The next most difficult hurdle is concern over an operation that actually requires little skill: the annealing, or slow cooling, of the finished work. A11 glass, including Pyrex, must be annealed after it has been heated to the plastic state. Unless the hot glass cools slowly, strains develop that shatter the work. Beginners in glassblowing tend to become so carried away by their new skill that they slight the annealing and thus court failure. The easy technique of annealing will be explained in detail. Follow the directions and success will be yours.

The essential equipment for manipulating hot glass need not cost more than $10. Included is a propane torch of the kind sold by hardware stores for doing odd jobs around the house, an eight-inch pair of tweezers, an assortment of corks for plugging the glass tubes, a few sheets of asbestos paper, three feet of soft rubber tubing with a bore of about eight millimeters, a box of absorbent cotton, a small three-edged file and a small stock of soft glass tubing that ranges in size from six to 10 millimeters. Glass tubing can be ordered through drugstores from scientific supply houses. It normally comes in four-foot lengths and costs about $1 a pound.

Begin by making a flexible blowpipe, which consists of the rubber tubing fitted with a glass mouthpiece at one end and a blowing cork at the other To make the pipe, begin by selecting a length of six-millimeter glass tubing. Grasp the piece near one end by your left hand (if you are right-handed) and brace it against the edge of the bench. With the file in your other hand make a transverse nick three inches from the end of the glass. The nick need not be deep. Use the side of your left thumb to guide the file and complete the nick with a single, inch-long forward thrust of the file; a pressure on the file of about eight ounces is adequate. Then moisten the nick with the tip of your tongue, grasp the tube with both hands an inch or so from the nick, point the nick away from you and break the tubing by simultaneously pulling the tube apart and bowing it toward you. The force of the pull should be on the order of five pounds. With luck the tube will part as a clean, right-angled break. Make two tubes, each three inches long.

Next set up the propane torch. The kit will doubtless include two burners, one for producing a broad, bushy flame and the other designed to give a pointed flame characterized by a light blue cone at the center. All propane burners have one or more ports for entraining air to support the flame. Make from sheet metal a sliding ring that can be used at certain times to close the air port. When the port is closed, the torch burns with a smoky flame that is used for annealing.

Fit the torch with the burner that produces the pointed flame. Light the torch and adjust the gas for a clear blue flame about two inches long. Holding one of the three-inch tubes between your thumb and forefinger, move the free half of the tube into and out of the hot region just beyond the visible flame at the rate of about one stroke per second and simultaneously rotate the tube 180 degrees on its axis. The object is to heat that portion of the glass gradually to a temperature of several hundred degrees. After some f30 seconds move the tube slowly into the flame and confine the heat to the last quarter. After another 10 seconds bring the hot end of the tube to the center of the flame just beyond the light blue cone. Stop the stroke but continue the rotation and tilt the tube so that the open end faces the burner at an angle of about 45 degrees. By now the hot glass should be coloring the flame a bright yellow. In a few more seconds the cut end will reach a red heat. Watch the glass as the end softens. It will lose its sharp edge and flow to a nicely rounded surface.

When the edge becomes fully rounded, quickly return the tube to the hot region just beyond the flame. Resume moving the glass into and out of the heat. The object now is to cool the glass gradually, just as it was gradually heated to the plastic state. After a few seconds, and without removing the glass from the heat, use your free hand to close the air port of the burner with the sliding ring. Continue to stroke and rotate the glass while a thick coating of soot forms on the work.

In the meantime take with your free hand a wad of cotton the size of a fist. When soot fully covers the hot glass, plunge the end of the tube into the wad of cotton, wrap the cotton around the hot sides and lay the piece aside to cool. The cotton in contact with the hot glass will retard the rate of cooling and complete the annealing. It is well to hold the cotton in a fold of asbestos paper that can be closed to smother the flame if the material ignites.

Soot and charred cotton can be removed with soap and water after the glass cools. Similarly "fire-polish" the remaining three ends of the two tubes. Finally, slip one tube part way into one end of the rubber tubing to serve as a mouthpiece. Then drill an axial hole for the second tube through a cork that makes a snug fit with the 10-millimeter glass tubing and join the cork to the rubber tubing by means of the second tube.

A piece of equipment you may want to make once you have the blowpipe is a test tube. Cut off a 10-inch length of 10-millimeter glass tubing, insert the blowing cork in one end, grasp the mouthpiece between your lips and gradually heat the outer three inches of the glass, following the same procedure as before until the hot glass begins to color the flame. Then open the gas valve to produce a four-inch flame. While continuously rotating the glass, heat the outer half-inch to redness. Keep the tube in a horizontal position. As the glass softens, the open end will begin to shrink and perhaps to droop. Control the rotation to bring the droop to the top and prevent the glass from sagging again.

When the color of the glass has turned to orange, remove the tube from the flame, grasp the hot end with the tweezers and pull it quickly away from the solid glass. This will close the tube in the form of a tapered end. A glass thread will stream from the point of the taper. Pass the thread through the flame about two inches from the tip of the tube. The thread will melt. Discard the outer portion. Return the tapered end to the edge of the flame. Grasp the remaining part of the thread with the tweezers and pull again. Melt off the excess thread as before, this time close to the tapered tip. Now hold the tapered end of the tube in the flame at a steep angle. Continue to rotate the work. As the glass softens and flows, the tapered point will become rounded; a thick lens of molten glass will form.

When the thickness of the lens has grown to about an eighth of an inch, remove the work from the fire and invert the tube at eye level. The thin portion of the glass at the edge will quickly cool to below redness. At this point blow gently into the mouthpiece and increase the pressure until the thick, hot glass expands into a hemisphere. Some people report that they can achieve better control of the glass by using a series of short puffs instead of maintaining constant pressure. If the glass solidifies before the hemispherical shape is achieved, return it to the fire. After the lens heats to redness remove the piece from the flame, wait until the thin portions cool and try again. The thin regions of the wall cool more quickly than the thick ones. Hence the delayed blowing tends to produce walls of uniform thickness. After the closure has been made anneal the glass by closing the air port. Hold the hot end of the glass in the smoky flame until it is heavily coated with soot and then wrap it in cotton. After the work cools fire-polish the open end. You have a test tube!

If desired, the test tube can be converted into an ampoule. Plug the open end with the blowing cork and heat a section of the tube uniformly about two inches away from the cork. During this operation the closed end must be supported by your left hand so that the tube will not bend when the glass softens. Find the balance point of the outer portion and grasp the tube between the thumb and forefinger of your left hand. Rest the blowing end lightly in a V made by pressing together the thumb and forefinger of your right hand. When the tube softens, suck gently on the mouthpiece. A constriction will form around the tube in the softened area. Try to make the constriction about two millimeters deep. Anneal and cool the glass.

You now have an ampoule for holding liquids or other specimens. Place the specimen inside the ampoule. To seal the container first tilt the propane torch so that the flame burns horizontally. Hold the ampoule vertically and rotate the constriction in the flame. When the glass softens, insert one point of the tweezers inside the open end to prevent the glass from drooping. After the glass reaches a distinct red color close the tweezers and pull the solid end away while continuing to rotate the ampoule. The thin thread will melt in the flame. Gradually lower the closed, tapered end out of the flame and, while maintaining the ampoule in the vertical position, coat the hot glass with soot and anneal in cotton. If the ampoule contains a liquid that would be altered by heat, the lower half of the glass can be kept in a container of ice water throughout the sealing operation.

It is easy to join two glass tubes of equal diameter or to make a T joint, particularly between tubes of unequal size when the vertical leg of the T is the smaller tube. First close one end of each tube to be joined by the procedure outlined for making a test tube Then, instead of annealing the hemisphere, heat it to a bright yellow, remove the glass from the fire and blow quickly and strongly into the open end [see top illustration in Figure 2]. A large bubble of glass will form and burst into fragments so thin that they glow with the iridescence of a soap bubble. Brush the fragments from the end thus flared. Prepare one end of the other tube in the same way. Close the nonflared end of one tube with a cork and fit the second tube with the blowing cork. Rotate both flared ends in the fire until the ragged edges heat to redness. Remove from the fire, quickly lift the ends to eye level and move them into contact as a matching joint. The redness need not extend more than a sixteenth of an inch into the glass.

Some workers prefer to hold the tubes at the angle of a shallow V so that initial contact is made at one point on the rim of each tube. This point of adhesion then serves as a hinge for guiding the ends together. Both tubes must be grasped and rotated synchronously at their respective balance points. This too may sound difficult. It is not. Hot glass behaves like heavy glue. It is just as sticky and strong. If the tube tends to bend at the joint, a slight pull, together with a corrective bending force, will put it right.

The straight joint is now rotated in the fire. As the glass heats, the joint will tend to shrink and thicken. When the glass becomes plastic, remove the work from the fire, allow the thinner portions to cool and then blow until some part of the joint expands to the diameter of the unheated wall. Now inspect the joint for the lowest, most shrunken portion. Without rotating the tube, heat this portion, remove the work from the fire, let the thin portions cool, then blow to expand this portion to the uniform diameter of the tube. Continue to work around the joint locally in this manner until its size and shape match that of the tubing. If a ragged accumulation of glass from the flared ends appears at any point, heat it locally and, while the glass is in the flame, alternately suck and puff gently while inspecting the heated portion. The hot glass will alternately shrink and expand, an action that will cause the ragged portions to merge with the neighboring glass. When the joint is complete, carefully anneal the work.

To make a T joint, stopper one end of the piece that will serve as the cross of the T and fit the blowing cork into the other end. Heat the middle of the tube gradually all around in a four-inch flame by the combined motions of stroking and rotating the glass in the flame. Experience will now enable you to judge when the glass approaches the plastic state. Immediately before this state is reached reduce the flame to about two inches and hold the glass still to heat a point at the middle of the tube. The heated spot should just make contact with the tip of the inner cone of the flame. When the glass begins to color the flame yellow, remove the tube and blow gently until the softened glass bulges slightly at the heated area [see Figure 3]. Do not wait for part of the area to cool. When a slight bulge forms, return the glass to the fire, reheat and blow again. The object is to blow a bulge equal in height to the radius of the tube. The width of the bulge at the level of the unheated portion must equal the diameter of the tube that will form the leg of the T. These proportions are easily judged by eye. The novice is advised not to try to form the bulge with a single heating of the glass. Puff it out in easy stages.

When the desired size has been achieved, return the bulge to the flame and heat it strongly in the middle until the spot of red-hot glass is slightly smaller than the inner bore of the tube that will become the leg of the T. Then remove the work from the fire and blow off a bubble. Brush the glass fragments from the edges of the hole thus formed. Gradually reduce the temperature of the heated portion by moving the glass in and out of the hot region beyond the colored part of the flame, but do not anneal. Next similarly flare one end of the tube that will serve as the leg of the T. The flared end is then joined to the flared hole by much the same procedure as was used for making the straight joint. The edges are heated, one on each side of the flame, until the glass becomes red all around to a depth of about a sixteenth of an inch. The mating edges are then pressed together lightly and the joint is returned to the fire. Allow the flame to play on the solid neighboring portions of the glass occasionally so that they will not become appreciably cooler than the joint.

The object now is to heat the joint locally and blow the mating faces slightly outward into a conical fairing. During this operation the leg of the T must be supported at right angles with respect to the crosspiece. Only a slight force need be applied because the glass is fairly solid except at the heated area. Local heating and blowing are continued until the fairing is complete. With experience you will find that glass becomes plastic enough for blowing curves like this before it reaches a red heat. Use the lowest temperature that will accomplish any desired result. Working at minimum temperature not only makes the job easier but also avoids a destructive effect known to glassblowers as devitrification. Devitrification occurs when glass is maintained above a certain critical temperature for extended periods. The ingredients of the glass tend to separate from one another and clump in the form of crystalline masses. Devitrified glass loses its transparency and other desirable physical properties; it looks frosted and burned.

Occasionally experiments require electrical connections to be made between the inner and outer portions of a sealed glass vessel. They are easy to install after the beginner has made a few T joints. First, flare the end of a piece of tubing by exploding a bubble then heat and flatten the other end slightly, as shown in the accompanying drawing [Figure 4]. For the conducting leads I frequently salvage the filaments from old vacuum tubes or discarded lamp bulbs. The bulb is placed in a strong paper bag and squeezed between the jaws of a vise until the glass breaks from the leads. The wires are then arranged in the flattened glass stem as illustrated. The stem is first brought gradually up to temperature to prevent cracking. The wires are next heated to a bright yellow by concentrating the hottest part of the flame on the metal at the point where it enters the tube. The glass is now heated to redness and squeezed in contact with the wires by means of the tweezers. Heating is continued until the glass reaches an orange color and "wets" the metal. If the wires are not first heated to a bright yellow the metal will react with the molten glass to produce tiny bubbles: literal a froth that ruins the glass-to-metal seal.

When completed, the seal must be carefully annealed. I am told that filament from lamps and vacuum tubes is not recommended for use with soda lime or lead glass because its coefficient of expansion differs from that of the glasses enough to invite cracks and leaks, but I have experienced no difficulty when the glass has been fully annealed. Incidentally, copper will seal to soft glass splendidly when the metal is reduced to a foil on the order of .005 inch thick. Suitable lengths of wire thick enough to carry the anticipated current are simply pounded flat with a hammer over the length that comes in contact with the glass and heated to the same temperature as the glass when the seal is made. The worker must be careful not to overheat and melt the copper. The copper takes on a distinctive and easily recognized color along the portions that are wetted by the molten glass. I have used copper seals successfully for firing barium getters: devices that evaporate a thin film of metallic barium onto the internal surface of the vacuum tube for absorbing unwanted gases. Getters are customarily fired by induction heating, which requires a source of high-frequency current. Sealed leads enable the amateur without access to induction-heating equipment to accomplish the same result with an automobile storage battery or a doorbell transformer.

What about bending glass tubes and blowing large bulbs? Simple bends are no problem, particularly in the case of tubes less than eight millimeters in diameter. To make bends, fit the torch with the burner that produces a broad, bushy flame. If possible, add a batwing attachment to spread the flame in a fan-shaped pattern. Then, supporting the tube at positions about a quarter of its length from each end, bring the center portion slowly up to temperature; let the glass sag in the form of a natural bend that can be stopped at any angle. If the bend is desired at some point other than the center, judge the approximate balance points for supporting the glass and proceed in the same way. When bending tubes, as in most operations with hot glass, the heat must be evenly distributed by rotating the glass. Tubes of greater diameter are bent by gradual steps. The glass tends to buckle along the inner radius of the bend or, conversely, to flatten along the outer radius, particularly in the case of large tubes with thin walls. These imperfections are continually corrected by local heating and blowing [see Figure 5].

As for blowing large bulbs, my advice is: Do not try unless you really can trace your ancestry to Venice. If you need a large bulb, make one from a round-bottomed flask.

Bibliography

MANUAL OF LABORATORY GLASS-BLOWING. R. H. Wright. Chemical Publishing Co., Inc., 1943.

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