GLASS.

The general term glass is employed by chemists to denote all mineral substances which, on the application of heat, pass through a state of fusion into hard and brittle masses, and which, though not always transparent, exhibit a lustrous fracture when broken. The glass of commerce, however, to which our remarks are restricted, or the transparent and artificial substance which is usually distinguished by the generic name, is produced by the igneous fusion of siliceous earth with certain alkaline earths or salts, or with metallic oxides.

The etymology of the word has been much disputed. It is derived by some from the Latin glacies, ice, its resemblance to which is thought to have suggested the title. Others have remarked, that the common Latin designation of this substance is vitrum ; and as the Romans applied this term, in common with the word glastum, to the plant which we call woad, they have deduced it from the latter of these, either because the ashes of this plant were used in the manufacture of glass, or because it exhibited something of the bluish colour which is procured from woad. Glassum, the name given to amber by the ancient Gauls and Britons, has also been assigned as the origin of the word. But none of these etymons appears very satisfactory. The most plausible theory is that which derives the term from the Saxon verbs glis-nian, or the German gleissen, splendere, which are probably contractions of the Anglo-Saxon ge-lixan, to shine, to be bright. This view is in a great degree confirmed by the sense in which the term glass and its derivatives are employed by our older writers, who frequently apply it to shining or glittering substances, without reference to colour or transparency. The discussion of this point, however, as will be easily seen, is of very little importance.

The origin of the manufacture of glass is involved in the most impenetrable obscurity, and has occasioned much learned, but, at best, doubtful speculation. The art is undoubtedly of the highest antiquity. Some even assert that it was coeval with the art of pottery or brickmaking. But this is only a conjectural supposition, founded chiefly on the fact, that it is impossible to excite a strong fire, such as is necessary in metallurgic operations, without vitrifying part of the bricks or stones of which the furnace is built. But it is more than likely, that such imperfect vitrifications might have been observed long before the idea occurred of taking advantage of the effects which they displayed, in manufacturing the useful article of which we are now about to treat. There can be no doubt that the art of making glass was well known to the Egyptians. Glass beads and other ornaments made of that substance, skilfully manufactured and beautifully coloured, have been found adorning many of their mummies which are known to be upwards of three thousand years old. Neri adduces a passage from the book of Job (chap, xxviii. verse 17), to prove that glass was known to the Jews at the time when that book was written. But, unfortunately for his theory, the word which, following the Vulgate and the Septuagint, he translates glass, and which in our version is rendered crystaĻ means simply, in the Hebrew root, any diaphanous or shining substance, and is often used, according to the fancy of translators, to denote any body which is beautiful or transparent. The two problems of Aristotle, “Why do we see through glass ?” and “Why is glass malleable ?” if genuine, which is very doubtful, comprise the earliest mention of the substance on record. The Ssaβς of Herodotus, which he describes as used by the Ethiopians in their cemeteries, has been variously interpreted amber or rock crystal. Both mean ings are, however, questionable; and it is plain he refers to a natural, not an artificial production; for he speaks of it as being dug up in large quantities. The same word is used by Aristophanes, a writer of the same century (the fifth before Christ), in his Nubes ; but, in the passage referred to, it may mean any stone which will permit die transit of the sun’s rays to form a burning medium; and though it is by some thought improbable that glass would have been called a stone, it is yet worthy of remark that Shakspeare applies the same epithet to it in his King Lear, act v. sc. iii. Theophrastus, who flourished not above a century after Aristotle (303 a. c.), speaks of the sand of the river Belus as used in the manufacture of glass, which he certainly distinguishes by the name υsλoς; and from that date the knowledge of the substance seems to have been pretty generally diffused. A century afterwards we hear of Archimedes having employed burning glasses, amongst other instruments, for the defence of Syracuse against the assaults of the Romans under Marcellus, by means of which he set the enemy’s ships on fire. The silence of Polybius, Plutarch, and Livy, as to this circumstance, may be held to argue against its accuracy; but modern experiments have shown, that it may not be wholly a fabrication, worthy only to be classed in that ample category of marvels, which

Graecia mendax

Audet in historia.

Buffon constructed a burning glass composed of about four hundred glass planes, each six inches square, so as to form a concave mirror capable of melting silver at the distance of fifty feet, and lead and tin at the distance of one hundred and twenty feet, and of igniting wood at the distance of two hundred feet. This has rendered the feat of the “Isthmiacis decus immortale colonis,” as he is styled by Silius Italicus, less incredible than it appeared before; but yet it is not impossible that his burning mirror was composed of polished steel. If the statement as to the celebrated sphere of the same philosopher having been constructed of glass can be relied on, the art of manufacturing glass must have been carried to a high degree of excellence upwards of two centuries before the Christian era.

A passage in Pliny, which attributes the discovery of glass to accident, has often been quoted. A merchant vessel, laden with nitre or fossil alkali, was driven ashore on the coast of Palestine, near the mouth of the river Belus, a small stream running from the foot of Mount Carmel in Galilee, into the Mediterranean. The mariners, unable to procure stones to rest their cooking vessels upon, used pieces of their cargo instead. The fire reduced the alkali to a soft state, and incorporating with the river sand, it melted down into a vitreous stream. The circumstance was communicated to the inhabitants of the district, who availed themselves of the hint, and engaged in the manufacture of glass. The story is plausible, and it is no extravagant credulity to believe, that the world has been indebted for this, as well as for many other of its most valuable discoveries, to what, in common language, is called accident. It is certain, however, from passages in the works of Strabo and Josephus, that the sand for about half a mile round the mouth of the river Belus was peculiarly adapted to the manufacture of glass. In fact, it was much used in the glass-houses of Tyre and Sidon. Some antiquaries maintain that the first glass-house was erected at Diospolis, the ancient capital of the Thebaid; but that distinction ů generally considered as belonging to the above-mentioned towns. Alexandria, another city of the Phoenicians, was long and highly celebrated for its glass, and at one time [7:10:563]furnished great part of the glass ware which was used at Rome. To these places the art was exclusively confined for some centuries. The expense of the manufacture probably prevented an extensive use of the substance, and its application to the ordinary domestic purposes which it now serves. It was considered as an article of luxury, and appeared chiefly in the form of urns or drinking cups. These latter were often of the most elaborate workmanship, and exquisitely embellished with raised, chased, or ornamented figures. The cups and cinerary urns which have survived the shock of time and accident, prove that the ancients possessed great skill in glass-blowing, and are marked by a delicacy of workmanship which modern art has not excelled. The Barberini or Portland vase, composed of deep-blue glass, with figures of a delicate white opaque substance raised on it in relief, is a splendid specimen of ancient skill in this department. It was found in the tomb of Alexander Severus, who died a.d. 235. Several other ancient glass vessels of great beauty are preserved in the British Museum.

Though extensively used by the Romans, we do not find any mention of glass in any of their writers before Lucretius. From the allusions to its lustre and transparency which occur in the writers of the Augustan age, glass would seem to have been then manufactured in considerable purity. But, perhaps, the word usually translated glass may rather refer to some natural production. For, though we know that an excellence in colouring glass was attained at an early period, which has never been surpassed, yet the art of producing it devoid of colour, the most difiicult process in glass-making, since it is easily affected by extraneous substances, was then scarcely known. Many years afterwards, the Emperor Nero paid 6000 sestertia, or about L.50,000 of our money, for two small drinking cups with handles, the chief excellence of which consisted in their being colourless. The glass in common use was of very inferior quality, and composed for the most part of coarse thick metal, sometimes white, but generally of a bluish colour.

From the circumstance of coloured glass beads and amulets having been found in druidical remains in this country, it has been argued by Pennant and others, that the art of making glass was known in Britain before its invasion by the Romans. It can hardly, however, as we think, be believed, that a people, who had made very trifling advances in civilization, and who, it is known, were entirely unacquainted with any other art, should be found not only conversant with the manufacture of glass, a complicated and highly ingenious process, but should excel in it; for the beads and amulets spoken of are of exquisite workmanship, and beautifully coloured in imitation of the rarest and most precious stones. There seems little doubt, therefore, that the ancient Britons procured these in the course of traffic with the Syrians, who visited the island, as we do those in the South Seas, to drive a trade with their savage inhabitants in toys and trinkets, giving them these in exchange for skins or other natural productions. By whatever means, however, these ornaments came into Britain, it is certain that they were in extensive use, though principally for religious purposes, long prior to the Roman invasion, as they are found in barrows or tumuli of a much older date. One at Stonehenge, in particular, on being opened, was found to be filled with them.

Glain Neidyr, or druidical glass rings, generally about half as wide as our finger rings, but much thicker, have frequently been found. The vulgar superstition regarding these was, that they were produced by snakes joining their heads together and hissing, when a kind of bubble like a ring was formed round the head of them, which the others, continuing to hiss, blew on till it came off at the tail, when it immediately hardened into a glass ring. Success was thought to attend any one who was fortunate enough to find one of those snake stones. They were evidently beads of glass employed by the Druids, under the name of charms, to deceive the vulgar. They are usually of a green colour, but some of them are blue, and others variegated with wavy streaks of blue, red, and white.

Glass utensils have been found in Herculaneum, which city was destroyed by an eruption of Mount Vesuvius in the reign of Titus (a. d. 79). A plate of glass also found there has occasioned much speculation as to its uses. Similar plates, to which Pliny gives the name of vitre<e camera, seem to have been employed in a manner not very well understood by us, as wainscotting for their rooms. It is disputed whether or not glass was used in Herculaneum for windows.

In the reign of Tiberius, a company of glass manufacturers established themselves in Rome, and had a street assigned them near the Porta Capena. The articles of their manufacture were few in number and of inferior quality, neither did they make rapid improvements in their art, notwithstanding the large prices which were then given for glass of foreign manufacture in the imperial city. In the year 220, they had increased so much in importance, and the produce of their manufacture was so considerable, that an impost was laid on it by Alexander Severus. Dion Cassius and Petronius Arbiter concur in their account of the discovery of malleable or ductile glass by a celebrated Roman architect, whose success in the restoration to its position of a porticowhich leaned to one side, had roused the envy and jealousy of Tiberius, and occasioned his banishment from Rome. Thinking that his discovery would disarm the emperor’s wrath, the artist appeared before him bearing a glass vessel, which he dashed upon the ground. Notwithstanding the violence of the blow, it was merely dimpled, as if it had been brass. Taking a hammer from his breast, he then beat it out into its original shape; but instead of giving him the reward which he had expected, the emperor ordered the unfortunate artisan to be beheaded, remarking, that if his discovery were known, gold would soon be held of as little value as common clay. This is probably another version of the story told by Pliny, of an artificer who made the same discovery, and whose workshop was demolished by those who had an interest in preventing the introduction of an article which would lower the value of gold, silver, and brass. Although it might not be justifiable to give unqualified disbelief to these stories, yet the knowledge we at present possess would restrict the possibility of such a discovery within the narrowest limits. The union of the properties of malleability and vitrification seems to be incompatible. Some metallic substances, by the application of intense heat, are reduced to the state of glass, but at the same time lose their malleability; which fact would seem to imply that it is impossible to communicate the latter property to glass. The extraordinary stories above mentioned have, however, been rationally enough explained by modem chemists. It has been observed by Kunckel, that a composition having a glossy appearance, and sufficiently pliant to be wrought by the hammer, may be formed; and by Neumann, that, in the fusion of muriate of silver, a kind of glass is formed, which may be shaped or beaten into different figures, and may be ^renounced in some degree ductile. Blancourt, in his L’Art de la Verrerie, mentions an artist who presented a bust of ductile glass to the Cardinal Richelieu, minister of Louis XIII. But he does not seem to have been more fortunate than his predecessors; for he was doomed to imprisonment for hfe, for “the politic reasons,” as Blancourt with much simplicity observes (we quote from the translation published in 1699), “which, it is believed, the cardinal entertained from the consideration of the consequences [7:10:564]of that secret,” which no doubt led him to fear lest the established interests of French glass manufacturers might be injured by the discovery. From expressions used by Blancourt in other parts of his work, we think, that by malleable glass, such as was produced by this artist, he understood some composition similar to those which Kunckel and Neumann discovered, and was not very exact in limiting the term to that vitreous substance which we now generally understand when we speak of glass.

The precise period at which the making of windowglass came into practice is not now certainly known. The Roman windows were filled with a semitransparent substance, called lapis specularis, a fossil of the class of talcs, which readily splits into thin smooth laminae or plates. This substance is found in masses of ten or twelve inches in breadth, and three in thickness; and, when sliced, very much resembles horn, instead of which it is to this day often employed by lantern makers. The Romans were chiefly supplied with this article from the island of Cyprus, where it abounds. So good a substitute for glass is it said to have been, that, besides being employed for the admission of light into the Roman houses, it was also used in the construction of hot-houses, for raising and protecting delicate plants; so that, by using it, the Emperor Tiberius had cucumbers at his table throughout the whole year.

There is no positive mention of the use of glass for windows before the time of Lactantius, at the close of the third century. But the passage in that writer which records the fact (De Opif. Dei, cap. 8), also shows that the lapis specularis still retained its place. Glass windows are distinctly mentioned by St Jerome, as being in use in his time (a. d. 422). A century later, the windows of the church of St Sophia at Constantinople are represented by Paulus Silentiarius as being filled with glass. After this period we meet with frequent mention of them. Joannes Philippinus (a. d. 630) states that glass was fastened into the windows with plaster.

The Venerable Bede asserts that glass windows were first introduced into England in the year 674, by the Abbot Benedict, who brought over artificers skilled in the art of making window-glass, to glaze the church and monastery of Wearmouth. Other authorities attribute the introduction of this luxury to Bishop Wilfred junior, who died in 711. It would thus appear, that glass windows were first introduced into England either about the end of the seventh or the beginning of the eighth century. The use of window-glass, however, was then, and for many centuries afterwards, confined entirely to buildings appropriated to religious purposes; but in the fourteenth century it was so much in demand, though still confined to sacred edifices and ornamental purposes, that glazing had become a regular trade. This appears from a contract entered into by the church authorities of York Cathedral in 1338, with a glazier to glaze the west windows of that structure; a piece of work which he undertook to perform at the rate of sixpence per foot for white glass, and one shilling per foot for coloured. Glass windows, however, did not become common in England till the close of the twelfth century. Until this period they were rarely to be found in private houses, and were deemed a great luxury, and a token of great magnificence. The windows of the houses were till then filled with oiled paper, or wooden lattices. In cathedrals, these and sheets of linen supplied the place of glass till the eighth century; in meaner edifices lattices continued in use till the eighteenth.

lhe glass of the Venetians was superior to any made elsewhere, and for many years commanded the market of nearly all Europe, lheir most extensive glass-works were established at Murano, a small village in the neighbourhood of Venice l but the produce was always recognised by the name of Venetian glass. Baron von Lowhen, in his Analysis of Nobility in its Origin, states that, “so useful were the glass-makers at one period in Venice, and so great the revenue accruing to the republic from their manufacture, that, to encourage the men engaged in it to remain in Murano, the senate made them all burgesses of Venice, and allowed nobles to marry their daughters; whereas, if a nobleman marries the daughter of any other tradesman, the issue were not reputed noble.”

The skill of the Venetians in glass-making was especially remarkable in the excellence of their mirrors. Beckmann, who has minutely investigated the subject, is of opinion that the manufacture of glass mirrors certainly was attempted, but not with complete success, in Sidon, at a very early period; but that they fell into disuse, and were almost forgotten until the thirteenth century. Previously to this period, plates of polished metal were used at the toilette; and in the rudeness of the first ideas which suggested the substitution of glass, the plates were made of a deep black colour to imitate them. Black foil even was laid behind them, to increase their opacity. The metal mirrors, however, remained in use long after the introduction of their fragile rivals, but at length they wholly disappeared; a result effected chiefly by the skill of the Venetians, who improved their manufacture to such a degree that they speedily acquired a celebrity which secured an immense sale for them throughout all Europe.

From Italy the art of glass-making found its way into France, where an attempt was made, in the year 1634, to rival the Venetians in the manufacture of mirrors. The first essay was unsuccessful, but another, made in 1665, under the patronage of the celebrated Colbert, in which French workmen, who had acquired a knowledge of the art at Murano, were employed, had better fortune. But a few years afterwards, this establishment, which was situated in the village of Tourlaville, near Cherbourg, in Lower Normandy, was also threatened with ruin by a discovery, or rather improvement, in the art of glass-making, effected by one Abraham Thevart. This improvement consisted in casting plates of much larger dimensions than it had hitherto been deemed possible to do. Thevart’s first plates were, cast at Paris, and astonished every artist by their magnitude. They were eighty-four inches long and fifty inches wide, whereas none previously made exceeded forty-five or fifty inches in length. Thevart was bound by his patent to make all his plates at least sixty inches in length and forty in breadth. In 1695 the two companies, Thevart’s and that at Tourlaville, united their interest, but were so unsuccessful, that, in 1701, they were unable to pay their debts, and were, in consequence, compelled to discharge most of the workmen, and abandon several of their furnaces. Next year, however, a company was formed, under the management of Antoine d’Agincourt, who re-engaged the discharged workmen; and the works realised considerable profits tothe proprietors, a circumstance which is attributed wholly to the prudent management of D’Agincourt/

Early in the fourteenth century the French government made a concession in favour of glass-making, by decreeing, that not only should no derogation from nobility follow the practice of the art, but that none, save gentlemen, or the sons of noblemen, should venture to engage in any of its branches, even as working artisans. This limitation was accompanied by a grant of a royal charter of incorporation, conveying important privileges, under which the occupation became eventually a source of great wealth to several families of distinction.

It has been said that the manufacturing of windowglass was first introduced into England in the year 1557. But a contract, quoted by Horace Walpole, in his Anecdotes of Painting, proves that this article was made in England upwards of a century before that period. This curious [7:10:565]document is dated in 1439, and bears to be a contract between the Countess of Warwick and John Prudde of Westminster, glazier, whom she employed, with other tradesmen, to erect and embellish a magnificent tomb for the earl, her husband. John Prudde is thereby bound to use “no glass of England, but glass from beyond seas a stipulation which, besides showing that the art of making window-glass was known and practised in England in the fifteenth century, seems also to indicate that it was inferior to what could be obtained from abroad. The finer sort of window-glass was made at Crutched Friars, London, in 1557. The first flint-glass made in England was manufactured at the Savoy House, in the Strand; and the first plate-glass, for looking-glasses, coach windows, and similar purposes, was made at Lambeth, by Venetian workmen, brought over in 1670 by the Duke of Buckingham.

The art of glass-making was introduced into Scotland in the reign of James VI. An exclusive right to manufacture it within the kingdom, for the space of thirty-one years, was granted by that monarch, to Lord George Hay, in the year 1610. This right his lordship transferred in 1627, for a considerable sum, to Thomas Robinson, merchant-tailor in London, who again disposed of it for L.250, to Sir Robert Mansell, vice-admiral of England. The first manufactory of glass in Scotland, an extremely rude one, was established at Wemyss in Fife. Regular works were afterwards commenced at Prestonpans, Leith, and Dumbarton. Crown-glass is now manufactured at Warrington, St Helens, Eccleston, Old Swan, and Newton, Lancashire; at Birmingham, Hunslet near Leeds, and Bristol. It is also manufactured of excellent quality on the Tyne and Wear. Great improvements have recently been made in the manufacture of crown-glass; and we believe this article, as made in England, is superior in quality to that of any other nation.

There is good reason for supposing that the art of staining or painting glass is coeval with the art of making glass itself, since, as has been noticed above, it is difficult to make it without colour. The possibility of subjecting this propensity to the will of the manufacturer must have very obviously presented itself, although it certainly requires much art and chemical knowledge to produce perfect specimens of this description of manufacture. This perfection seems, however, to have been attained at a very early period. It is certain that the art was known in Egypt at least 3000 years ago. We have already mentioned the beautiful imitations of precious stones, found adorning mummies which are known to have existed for that time. We meet with frequent mention of specimens of Eastern workmanship of consummate beauty, upon which great value was placed. The Emperor Hadrian, whilst at Alexandria, was presented by an Egyptian priest, with two glass cups, which sparkled with colours of every kind, and which had been used in the service of the temple. These he prized so highly that he ordered that they should be produced only on festivals and great solemnities. The works of Cayius and Winkelmann furnish some striking instances of ancient skill in the formation of pictures by means of delicate glass fibres of various hues, which, after being fitted together with the utmost nicety, were conglutinated by fusion into a solid mass. The art of combining the various colours, so as to produce pictures, such as is now practised, is comparatively of recent date. The earliest specimens of this kind of work discover a factitious joining of different pieces of glass, differently tinged, and so arranged as, by a species of mosaic work, to produce the figure or figures wanted. The various pieces are held together generally by a vein of lead, run upon the back of the picture, precisely at their junction.

But modern ingenuity has superseded this clumsy expedient; and every colour used in painting can now be introduced into one entire sheet. For a long period the pictured glass used in cathedrals was merely painted on the surface, the art of incorporating the colours with the glass by fusion, the method now practised, being unknown till the beginning of the fifteenth century. This great improvement is ascribed to a painter of Marseilles, who went to Rome during the pontificate of Julius II.; but his discovery went no further than the producing of different colours on different pieces of glass, and having them afterwards united in the old fashion. This art was, at a lat⅛r period, greatly improved by Albert Durer and Lucas of Leyden, the latter of whom brought it nearly to the state in which it is now practised.

The first painted glass executed in England was in the time of King John; previously to this, all stained or painted glass was imported from Italy. The next notice of it occurs in the reign of Henry III. The treasurer of that monarch orders that there be painted, on three glass windows in the chapel of St John, a little Virgin Mary holding the child, and the Trinity, and Saint John the Apostle. Some time after, he issues another mandate for two painted windows in the hall.

Even at this early period, however, England boasted of eminent native artists in glass painting, amongst the first of whom was John Thornton, glazier of Coventry. This person was employed, in the time of Henry IV., by the dean and chapter of York cathedral, to paint the eastern window of that splendid edifice; and for the beautiful and masterly workmanship which he exhibited in this specimen of his skill, he received four shillings per week of regular wages. He was bound to finish the work in less than three years, and to receive, over and above the weekly allowance, one hundred shillings for each year; and if the work was done to the satisfaction of his employers, he was to receive, on its completion, a further sum of L.10.

From this period downwards there have been many skilful native artists, although the Reformation greatly impeded the progress of the art, by banishing the ungodly ostentation of ornamented windows from churches; indeed, so serious was this interruption, that the art had nearly altogether disappeared in the time of Elizabeth. Amongst the most eminent glass painters who first appeared upon the revival of the art, were Isaac Oliver, born in 1616, and William Price, who lived about the close of the seventeenth century. This last person was for many years the only glass painter in England. He is said to have discovered, what is to this day a desideratum in the art of glass staining, the secret of producing a rich, clear, bright, and transparent red, the most difficult to strike, and the most expensive of all the colours employed in glass painting. But Price having died soon after making this discovery, his secret died with him. This artist was succeeded by a person at Birmingham, who, in 1757, fitted up a window for Lord Lyttleton, in the church of Hajely. To him succeeded one Peckit of York, who attained considerable eminence in the art.

During all this time, however, and indeed until a comparatively recent date, painted glass was regarded as too costly and too magnificent an article to be otherwise employed than in decorating religious edifices or the palaces of nobles; and even in the latter case it was but sparingly used. Modern improvement has now placed this beautiful ornament within the reach of very ordinary circumstances; and, when this is considered, it must excite a strong feeling of surprise to find how little so elegant a luxury is even yet in demand. The art of staining glass is now practised with very great success. The colours of modern artists, we venture to allege, notwithstanding what is often urged to the contrary, equal in variety and richness those of the ancients; and, with the superior knowledge which we now possess of the principles of drawing[7:10:566], and of bringing several colours together on a single sheet, encouragement alone is wanting to attract artists ' of talent and inventive genius to the pursuit of the art, and to carry it to a greater height of excellence than it has ever reached in the hands of their predecessors.

MANUFACTURE OF CROWN-GLASS.

In order to secure success to his operations, the glass manufacturer must bestow the utmost care upon the erection of his furnaces. They must be well and substantially built, of the best materials, of the most approved construction, and under the direction of a builder of tried skill and extensive experience. A false economy in these respects cannot fail of leading to the most ruinous results.

Crown-glass is the best kind of glass now employed in the glazing of windows, and is so called to distinguish it from the common, broad, or spread glass, which was in use before the introduction of crown-glass, but which, on account of its inferior quality, is now rarely used. In the manufacture of crown-glass the following furnaces and arches are required, viz. calcar arch, main furnace, bottoming hole, flashing furnace, nose hole, and annealing kiln.

A Calcar Arch for burning frit is a common reverberatory furnace, and is about ten feet long, seven feet wide, and two feet high. A building ground plan and elevation of one is shown in Plate CCLXVI. figs. 4 and 5. The crown and sides are built of fire-brick, and the other parts of common brick. The bottom should be carefully joined and cemented, as the salt is apt to ooze through it.

The Main or Glass-making Furnace is an oblong square, built in the centre of a brick cone, large enough to contain within it two or three pots at each side of the grateroom, which is either divided, as shown in the plan, or runs the whole length of the furnace, as the manufacturer likes. Plate CCLXVI. fig. 3 is a ground plan, fig. 2 is an end elevation, and fig. 1 a front elevation, of a six-pot furnace. 1, 2, 3, fig. 1, are the working holes for the purpose of ventilation, for putting in the materials, and for taking out the metal to be wrought. 4, 5, 6, 7 are pipe holes for warming the pipes before beginning to work with them. 8, 9, 10 are foot-holes for mending the pots and sieges. 11 is a bar of iron for binding the furnace, and keeping it from swelling.

The arch, as shown in fig. 2, is of an elliptic form. A barrel arch, that is, an arch shaped like the half of a barrel cut longwise through the centre, is sometimes used. But this soon gives way when used in the manufacture of crown-glass, although it does very well in the clay-furnace for bottle-houses.

The best stone for building furnaces is fire stone from Coxgreen, in the neighbourhood of Newcastle. Its quality is a close gree, and it contains a greater quantity of talc than the common fire stone, which seems to be the chief reason of its resisting the fire better. The great danger in building furnaces is, lest the cement at the top should give way with the excessive heat, and, by dropping into the pots, spoil the metal. The top should therefore be built with stones only, as loose as they can hold together after the centres are removed, and without any cement whatever. The stones expand and come quite close together when annealing; an operation which takes from eight to fourteen days at most. There is thus less risk of any thing dropping from the roof of the furnace.

The inside of the square of the furnace is built either of Stourbridge fire-clay annealed, or the Newcastle fire-stone, to the thickness of sixteen inches. The outside is built of common brick about nine inches in thickness.

The furnace is thrown over an ash-pit, or cave as it is called, which admits the atmospheric air, and promotes the combustion of the furnace. This cave is built of stone until it comes beneath the grate-room, when it is formed of fire brick. The abutments are useful for binding and keeping the furnace together, and are built of masonry. The furnaces are stouoy clasped with iron all round, to keep them tight. In four-pot furnaces this is unnecessary, provided there be four good abutments.

Bottoming Hole. Plate CCLXVII. figs. 3 and 4. Elevation and ground plan. The interior is of common firebrick, the mouth either of common fire-brick or Stourbridge clay, and the outside entirely of common brick.

Flashing Furnace. Figs. 5 and 7. Elevation and ground plan. The outside is built of common brick, the inside of fire-brick, and the mouth or nose of Stourbridge fire-clay.

Nose Hole. Fig. 6. Elevation. This is a small aperture off the flashing furnace, and of the same materials. Fig. 7, ground plan.

Annealing Kiln. Figs. 8 and 9. It is built of common brick, except around the grate-room, where fire-brick is used.

The materials of which crown-glass is usually composed are kelp and fine white sand. Pearl ashes, or certain other alkalis, sometimes supply the place of the former of these substances. The quality of kelp is extremely various. That from Orkney is superior to what is made in Ireland, the Hebrides, or the lower parts of Scotland. It is found to contain less alkali, and to produce glass of a better colour.^[1. Some eminent chemists assert that, although the usual quantity of kelp be added in the manufacture of glass, the weight of the glass produced is nothing more than the original weight of the sand. But this is not the case with the Orkney kelp, for though it has less alkali, it contains more insoluble matter than the west Highland kelp, and of course produces a larger quantity of glass. The west Highland yields glass of very inferior colour to that procured by the use of Orkney kelp. ] For the glass-maker’s purposes the kelp of the Orkneys is decidedly the best. It is freer from sulphur than the others, the presence of which makes the glass green, crude, and fretful. The following is the course pursued in the preparation of kelp. The fuci are cut from the rocks in the months of May, June, and July. They are then brought to the shore, and, after being spread out and dried, are thrown into a pit lined with stones, in which a large fire of peat has been previously kindled. On this fire the weed is heaped from time to time, until a large mass is accumulated, and the whole is reduced to a state of fusion. It is then well mixed and levelled, and allowed to cool. When sufficiently cold, it is taken from the pit, and broken into portable masses, for the convenience of transportation. To prevent the dissipation of the alkali, a thing very apt to occur, the greatest care is necessary in every part of this process; in the gathering and drying, as well as in the burning of the fuci; in the treatment of the mass whilst in a state of fusion; and in its exposure to the atmosphere during these operations. Kelp burners are but too frequently guilty of carelessness in this respect. In some places they burn the fuci in pits which are not lined with stones, and, of consequence, sand and earthy substances mingle with the fused mass.^[2. The best mode of preparing kelp, as invented by Colonel Fullarton,' is by burning it in a reverberatory furnace, and throw, ing it down in the form of cakes, in the same manner as frit, which we shall afterwards have occasion to describe. When so prepared, it is more fit for the glass manufacturer, being free of extraneous matter. This method is now employed by extensive makers of kelp in Ireland. ] It is no uncommon thing for the makers to increase the weight of kelp intentionally, foolishly thinking to procure a high-cr[7:10:567] price for it by so doing. Such adulteration is, however, at once detected by the kelp merchant, and the article, which might otherwise have brought a good price, is reduced to less than a third of its value. The inferiority of the Lowland kelp to that of Orkney and the .Hebrides, may with safety be attributed as much to this practice as to the inferiority of the fuci. Some idea, but at best a very uncertain one, of the quality of kelp may be formed by the examination of its external appearance. A chemical analysis of its properties can alone give security to the manufacturer. In preparing it for the manufacture of glass, it is first broken into small pieces, either by the hand or by a machine called a stamper. It is then put into a mill and ground into a fine powder, stones and all other extraneous matter being picked out. The powder is afterwards passed through brass wire sieves.

With regard to the other component part of windowglass, namely, sand, that of the best description is procured from Lynn Regis, in Norfolk. That procured from Alum Bay, on the western coast of the Isle of Wight, is also of excellent quality. The superiority of this sand arises from the circumstance of its containing a greater quantity of minute transparent crystals than is found in the sand of any other place in the country. In preparing the sand, it is usually washed in a large vat with boiling or cold water, until the water runs off quite clear. The sand is then put into a calcining arch, where it is subjected to a strong heat for twenty-four hours. During this time it is kept red hot, and immediately on being taken out is plunged into pure cold water. This has the effect of dividing the particles of sand, and making it unite more readily with the alkali during the process of calcining. Some use nitre during this process, which consumes any sulphureous matter that may be present, or extraneous substances of an animal or vegetable nature, and reduces them to an earth not injurious to glass. When this operation is completed, it is removed into the mixing room, where the proportions of material are adjusted and mingled together, previously to their being fritted or calcined. Here the materials, the sand and the kelp powder, are carefully proportioned, generally in the degree of eleven of kelp to seven of sand, some manufacturers using eleven to eight, which are mixed up according to the judgment of the mixer. The majority of glass manufacturers are now giving up the use of kelp. Within the last few years the improvements in the manufacture of carbonate of soda have been very great, while it has also fallen considerably in price. Instead, therefore, of using such an impure alkali as kelp with sand, carbonate of soda with sand and lime is employed, which gives glass of as good a colour as plate, and is attended with many other advantages which the old materials do not possess. Manufacturers, instead of kelp, purchase sulphur, and with it make sulphuric acid. With sulphuric acid and muriate of soda they make sulphate of soda, to which lime, coal, &c. are added, and thus produce carbonate of soda, which, with sand and lime, is made into glass. The operations for preparing these materials are carried on within their own premises by several extensive glass manufacturers. The following mixture has been found to produce an excellent quality of glass :—3 cwts. Lynn sand; 2∣ ditto carbonate of soda; 14 lbs. nitre; 14 ditto lime; 7 ditto charcoal; one fourth of the above weight of cullet.

This mixture will make a very excellent glass when the furnace is kept at a proper heat. The proportions must, of course, be regulated in some degree by the heat which the furnace attains. The addition of any other ingredient will injure the quality and colour of the glass. It may be either fritted or not before being put into the pots. The use of this mixture saves coals, time, and wages, as the founding occupies from sixteen to twenty hours only, whilst in other cases the time occupied by this process is from twenty to twenty-four hours. It can also be blown to a thinner and finer substance, and is thus liable to a less duty. When the sand and kelp are thoroughly mixed, the compost is put into a calcining arch or reverberatory furnace, where it is subjected to a heat so strong as to reduce it to a semifluid state. Whilst in this state, it is stirred without intermission, to prevent the formation of knots containing more sand than the rest of the batch, an effect resulting from the dissipation of the alkali by excess of heat. The process of calcining requires more or less time according to the varying properties of the ingredients composing the batch. From three to four hours is the time usually occupied by each batch. The frit, as the substance is now called, is taken from the furnace, spread upon a plate of iron whilst yet hot, and, before it becomes quite cold, divided into large cakes. In the opinion of many, it cannot be too old for use; as when new the glass made from it is full of what are called seeds. It is commonly kept about six months by opulent manufacturers. The last operation consists in throwing the frit into the melting pots, which are of the form represented in Plate CCLXVIII. fig. 2.

To prevent stones or clay from the furnace falling into the pot, those used in making flint-glass are always covered in on the top; and the same thing has been tried in crown pots, made with two openings, one in the front and one in the back, the back one to be plugged up when beginning to work from the front of the pot. This method succeeded very well, but was abandoned from the length of time it required; a circumstance which more than counterbalanced its advantages.

These pots or crucibles are made of the finest clay. That from Stourbridge in Worcestershire is considered the best. Great care is requisite in the selection, and in cleansing it from extraneous particles, the presence of which, even in the smallest degree, will injure the pot. A fine powder procured by grinding old crucibles is generally mixed, in a proportion seldom larger than a fourth, with what řs termed the virgin clay. This mixture dries more rapidly, contracts less while drying, and presents a firmer resistance to the action of the fire and alkali used in the composition of glass than the mere unmixed clay. These ingredients having been mixed, they are wrought into a paste in a large trough, and carried to the pot loft, covered in such a way as to exclude dust and other minute particles. Here a workman kneads this paste by tramping it with his naked feet, turning it from time to time until it becomes as tough as putty. It is then made into rolls, and wrought, layer upon layer, into a solid and compact body, every care being taken to keep it free of vacuities, as latent air would, by its expansion in the furnace, cause an immediate rupture of the pots.

After pots are made, very great care is necessary to bring them to the proper state of dryness before taking them to the annealing or pot arch. In drying they commonly shrink about two inches in the circumference. When pots are made during summer, the natural tem∣>crature is sufficient. In winter they are kept in a temperature of from fifty to fifty-five degrees Fahrenheit. They remain in the room where they are made for a period varying from nine to twelve months. Being afterwards removed to another apartment, where the heat is from eighty to ninety degrees Fahrenheit, they are kept there for about four weeks. They are then removed for four or five days, more or less, according to their previous state of dryness, to the annealing arch (of which figs. 1 and 2, Plate CCLXVII. is an elevation and ground plan), which is gradually and cautiously heated up till it reaches the temperature of the working furnace, whither, after being sufficiently annealed, they are carried as quickly as possible. Pots last upon an average from eight to ten weeks. Their value is usually estimated [7:10:568] at L.8 or L.10 each. A section of a crown-glass pot, showing their usual dimensions, is given at Plate CCLXVIΠ. fig· 2 ∙

To the frit thrown into these pots there is added a proportion, about an eighth, of cullet or broken crown-glass. After this has been done, the furnace is raised to the highest possible degree of heat. The pots are filled every third hour or so, according as the frit melts, till they are completely full. The intensity of the heat is then increased, if possible, till the metal, as it is now called, is reduced to fine liquid glass, which is then ready for the operations of the workman.^[3. The tanăher or glass gall is removed while the furnace is at its extreme degree of heat. ] From twenty-four to thirty hours in all are required for this process, which is called founding.'^[4. A piece of wood about eleven inches long by seven broad, with a hole three inches by one inch, forms an excellent protection to the eyes from the heat to which they are exposed when examining the metal in the pots. ]

The furnace is slackened for about two hours, and the metal being now in a workable state, the first operator who approaches the furnace in which the pot of liquid glass is placed is the »kimmer, who skims off all crude or extraneous substances from the surface of the metal. Next follows the gatherer, who is provided with an iron pipe or tube, six or seven feet in length, and of this shape.

Having previously heated that end of the tube which takes up the glass, he dips it into the pot of metal; and by turning it gently round, gathers about one and a half pound of liquid glass on the end of it. Having allowed this to c∞l for a little, he again dips it into the pot, and gathers an additional quantity, of from two and a half to three pounds. This is also permitted to cool as before, when the operation of dipping is again repeated, and a sufficient quantity of metal, from nine to ten pounds weight, is gathered, to form what is technically called a table or sheet of glass. The rod, thus loaded, is held for a few seconds in a perpendicular position, that the metal may distribute itself equally on all sides, and that it may, by its own weight, be lengthened out beyond the rod. The operator then moulds the metal into a regular form, by rolling it on a smooth iron plate, called the “marver,” a term corrupted from the French word marbre.

He then blows strongly through the tube, when his breath penetrating the red-hot mass of glass, causes it to swell out into a hollow pear-shaped vessel, thus:

The tube with the elongated sphere of glass at the end of it is then handed to the blower, who heats it a second and third time at the furnace, pressing the end, between each blowing, against the bullion bar, so called from the part thus pressed forming the centre of the sheet or bull’s eye, thus:

By the dexterous management of this operation, the glass is brought into a somewhat spherical form.

The blower heats a third time at the bottoming hole, and blows the metal into a full-sized globe thus:

When this part of the process has been completed, and the glass has been allowed to cool a little, it is rested on the casher box, and an iron rod, called a “pontiľ or punty rod, i on which a little hot metal has been previously gathered, is applied to the flattened side, exactly opposite the tube, which is detached by touching it with a piece of iron, dipped beforehand in cold water, leaving a circular hole in the glass of about two inches diameter. The operation of attaching the punty is shown by the following plate.

Taking, hold of the punty rod, the workman presents the glass to another part of the furnace called the “nose hole,” [7:10:569]where the aperture made by its separation from the tube is now presented and kept until it has become sufficiently ductile to fit it for the operation of the flashing furnace. Whilst here, it is turned dexterously round, slowly at first, and afterwards with increasing rapidity; and the glass yielding to the centrifugal force, the aperture just mentioned becomes enlarced.

The workman, taking great care to preserve, by a regular motion, the circular figure of the glass, proceeds to whirl it round with increasing velocity, until the aperture suddenly flies open with a loud ruffling noise, which has been aptly compared to the unfurling of a flag in a strong breeze; and the glass becomes a circular plane or sheet, of from four to four and a half feet diameter, of equal thickness throughout, except at the point called the bullion or bull’s eye, where it is attached to the iron rod. The following figure will give some idea of this very beautiful part of the process.

wi⅛ h . mSret⅛e"y∖S l Ω ,n < ’ ⅛ i " "'°' cd ™ uod - to i - f -∙ k ' -i⅛S Ŵ£

nealing arch, where it is rested on a bed of sand, and detached from the punty rod by a shears. The sheet or tablé is then lifted on a wide pronged fork, called a faucet, and put into the arch to be tempered, where it is ranged with many others set up edgewise, and supported by iron frames to prevent their bending. From four to six hundred tables are placed in one kiln.

A sketch of the interior of a crown-glass house, during the progress of these operations, has been given in Plate CCLXVIII. fig. 1.

The kiln having been clayed up, the fire is permitted to die out, and the heat diminished as gradually as possible. When the glass is properly annealed, and sufficiently cold to admit of its being handled, it is withdrawn from the oven, after the removal of the wall built into the front of the arch, and is then quite ready for the glazier’s use. It is first, however, removed to the manufacturer’s warehouse, where the circular sheets are cut into halves, and assorted into the different qualities, known to the tradesmen by the names of seconds, thirds, and fourths.

We conclude our remarks on the manufacture by observing, that the quality of glass does not depend upon the mixtures alone, but also upon the treatment it receives after it has been made, the quality of the coals, and management of the furnaces. Cleanliness in every department of the manufacture, a general knowledge of chemistry, and of the art in all its details, with the most unremitting industry, and skill in the direction and government of the operatives, are all essentially necessary for the production of good glass.

MANUFACTURE OF BRITISH SHEET-GLASS.

This article is only manufactured by Messrs Chances and Hartleys, of West Bromwich, near Birmingham, who, after having visited the glass manufactories of France, Belgium, and Germany, commenced, in 1832, the making of British sheet. The principle upon which it is manufactured is the same as that acted upon in the making of common or green glass·. The metal is formed into cylinders, and then flattened into sheets. The French, Belgians, and Germans, having pursued this system for the last fifty years in making their window-glass, have much improved the old mode of making it; and-as the parties who are now manufacturing this article in England are crown-glass makers, and have imported all the improvements adopted in the making of sheet-glass in France, Belgium, and Germany, and combined with these the improvements which their experience as crown-glass makers had taught them during the same period, they have surpassed the French, Belgians, and Germans in sheet-glass, and can now compete with them in all parts of the world.

There is no crown-glass made in France; and their window-glass, though superior to our broad or common glass, is not equal to the British sheet. In Germany there is little crown-glass made, and that of a very inferior quality. The greater part of the glass made in that country is sheet, and it is of much better quality than the French or Belgian. In Germany a common sort of glass is made, in the following manner: Three or four workmen form a partnership, and, having fixed upon a place in the woods where clay and sand are easily met with, they proceed to build a glass-house with wood and clay. They then make the pots, and, from the ashes of the wood which they burn, obtain potash, which, after it has been mixed with sand, they melt into glass. They blow the metal into cylinders, flatten it into sheets, cut, pack, in short, perform the whole operations from first to last, themselves. A good deal of the best of the glass made in this manner is sent to Nuremburg, where it is polished and sent into Holland. Some of it is sometimes smuggled into this country, and is known by the designation of Dutch glass.
[7:10:570]

The expense of making British sheet is about the same t as making crown-glass, excepting in the case os large squares, when it is much less; in crown-glass it is very difficult to get a square 34 × 22, but in sheet-glass the common size is 40 × 30; nay, sheets are sometimes made as large as 50 × 36. Its other advantage over crown is, that it has none of that wavy or curved appearance, by which the vision is so much distorted in crown-glass; but, at the same time, sheet-glass has rather an unpleasant appearance when viewed from the outside of a building, in consequence of an unevenness of surface, technically termed cockled; when viewed, however, from the inside, it is difficult to distinguish it from plate-glass.

The materials employed in the making of sheet-glass are the same as those used in the making of crown-glass. The large melting furnace is also very similar; in France and Belgium they usually contain six or eight pots, but at the manufactory at West Bromwich the furnace contains ten pots, each containing seven cwts. of metal, which requires fourteen hours to melt.

In a line with each pot, and four feet from the ground, are erected ten stages, with an open space between each, of about two feet, through which the workman swings his glass when making cylinders. When the metal is ready for working, the ten workmen take their stations, each having his own pot and stage, and also an assistant, and commence making the cylinders, as follows: After gathering the quantity of metal required (which varies from three to twenty pounds), the workman places it in a horizontal position upon a wooden block, which has been hollowed, so that, when the workman turns the metal, it shall form it into a solid cylindrical mass. In the mean time, the assistant, with a sponge in his hand, and a bucket of water by his side, lets a fine stream of water run into the block, which keeps the wood from burning, and also gives a brilliancy to the surface of the glass. The water, the moment it comes in contact with the glass, is raised to the boiling point, and, in that state, does no injury to the metal; but it is only when the metal is at a high temperature that such is the case; for, whenever the glass is cooled to a certain degree, it immediately cracks upon coming in contact with water. When the workman perceives that the mass of metal is sufficiently formed and cooled, he raises the pipe to his mouth at an angle of about seventy-five degrees, and commences blowing it, at the same time continuing to turn it in the wood block, till he perceives the diameter to be of the requisite dimensions, which are usually about ten inches. He then reheats this cylindrical mass, and, when it is sufficiently softened, commences swinging it over his head, continuing to reheat and swing till he has made it the desired length, which is commonly about forty inches. It is now in a cylindrical form, forty inches long and ten inches in diameter, one end being closed, and the other having the pipe attached to it. The workman now begins to open the end which is closed, for which purpose he encloses the air in the cylinder, by stopping the aperture of the pipe with his finger; and then placing the closed end of the cylinder towards the fire, it becomes softened, while at the same time the air within is expanding, and, in about thirty seconds, the glass becomes too soft to retain it, and bursts, a small aperture being formed at the point of the cylinder. The workman then turns the cylinder round very quickly, and, by keeping it warm at the same time, flashes it out perfectly straight; the other end, which is attached to the pipe, has now to be cut off. This is done in the following manner: The workman having gathered a small quantity of metal on the pontil, draws it out into a thread of about one eighth of an inch in diameter, laps it round the pipe end of the cylinder, and, after letting it remain there for about five seconds, withdraws it suddenly, and immediately applies a cold iron to the heated part, which occasions such a sudden contraction, that it cracks off where the hot string of glass has been placed round it. The workman having now formed a perfect cylinder of forty inches in length and ten in diameter, has, before it can be flattened, to split it on one side, so that it can be opened out; but before doing this, he is obliged to let it cool, and then, laying the cylinder horizontally upon a bench, draws a red hot iron two or three times along the inner surface. The cylinder, thus heated, immediately splits along the heated part, owing to the expansion of the glass when heated, its cylindrical form preventing its breaking at the point of expansion.

The blower having now completed his cylinder, hands it over to the flattener to make it into a flat sheet; to accomplish which, two furnaces are built together, the one for flattening the cylinders, the other for annealing the sheets, the former being kept at a much higher temperature than the latter. The cylinder, after being gradually reheated, is placed in the centre of the flattening furnace, upon a smooth stone, with the cracked side upwards. In a short time it becomes softened with the heat, and by its own weight falls out into a flat square sheet of forty inches by thirty. The flattener, with a piece of charred wood, rubs it quite smooth, and then places it in the annealing arch, where it remains about three days to be annealed. A workman will make sixty cylinders 40 × 50 in one day; and a flattener can flatten the same number in the same time. This glass can be made of any thickness from one twentieth to half an inch; but, in consequence of the heavy duty in this country, it is seldom made more than one twelfth of an inch in thickness, crown-glass being about one sixteenth.

The same enterprising company also manufacture large oval and square British shades, used for covering French clocks and other ornaments, and commonly called French shades. We believe they are the only persons in this country who do so, as, till within the last two years, we have been supplied entirely from France. From their being made in England, and greatly decreased in price (being fifty per cent, cheaper than when imported from France), this beautiful ornament is now becoming very generally used.

There is another species of glass called broad or common ¼'indow-glass, which is formed of the coarsest materials. The ingredients usually employed are, six measures of soap-boilers’ waste, three of kelp, and three or four of sand. After these have been fritted for from twenty to thirty hours, they are removed while red hot to the pots in the working furnace, where, in the space of from twelve to fifteen hours, they are reduced to a fluid state. The metal is taken out in the manner already described, and blown into globes about a foot in diameter. A piece of iron dipped in cold water is run along them, and produces a crack nearly rectilinear; and, while yet warm and ductile, these spheres are opened out and flattened on a smooth iron plate at the mouth of the furnace.

MANUFACTURE OF PLATE-GLASS.

This description of glass may be manufactured in the same manner as broad window-glass, or by casting the materials in a state of fusion upon a flat surface. Little correct information has been published relative to the manufacture of plate-glass, from the reluctance of proprietors to permit their works to be examined by individuals capable of giving an intelligible account of them. If such are permitted to scan the mysteries, they are generally restricted to keep secret the information which they have acquired. The late Mr Parks the chemist, however, seems to have been exempted from this condition, and having visited the premises of the British Plate-Glass Company, at Ravenhead, in Lancashire, he has published a short account of the process as there carried on. Besides the [7:10:571]above manufactory, there are other two in Great Britain; Messrs Cookson and Co.’s, South Shields, and W. A. A. West’s, Eccleston, near St Helen’s; and plate-glass is also made at the celebrated works at St Gobain in France, besides other places upon the Continent

The following is Loysel’s account of the relative proportions of the materials used at St Gobain, in the manufacture of plate-glass. White sand, 100 parts; carbonate of lime, 12 ditto; soda, 45 to 48 ditto; fragments of glass of like quality, 100 ditto; oxide of manganese, ⅜. The following proportions of ingredients are said to produce the best description of this article. Lynn sand which has been well washed and dried, 720 parts; alkaline salt containing 40 per cent, of soda, 450 ditto; lime slaked and sifted, 80 ditto; nitre, 25 ditto; broken plate-glass, 425 ditto. These quantities produce one pot of metal which yields 1200 pounds of glass.

Great nicety must be observed in conducting the processes of this manufacture. The materials must be selected with the utmost care. The sand should be of the whitest and finest description, and well washed and passed through a sieve, previously to being mixed with the other ingredients. Soda is always preferred to potash, because it imparts a higher degree of fluidity to the glass, and also because the impurities which it contains are more easily dissipated by the heat. Lime acts as a flux, and manganese has the effect of giving a slightly reddish hue to the mixture by which the colours of the other materials are neutralized, so that scarcely any appreciable tint remains. Cobalt is likewise used in some manufactories, much for the same purpose as manganese. The broken glass, or cullet as it is technically called, is those fragmentary portions which are cut from the plates when they are squared, or that which may flow over in the process of casting. The sand, lime, soda, and manganese, being properly mingled in the proportions above given, are fritted in small furnaces, where the temperature is gradually raised to a red or white heat, and there maintained until no more vapour is evolved, nor change undergone by the mixture. This process occupies six hours, and after its completion the other ingredients are added, consisting of cullet and cobalt. At StGobain there are twokindsofcrucibles employed; the one in which the glass is melted is called a pot, and has the shape of an inverted truncated cone; the other is entitled acwretfe,∙ it is kept empty in the furnace, and exposed to the full degree of heat. Forty hours are requisite to vitrify the materials properly, and bring the glass to a fit state for casting· The pots are skimmed in the manner already described. When the liquid mass has been properly refined, the cuvette is filled by a copper ladle, and after sufficient time is allowed for the bubbles created by this disturbance to escape, it is removed to the table where the plates are cast. Copper was the metal of which tables were formerly constructed; but cast iron has now been found to answer the purpose completely, and it is greatly superior to copper in this respect, that it remains uninjured during all the sudden transitions of temperature to which it must be subjected. The British Plate-Glass Company were the first to introduce this improvement. They procured a plate fifteen feet long nine feet wide, and six inches thick. The sides are provided with metallic ribs, the depth being exactlv the measure of thickness which it i s desired the glass should be of. During the casting there is a similar rib temporarily attached to the lower end of the table. The cuvette being filled with melted glass, it ⅛ withdrawn from the furnace by means of a crane, taken to the upper end of the casting table, and after being properly scummed, and elevated to a sufficient height by means of a crane, it is emptied of its contents. The surface of the melted matter is then smoothed by means of a large hollow copper cylinder, which extends across the table, resting upon the side ribs. This is set in motion, and rolled over the glass, by which process it is spread out into a sheet of uniform breadth and thickness. When the plate has become completely hardened, it is carefully inspected, to see that no flaws or bubbles appear on the surface. Should any be found, the sheet is immediately divided by cutting through them. It is afterwards removed to the annealing oven, where it is placed in a horizontal position, and remains for about fifteen days. When glass is in a high state of fluidity it is liable to be injured even by a draught of air, so that the apartment must be kept as free as possible from disturbance. The opening or shutting of a door, by setting the air in motion, might impair the value of the plate. After having been withdrawn from the annealing oven, they have to undergo the operations of squaring, grinding, polishing, and silvering, before they are fit for the market. These processes have thus been described by a late writer upon the subject.

“The first process—that of squaring and smoothing the edges—is performed by passing a rough diamond along the surface of the glass, guided by a square rule; the diamond cuts to a certain depth into the substance, when, by gently striking the glass with a small hammer underneath the part which is cut, the piece comes away; and the roughnesses of the edge then left are removed by pincers. The plate is then taken to the grinding apartment.

“The next step is to imbed each of the plates upon a table or frame adjusted horizontally, and made of either freestone or wood, cementing the glass securely thereto by plaster of Paris. One plate being then reversed and suspended over another, the material employed in grinding their surfaces is introduced between the two, and they are made to rub steadily and evenly upon each other by means of machinery set in motion by a steam-engine.” River sand and water were formerly used for the purpose of abrading the surface, but ground flint is now substituted, as answering the purpose better. When one side of each plate has been sufficiently ground, it is loosened from the frame, and turned over, so as to present the other surface to be ground in the same manner. Some degree of pressure is employed, by loading the upper plate with weights, as the grinding of each side approaches to completion. The process thus described used formerly to last during three entire days, but this time is now much abridged. The greatest attention is required in order to finish with the surfaces perfectly level and parallel, for which end a rule and plumb-line are employed.

By means of this grinding, the plates will have been made level; but they are too rough to receive a polish. To fit them for this, they must again be ground with emery powder of increasing degrees of fineness. The preparation and sorting of this powder are effected in the following simple and ingenious manner :—“A considerable quantity of emery is put into a vessel containing water, and is stirred about violently until the whole is mechanically mixed with the water. Emery is absolutely insoluble by such means; and if the mixture were left at rest during a sufficient time, the whole would subside in layers; the coarsest and heaviest particles sinking first, and so on successively, until the very finest particles would range themselves as the upper stratum. Previously to this, however, and while these finest grains are still suspended in the water, it is poured off into a separate vessel, and the emery is there allowed to settle. A fresh supply of water is poured into the first vessel, the contents of which are again violently agitated, and allowed partially to subside as before. A shorter interval is allowed for this than in the first case; and then the liquor is poured off into a third vessel, by which means emery of the second degree of fineness is separated. This operation is repeated in order to obtain powders having four different degrees of fineness. The deposits are then separately dried upon a stove to a consistence proper [7:10:572]for making them up into small balls, in which form they are delivered to the workmen.

“In this further rubbing together, or, as it is called, smoothing of the glass plates, it must be understood that the coarsest emery is first used, and so on, substituting powders having increasing degrees of fineness as the work proceeds.”^[5. Glass Manufacture, Lardner’s Cabinet Cyclopaedia., No. 26. ]

These processes finished, the glass, although perfectly even, appears opaque or deadened on the surface, and requires polishing. This is effected in the following manner. A piece of wood is covered with numerous folds of woollen cloth, the layers being divided by some carded wool interposed between each, the whole forming a tolerably hard but elastic cushion, which is furnished with a handle. The plate is laid upon a bed of plaster, as already described, and the cushion being wetted, is covered with the red oxide of iron (the colcothar of commerce), and moved backwards and forwards upon the surface of the plate. Lastly, if the glass be intended for mirrors, it is silvered, that is, covered on one side with a thin coating of mercury.

The process of blowing plate-glass differs so slightly from the methods used in producing broad glass that they need not be here repeated. Any difference that does exist, arises from the great bulk and weight of the mass of glass operated upon.

STAINED AND PAINTED GLASS.

The invention of the art of colouring glass, that is, imbuing it throughout with any particular tint, seems, as we have already had occasion to remark, to be nearly coeval with the discovery of the art of making the substance itself, Egyptian ornaments, and druidical beads, of the highest antiquity, as already mentioned in our historical notices, having been found of numerous and surprisingly beautiful colours. The art of staining and painting glass so as to form pictorial representations of objects, is, however, of comparatively recent date. The precise period of its introduction is unknown, but it is at least certain that it has existed for many centuries in different degrees of excellence, and that it has moreover been gradually advancing in improvement. The earliest record which we possess concerning the existence of this beautiful art is of the age of Pope Leo III., that is, about the year 800, a period which witnessed the erection of many of the most magnificent ecclesiastical edifices upon the Continent. Several centuries, however, elapsed before stained glass came into general use. The earliest specimens with which we are acquainted differ from those of more recent date, inasmuch as they are composed of small pieces of glass imbued throughout with colour during the process of manufacture, and to which artists give the name of pot metal.

These were united by joinings of lead after having been cut to the required shapes, and are thus made to form a species of Mosaic work; but, as might be expected, they do not possess that breadth of colour, or truth and delicacy of effect, which the improvements of more modern artists have enabled them to communicate to their works. The manufacture of stained glass was for many centuries confined to the Continent, and was nowhere prosecuted with so much success as at Venice. At the same period that the pointed style of architecture reached the meridian of its splendour in England, we find native artists who were fully qualified to do justice to the grandeur of the buildings they were called on to adorn, and not unworthy of being put in competition with their most successful foreign rivals. In proof of this we need only refer to the windows of York Minster, of the collegiate chapels and halls of Oxford, and those of the chapel of King’s College, Cambridge. A pretty general, but very erroneous, idea exists with regard to the superiority of the colours employed in ancient glass painting over those in use at the present day. It is believed by many, and taken for granted by others, that not only is the same brilliancy unattainable by modem skill, but that the art of producing the colours themselves is itself entirely lost. This is a very mistaken notion; for not only are those now employed as brilliant and durable as the colours of the ancients, but others have been added, which they most probably did not know how to produce, or at least did not use. Amongst these are pink, straw colour, and other compound tints. That indiscriminating veneration for the antique, which has so often and so seriously interfered with the claims of modern art to the encouragement to which it is entitled, has operated against glass painting and staining with perhaps fully more force than against any other art. It has induced a belief that excellence in the art is confined to the ancients; and that all the productions of the modern glass stainer and painter are mere flimsy imitations of those of the artists of the fourteenth and fifteenth centuries. Nothing farther is necessary to convince any one of the injustice and absurdity of this notion, than a visit to the warehouse or workshop of any respectable glass painter and Stainer of the present day.

The following observations are deserving of the attention both of artists and others less immediately interested in the subject. “Captivated with the vivid hues of the antique, persons have remarked, with more frequency than judgment, ‘ such colours certainly cannot be equalled in the present day.’ With the exception, perhaps, of its reference to the ancient ruby, which appears to have been lost for the past two centuries, this assertion may be altogether denied; and more than this, besides commanding all the resources of the old artists, ours of the present day are in possession of many practical advantages which the former had not, among which is that of their being able to lay every variety of colour on one and the same piece of glass, and thus often to execute with nicety in ten pieces a subject on which a hundred formerly would not have been so well bestowed. With these opportunities, and with greatly improved ideas upon the science of painting in general, our artists should be qualified for productions of a higher order than those of their ancient predecessors; and it remains, then, to be asked, on what account they fail to imitate their works with success. The chief occasion of this may be found in the imperfect acquaintance which the modern artists have with the principles of old English design, both pictorial and architectural. They seem to forget that these principles are totally distinct from those observed in the paintings of the later Italian masters, whose figures and groups are not unfrequently copied for the embellishment of our windows in the pointed style, but always without success. Strict simplicity, solemn dignity, and appropriate costume, are primary points for consideration in imitating the figures of our old windows. In the imitation also of those architectural decorations with which such figures were anciently surrounded, it is worse than puerile to suppose that mouldings and foliage borrowed from the Greek and Italian systems can by any possibility be so arranged as to bear a resemblance to the characteristic features of the old English style, a style as isolated as it is magnificent. Errors such as these can only be avoided by means of patient and attentive study, both of actual remains and of the numerous explanatory works now rendered generally accessible by the invaluable labour of able antiquaries and [7:10:573]draughtsmen. But after an adequate acquaintance with the principles of design is attained, those of execution are no less to be observed; all the subjects of the painting, and especially those of an architectural character, being relieved more by strong black outline and variety of colour than by the use of decided shadows. Modem imitations are frequently injured by displaying too much of modern art; the work, by being too highly finished, and that in few pieces, often loses the glittering effect that attends its more simple prototype. An additional precaution to be observed consists in the avoidance of excessive depth of colour; a consideration, the neglect of which has given to some modern imitations a vulgar, gaudy appearance, totally dissimilar to the equalised and subdued, yet lively colouring, observed in the best specimens of the middle ages."

The want of harmony here complained of is too often and offensively manifest in modern public buildings, which have been fitted up with stained-glass windows, and where the absurd incongruities of style have marred rather than increased the elegance of the structure. Florid and meaningless mouldings, scrolls and foliage, are seen disturbing the simple beauty of Grecian architecture, or the austere and majestic grandeur of a Gothic pile; so that people seem to have been careless as to the character or propriety of the ornament, provided they did but procure something in the shape of stained glass. No doubt much of this evil is to be attributed to the unskilfulness of the artists employed, few of whom are possessed of a sufficient knowledge of design, or the more intricate principles of the pictorial art, to enable them to suit their embellishments to edifices of a noble or commanding character. Disadvantages inherent in the art itself tend to debar from engaging in it men whose talents as painters ensure them patronage and success by adhering to the palette and the canvass. The expense of materials; the tedious and precarious nature of the process, where the artist cannot watch with satisfaction the effect which each additional stroke of his brush gives to the work as it grows beneath his hand, but must await in suspense the result of its exposure in the kiln; the difficulty of introducing such works of art to general notice; and the uncertainty of ultimate remuneration for his labours; prevent the man of genius from prosecuting an art, which nevertheless presents the most favourable opportunities for the expression of his ideas. There are not wanting, however, specimens of distinguished excellence produced by modern skill in the higher branches of this art, especially in the historical department. They certainly do not possess that mellow richness of colour, that calm and imposing grandeur of character, which distinguishes many of the older productions, but they excel them in accuracy of drawing, and in details; and while we know that it is perfectly possible to combine the various excellencies of former artists with those of a more recent date, we do not despair of seeing greater triumphs achieved in the art of staining glass than have yet been effected. Every variety of subject may be executed in this style. To the landscape painter it offers peculiar advantages, from the powerful contrasts in light and shade which it enables him to produce; while to the painter of flowers it affords a means wholly unequalled for displaying the delicacy and at the same time splendour of cofour, which his prototypes in nature place before his eyes.

It is somewhat strange, that while almost every other art has been called on to contribute in some way or other to our domestic comforts, or to the adornment of our dwellings, that of glass painting and staining, though its productions are not more expensive than many of these, and are certainly not inferior in elegance, should yet be but rarely applied to domestic purposes, where it could be employed with admirable effect. When stained glass has been used in the embellishment of the mansions of the middling, or even of the upper classes, it has been hitherto in a great measure confined to hall and staircase windows, and to windows placed in similar situations; but its use might be much extended, with great advantage in point of ornamental beauty.

If, for instance, the windows of a room were filled with stained glass, whose prevailing tints should harmonize with the predominating colour in the apartment, a striking and singularly pleasing effect would be produced. Where the expense, or any other objection, might be urged against figured glass, in which masses of brilliant colour are employed, that description which is plain and of one tint, such as brown, ruby, pale yellow, or similar hues, might be substituted; and if selected with reference to the prevailing colour of the interior with the view of either heightening or deepening its complexion, a warmth and richness of tint would be thrown around the apartment, such as no other contrivance of art can communicate. And where both of these descriptions of coloured glass might be found unsuitable, either from the expense or the too great exclusion of light, there is a third description of ornamental glass which might be employed, and which is not subject to these objections. This is a pale tracery, resembling lacework, which, when properly executed, is exceedingly beautiful, and is marked by an appearance of singular lightness and elegance. The introduction of such windows as might thus be produced would greatly enhance the beauty of our domestic architecture.

Except in the name, painting on glass has no resemblance to any other department of the pictorial art but that of porcelain. Both the colours, and the process of their application throughout, are entirely different. While animal and vegetable substances are freely used as colouring matter in every other department of the pictorial art, they are wholly excluded in that of glass painting, where all the pigments used are subjected, after being laid on, to the operation of fire, to make them penetrate the body of the glass, or become fused on its surface—a process which would wholly destroy the colouring properties of such substances. All the colours employed in glass painting and staining are oxides of metals or minerals, as gold, silver, cobalt, which not only stand the fire, but require the powerful interference of that agent to bring out their brilliancy and transpareAcy. Some colours, on the application of heat, penetrate the body of the glass, and, from this circumstance, are called stains; while others, being mixed with a vitreous substance called flux, become fused or vitrified on the surface. The former produces a variety of colours, and all of them are perfectly transparent. The produce of the latter are only semitransparent, but they may be made to yield any colour or tint required.

In preparing these colours, the most important point to be attended to is, to have all those that are to be used at the same time of an equal degree of softness. To attain this, those that are hard, and require a great degree of heat to make them effective, must be fixed first; leaving the soft colours, for which a slight heat only is necessary, to the last. If used promiscuously, and without regard to this precaution, some of the colours would be rendered too fluid, while others would be insufficiently fused, and the work in consequence spoiled. It is likewise of great importance to make a proper selection of glass for the purposes of staining and painting, as one kind will assimilate more freely with one colour than with another. The description of glass generally chosen for painting of staining is the best crown-glass.

It does not seem necessary to enter at length here inte the details of the process of glass painting and staining, as our account of the processes and mixtures must of ne-ceβsity[7:10:574] be almost entirely a recapitulation of what has been more fully and satisfactorily stated in other works, than the space allotted to this part of the subject would permit us to do; while all such details would be nearly useless for any practical purpose, there being scarcely a possibility of either communicating or acquiring such a knowledge of the art of glass painting or staining as would enable any person to understand or practise it successfully, unaided by personal observation and extensive experience.

CROWN-GLASS CUTTING.

This is a subject of very great importance to the glazier. Since the sheets of glass are circular, and the panes of glass most commonly used of a square or oblong shape, it is evident that there must some waste result from cutting up a sheet. The grand object of the glass-cutter, therefore, in the first place, is not only to cut up the sheet into I>anes which shall produce the least waste, but also, since the sheets of glass vary in size, to know what panes can be obtained from a sheet of a given diameter. To the glass-cutter this is well known, and to the general reader it can scarcely be explained so as to be completely understood. The utensils necessary for this purpose are a cross cutting board and a breaking out board, both of which are graduated or indented with brass lines in such a manner as to show through the sheet of glass, when laid on the cutting table, how many panes of one size, and how many of another, may be obtained from a sheet of given dimensions: squares and laths, on which feet, inches, and fractional parts are marked, are also necessary.

Crown-glass always cuts best in a warm temperature. When it happens to be hard, which is seldom the case (a bad diamond, or not knowing how to use a good one, being the usual cause of breakage), it has been found to cut better after warming the glass before the fire, which has the effect of diminishing the tension of the ill-annealed glass.^[6. To glaziers and others who may wish to obtain a thorough acquaintance with the subject, we would recommend a small volume lately published, entitled Crown-Glatt cutter and Glasiert Manual, by william Cooper, Edinburgh, Glass-cutter and Stained-Glass Maker, &c. in Ordinary to the Kins for Scotland. ]