Mention being made of metallick glasses, which belongs to the perfection of metals, I am forced to say somthing also of other Amausa, or coloured glasses, which are called Gems, and are worn for beautifying, which though it be not profitable, yet it is a delightful labour, which knowledge, as well noble as ignoble have long sought, not for gain, but recreation sake, erring from the true way (although prolixly described in many tongues) through ignorance of the art to render crystal or flint fusible, and colouring it, being content with lead glasses made of one part of crystals, or flints, and three or four parts of minium or ceruse, glass of no worth, as not only very soft and unapt for polishing, but also heavier than it ought by means of the lead, and having a yellow or green colour, for every glass made of crystal or flint, and minium or ceruse by themselves, viz. without the addition of other colours, gets a yellow colour from the Lead, hindering and altering other mixt colours; therefore a good stone is not made this way of lead and flint, but Leaden glasses of this sort, Venice glass, Ashes of tin, and colours being added to them, be used diversly of the goldsmiths, namely to colour gold, otherwise of no moment.
Therefore I will give another preparation, namely out of flints &c. crystals alone without minium and ceruse, with metallick colours, having the colour and elegancy of excellent stones; but not harder than glass; for although crystal is harder than iron, yet by melting it is deprived of its hardness in some measure, and is made like to glass, yet so much hardness reserved, as serves to write on another glass, which glasses are easily polished, and in all things and by all, most like, hardness excepted, to natural stones; with which not only various kinds of stones may be made, and other gold, silver, and wooden works or pictures adorned; but also diverse supellectils, as salts, hafts or hilts, cups, &c. and also images and antiquities may be formed (by fusion) like to those cut out of gems by the hand of an ingenious workman, most delightful.
They are made after this manner: first you must look for flints and crystals not coloured, but very white, gathered out of sand or streams, which you must heat in a covered crucible, and quench them glowing hot in cold water, that they may crack and may be pulverised; otherwise they are so hard that when they are powdred, they take part of the morter and so are defiled; therefore it is worth your labor to handle them well. Afterward ℞ of flints prepared, and the purest salt of Tartar, made in glased vessels, but not in copper or iron, equal parts, mingle them and keep them for use.
And if thou wilt make this mass into a gem, you must first mingle some colour (what you desire) afterward so long place it (being put into a clean covered crucible scarce half full) in a very strong fire, till all the salt of Tartar hath evaporated, and the flint together with the colour come into substance fusible like glass: you must then put a small clean iron wier, and draw out a little of the melted mass for tryal; whether it have stood long enough in the fire, whether there be yet pustles and little sands, or whether it being exactly melted, it shall descend to the bottom, which done, you must take off the crucible, and place it under some hot iron or earthen vessel, that it may wax cold with the melted stone; otherwise the mass will be broken in the crucible into very small parts, and would be unfit for greater works: neither must you pour out the melted mass for fear of the attraction of aire, and pustles to arise thence. But being willing to make out of the Mass by Fusion, not Engraving Money or Images; there is no need to leave the mass in the crucible to cool, but presently to pour it out hot in a copper morter, and nothing will stick to the crucible, but all the mass will be poured out without any waste: And this mass, if thou wilt, thou maist powder or break into very small bits for fusion and impression. But the mass when cooled in the crucible, is to be taken by breaking the crucible, and to be reduced into greater or lesser stones by cutting: but melting for money or images; you must place the money or image, which you will imitate, with the backside or hinder-part downward in an iron Ring, a Fingers breadth broad of greater capacity than the money, upon a stone or plain wood, and sprinkle on a little Tripoly, or fine Sand, through a cloth, namely, as much as sufficeth to cover the mold, and upon this to put more, well moistened with water, like ashes of cupels, and to press it, being most tenacious, firmly to the mold, but warily, lest the mold be moved; which done, you must turn the ring, and with a knife lift up the mold, and to take it, being lifted up with ones hands or tongs, the image being left in the sand, to be dryed by heat of the Sun or Fire. Afterward to cast the image, place the ring with the image impressed in the sand under a tile, and administer a strong fire, that the whole ring, with the sand, and the image in the sand may be very hot: then take off the ring, to see if the image have suffered any loss; which, if it have not, you must put upon it so much of the aforesaid glass, coursly beaten, as sufficeth in the fusion to fill the image impressed on the sand; which done, put the ring again under the tile, and administer a fire of fusion, till the glass melt in the ring; to which, touch with a smooth iron and light, (with a handle) being hot the ring being taken first out of the furnace with tongs, pressing the glass well to the mold; and then place it under a hot iron, or earthen vessel to cool; and being cold, take the image from the mold, which answers to it in all things, if thou hast aright proceeded, exactly representing the Carvers art, or a seal impressed on a jewel, which excellent work is most fit to feign, and represent Antiquities and Rarities.
The colouring of the aforesaid mass follows, by which it is made most like to Gems.
It behooveth that colours be taken from metals and minerals, namely from Copper, Iron, Gold, Silver, Wismuth, Magnesia and Granate; of other colours I know nothing of certainty, Copper commonly makes a colour green like the Sea, Copper with Iron, grass-green; Granate a smaragdine colour, Iron yellow or jacynth; Gold the best skie colour; Wismuth common skie colour; Magnesia Amethystine, mixt, they give other colours; E. gr. Gold mixt with Silver gives an Amethyst colour; Iron and Copper, a pale green; Wismuth and Magnesia, a purple; Silver and Magnesia, various colours like an Opal.
Images are also made of divers colours, if the masses of diverse colours be broken into bits and mixt, be put upon the Mold, &c. And if thou desirest an opac mass (green, red, skie colour, &c.) add a little calx of Tin darkning, on which as on a Basis the colours insist. For example; in making a Turcoise stone or a Lazulus, mingle with the Azure made of the silver Marcasit or Zafora (to colour the mass) the calx of Tin, that they may melt together, and before the impression be made, put upon the Mold some prepared gold, then spread and put upon this the aforesaid glass; and the fusion and impression being made, will be made thence a stone having golden veins like lapis Lazulus very delightful; But there must be a calx of Gold not losing its splendor in the fire, such as is made by Mercury, or that which is better, which is precipitated out of Aqua Regia: of which above.
Of the preparation of the colours for colouring the mass of Flints and Crystals.
The plates of copper often heated, are to be quenched in cold water of which more in the Fifth part, from three to six grains of it may be mixed with ℥ j. of the mass for a Sea-green colour. Iron is reduced into crocus by reverberation; of which from four to ten grains are added to the mass for a yellow or Jacynth colour; Silver is dissolved in Aqua fortis, and precipitated with the liquor of Flints after it is edulcorated and dryed, whereof from one to six grains, added to ℥ j. of the mass, they make mixt colours.
Gold is dissolved in Aqua Regia, edulcorated and dryed, precipitated first with liquor of Flints, whereof from grain four to ℈ ss. mixt with one ounce of the mass, make a most elegant Saphire. And if from three to six of that soluble ruby made of the Gold, and the nitrous Regulus Martis be added to ℥ j. of the mass, they make a very polite ruby: Magnesia pulverised, whereof, from six to fourteen grains, to ℥ j. of the mass, make an Amethyst.