« ΠροηγούμενηΣυνέχεια »
sand, soda, lime, and manganese are first mixed together with great care, and are fritted in small furnaces built for this purpose, the heat being gradually raised to a full red-white, and kept at this point with frequent stirring till the materials undergo no further change, nor give any kind of vapor. The azure and the glass fragments, being already perfectly vitrified, are not added till towards the end of the process, which lasts about six hours. The glass-house for this manufacture differs in several particulars from the common houses for blowing glass, being about eighteen feet long and fifteen wide, made of good bricks. They are particularly distinguished from the common furnaces by containing two kinds of crucibles; the larger ones, called * pots,' are in the form of an inverted and truncated cone, and in these the glass is melted. The others are smaller, called 'cuvettes.' Another essential part of this furnace is the flat table (of which there is one corresponding with each pot) on which the glass is cast. These tables are of copper-plate, about ten feet by six, supported by masonry; and contiguous to each, on the same level, are flat ovens heated from underneath, upon which the glass when cast and sufficiently cooled may be slid without difficulty from off the table, and theu annealed. The tops of the flat oven and the table are on a level with the corresponding opening of the furnace, whence the cuvettes are withdrawn. When the glass is throughly melted, and fine, the cuvette is filled in the following way: the workman takes a copper ladle about ten inches in diameter, and fixed to an iron handle seven feet long, plunges it into the glass-pot, brings it up full of melted glass and empties it into the cuvette, the ladle being supported at the bottom by a strong iron rest held by two other workmen, lest the red-hot copper should bend and give way with the weight of the glass within. The cuvette being filled is suffered to remain in the furnace for some hours, that the bubbles formed by this disturbance of the glass may have entirely disappeared, and the samples taken out from time to time become quite clear and limpid. The door of the furnace is now opened, the cuvette is slid out and pulled upon a low iron cradle, and immediately drawn on to the side of the copper table, where it is hoisted by a tackle and iron chains, and overset upon a table, on which a thick flood of melted glass flows and spreads in every direction to an equal thickness. It is then made quite smooth and uniform at the surface, by passing over it a heavy hollow roller or cylinder of copper made true and smooth by turning, after it is cast, and weighing about 500 pounds. At the same time, the empty cuvette is returned by the iron cradle to its proper place within the furnace. The number of workmen required for the whole process of casting is at least twenty, each of which has his separate employment. The plate being cast, the inspector examines whether there are any bubbles on any part of the surface, and if found, the plate is immediately cut up through them. The plate being now cool is slid by an iron instrument from the casting table to the contiguous annealing oven, previously well heated, and is carefully taken up and ranged within it. Each
oven will contain six entire plates, and, when full, all the openings are stopped with clay, and the plates allowed to remain there for ten days or a fortnight, to be thoroughly annealed. When fit to be taken out of the annealing oven they are sent away to receive all the subsequent operations of polishing, silvering, fee; but first the edges are cut smooth and squared. This is done by a diamond, which is passed along the surface of the glass upon a square ruler in the manner of glaziers, and made to cut into the substance of the glass to a certain depth. The cut is opened by gently knocking with a small hammer on the under side of the glass just beneath, and the piece comes off, and the roughnesses are removed by pincers The plate is then finished as far as the glass-house business is concerned. The glass is now to be polished, which is done with sand and water; the glass being first fastened down to a wooden frame, with plaster of Paris, the operation is performed by means of another glass fastened in a frame, which is made to rub upon the other, wet sand being interspersed between the two. As the surfaces of the plates wear down, the sand is used finer and finer. Emery is next used of two or three degrees of fineness, which brings the glass to an even surface, but it is still perfectly opaque. To render it transparent, colcothar, which is the residue left in the retorts of the aquafortis makers, is applied. The polishing instrument is a block of wood, covered with several folds of cloth and carded wool, so as to make a firm elastic cushion. This block is worked by the hand; but, to increase the pressure of the polisher, the handle is lengthened by a wooden spring, bent to a bow three or four feet long, which, at the other extremity, rests against a fixed point to a beam placed above. The plate is now fastened to a table with plaster, covered with colcothar, and the polisher begins his operation by working it backwards and forwards over the surface of the plate till one side is done; then the other is to be polished in the same manner.
Crown-glass is the name given to the best window glass, the composition of which varies very considerably: but a good glass of this kind may be made with 200 parts of soda, 300 of fine sand, 33 of lime, and from 250 to 300 of the ground fragments of glass that has already been worked. A small quantity of arsenic is sometimes added to facilitate the fusion. Zafre, or the oxide of cobalt, with ground flint, is often used to correct the dingy yellow which the inferior kind of crown-glass naturally acquires. The manufacture of the common window glass, though made by blowing, is carried on very differently from that of the common flint glass articles, as the object is to produce a large flat and very thin plate, which is afterwards to be cut by the glazier's diamond into the required shapes and sizes. It is difficult to convey to the reader a proper and precise idea of the process by mere description, but it may be mentioned, that the workman takes a large mass of glass on the hollow iron rod, and by rolling it on an iron plate, and swinging it backwards and forwards, causes it to lengthen by its own weight into a cylinder, which is then rendered hollow by blowing with a fores of breath till it is brought out to the requisite thickness. The hollow cylinder is then opened by holding it to the fire, which by expanding the air confined within it (the hole of the iron rod being stopped) bursts it at its weakest part; and when still soft it is ripped up through its whole length by iron shears, opened out into a flat surface, and then it is finished by annealing as usual.
Common green bottle glass is another kind, which is made almost entirely of sand, lime, and sometimes clay, alkaline ashes of any kind, according as cheapness or convenience direct, and more especially of kelp in this country; of barilla, varec, and the other varieties of soda, in France; and of wood ashes in many parts of Germany, and in North America. The following composition has been given as a good and cheap material for bottle-glass, 100 parts of common sand, 30 of varec (a coarse kind of kelp made on the western coasts of France) 160 of the lixiviated earth of ashes, 30 of fresh wood ash, 80 of brick clay, and about 100 of broken glass. Bottle-glass is * very hard, well vitrified glass, not very heavy relatively to its bulk, and being fused at a very high degree of heat, and from other circumstances, it resists the corrosive action of all liquids much better than flint-glass. Besides being used for wine and beer bottles, it is much employed for very large retorts, subliming vessels, and other processes of chemistry, for which it is admirably adapted, being able to bear as much as a pretty full red-heat, without melting or sinking down into a shapeless lump.
Composition for white and crystal-glass.—To make crystal-glass, take of the whitest terso, pounded small, and sifted as fine as flour, 200lbs.; of the bait of polverine 130lbs.; mix them together, and put them into the furnace called the calcar, first heating it. For an hour keep a moderate fire, and keep stirring the materials with a proper rake, that they may incorporate and calcine together; increasing the fire for five hours; after which the matter is taken out, being sufficiently calcined, and is called frit. After this, remove it immediately from the calcar to a dry place, and cover it up from dust, for three or four months. Now, to make the crystal glass, take of the above crystal frit, called also bollito, and set it in the melting pots in the furnace, adding to it a due quantity of manganese; when the two arc fused, cast the flour into fair water, to clear it of the salt called sandiver, which would otherwise make the crystal obscure and cloudy. This washing must be repeated again and again, till the crystal be fully purged; or this scum may be taken off by proper ladles. Now set it to boil for four, five, or six days; which being finished, see whether it have manganese enough, and, if it be yet greenish, add more manganese at discretion, by little and little at a time, taking care not to over dose it, because it will incline it to a blackish hue. Let it clarify, and become of a shining hue; which done, it is fit to be used, and blown into vessels of any kind. Or, 120 parts of fine sand, 40 of purified pearl-ash, 35 of litharge, 13 of nitre, and a small quantity of black oxide of manganese, make a good glass.
Compositions for flint-glass.—Flint glass, as it
is usually called by us, is of the same general kind with that which, in other places, is called crystal-glass. It has this name from its having been originally made with calcined flints, before the use of white sand was understood ; and it has retained this name, though there are now no flints used in its composition. This glass differs from the crystal-glass in having lead in its composition to flux it, and white sand for its body, whereas the fluxes used in the other are salts or arsenic, and the body consists of tarso, white river pebbles, and such kinds of stones. To the lead and white sand a due proportion of nitre is added, and a small quantity of magnesia. The most perfect kind of flint-glass is made by fusing, in a very strong fire, 120 lbs. of white sand, 50 lbs. of red-lead, 40 lbs. of the purest pearlash, 20 lbs. of nitre, and 5oz. of magnesia. Another composition of flint-glass is said to consist of the following: 120 lbs. of white sand, 54 lbs. of the purest pearl-ash, 36 lbs. of red-lead, 12 lbs. of nitre, and 6 oz. of magnesia. To either of the above compositions a pound or two of arsenic may be added, to increase the flux of the composition. A still cheaper flint-glass may be made with 120 lbs. of white sand, 35 lbs. of the best pearl-ash, 40 lbs. of red-lead, 13 lbs. of nitre, 6 lbs. of arsenic, and 4 oz. of magnesia; or, instead of the arsenic, may be substituted 15lbs. of common salt; but this will make it more brittle than the other. But the cheapest of all the compositions hitherto employed, consists of 120 lbs. of white sand, 30 lbs. of red-lead, 20 lbs. of the best pearl-ash, 10 lbs. of nitre, 15 lbs. of common salt, and 6 lbs. of arsenic. Or, 100 parts of sand, 80 to 85 of red-lead, 35 to 40 of pearl-ash, 2 or 3 of nitre, and 1 oz. of manganese. The oxide of lead may be reduced in this glass.
Of silvering glass.—Glass when smoothed and polished does not acquire the property of reflecting objects till it has been silvered, as it is called, an operation effected by means of an amalgam of tin and quicksilver. The tin-leaf employed must be of the size of the glass, because, when pieces of that metal are united by means of mercury, they exhibit the appearance of lines. Tin is one of those metallic substances which become soonest oxidated by the means of mprcury. If there remains a portion of that oxide or calx, of a blackish gray color, on the leaf of tin, it produces a spot or stain in the mirror, and that part cannot reflect objects presented to it: great care, therefore, is taken in silvering glass to remove the calx of tin from the surface of the amalgam. The process is as follows:—The leaf of tin is laid on a very smooth stone table, and, mercury being poured over the metal, it is extended over the surface of it by means of a rubber made of bits of cloth. At the same moment the surface of the leaf of tin becomes covered with blackish oxide, which is removed with the rubber. Mora mercury is then poured over the tin, where it remains at a level to the thickness of more than a line, without runiiing off. The glass is applied in a horizontal direction to the table at one of its extremities, and being pushed forwards it drives before it the oxide of tin which is at the surface of the amalgam. A number of leaden weights, covered with cloth, are then placed on the glass which floats on the amalgam, in order to press it down. Without this precaution the glass would exhibit the interstices of the crystals resulting from the amalgam. These crystals have the form of large square laminae irregularly disposed.
To obtain leaves of tin, which are sometimes six or seven feet in length, with a proportionate breadth, they are not rolled but hammered after the manner of gold-beaters. The prepared tin is first cast between two plates of polished iron, or between two smooth stones not of a porous nature, such as thunder-stone. Twelve of these plates are placed over each other; and they are then beaten on a stone mass with heavy hammers, one side of which is plain and the other rounded. The plates joined together are first beaten with the latter: when they become extended the number of the plates is doubled, so that they amount sometimes to eighty or more. They are then smoothed with the flat side of the hammer, and are beaten till they acquire the length of six or seven feet, and the breadth of four or five. The small block of tin from which they are formed is at first ten inches long, six in breadth, and a line and a quarter in thickness. When the leaves are of less extent, and thin, from eighty to 100 of them are smoothed together.
Tin, extracted from the amalgam which has been employed for silvering glass, exhibits a remarkable peculiarity. When fused in an iron pan, its whole surface becomes covered with a multitude of tetrahedral prismatic crystals, two or three lines in length, and a quarter of a line in thickness. The interior of these pieces of tin, when cut with a chisel, have a grayer tint than pure tin, which is as white as silver. The latter crystallises also by cooling; but it requires care. When it begins to be fixed, decant the part which is still in fusion, and there will remain at the bottom of the crucible beautiful crystals of a dull white color, which- appeared to me to be cubes or parallelopipedons.
Painting on glass.—The primitive manner of painting on glass was very simple, and, of consequence, very easy; it consisted in the mere arrangement of pieces of glass of different colors, in some sort of symmetry; and constituted a kind of what we call Mosaic work. Afterwards, when they came to attempt more regular designs, and even to represent figures raised with all their shades, their whole address went no farther than to the drawing the contours of the figures in black, with water colors, and 'etching the draperies after the same manner, on glasses of the color of the object intended to be painted. For the carnations they chose glass of a bright red; upon which they designed the principal lineaments of the face, &c, with black. At last, the taste for this sort of painting being considerably improved, and the art being found applicable to the adorning of the churches, basilicas, &c, they found means of incorporating the colors with the glass itself, by exposing them to a proper degree of fire, after the colors had been laid on.
Those beautiful works, among the painters in glass, which were made in the glass-house, were of two kinds: in some the color was diffused through the whole body of glass; in others, which
were the more common, the color was only Op one side, scarcely penetrating within the substance abbve one-third of a line; though this was, more or less, according to the nature of the color, the yellow being always found to enter the deepest. These last, though not so strong and beautiful as the former, were of more advantage to the workmen ; because, on the same glass, though already colored, they could show other kinds of colors, where there was occasion to embroider draperies, enrich them with foliages, or represent other ornaments of gold, silver, &c. In order to this, they made use of emery: grinding, or wearing down the surface of the glass, till such time as they were got through the color to the clear glass: this done, they applied the proper colors on the other side of the glass. By this means the new colors were prevented from running, and mixing among the former, when the glasses came to be exposed to the fire, as will hereafter be shown.
When the intended ornaments were to appear white, or silvered, they contented themselves to bare the gloss of its color with emery, without applying any new color at all; and it was in this manner that they wrought the lights and heightenings on all kinds of colors. The painting with vitreous colors on glass depends entirely on the same principles as painting in enamel, and the. manner of executing it is likewise the same, except that in this, the transparency of the colors being indispensably requisite, no substances can be used to form them but such as vitrify perfectly: and, therefore, the great object is to find a set of colors which are composed of such substances, as, by the admixture of other bodies, may promote their vitrification and fusion; are capable of being converted into glass; and melting in that state, with less heat than is sufficient to melt such other kinds of glass as may be chosen for the ground, or body to be painted; to temper these colors, so as to make them proper to be worked with a pencil, and to burn or reduce them by heat, to a due state of fusion, without injuring or melting the glass which constitutes the body painted. The first thing to be done, in order to paint on glass, in the modern way, is to design, and even color, the whole subject on paper. Then they make choice of pieces of glass proper to receive the several parts, and proceed to divide or distribute the design itself, or the paper it is drawn on, into pieces suitable to those of glass; having always a view that the glasses may join in the contours of the figures, and the folds of the draperies; that the carnations and other finer parts may not be damaged by the lead wherewith the pieces are to be joined together. The distribution being made, they mark all the glasses, as well as papers, with letters or numbers, that they may be known again; which done, applying each part of the design on the glass intended for it, they copy or transfer the design upon this glass, with the black color, diluted in gum-water, by tracing and following all the lines and strokes as they appear through the glass with the point of a pencil.
When these first strokes are well dried, which happens in about two days, the work being only in black and white, they give it a slitrhi wash over with urine, gum arabic, and a little black; and this several times repeated, according as the ■hades are desired to be heightened; with this precaution, never 10 apply a new wash till the former is sufficiently dried. This done, the lights and risings are given, by rubbing off the color in the respective places with a wooden point, or the handle of the pencil.
As to the other colors above-mentioned, they are used with gum-water, much as in painting in miniature, taking care to apply them lightly, for fear of effacing the outlines of the design; or even, for the greater security, to apply them on the other side, especially yellow, which is very pernicious to other colors by blending therewith.
And here to J, as in pieces of black and white, particular regard must be always had not to lay color on color, or lay on a new lay till such time as the former are well dried. It may be added, that the yellow is the only color that penetrates through the glass, and incorporates therewith by the fire; the rest, and particularly the blue, which is very difficult to use, remaining on the surface, or at least entering very little. When the painting of all the pieces is finished, they are carried to the furnace or oven, to anneal or bake the colors. The furnace here used is small, built of brick, from eighteen to thirty inches square. At six inches from the bottom is an aperture to put in the fuel, and maintain the fire. Over this aperture is a grate, made of three square bars of iron, which traverse the furnace, and divide it into two parts. Two inches above this partition is another little aperture, through which they take out pieces to examine how the operation goes forward. On the grate is placed a square earthen pan, six or seven inches deep, and five or six inches less every way than the perimeter of the furnace. On one side hereof is a little aperture, through which to make the trials, placed directly opposite to that of the furnaces destined for the same end. In this pan are the pieces of glass to be placed in the following maiiiipr.1—First, the bottom of the pan is covered with three strata, or layers, of quicklime pulverised; those strata being separated by two others of old broken glass, the design whereof is to secure the painted glass from the too intense heat of the fire. This done, the glasses are laid horizontally on the last, or uppermost layer of lime. The first row of glass they cover over with a layer of the same powder an inch deep; over this they lay another range of glasses; and thus alternately till the pan is quite full, taking care that the whole heap always ends with a layer of the lime-powder.
The pan thus prepared, they cover up the furnace with tiles on a square table of earthenware, closely luted all round, only having five little apertures, one at each corner, and another in the middle to serve as chimneys.
Things thus disposed, there remains nothing but to give the fire to the work. The fire for the first two hours must be very moderate, and must be increased in proportion as the coction advances for the space often or twelve hours, in which time it is usually completed. At last the fire, which at first was only of charcoal, is to be of dry wood; so that the flame covers the whole pan, and even issues out at the chimneys. During the last hours they make assays from time to
time by taking out pieces laid for that purpose, through the little aperture of the furnace and pan, to see whether the yellow be perfect, and the other colors in good order. When the annealing is thought sufficient, they proceed with great haste to extinguish the fire, which otherwise would soon burn the colors and break the glasses.
Mr. Collins, glass-manufacturer of the Strand, near Temple Bar, has furnished the following observations and receipts on this subject:—
'Enamel colors and painting on glass,' says Mr. Collins, 'differ totally from all others, it being requisite on glass that the colors used should appear transparent, and bear (without blistering in the kiln) to be laid on very thick. In every other style of enamel painting the fluxes must be so compounded as to bring the beauty of the color on the surface, and they do not require to be any thing like the substance compared to those used on glass.
1 Crown window glass is the best for the purpose of enamelling upon, its principal composition or base being silex, which is not only the best substance for receiving colors, but also by far the best as the base for the fluxes. The best fluxes are obtained from finely calcined flints, lead and salts forming the fusing matter; these latter must be carefully used in various proportions, as the colors or oxides require.
'Receipts for the colors.—From gold only are prepared any pink or rose colors, although it has often been asserted that the French have prepared them from iron, which may sometimes answer for an orange-red, but will never produce a pink; and is very far (even as a red) from being so fixed a color as those made from gold, although it has been stated to be more so. In fact, a color being well fixed (on the contrary) depends as much upon the properties of the flux being rightly prepared to receive it as on the oxide or coloring matter itself, which experiment only can firmly elucidate.
'All metals should be as far removed from their metallic state as possible, and when in that state from which it would be the most difficult to restore it, it is best calculated for the purpose, therefore gold precipated by tin is better than that by an alkali, being a much more perfect oxide. Besides that. tin is the firmest and best base for receiving and holding the color struck from gold.
'In combining the fluxes, so that they shall bear the greatest possible affinity for the oxides intended, rests the principal art of color-making.
'In the solutions of gold and tin it is best to use more of the nitric and as little of the muriatic acid as possible, and the larger the proportion of metal that can be dissolved in a certain portion of acids the better.
'In the solution of gold the beauty of the color rests principally in the precipitate; to obtain the best, use the water as hot as possible; into about a pint of which drop a little gold (about fifteen or twenty drops), then the tin most carefully, by a drop at a time until it becomes as nearly as possible the color of port-wine at the edge of the basin ; it will then instantly precipitate itself. Wash it several times with very hot water; it must now be mixed with its flux before it is suffered to dry.
'Rose color should always be made from an oxide that inclines to the pink (as it occasionally differs); the flux should contain scarcely any lead, a small portion of silver is then added, and the whole finely ground before dry.
'I have entered at greater length into the pink and rose colors produced from gold than on any others, they being by far the most difficult to produce, and should never be made but on a bright and clear sun-shiny day, which I am persuaded has great influence on the preparation, as you never can produce this color good with a damp atmosphere or a cloudy sky.
'Blue is made from cobalt; the best is that prepared by fire, as in Staffordshire, being more condensed than that which is prepared by the acids. It is then fused with borax ground fine and washed several times; when dry, mixed with the flux and melted together.
'Purple is made from an oxide that incline; to the blue, and the flux may contain a much larger portion of lead, &c, as the rose color, only omitting the silver.
'Yellows are made from varied proportions of the oxides of antimony and lead. Tin is best omitted and silex used in its place; the whole to be well melted.
* Orange. Prepared as the yellow, only introducing a small quantity of the purple oxide of gold, and melted as yellow.
'Brown is made from manganese and antimony ground with the flux, and well melted together.
'Black is best when made from good iron scales and oxide of cobalt, with a little of the darkest possible purple oxides of gold, mixed with the flux and melted together.
'Green is made from, copper oxidated by fire, jnited with the flux, and well melted. It is then mixed with yellow to produce a grass green, and with white enamel (made by arsenic) to produce a blue-green.
'White, which is seldom used on glass, is made from arsenic mixed with the flux, and when in a state of fusion kept well covered. Tin
is also considered, for some purposes, the only thing from which a good fixed while can be made, but all that I have yet seen made in this country is very bad. The Venetian white enamel only can be depended on, which latter more particularly applies to enamelling on copper. The hard white enamel is but very little understood in this country. By some its base is stated to be the oxide of tin, but it is very doubtful. This is that substance used as the first ground or coating of the copper-plates for enamel painting, over which a somewhat more transparent and softer enamel (termed flux) is laid, which, melting sooner than the first, is better adapted for receiving the colors. In this style of painting so little can be done before it is necessary to fire the picture, that it frequently requires a dozen fires to complete a painting.
'Ruby. That produced by the ancients is what has made the greatest noise, the art of making which being considered lost, and for this reason principally admired. But this is an error, as that beautiful color is now made in as great perfection as ever, and equally well understood. Ruby may be made either from gold or copper. When made from the latter the color is liable to change by various degrees of heat, any thing above a red heat totally dissipating it. That made from gold is perfectly fixed, though not quite so deep a tint; with this latter, antimony, iron, and silver are used. With the copper red tartar.
'Painting on glass requires infinitely more care in burning than enamel, both on account of the superior size and brittleness of the substance; it therefore requires many hours annealing.
• In the preparing of glass and enamel colors there is great difference; but the oxides or coloring matters are alike in all, excepting the yellow, which on glass is produced from silver, on enamel from antimony. A fine red is produced on glass by the union of silver and antimony.'
GLAS (John), M. A., the founder of that sect of Independents commonly distinguished in Scotland by the name of Glassites, and in England by that of Sandemanians. See IndePendents. His father was minister of Abernethy; and he was born at his father's manse in 1695 He was educated and obtained the degree of M.A. at St. Andrews; and was, when a very young man, ordained minister of Tealing, near Dundee. His doctrines and ministry were remarkable, and drew much attention; and, even while connected with the establishment, his peculiar ideas of the purity of church communion and government, were observable In 1727 he published a work, entitled The Testimony of the King of Martyrs, in which his Independent principles were fully developed. In that treatise lie maintains the inconsistency of any connexion between civil establishments and the church of Christ, which is not of this world. This publication, with other concurring circumstances, produced a long controversy between Mr. Glas and
the Synod of Angus and Mearns. So far, however, from retracting or trimming in his principles, he still more openly avowed and maintained them, in a tract published the following year, entitled, A Congregation subject to no Jurisdiction under Heaven. He was deposed in 1728, and immediately afterwards connected himself with a few poor people who adhered to him; and, although his adherents have never been numerous, the doctrines which he taught have been generally spread through Britain and many parts of America. He was subjected to great poverty for many years, in supporting a numerous family of thirteen children; and died at Dundee, in November, 1773, highly valued by his friends, and respected by all who knew him. He published many religious Tracts, which were collected and printed in 4 vols. 8vo„ and since reprinted in 5 vols. The leading doctrines in these works, are, 1. The divinity of Christ, and the perfection of his righteousness to justify the ungodly; 2. Purity of Christian com