These clays are highly refractory, and compare very favorably with the clays of Belgium, which are said to be the best in Europe. The analysis of the Belgian clay shows —

In Greenup County, Ky., clays of good quality are also found, but unfortunately they contain larger proportions of lime, alkalies, and especially magnesia; the latter has a tendency to lessen very materially the refractory quality of clay. Other clays, varying in quality, are found in Edmonson, Lincoln, and Madison Counties. Many of the latter clays, though containing impurities, may be made useful by a judicious selection and washings.

The clays of New Jersey have been tried, especially those from the Amboy district, but so far they have not been used with success for pot making, although they answer very well for the construction of furnaces. Whether these clays, like many others, could not be improved by washing is certainly worth trying. In fact, I believe we have as good clays in this country as we need, and many of the inferior or impure qualities may be prepared as china manufacturers do, so as to render them suitable for pot making.

The clays of Europe are principally drawn from Forges-les-Eaux, in France; Andennes, Belgium; Stourbridge, England; Klingenberg, Germany. They average the following proportions:

Plate-glass.

The manufactories of plate-glass are principally located in France, Belgium, England, and Germany.

In France, St. Gobain, the pioneer and the largest establishment, was founded in 1695, in the castle of that name. Chemical works belonging to this company are erected at Chauny, near St. Gobain. The manufactories of Cirey and Montluçon also belong to this company. There are three other plate works — at Recquignies and Jeumont, owned by

the Belgian Company of Ste. Marie d’Oignies, at Floreffe, and Aniche. Quite recently another factory has been erected at St. Denis, near Paris, for the manufacture of plain and silvered plates, rough plates, etc. MM. J. Grelley & Co. are the proprietors.

In Belgium there are five plate-glass works, at Sainte Marie d’Oignies, Floreffe, Courcelles, Roux, near Charleroi, and at Aurelais. Each of these works has laboratories attached. Belgium, in 1867, manufactured only 119,000 yards of plate-glass, but at the present day the average has reached an annual production of 357,000 square yards. Three-fourths of the production are exported to the Netherlands, England, and the United States. Prices since 1867 have declined 27 per cent. The exportation, which was then about $350,000, had reached $760,000 in 1877.

England possesses six or seven plate works. The oldest, St. Helen’s, was founded in 1773, and is the first establishment which introduced machinery for grinding and polishing plates. In Germany are to be found the works of Stolberg, near Aix-la-Chapelle, and Waldhof, near Mannheim, founded and owned by the St. Gobain works. These were established to supply the local demand of the country. Three more factories have since been established, one at Walburg, one at Cramplan, and the other at Freden. These, however, so far, are but comparatively small works. The production of Germany is scarcely one-tenth of that of France, Belgium, and England together.

Russia has one plate-glass factory at Dorpat. Bohemia has one also at Stockau.

In 1860 the annual production of the European manufacturers was 992,000 yards, in 1867 1,100,000, and in 1877 1,800,000 yards, representing a value of about $12,000,000. Out of this amount France makes 600,000 yards, four-fifths of which are made at St. Gobain; England makes 600,000 yards; Belgium, 250,000 yards; the remainder, 350,000 yards, is made in the other countries named. This amount of rough and polished plate-glass, if laid on the ground, would cover about 372 acres. This would make a very respectable hot-house.

French and Belgian plates, it cannot be denied, are of a superior quality. English glass generally has a green tint which, although it may not be objectionable for plain window plates, is not as well adapted for silvering. This defect is attributable to the quantity of iron contained in British sand.

In Bohemia and Bavaria several establishments still manufacture blown plate-glass, but since the articles produced are quite inferior, this industry tends to disappear, and I do not think it worth the while to give any description of the processes used.

In England a certain kind of plate called patent glass is manufactured. This glass is blown in a similar manner to cylinder window glass, but is flattened with particular care to obtain as even and smooth a surface as possible; it is ground and polished somewhat in the same manner as cast plates. This quality is much sought after by photographers, and is also used for window panes and as a cheaper substitute for heavier plates for mirrors, car windows, and in ornamenting furniture.

The mixture generally used by manufacturers at the present day does not vary materially from the following:

The quantity of cullets (broken glass) to be added varies according to the quantity a manufacturer has at his disposal. The materials used should be very pure; the sulphate of soda should especially be well refined. Of late the old-style direct fire furnace has been replaced by those of Siemens and Boëtius. In the arrangement of a casting house the Siemens furnace is placed at one end, and the building does not require to be as wide as with other furnaces. These furnaces contain 8, 12,16, or 24 pots, according to the size; they are rectangular in form. Two rows of bricks support the arches, which are placed far enough apart to permit the passage of the pots. The regenerators are placed immediately below the furnace; the bricks in the passages generally require to be renewed every six months. As these furnaces are now well known, I will not give a particular description of their construction.

The Boëtius furnace is built above ground, nearly in the same manner as the Siemens. Under the head of “Furnaces” I will describe this furnace more at length.

On the sides of the casting house are ranged the annealing ovens or kilns, and those for heating the pots. The casting table is placed in the middle, running upon a railway. Alongside is another railway to carry the small running table upon which the plate is pushed when rolled, previously to pushing it into the annealing furnace or kiln.

On the side opposed to this table the crane for raising the pots is placed upon a truck running over a railway. All these railways run the whole length of the casting house.

The operation of casting a plate is as follows:

The glass in the pots having attained the proper degree of liquidity and having received a thorough melting and refining, the fire is slackened to render the mass somewhat viscous by cooling. The doors in front of the pots are lifted or taken away, the workmen, with a long pair of iron pincers, take hold of the pot in the furnace, bring it upon an iron truck or carriage, and at a dog-trot carry it under the crane. The impurities or glass gall upon the surface of the glass are now scraped off, the pot carefully wiped on the outside with a wet cloth to prevent dirt from falling upon the casting table. The pot is now seized by a pair of strong iron tongs or nippers and raised over the table by means of the crane. The casting table is a large cast-iron slab, well polished, mounted upon a carriage running over a railway. Upon this table two iron rules of the thickness of the required plate are now laid on each side. The pot suspended above is now tilted over and the glass poured upon the table. A heavy iron roller is now passed over the glass, the ends of which rest upon the thickness rules. The roller is rolled again back to its original position. During the rolling, if any impurities are detected in the glass when yet plastic, they are removed with suitable instruments. The plate having reached a sufficient degree of solidity by cooling, it is now pushed upon the small running table, and from that transferred to the annealing kiln. The door of the kiln having been closed air-tight, the glass is allowed to remain for about twenty-four hours, when small openings are gradually made until, at the expiration of three or four days, the plate is sufficiently cooled to be taken out without running the risk of breaking it.

The furnaces or kilns for annealing are simply arched furnaces containing one or two fire-places, but of late years but one has been used. In order to hasten the cooling of the kilns the bottom is so made as to reserve passages made of longitudinal walls or clay pipes, covered with the bottom bricks. These passages are open to the cold air and hasten the cooling very rapidly.

Under the head of “Flattening ovens,” in the article on “Window glass,” I have described several systems of furnaces which, I think, could be used with advantage and economy, to say nothing of the ease of handling the plates in the annealing ovens. I do not see any reason, either,

why gas should not be used as a fuel in this operation, as well as for the melting of glass. The time required for the heating and the cooling of the plates would be much shorter, and the annealing more uniform on both surfaces. Although many mechanical improvements have been made in working glass, we have remained too conservative altogether in this country in the use of furnaces. I notice of late, however, that several manufacturers of hollow ware are beginning to appreciate the advantages of gas furnaces, and several have already been erected in Pittsburgh, Pa., and elsewhere. It cannot be understood, for instance, why we should not adopt them generally when we are assured of an economy of 40 to 50 per cent, in fuel by the use of the Siemens furnace, and 30 per cent. with the Boëtius system.

In regard to the grinding, smoothing, and polishing of plates I shall say but little, as it is very difficult to convey a proper description without the use of drawings of the mechanical means employed.

The first operation consists of grinding the rough plate having an uneven surface, by means of a large wooden table suspended to the ceiling. This table has its lower face covered over with several blades of cast-iron, screwed to the wooden frame. Immediately under the upper frame is placed another large table, upon which the plate is cemented with plaster. Sand and water are thrown upon the rough plate, the upper table is lowered, and is made to travel back and forth over the surface of the glass plate by means of steam-power. The sand is replaced by emery, gradually increasing in fineness, until the plate, by continued grinding, is sufficiently smooth. Of late years, in Germany and Belgium, a new style of polishing machinery has been adopted. It consists of a large wooden table to which a back-and-forth motion is imparted; also two large iron or wooden frames, to which are secured cast-iron blades. These frames receive a circular translatory motion. This system does double the quantity of work produced by the preceding machine. Another system, an American invention, was put in practical operation at first in England, but is now used in France and Germany. A circular plate of cast iron is screwed upon the upper end of a vertical shaft so as to revolve with it. Above this table frames are arranged to hold the plates of glass, which are laid in a bed of plaster. These frames also revolve on their centers by the friction of the table upon the glass, of course slowly, but so as to present each side of the plate they hold to an

equal amount of rubbing, as they are never nearer to the center of the table or farther from it.

The same machine is used for polishing, by screwing to the cast-iron table rings covered with felt.

The second operation is the smoothing, and is similar to the preceding, but the rubbing substance is a piece of plate glass instead of iron. This operation is now carried on by machinery to imitate the old hand process.

The glass plate to be smoothed is fastened upon a table, receiving a slow back-and-forth motion. A motion is imparted to the upper platen by means of two levers moving a wooden box, to which is fastened a piece of plate glass, which bears upon the lower plate and is drawn back and forth. As one of the levers acts before the other, the upper plate is moved from right to left and again from left to right, while it is drawn forward and back.

The third operation is the polishing, which is given by means of colcothar or red oxide of iron. The plate is fastened to a table, receiving a back-and-forth motion; a couple of brushes lined with felt, having a motion perpendicular to the table, rub over the surface of the plate and, by means of colcothar as a polishing powder, give it a final polish. It takes from eight to ten hours to polish one side of a plate of five to six superficial yards. Another style of polishing machine is sometimes used, having a translatory circular motion both in the table and the upper polishing platens.

These three operations add considerably to the cost of plate glass; it is, therefore, essential to a manufacturer to have his plates flattened as evenly as possible so as to save in the thickness to be ground away to obtain a perfectly plane surface.

The price of plate glass from St. Gobain was, in 1862 and 1875:

These prices show a rise of 30 per cent, in consequence of the additional taxes imposed by the French Government, The price of plate glass rises rapidly with the dimensions.

The following are the prices in France for “third-choice” plates, which are said to be sold in the largest quantities:

There are also wide differences in prices according to quality — a plate of 5 meters, for instance, of “second choice” is worth $11 per superficial meter, while the same, “first choice,” is worth $12.68.

Silvering. — Silvered plate glass is produced by causing a slight coating of mercury to adhere to the glass. To obtain this result mercury must be retained by a metallic medium; it is, therefore, amalgamated with tin. Mercury, owing to its power of reflecting light very brightly, has been chosen as the best medium.

The operation of silvering is briefly as follows:

Upon a very smooth stone table a sheet of very thin tin is spread very carefully, so as to prevent all wrinkles. Upon this sheet mercury is rubbed all over, then as much mercury as the sheet will retain is poured over it. The glass plate is now carefully slipped over the edge of the stone table, as near as possible to the mercury, and lowered on to it. All the parts previous to this operation have been carefully cleaned, and the plate is handled with pieces of tissue paper, to prevent the introduction of dirt. The plate is now covered with a cloth and loaded with weights to expel the surplus mercury. When the plate has been so weighted, the table is slightly inclined, and gradually increasing the inclination from time to time, until the mercury has been sufficiently drained; this generally requires twenty-four hours. The plate is now carefully taken up and carried over to an inclined wooden table, which is depressed gradually more and more to finish draining the mercury until the plate is supposed to be dry.

This is the process which has been heretofore followed altogether, but of late plates have been silvered with a solution of silver. Mercury has deplorable effects upon the health of workmen, as they are exposed to its dangerous emanations; these are rapidly absorbed by the skin and produce the well-known and terrible mercurial poisoning. It is hoped, therefore, that mercury will be abandoned, and the new silvering process described below will be adopted in its place. Several methods have been proposed for silver solutions, all springing, However, from the discovery of Liebig, that aldehyde (produced by a partial oxidation of alcohol) when heated with nitrate of silver, the revivified metal covers the glass with a brilliant metallic coating. It is not my purpose to trace the different improvements made

by Drayton and Pettitjean, but I will briefly indicate the process of the latter, which is now altogether used by the St. Gobain works with perfect success.

The operation is very similar to silvering with mercury. The table, instead of being a stone, is a hollow sheet-iron table, made quite smooth on its upper surface, and containing inside water capable of being heated by means of steam, to bring the temperature to 95 — 104 degrees. Preparatory to silvering the glass it should be thoroughly cleaned. The table being ready, a piece of oil-cloth is spread over it, and upon this is laid a piece of cotton cloth. The plates are now put upon these cloths, and the following solutions are poured over them:

Liquor No. 1. Dissolve in a liter of water 100 grams of nitrate of silver; add 62 grams of liquid ammonia of 0.880 density; filter and dilute with sixteen times its volume of water. Then pour in this liquor 7.5 grams of tartaric acid dissolved in about 30 grams of water.

Liquor No. 2. This liquor is precisely the same as the other, with the exception that the quantity of tartaric acid is doubled, say 15 grams.

First pour of liquor No. 1 upon the plates as much as will remain upon the surface without running over. The heat of the table is now increased gradually to 95 — 104 degrees Fahrenheit, and in about thirty minutes the glass is covered over with a metallic coating. The table is now inclined and the plates washed with water, which carries off the surplus silver. The table is again raised, and liquor No. 2 is now poured over; in about a quarter of an hour another coat is deposited, which covers the glass completely. The plates are again washed; then they are carried to a slightly heated room, where they are gradually dried.

This operation, as will be seen, is quite simple, and is generally performed by women. The silver carried off in washing and that contained in the cloths is recovered again. Since glass silvered by this process is liable to be altered when exposed to the air, and the coating may become easily detached if not covered over with a protecting coat of paint, the silver pellicle is covered with an alcoholic copal varnish, put on with a brush, and when this is dry a coat of red-lead paint is put on.

Plates silvered by this means have more brilliancy than with mercury, but as there is a slight tinge of yellow given to objects reflected by these mirrors, they were at first objected to. This objection has passed away, however, to a great extent, and the yellow reflection has been obviated by

giving a slight coloration to the glass. I have not been able to get positively the relative costs of both processes; it is said, however, that the new silver process costs about 30 cents per square meter. Inasmuch as such works as the St. Gobain have adopted it, and as the terrible disorders caused by mercury may be thus avoided, there should be no hesitation in adopting this new process everywhere.

The use of platina has been tried for a reflecting surface, but owing to the somber appearance of reflected objects by looking-glasses prepared with it, it has not met with a commercial success. I do not think, therefore, that it is worth the while to describe the process.

Rolled plate. — England manufactures large quantities of this glass, and lately France and Belgium have taken it up.

Instead of being cast with the costly apparatus necessary for plate glass of large sizes, this glass is cast in a very simple manner. A basin or dipper is introduced into the glass and filled up; this dipper is suspended upon a hook placed in front of the pot, to enable the workmen to dip and withdraw it with facility when it is full. This dipper is carried over an iron table, and by giving a blow upon it all the cooled filaments and pieces attached to the outside fall off. The dipper is now emptied over the casting table, the thickness of the glass being regulated by metallic pieces or rules. The roller is passed over the mass by the workmen, back and forth. These plates are sometimes imprinted with quadrangular ribs, very close to one another, in order to hide the defects or air bubbles which are likely to occur with this mode of dipping glass. These plates are usually cast about one-eighth of an inch thick. They are used for covering hot-houses, for door panels, and for windows.

{Margin note: Plate-glass making in America}

Rough plates have already been produced in large quantity in America, but the pioneer establishments in polished and silvered plates have so far been quite unfortunate. There is no reason for this state of affairs. We have materials in abundance, and very cheap. Our labor has decreased in cost so as to put us almost on a par with Europe. Much of the work in a plate-glass manufactory is done mechanically, and therefore does not cost any more than in Europe. We have the ingenuity to devise and invent our own tools, as is shown by the fact that the polishing machinery now used in Europe is of American origin. Under these circumstances we must, however, come to the reluctant conclusion that the causes of repeated failures in plate-glass making in this country must be attributed to the want

of technical knowledge of those engaged in it. In fact, I know personally of the failure of two establishments solely attributable to this cause. This country must and can supply itself with plate glass, and the day is not distant when works properly managed will drive out the imported article. We have been successful in many other branches of glass-making. Window glass is now made very extensively; why not plate glass, which is almost the same industry? A good field is open for success to any one who will embark in this industry, provided he is supplied with the proper technical and practical knowledge.

Tempered or toughened glass.

It is not very long since the discovery of M. Alfred de la Bastie filled all our newspapers with paragraphs, more or less ridiculous, about the properties of this glass. Some claimed it was malleable; others that it could not be broken. In fact, tempered glass was called upon to supersede all other materials. The excitement being over, tempered glass may now take its rank among valuable inventions, subject, however, to many defects in its present state.

The process of tempering glass, as is well known, consists in heating a piece of glass, say a window pane, to such a degree as to approach malleability, but not hot enough to lose its shape; the glass in this state is instantly plunged into a bath composed of fatty and resinous matter, which is heated and maintained liquid at a temperature ranging from 300° to 600°, according to the quality of the glass. The difference of temperature between the malleable state, about 1,400°, and that of the bath constitutes the temper.

Glass in the plastic state, when plunged into cold water, will fly to pieces if dropped indiscriminately, but if a piece of very fluid glass is allowed to fall into water in the shape of a tear or drop, it will be perceived that the outside of the glass cools at once, while the inside remains partly fluid for some time, as can be distinguished by the red color showing through the water. This cooling will continue until the mass is perfectly solid. This indicates that the outside layer becomes at once condensed by cooling, while the inside remains fluid and consequently more distended. This cooling process goes on, the outside layer compressing the next adjoining, until the whole mass is thoroughly cooled. This peculiar form and state of glass is known as Prince Rupert’s drops. Though a hard blow may be struck upon the thick part of these drops, it has no perceptible effect, but if the tail or thin end is ruptured the whole mass instantly

Window glass.

Although window glass, at the present time, is universally used, it is but a few hundred years since it has come into general use. Although doubt has existed, as to whether window glass was used by certain ancient peoples, it is now an established fact that glass windows were known, if not to the Greeks, at least to the Romans, before the Christian era. The discoveries made at Pompeii and Herculaneum of windows set with glass have removed all doubt upon the antiquity of the use of this material for transmitting light. M. Bontemps examined some of the window frames found at Pompeii, and he gives it as his opinion that the panes, instead of having been blown, must have been cast in metallic frames, probably of bronze. These panes show very irregular thickness; a conclusive evidence that the glass could not have been blown. Thus, sixteen centuries before casting plate-glass, as by the present method, the Romans were using a process very similar in its nature. The Pompeiian glass, upon analysis, shows the following ingredients :

It would seem that the use of glass for windows at Pompeii was not extended to a great degree, for but few specimens have been found.

The use of glass for windows of dwellings was first introduced in the fourteenth century. They were first made of small pieces of glass leaden cames. The use of glass incased in wooden frames was first introduced under the reign of Louis XIV of France; the pieces of glass were very small. In England it was only during the twelfth century that glass panes were introduced. Two centuries ago glass windows were very rare. In 1661, in Scotland, windows were only to be found in the principal chambers of the king’s palaces. Le Vieil says that towards the end of the twelfth century Englishmen did not know what a glass-house or a window was. Workmen at that time were brought from

{Glass-study note: Remainder of paragraph was on page 345}

France to make windows, and brought the glass with them. Windows were then described as “new things” in that country, “necessary against rain and birds.”