II. Working or blowing of Window or Table GLASS. The method of working round glass, or vessels of any sort, is in every particular applicable to the working of window or table glass, till the blowing iron has been dipped the fourth time. But then instead of rounding it, the workman blows, and so manages the metal upon the iron plate, that it extends two or three feet in the form of a cylinder. This cylinder is put again to the fire, and blown a second time, and is thus repeated till it is extended to the dimensions required, the side to which the pipe is fixed diminishing gradually till it ends in a pyramidal form; so that, to bring both ends nearly to the same diameter, while the glass is thus flexible, he adds a little hot metal to the end opposite the pipe, and draws it out with a pair of iron pincers, and immediately cuts off the same end with the help of a little cold water as before.
The cylinder being now open at one end, is carried back to the bocca; and there, by the help of cold water, it is cut about eight or ten inches from the iron pipe or rod; and the whole length at another place, by which also it is cut off from the iron rod. Then it is heated gradually on an earthen table, by which it opens in length; while the workman, with an iron tool, alternately lowers and raises the two halves of the cylinder; which at last will open like a sheet of paper, and fall into the same flat form in which it serves for use; in which it is preserved by heating it over again, cooling it on a table of copper, and hardening it 24 hours in the annealing furnace, to which it is carried upon forks. In this furnace an hundred tables of glass may lie at a time, without injury to each other, by separating them into tents, with an iron shiver between, which diminishes the weight by dividing it, and keeps the tables flat and even.
Of window or table glass there are various sorts, made in different places, for the use of building. Those most known among us are given us by the author of the Builder's Dictionary, as follows:
1. Grown, of which, says Neri, there are two kinds, distinguished by the places where they are wrought; viz. Ratcliff crown glass, which is the best and clearest, and was first made at the Bear garden, on the Bankside, Southwark, but since at Ratcliff: of this there are 24 tables to the case, the tables being of a circular form, about three feet six inches in diameter. The other kind, or Lambeth crown glass, is of a darker colour than the former, and more inclining to green.
The bell window or crown glass is made of white sand 60 pounds, of purified pearl ashes 30 pounds, of saltpetre
Glas. saltpetre 15 pounds, of borax one pound, and of arsenic half a pound. If the glass should prove yellow, magnesia must be added. A cheaper composition for window glass consists of 60 pounds of white sand, 25 pounds of unpurified pearl ashes, 10 pounds of common salt, 5 pounds of nitre, 2 pounds of arsenic, and one ounce and a half of magnesia. The common or green window glass is composed of 60 pounds of white sand, 30 pounds of unpurified pearl ashes, 10 pounds of common salt, 2 pounds of arsenic, and 2 ounces of magnesia. But a cheaper composition for this purpose consists of 120 pounds of the cheapest white sand, 30 pounds of unpurified pearl ashes, 60 pounds of wood ashes, well burnt and sifted, 20 pounds of common salt, and 5 pounds of arsenic.
2. French glass, called also Normandy glass, and formerly Lorraine glass, because made in those provinces. At present it is made wholly in the nine glass works; five whereof are in the forest of Lyons, four in the county of Eu; the last at Beaumont near Rouen. It is of a thinner kind than our crown glass; and when laid on a piece of white paper, appears of a dirtyish green colour. There are but 25 tables of this to the case.
3. German glass is of two kinds, the white and the green: the first is of a whitish colour, but is subject to those small curved streaks observed in our Newcastle glass, though free from the spots and blemishes thereof. The green, besides its colour, is liable to the same streaks as the white, but both them are straighter and less warped than our Newcastle glass.
4. Dutch glass is not much unlike our Newcastle glass either in colour or price. It is frequently much warped like that, and the tables are but small.
5. Newcastle glass is that most used in England. It is of an ash colour, and much subject to specks, streaks, and other blemishes; and besides is frequently warped. Leybourn says, there are 45 tables to the case, each containing five superficial feet: some say there are but 35 tables, and six feet in each table.
6. Phial glass is a kind betwixt the flint glass and the common bottle or green glass. The best kind may be prepared with 120 pounds of white sand, 50 pounds of unpurified pearl ashes, 10 pounds of common salt, 5 pounds of arsenic, and 5 ounces of magnesia. The composition for green or common phial glass consists of 120 pounds of the cheapest white sand, 80 pounds of wood ashes well burnt and sifted, 20 pounds of pearl ashes, 15 pounds of common salt, and 1 pound of arsenic.
The common bottle or green is formed of sand of any kind fluxed by the ashes of burnt wood, or of any parts of vegetables; to which may be added the scoria or clinkers of forges. When the softest sand is used, 200 pounds of wood ashes will suffice for 100 pounds of sand, which are to be ground and mixed together. The composition with the clinkers consists of 170 pounds of wood ashes, 100 pounds of sand, and 50 pounds of clinkers or scoria, which are to be ground and mixed together. If the clinkers cannot be ground, they must be broke into small pieces, and mixed with the other matter without any grinding.
III. Working of Plate or Mirror Glass. 1. The materials of which this glass is made are much the
same as those of other works of glass, viz. an alkali, salt and sand.
The salt, however, should not be that extracted from pulverine or the ashes of the Syrian kali, but that from BARILLA, growing about Alicante in Spain. It is very rare that we can have the barilla pure; the Spaniards in burning the herb make a practice of mixing another herb along with it, which alters its quality; or of adding sand to it to increase the weight, which is easily discovered if the addition be only made after the boiling of the ashes, but next to impossible if made in the boiling. It is from this adulteration that those threads and other defects in plate glass arise. To prepare the salt, they clean it well of all foreign matters; pound or grind it with a kind of mill, and finally sift it pretty fine.
Pearl ashes, properly purified, will furnish the alkali salt requisite for this purpose; but it will be necessary to add borax or common salt, in order to facilitate the fusion, and prevent the glass from stiffening in that degree of heat in which it is to be wrought into plates. For purifying the pearl ashes, dissolve them in four times their weight of boiling water, in a pot of cast iron, always kept clean from rust. Let the solution be removed into a clean tub, and remain there 24 hours or longer. Having decanted the clear part of the fluid from the dregs or sediment, put it again in the iron pot, and evaporate the water till the salts are left perfectly dry. Preserve them in stone jars, well secured from air and moisture.
Pearl ashes may also be purified in the highest degree, so as to be proper for the manufacture of the most transparent glass, by pulverizing three pounds of the best pearl ashes with six ounces of saltpetre in a glass or marble mortar, till they are well mixed; and then putting part of the mixture into a large crucible, and exposing it in a furnace to a strong heat. When this is red hot, throw in the rest gradually; and when the whole is red hot, pour it out on a moistened stone or marble, and put it into an earthen or clean iron pot, with ten pints of water; heat it over the fire till the salts be entirely melted; let it then stand to cool, and filter it through paper in a pewter cullender. When it is filtered, put the fluid again into the pot, and evaporate the salt to dryness, which will then be as white as snow; the nitre having burnt all the phlogistic matter that remained in the pearl ashes after their former calcination.
As to the sand, it is to be sifted and washed till such time as the water come off very clear; and when it is well dried again, they mix it with the salt, passing the mixture through another sieve. This done, they lay them in the annealing furnace for about two hours; in which time the matter becomes very light and white: in this state they are called frit or frutta; and are to be laid up in a dry clean place, to give them time to incorporate: they lie here for at least a year.
When they would employ this frit, they lay it for some hours in the furnace, adding to some the fragments or shards of old and ill made glasses; taking care first to calcine the shards by heating them red hot in the furnace, and thus causting them into cold water. To the mixture must likewise be add-
Glas. ed mangatefe, to promote the fusion and purification.
The best composition for looking glafs plates confifts of 60 pounds of white fand cleaned, 25 pounds of purified pearl ashes, 15 pounds of faltpetre, and 7 pounds of borax. If a yellow tinge fhould affect the glafs, a fmall proportion of magnelia, mixed with an equal quantity of arfenic, fhould be added. An ounce of the magnelia may be firft tried; and if this proves infufficient, the quantity fhould be increafed.
A cheaper composition for looking glafs plate confifts of 60 pounds of the white fand, 20 pounds of pearl ashes, 10 pounds of common falt, 7 pounds of nitre, 2 pounds of arfenic, and 1 pound of borax. The matter of which the glaffes are made at the famous manufacture of St Gobin in France, is a composition of folder and of a very white fand, which are carefully cleaned of all heterogeneous bodies; afterwards walhed for feveral times, and dried fo as to be pulverized in a mill, confifting of many peffles, which are moved by horfes. When this is done, the fand is lifted through filk fieves and dried.
The matter thus far prepared is equally fit for plate glafs, to be formed either for blowing or by cafting.
The largeft glaffes at St Gobin are run; the middle fixed and fmall ones are blown.
2. Blowing the plates. The workhoufes, furnaces, &c. ufed in the making of this kind of plate glafs, are the fame, except that they are fmall, and that the carquaffes are difpofed in a large covered gallery, over againft the furnace, as thofe in the following article, to which the reader is referred.
After the materials are vitrified by the heat of the fire, and the glafs is fufficiently refined, the workman dips in his blowing iron, fix feet long, and two inches in diameter, fharpened at the end which is put in the mouth, and widened at the other, that the matter may adhere to it. By this means he takes up a fmall ball of matter, which flicks to the end of the tube by constantly turning it. He then blows into the tube, that the air may fwell the annexed ball; and carrying it over a bucket of water, which is placed on a fupport at the height of about four feet, he fprinkles the end of the tube to which the matter adheres, with water, ftil turning it, that by this cooling the matter may coalefce with the tube, and be fit for fuffaining a greater weight. He dips the tube again into the fame pot, and proceeds as before; and dipping it into the pot a third time, he takes it out, loaded with matter, in the fhape of a pear, about ten inches in diameter, and a foot long, and cools it at the bucket; at the fame time blowing into the tube, and with the affiftance of a labourer, giving it a balancing motion, he caufes the matter to lengthen; which, by repeating this operation feveral times, affumes the form of a cylinder, terminating like a ball at the bottom, and in a point at the top. The affiftant is then placed on a stool three feet and a half high; and on this stool there are two upright pieces of timber, with a crofs beam of the fame, for fupporting the glafs and tube, which are kept in an oblique pofition by the affiftant, that the mailer workman may with a puncheon fet in a wooden handle, and with a mallet, make a hole in the maf; this hole is drilled at the centre of the ball that terminates the cylinder, and is about an inch in diameter.
VOL. IX. Part II.
Glas. When the glafs is pierced, the defects of it are perceived; if it is tolerably perfect, the workman lays the tube horizontally on a little iron treffel, placed on the fupport of the aperture of the furnace. Having expofed it to the heat for about half a quarter of an hour, he takes it away, and with a pair of long and broad fhears, extremely fharp at the end, widens the glafs, by infinuating the fhears into the hole made with the puncheon, whilft the affiftant, mounted on the stool, turns it round, till at laft the opening is fo large as to make a perfect cylinder at bottom. When this is done, the workman lays his glafs upon the treffels at the mouth of the furnace to heat it; he then gives it to his affiftant on the stool, and with large fhears cuts the maf of matter up to half its height. There is at the mouth of the furnace an iron tool called pontil, which is now heating, that it may unite and coalefce with the glafs juft cut, and perform the office which the tube did before it was feprated from the glafs. This pontil is a piece of iron fix feet long, and in the form of a cane or tube, having at the end of it a fmall iron bar, a foot long, laid equally upon the long one, and making with it a T. This little bar is full of the matter of the glafs, about four inches thick. This red hot pontil is prefented to the diameter of the glafs, which coalefces immediately with the matter round the pontil, fo as to fupport the glafs for the following operation. When this is done, they feprate the tube from the glafs, by ftriking a few blows with a chiffel upon the end of the tube which has been cooled; fo that the glafs breaks directly, and makes this fepration, the tube being difcharged of the glafs now adhering to the pontil. They next prefent to the furnace the pontil of the glafs, laying it on the treffel to heat, and redder the end of the glafs, that the workman may open it with his fhears, as he has already opened one end of it, to complete the cylinder; the affiftant holding it on his stool as before. For the laft time, they put the pontil on the treffel, that the glafs may become red hot, and the workman cuts it quite open with his fhears, right over againft the fore-mentioned cut; this he does as before, taking care that both cuts are in the fame line. In the mean time, the man who looks after the carquaffes comes to receive the glafs upon an iron fhovel two feet and a half long without the handle, and two feet wide, with a fmall border of an inch and a half to the right and left, and towards the handle of the fhovel. Upon this the glafs is laid, flattening it a little with a fmall flick a foot and a half long, fo that the cut of the glafs is turned upwards. They feprate the glafs from the pontil, by ftriking a few gentle blows between the two with a chiffel. The glafs is then removed to the mouth of the hot carquaffe, where it becomes red hot gradually; the workman, with an iron tool fix feet long, and widened at the end in form of a club at cards four inches long, and two inches wide on each fide, very flat, and not half an inch thick, gradually lifts up the cut part of the glafs to unfold it out of its form of a flattened cylinder, and render it fmoath, by turning it down upon the hearth of the carquaffe. The tool already defcribed being infinuating within the cylinder, performs this operation by being pufhed hard againft all the parts of the glafs. When the glafs is thus made quite fmoath, it is pufhed to the bottom of the
5 C
Glas. carquaisse or annealing furnace with a small iron raker, and ranged there with a little iron hook. When the carquaisse is full, it is stopped and cemented as in the case of run glasses, and the glass remains there for a fortnight to be annealed; after which time they are taken out to be polished. A workman can make but one glass in an hour, and he works and rests for six hours alternately.
Such was the method formerly made use of for blowing plate glass, looking glasses, &c.; but the workmen, by this method, could never exceed 50 inches in length, and a proportional breadth, because what were larger were always found to warp, which prevented them from reflecting the objects regularly, and wanted substance to bear the necessary grinding. These imperfections have been remedied by the following invention of the Sieur Abraham Thevart, in France, about the year 1688.
3. Casting or Running of Large Mirror Glass Plates. The furnace is of a very large dimension, environed with several ovens, or annealing furnaces, called carquaisse, besides others for making of frit and calcining old pieces of glass. This furnace, before it is fit to run glass, costs 3500l. It seldom lasts above three years, and even in that time it must be refitted every six months. It takes six months to rebuild it, and three months to rest it. The melting pots are as big as large hogheads, and contain about 2000 weight of metal. If one of them bursts in the furnace, the loss of the matter at a time amounts to 250l. The materials in these pots are the same as described before. When the furnace is red hot, these materials are put in at three different times, because that helps the fusion; and in 24 hours they are vitrified, refined, settled, and fit for casting. A is the bocca, or mouth of the furnace; B is the cistern that conveys the liquid glass it receives out of the melting pots in the furnace to the casting table. These cisterns are filled in the furnace, and remain therein six hours after they are filled; and then are hooked out by the means of a large iron chain, guided by a pulley, placed upon a carriage with four wheels marked C, by two men. This carriage has no middle piece; so that when it has brought the cistern to the casting table D, they slip off the bottom of the cistern, and out rushes a torrent of flaming matter upon the table: this matter is confined to certain dimensions by the iron rulers EE, which are moveable, retain the fluid matter, and determine the width of the glass; while a man, with the roller F resting on the edge of the iron rulers, reduceth it as it cools to an equal thickness, which is done in the space of a minute. This table is supported on a wooden frame, with trusses for the convenience of moving to the annealing furnace; into which, strewed with sand, the new plate is shoved, where it will harden in about 10 days.
What is most surprising throughout the whole of this operation, is the quickness and address wherewith such massy cisterns, filled with a flaming matter, are taken out of the furnace, conveyed to the table, and poured therein, the glass spread, &c. The whole is inconceivable to such as have not been eye witnesses of that surprising manufacture.
As fast as the cisterns are emptied, they carry them back to the furnace and take fresh ones, which they empty as before. Thus they continue to do so long as
there are any full cisterns; laying as many plates in each carquaisse as it will hold, and stopping them up with doors of baked earth, and every chink with cement, as soon as they are full, to let them anneal, and cool again, which requires about 14 days.
The first running being dispatched, they prepare another, by filling the cisterns anew from the matter in the pots; and after the second, a third; and even a fourth time, till the melting pots are quite empty.
The cisterns at each running should remain at least six hours in the furnace to whiten; and when the first annealing furnace is full, the casting table is to be carried to another. It need not here be observed, that the carquaisse, or annealing furnaces, must first have been heated to the degree proper for them. It may be observed, that the oven full, or the quantity of matter commonly prepared, supplies the running of 18 glasses, which is performed in 18 hours, being an hour for each glass. The workmen work six hours, and are then relieved by others.
When the pots are emptied, they take them out, as well as the cisterns, to scrape off what glass remains, which otherwise would grow green by continuance of fire, and spoil the glasses. They are not filled again in less than 36 hours; so that they put the matter into the furnace, and begin to run it every 54 hours.
The manner of heating the large furnaces is very singular; the two tifers, or persons employed for that purpose, in their shirts, run swiftly round the furnace without making the least stop: as they run along, they take two billets, or pieces of wood, which are cut for the purpose: these they throw into the first tiffart; and continuing their course, do the same for the second. This they hold without interruption for six hours successively; after which they are relieved by others, &c. It is surprising that two such small pieces of wood, and which are consumed in an instant, should keep the furnace to the proper degree of heat; which is such, that a large bar of iron, laid at one of the mouths of the furnace, becomes red hot in less than half a minute.
The glass, when taken out of the melting furnace, needs nothing farther but to be ground, polished, and foliated.
4. Grinding and Polishing of Plate Glass. Glass is made transparent by fire; but it receives its lustre by the skill and labour of the grinder and polisher; the former of whom takes it rough out of the hands of the maker.
In order to grind plate glass, they lay it horizontally upon a flat stone table made of a very fine grained freestone; and for its greater security they plaster it down with lime or stucco; for otherwise the force of the workmen, or the motion of the wheel with which they grind it, would move it about.
This stone table is supported by a strong frame A, made of wood, with a ledge quite round its edges, rising about two inches higher than the glass. Upon this glass to be ground is laid another rough glass not above half so big, and so loose as to slide upon it; but cemented to a wooden plank, to guard it from the injury it must otherwise receive from the scraping of the wheel to which this plank is fastened, and from the weights laid upon it to promote the grinding or triture of the glasses. The whole is covered with a wheel B, Glas. Plate GCLXVII. made
Glass. made of hard light wood, about six inches in diameter, by pulling of which backwards and forwards alternately, and sometimes turning it round, the workmen, who always stand opposite to each other, produce a constant attrition between the two glasses, and bring them to what degree of smoothness they please, by first pouring in water and coarse sand; after that, a finer sort of sand, as the work advances, till at last they must pour in the powder of smalt. As the upper or incumbent glass polishes and grows smoother, it must be taken away, and another from time to time put in its place.
This engine is called a mill by the artists, and is used only in the largest sized glasses; for in the grinding of the lesser glasses, they are content to work without a wheel, and to have only four wooden handles fastened to the four corners of the stone which loads the upper plank, by which they work it about.
When the grinder has done his part, who finds it very difficult to bring the glass to an exact plainness, it is turned over to the polisher; who, with the fine powder of tripoli stone or emery, brings it to a perfect evenness and lustre. The instrument made use of in this branch is a board, cc, furnished with a felt, and a small roller, which the workman moves by means of a double handle at both ends. The artist, in working this roller, is assisted with a wooden hoop or spring, to the end of which it is fixed: for the spring, by constantly bringing the roller back to the same points, facilitates the action of the workman's arm.
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 are 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, a part of it in the frit, and the rest in the melted metal.
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 alummen 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 alummen 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 such an equal quantity; powder and mix them well; then take 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 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
Glàs. the philosopher's stone, may be prepared in the following manner: take of the second composition for hard glàs 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 glàs of a deep transparent yellow brown colour. Powder this glàs, and add to it two pounds of powdered glàs of antimony; grind them together, and thus mix them well. Take part of this mixture, and rub into it 8 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 glàs. 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 glàs. This kind of glàs 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 glàs, will make it represent the semi-opaque 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 glàs, but the best is the following.
Dissolve four ounces of fine leaf silver in a glàs 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 glàs 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 glàs 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 glàs; and let it remain an hour between each time of putting in the powders. After all are in, let it stand 24 hours; then let the glàs be well mixed, and take an assay 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, foot of the chimney two ounces, crocus martis made with brimstone, halt an ounce; let these be well powdered and mixed, and put them by degrees into the glàs at six times, waiting a little while between each putting in. When the whole is put in, let the glàs boil and settle for 24 hours; then make a little glàs body of it; which put in the furnace many times, and see if the glàs 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, foot, 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.
Chrysolite colour may be made of ten pounds of either of the compositions for hard glàs described above, and six drachms of calcined iron.
Red cornelian colour may be formed by adding one pound of glàs 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 crystall 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 fallow, or any other soft kind. Precipitates of silver baked on glass will stain it yellow, and likewise give 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 foot 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 fusibility 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 tals 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 pulverine, four pounds of common pulverine 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
Glas. 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 fine 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
Glas. 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 Kuckel. The gold is directed to be dissolved in a mixture of one part of spirit of salt and three of aquafortis, 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 opaque 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 opaque 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.