A History of Inventions, Discoveries, and Origins, Volume I (of 2)

Seneca, in his ninetieth epistle, in which he judges too philosophically, that is, with too little knowledge of the world, in regard to the value of labour, mentions one Democritus who had discovered the art of making artificial emeralds309; but in my opinion this discovery consisted in giving a green colour by cementation to the natural rock crystal: and this art I imagine was treated of in that book, the name of which Pliny, through an over-anxious care lest the deception should become common, does not mention310. For colouring crystal and glass, so as to resemble stones, Porta311, Neri312, and others have, in modern times, given directions which are, however, not much used, because the crystal is thereby liable to acquire so many flaws that it cannot be easily cut afterwards, though, as Neri assures us, these by attention may sometimes be avoided.

It is worthy of remark, that in some collections of antiquities at Rome, there are pieces of coloured glass which were once used as jewels. In the Museum Victorium, for example, there are shown a chrysolite and an emerald, both of which are so well executed, that they are not only perfectly transparent and coloured throughout, but neither externally nor internally have the smallest blemish, which certainly could not be guarded against without great care and skill.

What materials the ancients used for colouring glass, has not been told to us by any of their writers. It is, however, certain that metallic oxides only can be employed for that purpose, because these pigments withstand the heat of the glass furnaces; and it is highly probable that ferruginous earth, if not the sole, was at least the principal substance, by which not only all shades of red, violet, and yellow, but even a blue colour, could be communicated, as Professor Gmelin has shown313. Respecting the red, of which only I mean here to speak, there is the less doubt, as, at present, sometimes an artificial, and sometimes a natural, iron ochre is often employed for that purpose. For common works this is sufficient; but when pure clear glass, coloured strongly throughout with a beautiful lively red, free from flaws, and in somewhat large pieces, is required, iron is not fit, because its colour, by the continued heat necessary for making glass, either disappears or becomes dirty and almost blackish314.

In the last century, some artists in Germany first fell upon the method of employing gold instead of iron, and of thereby making artificial rubies, which when they were well set could deceive the eye of a connoisseur, unless he tried them with a diamond or a file. The usual method was to dissolve the gold in aqua regia, and to precipitate it by a solution of tin, when it assumed the form of a purple-coloured powder. This substance, which must be mixed with the best frit, is called the precipitate of Cassius, gold-purple, or mineral-purple315.

This Cassius, from whom it takes its name, was called Andrew, and because both the father and the son had the same christian name, they have been often confounded with each other. The father was secretary to the duke of Schleswig, and is not known as a man of letters; but the son is celebrated as the inventor or preparer of the gold-purple, and of a bezoar-essence. He took the degree of doctor at Leyden, in 1632, practised physic at Hamburg, and was appointed physician in ordinary to the bishop of Lubec. As far as I know, he never published anything respecting his art; but this service was rendered to the public by his son, who was born at Hamburg, and resided as a physician at Lubec. He was the author of a well-known treatise, now exceedingly scarce, entitled Thoughts concerning that last and most perfect work of nature, and chief of metals, gold, its wonderful properties, generation, affections, effects, and fitness for the operations of art; illustrated by experiments316.

From this work, it will be easily understood why the author does not give himself out as the inventor of the gold-purple317, which he is commonly supposed to be, at which Lewis is much astonished. It is seen also by it that Leibnitz calls him improperly a physician at Hamburg, having probably confounded the father and son together318. Upon the whole, it is not proved that any of the Cassius’s was the inventor of the above precipitate, else it would certainly not have been omitted319 in this treatise; and mention of gold-purple is to be found in the works of several old chemists320.

Something of this kind has, doubtless, been meant by the old chemists, when they talk of red lions, the purple soul of gold, and the golden mantle; but what they wished to conceal under these metaphors, I am not able to conjecture. In the year 1606, when Libavius published his Alchemy, the art of making ruby-glass must have been unknown. He indeed quotes an old receipt for making rubies; and conjectures, that because the real stones of the same name are found in the neighbourhood of gold mines, they may have acquired their colour from that metal; and that by means of art, glass might be coloured by a solution of gold321. The later chemists, however, and particularly Achard, found no traces of gold, but of iron, in that precious stone322.

Neri, who lived almost at the same time as Libavius323, was better acquainted with the gold-purple, though his receipt is very defective. According to his directions, the gold solution must be evaporated, and the residue suffered to remain over the fire until it becomes of a purple colour. One may readily believe that this colour will be produced; but glass will scarcely be coloured equally through by this powder, and perhaps some of the gold particles will show themselves in it. Kunkel affirms, and not without reason, that something more is necessary to make rubies by means of gold; but he has not thought proper to tell us what it is324.

Glauber, who wrote his Philosophical Furnace325 about the middle of the seventeenth century, appears to have made several experiments with the gold-purple. He dissolved the metal in aqua regia; precipitated it by liquor of flint, and melted into glass the precipitate, which contained in it abundance of vitreous earth326.

None, however, in the seventeenth century, understood better the use and preparation of gold-purple than John Kunkel, who, after being ennobled by Charles XI., king of Sweden, assumed the name of Löwenstiern. He himself tells us, that he made artificial rubies in great abundance, and sold them by weight, at a high price. He says, he made for the elector of Cologne a cup of ruby-glass, weighing not less than twenty-four pounds, which was a full inch in thickness, and of an equally beautiful colour throughout. He employed himself most on this art after he engaged in the service of Frederic William, elector of Brandenburg, in the year 1679. At that time he was inspector of the glass-houses at Potsdam; and, in order that the art of making ruby-glass might be brought to perfection, the elector expended 1600 ducats. A cup with a cover, of this manufacture, is still preserved at Berlin. Kunkel, however, has nowhere given a full account of this art. He has only left in his works a few scattered remarks, which Lewis has collected327.

In the year 1684, earlier than Cassius, John Christian Orschal wrote his well-known work, Sol sine veste328, in which he treats, more intelligibly than any one before him, of the manner of making ruby-glass. He, however, confesses that Cassius first taught him to precipitate gold by means of tin; that Cassius traded in glass coloured with this precipitate, and that a good deal of coloured glass was then made at Freysingen, but that the art was kept very secret. As Orschal deserves that his fate should be better known, I shall here mention the following few particulars respecting him. About the year 1682 he was at Dresden, in the service of John Henry Rudolf, from whom he learnt many chemical processes, and particularly amalgamation, by which he gained money afterwards in Bohemia. After this he was employed at the mines in Hesse; but he brought great trouble upon himself by polygamy and other irregularities, and died in a monastery in Poland.

Christopher Grummet, who was Kunkel’s assistant, wrote, in opposition to Orschal, his known treatise, Sol non sine veste, which was printed at Rothenburg, in 1685329. In like manner, an anonymous author printed against Orschal, at Cologne, in 1684, another work, in duodecimo, entitled Apelles post tabulam observans maculas in Sole sine veste. The dispute, however, was not so much concerning the use of gold-purple, as the cause of the red colour, and the vitrification of gold.

It is worthy of remark, that Kunkel affirms he could give to glass a perfect ruby-red colour without gold; which Orschal and most chemists have however doubted. It is nevertheless said, that Krüger, who was inspector of the glass-houses at Potsdam, under Frederic William king of Prussia, discovered earlier the art of making ruby-glass without gold, and that a cup and cover of cut glass made in this manner is still preserved at Berlin.

Painting on glass and in enamel, and the preparation of coloured materials for mosaic work, may, in certain respects, be considered as branches of the art of colouring glass; and in all these a beautiful red is the most difficult, the dearest, and the scarcest. When the old master-pieces of painting on glass are examined, it is found either that the panes have on one side a transparent red varnish burnt into them, or that the pieces which are stained through and through, are thinner than those coloured in the other manner330. It is therefore extremely probable that the old artists, as they did not know how to give to thick pieces a beautiful transparent red colour, employed only iron, or manganese, which pigment, as already observed, easily becomes in a strong heat blackish and muddy331. Enamel-painters, however, were for a long time obliged to be contented with it. A red colour in mosaic work is attended with less difficulty, because no transparency, nay rather opacity, is required. At Rome those pieces are valued most which have the beautiful shining red colour of the finest sealing-wax. We are told by Ferber that such pieces were at one time made only by a man named Mathioli, and out of a kind of copper dross; at present (1792), there are several artists in that city who prepare these materials, but they are not able to give them a perfect high colour332.

[Of late years the interesting art of painting on glass has attracted considerable attention; lovers of the fine arts, antiquaries, and chemists, have contributed to its perfection, and have sought to ascertain by what methods their predecessors were able to give those beautiful and brilliant tints to their productions, many of which have been so wantonly destroyed by the barbarity of the last century333. One of the most ingenious essays that has been written on the subject, is that published by an anonymous correspondent in the Philosophical Magazine for December 1836, which we subjoin in elucidation of our present knowledge on the subject.

On the Art of Glass-Painting. By a Correspondent

It is a singular fact, that the art of glass-painting, practised with such success during the former ages from one end of Europe to the other, should gradually have fallen into such disuse, that in the beginning of the last century it came to be generally considered as a lost art. In the course of the eighteenth century, however, the art again began to attract attention, and many attempts were made to revive it. It was soon found by modern artists, that by employing the processes always in use among enamel-painters, the works of the old painters on glass might in most respects be successfully imitated; but they were totally unable to produce any imitation whatever of that glowing red which sheds such incomparable brilliancy over the ancient windows that still adorn so many of our churches334. For this splendid colour they possessed no substitute, until a property, peculiar to silver alone among all the metals, was discovered, which will presently be described. The art of enamelling on glass differs little from the well-known art of enamelling on other substances. The colouring materials (which are exclusively metallic) are prepared by being ground up with a flux, that is, a very fusible glass, composed of silex, flint-glass, lead, and borax: the colour with its flux is then mixed with volatile oil, and laid on with the brush. The pane of glass thus enamelled is then exposed to a dull red heat, just sufficient to soften and unite together the particles of the flux, by which means the colour is perfectly fixed on the glass. Treated in this way, gold yields a purple, gold and silver mixed a rose-colour, iron a brick-red, cobalt a blue335; mixtures of iron, copper and manganese, brown and black. Copper, which yields the green in common enamel-painting336, is not found to produce a fine colour when applied in the same way to glass, and viewed by transmitted light; for a green therefore recourse is often had to glass coloured blue on one side and yellow on the other. To obtain a yellow, silver is employed, which, either in the metallic or in any other form, possesses the singular property of imparting a transparent stain, when exposed to a low red heat in contact with glass. This stain is either yellow, orange, or red, according to circumstances. For this purpose no flux is used: the prepared silver is merely ground up with ochre or clay, and applied in a thick layer upon the glass. When removed from the furnace the silver is found not at all adhering to the glass; it is easily scraped off, leaving a transparent stain, which penetrates to a certain depth. If a large proportion of ochre has been employed, the stain is yellow; if a small proportion, it is orange-coloured; and by repeated exposure to the fire, without any additional colouring matter, the orange may be converted into red. This conversion of orange into red is, I believe, a matter of much nicety, in which experience only can ensure success. Till within a few years this was the only bright red in use among modern glass-painters; and though the best specimens certainly produce a fine effect, yet it will seldom bear comparison with the red employed in such profusion by the old artists.

Besides the enamels and stains above-described, artists, whenever the subject will allow of it, make use of panes coloured throughout their substance in the glass-house melting-pot, because the perfect transparency of such glass gives a brilliancy of effect, which enamel-colouring, always more or less opake, cannot equal. It was to a glass of this kind that the old glass-painters owed their splendid red. This in fact is the only point in which the modern and ancient processes differ, and this is the only part of the art which was ever really lost. Instead of blowing plates of solid red, the old glass-makers used to flash a thin layer of red over a substratum of plain glass. Their process must have been to melt side by side in the glass-house a pot of plain and a pot of red glass: then the workman, by dipping his rod first into the plain and then into the red glass pot, obtained a lump of plain glass covered with a coating of red, which, by dexterous management in blowing and whirling, he extended into a plate, exhibiting on its surface a very thin stratum of the desired colour337. In this state the glass came into the hands of the glass-painter, and answered most of his purposes, except when the subject required the representation of white or other colours on a red ground: in this case it became necessary to employ a machine like the lapidary’s wheel, partially to grind away the coloured surface till the white substratum appeared.

The material employed by the old glass-makers to tinge their glass red was the protoxide of copper, but on the discontinuance of the art of glass-painting the dependent manufacture of red glass of course ceased, and all knowledge of the art became so entirely extinct, that the notion generally prevailed that the colour in question was derived from gold338. It is not a little remarkable that the knowledge of the copper-red should have been so entirely lost, though printed receipts have always existed detailing the whole process. Baptista Porta (born about 1540) gives a receipt in his Magia Naturalis, noticing at the same time the difficulty of success. Several receipts are found in the compilations of Neri, Merret and Kunckel, from whence they have been copied into our Encyclopædias339. None of these receipts however state to what purposes the red glass was applied, nor do they make any mention of the flashing. The difficulty of the art consists in the proneness of the copper to pass from the state of protoxide into that of peroxide, in which latter state it tinges glass green. In order to preserve it in the state of protoxide, these receipts prescribe various deoxygenating substances to be stirred into the melted glass, such as smiths’ clinkers, tartar, soot, rotten wood, and cinnabar.

One curious circumstance deserves to be noticed, which is, that glass containing copper when removed from the melting-pot sometimes only exhibits a faint greenish tinge, yet in this state nothing more than simple exposure to a gentle heat is requisite to throw out a brilliant red. This change of colour is very remarkable, as it is obvious that no change of oxygenation can possibly take place during the recuisson.

The art of tinging glass by protoxide of copper and flashing it on crown-glass, has of late years been revived by the Tyne Company in England, at Choisy in France340, and in Suabia in Germany, and in 1827 the Academy of Arts at Berlin gave a premium for an imperfect receipt. To what extent modern glass-painters make use of these new glasses I am ignorant; the specimens that I have seen were so strongly coloured as to be in parts almost opake, but this is a defect which might no doubt be easily remedied341.

I shall now conclude these observations by a few notices respecting glass tinged by fusion with gold, which, though never brought into general use among glass-painters, has I know been employed in one or two instances, flashed both on crown- and on flint-glass. Not long after the time when the art of making the copper-red glass was lost, Kunkel appears to have discovered that gold melted with flint-glass was capable of imparting to it a beautiful ruby colour. As he derived much profit from the invention, he kept his method secret, and his successors have done the same to the present day. The art, however, has been practised ever since for the purpose of imitating precious stones, &c., and the glass used to be sold at Birmingham for a high price under the name of Jew’s glass. The rose-coloured scent-bottles, &c., now commonly made, are composed of plain glass flashed or coated with a very thin layer of the glass in question. I have myself made numerous experiments on this subject, and have been completely, and at last uniformly, successful, in producing glass of a fine crimson colour. One cause why so many persons have failed in the same attempt342, I suspect is that they have used too large a proportion of gold; for it is a fact, that an additional quantity of gold, beyond a certain point, far from deepening the colour, actually destroys it altogether. Another cause probably is, that they have not employed a sufficient degree of heat in the fusion. I have found that a degree of heat, which I judged sufficient to melt cast-iron, is not strong enough to injure the colour. It would appear, that in order to receive the colour, it is necessary that the glass should contain a proportion either of lead, or of some other metallic glass. I have found bismuth, zinc, and antimony to answer the purpose, but have in vain attempted to impart any tinge of this colour to crown-glass alone.

Glass containing gold exhibits the same singular change of colour on being exposed to a gentle heat, as has been already noticed with respect to glass containing copper343. The former when taken from the crucible is generally of a pale rose-colour, but sometimes colourless as water, and does not assume its ruby colour till it has been exposed to a low red-heat, either under a muffle or in the lamp. Great care must be taken in this operation, for a slight excess of fire destroys the colour, leaving the glass of a dingy brown, but with a blue transparency like that of gold-leaf. These changes of colour have been vaguely attributed to change of oxygenation in the gold; but it is obviously impossible that mere exposure to a gentle heat can effect any chemical change in the interior of a solid mass of glass, which has already undergone a heat far more intense. In fact I have found that metallic gold gives the red colour as well as the oxide, and it appears scarcely to admit of a doubt, that in a metal so easily reduced, the whole of the oxygen must be expelled long before the glass has reached its melting-point. It has long been known that silver yields its colour to glass while in the metallic state, and everything leads one to suppose that the case is the same as to gold.

There is still one other substance by means of which I find it is possible to give a red colour to glass, and that is a compound of tin, chromic acid, and lime; but my trials do not lead me to suppose that glass thus coloured will ever be brought into use.

* * * * *

With respect to the production of artificial gems, they are now made abundantly of almost every shade of colour, closely approximating to those which occur in nature, excepting in hardness and refractive power. They are formed by fusing what is called a base with various metallic oxides. The base varies in composition: thus, M. Fontanieu makes his by fusing silica with carbonate of potash, carbonate of lead and borax. M. Donault Wieland’s consists of silica, potash, borax, oxide of lead, and sometimes arsenious acid. Hence the base differs but little in composition from glass. By fusing the base with metallic oxides, the former acquires various tints. Thus with oxide of antimony the oriental topaz is prepared; with oxide of manganese and a little purple of cassius, the amethyst; with antimony and a very small quantity of cobalt, the beryl; with horn silver (chloride of silver), the diamond and opal: the oriental ruby is prepared from the base, the purple of cassius, peroxide of iron, golden sulphuret of antimony, manganese calcined with nitre and rock crystal.]

SEALING-WAX

Writers on diplomatics mention, besides metals, five other substances on which impressions were made, or with which letters and public acts were sealed, viz. terra sigillaris, cement, paste, common wax, and sealing-wax344. The terra sigillaris was used by the Egyptians, and appears to have been the first substance employed for sealing345. The Egyptian priests bound to the horns of the cattle fit for sacrifice a piece of paper; stuck upon it some sealing-earth, on which they made an impression with their seal; and such cattle only could be offered up as victims346.

Lucian speaks of a fortune-teller who ordered those who came to consult him to write down on a bit of paper the questions they wished to ask, to fold it up, and to seal it with clay, or any other substance of the like kind347. Such earth seems to have been employed in sealing by the Byzantine emperors: for we are told that at the second council of Nice, a certain person defended the worship of images by saying, no one believed that those who received written orders from the emperor and venerated the seal, worshiped on that account the sealing-earth, the paper, or the lead348.

Cicero relates that Verres having seen in the hands of one of his servants a letter written to him from Agrigentum, and having observed on it an impression in sealing-earth (cretula), he was so pleased with it that he caused the seal-ring with which it was made to be taken from the possessor349. The same orator, in his defence of Flaccus, produced an attestation sent from Asia, and proved its authenticity by its being sealed with Asiatic sealing-earth; with which, said he to the auditors, as you daily see, all public and private letters in Asia are sealed: and he showed on the other hand that the testimony brought by the accuser was false, because it was sealed with wax, and for that reason could not have come from Asia350. The scholiast Servius relates, that a sibyl received a promise from Apollo, that she should live as long as she did not see the earth of the island Erythræa where she resided; that she therefore quitted the place, and retired to Cumæ, where she became old and decrepid; but that having received a letter sealed with Erythræan earth (creta), when she saw the seal she instantly expired351.