Colouring of GLASS. That the colours given to glass may have their full beauty, it must be observed, that every pot when new, and first used, leaves a foulness in the glass from its own earthy parts; so that a coloured glass made in a new pot can never be bright or perfectly fine. For this reason, the larger of these, when new, may be glazed with white glass; but the second time of using the pots lose this foulness. The glazing may be done by reducing the glass to powder, and moistening the inside of the pot with water; while it is yet moist, put in some of the powdered glass, and shake it about, till the whole inner surface of the pot be covered by as much as will adhere to it, in consequence of the moisture. Throw out the redundant part of the powdered glass; and the pot being dry, set it in a furnace sufficiently hot to vitrify the glass adhering to it, and let it continue there some time; after which, care must be taken to let it cool gradually. Those pots which have served for one colour must not be used for another; for the remainder of the old matter will spoil the colour of the new. The colours must be very carefully calcined to a proper degree; for if they were calcined either too much or too little, they never do well; the proper proportion, as to quantity, must also carefully be regarded, and the furnaces must be fed with dry hard wood. And all the processes succeed much the better if the colour be used dividedly, that is, part of it in the frit, and the rest in the melted meltal.
A hard glass, proper for receiving colours, may be prepared by pulverizing 12 pounds of the best sand cleansed by washing in a glass or flint mortar, and mixing seven pounds of pearl ashes or any fixed alkaline salt purified with nitre, one pound of saltpetre, and half a pound of borax, and pounding them together. A glass less hard may be prepared of twelve pounds
of white sand cleansed, seven pounds of pearl ashes purified with saltpetre, one pound of nitre, half a pound of borax, and four ounces of arsenic prepared as before.
Amethyst colour. See Purple below, and the article AMETHYST.
Balas colour. Put into a pot crystal frit, thrice washed in water; tinge this with manganese, prepared into a clear purple; to this add alumina cativum, sifted fine, in small quantities, and at several times: this will make the glass grow yellowish, and a little reddish, but not blackish, and always dissipates the manganese. The last time you add manganese give no more of the alumina cativum, unless the colour be too full. Thus will the glass be exactly of the colour of the balas ruby. See Ruby GLASS.
The common black colour. The glassmakers take old broken glass of different colours, grind it to powder, and add to it, by different parcels, a sufficient quantity of a mixture of two parts zaffer and one part manganese: when well purified, they work it into vessels, &c.
Glass beads are coloured with manganese only.
Black velvet colour. To give this deep and fine colour to glass, take of crystalline and pulverine frit, of each 20 pounds; of calx of lead and tin, four pounds; set all together in a pot in the furnace, well heated: when the glass is formed and pure, take steel well calcined and powdered, scales of iron that fly off from the smith's anvil, of each an equal quantity; powder and mix them well; then put six ounces of this powder to the above-described metal while in fusion: mix the whole thoroughly together, and let them all boil strongly together; then let it stand in fusion 12 hours to purify, and after this work it. It will be a most elegant velvet black.
There is another way of doing this, which also produces a very fair black. It is this: take a hundred weight of rochetta frit, add to this two pounds of tartar and six pounds of manganese, both in fine powder; mix them well; and put them to the metal while in fusion, at different times, in several parcels; let it stand in fusion after this for four days, and then work it.
A glass perfectly black may also be formed by adding to ten pounds of either of the compositions for hard glass above described, one ounce of zaffer, six drachms of manganese, and an equal quantity of iron strongly calcined.
Blue colour. A full blue may be made by adding six drachms of zaffer and two drachms of manganese to ten pounds of either of the compositions for hard glass, described above. For a very cool or pure blue glass, half an ounce of calcined copper may be used instead of the manganese, and the proportion of zaffer diminished by one half. Glass resembling sapphire may be made with ten pounds of either of the compositions for hard glass, three drachms and one scruple of zaffer, and one drachm of the calx cassii or precipitation of gold by tin; or, instead of this latter ingredient, two drachms and two scruples of manganese. Or a sapphire-coloured glass may be made by mixing with any quantity of the hard glass one-eighth of its weight of smalt. A beautiful blue glass is also produced from the oxide of cobalt.
Venetian brown, with gold spangles, commonly called
Glass. the philosopher's stone, may be prepared in the following manner: take of the second composition for hard glass above described, and of the composition for paste, of each five pounds, and of highly calcined iron an ounce; mix them well, and fuse them till the iron be perfectly vitrified, and has tinged the glass of a deep transparent yellow brown colour. Powder this glass, and add to it two pounds of powdered glass of antimony; grind them together, and thus mix them well. Take part of this mixture, and rub into it 80 or 100 leaves of the counterfeit leaf gold called Dutch gold; and when the parts of the gold seem sufficiently divided, mix the powder containing it with the other part of the glass. Fuse the whole with a moderate heat till the powder run into a vitreous mass, fit to be wrought into any of the figures or vessels into which it is usually formed; but avoid a perfect liquefaction, because that in a short time destroys the equal diffusion of the spangles, and vitrifies, at least in part, the matter of which they are composed; converting the whole into a kind of transparent olive-coloured glass. This kind of glass is used for a great variety of toys and ornaments with us, who at present procure it from the Venetians.
Chalcedony. A mixture of several ingredients with the common matter of glass, will make it represent the semi-opake gems, the jaspers, agates, chalcedonies, &c. The way of making these seems to be the same with the method of making marbled paper, by several colours dissolved in several liquors, which are such as will not readily mix with one another when put into water, before they are cast upon the paper which is to be coloured. There are several ways of making these variously coloured glasses, but the best is the following.
Dissolve four ounces of fine leaf silver in a glass vessel in strong aquafortis; stop up the vessel, and set it aside.—In another vessel, dissolve five ounces of quicksilver in a pound of aquafortis, and set this aside.—In another glass vessel, dissolve in a pound of aquafortis three ounces of fine silver, first calcined in this manner: amalgamate the silver with mercury, mix the amalgam with twice its weight of common salt well purified; put the mixture in an open fire in a crucible, that the mercury may fly off, and the silver be left in form of powder. Mix this powder with an equal quantity of common salt well purified, and calcine this for six hours in a strong fire; when cold, wash off the salt by repeated boilings in common water, and then put the silver into the aquafortis. Set this solution also aside.—In another vessel, dissolve in a pound of aquafortis three ounces of sal ammoniac; pour off the solution and dissolve in it a quarter of an ounce of gold. Set this also aside.—In another vessel, dissolve three ounces of sal ammoniac in a pound of aquafortis; then put into the solution cinnabar, crocus martis, ultramarine, and ferretto of Spain, of each half an ounce. Set this also aside.—In another vessel, dissolve in a pound of aquafortis three ounces of sal ammoniac; then put into it crocus martis made with vinegar, calcined tin, zaffer, and cinnabar, of each half an ounce; let each of these be powdered very fine, and put gently into the aquafortis. Set this also aside.—In another vessel, dissolve three ounces of sal ammoniac in a pound of aquafortis, and add to it brass calcined with brimstone, brass thrice
calcined, manganese, and scales of iron which fall from the smith's anvil, of each half an ounce; let each be well powdered, and put gently into the vessel. Then set this also aside.—In another vessel, dissolve two ounces of sal ammoniac in a pound of aquafortis, and put to it verdigrise an ounce, red lead, crude antimony, and the caput mortuum of vitriol, of each half an ounce; put these well powdered leisurely into the vessel, and set this also aside.—In another vessel, dissolve two ounces of sal ammoniac in a pound of aquafortis, and add orpiment, white arsenic, painters lake, of each half an ounce.
Keep the above nine vessels in a moderate heat for 15 days, shaking them well at times. After this pour all the matters from these vessels into one large vessel, well luted at its bottom; let this stand six days, shaking it at times; and then set it in a very gentle heat, and evaporate all the liquor, and there will remain a powder of a purplish green.
When this is to be wrought, put into a pot very clear metal, made of broken crystalline and white glass that has been used; for with the virgin frit, or such as has never been wrought, the chalcedony can never be made, as the colours do not stick to it, but are consumed by the frit. To every pot of 20 pounds of this metal put two or three ounces of this powder at three several times; incorporate the powder well with the glass; and let it remain an hour between each time of putting in the powders. After all are in, let it stand 26 hours; then let the glass be well mixed, and take an essay of it, which will be found of a yellowish blue; return this many times into the furnace; when it begins to grow cold, it will show many waves of different colours very beautifully. Then take tartar eight ounces, soot of the chimney two ounces, crocus martis made with brimstone, half an ounce; let these be well powdered and mixed, and put them by degrees into the glass at six times, waiting a little while between each putting in. When the whole is put in, let the glass boil and settle for 24 hours; then make a little glass body of it; which put in the furnace many times, and see if the glass be enough, and whether it have on the outside veins of blue, green, red, yellow, and other colours, and have, beside these veins, waves like those of the chalcedonies, jaspers, and oriental agates, and if the body kept within looks as red as fire.
When it is found to answer this, it is perfect, and may be worked into toys and vessels, which will always be beautifully variegated; these must be well annealed, which adds much to the beauty of their veins. Masses of this may be polished at the lapidary's wheel as natural stones, and appear very beautiful. If in the working the matter grow transparent, the work must be stopped, and more tartar, soot, and crocus martis, must be put to it, which will give it again the necessary body and opacity, without which it does not show the colours well.
Chrysodite colour may be made of ten pounds of either of the compositions for hard glass described above, and six drachms of calcined iron.
Red cornelian colour may be formed by adding one pound of glass of antimony, two ounces of the calcined vitriol called scarlet ochre, and one drachm of manganese or magnesia, to two pounds of either of the compositions.
Glass. positions for hard glass. The glass of antimony and magnesia are first fused with the other glass, and then powdered and ground with the scarlet ochre: the whole mixture is afterwards fused with a gentle heat till all the ingredients are incorporated. A glass resembling the white cornelian may be made of two pounds of either of the compositions for hard glass, and two drachms of yellow ochre well washed, and one ounce of calcined bones: grind them together, and fuse them with a gentle heat.
Emerald colour. See Green below.
Garnet colour. To give this colour to glass, the workmen take the following method. They take equal quantities of crystal and rochetta frit, and to every hundred weight of this mixture they add a pound of manganese and an ounce of prepared zaffer: these are to be powdered separately, then mixed and added by degrees to the frit while in the furnace. Great care is to be taken to mix the manganese and zaffer very perfectly; and when the matter has stood 24 hours in fusion, it may be worked.
Glass of this kind may be made by adding one pound of glass of antimony, one drachm of manganese, and the same quantity of the precipitate of gold by tin, to two pounds of either of the compositions for hard glass; or the precipitate of gold may be omitted, if the quantities of the glass of antimony and manganese be doubled.
Gold colour. This colour may be produced by taking ten pounds of either of the compositions for hard glass, omitting the saltpetre; and for every pound adding an ounce of calcined borax, or, if this quantity doth not render the glass sufficiently fusible, two ounces; ten ounces of red tartar of the deepest colour; two ounces of magnesia; and two drachms of charcoal of sallow, or any other soft kind. Precipitates of silver baked on glass will stain it yellow, and likewise give it a yellow colour on being mixed and melted with 40 or 50 times their weight of vitreous compositions; the precipitate from aquafortis by fixed alkali seems to answer best. Yellow glasses may also be obtained with certain preparations of iron, particularly with Prussian blue. But Dr Lewis observes, that the colour does not constantly succeed, nor approach to the high colour of gold, with silver or with iron. The nearest imitations of gold which he has been able to produce have been effected with antimony and lead. Equal parts of the glass of antimony, of flint calcined and powdered, and of minium, formed a glass of a high yellow; and with two parts of glass of antimony, two of minium, and three of powdered flint, the colour approached still more to that of gold. The last composition exhibited a multitude of small sparkles interspersed throughout its whole substance, which gave it a beautiful appearance in the mass, but were really imperfections, owing to air bubbles.
Neri directs, for a gold yellow colour, one part of red tartar and the same quantity of manganese, to be mixed with a hundred parts of frit. But Kunckel observes, that these proportions are faulty; that one part, or one and a quarter, of manganese, is sufficient for a hundred of frit; but that six parts of tartar are hardly enough, unless the tartar is of a dark red colour, almost blackish; and that he found it expedient to add to the tartar about a fourth of its weight of powdered charcoal. He
adds, that the glass swells up very much in melting, and that it must be left unstirred, and worked as it stands in fusion. Mr Samuel More, in repeating and varying this process in order to render the colour more perfect, found that the manganese is entirely unessential to the gold colour; and that the tartar is no otherwise of use than in virtue of the coaly matter to which it is in part reduced by the fire, the phlogiston or inflammable part of the coal appearing in several experiments to be the direct tinging substance. Mr Pott also observes, that common coals give a yellow colour to glass; that different coaly matters differ in their tinging power; that caput mortuum of soot and lamp black answer better than common charcoal: and that the sparkling coal, which remains in the retort after the rectification of the thick empyreumatic animal oils, is one of the most active of these preparations. This preparation, he says, powdered, and then burnt again a little in a close vessel, is excellent for tinging glass, and gives yellow, brown, reddish, or blackish colours, according to its quantity; but the frit must not be very hard of fusion, for in this case the strong fire will destroy the colouring substance before the glass melts: and he has found the following composition to be nearly the best; viz. sand two parts, alkali three parts; or sand two, alkali three, calcined borax one; or sand two, alkali two, calcined borax one: and though saltpetre is hardly used at all, or very sparingly, for yellow glasses, as it too much volatilizes the colouring substance; yet here for the most part a certain proportion of it, easily determined by trial, is very necessary; for without it the concentrated colouring matter is apt to make the glass too dark, and even of an opaque pitchy blackness. It does not certainly appear that there is any material diversity in the effects of different coals, the difference being probably owing to the different quantities of the inflammable matter which they contain; so that a little more shall be required of one kind than of another for producing the same degree of colour in the glass. Nor does the softness or insubility of the frit appear to be in any respect necessary.
Gold-coloured spangles may be diffused through the substance of glass, by mixing the yellow tincture with powdered glass, and bringing the mixture into fusion.
Green. This colour may be imparted to glass by adding three ounces of copper precipitated from aquafortis, and two drachms of precipitated iron, to nine pounds of either of the compositions for hard glass. The finest method of giving this beautiful colour to glass is this: Take five pounds of crystalline metal that has been passed several times through water, and the same quantity of the common white metal of polverine, four pounds of common polverine frit, and three pounds of red lead; mix the red lead well with the frit, and then put all into a pot in a furnace. In a few hours the whole mass will be well purified: then cast the whole into water, and separate and take out the lead; then return the metal into the pot, and let it stand a day longer in fusion; then put in the powder of the residuum of the vitriol of copper, and a very little crocus martis, there will be produced a most lively and elegant green, scarce inferior to that of the oriental emerald. There are many ways of giving a green to glass, but all are greatly inferior to this.—To make a sea green, the finest crystalline glass only must be used, and no manganese must
must be added at first to the metal. The crystal frit must be melted thus alone; and the salt, which swims like oil on its top, must be taken off with an iron ladle very carefully. Then to a pot of twenty pounds of this metal add six ounces of calcined brass, and a fourth part of the quantity of powdered zaffer: this powder must be well mixed, and put into the glass at three times; it will make the metal swell at first, and all must be thoroughly mixed in the pot. After it has stood in fusion three hours, take out a little for a proof: if it be too pale, add more of the powder. Twenty-four hours after the mixing the powder the whole will be ready to work; but must be well stirred together from the bottom, lest the colour should be deepest there, and the metal at the top less coloured, or even quite colourless. Some use for this purpose half crystal frit and half rochetta frit, but the colour is much the finest when all crystal frit is used.
Lapis lazuli colour. See Lapis LAZULI.
Opal colour. See OPAL.
Purple of a deep and bright colour may be produced by adding to ten pounds of either of the compositions for hard glass, above described, six drachms of zaffer and one drachm of gold precipitated by tin; or to the same quantity of either composition one ounce of manganese and half an ounce of zaffer. The colour of amethyst may be imitated in this way.
Red. A blood-red glass may be made in the following manner: Put six pounds of glass of lead, and ten pounds of common glass, into a pot glazed with white glass. When the whole is boiled and refined, add by small quantities, and at small distances of time, copper calcined to a redness as much as on repeated proofs is found sufficient: then add tartar in powder by small quantities at a time, till the glass is become as red as blood; and continue adding one or other of the ingredients till the colour is quite perfect.
Ruby. The way to give the true fine red of the ruby, with a fair transparency, to glass, is as follows: Calcine in earthen vessels gold dissolved in aqua-regia; the menstruum being evaporated by distillation, more aqua regia added, and the abstraction repeated five or six times, till it becomes a red powder. This operation will require many days in a hot furnace. When the powder is of a proper colour, take it out: and when it is to be used, melt the finest crystal glass, and purify it by often casting it into water; and then add, by small quantities, enough of this red powder to give it the true colour of a ruby, with an elegant and perfect transparency.
The process of tinging glass and enamels by preparations of gold was first attempted about the beginning of the last century. Libavius, in one of his tracts entitled Alchymia, printed in 1606, conjectures that the colour of the ruby proceeds from gold, and that gold dissolved and brought to redness might be made to communicate a like colour to factitious gems and glass. On this principle Neri, in his Art of Glass, dated in 1611, gives the process above recited. Glauber in 1648 published a method of producing a red colour by gold, in a matter which is of the vitreous kind, though not perfect glass. For this purpose he ground powdered flint or sand with four times its weight of fixed alkaline salt: this mixture melts in a moderately strong fire, and when cool looks like glass, but exposed to the air
runs into a liquid state. On adding this liquor to solution of gold in aqua-regia, the gold and flint precipitate together in form of a yellow powder, which by calcination becomes purple. By mixing this powder with three or four times its weight of the alkaline solution of flint, drying the mixture, and melting it in a strong fire for an hour, a mass is obtained of a transparent ruby colour and of a vitreous appearance; which nevertheless is soluble in water, or by the moisture of the air, on account of the redundancy of the salt. The Honourable Mr Boyle, in a work published in 1680, mentions an experiment in which a like colour was introduced into glass without fusion; for having kept a mixture of gold and mercury in digestion for some months, the fire was at last immoderately increased, so that the glass burst with a violent explosion; and the lower part of the glass was found tinged throughout of a transparent red colour, hardly to be equalled by that of rubies.
About the same time Cassius is said to have discovered the precipitation of gold by tin, and that glass might be tinged of a ruby colour by melting it with this precipitate; though he does not appear, says Dr Lewis, from his treatise De Auro, to have been the discoverer of either. He describes the preparation of the precipitate and its use; but gives no account of the manner of employing it, only that he says one drachm of gold duly prepared will tinge ten pounds of glass.
This process was soon after brought to perfection by Kunckel; who says, that one part of the precipitate is sufficient to give a ruby colour to 1280 parts of glass, and a sensible redness to upwards of 1900 parts; but that the success is by no means constant. Kunckel also mentions a purple gold powder, resembling that of Neri; which he obtained by inspissating solution of gold to dryness; abstracting from it fresh aqua-regia three or four times, till the matter appears like oil; then precipitating with strong alkaline ley, and washing the precipitate with water. By dissolving this powder in spirit of salt and precipitating again, it becomes, he says, extremely fair; and in this state he directs it to be mixed with a due proportion of Venice glass.
Orschal, in a treatise entitled Sol sine Veste, gives the following process for producing a very fine ruby. He directs the purple precipitate made by tin to be ground with six times its quantity of Venice glass into a very fine powder, and this compound to be very carefully mingled with the frit or vitreous composition to be tinged. His frit consists of equal parts of borax, nitre, and fixed alkaline salt, and four times as much calcined flint as of each of the salts; but he gives no directions as to the proportion of the gold precipitate or mode of fusion. Hellot describes a preparation, which, mixed with Venice glass, was found to give a beautiful purple enamel. This preparation consists of equal parts of solution of gold and of solution of zinc in aqua-regia mixed together, with the addition of a volatile salt prepared from sal ammoniac by quicklime, in sufficient quantity to precipitate the two metals. The precipitate is then gradually heated till it acquires a violet colour. However, though a purple or red colour, approaching to that of ruby, may, by the methods above recited, be baked on glass or enamels, and introduced into the mass by fusion, the way of equally diffusing such
Glass. such a colour through a quantity of fluid glass is still, says Dr Lewis, a secret. The following process for making the ruby glass was communicated to Dr Lewis by an artist, who ascribed it to Kunckel. The gold is directed to be dissolved in a mixture of one part of spirit of salt and three of aqua fortis, and the tin in a mixture of one part of the former of these acids with two of the latter. The solution of gold being properly diluted with water, the solution of tin is added, and the mixture left to stand till the purple matter has settled to the bottom. The colourless liquor is then poured off, and the purple sediment, while moist and not very thick, is thoroughly mixed with powdered flint or sand. This mixture is well ground with powdered nitre, tartar, borax, and arsenic, and the compound melted with a suitable fire. The proportions of the ingredients are 2560 parts of sand, 384 of nitre, 240 of tartar, 240 of borax, 28 of arsenic, five of tin, and five of gold.
Topaz Colour. Glass resembling this stone may be made by pulverizing ten pounds of either of the compositions for hard glasses with an equal quantity of the gold-coloured glass, and fusing them together.
White opake and semitransparent glass may be made of ten pounds of either of the compositions for hard glass, and one pound of well calcined horn, ivory, or bone; or an opake whiteness may be given to glass by adding one pound of very white arsenic to ten pounds of flint glass. Let them be well powdered and mixed by grinding them together, and then fused with a moderate heat till they are thoroughly incorporated. A glass of this kind is made in large quantities at a manufactory near London; and used not only for different kinds of vessels, but as a white ground for enamel in dial plates and snuff boxes, which do not require finishing with much fire, because it becomes very white and fusible with a moderate heat.
Yellow. See Gold colour above.
Painting in GLASS. The ancient manner of painting in glass was very simple: it consisted in the mere arrangement of pieces of glass of different colours in some sort of symmetry, and constituted what is now called mosaic work. See MOSAIC.
In process of time they came to attempt more regular designs, and also to represent figures heightened with all their shades; yet they proceeded no farther than the contours of the figures in black with water colours, and hatching the draperies after the same manner on glasses of the colour of the object they designed to paint. For the carnation, they used glass of a bright red colour; and upon this they drew the principal lineament of the face, &c. with black.
At length, the taste for this kind of painting improving considerably, and the art being found applicable to the adorning of churches, basilicas, &c. they found out means of incorporating the colours in the glass itself, by heating them in the fire to a proper degree; having first laid on the colours. A French painter at Marseilles is said to have given the first notion of this improvement, upon going to Rome under the pontificate of Julius II.; but Albert Durer and Lucas of Leyden were the first that carried it to any height.
This art, however, has frequently met with much interruption, and sometimes been almost totally lost; of
which Mr Walpole gives us the following account, in his Anecdotes of Painting in England.
"The first interruption given to it was by the reformation, which banished the art out of churches; yet it was in some measure kept up in the eschuthecons of the nobility and gentry in the windows of their seats. Towards the end of Queen Elizabeth's reign it was omitted even there; yet the practice did not entirely cease. The chapel of our Lady at Warwick was ornamented anew by Robert Dudley earl of Leicester, and his countess, and the cipher of the glass-painter's name yet remains, with the date 1574: and in some of the chapels at Oxford the art again appears, dating itself in 1622, by the hand of no contemptible master.
"I could supply even this gap of 48 years by many dates on Flemish glass; but no body ever supposed that the secret was lost so early as the reign of James I. and that it has not perished since will be evident from the following series, reaching to the present hour.
"The portraits in the windows of the library at All Souls, Oxford. In the chapel at Queen's College there are twelve windows dated 1518. P. C. a cipher on the painted glass in the chapel at Warwick, 1574. The windows at Wadham's College; the drawing pretty good, and the colours fine, by Bernard Van Linge, 1622. In the chapel at Lincoln's Inn, a window, with the name Bernard, 1623. This was probably the preceding Van Linge. In the church of St Leonard, Shoreditch, two windows by Baptista Sutton, 1634. The windows in the chapel at University College, Hen. Giles pinxit, 1687. At Christ Church, Isaac Oliver, aged 84, 1700. Window in Merton Chapel, William Price 1700. Windows at Queen's New College, and Maunlin, by William Price, the son, now living, whose colours are fine, whose drawing is good, and whose taste in ornaments and mosaic is far superior to any of his predecessors; is equal to the antique, to the good Italian masters, and only surpassed by his own singular modesty.
"It may not be unwelcome to the curious reader to see some anecdotes of the revival of taste for painted glass in England. Price, as we have said, was the only painter in that style for many years in England. Afterwards one Rowell, a plumber at Reading, did some things, particularly for the late Henry earl of Pembroke; but Rowell's colours soon vanished. At last he found out a very durable and beautiful red; but he died in a year or two, and the secret with him. A man at Birmingham began the same art in 1756 or 1757, and fitted up a window for Lord Lyttleton, in the church of Hagley; but soon broke. A little after him, one Peckitt at York began the same business, and has made good proficiency. A few lovers of that art collected some dispersed panes from ancient buildings, particularly the late Lord Cobham, who erected a Gothic temple at Stowe, and filled it with arms of the old nobility, &c. About the year 1753, one Asciotti, an Italian, who had married a Flemish woman, brought a parcel of painted glass from Flanders, and sold it for a few guineas to the Honourable Mr Bateman, of Old Windsor. Upon that I sent Asciotti again to Flanders, who brought me 450 pieces, for which, including the expence of his journey, I paid him thirty-six guineas. His wife made more journeys for the same par-
Glass. pose; and sold her cargo to one Palmer a glazier in St Martin's lane, who immediately raised the price to one, two, or five guineas for a single piece, and fitted up entire windows with them, and with mosaics of plain glass of different colours. In 1761, Paterson, an auctioneer at Essex house in the Strand, exhibited the two first auctions of painted glass, imported in like manner from Flanders. All this manufacture consisted in rounds of Scripture stories, stained in black and yellow, or in small figures of black and white; birds and flowers in colours, and Flemish coats of arms.
The colours used in painting or staining of glass are very different from those used in painting either in water or oil colours.
For black, take scales of iron, one ounce; scales of copper, one ounce; jet, half an ounce: reduce them to powder, and mix them. For blue, take powder of blue, one pound; sal nitre, half a pound: mix them and grind them well together. For carnation, take red chalk, eight ounces; iron scales, and litharge of silver, of each two ounces; gum arabic, half an ounce; dissolve in water: grind all together for half an hour as stiff as you can; then put it in a glass and stir it well, and let it stand to settle 14 days. For green, take red lead one pound; scales of copper, one pound; and flint, five pounds: divide them into three parts; and add to them as much sal nitre; put them into a crucible, and melt them with a strong fire; and when it is cold, powder it, and grind it on a porphyry. For gold colour, take silver, an ounce; antimony, half an ounce: melt them in a crucible; then pound the mass to powder, and grind it on a copper plate; add to it yellow ochre, or brick dust calcined again, 15 ounces; and grind them well together with water. For purple, take minium, one pound; brown stone, one pound; white flint, five pounds: divide them into three parts, and add to them as much sal nitre as one of the parts; calcine, melt, and grind as you did the green. For red, take jet, four ounces; litharge of silver, two ounces; red chalk, one ounce: powder them fine, and mix them. For white, take jet, two parts; white flint, ground on a glass very fine, one part: mix them. For yellow, take Spanish brown, ten parts; leaf silver, one part; antimony, half a part: put all into a crucible, and calcine them well.
In the windows of ancient churches, &c. there are to be seen the most beautiful and vivid colours imaginable, which far exceed any of those used by the moderns, not so much because the secret of making those colours is entirely lost, as that the moderns will not go to the charge of them, nor be at the necessary pains, by reason that this sort of painting is not now so much in esteem as formerly. Those beautiful works which were made in the glass houses were of two kinds.
In some, the colour was diffused through the whole substance of the glass. In others, which were the more common, the colour was only on one side, scarce penetrating within the substance above one-third of a line; though this was more or less according to the nature of the colour, 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, by reason that on the same glass, though already coloured, they could show other kinds of colours where
there was occasion to embroider draperies, enrich them with foliage, 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 colour to the clear glass. This done, they applied the proper colours on the other side of the glass. By these means, the new colours were hindered from running and mixing with the former, when they exposed the glasses to the fire, as will appear hereafter.
When indeed the ornaments were to appear white, the glass was only bared of its colour with emery, without tinging the place with any colour at all; and this was the manner by which they wrought their light and heightenings on all kinds of colour.
The first thing to be done, in order to paint or stain glass, in the modern way, is to design, and even colour, the whole subject on paper. Then they choose such pieces of glass as are clear, even, and smooth, and proper to receive the several parts; and proceed to distribute the design itself, or papers it is drawn on, into pieces suitable to those of the glass; always taking care 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 impaired by the lead with which the pieces are to be joined together. The distribution being made, they mark all the glasses as well as papers, that they may be known again: which done, applying every part of the design upon the glass intended for it, they copy or transfer the design upon this glass with the black colour 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 strokes are well dried, which will happen in about two days, the work being only in black and white, they give a slight wash over with urine, gum arabic, and a little black; and repeat it several times, according as the shades are desired to be heightened; with this precaution, never to apply a new wash till the former is sufficiently dried.
This done, the lights and risings are given by rubbing off the colour in their respective places with a wooden point, or the handle of the pencil.
As to the other colours 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 the other colours, by blending therewith. And here too, as in pieces of black and white, particular regard must always be had not to lay colour on colour, 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 colour 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 colours.
The furnace here used is small, built of brick, from 18 to 30 inches square. At six inches from the bottom is an aperture to put in the fuel and maintain the fire.
Glass. 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 coction 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 the other side hereof is a little aperture, through which to make 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 manner: First, The bottom of the pan is covered with three strata or layers of quicklime pulverized; 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; and over this they lay another range of glasses, and thus alternately till the pan is quite full; taking care that the whole heap always end with a layer of the lime powder.
The pan being thus prepared, they cover up the furnace with tiles, on a square table of earthen ware, closely luted all round; only leaving 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 of ten or twelve hours; in which time it is usually completed. At last the fire, which at first was 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 essays, from time to time, by taking out pieces laid for the purpose through the little aperture of the furnace and pan, to see whether the yellow be perfect, and the other colours in good order. When the annealing is thought sufficient, they proceed with great haste to extinguish the fire, which otherwise would soon burn the colours, and break the glasses.
Glass Balls, which are circular, or otherwise shaped hollow vessels of glass, may be coloured within, so as to imitate the semipellucid gems. The method of doing it is this: make a strong solution of ichthyocola, or isinglass, in common water, by boiling; pour a quantity of this while warm into the hollow of a white glass vessel; shake it thoroughly about, that all the sides may be wetted, and then pour off the rest of the moisture. Immediately after this, throw in red lead, shake it and turn it about, throw it into many places with a tube, and the moisture will make it stick and run in waves and pretty figures. Then throw in some of the painters blue smalt, and make it run in waves in the ball as the red lead; then do the same with verdigrise, next with orpiment, then with red lake, all well ground; always casting in the colours in different places, and turning the glass, that the moisture within may run them into the waves. Then take fine plaster of Paris, and put a quantity of it into the ball; shake it also nimbly about; this will everywhere stick firmly to the glass, and give it a strong inner coat, keeping all the
colours on very fairly and strongly. These are set on frames of carved wood, and much esteemed as ornaments in many places.
Glass Drop. See RUPERT'S Drops.
Engraving on Glass. Professor Beckmann has proved, that so early as the year 1670 the art of etching upon glass was discovered by Henry Schwanhard, son of George Schwanhard, who was a celebrated glass-cutter, patronized by the emperor Ferdinand III. about the middle of the last century. At the time of his death, 1697, the father practised his art at Prague and Ratisbon. Whether the son followed the same business at the same towns, or removed to Nuremberg, is not very evident; but in the year above mentioned, some aqua-regia (nitro-muriatic acid) having accidentally fallen on his spectacles, he was surprised to find the glass corroded by it, and become quite soft. He thus, it is said, found himself in possession of a liquid by which he could etch writing and figures upon plates of glass.
But it is probable, as Beckmann seems to think, that he had discovered the fluoric acid itself; for in the year 1725 there appeared in a periodical work the following receipt for making a powerful acid, by which figures of every kind can be etched upon glass.
"When the spiritus nitri per distillationem has passed into the recipient, ply it with a strong fire, and when well dephlegmated, pour it, as it corrodes ordinary glass, into a Weldenberg flask. Then throw into it a pulverized green Bohemian emerald, otherwise called hesphorus, (which, when reduced to powder, and heated, emits in the dark a green light), and place it in warm sand for 24 hours. Take a piece of glass well cleaned, and freed from all grease by means of a ley; put a border of wax round it, about an inch in height, and cover it all over with the above acid. The longer you let it stand so much the better; and at the end of some time the glass will be corroded, and the figures which have been traced out with sulphur and varnish will appear as if raised above the pane of glass.
That the Bohemian emerald or hesphorus mentioned in this receipt it green sparry flour, cannot, says the professor, be doubted; and he seems to have as little doubt of the receipt itself having passed from Schwanhard and his scholars to the periodical work of 1725, from which it was inserted in the Oekonomische Encyclopedie of Krunitz. This supposition certainly acquires a considerable degree of probability from the similarity of Schwanhard's method of etching to that which is here recommended, and which is so different from what is now followed. At present, the glass is covered with a varnish either of isinglass dissolved in water, or of turpentine oil mixed with a little white lead, through which the figures to be etched are traced as on copper; but Schwanhard, when he had drawn his figures, covered them with varnish, and then by his liquid corroded the glass around them. His figures, therefore, when the varnish was removed, remained smooth and clear, appearing raised from a dim or dark ground; and M. Beckmann who persuaded some ingenious artists to make trial of this ancient method of etching, declares, that such figures have a much better effect than those which are cut into the glass.
Foliating of Glass. See FOLIATING and LOOKING-glass.
Glass of Lead, a glass made with the addition of a large quantity of lead, of great use in the art of making counterfeit gems. The method of making it is this: Put a large quantity of lead into a potter's kiln, and keep it in a state of fusion with a moderate fire, till it is calcined to a gray loose powder; then spread it in the kiln, and give it a greater heat, continually stirring it to keep it from running into lumps; continue this several hours, till the powder become of a fair yellow; then take it out, and sift it fine: this is called calcined lead. Take of this calcined lead 45 pounds, and crystalline or other frit 12 pounds; mix these as well as possible together; put them into a pot, and set them in the furnace for ten hours; then cast the whole, which will be now perfectly melted, into water; separate the loose lead from it, and return the metal into the pot; and after standing in fusion 12 hours more, it will be fit to work. It is very tender and brittle, and must be worked with great care, taking it slowly out of the pot, and continually wetting the marble it is wrought upon.
It is well known that ceruse or white lead, minium, litharge, and all the other preparations and calces of lead, are easily fused by a moderate fire, and formed into a transparent glass of a deep yellow colour. But this glass is so penetrating and powerful a flux, that it is necessary to give it a greater consistence, in order to render it fit for use. With this view, two parts of calx of lead, e. g. minium, and one part of sand or powdered flints, may be put into a crucible of refractory clay, and baked into a compact body. Let this crucible, well closed with a luted lid, be placed in a melting furnace, and gradually heated for an hour, or an hour and a half; and afterwards let the heat be increased so as to obtain a complete fusion, and continued in that state for the same time: let the crucible remain to cool in the furnace; and when it is broken a very transparent yellow coloured glass will be found in it. Some add nitre and common salt to the above mixture, because these salts promote the fusion and the more equal distribution of the sand. This glass of lead has a considerable specific gravity, and its lowest part is always the heaviest. It is an important flux in the assays of ores to facilitate their scorification.
Glass of lead is capable of all the colours of the gems in very great perfection. The methods of giving them are these: for green, take pulverine frit 20 pounds, lead calcined 16 pounds; sift both the powders very fine; then melt them into a glass, separating the unmixed lead, by plunging the mass in water; after this return it into the pot, and add brass thrice calcined six ounces, and one pennyweight of crocus martis made with vinegar; put this in at six different times, always carefully mixing it together, and take a proof of it; when the colour is right, let it stand eight hours, and then work it. If instead of the calcined brass the same quantity of the caput mortuum of the vitriolum veneris be used, the green is yet much finer.
For topaz colour, take crystal frit 15 pounds, calcined lead 12 pounds; mix them well together, by sifting the powders through a fine sieve; then set them in a furnace not too hot, and separate the superfluous
unmixed lead, by casting the whole into water; repeat this twice: then add half gold yellow glass, and let them incorporate and purify, and they will be of the true and exact colour of the oriental topazes.
For sea green, take crystal frit 16 pounds, calcined lead 10 pounds; mix and sift them together, and set them in a pot in a furnace; in 12 hours the whole will be melted; then cast it into water, and separate it from the loose lead; put them into the furnace again for eight hours; then separate the loose lead by washing a second time, and return it to the pot for eight hours more.
Muscovy Glass. See MICA, MINERALOGY Index.
Painting on Glass by means of Prints. See BACK-painting.
Glass Porcelain, the name given by many to a modern invention of imitating the china ware with glass. The method given by M. Reaumur, who was the first that carried the attempt to any degree of perfection, is shortly this: The glass vessels to be converted into porcelain are to be put into a large earthen vessel, such as the common fine earthen dishes are baked in, or into sufficiently large crucibles; the vessels are to be filled with a mixture of fine white sand, and of fine gypsum or plaster stone burnt into what is called plaster of Paris, and all the interstices are to be filled up with the same powder, so that the glass vessels may nowhere touch either one another, or the sides of the vessel they are baked in. The vessel is to be then covered down and luted, and the fire does the rest of the work; for this is only to be put into a common potter's furnace, and when it has stood there the usual time of the baking the other vessels, it is to be taken out, and the whole contents will be found no longer glass, but converted into a white opaque substance, which is a very elegant porcelain, and has almost the properties of that of China.
The powder which has served, once will do again as well as fresh, and that for a great many times: nay, it seems, ever so often. The cause of this transformation, says Macquer, is probably that the vitriolic acid of the gypsum quits its basis of calcareous earth, and unites with the alkaline salt and saline earth of the glass, with which it forms a kind of salt, different from the calcareous selenite, by the interposition of which matter the glass acquires the qualities of porcelain.
Glass Pots, the vessels in the glass trade used for melting the glass. Those for the white glass works are made of a tobacco pipe clay, brought from the isle of Wight, which is first well washed, then calcined, and afterwards ground to a fine powder in a mill; which being mixed with water, is then trod with the bare feet till it is of a proper consistence to mould with the hands into the proper shape of the vessels. When these are thus made, they are afterwards annealed over the furnace. Those for the green glass works are made of the nonsuch, and another sort of clay from Staffordshire; they make these so large as to hold three or four hundred weight of metal. And besides these, they have a small sort called piling pots, which they set upon the larger, and which contain a finer and more nice metal fit for the nicest works.
The clay that is used for this purpose should be of the purest and most refractory kind, and well cleansed from all sandy, ferruginous, and pyritous matters; and
to this it will be proper to add ground crucibles, white sand, calcined flints duly levigated, or a certain proportion of the same clay baked, and pounded not very finely. The quantity of baked clay that ought to be mixed with the crude clay, to prevent the pots from cracking when dried, or exposed to a great heat, is not absolutely determined, but depends on the quality of the crude clay, which is more or less fat. M. D'Antic, in a memoir on this subject, proposes the following method of ascertaining it: The burnt and crude clay, being mixed in different proportions, should be formed into cakes, one inch thick, and four inches long and wide. Let these cakes be slowly dried, and exposed to a violent heat, till they become as hard and as much contracted as possible, and in this state be examined; and the cake, he says, which has suffered a diminution of its bulk equal only to an eighteenth part, is made of the best proportions. He observes, in general, that most clays require that the proportion of the burnt should be to the fresh as four to five.
Tin Glass, the same with Bismuth. See BISMUTH, CHEMISTRY Index.
GLASSES are distinguished, with regard to their form, use, &c. into various kinds, as drinking glasses, optical glasses, looking glasses, burning glasses, &c.
Drinking Glasses, are simple vessels of common glass or crystal, usually made in form of an inverted cone.
Each glass consists of three parts, viz. the bowl, the bottom, and the foot; which are all wrought or blown separately.
Nothing can be more dexterous and expeditious than the manner of blowing these parts: two of them opened, and all three joined together. An idea is only to be had thereof, by seeing it actually done. For the method of gilding the edges of drinking glasses, see GILDING on Enamel and Glass.
Optical Glasses. See OPTICS.
The improvements hitherto made in telescopes by means of combining lenses made of different kinds of glass, though very great, are yet by no means adequate to the expectations that might reasonably be formed, if opticians could fall on any method of obtaining pieces of glass sufficiently large for pursuing the advantages of Mr Dollond's discovery. Unfortunately, however, though the board of longitude have offered a considerable reward for bringing this art to the requisite perfection, no attempt of any consequence has hitherto been made. Mr Keir is of opinion, that the accomplishment of this is by no means an easy task; as it requires not only a competent knowledge of the properties of glass fitted for the purpose (the faults not being evident to common inspection), but a considerable degree of chemical knowledge is also necessary in order to invent a composition by which these faults may be avoided; and lastly, a kind of dexterity in the execution of the work, which can only be acquired by practice. Our author, however, thinks, that if the subject were more generally understood, and the difficulties more fully pointed out, for which purpose he makes the following remarks, the end may be more easily accomplished.
1. The rays of light passing through a glass lens or prism, or through any other medium of unequal thickness, are refracted; but not in an equal manner, the blue, violet, &c. being more refracted than the red.
2. Hence it happens, that the rays of light, when refracted by a common lens, do not all unite in one focus, but in reality form as many different foci as there are colours; and hence arise the prismatic colours, or irises, which appear towards the borders of the image formed by the common convex lenses, and which render the vision extremely indistinct.
3. The indistinctness of vision produced by this cause, which is sensible in telescopes of a small aperture, increases in so great a proportion, viz. as the cubes of the diameters, that it seemed impossible to increase the power of dioptric telescopes greatly, without extending them to a very inconvenient length, unless this confusion of colours could be corrected.
4. It was known that different transparent bodies possessed different degrees of refractive power; and until Mr Dollond discovered the contrary, it was supposed, that the refractions of the coloured rays were always in a determined ratio to one another. On this supposition it seemed impossible to correct the faults of refracting telescopes: for it was supposed, that if the dispersion of light produced by a convex lens were counteracted by another lens or medium of a concave form, the refraction would be totally destroyed; and this indeed would be the case, if the two mediums were made of the same matter; and from some experiments made by Sir Isaac Newton, this was supposed to be actually the case in all substances whatever.
5. From considering that the eyes of animals are formed of mediums of different colours, it occurred first to Mr David Gregory, the celebrated professor of astronomy at Oxford, and then to Mr Euler, that, by a combination of mediums which had different refractive powers, it might be possible to remedy the imperfections of dioptric telescopes. It does not, however, appear, that either of these gentlemen understood the true principle on which these phenomena depend. Mr Euler executed his idea by forming a compound object lens from two glass lenses with water interposed, but his attempt was not attended with success. Mr Dollond, however, was led by some arguments adduced by Mr Klingenstierna of Sweden, to repeat one of Sir Isaac Newton's experiments, and which had induced even that great philosopher himself to suppose that the improvement afterwards executed by Mr Dollond was impossible. This experiment was made by Sir Isaac Newton, by placing a glass prism within a prismatic vessel filled with water, in such a manner that the rays of light which were refracted by the glass prism should pass through and be refracted in a contrary direction by the water prism. In this manner the refraction of the light was entirely destroyed. But when Mr Dollond repeated the experiment, he found, that, contrary to his own expectations, when the angles of the two prisms were so proportioned that they counteracted each other's mean refraction, then colours appeared; and on the other hand, when they were so proportioned that the dispersion of the coloured rays was counteracted, the mean refraction still subsisted; which evidently proved, that the mean refractive and dispersive powers of glass and water were not proportional to one another.
6. To apply this to the proposed improvement, Mr Dollond examined several kinds of glass. Crown glass was found to possess the smallest dispersive power in proportion to its refraction; while flint glass possessed
the greatest dispersive power in proportion to its refraction, which was also very great. On comparing these two exactly together, he found, that a wedge of white flint glass whose angle was about 25 degrees, and another of crown glass whose angle was 29 degrees, refracted very nearly alike. He found also, that, when the wedges were ground to such angles, the refraction produced by the flint glass was to that produced by the crown glass nearly as two to three; the refracted light was then free from colour. On measuring the general refracting powers of these two glasses, he found, that in flint glass, the sine of incidence of the rays was to the sine of mean refraction as 1 to 1.583; and that in crown glass, the sine of incidence was to the sine of mean refraction as 1 to 1.53.
The methods of determining the different refractive powers of glass are given under the article OPTICS. Here we shall only observe, that two kinds of glass are necessary for the construction of achromatic telescopes; one of which shall possess as small, and the other as great, dispersive powers, relative to their mean refracting powers, as can be produced. The difference of glasses in this respect depends on the quality of the ingredients employed in their composition. Crown glass, which is composed of sand melted by means of the ashes of sea weeds, barilla or kelp, both which fluxes are known to consist of vegetable earth, alkali, and neutral salt, is found to give the smallest dispersive power. Plate glass, which consists of sand melted by means of fixed vegetable alkali, with little or no vegetable earth, gives a greater dispersive power; but both these give much less than flint glass, which consists of sand melted by means of minium and fixed alkali. It appears, therefore, that the dispersion of the rays is greatest when minium, or probably other metallic calces, are made use of; and that alkalies give a greater power of dispersion than vegetable or other earths. Mr Zieher of Petersburg, however, informs us, that he has made a kind of glass, much superior in this respect to flint glass; but it does not as yet appear whether it be more fit for optical purposes than that commonly made use of. There seems no difficulty in augmenting the dispersive power, as that is found to depend on the quantity of minium or other flux: but thus we unfortunately increase also the capital fault to which flint glass and all compositions of that kind are subject; namely, the being subject to veins or small threads running through it. By these, even when so small as to be imperceptible to the naked eye, the rays which fall on them are diverted from their proper direction, and thereby render the images confused. This is owing to the greater density of the veins, as appears by their image being received on white paper, when the glass is held between the paper and the sun or a candle at a proper distance. The rays of light being then made to converge by the superior density of the veins, their images will appear as bright lines bordered with obscure edges on the paper. Flint glass is so much subject to this kind of imperfection, that it is with difficulty the opticians can pick out pieces of the size commonly used from a large quantity of the glass. It is farther to be regretted, that the minium which produces the greatest dispersive power, is likewise the very substance which renders flint glass much more subject to these imperfections than any other. The
reason is, that the sand and earthy matters mix uniformly in fusion; and having not only a considerable degree of affinity towards each other, but also being not much different from each other, they are not apt to separate. On the other hand, when such a heavy substance as minium is added to these earthy substances, though it has a pretty strong tendency to unite with the earthy substances, it has none with the fixed alkali, which is another ingredient in this glass. Hence some parts of the glass will contain more metallic matter than the rest; particularly that near the bottom of the pot, which is so full of large veins as to be applied only to the making of wares of little value. The veins in this case are formed by the descent of the minium at the bottom, which in its passage forms threads or veins by dragging other parts of the glass along with them.
The correction of this fault appears therefore to be very difficult. M. Macquer informs us, that he had in vain tried to remove it by very long fusion and a fierce fire; which indeed others have found by experience not to correct, but to augment the evil. Mr Keir is of opinion that some new composition must be discovered, which, along with a sufficient refractive power should possess a greater uniformity of texture; but he is likewise of opinion, that scarce any alteration in this respect could be made without injuring the colour of the glass. For optical purposes, however, our author does not think that an alteration in the colour of the ingredients would be very detrimental. "I am convinced (says he), that glasses sensibly tinged with colour, might transmit as much or more light than the best flint glass. For the colourless appearance of flint glass is an optical deception. The minium gives it a considerable tinge of yellow, and the alkali inclines it to a bluish cast, besides the colour arising from a greater or less impurity of the materials; so that the glass would actually be very sensibly coloured, unless by the addition of manganese, which is known to give a purplish red. Thus the other tinges are counteracted, but not effaced or destroyed as has been frequently imagined. By the mixture of the three principal colours, red, yellow, and blue, more or less exactly counterpoised, a certain dark shade is introduced, in which, as not any one of the colours predominates, no coloured tinge appears, but the effect is merely a diminution of the transparency of the glass, which, however, is too small for ordinary observation." Mr Keir is even of opinion, that a certain tinge of yellow would in many cases be of service, because it would exclude some of the blue rays, which being most refrangible are most injurious to the distinctness of vision.
Very considerable difficulties, however, must arise in attempting improvements of this kind; as the experiments must all be tried on a very large scale. This is not only attended with a very heavy expence in itself on account of the quantity of materials employed, but from the heavy duty of excise, which is rigorously exacted whether the glass be manufactured into saleable articles or not. It is observed in the manufacture of every kind of glass, that the glass in the middle of the area or transverse section of a pot is much purer and freer from veins and other imperfections than the part which is near the sides, and that the glass at the bottom is the worst of all. Consequently it is chiefly in large pots, such as are used in manufactures, that there
Glass, Glastonbury is a probability of success. Very fine and beautiful glasses, called paste and artificial gems, may be made in smaller pots or crucibles; but this glass is suffered to cool and subside in the vessel, by which means the contiguous parts are more uniform in their texture than can be expected in a piece of glass taken out of the pot while hot in the common way, by making it adhere and twist round an iron rod or pipe. But although the method of allowing the glass to cool in the pots is very advantageous for the purposes of the jeweller, it is by no means applicable to those of the optician. Glass cooled in that gradual manner, suffers some degree of crystallization or peculiar arrangement of its parts; the consequence of which is, that the rays of light undergo certain refractions independent on the form of the glass, which greatly affect the distinctness of vision in telescopes.
Musical GLASSES. See HARMONICA.
Looking GLASSES. See LOOKING GLASS, MIRROR, and FOLIATING.
Burning GLASS. See BURNING GLASS.
Weather GLASS. See BAROMETER.
Copping GLASS. See SURGERY.
Hour GLASS. See HOUR GLASS.
Watch GLASS. See WATCH.