|Paste gem and gold finger ring|
Museum of London, 1600s
Next, he works the fractured quartz, which should "crumble and decompose like refined sugar." He grinds it on the porphyry stone with a muller to reduce it to the finest powder; "like sifted grain flour. […] Grind it one time, grind it again and then make another pass. Always put only a little onto the porphyry, which is to say half a spoonful at a time." In another part of the book, Neri recounts a glassmaker's proverb which is apt for this process: "A fine sieve and dry wood bring honor to the furnace."
He mixes in a quantity of "minium" which is an orange or red mineral consisting of lead oxide. This material acts as a "flux," which dramatically lowers the melting temperature of the quartz, and at the same time creates the highly refractive "fire" for which gems are known. In an improvement to this "standard" way of making paste gems, Neri substitutes "Sugar of Saturn" –what we know as lead acetate. This is a water soluble (and highly toxic) lead compound which can be produced in the laboratory, in an extremely purified state. Chemically, minium and lead acetate will reduce the same way in glass; the advantage of the latter is in its higher purity.
The last part of the formula is to mix in various pigments to give the glass the characteristic colors of different gems. Neri uses "zaffer" (cobalt oxide) for sapphires, copper for emeralds, etc. He presents a number of possibilities, even using real gems as pigment. He also gives advice of how to achieve the correct level of tint for different applications:
Be aware that the colors for the pastes described above can be made more or less saturated according to your will, and inclination, and according to the works in which they will be used. In order to make small stones for rings, the color must be deeper, for larger stones use less color, for earrings, and pendants, concentrate the color. Remit this matter to the discretion of whoever is doing the work. There are no true rules here; the rules given by me above serve only to illuminate the intellect of the curious artisan. To this end, you can always invent and find better colors than the ones that I describe here, other than verdigris, zaffer, and manganese. The curious and noble spirit will be able to get a marvelous red from gold, yet another beautiful red from iron, a most beautiful green from copper, a golden yellow color from lead, a celestial one from silver and a sky blue even more beautiful from the garnets of Bohemia. You will find the small garnets inexpensive, and you can extract their tincture, as I have done many times in Flanders. These materials result in notable effects, and you may do the same with rubies, sapphires, and other gems. All of these things are manageable by a practical chemist. To require me to write such things here would be too much of a tedious matter, especially since I wish to speak succinctly in the present work. However, with the colors described above, you can make very appealing pieces.The mixed "pasta" is dried and loaded into small terracotta pots, covered with a lid, and sealed shut with a special mixture of clay called "lute." Many of these pots can be prepared and fired at the same time in a pottery furnace. The pots are fired continuously for twenty-four hours or longer. They are allowed to cool slowly, along with the furnace. The lid is then unsealed and lifted very carefully:
Because it is such a significant matter, I will repeat once more, that when the pastes are not cooked or purified enough, you should return them to cook again in the same crucibles, taking care not to break them. If the pastes are not satisfactorily cooked and purified, putting them into other crucibles will cause many impurities to form. They will have adhesions from the crucibles, and be loaded with dirt; they will be useless, and not usable for any work. When they are not cooked or purified well, then you should not break the crucibles in any way, but re-lute and return them to the furnace or kiln to cook again. Then they will become pure, and beautiful, to be used in any kind of work that you might want to make.