Untitled Document

03 October 2003

Glass Blowing: Making a Test Tube

by C. L. Stong
Excerpted from "Scientific American's The Amateur Scientist", first published May, 1964.

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.

Figure 1. Equipment for glass blowing. Click image to enlarge

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.

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!