiral may be uniform and moderately smooth. When the original piece of tube has been used up, another piece is sealed on to the open end, and the operation continued as far as may be required.

[Ill.u.s.tration: FIG. 16.--Making a spiral.]

GROUND JOINTS

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 gla.s.s 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 stopc.o.c.ks 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.

[Ill.u.s.tration: 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 stopc.o.c.k.) If many such joints are to be made, it will pay to have a little sleeve of bra.s.s 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 bra.s.s 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 stopc.o.c.ks.

SEALING IN PLATINUM WIRE

Very often it is necessary to seal platinum wire into the wall of a tube. Professional gla.s.s-blowers usually use a special sort of gla.s.s ("Einschmelzglas") which is usually a lead gla.s.s, 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 gla.s.s is sealed into the tube in such a way that it joins the platinum to the gla.s.s of the tube.

To do this, the small globule of special gla.s.s 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 gla.s.s 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 gla.s.s 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 gla.s.s surfaces fused together, taking care to use only the end of the hissing flame, if the special gla.s.s contains lead. (See Chapter I, page 1.) The whole circ.u.mference 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 sc.r.a.p of gla.s.s, by heating the gla.s.s and thrusting the wire into it a very little way. Using this piece of gla.s.s 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 gla.s.s 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 circ.u.mference of the tube is heated at that point and annealed as usual.

[Ill.u.s.tration: FIG. 18.]

The use of this special gla.s.s 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 gla.s.s, as a lump of it will usually cause the tube to crack on cooling. When such gla.s.s 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 sc.r.a.p of gla.s.s 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 circ.u.mference of the tube at that point.

Any gla.s.s adhering to the end of the platinum wire, where the sc.r.a.p of gla.s.s was sealed on for a handle, may be removed when the gla.s.s has cooled by crus.h.i.+ng it carefully with a pair of pliers.

SEALING VACUUM TUBES

Tubes which have been evacuated usually are sealed off while they are still connected to the vacuum pump. The connection should be through a small, rather thick-walled tube. When this is to be sealed, it is slowly heated toward the softening point. As the gla.s.s just begins to soften, the air-pressure will force it in, and care must be taken that the softening is uniform over the whole circ.u.mference of the tube. As the shrinking goes on, the tube is gently drawn out to make a thick-walled cone at that place, and the end is drawn off as soon as the tube is sealed. The princ.i.p.al point to be guarded is the thickness of the walls of the cone, and uniform heating. A thin place or a hot place will give way under the air-pressure and be sucked into the tube.