Glass , (Sax. gloes), in chemistry, any product of fusion having the peculiar lustre known as vitreous, hard and brittle, whether transparent or not; in common use, the transparent product derived from the fusion of silica with an alkali to which lime or a metallic oxide is added. No material invented by man is to be compared with glass in the service it has rendered. To its aid, applied in a thousand different forms, the sciences, particularly chemistry and astronomy, are essentially indebted for their advancement; and its uses in common life render it no less important to the daily wants of mankind. The purity of its material causes the presence of foreign substances to be instantly detected, and it is consequently the most cleanly substance, and especially suited for vessels for holding and keeping liquids. It resists the action of nearly all the powerful chemical reagents; and but for this substance many of them would never have been known, nor could they now be made and kept. - Nothing definite is known concerning the discovery of the art of glass making or the early history of its manufacture.
The statement made by Pliny that some Phoenician mariners having landed on the banks of a small river in Palestine, " and finding no stones to rest their pots on, they placed under them some masses of nitrum [soda, as is supposed], which, being fused by the heat with the sand of the river, produced a liquid and transparent stream," is not generally accepted as showing the origin of glass. A stronger heat than could be obtained from an open fire would be required to effect this result. Nor is much more credit to be attached to his accounts respecting the production of a glass of malleable character, which when thrown upon the ground was merely indented, and could be restored to shape with a hammer, as if it were brass. Some metallic salts, as chloride of silver, possess ductility at the same time with a glossy appearance, and of one of them the articles referred to may perhaps have been made; but all modern experience is opposed to the possibility of a vitrified body being malleable. It has been established with certainty that the art was practised among the Egyptians at a very early period.
Paintings on a tomb at Beni Hassan, supposed to date from the reign of Osortasen I., about 3,000 B. C, represent Theban glass blowers at work with blowpipes very similar to those used at the present day. A necklace bead of material similar to the modern crown glass was found at Thebes, bearing the name of the queen of Thothmes III., who reigned about 1500 B. C, inscribed in hieroglyphics. In the British museum there is an interesting ancient Egyptian specimen in the form of a small bottle of opaque light-blue glass, on which are painted in yellow the names and titles of the same monarch. Ornaments imitating precious gems in color and beauty show that the art had been brought to a high degree of perfection by the Egyptians. Not only was glass used by them in making drinking vessels, but also for mosaic work, the figures of deities and sacred emblems, and even for coffins, in all of which they attained excellent workmanship and surprising brilliancy of color. The glass works of Alexandria, in operation in the time of Strabo and Pliny, were famous among the ancients. According to Theophrastus, the processes of cutting or grinding, of gilding and coloring, were in use 370 years B. C. Articles of exquisite workmanship were produced, but of great cost, and known only as luxuries.
Vases and cups, some enamelled and beautifully cut and wrought with raised figures, and some remarkable for the brilliancy of their colors, were furnished to the Romans. From the Egyptians the Phoenicians are supposed to have received the art, which flourished at a very early period at Sidon and Tyre. In the ruins of Nineveh glass lenses, vases, bottles, etc, have been found; but there is no indication of the use of glass for windows. A small vase of transparent green glass, on which are engraved in outline a lion and the name and titles of the Assyrian monarch Sargon, 719 B. C, is preserved in the British museum, and is regarded as the earliest dated specimen of transparent glass. It was found in the palace of Nimrud in Nineveh. That the manufacture of glass was extensively practised by the ancient Greeks, and that they had acquired great skill in the art, are shown by the remarkable collection of specimens taken by Cesnola from the tombs at Bali on the island of Cyprus in 1866-'70, and deposited in the metropolitan museum of art, New York, in 1872. This collection of Greek glass, the most extensive known, comprises 1,700 articles, not only plain and simple, but various in form and color, and iridescent and incrusted.
There are plates plain, fluted, and with handles, in the various colors and in different shades of the same color. There is a great variety of ornamental cups and vases, and bottles of all sizes and shapes known to any people. (See Cesnola.) The manufacture of glass was introduced into Rome in the time of Cicero. During the reign of Nero great improvements were made and great skill was attained in the production of ornamental articles. At this early period only articles of luxury were produced, chiefly vases and cups for the tables of the wealthy, or urns and lachrymatories for their tombs. In the 3d century articles of glass were in common use. Numerous specimens of Roman glass have been found in the ruins of Herculaneum and Pompeii. From these it appears that glass was used for admitting light to dwellings in Pompeii, although other houses had window frames filled with a kind of transparent talc. The great perfection which the art had attained among the Romans is attested by the celebrated Barberini or Portland vase in the British museum, said to be the most beautiful example known of glass of two layers.
This vase was found about the middle of the 16th century in a marble sarcophagus near Rome, and is supposed to have been made as early as 138 B. C. After having been for more than two centuries the principal ornament in the Barberini palace in Rome, it was purchased by the duke of Portland for £1,029, and placed in the British museum. Here it was broken by a madman into many pieces, which were afterward joined together with great skill. The vase is about 10 inches high, and is composed of two layers of glass, the under one being of a deep blue color and the other of opaque white. The raised figures appear in white upon a beautiful background of blue, and by some are supposed to represent the marriage of Peleus and Thetis. - In the 13th century, and for several centuries after, the Venetian was the best and the most famous glass in commerce. The principal works were at Murano, one of the islands adjacent to Venice. Here the manufacture was long successfully prosecuted, being sustained by the fostering care of the government, and its workmen being invested with extraordinary privileges. Glass mirrors were probably first made here, and they became famous all over Europe, gradually taking the place of the mirrors of polished metal which were before in use.
Many of the ornamental objects they produced were exceedingly ingenious, and are reproduced and admired even at this day. The Bohemians next acquired reputation in this art; and owing to the purity of the materials found in abundance in their country, as well as to their skill, their wares still continue famous. The superiority of the Bohemians was evinced especially in the production of white glass, made with pure quartz and lime and the potash obtained by burning the trees of their immense forests. This glass was for a long time held in the highest estimation, but was destined to lose its fame when flint glass with lead was produced in England. The engraved glass of Bohemia became especially celebrated. The French, perceiving the importance of the business, early imitated the example of the Venetians, and gave extraordinary encouragement to any of the nobility who would prosecute the manufacture. In 1634 attempts were made to produce mirrors from blown glass, as was practised so successfully by the Venetians; but about the year 1666 it was found necessary to procure workmen from Venice. Works were then erected at Tourlaville near Cherbourg, which was selected from the resemblance of the locality to that of the works at Murano. In 1688 Abraham Thevart introduced in Paris the method of making large plates by casting the glass instead of blowing; he thus produced heavy plates measuring 84 inches by 50, while those previously made had barely reached in length the smaller figure named, and were necessarily thin.
In 1665 the manufacture of glass was established at St. Gobain. In the 18th century the business became very successful, and has continued so to the present time, the products of the establishment ranking among the first in quality in the world. - The first positive allusions to the use of glass for windows were made by Lactantius about the close of the 3d century, and by St. Jerome about the close of the 4th. It is asserted by the Venerable Bede that glass windows were first introduced in England in 674 by the abbot Benedict; but at this time and for many centuries afterward the use of window glass was limited to ecclesiastical structures. Colored window glass is known to have been used in churches in the 8th century; but for private houses glass long continued to be a rarity, and in England in the 12th century houses provided with glass windows were regarded as magnificent. Even in the 16th century in England and the 17th in Scotland only the dwellings of the wealthy were provided with glass.
The manufacture of window glass, according to an old builder's contract brought to light by Horace Walpole, and copied into his "Anecdotes of Painting," was conducted in England as early as 1439; but a decided preference was given to that "from beyond the seas." It was commenced in London in 1557; and soon afterward flint glass also was made there. The production of plate glass was undertaken in 1670 at Lambeth by the duke of Buckingham, who imported Venetian workmen. The government encouraged the enterprise by a bounty upon the glass intended for exportation; and under this protection, also extended to the different branches of the manufacture, by which the cost was reduced from 25 to 50 per cent., many other glass factories sprung up in different parts of the kingdom; but their prosperity and the progress of the art were afterward greatly checked by the excise duties imposed, and the surveillance of crown officers over all the operations of the works. The bounties and the duties, with their annoying restrictions, were abolished in 1845, when the suddenly increased demand for home consumption brought into existence many more establishments.
Their capacity for production became immense, as is shown by the fact that the firm of Chance and co. executed the large order in sheet glass for the crystal palace in 1851 without materially affecting their ability to fill their general orders. The quality of the English crown glass is unrivalled. - Glass appears to have been one of the earliest branches of manufacture introduced into the United States; but to what extent it was carried on in early times is unknown. In Salmon's "Modern History" (London, 1746), vol. iii., p. 440, mention is made of glass works which were commenced at Jamestown, Va., and the completion of which was interrupted by the Indian massacre of March 22, 1622; and in Howe's "Historical Collections of Virginia," p. 39, is a quotation from "Smith, book iv., p. 18," in which, under date of 1615, it is said that "for a long time the labor of the colony had been misdirected in the manufacture of ashes, soap, glass, and tar, in which they could by no means compete with Sweden and Russia." In Felt's "Annals of Salem," Mass., reference is made to the "Glasshouse Field," so named from the fact that in 1639 and 1640 several acres of land were appropriated to Ananias Conklin and others for the purpose of aiding them in the manufacture of glass, which was carried on for a considerable period.
About 1750 works were established by Germans at Germantown, Mass. (now a part of Quincy), for the manufacture of bottles, but they were burnt before the revolution. But the first glass factory in the United States of which we have a precise account was built by Mr. Robert Hewes of Boston, in the town of Temple, N. II., in 1780. It appears that the works were established there on account of the cheapness of fuel and labor. In the winter of 1780-'81 they were destroyed by fire. From a reference to this subject by Washington in his diary (1789) it would appear that glass was made at that time in New Haven. It is believed that in Salem and in Hewes's works only bottles and ordinary ware were made, and that the first window glass was manufactured in Boston. In 1787 a company was incorporated for the manufacture of crown glass, and after numerous embarrassments the first glass was made in 1793, under the superintendence of a German named Lindt. The shares of the company attained a high value, and the Boston crown glass became celebrated for its excellence.
The subsequent failure of the company was owing to the mismanagement of a board of directors who attempted to substitute American for German clay, and made other expensive and unsuccessful experiments; among these was the expansion of their business by the erection of other works for making thin crown glass at South Boston and sheet glass at Chelmsford. Works were established by the New England crown glass company for the manufacture of that article in East Cambridge about 1825, and others for bottles and for flint glass about the same period. Other crown glass works were erected in New York and other states at subsequent periods, but all were discontinued many years ago. The New England glass company, established in 1817 at East Cambridge for the manufacture of flint glass, is still in existence, and has gained a wide reputation for the excellence of its wares. Besides these works, the chief establishments for the manufacture of flint glass in the United States are in Sandwich, Mass., Brooklyn, N. Y., and Pittsburgh, Pa., and its vicinity. Sheet glass is made in Lanesbor-ough, Mass., New Jersey, New York, Pennsylvania, and in a few places in the western states.
The first plate glass manufactory was established at Cheshire, Berkshire co., Mass., about 1853. The company afterward removed their works to Lenox in the same county, and became known as the "Lenox Rough Plate Glass Company." They have the machinery for making polished plate glass, but have not yet produced it in large quantities. Henry R. Schoolcraft was employed in his youth in the works at Cheshire, and in 1817 he published a treatise entitled "Vitreology," designed to exhibit the application of chemistry to this art. - Glass is a chemical compound of variable ingredients, different substances of similar character replacing each other to produce its varieties. Silicic acid or silica is its principal element, which combines with the potash, soda, oxide of lead, lime, alumina, and other substances that may be added, to produce silicates of these bases. By the manufacturer the bases are classed as fluxes. Boracic acid may take the place of silicic acid to produce vitreous borates or glass. The proportions of the bases named admitting in their use of indefinite variations, a wide scope is given for the exercise of the skill of the manufacturer in producing any particular quality of glass.
The metallic oxides also afford him abundant resources for imparting any desired hue to his product, according as these are judiciously selected and introduced. The important requisite in all the varieties of glass is a fusible compound, which solidifies on cooling into a transparent mass, without assuming a crystalline structure. Such a substance is a product of the process of reducing metallic ores. The compounds produced by the glass manufacturer range from the most fusible combinations of one part of silica with two or three of soda or potash, which melt at a cherry-red heat and dissolve in cold water, to the hard and refractory silicates of lime and alumina, some of which require the powerful heat of a furnace to soften them. Potash especially increases the fusibility of glass; the oxides of lead and of zinc, and to some extent barytes, produce a similar effect, while they also add to its softness, its lustre, its specific gravity, and its power of refracting light, and do not interfere with its perfect freedom from color, unless the lead be used in excess, when it gives a yellowish tinge.
Iron, in the state of the silicate of the protoxide, imparts a dark green color; but by the addition of a small quantity of binoxide of manganese (MnO2) the color disappears, as the protoxide is converted into the sesqui-oxide (Mn2O3), and the manganese, losing one atom of oxygen, becomes MnO. Other metallic oxides, as those of uranium, copper, silver, and gold, are also employed to give intense colors. Without reference, however, to substances used for imparting or removing colors, the essential materials of ordinary glass may be regarded as silica and boracic acid, the alkalies, lime, and oxide of lead. The varieties of glass are classified by Dr. Knapp as follows: 1. Bottle glass, including the varieties worked into hollow vessels and tubes, as common bottles, glass for medicinal bottles, white bottle glass for vials, tumblers, tubes, etc. The dark-colored varieties are distinguished for their large proportion of oxide of iron and alumina, and none contain oxide of lead. The white bottle glass contains silica, soda or potash, and lime. 2. Window glass, including English crown and cylinder or sheet glass; this is a silicate of potash or soda, lime, and alumina. 3. Plate glass, differing from the preceding only by the greater purity and freedom from color of the materials. 4. Flint glass, used for grinding, etc, composed of silica, potash, and oxide of lead. 5. Crystal, for optical purposes and table ware, consisting of silica or boracic acid, potash, and more lead than the preceding. 6. Strass, the paste used for imitations of precious stones; it contains much oxide of lead, and also metallic oxides used for the colors. 7. Enamel, composed of silica, soda, and oxide of lead, but rendered opaque by oxide of tin or antimony, which f&rm a stannate or antimoniate with the soda.
To these may be added the soluble glass, which is a simple silicate of soda or of potash, or a mixture of the two silicates. The following analyses of several kinds of glass are from Knapp's "Chemical Technology:"
Fig. 1. - Theban Glass Blowers.
Fig. 2. - Blue Glass Bottle with Name of Thothmes III.
Fig. 3. - Green Glass Vase with Name of Sargon.
FIG. 4. - Cesnola Collection of Cypriote Glass in the Metropolitan Museum of Art, New York.
Fig. 5. - 1. The Portland Vase. 2. Opposite figures enlarged. 3. Device on bottom. 4, 4. Devices on handles.
Fig. 6. - Venetian Glass Bottle.
Fig. 7. - Engraved Bohemian Drinking Glass.
VARIETIES OF GLASS.
Oxide of iron.
Oxide of lead.
Bottle glass, French.......
Medicinal glass, French............
Window glass, French.............
• • • <
Plate glass, french........
Berthier .. .
White glass, goblet, Bohemia.......
" French fusible tubing..
Crystal, London optical............
Berthier .. .
Flint glass, Guinand...........
Bohemian hard glass tubing........
The later editions of Dr. Knapp's work give the following more recent analyses by Peligot:
VARIETIES OF GLASS.
Oxide of tin.
Oxide of lead.
Oxide of iron.
Bohemian opal glass...
The second of these is a remarkable glass, being a simple silicate of potash with 10 per cent. more silica than is contained in Fuchs's soluble glass. (See Glass, Soluble.) Particles of glass are dispersed through the semi-transparent, imperfectly melted mass. The compound is not attacked by boiling water, and does not attract moisture from the air. The ingredients of glass appear to be in the proportions of chemical equivalents - results, however, obtained by practice and not by mixtures made with this view. Various causes affect the stability of the combinations and the qualities of the compounds. The alkali in window glass powdered and moistened is detected by its action upon turmeric paper, and may be partially dissolved out by long continued digestion in boiling water. Atmospheric agents sometimes remove it in part from window panes, leaving a film of silica or silicate of lime. The glass of stable windows is liable to change its appearance, and assume prismatic colors, from the action of the ammoniacal vapors upon the silica. Changes in the degree of oxidation of its metallic ingredients, which are sometimes induced by atmospheric causes, are also attended by changes of colors.
Long continued cooling has the effect of changing the structure, causing it to lose its transparency and become devitrified. Its ingredients form among themselves a new arrangement of their particles, and compounds are produced which assume a crystallized structure. By remelting, the vitreous character may be restored, though with a loss of a portion of potash which was volatilized in the devitrification. In making articles of glass, and especially bottles, it is necessary to guard against this tendency to crystallize, and shorten the process of annealing on account of it. Devitrified glass was first described by Reaumur, and has hence been called Reaumur's porcelain. In consequence of the ease with which it may be made into any shape, and its tenacity and refractory nature, not unlike porcelain itself, it has been thought that it may be employed as a cheap substitute for this material, especially in many articles used in chemical laboratories. - The specific gravity of glass varies with its composition, from 2.4 to about 3.6, although optical glass of greater specific gravity is sometimes made, amounting in some instances to 5. Its density and also its refractive property are increased with the proportion of oxide of lead it contains.
Brittleness is a quality that limits the alteration of the shape of glass within narrow bounds, after it has cooled; but when softened by heat while it is highly tenacious, no substance is more easily moulded into any form, and it can be blown by the breath into hollow vessels of which the substance is so thin that they may almost float in the air. It may also be rapidly drawn out into threads of several hundred feet in length; and these have been interwoven in fabrics of silk, producing a beautiful effect. In the soft plastic state it may be cut with knives and scissors like sheets of caoutchouc. It is then inelastic like wax; but when cooled its fibres on being beaten fly back with a spring, and hollow balls of the material have, when dropped upon the smooth face of an anvil from the height of 10 or 12 ft., been found to rebound without fracture to one third or one half the same height. It has the valuable property of welding perfectly when red hot, and portions brought together are instantly united. When moderately heated it is readily broken in any direction by the sudden contraction caused by the application of a cold body to its surface.
It is also divided when cold by breaking it along lines cut to a slight depth by a diamond, or some other extremely hard-pointed body of the exact form suited for this purpose; and it may be bored with steel drills, provided these are kept slightly moistened with water, which forms a paste with the powder produced. Oil of turpentine, either alone or holding some camphor in solution, is also used for the same purpose. Copper tubes fed with emery also serve to bore holes in glass. Acids and alkalies act upon glass differently according to its composition, and reference should be made to this in storing different liquids in bottles. Silicate of alumina is readily attacked by acids, and bottles in which this is in excess are soon corroded even by the bitartrate of potash in wine, and by the reaction the liquor itself is contaminated. A glass that loses its polish by heat is sure to be attacked by acids. Oxide of lead when used in large proportion is liable to be in part reduced to a metallic state by different chemical reagents, and give a black color to the glass.
All glasses are attacked by hydrofluoric acid. - In 1863 a series of experiments showing the action of sunlight on glass was begun, and has since been continued, by Mr. Thomas Gaffield, a merchant of Boston, whose collection of authorities on glass and kindred subjects is more complete than any other in this country. As early as 1824 Prof. Faraday had noticed a change in color produced in glass containing oxide of manganese when exposed to the sun's rays, and this effect was attributed to the action of solar light on that ingredient. Mr. Gaffield's experiments, embracing about 80 different kinds of glass, colored and uncolored, of English, French, German, Belgian, and American manufacture, have proved that this remarkable phenomenon is not limited to glass containing oxide of manganese, but extends to almost every species of glass. That the effect is not due to heat, but solely to the actinic rays of the sun, is shown by the fact that no change of color is produced in the glass when it is exposed to heat; while on the contrary, after the discoloration has been produced by solar light, the colors thus acquired disappear under the action of heat, and the glass assumes its normal color.
This process may be repeated indefinitely, the change of color being produced by solar light, and the original color restored by heat. It was also shown that the effect was not produced by air or moisture. In some specimens the change was more easily effected than in others; in some days were sufficient, in others years were required; but in almost all the change was produced. "It is very interesting," says Mr. Gaffield, "to witness any one of these series of specimens, showing, as in one of white plate, a gradual change, commencing in a day or a few days in summer, from greenish or bluish white to a yellowish white or light yellow, a deep and deeper yellow, until it becomes a dark yellow or gold color; and in some Belgian sheet specimens a gradual change, commencing in a few weeks in summer, from brownish yellow to deeper yellow, yellowish pink, pink, dark pink, purple, and deep purple." The following statement shows the changes produced in nine different kinds of window glass by one year's exposure to the sun's rays:
* Relation between the oxygen of the acid and the total amount of oxygen in the bases.
KIND OF GLASS.
Color before exposure.
Color after exposure.
French white plate___
German crystal plate ..
American crystal sheet.
Light bluish white.
" .. "
Lighter " "
Light yellowish green.
" ordinary sheet.
The colors named above are given from an observation of the glass edgewise, when a body of color several inches in depth is seen, whereas the usual thickness of the glass varies from one fourteenth to one quarter of an inch, and shows its color easily only when a white curtain or paper is placed behind it. The partial or entire disuse of oxide of manganese in many window-glass manufactories of late years, while it has produced an article not so light in color, has made one more permanent, which the action of sunlight changes but little, if any, in color or shade. Mr. Gaffield's experiments were also extended to showing the comparative power of the different kinds of glass to transmit the actinic rays of the sun. Of colored glasses, blue was found to transmit the most and red and orange the least. - The crude materials employed in the manufacture of glass are selected with more or less care, according to the quality of the articles to be produced. The three principal elements of which crown and sheet glass are composed are silica, soda, and lime. Of these by far the largest element is silica, which is now universally supplied in the form of sand. English crown and sheet glass generally contains about 73 per cent. of silica, and 13 each of soda and lime.
On the continent less sulphate is used than in England; the component parts of foreign sheet glass may be stated at 74 per cent. of silica, 11 of soda, and 14 of lime. In both cases the remainder consists of alumina and oxide of iron. To the above ingredients it is generally the custom to add a small quantity of arsenic to assist in oxidizing any carbonaceous impurities and to promote the decomposition of the other materials, and of peroxide of manganese to peroxidize and thus reduce the coloring property of the oxide of iron present. Silica is obtained in the form of quartz sand from sea beaches and from the disintegration of quartzose rocks in the interior. It was in England once procured from flints calcined and ground to powder, whence the name flint glass. The purest and best sand in the world for manufacturing glass is from Lanesborough, Mass., and other portions of Berkshire county. Some of it is exported to Europe, where it is known as the "Berkshire white sand," and there used in making the best qualities of glass. The grains are remarkable for their purity; in the mass they appear white, but under the microscope each grain is limpid like a clear quartz crystal. Other qualities are procured in various parts of the country.
Next to the American sand in quality is that obtained from Fontainebleau in France, and much used by the French manufacturers. It is almost entirely free from iron, and is well adapted for the manufacture of white glass. The sand used by the extensive establishment of Chance and co., near Birmingham, England, is from Leighton Buzzard, Bedfordshire. Lime may be used either in the state of quicklime or in limestone of the purest qualities. Common wood ashes have been used to furnish potash, and ashes of sea plants to furnish soda; but these have been replaced by the crude alkalies obtained from them and other sources, and for some purposes refined pearlash is employed. The carbonate of soda is also extensively prepared from common salt; and at Newcastle, England, black bottles are made from rock salt and sand from the bed of the river, with carbonate of lime of the soap works and the tank waste of the alkali makers. Sulphate of soda, the waste product of many chemical works, is successfully used, except for plate glass. Although glass can be produced from sand and alkali without any other addition, lime is a very important element, as giving to it hardness and insolubility.
In flint glass this ingredient is replaced by lead, which gives greater brilliancy to the glass than lime, but, in consequence of the difference between its specific gravity and that of the other materials, is the cause of innumerable stria). Saltpetre and binoxide of manganese and arsenic also are often introduced into the mixtures with the view of promoting the same object. Alumina and oxide of iron are commonly not intentionally used; they come from the impurities of the other materials. Waste glass, called cullet, forms a considerable proportion of the raw materials in some works; it promotes the fusion and the chemical union of the silica and bases mixed with it, but must be well sorted, so that no qualities be introduced inferior to that intended to be made. - In melting glass, the raw materials, thoroughly ground, mixed together, and sifted, are well incorporated with from one quarter to one third of their weight of broken glass before being introduced into the melting pots. These are already heated to a white heat in the furnace, and receive only two thirds of a charge at a time, more being added as the first portion melts down.
The pot being at last filled with the melted "metal," the heat is raised as rapidly as possible, and the progress of the operation is judged of by the workmen dipping iron rods from time to time into the mixture and examining the appearance of the drops withdrawn. A nearly homogeneous product, which becomes transparent on cooling, indicates that the most refractory ingredients have been all dissolved. Their mixture has been facilitated by the continual disengagement of carbonic acid gas, which in its escape caused the whole to be thrown into ebullition. Some of the gas remains in the mass, rendering it spongy and full of vesicles. Unless in the manufacture of the finer qualities of glass, for which the purest materials are employed, there is also a scum, called glass gall or san-diver, floating upon the surface, consisting of the insoluble matters, and the sulphates of soda and lime not taken up by the mixture. This is removed by ladling, and the "metal" is next fined, which is done by increasing the heat to the highest degree, and keeping the contents of the pots in a state of perfect fluidity from 10 to 30 hours; in this time the bubbles disappear and the insoluble matters settle to the bottom.
The furnace is then allowed to cool until the metal has become viscid, so that it may be taken out and worked; and it is afterward kept at sufficiently high temperature to maintain the glass in this condition, that it may be used as required. The arrangements of the great circular glass furnaces, with their central fire surrounded with eight to twelve pots, each reached by its own arch under the general dome, admit of enough material being melted at once to employ all hands the first four working days of the week, the men working day and night in six-hour shifts. The materials of the furnaces and pots, in order that they may withstand the excessive heat and the action of the various melted ingredients, must be carefully selected from the most refractory substances, and the work must be most skilfully executed. The construction of the great melting pots is an object of special solicitude, and the placing of a new one in the furnace while this is in operation is a task of no little apparent difficulty and danger. In England they are made of the best Stourbridge fire clay, mixed with about one fifth part of ground potsherds. The work is done entirely by hand, no machinery having yet been invented for that purpose.
An average-sized pot is about 4 ft. high, 4 ft, in diameter at top, and somewhat smaller at the bottom, and will contain about 25 cwt. of melted glass. The average duration of a pot in the furnace is about eight weeks. In the case of window and ordinary bottle glass, the pot is a plain round vessel open at the top; but in melting flint glass, it being necessary to protect the metal from all external impurities, the top of the pot is made in the form of an arch or hood, with a small opening on one side near the top, which corresponds with the nose hole of the furnace, and from which the workman withdraws the melted glass. Ordinarily two kinds of furnaces are used in addition to the annealing oven, one for melting the glass, and the other for reheating it at different stages during the process of manufacture. One of the most important improvements in the manufacture of glass has been the adoption of the Siemens regenerating gas furnace. (See Furnace.) The novelty of this system consists in taking up the waste heat from the furnace in large chambers, and using it for raising to a higher temperature the elements of combustion.
The whole of the fuel, except the inorganic portions, is converted into gas, not in the furnace itself, but in adjacent " producers." The gas and air passing through separate chambers, and having each been heated to a high degree in the waste-heat chambers, meet on entering the furnace, and there ignite, producing a heat of wonderful intensity. The advantages of this system are a greater intensity of heat produced from less fuel, and, what is very important in the manufacture of glass, a degree of cleanliness which cannot be attained by the older methods of melting. The intensity of the heat produced is indicated by the fact that in a sheet-glass furnace containing 1,800 cubic feet, materials for about 16 tons of glass in eight large pots are melted and refined into a liquid mass in 25 hours. - Such is a mere outline of the means employed to bring the materials of glass into their desired combination. The production of each kind of glass is a separate branch of manufacture, involving many curious details and processes, too numerous even to be named in this account.
The tools employed are few and simple, and differ but little from those described in the work of Blancourt "On the Art of Glass," published in London in 1699. The first in importance is the pipe or blowing tube, made of wrought iron, 4 or 5 ft. long, with a bore from 1/4 to 1 in. in diameter, a little larger at the mouth end than at the other. It is a long hand, partly covered with wood, with which, the end. being heated red hot, the workman reaches into the pot of melted matter and gathers up the quantity he requires, and which afterward holds the article in the manipulations to which he subjects it; and it is at the same time the air tube through which the breath is forced to expand the vessel, or through which water is sometimes blown to produce the same effect by the steam it generates. A solid rod of iron, called a punty or pontil, serves to receive the article upon its end when freed from the pipe, adhesion being secured by the softness of the glass or by a little red-hot lump already attached to the punty.
Spring tongs, like sugar tongs, are used to take up bits of melted glass; and a heavier pair, called pucellas, furnished with broad but blunt blades, serve to give shape to the articles as the instrument in the right hand of the workman is pressed upon their surface, while, seated upon his bench, he causes with his left hand the rod holding the article to roll up and down the two long iron arms of his seat, upon which it is laid horizontally before him. At the same time the vessel is also shaped from the interior as well, and is occasionally applied to the opening of the furnace to soften it entirely or only in some part to which greater distention is given by blowing. The pucellas are sometimes provided with blades of wood, as at 4, fig. 9. Another important instrument is a pair of shears, with which a skilful workman will cut off with one clip, the top of a wine glass, as he twirls it round with the rod to which it is attached held in the left hand. The edge softened in the fire is then smoothed and polished.
Besides these a wooden utensil called a battledore is employed, with which the glass is flattened by beating when necessary; compasses and calipers and a measure stick are at hand for measuring; and a slender rod of iron forked at one end is used to take up the articles, and carry them when shaped to the annealing oven, in which they are left for some time to be tempered. (See Annealing.) The marver (Fr. marbre, marble) is a smooth polished cast-iron slab, upon the surface of which the workman rolls the glass at the end of his tube in order to give it a perfectly circular form. Those used in the manufacture of common black bottles are furnished on one edge with several concavities, in which the mass of metal taken from the melting pot is first roughly shaped as it is rolled over and over and made to swell by gentle blowing. One of the most ordinary forms into which glass is manufactured is that of bottles, which are made in moulds by the process of blowing, the kind of glass generally used being the ordinary green or window glass, and flint glass.
The method of making bottles is described and illustrated in the article Bottle. Bottles for champagne and aerated waters are made of extraordinary strength, and are sometimes tested by the pressure of water before being used. - Of the various kinds of glass in common use, none require more care to insure the purity of the materials employed than the crystal or flint glass, of which are made many choice articles for domestic purposes, some of which are subjected to the processes of cutting or grinding and polishing. It possesses the properties of great transparency and high refractive power, which fit it for lenses for optical instruments. Flints calcined and ground were formerly used to furnish the silica, but pure sand is now generally used in its stead. Oxide of lead enters largely into its composition, and to this are due its brilliancy, density, and comparative softness. The oxide should be especially prepared to insure its purity. Oxide of zinc has been found to produce similar effects. The fusion must be rapid and at intense heat, and this must be reduced as soon as the metal is thoroughly melted and refined by the escape of the bubbles of gas, or the product acts upon the alumina and iron of the pot, and is thus so contaminated as to be worthless.
The furnace is usually circular in form, and contains from four to ten pots, in front of each of which there is an opening for the workman. In the manufacture of articles of domestic use made of flint glass two processes are in use, blowing and pressing, the latter being very common in the United States. By the former method a mould is sometimes used, as in the case of bottles, when the operations are similar to those described in working ordinary green glass; or the article may receive its symmetrical form from the skill of the workman unaided by any mould. This process may be illustrated by describing how a wine glass in three parts is made. The workman, having gathered on the end of a blowpipe the requisite amount of glass (1, fig. 10), rolls it on the marver and expands it by blowing into the tube until it assumes the form shown at 2, and afterward, being flattened at the end with the battledore, that at 3. A lump of glass is now attached to the flat end of the bowl (4), which the workman with the pucellas, while rotating the pipe on the long arms of the chair in which he sits, transforms into the shape shown at 5. A globe is now attached to the end of this stem (6), which is afterward opened and flattened into the form represented at 7. A punty tipped with a small knob of hot glass is next stuck to the foot of the wine glass, which is severed from the blowpipe at the dotted line shown at 8. The top of the glass is then trimmed with shears (9), after which it is flashed and finished as at 10. It is now severed from the end of the punty by a sharp blow and carried by a boy to the annealing oven on the end of a forked rod.
In the manufacture of articles by the method of pressing, a hollow mould is used made of steel or iron, with its interior surface so designed as to give the object the required shape and figuration. This mould may be in one piece or consist of several parts, which are opened when the moulded glass is taken out. The process will be illustrated by describing the production of a tumbler. A lump of glass is gathered from the pot on the end of a punty by the "gatherer," and being held over the open mould, a sufficient quantity is cut off with a pair of scissors by another workman and drops into the mould. This is now pushed under a hand press, and a smooth iron plunger is brought down into the mould with such force that the hot glass is made to till the entire space between the inside of the mould and the plunger, whose size and shape are the same as those of the interior of the tumbler. The plunger being raised up, the mould is taken from the press and turned over, when the tumbler is made to drop out bottom side up.
A punty with a piece of hot glass at one end is now attached to the bottom of the tumbler, which is heated at another furnace and smoothed by being skilfully rubbed with a wooden tool while rotated on the arms of the workman's chair; after which it is taken on a fork to the annealing oven. By this process articles can be produced with a rapidity not attainable in the case of blown glass, and therefore with less cost; but the latter is generally preferred. - The glass commonly used for window panes is one of the hardest varieties, and of unsuitable quality for shaping into vessels or manufacturing by cutting or grinding. Besides plate glass, which is also used for windows of a more expensive character, there are two kinds of window glass, known as crown and sheet from the different processes of manufacture; the former being first blown into a globe or sphere and flattened out into a circular disk, while the latter is formed into a cylinder which is afterward opened out into a sheet. In making crown glass, the workman gathers from the pot on the end of a blowpipe the requisite amount of molten glass, which is usually about 9 lbs.
The pipe being cooled to admit of handling, the lump is rolled upon the marver to give it a conical form, and a boy blowing at the same time through the tube causes the glass to swell. It is now heated by holding it in the furnace, and is then again rolled and enlarged by blowing. The most of the glass is worked down to the end of the conical or pear-shaped lump, the upper part being hollow. The solid end is called the bullion. This being softened in the furnace, the tube is laid across a rest and twirled around, while the glass is blown into a globe. During the expansion it is important to keep the bullion point in a line with the axis of the pipe. This is done by a boy holding against the bullion point a piece of iron terminating in a small cup, while the workman constantly twirls and blows through the pipe resting upon an iron support. The globe at the end of the tube is now pointed toward the flame of the furnace, and being constantly twirled, the end toward the fire flattens out, the bullion point still forming a prominence of thicker metal in the centre. To this centre a punty with a lump of molten glass at its end is next attached, and the blowing pipe is separated by applying a piece of cold iron around the nose.
As it breaks away it takes a portion of glass with it, leaving a circular opening. Taken up by the punty, the glass is held with the nose (or portion to which the blowing pipe had been attached) presented to the nose hole of the furnace. Here it is softened almost to melting, while it is all the time twirled around; it is then presented to the flame issuing from the great circular opening of the flashing furnace, the man holding it being protected from the fire by a covering over his head and face. Rapidly revolving in this flame, the opening in the end grows larger; the heated air within prevents the two opposite faces of the flattened spheroid from coming together, and the centrifugal force is constantly enlarging its diameter. The opening rapidly increases, until the glass becomes a flat circular disk, which being removed from the fire is kept rapidly revolving until it is cool enough to retain its form. The punty is then cracked off, and the disk or table is removed upon a fork to the annealing oven and set upon edge with the rest, arranged in rows and supported by iron rods so as nut to press against each other, and the thicker part in the centre, called the bullion point or bull's-eye, also keeping the tables apart and open for the circulation of air.
The annealing is completed in from 24 to 48 hours. Tables are thus commonly made of 54 inches diameter, and some have been produced of 70 inches; but the difficulty of manipulation and the uncertainty of the result render the making of very large sizes unprofitable. A pot containing half a ton commonly produces 100 tables; and in the crown glass houses it is customary to empty eight such pots in three days every week. From the annealing kiln the tables are taken to the warehouse and sorted according to their different qualities and defects. Each one is then laid in turn upon a "nest" or cushion, and is divided by the diamond into two pieces, the larger one containing the bull's-eye. These are next cut up into rectangular panes. The shape and the bull's-eye involve considerable waste in cutting; and numerous other defects arc found in many of the sheets. These, however, are compensated for by the remarkable brilliancy of surface peculiar to glass made in this way, which is attributed by some to the influence of the mar-ver, and by others to the effect produced by flashing the surface.
Crown glass is also free from the undulations, or cockles, which often disfigure the surface of glass made by the cylindrical process. - In the manufacture of sheet glass two furnaces are generally used, one for melting or making the glass, and the other for reheating it during the process of blowing. The latter is usually of an oblong form, with four, five, or six holes on each side for as many workmen. On each side of this furnace is a pit about 7 ft. deep, 16 ft. wide, and as long as the furnace; over this at intervals of about 2 ft. are erected in front of each hole of the furnace wooden stagings or platforms, upon which the workman stands when swinging the cylinder to and fro and over his head. The manufacture of this kind of glass may be divided into three processes: 1, blowing the cylinder; 2, flattening it out into a sheet; 3, polishing the sheet. The first step is to gather from the pot a lump of melted glass of the required weight, which experience enables the workman to do with great accuracy. Dipping the end of a blowpipe into the melted metal and twirling it round, he gathers a pear-shaped lump of 2 or 3 lbs. After this has cooled to a dull red, it is again dipped into the glass in the pot, and a larger amount withdrawn.
Thus by degrees a sufficient quantity is collected, usually about 20 lbs., to produce a sheet of glass of the required size. When this mass has become somewhat cooled, the workman places it in a block of wood so hollowed as to allow the lump of glass when placed upon it to be blown to the required diameter of the cylinder. Here, while a stream of cold water is turned upon the block to prevent the wood from being burnt and the glass from being scratched, the workman revolves the pipe, and blows through it, occasionally raising it to an angle of about 75°, until he has formed a hollow pear-shaped mass, with its largest diameter, which is the same as that of the finished cylinder, next to the pipe. It is now taken to the blowing furnace, where after being heated it is swung to and fro in the pit and round in a vertical plane over the head of the workman, who stands upon the platform above mentioned and keeps the lengthening cylinder full of air by occasionally blowing through the tube. Uniformity of thickness and of diameter, which was de-termined by the wooden block, is secured by the skill of the workman, who when the metal runs out too freely holds the cylinder vertically above his head, still keeping it well filled with air.
This operation is skilfully continued until a cylinder is produced about 11 in. in diameter and about 50 in. long, closed at one end and attached to the blowpipe at the other. The next step is to open the end of the cylinder, which the workman does by filling it with air and, after closing the aperture of the pipe with his thumb, exposing the end to the heat of the furnace. The heat expands the air in the cylinder, which bursts open at the end where the glass is the softest. The aperture thus made is widened to the required diameter by rapidly revolving the cylinder at the furnace hole, the pipe resting on an iron support, and subsequently holding it in a vertical position with the open end downward until the glass is cooled sufficiently to retain its shape. The cylinder-is now laid upon a wooden rest, or trestle, and detached from the pipe by touching with a piece of cold iron the pear-shaped neck near the nose of the pipe, and gently striking the pipe; an opening about three inches in diameter is thus formed. This end, the cap of the cylinder, is now taken off by winding around it a thread of hot glass, and after removing it applying a piece of cold iron to any point which the thread covered.
After trimming the other end by cutting off about two inches in length with a diamond, the cylinder is split open longitudinally by drawing along its inside surface a diamond attached to a long handle and guided by a wooden rule. Formerly this splitting was done with a red-hot iron, which is still sometimes used. The cylinder is now taken to the flattening oven, where it is placed, with the slit uppermost, upon the flattening stone, from the irregularities of whose surface it is protected by a sheet of glass. The cylinder soon becomes heated and opens out into a wavy sheet, the movement being accelerated by the iron rod of the workman. The surface of the sheet is next rubbed with a piece of wood attached to the end of an iron rod for the purpose of removing the irregularities of the surface. The flattening stone is now moved on wheels to the adjoining annealing oven, where the sheets are placed for annealing, which usually requires from 24 to 36 hours. From the annealing oven the sheets are taken to the warehouse, where they are smoothed, polished, assorted, and cut into panes of the required dimensions.
The former method of grinding and polishing sheet glass by imbedding the sheets in plaster of Paris proved inadequate to remove the defects in the glass consequent upon the mode of manufacture. The chief of these was the undulating or wavy appearance of the surface, called cockles, which was attributed to the difference of diameter between the inner and outer surfaces of the cylinder, and which caused objects seen through the glass to be distorted. Notwithstanding the glass was made very thick after the superficial roughness was removed, the result was a thin sheet much inferior to plate glass. The ingenious process devised by Mr. James Chance for producing patent plate glass, which is now used in England and most factories on the continent, is one of the most important improvements in the manufacture. By removing the thin outer surface of the glass by this method, an evenness and a polish are secured, even on the thinnest sheet, which make it in many respects equal to plate glass, and far superior to the sheet glass produced by the old process. The improved method consists in placing the sheet to be ground and polished upon a flat surface covered with a piece of damp soft leather or cotton cloth.
A slight pressure applied to the glass causes it to adhere to the surface of cotton or leather, and by thus producing a vacuum the entire sheet is firmly maintained in a flat position by atmospheric pressure. The exposed surfaces of two sheets fixed in this manner are rubbed against each other in a horizontal position by machinery, emery and water being constantly supplied to keep up the friction. Both sides of the sheet are polished in this manner, with only a slight diminution of the thickness of the glass. After the removal of the sheets from these surfaces, they resume by their own elasticity their original shape, which is often more or less curved. The final polish is given to the sheets by a process similar to that used in polishing plate glass. In each process through which the glass has passed, it was exposed to some imperfection, and some of the sheets bear the peculiar defects of them all and are of little value; others are suitable for inferior uses, and but few are perfect. The wide difference between the quality of the best and the worst sheets is indicated by the fact that the former are valued at three times more than the latter. The same kind of material is used in the production of both crown and sheet glass.
The remarkable brilliancy of surface of the former gives to it a certain advantage over sheet glass; but the larger size easily attained in making the latter gives it the supremacy in commerce. Of crown glass it is difficult to obtain panes of 34x22 in., while the usual size of the sheets of cylinder glass is 47 x 32 in., and cylinders are occasionally blown 77 in. in length, requiring about 38 lbs. of glass. The largest sizes are only produced by the most skilful workmen. The relative antiquity of the two processes of making crown and sheet glass is involved in no little obscurity. The cylindrical process is the only one mentioned by Theophilus, who is supposed to have lived in the 12th century, and this method was long retained by the Venetians and the Bohemians, as being best adapted to the production of their colored glasses on account of the uniformity of thickness and of color secured. But in the north of Germany, France, and England, it fell into disuse, and the rotary principle prevailed exclusively. Subsequently the latter was abandoned on the continent, but held its supremacy in England, where crown glass was used for houses of the better class, while the use of sheet glass was limited to inferior dwellings.
In 1832 the improved process of making cylinder glass was introduced into England from France, and subsequently the improved method above mentioned of polishing the sheets was adopted. The cylindrical method is the one now in general use in England, much of the glass being known in commerce as patent plate. - The building or factory for the manufacture of plate glass is generally of very large size. That of the British plate-glass works at Ravenhead, where it is called the foundery, is 339 ft. long by 155 wide; while the famous halle of St. Gobain in France is 174 by 120 ft. In the centre is the square melting furnace, with openings on two parallel sides for working purposes, while along two sides of the great building are arranged anneal-ing ovens, which are sometimes 30 by 20 ft. in order to receive the immense plates that are to be annealed. Two kinds of pots are used, the ordinary one, open at the top, for melting the glass, and cisterns or cuvettes, in which the molten glass is carried to the casting table. In France the cuvette is usually of a quadrangular form, with a groove in each of its sides, or, as in the case of the larger cisterns, in two parallel sides, in which the tongs or iron frame are fitted when the cuvette is moved.
Between each two pots in the furnace are placed, according to their size, one or more cuvettes. In some establishments the cuvette is not now used, the metal being poured from the pot in which it is melted on to the casting table. In France 16 hours are allowed for the melting, and the same time for the metal to remain in the cuvettes; but the latter term is often extended in order that the aeriform bubbles may escape and the excess of soda become volatilized. Toward the last the temperature is allowed to fall, and the glass then acquires the slight degree of viscidity suitable for casting. The molten glass is transferred from the puts into the adjacent cuvettes by means of wrought-iron ladles with long handles. When the glass is in the proper condition to be cast, the "tongs carriage," consisting of two powerful bars of iron united like two scissors blades, and resting upon two wheels, is pushed into the opening made in the furnace, and the cuvette is clamped in the quadrant formed at the extremity of the tongs, two workmen manipulating the handles at the other extremity. The cistern thus taken from the furnace, while tilled with molten glass, is placed on another carriage and quickly conveyed to the casting table.
This consists of a massive slab, usually of cast iron, supported by a frame, and generally placed at the mouth of the annealing oven. At the Thames works in England the casting plate is 20 ft. long, 11 ft. broad, and 7 in. thick. Formerly these tables were of bronze, and the great slab of St. Gobain of this alloy weighed 50,000 lbs.; but cast iron was found less liable to crack, and is now generally used for this purpose. On each side of the table are ribs or bars of metal, which keep the glass within proper limits, and by their height determine the thickness of the plate. A copper or bronze cylinder about a foot in diameter, resting upon these bars, extends across the table. After being heated by hot coals placed upon it, the table is carefully cleaned preparatory to casting. The cistern containing the melted glass is raised from the carriage on which it was brought from the furnace by means of a crane, its outside carefully cleaned, and the glass skimmed with a copper sabre. The cuvette is now swung round over the table, over which a roller covered with cloth is drawn to remove all impurities, and the molten glass poured out in front of the cylinder, which being rolled from one extremity of the table to the other spreads out the glass in a sheet of uniform breadth and thickness.
The operation is a beautiful one from the brilliancy of the great surface of melted glass, and the variety of colors exhibited upon it after the passage of the roller. While the plate is still red hot about two inches of its end is turned up like a flange, against which an iron rake-like instrument is placed, and the plate is thrust forward into the annealing oven, the temperature of which is that of dull redness. Another plate is now immediately cast upon the hot table, and the annealing oven when filled is closed and left for about five days to cool. The process of casting is done so systematically and with such despatch in a well regulated establishment, that the glass has been taken from the furnace, cast, and put into the annealing oven in less than live minutes. From the annealing oven the plates are taken to the warehouse, where they are carefully examined to see how they may he cut to the best advantage. In different manufactories and at different times various processes have been in use for grinding and smoothing the surface of plate glass, but the principle has been the same in all, viz.: rubbing the surface to be smoothed with another surface either of glass or iron, and at the same time applying sand or emery of different degrees of fineness and water between the two impinging surfaces.
One of the most approved methods of grinding and smoothing the plates was introduced into England in 1856, and adopted in the British plate-glass works. This apparatus consists of a revolving table, 20 ft. in diameter, fixed upon a strong cast-iron spindle, and capable of running at an average speed of 25 revolutions a minute. Above the table frames are arranged to hold the plates of glass, which are laid in a bed of plaster of Paris, with the face to be polished resting upon the table. These frames also revolve on their centres by the friction of the table upon the glass, slowly, but so as to present each side of the plates they hold to an equal amount of rubbing as they are moved nearer to the centre of the table or further from it. Sand and water are applied to facilitate grinding down the glass. The grinding by this process is found to be even and equal, and the machinery to work smoothly and steadily from the facility with which the plates accommodate themselves to the power applied. After grinding they are smoothed with emery powder of finer and finer qualities, and are thus prepared for polishing. By the process above described the grinding and smoothing are done by the same machine; but formerly two sets of apparatus were required for this purpose.
By grinding the surface of the plate is made true, but presents a rough appearance which is removed by the process of smoothing, At this stage it is somewhat opaque, and this defect disappears after the final process of polishing. This is performed chiefly by machinery. The plate of glass having been fixed upon the table by means of plaster of Paris, the surface is subjected to the action of a series of wooden blocks covered with felt and attached to a frame by which they are made to move over the surface of the glass. At the same time a polishing powder, generally red oxide of iron, is applied, while the friction may be increased by adding weight to the rubbers. Polishing sometimes brings out defects which were before concealed; the plates are consequently again assorted, and, if need be, reduced to smaller sizes. (For the methods of silvering them, see Mirror.) Bending the large plates or the smaller sheets of glass for the purpose of fitting them for bow windows, etc, is an especial branch of the manufacture. A core of refractory material and suitable shape is introduced upon the floor of the furnace; and upon this is laid the sheet to be bent, which as it softens by gravity conforms itself to the shape of the bed upon which it is laid.
The value of plate glass varies greatly with the size. In the United States the price of a plate of standard British or French glass, 5 x 3 ft., is about $35; but when the dimensions are double, the plate being 10 x 6 ft., the price is increased to about $175. A plate 14 x 8 ft. is valued at about $500. - No glass is of such importance in the arts as that of which the lenses of optical instruments are made. Both flint and crown glass are applied to this use, but each of them has its defects. The former, from the great difference in the densities of its ingredients, is with much difficulty obtained of homogeneous structure, an essential requisite in all glass used for optical purposes; and the latter is difficult to procure of uniform composition and texture, from the high temperature required for its fusion and the consequent tendency to devitrify in cooling; or if this is obviated by an increased proportion of alkali in the composition, the excess of this causes attraction of moisture from the air and a damp surface to the lens. The best flint glass is subject to defects, chief among which are undulatory appearances called striae, resulting from a want of uniform density in the glass, and tending to refract and disperse in different directions the rays of light passing through it.
These defects are of great importance when the glass is to be used for optical purposes. In 1753 John Dollond, an English optician, first began the construction of achromatic object glasses, formed of two kinds of glass of different density, in accordance with the theory announced not long before by Euler. For this purpose Dollond used fragments of flint and of crown glass, but did not succeed in making object glasses with a larger aperture than 2 or 3 in. in diameter; and when the need of telescopes of greater magnifying power was strongly felt, it was difficult to produce flint glass sufficiently free from striae for a lens 4 in. in diameter. The scientific bodies of France and England offered prizes for the attainment of this result, and the most renowned glass manufacturers at the end of the last and the beginning of the present century endeavored to solve the problem. This was done by Guinand of Switzerland, a man not conversant with science, nor even a glass manufacturer, but an optician. By methods of his own he made the furnaces, crucibles, and mixtures he employed, and produced the glass, which he shaped and polished, giving without knowledge of mathematics the requisite proportion to the curves of its surface, and completed lenses of flint glass of great perfection of structure, 9 in. in diameter.
The secret of his success in making the glass is believed to have consisted in keeping the mixture agitated by stirring when at its greatest liquidity, and then suffering it to cool and anneal in the pot. From the most perfect portions of the comparatively homogeneous mass thus obtained, the lenses were cut out by a process similar to that of sawing blocks of stone. By one of the sons of Guinand the secret was imparted to M. Bontemps; and in 1828 lenses were made in France of 12 to 14 in. diameter. In 1848 Bontemps went to England, and in conjunction with the Messrs. Chance and co. made disks of flint and of crown glass larger than any before produced. At the exhibition in London in 1851, a disk of flint glass was exhibited by Messrs. Chance and co. 29 in. in diameter and weighing 2 cwt.; and at the Paris exposition in 1855 they exhibited one of the same diameter made of crown glass. One of these was afterward sold to the French government for £1,000. They are of pure color, and of such homogeneous structure that the light is transmitted without polarization.
Prof. Faraday, one of a committee appointed by the astronomical society of London to experiment upon the means of producing optical lenses, while Guinand's secret method of making these 6 in. in diameter was exciting the admiration of the scientific world, discovered the heavy glass called by his name (composed of protoxide of lead 104 lbs., silicate of lead 24, and dry boracic acid 25), which has proved of considerable importance in investigations connected with the polarization of light; but its liability to change unfits it for general optical uses. Lenses both of flint and of crown glass are used in the object glasses of achromatic telescopes, serving by their combination to counteract the unequal tendency of each to disperse the rays of light. It seems to be conceded by scientific men that the glass best adapted to achromatism would be a flint glass possessing a smaller refractive power and a larger dispersive index, and a crown glass having, conversely, a greater refractive power and a less dispersive index. - The annual production of plate glass in Europe may be stated in round numbers at upward of 10,000,000 sq. ft., of which about 4,000,000 sq. ft., valued at about 28,000,000 francs, is produced in France, 3,750,000 in England, 1,500,000 in Germany, and 1,000,000 in Belgium. The industry is limited to a few large establishments, there being six each in France and England, and two each in Germany and Belgium. In addition to the above, large quantities of rough plate glass are made in England for horticultural and other cheap purposes.
About 15,000,000 sq. ft. of window glass, of the value of about 15,-000,000 francs, is produced annually in France, and about 100,000,000 bottles, valued at about 20,000,000 francs; the production of flint glass amounts to about 15,000,000 francs, and of ordinary table glass about the same. The en-tire production of the country exceeds 75,000,-000 francs. The exports of glass from England in 1872 were 2,131,924 sq. ft. of plate glass, valued at £243,780; 113,004 cwt. of flint, valued at £300,484; 760,836 cwt, of common bottles, valued at £373,138; and other kinds of glass to the value of £204,593. The latest statistics on the manufacture of glass in the United States are afforded by the census of 1870, as follows:
Fig. 8. - Melting Pots.
Fig. 9. - Tools used in Glassmaking. 1. Pipe or blowing tube. 2. Pucellas. 3. Shears. 4. Pucellas with wooden blades. 5. Spring tongs. 6. Battledore.
Fig. 10. - Glass Makers Chair.
Fig. 11. - Process of Making a Wine Glass.
Fig. 12. - Hand Press.
Fig. 13. - Blowing Cylinder Glass.
Fig. 14. - The Cylinder in Different Stages of Manufacture.
Fig. 15. - Casting Table.
No. of establishments.
Ware, not specified.
The establishments were chiefly in Pennsylvania, New York, New Jersey, and Ohio. Of the five manufactories of plate glass, three were in Ohio and one each in New York and New Hampshire. Not included in this statement is the Lenox rough plate glass company at Lenox, Mass. The importations of glass and glass ware into the United States for the year ending June 30, 1873, amounted to $5,834,712, including cylinder, crown, and common window, $2,759,728; cylinder and crown polished, $21,217; fluted, rolled, or rough plate, $34,-180; cast polished plate not silvered (2,482,-359 sq. ft.), $1,550,857; cast polished plate silvered (2,392,274 sq. ft.), $823,076; other manufactures, $2,230,980. Of the cylinder, crown, or common window, $2,181,044 worth came from Belgium and $451,223 from England; of the cast polished plate not silvered, 1,955,000 sq. ft., valued at $1,252,991, from England, 246,-098 sq. ft., valued at $155,450, from Belgium, and 39,047 sq. ft., worth $22,963, from France; of the silvered plate, 2,297,049 sq. ft., valued at $704,913, was the production of England.