Part 1
Experimental Glass Blowing FOR BOYS
BY CARLETON J. LYNDE, PH. D. Professor of Physics MacDonald College, Quebec Province, Canada
Prepared under the direction of ALFRED C. GILBERT YALE UNIVERSITY, 1909
EXPERIMENTAL GLASS BLOWING
Boys, glass tubes are made in the sizes shown in Fig. 2, and in larger sizes. You will use sizes 2, 4, and 6 in the following =experiments=.
Experiment 1. Fun bending glass.
[Illustration:
FIG. 2
SIZES OF GLASS TUBING ]
Hold a piece of No. 2, with both hands, in the flame of the alcohol lamp, and turn it constantly (Fig. 3). Do you find that when the glass becomes nearly red hot, it becomes soft and bends easily?
[Illustration:
FIG. 3
HEATING GLASS TO SOFTEN IT ]
Take the tube out of the flame, bend it into any shape you wish (Fig. 4), and allow it to cool. Do you find that the glass hardens when it cools and retains the bent shape?
Heat the tube near the first bend, turn it constantly, take it out of the flame, and make another bend.
Repeat this and make all kinds of fantastic shapes.
Place all hot glass on the cooling blocks, not on the table.
Glass is used in many, many ways by the human race; for example, to make bottles, tumblers, window glass, and so on, and all of these uses depend upon the facts which you have just illustrated, namely, that glass becomes soft when heated and hard when cooled again.
[Illustration:
FIG. 4
BENDING GLASS ]
THE LAMP
The wick should be cut straight across and should project above the wick holder about ⅛ inch (Fig. 5), or a little more if you require more heat. Burn wood alcohol or grain alcohol, because they give flames without soot or smoke. Fill the lamp to within a ½ inch of the top only; it will burn one hour. The hottest part of the flame is not down close to the wick, as most beginners suppose, but up just beneath the tip.
[Illustration:
FIG. 5
THE LAMP ]
Buy your alcohol at a drug store in quantities of one pint or more. When you are through experimenting for the day pour the alcohol from the lamp back into the pint bottle and cork the bottle tightly. Alcohol left in the lamp gradually evaporates and is lost.
Do not let the lamp stand with alcohol in it for any considerable time—overnight for example—because fuel alcohol contains water and when it evaporates from the wick, the alcohol evaporates first and leaves the water in the wick. Then when you try to light the wick again, you will find that you cannot do so, because, of course, water does not burn. If this happens to you, take the wick out, dry it, and start the lamp again.
[Illustration:
FIG. 6
MAKING A SCRATCH ]
It is perfectly safe to use kerosene in the lamp, but it gives a very smoky flame which deposits soot on the glass and fills the air with soot particles. Your mother will object very strenuously to this because the soot particles settle and blacken everything. Burn alcohol only, at least in the house.
Experiment 2. To cut glass tubing.
[Illustration:
FIG. 7
BREAKING THE TUBE ]
Cut off a six-inch length of No. 2 as follows: Lay the tube flat on the table, mark the six-inch length and draw the file across the tube at this point, pressing hard enough to make a good scratch (Fig. 6). Grasp the tube with both hands near the scratch, as in Fig. 7, pull apart and bend slightly. Do you find that the tube breaks across easily?
Repeat this with No. 4 and No. 6 tubes.
Experiment 3. To make the edges smooth.
[Illustration:
FIG. 8
MAKING THE EDGES SMOOTH ]
Hold one end of the six-inch piece of No. 2 in the tip of the flame (Fig. 8), and turn constantly until it is just red hot. Take it out and let it cool on the blocks. Do you find that the edges are smooth?
Repeat with the other end.
Repeat with both ends of the six-inch piece of No. 4.
If thick glass is heated quickly it may crack, because the hot exterior expands more quickly than the cooler interior and produces internal strains.
[Illustration:
FIG. 9
THE BLOWPIPE FLAME ]
The No. 6 tube is comparatively thick and should be heated =gradually= as follows: Hold the end in the flame for about 1 second, then withdraw it for about 1 second; hold it in the flame again for 1 second, and withdraw it for 1 second. Repeat this eight or ten times, then hold and turn it in the flame until red hot.
Smooth both ends of the No. 6 piece in this way.
Experiment 4. Practice with the blowpipe.
Hold the small end of the blowpipe just inside the flame at one edge, about ⅛ inch above the wick (Fig. 9), and blow air through the flame parallel to the top of the wick.
Keep your mouth closed on the blowpipe, =breathe through your nose=, and =practice keeping a steady stream of air going for a long time=. You will be able to do this with a little practice.
Do you observe that the blowpipe flame is pointed, also that it is made up of a pointed cone inside and a lighter-colored cone outside? The hottest part of the flame is inside the outer cone just beyond the point of the inner cone.
[Illustration:
FIG. 10
CLOSING ONE END OF A TUBE ]
The blowpipe flame is hotter than the lamp flame because the heat of the burning alcohol is concentrated at one point by means of the air blast, and because the alcohol is more completely burned by the extra air.
Experiment 5. To close the end of a small tube.
Hold one end of a piece of No. 2 tube in the blowpipe flame (Fig. 10), turn it slowly, and heat until the end closes. Does it close nicely?
Close one end of a piece of No. 4 in the same way.
You can close No. 6 tubing in this way, but it leaves a large lump of glass which may crack on cooling or on reheating. You will practice closing No. 6 tubing later.
The “why” of it
The glass becomes soft when heated because it becomes almost a liquid, and if it is heated sufficiently it becomes entirely a liquid. In this respect it acts very much as pitch, rosin, and wax act when heated by the sun or by a fire.
[Illustration:
FIG. 11
MAKING A GLASS BUBBLE ]
The end of a glass tube becomes smooth, or closes entirely, when heated, for the following reason: The surface of any liquid tries to take the smallest possible area (this is explained in detail under “Surface Tension” in the Gilbert book on “Experimental Mechanics”), for example, a small particle of water takes the shape of a drop, a sphere, and the surface of a sphere has the least area for a given amount of water. Now when the end of the glass tube is heated it becomes a liquid, and the surface of this liquid contracts the glass into a smooth rounded surface of least area. If the tube is heated still more, the surface contracts still more and closes the end.
Experiment 6. Fun blowing glass bubbles.
Smooth one end of a piece of No. 2 tube and allow it to cool. Close the other end in the blowpipe flame, turn it slowly, and heat until it is very hot. Take the tube out of the flame, put the smooth end into your mouth quickly, and blow as hard as you can (Fig. 11). Do you get a fine big glass bubble which bursts with a pop?
If you get only a small bulb at the first trial, heat the end, and try again. Do you find that the bulb shrinks when heated but blows out again readily?
[Illustration:
FIG. 12
BLOWING A BULB ]
When you get a big bubble, place the bubble end of the tube on a cooling block and break all the thin glass away from the tube by striking it with the file or blowpipe. Then close the end and blow another bubble.
Repeat until you can blow bubbles easily.
Repeat with a piece of No. 4 tube.
BUBBLE COLORS
[Illustration:
FIG. 13
A WATER BALLOON ]
Do you find that the thin glass of the bubbles shows colors, especially in sunlight, just as soap bubbles do? You boys who have had the Gilbert set on “Light Experiments” will know that these colors are due to “interference.” The colors produced by a thin film of oil on water are also produced by “interference.”
Experiment 7. To make water balloons.
Close one end of the No. 2 tube in the blowpipe flame again and while it is still hot blow carefully into the open end until you have a bulb about ½ inch in diameter (Fig. 12). Now let it cool. Make a scratch with the file about ¼ inch from the bulb, break the tube at this point (Fig. 13), and smooth the rough edge.
[Illustration:
FIG. 14
THE BALLOON SINKS AND RISES ]
Put the bulb in a tumbler of water. Does it float? If not, make another balloon with a larger bulb.
Experiment 8. Magic.
Find a large bottle made of clear glass, the neck of which will fit your solid rubber stopper.
Fill the bottle with water =to overflowing=, insert the balloon, and then the stopper.
Now press down hard on the stopper. Does the balloon sink in a most magical manner (Fig. 14)?
Release the stopper. Does the balloon rise in an equally magical manner?
[Illustration:
FIG. 15
A BALLOON RACE ]
Experiment 9. Balloon races.
Make another water balloon. Put the two balloons together in the bottle filled to overflowing with water.
Insert the stopper and press down hard. Do the balloons sink (Fig. 15), and does one sink more quickly than the other?
Release the stopper. Do the balloons rise, and does one rise more quickly than the other?
The most buoyant balloon sinks last and rises first.
The “why” of it
[Illustration:
FIG. 16
DRAWING A THIN TUBE ]
You boys who have the Gilbert set on “Hydraulic and Pneumatic Engineering” will know the “why” of the last three experiments. Any body floats in water if it is lighter than an equal volume of water, and it sinks if it is heavier than an equal volume of water. Water is practically incompressible but air is very compressible: thus when you press down on the stopper, you force water into the balloon and compress the air in it; when you release the stopper, the compressed air in the balloon expands and drives the water out. When the weight of the balloon and the weight of the water in it are together greater than the weight of water displaced by the balloon, the balloon sinks; when they are less, it rises.
Experiment 10. Fun with thin tubes.
Hold a piece of No. 2 tubing in the lamp flame and turn it constantly. When it is red hot and soft, =take it out of the flame= and pull your hands apart until the tube is stretched ten or twelve inches (Fig. 16). Is the tube in the shape shown in Fig. 17?
[Illustration:
FIG. 17
A GLASS TUBE STRETCHED ]
Allow the tube to cool, break the large ends away from the thin tube, place one end of the thin tube in a glass of water, and blow into the other end to make air bubbles in the water (Fig. 18). If you can do so, it is a real tube.
[Illustration:
FIG. 18
AIR THROUGH TUBE ]
Does the thin tube bend easily and does it spring back when released?
Repeat the experiment with another piece of No. 2 tubing, but make the thin tube as long as you can.
Can you blow air through the thin tube, and does it bend very easily indeed?
Repeat with a piece of No. 4 tubing.
These thin hairlike tubes are called “capillary” tubes, from the Latin word =capillus=, meaning a hair.
Experiment 11. Magic.
[Illustration:
FIG. 19
WATER RUNS UPHILL ]
You have always heard that water runs =downhill=, but you will now see it run =uphill= and remain there in a most =magical= manner.
Cut off 5-inch lengths of No. 6, No. 4, and No. 2 tubing, stand them side by side in a glass full of water (Fig. 19), and move them up and down in the water to wet the inside of the tubes.
Now look at the water level in each of the tubes. Is it above the level of the water in the glass, and is it higher the smaller the inside diameter of the tube, that is, is it higher in the No. 2 than in No. 4, and in No. 4 than in No. 6?
Now take the thin capillary tube which has the largest inside diameter, place one end in the glass of water, suck it full of water and blow it out. Now with one end in the glass of water notice quickly how the water rises inside the tube. Does it run =uphill= in a most magical manner (Fig. 20), and does it remain there?
[Illustration:
FIG. 20
WATER RUNS UP TUBE ]
Repeat this with your other capillary tubes. Does the water run uphill in each, and does it rise higher the smaller the inside diameter of the tube?
The “why” of this is explained in Gilbert’s “Experimental Mechanics” under “Capillarity.”
WHAT IS GLASS?
Common glass is made from three substances with which you are all more or less familiar; namely, sand, sodium carbonate (washing soda), and lime.
If sand and soda or potash are mixed and heated to a high temperature, they melt together and produce a glass which dissolves in water. This is known as “water glass” and it is used in many ways: to preserve eggs, to cement fire bricks, to make fireproof cement, and so on. If, however, lime is added and the mixture is heated to a high temperature, a glass is produced which is not soluble in water. This is the glass you know.
The three most common kinds of glass are: Venetian glass, made from sand, soda, and lime; Bohemian glass, from sand, potash, and lime; and crystal or flint glass, from sand, potash, and lead oxide.
[Illustration:
FIG. 21
SECOND STEP IN MAKING WINDOW PANES ]
[Illustration:
FIG. 22
IRONING THE CYLINDERS FLAT ]
HOW ARE THINGS MADE OF GLASS?
The glass mixture is heated to a high temperature in fire clay pots or tanks in large ovens. The surface is skimmed from time to time and the heating is continued until all air bubbles have escaped from the mixture, usually about three days.
The glass is now quite fluid and it is allowed to cool somewhat until it is viscous; then the objects are made by blowing, pressing, or rolling, as described below.
The finished articles are finally “annealed,” that is, they are placed while still hot in a second hot oven, which is then sealed and allowed to cool slowly, for four or five days or for as many weeks, according to the kind of glass.
If a glass object cools quickly, it cools more rapidly on the surface than in the interior. This produces a condition of strain in the glass and the object may drop to pieces when jarred or scratched. This condition of strain is avoided by allowing the objects to cool very slowly, that is, by annealing.
WINDOW GLASS
Window glass is blown in exactly the same way as you have blown glass balloons; the process is illustrated in Fig. 1.
The glass mixture is heated for about three days in fire clay pots and is allowed to cool until it is viscous. The glass blower then attaches a lump of the viscous glass to the end of a straight iron blowpipe about five feet long and blows a bulb. He then reheats the glass and blows a larger pear-shaped bulb and in doing so rests the glass on a pear-shaped mold of charred wood (see center of Fig. 1). He again reheats the glass, holds the pear-shaped bulb over a pit, and blows a long cylinder (see left of Fig. 1).
The ends of the cylinder are now cut off and the edges are smeared with molten glass to prevent splitting (see right, Fig. 21). The cylinder is next cut lengthwise with a diamond (center, Fig. 21), and is placed in a second hot oven, where it is ironed out flat (Fig. 22).
[Illustration:
FIG. 23
BOTTLES BLOWN IN A MOLD ]
The flat sheets are finally annealed in a third oven for a number of days and are then cut into panes, sorted, and packed.
GLASS TUBES
[Illustration:
FIG. 24
ROLLING PLATE GLASS ]
The glass tubes with which you do the experiments in this book are made in the same way as window glass up to the stage of blowing the cylinder; then the blower’s helper attaches an iron rod to the opposite end of the cylinder (see right of Fig. 1), and the blower and helper walk backward away from each other to pull the cylinder into a tube. Of course, they use a small amount of glass to make small tubes, and larger amounts for large tubes.
MOLDED GLASS
Many articles of glass are made by blowing the glass in molds. Bottles are made in this way (Fig. 23), and large machines are now in use which mold many bottles at one time in this way.
PRESSED GLASS
Many articles are made by pressing glass into molds, that is, the molten glass is poured into molds and is pressed against the sides of the mold by means of a plunger. Imitation cut glass is pressed in this way.
PLATE GLASS
The large sheets of plate glass used in store windows are not blown, but rolled. The molten glass is poured from the fire clay pots upon a cast-iron table and is rolled flat by means of a large iron roller (Fig. 24). The glass is then in the shape of plate glass, but is rough on both sides. It is annealed for a number of days and then is ground smooth on both sides, first with coarse emery, then with finer and finer emery, and is finally polished with rouge. The result is the beautifully polished plate glass we see in large windows.
OPTICAL GLASS
The United States and Great Britain made great strides in the manufacture of optical glass during the war and there are now many kinds on the market. They are used in making the lenses, prisms, and mirrors for optical instruments.
Optical glass is made in much the same way as ordinary glass, but great care is taken: first, to see that the materials are pure; second, to stir the glass constantly, as it cools from the molten to the viscous state, to make it as uniform as possible; and third, to cool it very slowly in the annealing process, to avoid strains.
QUARTZ GLASS
[Illustration:
FIG. 25
A POLLYWOG ]
An entirely new glass has been placed on the market in quantity in recent years. It is made by melting very pure quartz sand at a temperature of 3000° F. and cooling it fairly rapidly. It has the very valuable property of expanding and contracting very, very slightly when heated and cooled. Thus there is practically no internal strain set up when it is heated or cooled quickly and it does not break. It can be heated red hot, for example, and then plunged into cold water without breaking. It is probable that this glass will be in universal use in a very few years.
Experiment 12. To make an acrobatic pollywog.
Smooth one end of a piece of No. 2 tube to put in your mouth, close the other end in the blowpipe flame, take it out and blow a bulb about ½ inch in diameter.
Allow the bulb to cool, then heat the tube about ¼ inch from the bulb and draw it out into a thin tube. Now bend the thin tube at right angles near the bulb and break it off (Fig. 25).
Place the bulb in water. Does it float? If not, blow another with a larger bulb.
Experiment 13. Magic.
[Illustration:
FIG. 26
ACROBATS ]
Place the pollywog in a bottle filled to overflowing with water, insert the solid rubber stopper, and press it down hard. Does the pollywog sink?
Now release the stopper quickly. Does the pollywog turn somersaults in a most magical manner (1, Fig. 26), and also rise?
Make one or two more pollywogs, place them all in the bottle together (2, Fig. 26), and entertain your friends with a pollywog circus.
The pollywog sinks when you press down on the stopper because you compress the air in it and force water in until it weighs more than the water it displaces.
[Illustration:
FIG. 27
DANCING POLLYWOGS ]
The pollywog rises when you release the stopper because the compressed air drives the water out until the pollywog weighs less than the water it displaces.
The pollywog turns a somersault because the water rushes out sidewise in one direction and forces the nozzle in the other direction.
Air may escape from the pollywog when it is turning a somersault; if so, water will take its place, and may make the pollywog too heavy to float. You can restore its buoyancy by sucking out the water.
Experiment 14. A dancing pollywog.
[Illustration:
FIG. 28
DRAWING GLASS SPIDER-WEBS ]
Make a pollywog as in Experiment 12, but bend its tail twice as shown in 1, Fig. 27; the nozzle is at one side and points sidewise.
[Illustration:
FIG. 29
THE SPIDER TRICK ]
Put it in the bottle full of water, then press down and release the stopper. Does it sink and rise, and does it also whirl around most beautifully as it rises?
Make another pollywog (2, Fig. 27), but bend its nozzle in the opposite direction. Does it whirl in a direction opposite to that of the first pollywog?
Put them in the bottle together and treat your friends to a pollywog dance.
The pollywog whirls because the water rushes out of the nozzle in one direction and forces the nozzle in the opposite direction.
Experiment 15. To make glass spider-web.
Heat the end of a piece of No. 2 tube in the blowpipe flame until it is melted and very hot. Now touch the end of another piece of glass to the melted glass, remove from the flame, and quickly pull the two pieces apart as far as you can (Fig. 28). Do you find that you have pulled part of the melted glass out into a very fine glass spider-web?
Repeat, but ask a friend to touch the second piece of glass to the first and run away as fast as he can.
Do you get a much finer spider-web?
Is the glass spider-web fairly strong and very flexible?
Experiment 16. The ancient spider trick.
[Illustration:
FIG. 30
ATTACHING A HANDLE ]
Attach an imitation spider—or the dead body of a real spider—to the end of the glass spider-web and surprise your friends, as shown in Fig. 29. The glass spider-web is much less visible than a thread for this purpose.
Experiment 17. To make working handles.