What Is the Difference Between Plate Glass and Window Glass?

How is plate glass made? These questions are asked very frequently. The two products are wholly unlike each other; and we wish to show wherein lies the difference. We shall tell how plate glass is made; and we hope to make it clear that great care, time and expense are involved in its manufacture.

The raw materials may be said to be virtually the same in plate glass as in window glass; the main difference being that in plate glass greater care is exercised in selecting and purifying the ingredients. Window glass is made with a blow-pipe. The work requires skill on the part of the operator; but the process is quite simple and rapid. And the result is, naturally, a comparatively ordinary and indifferent product. On the other hand, the superb quality of plate glass is owing to the elaborate method of producing it.

Commercial plate glass was first made in France somewhat more than two hundred years ago; although glass in one form or another has been in use for many centuries. Apparently glass was known in Egypt fully four thousand years ago.

The materials used are silica (white sand), carbonate of soda (soda ash), and lime. Other materials, as arsenic and charcoal, are used in small proportions, but the main ingredients are the first three named.

Probably it is little imagined that in the production of plate glass, mining is involved in two or more forms (namely silica and coal), also the quarrying of limestone, the chemical manufacture of soda ash on a large scale, the reduction and treatment of fire clay to its right consistency, an elaborate and expensive system of pot making; and the melting, casting, rolling, annealing, grinding and polishing of the glass.

In special uses, as in beveled plates and mirrors, two more elaborate processes must be added—beveling and silvering—all of which are performed under the direction of experts aided by a large amount of labor and expensive machinery.

Pots of fire clay take so important a part in the successful manufacture of plate glass that the subject deserves especial notice. The different clays after being mined are exposed to the weather for some time to bring about disintegration.

At the proper stage finely sifted raw clay is mixed with coarse, burned clay and water. This reduces liability of shrinkage and cracking. It is then “pugged,” or kneaded in a mill; kept a long time (sometimes a year) in storage bins to ripen; and afterwards goes through the laborious process of “treading.” Nothing has thus far been found in machinery by which the right kind of plasticity can be developed as does this primitive treading by the bare feet of men. The clay must be treaded, not once or twice, but many times. The building of pots is a slow, tedious and time-killing affair; but this is most essential.

Without extreme care, some elements used in the making of the pots might be fused into glass while undergoing the intense heat of the furnace; or they might break in the handling. The average pot must hold about a ton of molten glass, and the average furnace heat necessary is about 3,000° Fahrenheit. The work is not continuous. Each workman has several pots in hand at a time, and passes from one to another adding only a few inches a day to each pot, so that a proper interval for seasoning be given. After completion, comes the proper drying out of the pots; and this is another feature in which the greatest scientific care is required. No pot may be used until it has been left to season for at least three months, and even a year is desirable. And after all this trouble, the pot has but 25 days of usefulness. The pots form one of the heavy items of expense in plate glass manufacture; and upon their safety great things depend.

The pot, having been first brought to the necessary high temperature, is filled heaping full with its mixed “batch” of ground silica, soda, lime, etc. Melting reduces the bulk so much that the pot is filled three times before it contains a sufficient charge of metal. When the proper molten stage is reached the pot is lifted out of the furnace by a crane; is first carefully skimmed to remove surface impurities, and then carried overhead by an electric tramway to the casting table. This is a large, massive, flat table of iron, having as an attachment a heavy iron roller which covers the full width, and arranged so as to roll the entire length of the table. The sides of the table are fitted with adjustable strips which permit the producing of plates of different thicknesses. The pasty, or half-fluid glass metal is now poured upon the table from the melting pot, and the roller quickly passes over it, leaving a layer of uniform thickness. The heavy roller is now moved out of the way, and then by means of a stowing tool the red hot plate is shoved into an annealing oven. All of these stages of the work have to be performed with remarkable speed, and by men of long training and experience. The plates remain for several days in the annealing oven, where the temperature is gradually reduced from an intense heat at first, until at the end of the required period it is no hotter than an ordinary room.

When the plate is taken from the annealing oven it has a rough, opaque, almost undulating appearance on the surfaces. It is only the surface, however, for within it is as clear as crystal. First, it is submitted for careful inspection, so that bubbles or other defects may be marked for cutting out. It then goes to the cutter who takes off the rough edges and squares it into the right dimensions; and thence to the grinding room.

The grinding table is a large flat revolving platform made of iron, twenty-five feet or more in diameter. The plate must be carried from the annealing oven to the grinding machines, and thence to the racks, by men skilled in the art. Twenty men are required to carry the larger plates of glass, ten on each side, using leather straps and stepping together in perfect time. The lock-step is absolutely essential to prevent accident. The grinding table is prepared by being flooded with plaster of Paris and water; then the glass is carefully lowered, and a number of men mount upon the plate and tramp it into place until it is set. After this, greater security is obtained by pegging with prepared wooden pins; and then the table is set in motion. The grinding is done by revolving runners. Sharp sand is fed upon the table, and a stream of water constantly flows over it. After the first cutting by the sand, emery is used in a similar manner.

The plates are inspected after leaving the grinding room, and if any scratches or defects of any kind are found they are marked. Some of these can be rubbed down by hand. There are also, not infrequently, nicks and fractures found at this stage; and in such case the plate must again be cut and squared. Afterward comes the polishing, which is done on another special table. The polishing material is rouge, or iron peroxide, applied with water, and the rubbing is done by blocks of felt. Reciprocating machinery is so arranged that every part of the plate is brought underneath the rubbing surface.

The grinding and polishing has taken away from the original plate half of its thickness, sometimes more. There is no saving of the material; it has all been washed away. When to this waste is added the fact that fully half of the original weight of lime and soda has been released by the heat of the furnace, escaping into the atmosphere in fumes and acids, one may begin to understand something of the cost of converting the rough materials of sand, limestone and soda into beautiful plate glass.

In preparing plate glass for mirrors great care must be exercised in the selection of the plates. This selection bears reference not only to surface defects, but to the quality in general; defects which cannot ordinarily be seen are magnified many fold after the glass has received a covering of silver.

In the process of beveling, the plate passes through the hands of skilled workmen of five different divisions, namely: roughers, emeriers, smoothers, white-wheelers and buffers; and different abrasive materials are used in the order indicated by the titles. These materials are sand, emery, natural sandstone imported from England, pumice and rouge.

The roughing mill is a circular cast-iron disk about 28 inches in diameter, constructed so that the face or top of the mill revolves upon a horizontal plane at a speed of about 250 revolutions per minute. The sand is conveyed to the mill from above through a hopper simultaneously with a stream of water which is played upon the sand to carry it to the mill. The rougher places the edge of the plate upon the rapidly revolving mill, and the cutting of the bevel is done by the passage of the sand between the mill and the plate of glass. A bevel of any desired width may be produced. Pattern plates containing incurves, mitres, etc., require a practiced eye and great skill upon the part of the operator.

When the plate leaves the rougher’s hands the surface of the bevel has been ground so deep by the coarse sand that polishing at this stage is impossible. Consequently, in order to produce a surface fine enough to render it susceptible of a high and brilliant polish it must go through the various treatments we have mentioned. The emerier uses a fine grade of emery on a mill similar in construction to a roughing mill, which takes away considerable of the coarse surface given by the first cutting. Then it goes to the smoother, who reduces the roughness slowly by using a fine sandstone from England; then it goes to the white-wheeler who operates an upright poplar-wood wheel using powdered pumice stone as an abrasive; and then, as a last stage it reaches the buffer, whose method of operation is shown in the illustration. The buffer brings a high polish to the bevel by the use of rouge applied to thick felt which covers his wheel.

The plate, after leaving the beveling room, is again carefully examined for surface defects. These defects may consist of scratches caused inadvertently by permitting the surface of the plate to come into contact with the abrasive material. These scratches are removed by hand polishing, which must be skillfully done; otherwise the reflection will become distorted through over-polishing in a given area or spot. The plate is then taken to a wash table where the surface to be silvered is thoroughly washed with distilled water; after which it is taken to a table that is covered with blankets, and which is heated to a temperature of from 90° to 110°. The blanketing is to protect the plate from being scratched, and also to catch all of the silver waste. The silvering solution is nitrate of silver liquefied by a certain formula, and is poured over the plate; the fluid having an appearance which to the ordinary observer looks like nothing other than pure distilled water. Within a few minutes the silver, aided by a reactory, added prior to pouring, begins to precipitate upon the glass; the liquids remaining above, and thus preventing air and impurities from coming into contact with the silver. Such contact would produce oxidation. After the silver is precipitated the plate is thoroughly dried, shellacked and painted; after which it is ready for commercial use.

Until about 25 years ago, practically all mirrors were silvered with mercury. There have been two reasons for discouraging the use of mercury for silvering; one being its injuriousness to the health of the workmen. In some European countries stringent laws were enacted, stipulating that men should work only a certain number of hours.

Other hygienic stipulations, added to the fact that the use of mercury was already very expensive, have tended to replace that process by the use of nitrate of silver.

Why Is the Sky Blue?

This question puzzled every one who thought of it for a long time. Even astronomers, the men who make a business of studying the skies, and other learned men, puzzled their brains about it and searched for the answer long ago, until finally, as always happens when a lot of people study a subject, Professor John Tyndall, a noted scientist of the last century, discovered the answer. The explanation follows: All the light we have is sunlight, which is pure white light. This white light is made up of rays of light of different colors. These rays are red, orange, yellow, green, blue, indigo and violet. It takes all of these different rays of light to make our white sunlight, and when you separate sunlight into its original rays you always produce the rays of light in the above colors and in the same order. This is only true, however, when the sunlight is passed through an object which does not absorb any of its rays. This is the arrangement of the different colors of light found in the rainbow. The rainbow is formed by sunlight passing into raindrops or vapor in such a way as to divide the sunlight into the different colored rays of light. When the rainbow is formed none of the rays are absorbed by raindrops or vapor through which the sunlight passes. Some of these rays of light are known as short rays and others as long rays. But when sunlight meets other things besides those which make a pure rainbow, these other objects have the ability to absorb some of the rays of colored light, and they throw off the remainder. When these rays have been thrown off those which have been absorbed make many different combinations, and thus are produced all of the different colors we know, the various tints and shades of color, according to composition and size.

Now, then, to get back to the color of the sky, which is blue as we know. The sky or air which surrounds the earth is filled with countless tiny specks of what we may call dust—particles of solid things hanging or floating in the air. These specks are of just the size and quality that they catch and absorb part of the rays of light which form our sunlight and throw off the rest of the rays, and the part which has been absorbed forms the combination of color which makes our sky so beautifully blue. Sometimes you notice, of course, that the sky is a lighter or darker blue than at other times. This difference is due to the kind and condition of tiny specks in the air at the time, and to the direction or angle at which the sunlight strikes these tiny particles. This fact brings up a question which you have not asked, but which would come naturally as the result of your first.

What Makes the Colors of the Sunset?

The direction of the sun’s rays when they meet these large and small particles in the air has a great deal to do with the combination of colors that result as these objects absorb part of the rays and throw off others. The sky is the most beautiful blue when the sun is high in the sky. But when the sun is setting the light has a greater distance to travel through the belt of air which surrounds the earth than when it is high up over our heads. You know that if you stick a pin straight down into an orange it won’t go in very far before it is clear through the peel, but if you stick the pin into an orange along the edge it will go through a great deal more of the peel than the other way. That is the way it is with the sunset colors. The peel of the orange is a good representation of the belt of air which surrounds the earth. At sunset the light instead of coming straight down through the belt of air, thus meeting the eye through the shortest possible amount of air, strikes the air on a slant, and, therefore, travels through a great deal more air and closer to the earth to reach it, with the results that it meets a great many more of these little specks, besides all the smoke and other things that hang in the air near the ground, and we thus get many more colors, because some of the things in the air absorb some of the rays and others absorb very different rays when the light comes in this slanting way, and that is what makes the different colors in the sunset. For this reason sunsets are often richer and more beautiful in color when the air is not so pure, but has much dirt and other matter floating about in it.

Are There Two Sides to the Rainbow?

No, there is only one side to the rainbow. The rainbow is made by reflection of the rays of sunlight through drops of water in the air, but you can never see a rainbow unless you are between it and the sun. You could never see a rainbow if you were looking at the sun, and so if you are looking at a rainbow you can be certain that anyone on the other side of it could not see it, because they would have to be looking right at the sun. The rainbow is always opposite to the sun and there can never be two sides to it.

Do the Ends of the Rainbow Rest on Land?

The ends of the rainbow do not rest on anything. You see, the rainbow is only the reflection of the sun’s rays thrown back to us by the inside of the back of the raindrops, which are still in the sky after the rain. Of course, if any of the drops of water touched the ground they would cease to be raindrops and, therefore, could not reflect the rays of the sunlight. So, what we think of as the ends of the rainbow do not really exist at all. The rainbow is only a reflection of the rays of sunlight from countless drops of water in the air, which the sun’s rays must strike at a certain angle in order to reflect back the light so we can see it. Where the sun’s rays do not strike the drops of water at the right angle no light is reflected, and there is the end of the rainbow.

What Causes the Different Colors of the Rainbow?

The colors of the rainbow, which are always the same, and are shown in this order—red, orange, yellow, green, blue and violet—are sunlight broken up into its original colors. It takes all of these colors in the proportions in which they are mixed in the rainbow to make the pure sunlight. These are known as the prismatic colors. As shown in another answer to one of your puzzling questions, the rainbow is caused by the rays of the sun passing into drops of water in the air and reflected back to us with one part of the drop of water acting on it in such a way as to break up the pure sunlight into these prismatic colors. When a rainbow appears at a time when there is a great deal of sunlight, you will generally see two rainbows. The inner rainbow is formed by the rays of the sun that enter the upper part of the falling raindrops, and the outer rainbow is formed by the rays that enter the under part of the raindrops. In the inner or primary bow, as it is called, the colors beginning at the outside ring of color are red, orange, yellow, green, blue and violet, and being exactly reversed in the outer or secondary bow. The secondary bow is also fainter. You may sometimes see smaller rainbows, even if it has not been raining, when looking at a fountain or waterfall. These are caused in exactly the same way.

What Makes the Hills Look Blue Sometimes?

This is due to the fact that when the hills look blue you are looking at them at a distance, and there is a long stretch of air between you and the hills. This air is filled with countless particles of dust and other things, and what you see is not really blue hills, but the reflection of the sun’s rays from the little particles in the air striking your eye. The color is due to the angle at which the light from the sun strikes these particles, and is reflected back to your eye and partially due to the character of the particles in the air.

Do the Stars Really Shoot Down?

The answer is “No.” We have come to use the expression “shooting stars” commonly, but we should probably be more correct if we said “shooting rocks,” for the things we refer to commonly as “shooting stars” are more like rocks than anything else. If any of the real stars were to fall into the air surrounding the earth we should all be burned up by the great heat developed long before it actually hit the earth, which it would undoubtedly destroy.

The things that fall and leave a streak of light are really only pebbles, stones, rocks or pieces of iron and other substances that fall from some place into the earth’s air belt. When they strike the air at the speed at which they are falling the friction of the air makes a heat that causes them to become luminous, and by far the greater part of them is burned up before they get very near the earth. We call them meteorites. Sometimes, though rarely, one will manage to strike the earth, coming at such great speed and being so large that the air has not been able to burn it up completely, and it will strike the earth and sink deep down into the soil. In most museums can be seen such meteorites that have been dug up after striking the earth. These are constantly falling into the air surrounding the earth, but in the day-time their light is not strong enough to be seen while the sun is shining.

Will the Sky Ever Fall Down?

No, the sky can never fall down, because it is not made of the kind of things that fall. We have become used to thinking of it as the roof of the earth, a great dome-shaped roof, because in our little way of looking at things we compared the earth and what is above it with the houses in which we live. The sky is just space in which the heavenly bodies revolve in their orbits. We cannot really ever see sky. We see only the sun’s light reflected by the air belt which surrounds the earth. In this air belt are the clouds which do come closer to the land at times than at others, and this is apt to aid in giving us an incorrect impression of this.

What Is the Milky Way?

The “Galaxy,” or “Milky Way,” as it is popularly called, is a luminous circle extending completely around the heavens. It is produced by myriads of stars, as can be seen when you look at it through a telescope. It divides into two great branches at one point, which travel for some distance separately and then reunite. It has also several branches. At one point it spreads out very widely into a fanlike shape.

Why Do They Call It the Milky Way?

The stars in the group are so numerous that they present to the naked eye a whiteness like a stream of milk. To produce this effect there are not hundreds of stars, nor thousands of them, but actually millions of them.

When you stop to think that each one of these stars in the Milky Way is a sun like our own—some of them smaller, of course, but many of them much larger—you begin to realize how impossible it is for man to form any real idea of the magnitude and wonders of the earth. Here in the Milky Way are so many suns like our own sun that they together as we look at them form the particles of a path which makes the circle of the heavens, and yet are so far away that to the naked eye each of them looks to us like only one of countless drops of milk in a very large stream of milk that goes around the whole sky.

Why Don’t the Stars Shine in the Day-time?

The stars do shine in the day-time. If you will go down into a deep well or the open shaft of a deep mine and look up at the sky, of which you can see a circular patch at the top of the well, you will be able to see the stars in the day-time. The moon also shines in the day-time, on some part of the earth. At certain times during the month you can notice that the moon rises before the sun sets, and sometimes in the morning you can still see the moon in the sky after the sun is up. Usually you cannot see either the moon or the stars in the day-time, because the light from the sun is so bright and strong that the light of the stars and moon are lost in the brightness of the sun’s rays. When the moon is visible before the sun sets or after the sun has risen it is because the light of the sun is not so bright and strong at the beginning or close of daylight. If you are fortunate enough some time to witness a total eclipse of the sun you will be able to see the stars in day-time without having to go down into a deep well or mine shaft.

How Far Does Space Reach?

Space surrounds all earths, planets, suns, and extends for an infinite distance beyond each of them in all directions. It is impossible to measure in terms of human knowledge how far space extends. It is one of the things beyond the comprehension of the human mind, and for that reason man can never know in miles or the number of millions of miles how far it extends. Man has been able to measure the distance from the earth of some of the stars, and some of the nearest of them are millions of miles from the earth. Most of them are hundreds and even thousands of million miles away, and when we stop to think that space extends at least as far on the other sides of the stars as it does on this side, and even beyond that, we can readily understand that it is not only impossible to measure space, but also impossible to give in words any conception of what its limits might be.

There is one word—infinite—which we are forced to use in speaking of the extent of space. Infinite means “without end,” unbounded, and so man has come to use the word “infinite” in describing the extent of space, and that is as near as any one can describe it.

What Does Horse Power Mean?

The term “horse power” is used in describing the amount of power produced by an engine or motor. When man made the first engines he needed some term to use in describing the amount of power his engine could develop. Up to that time man had used the horse for turning the wheels of his machinery and the horse to him naturally represented the most powerful animal working for man. When engines came into use they replaced the horses because they were capable of developing many times the power of the horse. In finding an expression which would accurately convey to the mind of another the power of a particular engine, it was natural to say that this engine would do the work of five, ten or more horses, and as this described it accurately and in a way that was entirely clear, it became customary to describe the power of an engine as so many times the power of one horse.

To-day we still cling to the term “horse power” in describing the strength of the engine, although the horse-power unit used to-day is greater than the power of an average horse. To speak of an engine of one horse power to-day means an engine that has the power to lift 30,000 pounds one foot in one minute.