The Science Notebook
Gilbert Glass Blowing - Part II

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NOTE:  This book was published in as a manual to accompany the Gilbert Glassblowing Set as part of the "Boy Engineering" Series.  the exact copyright date is unknown, although based on information from "The Internet Archive" it is believed that this publication is in the public domain.  Many today would not consider glassblowing to be a safe activity for young people.    Therefore, this book is probably best appreciated for its historical value rather than as a source for current information and good experiments.  If you try anything here, please understand that you do so at your own risk.  See our Terms of Use.

Pages 11 - 27

GLASS BLOWING  11

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 ?

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,

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GLASS BLOWING  13

potash, and lime; and crystal or flint glass, from sand, potash, and lead oxide.




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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

GLASS BLOWING  15

(center, Fig. 21), and is placed in a second hot oven, where it is ironed out flat (Fig. 22).

The flat sheets are finally annealed in a third oven for a number of days and are then cut into panes, sorted, and packed.




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GLASS TUBES



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

GLASS BLOWING  17

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,

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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

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 1/2 inch in diameter.

Allow the bulb to cool, then heat the tube about 1/4 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.

GLASS BLOWING  19

Experiment 13. Magic.



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.

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.

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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.



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.

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.

GLASS BLOWING  21

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.



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.



You can save glass in many cases by attaching a short piece of glass to the piece you intend to work with, as follows: Heat an end of each piece in the lamp flame until red hot, press them together, remove from the flame, and hold until solid.  The short piece then serves as a working handle (Fig. 30) for the large piece.

Experiment 18. To close a large tube.

You closed small tubes in Experiment 5 by simply heating the end in the blowpipe flame. This method does not serve for

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large tubes, however, because it leaves a very large lump of glass which may crack on cooling or reheating.



Practice the following method of closing a large tube; first  with a piece of No. 4 tube, and then with a piece of No. 6: Attach a working handle to the end to be closed, heat the tube 1/2 inch from the end in the blowpipe flame, turn constantly, and when soft pull apart until the tube has the shape 1, Fig. 31. Heat, turn, and pull the end away to leave the tube as in 2. Heat the end and blow out until it has the shape 3. The end is now closed and the glass has about the same thickness as the remainder of the tube.

GLASS BLOWING  23

Experiment 19. To make a submarine.



Close one end of a piece of No. 2 tubing as described above, but leave the end somewhat pointed (1, Fig. 32). Heat the tube on one side at a distance 1/2 inch from the end and blow a bulb about 1/2 inch in diameter (2). Heat the tube 1/4 inch from the bulb, draw it down into a fine tube, and break off the tube, leaving a small hole in the end (3). Place the submarine in a glass of water, and if it floats it is complete.

Experiment 20. Magic.



Fill a bottle to overflowing with water, insert the submarine open end down, insert the solid rubber stopper and press down hard (Fig. 33). Does the submarine submerge?

Release the stopper. Does the submarine rise and does it also move forward?

Turn the bottle on its side and release the stopper quickly. Does the submarine shoot forward at a great rate (Fig. 34)?



The submarine acts in this magical manner for the reasons given in Experiment 9. When you press the stopper in, you compress the air in the submarine and force water in until the submarine weighs more than an equal volume of water and it sinks. When you release the pressure n the stopper, the compressed air forces the water out until the submarine becomes lighter than an equal volume of water and it rises. The water rushing out through the opening exerts pressure backward on the water in the bottle and the reaction drives the submarine forward.

Experiment 21. Fun with the submarine.

If your friends do not know about the little submarine, you can mystify them as follows: Tell them that submarines are

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just like other fish; namely, they 1ay eggs, and the little eggs hatch out after a certain number of days (of course, your friends will know that you are only joking). Pretend that you found one of these submarine eggs, hatched it out in lukewarm water, and that you have trained the baby submarine to do some simple tricks. For example, that you have trained it to submerge, rise, and attack, when you issue the commands "submerge," "rise," and "attack."

Tell them to watch the submarine carefully and to notice that it takes in water and submerges when you issue the command "submerge." Stand the bottle on the table, issue the command "submerge" and, while your friends are watching the submarine, press down on the stopper unknown to them.

Tell them to watch the submarine carefully again and to notice that it expels water and rises when you issue the command "rise." Issue the command and unknown to them release the pressure on the stopper slowly.

Repeat with the command "attack" and release the pressure quickly.

Experiment 22. A submarine battle.

Make a second submarine, place it in a large bot-

GLASS BLOWING  23

tle with the first submarine, turn the bottle on its side, and make the submarines manoeuver by moving the stopper in and out.



Finally arrange them so that they are on the bottom, facing each other bow to bow, two or three inches apart (1, Fig. 35), and release the stopper quickly. Do the submarines try to ram each other (2, Fig. 35) in a most realistic manner?

Experiment 23. To flare the end of a tube.



Heat the end of a piece of No. 2 tube until it is red hot, take out of the flame, hold the flaring wire inside the end, and press outward gently while you revolve the tube (1 Fig. 36). Do you find that the end is flared out (2, Fig. 36)?

Experiment 24. To make an air gun.



Take a full-length piece of Xo. 4 tube and flare both ends slightly. This is the air gun (Fig. 37).



Now to make an arrow, cut off the lighting end of a match and insert a pin in the other end (Fig. 38).

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Insert this arrow in the air gun and blow it out. Does it come out with considerable speed?

Experiment 25. A shooting match.



Draw a target on a piece of paper and hang it up, away from the wall or at the edge of the table, where there will be space behind for the arrows to pass through. Now shoot at the target with your air gun (Fig. 39). Do you find that the arrow makes holes in the target and sometimes goes right through?

The bull's-eye of a target is usually 1 inch in diameter, the next circle outside is 2 inches in diameter, the next 4 inches, and the outer circle 5 inches.

Get up a shooting match and keep track of the score made by each.

If the bull's-eye is cut anywhere by the arrow, the count is 5 points ; a cut anywhere inside or touching the 2-inch circle counts 4 points; anywhere inside or touching the next two circles counts 3 and 2 points respectively. The one who makes the highest score in five shots is the winner, It is more sanitary if each shooter has his own air gun and
arrows

GLASS BLOWING  27

Experiment 26. Height and distance contest.

Go outside and see which of you can shoot his arrow to the greatest height and to the greatest distance. Give each contestant five shots.

You can make fair estimates of the heights if you shoot up beside a building or tall tree.

Experiment 27. To make a pea shooter.

Take a full-length piece of No. 6 tubing, smooth both ends and flare them out slightly. This makes an excellent pea shooter. Try it with peas. Do you find that they come out with great speed?

Experiment 28. A pea-shooting match.
 



Make a target on a piece of paper, hang it up away from the wall or at the edge of the table, and shoot at it (Fig. 40). Do you find that the peas go right through the paper?

Arrange a match with your friends and keep track of the score as in Experiment 25.

Experiment 29. To make a good bend.



A good bend has the same diameter in the bend as in the remainder of the tube (1, Fig. 41). It is rather difficult to make


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