Dear Readers,

As you may have seen elsewhere, in mid February my wife and I suffered the loss of our home in a fire, in the hills of central Massachusetts. The good news is that we got out safely and had no animals in our care at the time. The fire crews were able to contain the fire from spreading, in what turned into a 3-alarm, 5-hour-long ordeal in subzero temperatures; they did amazing work, and no one was injured. The bad news is that all of my physical historical materials and research of 30 years have gone up in smoke. As a result I have decided to suspend this blog for the time being. It will remain online as a resource for those interested in the history of glass and glassmaking in the seventeenth century and beyond. I do intend to resume writing when I can, but for now my time and energy are required in getting us back on our feet.

Friends are providing temporary shelter for us nearby and our intention is to rebuild as soon as possible. To those who have reached out with a steady hand, to those who have opened their wallets, and offered advice in our time of need, we thank you from the bottom of our hearts. In what are already difficult times for all of us, you have made a huge difference in our lives.

Paul Engle
6 March, 2021

Wednesday, December 30, 2015

Ultramarine Blue

Scrovegni  Chapel, Padua
Frescos and ultramarine ceiling, Giotto 1306.
In his fifteenth century handbook for painters, Cennino Cennini said, "Ultramarine blue is a color illustrious, beautiful and most perfect, beyond all other colors; one could not say anything about it, or do anything with it, that its quality would not still surpass." The ancient Egyptians used ultramarine to decorate the sarcophaguses of their pharos. Later, Marco Polo reported that it was made at a lapis lazuli mine in Afghanistan. Its name alludes to these far-flung origins: ultra-marine = "beyond the sea." Venetians were probably the first in Italy to learn the extraction technique and import the raw lapis. Producing the rich blue pigment from the rock was no simple task; success required an elaborate set of steps. Because of the difficulty, for a time, an ounce of ultramarine was valued more highly than an ounce of pure gold. In the legal contracts drawn up for commissioned paintings, patrons often stipulated exact amounts of the precious material for the artist to use. Beyond its beauty, its presence in a painting signaled the wealth of its owner.

In the last part of his book, L'Arte Vetraria, Antonio Neri presents his recipes for a variety of paints, including one for ultramarine. In glassmaking, drinking goblets adorned with delicate paint-work raised their value and elevated them into the realm of art. Unlike enamels, which fired into the glass, most paint, including ultramarine could not survive the furnace, requiring application only after a piece was finished. The number of different paint and lake recipes in the book indicates Neri's familiarity with the craft. This, combined with his willingness to use other painter’s materials like "smalt" in his glass formulations, hints at a still unknown chapter in the alchemist's life. Perhaps, for a period in Antwerp, he worked directly with fine artists. Here is Neri’s ultramarine:

Take fragments of lapis lazuli, which you can find plentifully in Venice and at low prices. Get fragments that are nicely tinted a pretty celestial color and remove any poorly tinted fragments. Cull the nicely colored fragments into a pot and put it amongst hot coals to calcine. When they are inflamed throw them in fresh water and repeat this twice. Then grind them on a porphyry stone most impalpably to become like sifted grain flour. 
Take equal amounts, three ounces each, of pine pitch, black tar, mastic, new wax and turpentine, add one ounce each, of linseed oil and frankincense. I put these things in a clay bowl to warm on the fire until I see them dissolve and with a stirring rod, I mix and incorporate them thoroughly. This done, I throw them into fresh water, so they will combine into one mass for my needs.  
For every pound of finely powdered lapis lazuli, ground as described above, take ten ounces of the above gum cake. In a bowl over a slow fire, melt the gum, and when it is well-liquified throw into it, little by little, the finely powdered lapis lazuli. Incorporate it thoroughly into the paste with a stirring rod.
Cast the hot incorporated material into a vessel of fresh water and, with hands bathed in linseed oil, form a round cake, proportionately round and tall. You should make one or more other of these cakes from the quantity of the material. Then soak these cakes for fifteen days in a large vessel full of fresh water, changing the water every two days. In a kettle, you should boil clear common water and put the cakes in a well-cleaned, glazed earthen basin. Pour warm water over them and then leave them until the water has cooled. 
Empty out the water and pour new warm water over them. When it has cooled, pour again, replenishing the warmth. Repeat this many times over, so that the cakes unbind from the heat of the water. Now add new warm water and you will see that the water will take on a celestial color. Decant the water into a clean glazed pan, pour new [warm] water over the cake and let it color [the water].
When it is colored, decant it and pass it through a sieve into a glazed basin. Pour warm water over the cake, repeatedly until it is no longer colored. Make sure that the water is not too hot, but only lukewarm because too much heat will cause the blue to darken, hence this warning, which is very important. 
Pass all this colored water through a sieve into the basin. It still has the unctuosity of the gum, so leave it to stand and rest for twenty-four hours; all the color will go to the bottom. Then gently decant off the water with its unctuosity, pour clear water over it and pass it through a fine sieve into a clean basin. 
Pass the fresh water through the sieve with the color stirred-up so that this color still passes through and therefore a great part of the filth and unctuosity will remain in the sieve. Wash the sieve well and with new water again pass the color through. Repeat these steps three times, which ordinarily leaves all the filth on the blue resting in the sieve. Always wash the sieve each time, cleaning it of all contamination. Put the blue in a clean pan. Gently decant off the water and then leave it to dry. You will have a most beautiful ultramarine, as I have made many times in Antwerp. 
The amount per pound of lapis lazuli will vary. It depends on whether the lapis has more or less charge of color and on the beauty of its color. Grind it exceedingly fine on the porphyry stone, as described above and you will succeed beautifully.  
For a quite beautiful and sightly biadetto blue that mimics ultramarine blue, take ordinary blue enamel and grind it exceedingly fine over the porphyry stone, as above. Incorporate it into the gum cake with the dose described above and hold it in digestion in fresh water for fifteen days as with the lapis lazuli. Follow the directions for the lapis lazuli, in all and for all, until the end. These blues are not only useful to painters, but they also serve in order to tint glasses par excellence.

Monday, December 28, 2015

The Kabbalah

Kabbalistic Sephiroth Tree,
from Portae Lucis, Paulus Ricius (Trans.)
Augsburg, 1516.
Kabbalah is a form of mysticism practiced within the Jewish tradition. In the early seventeenth century, there was a great deal of interest in Kabbalistic teachings among Catholic alchemists and natural philosophers. It was recognized that Christian alchemy had its roots in Hermetic and earlier Arabic societies, (the word "alchemy" itself is of Arabic origin.) It was thought that the Jewish Kabbalah was yet another branch of the same traditions of relaying secret knowledge by word of mouth. 

In early modern Florence, Italy, there were some interesting connections between the Kabbalah and glassmaker, alchemist and Catholic Priest Antonio Neri. Here is Neri’s own description, of Kabbalah in his 1613 manuscript Discorso: 
Some call it Kabbalah: in ancient times fathers communicated it to their children only by voice, preserving [this knowledge] for posterity, not for history, but as simple tradition. Others finally gave it the name of 'wisdom' [sapienza] because they rightly believed it was impossible, without this art, to know perfectly the nature and the qualities of natural bodies. In order to achieve the end they wanted, which was the perfection of the bodies, they separated the pure from the impure through various chemical operations, which can all be reduced to six principal phases.*
He goes on to describe basic chemical operations that were thought to be fundamental to purifying materials, and ultimately to the production of the Philosopher's Stone. These techniques are the same as practiced in Christian alchemy, and Neri uses them in his glassmaking recipes. Clearly, he had more than a passing knowledge of the subject, and it is interesting to speculate on how he might have come to learn about Jewish alchemical traditions. 

Early seventeenth century Florence contained a city within a city: the Jewish Ghetto. A walled perimeter encircled what is now the Piazza della Republica. This was the mandated home for all of Florence's Jewish population. Each night, entrance gates were closed and locked from the outside. Within the Ghetto, residents were allowed to live and warship freely, even maintaining a Synagogue. In the daytime, the gates were opened, and residents were allowed to go about their business and leave the city with special passes. Among the Ghetto's most prominent residents was the family of alchemist Benedetto Blanis (c.1580-1647.) Blanis served as librarian to Medici prince Don Giovanni. Giovanni maintained an alchemical laboratory in his residence, which was run by Blanis, located only a short distance from where Antonio Neri was living when he first worked at the Casino di San Marco.  

Don Giovanni maintained a close relationship with Neri’s benefactor Don Antonio de' Medici. So close, in fact that when two of  Blanis' relatives were implicated in a gambling scheme, Don Antonio hid them at his residence and then spirited them away, out of Florence, in his own coach until matters cooled off. Furthermore, Blanis came from a family of doctors who must have been known to Neri's father, royal physician to Grand Duke Ferdinando. Antonio Neri was probably a couple of years older than Blanis, if they did not meet through mutual connections with the Medici family, then perhaps they met on the street. The walk for Neri, between his living quarters near Santa Trinita, and the Casino laboratories would have passed around or through the Ghetto, and the walk for Blanis to Don Giovanni's palazzo on Via Parione took him past Neri's front door. The paths of the two men may have crossed, but there is not direct evidence.

Of course, in the absence of hard facts, there are many other possibilities of how Antonio Neri might have become acquainted with Kabbalistic tradition. By taking a look at Blanis and his connections to the Medici family, we can at least see an area of cooperation between Jewish and Christian alchemists in what we might otherwise assume to be an inviolable separation.** 

* “Discorso sopra la Chimica: The Paracelsian Philosophy of Antonio Neri”, M.G. Grazzini / Nuncius 27 (2012), p. 337.
For more on Blanis, see Edward L. Goldberg, The Secret World of Benedetto Blanis. (2011).

** This post first appeared here on 6 January 2014.

Friday, December 25, 2015

Sal Ammoniac

Ammoniac crystals, Fan-yagnobskoe coal mine, Tadjikistan,
Photo (c) A. A. Evseev.
Here we will examine "sal ammoniac," a common alchemical ingredient used by Antonio Neri in many of his early seventeenth century preparations. In its pure form, it is a colorless crystalline material and is known to chemists as ammonium chloride. It does occur as a (rare) natural mineral, but it was also manufactured as early as the thirteenth century, as noted by alchemist Albertus Magnus in his De alchymia.[1] Neither he nor Neri provides a recipe for sal ammoniac, but other sources indicate that it was made by allowing urine to putrefy with common salt. French investigators documented another method used in Egypt in the eighteenth century. This scheme involved burning the dung of animals who fed on spring grasses and then sublimating the ammoniac out of the resulting soot. Sublimation occurs when a heated material goes directly from a solid to a gaseous state without ever becoming liquid. Sal ammoniac has this property; when heated it turns to a gas and upon cooling, turns back to a solid.
The usefulness of sal ammoniac in alchemy stems from the fact that when dissolved in water, which it does easily, it immediately dissociates into equal parts of ammonia and hydrochloric acid, which in turn will dissolve some metals, including tin, zinc, iron and (reluctantly) lead. Its most famous use was as an additive to the stronger acid aqua fortis (nitric acid). Together the two formed aqua regis which was strong enough to dissolve gold. At the time that Neri was working, the only known way to dissolve the most 'noble' of metals (gold) was with the 'king' of acids (aqua regis). Neri puts this knowledge to use in his recipe for ruby-red colored glass made with pure gold. His description is light on details, but he does clearly direct the reader to dissolve the precious metal in aqua regis, then gently evaporate away the acid to obtain the red pigment.

Elsewhere in Neri's glassmaking book, L'Arte Vetraria,[2] he uses sal ammoniac in the production of "alemagna blue" paint and in the tinting of natural rock crystal. 

Another of Neri's creations requiring sal ammoniac was Chalcedony glass. It had swirls of every color the glassmaker could produce. He achieved this feat by making extensive use of aqua regis to dissolve a long list of metals. He then gently evaporated off the acid, leaving ultrafine powdered metals, which he added as pigments to the glass melt. 
With this powder, I made a chalcedony in a glass furnace in Antwerp that was then run by a most courteous gentleman; Mr. Filippo Gridolfi. This chalcedony gave rise to work so nice and graceful, that it emulated true oriental agate, and in beauty and delightful colors by far exceeded it.
Today, chemical factories produce vast quantities of the materials used by Neri in his glassmaking exploits and in far higher purities. Having unlimited quantities of every conceivable chemical compound at our fingertips makes it difficult to appreciate the physical labor involved by seventeenth century alchemists, both in the preparation of the glass and in the production of the individual ingredients. The chalcedony glass recipe cited above must have taken workers many, many hours to produce and must have cost a small fortune. 

[1] Magnus 1958.
[2] Neri 1612.
[3] Glauber and others used the term 'sal ammoniac' to describe a related chemical (NH4)2SO4. When mixed with aqua fortis this forms a nitric-sulfuric acid solution, which does not form aqua regis, and does not dissolve gold.
*This post first appeared here 22 August 2014.

Wednesday, December 23, 2015

Faux Pearls

Johannes Vermeer
"Girl with a pearl earring" (1665-6)
Natural pearls, found inside various seashells, have been prized and worn as jewelry since antiquity. The pearl is formed as a secretion of the mollusk; it is the animal's response to an irritant, perhaps a sharp grain of sand, which has become lodged in its tissue. The secretion, called "nacre" is the same material from which the mollusk builds and enlarges its shell. Natural pearls are rare; large, well formed ones are even more so. A famous legend claims that Cleopatra used pearls to win a bet with Marc Antony: that she could spend ten million sesterces on a single meal. She literally drank pearls that had been ground up and dissolved in wine. Because of the difficulty in obtaining pearls, and their high demand among the wealthy, it is not surprising that like artificial gems, artificial pearls have enjoyed a brisk trade throughout history.

In Antonio Neri's era, the early seventeenth century, a number of recipes used glue, egg whites or other organic materials to simulate pearls. These had the obvious disadvantage of being susceptible to degradation by moisture and physical handling. Another alternative was to simulate pearls with glass, and on this count, Neri does not disappoint. Recipe number sixty in his 1612 book L'Arte Vetraria gives his prescription for artificial pearls. Here it is in its entirety:



In fused and clarified cristallo, add three or four portions of tartar from wine dregs. You must thoroughly calcine this tartar to a white color. Stir it thoroughly into the glass, and continue to add more tartar, also well calcined until it is white. Add four to six more portions, always stirring the glass thoroughly, continuing thus until the cristallo takes on a pearl color. In this recipe, I cannot give exact rules, because it is a matter of experience, which is gained through experimentation. Once obtained, you must work the color quickly, because it will dissipate. I have practiced and experimented with this method many times.

"Cristallo" is the exceptionally clear glass the Venetians developed, perfected and were renowned for throughout Europe. "Tartar" is a crystalline growth that forms on the inside of wine casks, what we now know as "cream of tartar." Occasionally, one might spot crystals at the bottom of bottles of wine. They are a rich source of potassium. Neri, the Venetians and others had used tartar as a glass flux over a period of centuries. Here, however, he is not using it as a flux, but as a colorant to give the glass the pearl's shimmering appearance. His claim to making many batches of this glass implies large numbers of artificial pearls were in circulation. Our glassmaker presents a second recipe, which does not make any mention of pearls, but oddly may have much more to do with the evolution of reproducing these treasures of the sea. Recipe number 114 is entitled "The Way to Tint Glass Balls, and Others Vessels of Clear Glass, From the Inside, In All Kinds of Colors, So They Will Imitate Natural Stones." Here, Neri spreads fish-glue on the inside surface of a blown globe of clear glass, followed by various pigments.

Even in his time, artificial pearls found their way into royal courts and onto the canvasses of master painters. The fashion-setting monarchs of France and Britain Catherine de' Medici and Elizabeth I were famous for their extravagant love of pearls. Elizabeth famously purchased faux pearls from Venetian glassmakers to adorn her garments. She commissioned many portraits donning her pearl studded creations. Referring to the famous painting by Johannes Vermeer, Lloyd Schwartz recently observed, "[T]he scholarship on Girl with a Pearl Earring reveals that the pearl isn't really a pearl […] the famous pearl is probably just glass painted to look like a pearl."* It is interesting to note that the painting was executed in 1665-6, within five years after three reprints of Neri’s book, two in Italian one in English, and only a couple of years before a Latin edition printed in Vermeer's own country.

Around 1680, a Parisian maker of rosary beads invented a type of artificial pearl consisting of a small hollow glass bead, painted on the inside with the iridescent discharge of fish scales mixed with glue. He then filled the beads with wax. Jacquin had apparently rediscovered the shimmering pearly residue of a specific fish. His innovation fueled a new industry; he called the precious pigment "essence d'orient." But the material had already been employed in eastern France in 1656 and according to other reports as early as the reign of Henry IV of France (1572–1610), which closely coincides with Antonio Neri's own lifetime. By 1716, scientists were investigating essence d'orient under a microscope. Rene Antoine Ferchault de Reaumur reported tiny, perfectly formed rectangular plates that reflect the light to cause the shimmering.**

Perhaps more interesting than who discovered what, is the exchange of ideas and the overlap of interest between an Italian alchemist, a British queen, a Dutch painter, a French jeweler and a biologist.


* Also see Anthony Bailey, A View of Delft: Vermeer Then and Now (London: Chatto & Windus, 2001), p. 123, 124.
** For an English summary see The Edinburgh Philosophical Journal October 1839-April 1840 (Edinburgh: Adam & Charles Black, 1840), v. 28, p. 114, 115.

Monday, December 21, 2015

Rise and Fall

"Merry Company," (1623)
Gerard van Honthorst
The first decade of the seventeenth century was a golden era for glass in Tuscany. The Venetian techniques brought to the region by Grand Duke Cosimo de' Medici in the 1570s had been assimilated. The pioneering work of his son, Francesco, in cross pollinating different crafts under one roof, was by now bearing fruit in unique items that included the handiwork of glass artisans. Grand Duke Ferdinando understood the value of glass as a source of prestige and was willing to invest in it. This was the environment in which Antonio Neri first learned to make glass. Delicate drinking glasses were the toast of the aristocracy throughout Europe. The material was critical to the advancement of chemistry, medicine and by the end of the decade astronomy. 

In 1602, Antonio Neri came to work in the shop of Niccolò Sisti in Pisa. While Sisti was making fancy glassware for the Medici court, the nearby Coscetti firm was supplying Pisa with everyday items. Coscetti made glassware for private homes, but also innkeepers, spice and perfume sellers, winemakers and a baker among others. Their wares included cruets for oil, saltcellars, carafes, drinking glasses, containers for holy water, reliquaries, gilded Venetian style cups and English style flasks. 

By the second decade, momentum started to shift and before long, the glass industry in Tuscany fell on hard times. Apparently the demand for glass could not support the number of factories that had started and the rapid succession of leadership in the duchy added uncertainty to patronage of the arts in general. 

Another factory in Pisa was run by Giovanbattista Guerrazzi, who had acquired the exclusive right to make Venetian style cristallo from Neri's old employer Sisti. In 1623, Guerrazzi had problems of a different sort, not directly related to the sales of glass. He appealed to Pisa’s Office of Rivers and Ditches, pleading with them to modify a recent ruling. He explained that he owned three houses next to his furnace, one for his family and the others functioning as sales space and housing for his workers. Since he was the exclusive maker of cristallo, he had employed a number of girls and women to decorate the delicate glassware, and a constant stream of the nobility showed up to watch the work being done. Guerrazzi's problem was that the Magistrate of Public Decency had recently published a list of seven places where women of "ill repute" were allowed to stay. One of these was located next door to his glassmaking operation. He begged for a change in the ruling, to move his new neighbors elsewhere.

The outcome of his appeal is not known, but Guerrazzi was succeeding in the glass business, and at the same time accelerating the demise of his competitors. He bought-out and demolished the furnaces of a number of other glassblowers and planned the same fate for the Coscetti operation, putting all the craftsmen there out of work. In the mid 1620s, after a quarter century of operation, the fires under Coscetti furnace were allowed to go out forever. Furnaces at Leghorn, Pistoia and Prato had shuttered, leaving only the one furnace in Pisa, two in Florence and two at the castle of Montaione. 

*This post first appeared here on 12 Dec 2013.

Friday, December 18, 2015

Neri the Scholar

Francesco Bartolozzi, Laurentian Library in the 18th cent.
(click to enlarge).
Whether one's chosen field was medicine, law, religion or alchemy, in the late fifteenth and early sixteenth century, books played as important a role in education as they do today. The schooling of Florentine priest and glassmaker Antonio Neri was no exception. The details of his training at seminary remain elusive, but there is no mistaking that his introduction to alchemy occurred well before his ordination as Catholic Priest around 1598. [1] Since his father was the celebrated physician to Grand Duke Ferdinando, there is a good chance that Antonio had access to wide variety of Medici resources, not the least of which was the famed Laurentian Library, designed by Michelangelo and run by Neri family friend Baccio Valore. [2]  

But we need not look as far as the Laurentian, which was only a ten-minute walk from the Neri household. Closer to home, in fact inside his home, there was the extensive library of his own father. At the turn of the century, it contained 477 volumes, spanning poetry, philosophy, the Greek classics, medicine, pharmacology, surgery, religion, even grammar. [3] We know this thanks to an inventory taken at the time of the physician’s death, leaving a list of titles that has survived the ages, even if the volumes themselves have long been dispersed or lost. At the time, outside of the royal family, it was probably one of the largest collections of books in private hands in Florence. Neri’s father had himself been in charge of the revision of the Ricettario Fiorentino, [4] the gold standard of doctors and apothecaries throughout Europe for medicinal prescriptions, published in 1597 and again without revision in 1623.

Antonio Neri is known best as an artisan who worked with his hands. No evidence has been found to place him at a specific monastery or university classroom. Nevertheless, what emerges from the details that we do have is a picture of a man who was steeped in a literary, scholastic tradition from an early age. His Mother’s father, Ser Francesco, held a degree in law as did her grandfather and great-grandfather. [5] Antonio’s father held a degree in medicine from the “Studio Fiorentino,” the forerunner of what today is the University of Florence.[6]  In addition, it would be reasonable to assume the household library included titles once owned by his grandfather Jacopo, a noted barber-surgeon who was known among the literati. It has been speculated that Jacopo’s best friend in the world [7] was poet Lodovico Domenichi, who wrote of his friend in a sonnet:

Marvel about you the people do,
Over how, one might say, almost stupidly,
So many lecturers and scholars admire you. [8]


Domenichi who had been appointed court historian by Grand Duke Cosimo I de’ Medici, goes on in the sonnet to name a list of mutual friends that includes poets, playwrights and intellectuals of the day.

In a similar way —that is through legal records—we know the contents and titles of the alchemical library of Neri’s sponsor, Don Antonio de’ Medici at the time of his death in 1621. These included several manuscripts by Neri himself, as well as his book on glassmaking. [9] However, one title did escape the attention of the bean-counters; sixty years after Don Antonio’s death, at the death of his son, Giulio, a handwritten book of recipes by Neri was found along with a box of elixirs. “Among them there was a booklet, entitled: Material of all the compounds of Priest Antonio Neri; there is a red dustcover, which says “experiments.” [10]

In his twenties, after a couple of years of making glass in Florence, Neri moved to Pisa where he assisted at the Medici’s furnace run by Niccolò Sisti. Pisa was home to a thriving university, with ample study possibilities, and Neri was proving himself a life-long researcher. From Pisa, in early 1604 he embarked on a seven-year long residence in Antwerp, where he stayed with his friend Emmanuel Ximenes. Ximenes was one of the wealthiest men in Flanders and maintained an extensive library in his palace. He owned many volumes devoted to the chemical arts. [11] In fact, his collection of books was probably the largest in the entire region. [12] Here too, the full list of books is preserved in an inventory compiled after the death in 1617 of Emmanuel’s wife, Isabella da Vega. 

Upon Antonio’s return to Italy, he published his glassmaking recipes in L’Arte Vetraria and then appears to have focused his attention on chemistry and medicine. In the last manuscript he is known to have written, within a year of his death, he writes of a recipe copied “from an old book, here in Pisa” in 1613. 


[1] In his manuscript Tesoro del Mondo devoted to “all of alchemy” (Neri 1598-1600) Neri self-identifies as a priest.
[2] Bartolomeo di Filippo di Niccolò Valori [il giovane] (1535–1606). He was keeper of the Laurentian, steward of the Medici herbal (simples) garden and an early director of the Accademia delle Arti del Disegno. He was a personal friend to Antonio Neri’s father and godfather to his first child (Antonio’s older sister) Lessandra.
[3] Bec 1984, pp. 299–310.
[4] Neri, Benadù, Rosselli, Galletti 1597, 1623.
[5] Ser Francesco di Ser Niccolò di Ser Antonio Parenti (1519 - ?)
[6] Fathers medical degree ref.
[7] . Garavelli 2004, p. 82, n. 186.
[8] Domenichi 1555, Stanza 7.
[9] Covoni 1892.
[10] Ibid p. 193.
[11] Duverger 1984.
[12] Dupré, Lüthy 2011, p. 272; Göttler, Dupré 2009.
* this post first appeared here on 31 Dec 2014.

Wednesday, December 16, 2015

Roasting the Frit

Diderot & d'Alembert, L'Encyclopédie (1772)Raking Out Roasted Frit
Making glass from raw materials involves several steps. In his 1612 book on glassmaking, L'Arte Vetraria, Antonio Neri breaks the process down into parts so that, "given a bit of experience and practice, as long as you do not purposely foul-up, it will be impossible to fail." Pure white sand, or preferably quartz river stones which Neri calls "tarso" is broken up and pulverized into a fine powder. The initial work can be done by heating the stones in a furnace, then dropping them into a vat of clean cold water, where they will fracture due to the thermal shock. The process was often repeated multiple times. From there, the pieces are pulverized in a stone mortar and pestle. Stone, because metal tools would contaminate the quartz, and in the end tint the glass. Finally, a powder is obtained by grinding with a stone tool on a flat granite "porphyry stone." This powdered quartz is the main ingredient of glass.

The second critical ingredient is the flux, what Neri calls "glass salt" or "soda." This can be obtained from mineral sources, but European glassmakers in the seventeenth century extracted all their salt from certain plants. The powdered quartz was mixed with the salt and a third ingredient, which is critical, lime. Lime is simply calcium oxide used by builders to make cement. It is nothing more than pulverized seashells roasted to a high temperature. Neri advises using two pounds of lime for every hundred pounds of salt. He specifies that it should be added to all his frit recipes, but it is not clear that he understood its critical importance; without lime, the glass would be subject to attack by mere water, eventually decomposing. This mixture of soda, lime and silica when heated in a kiln would chemically react forming "frit." The combined materials were raked around in a kiln for a long period (many hours) and finally formed nut sized pieces. It was cooled and heaped into piles in dry cellars where it was aged for a time. This is where some chemical "magic" in glassmaking takes place. The glass salt or soda dramatically lowers the melting temperature of the quartz, all the way down to a point that was easily achieved in a wood fired furnace. When a batch of glass was made, the aged frit was then melted in furnace crucibles and skimmed to remove excess salt, which floated on the surface; it could foul the glass, and smelled terrible. The melted glass, now ready to work, was sometimes colored and finally made into objects by gaffers. 

Neri obtained his glass salt from products shipped by traders from the Levant (eastern Mediterranean). It was supplied as the dried, partially charred remains of special plants that grow in arid seaside conditions; 'Kali' and 'Soda'. Shipping them this way cut down on weight and volume, and prevented rotting. These plants contain large amounts of sodium carbonates. This is a white powder, chemically identical to what we know as "washing soda." He advises, 
In buying either of these make sure it is richly salted. This may be determined by touching it with the tongue in order to taste its saltiness; but the surest way of all is to do a test in a crucible and to see if it contains much sand or stones, a thing common in this art and very well known by glass conciatori.
He crushes any large pieces of the product in a stone mortar, and sifts the result through a fine screen, ensuring that most of what remains is salt.  
As the common proverb of the art of glassmaking says: a fine sieve and dry wood bring honor to the furnace. Then with any of these sodas, 100 pounds of soda ordinarily requires 85 to 90 pounds of tarso.
Neri sets up large cauldrons of clean water over brickwork stoves, adds the plant product and boils the water. He strains the insoluble parts out and reduces the liquid by evaporation until crystals of the salt start to form on the surface. He skims these off and continues the process. Finally he carefully dries the product. Our glassmaker describes several variations of this process, including one in which he takes extreme measures to ensure the purity of the salt and clarity of the finished glass. In all, this is a task that could easily take several weeks to perform for the amount of frit to fill a single pot for the gaffer to work.

Not content with the established materials, our glassmaker experimented extensively with other plants: 
[U]se the husks and stalks of fava beans after the farmhands have thrashed and shelled them. With the rules and diligence prescribed for the Levantine polverino salt, extract the salt from this ash, which will be marvelous, and from which a frit can be made using well-sifted white tarso, as is described throughout this work. A very noble frit will result, which in the crucible will make a crystal of all beauty. The same may be made from the ashes of cabbages, or a thorn bush that bears small fruit, called the blackberry, even from millet, rush, marsh reeds, and from many other plants that will relinquish their salt. *
*These other plants produce potassium carbonate salts with similar properties to sodium carbonate.
** This post first appeared here 9 December 2013.

Monday, December 14, 2015

Glass Beads

Six-layer glass chevron trade beads
(photo attr. unknown)
One of the oldest applications of glass, perhaps the oldest, is the production of beads. That development took place about 5000 years ago, but in the history of beadmaking, glass is a relatively recent innovation. Before glass was developed, beads were made from clay, metal, wood, bone, shell and stone. Examples are included in some of the oldest human artifacts ever found, as old as 100,000 years, and that number has increased regularly with new discoveries. Moving closer to the present, in 1612 the first printed book on glassmaking was published in Italy and it contains numerous references to beads made of ordinary glass, Venetian style cristallo and what we would now call lead crystal. [2] 

In his 1612 book, L’Arte Vetraria, Antonio Neri describes preparing batches of 300 to 400 pounds of a blue-green colored glass destined for beadmakers [3]. The molten glass was drawn out into thin rods and cooled for use by workers who formed beads by winding the glass around a metal rod over an oil lamp. 
I demonstrated this method of making aquamarine in Florence in the year 1602, at the Casino, and I made many batches of it for beadmaking cane, which always resulted in a most beautiful color. Take note, than in Murano for beadmaking cane they take half crystal frit and half rocchetta frit, and nevertheless still get a nice aquamarine; but in pure crystal it is the most beautiful.
Elsewhere, we learn that Neri supervised the production of beadmaking cane, not only in Florence, but also in Pisa. At that time, glass beads were manufactured for use both within Europe and for use around the world as a trade currency. Locally, beads found use in devotional objects in the form of rosaries, where they were called ‘conterie’ and ‘paternostri’ (literally 'our fathers'). Outside of Europe, in Africa, the Americas, the Far East, and in China, glass beads were used in trade for sugar, spices and other goods. On the darker side of history, they were used to buy men, women and children for the slave trade.

The question of Neri’s involvement in the production of trade beads naturally arises, but it is a question not so easily answered. He makes no direct mention of the intended use of the beads made from his glass, but there are clues. On one hand, the Medici family, for whom Neri worked in Florence and Pisa regularly engaged in trade expeditions, on the other hand Neri himself was a Catholic Priest, with some obligation to the Church. His work could as easily have been destined for foreign shores as for the hands of the laity.

Neri’s good friend, Emmanuel Ximenes came from a family of traders, or “bankers” as they were called at the time. They financed expeditions to Africa, India and the Americas. Glass beads were not the only goods used in trade, but they were relatively inexpensive to manufacture and made a convenient ballast material for ships. Empty vessels needed something heavy in the cargo hold to keep the boat from riding too high in the water. Often rocks or sand was used, but barrelfuls of glass beads served the double purpose of a near universally accepted currency. It is easy to imagine Neri’s friendship with the wealthy banker facilitating trade deals, but no evidence has come to light that anything like this took place. 

Antonio Neri visited his friend Emmanuel Ximenes at his palace in Antwerp and stayed for about seven years. It might seem like an ideal opportunity for bead production, except that the port at Antwerp had been blockaded by their Dutch neighbors to the north for several years. They were fighting a bloody war for independence from Spain and any trade that did occur had to be routed to other ports. Emmanuel’s brother Duarte had a large shipment of sugar confiscated in this period and went to considerable trouble to have it returned. [4] This does not rule out a role for Neri in trade beads, but it does make it less likely than when he was living in Italy.

There is also no evidence that I am aware of that the Ximenes were involved in the slave trade. It is true that Emmanuel was a Knight of Saint Stephen, which essentially served as the Tuscan Navy. They regularly intercepted Ottoman pirate and military ships and when caught, the crew was generally pressed into slavery. However, this was not a profit driven activity and by the time Neri and Ximenes met, the later was well beyond the age of active service. 

[1] Old beads: http://www.sciencedaily.com/releases/2009/05/090505163021.htm
[2] Neri 1612, chs. 22, 26, 29, 36, 46, 47, 62, 64, 65.
[3] Ibid, ch. 22.
[4] For a full description of the 1602 Sugar Confiscation, see Roitman 2009, pp. 207–229.
* This post first appeared here 17 Dec 2014.

Friday, December 11, 2015

Fall from Grace

Felice Fontana
(Attribution unknown)
In 1790, two centuries after the life of glassmaker Antonio Neri, his name appeared in an unlikely place: in the annotations of a Swedish manuscript that was translated into Italian by one of Florence's leading scientists.[1] On second thought, perhaps its appearance is more unexpected than unlikely. Torbern Bergman was the celebrated Swedish chemist responsible for the manuscript and the translator was Felice Fontana, the founding director of Florence's Natural History Museum (La Specola). Fontana took the opportunity to annotate Bergman’s History of Chemistry in the Middle Ages with a list of Renaissance era materials collected in the Grand Ducal library in Florence (now the BNCF). Figuring prominently among the documents was alchemist Antonio Neri.

Below is an excerpt from one such letter, penned by Don Stefano Giraldi, the prior of San Pancrazio Church in Florence, addressed to "Your Excellency." In the passage notable characters are Don Antonio, Neri's long time benefactor — the Medici prince who ran the Casino di San Marco laboratory where the priest first made glass. Also, Francesco Orlando Lorenzi (Count Lorenz, active 1793) was third count of Lorenzana and the minister of France at the Florentine court in the time of Louis XV.


Among those who abused the credulity of the Prince D[on] Antonio by leading him to understand they knew how to make gold, the most famous was Florentine Priest Antonio Neri, who, if he could not make gold, did know how to make many other beautiful and useful things, and has secured eternal fame with his work on the Art of Glassmaking, which also found prestige in the Tuscan language…  
Of other works of Preist Neri, I do not know if they have been published or even if the manuscripts have been found. Uniquely, Father Maestro Arrighi, [Augustinian] Serviette and Alchemist, in 1735 bought books that Count Lorenz had sent to France. Among them was a large manuscript in quattro by Antonio Neri. He told me that it contained a method of making the philosopher's stone and that there was a preamble  in which Neri confessed to have copied this method from a manuscript found in an old library. He  tried [the recipe] several times and always  failed.  To raise hope for others to enjoy much treasure, he had copied the method by characters in a cipher he invented and burned the old original. P[reist] Arrighi never let me see the manuscript, which was four fingers high as he assured me that the bookseller had sold it to him for the price of 5 lire; and no doubt, there will be other things besides the process of the philosopher's stone, which occupied[only a]  few page.
This flaw in Priest Neri to pass himself off as the possessor of the philosopher’s stone detracted much from his esteem. Nearby, his countrymen were wise and enlightened, and therefore he never attained the good image he should have had in his country, his mercy was the real merit of so many other good things; indeed always absent [making] gold, though he wanted to give the impression that he could, he incurred various dangers, and was forced to spend some time wandering.[2]

This gem of a letter has many interesting facets. First, it establishes that 150 years after Neri’s death, his name was still on the lips of royal courtiers and his manuscripts commanded a dear price from booksellers. It also establishes a route that his unpublished work took in leaving Italy, through France. 

The letter also provides us with insight to the changing narrative about alchemy. Neri was being distanced from the less palatable aspects of his work. "[I]f he could not make gold, [he] did know how to make many other beautiful and useful things, and has secured eternal fame with his work on the Art of Glassmaking." His sponsor Don Antonio’s portrayal as gullible about the possibility of transmutation kept the prince's reputation intact, even if somewhat diminished. (Popular history seems to have little conscience when it comes to making those who came before seem stupid.) Both Neri and Don Antonio were dedicated experimenters and they knew the proof of transmutation lay in actually making gold and they tried relentlessly. The narrative presented in the letter makes an appealing, face-saving story line, but we need only cast a sideways glance for it to start crumbling. According to the writer,  Florentines were too sophisticated to be drawn into alchemical gold-making schemes, yet a little later we find out Neri was forced to "wander" Europe, chased out of Florence by those demanding the secret of transmutation. Which was it?

The work of Neri's countryman and contemporary Galileo had established the natural sciences as Tuscany’s great patrimony. In the mid eighteenth century, when the letter was written, experimenters were as eager as ever to separate themselves from the charlatans and mountebanks who sold miracle cures and fueled impossible dreams. By then, it was agreed by researchers that transmutation was not possible, but even in the nineteenth century, there was no theoretical foundation to back up this supposition, which made it a sore point. Because of the acclaimed glass book, Neri earned a place in the pantheon of Italy's 'great men', but at a cost. His now 'embarrassing' work on transmutation caused the rest of his legacy to be largely written out of the history books.

[1] Bergman, Tofani 1790. (Fontana was writing under the pseudonym Giuseppe Tofani.)
[2] Ibid, pp. 99 - 101.
* This post first appeared here 24 December 2014.

Wednesday, December 9, 2015

Neri's Travels

“Roma,” Antonio Neri,
from Tesoro del Mondo (Neri 1598–1600).
The length and breadth of Antonio Neri's travels are far greater in thumbnail biographies and off hand remarks than can be substantiated by actual documentation. While stories of the glassmaker's travels through Europe abound, the truth of the matter is that only a small number of his movements have been verified through contemporary materials. Even if a minority of the wanderings attributed to Neri are true, then he certainly was a man of the world. Writing nearly two centuries after his death, historian Giovanni Targioni-Tozzetti claimed the priest left Italy to elude "thugs" in Florence who hounded him for the secret of transmutation. Tozzetti says he fled to England first and then visited Spain, Holland and France. [1] No evidence has yet turned up to support any of this. 

Other accounts say he "traveled all over Europe" and that he deceitfully posed as a "common assistant" in order to learn scientific secrets that he could not gain access to by other means. [2] One story I have heard making the rounds among glass workers is that Neri was chased to the "gates of Prague" by assassins. This is most likely confusion with a similar story about Venetian glassmakers leaving Murano without state permission to ply their craft elsewhere.

There are four cities that Neri is confirmed to have visited: Florence, the city of his birth; Pisa where he worked at the glass furnace run by Niccolò Sisti; Antwerp, where he spent about seven years visiting his friend Emmanuel Ximenes and in Mechelen, at the Hospital of Malines, where he wrote about pioneering medical treatments in a letter to a friend back in Florence.[3] 

In addition, there are other locations that are strongly hinted at in various writings. In his Treasure of the World manuscript, Neri has an allegorical depiction in the form of a simplistic map showing “The Ways to Rome.” It depicts the walled enclave of the Vatican (see illustration) with various paths representing different chemical routes to transmutation. If nothing else, this leaves the door open to a personal familiarity with the eternal city. In his glass book, L’Arte Vetraria, Neri mentions a number of specific locations in northern Italy, but perhaps none as authoritatively as Venice. He comments about the materials and techniques specific to the glassmakers on Murano. There is little doubt that Neri was exposed to Venetian glass workers in Florence, Pisa and Antwerp, so they provide a perfectly plausible source for his knowledge of their distinctive techniques. This would be a sufficient explanation, except that there is also a letter written by his friend Emmanuel Ximenes, detailing a route for Neri's visit to Antwerp; a route that runs through Venice. Below is the passage from a letter, dated 5 December 1602. The glassmaker would be delayed by illness, but the following year he did make the journey. While it seems a good bet that he followed Ximenes' instructions that is another detail in need of confirmation.
Anyway, the lack of peace in these countries prevents me from recommending them for you to come or not, but if you make up your mind to come, God willing, you will have the same fortunes as we have. Besides, after your arrival, is not a marriage indissoluble, having no other bond than mutual affection? If you decide to come I would recommend that you should go with the courier from Florence to Venice, arriving in Venice in time that you would be able to accompany the merchants who come to the fair held in Frankfurt at mid Lent; you will stay there the length of the fair for fifteen days, which will not displease you for having seen it. After that, you would go in the company of other merchants to Cologne and then with them or others, by land or sea to Holland, ending up at this city. This sea, however, is nothing more than rivers. I recently went by land to Basel and from there by water ending here. But for Your Lordship,  who does not speak the German and Flemish languages, I would consider better the way that I say, with merchants from Venice to Frankfurt and then with others by water to arrive here. To this end, if you decide to come, upon giving me notice I will immediately send letters of recommendation to Venice to find a person who will help you to find company that must end up in Frankfurt and another for a friend in Frankfurt to get you started and perhaps it would be the same one with whom Guillelmo Reineri, servant of my brother Mr. Niccolò, came from here. Guillelmo usually goes to every fair by water up to Frankfurt, then back when it ends. He is close to me, a friend and very practical in his travels. This Renieri may give a report of the Frankfurt fair and also details of the voyage, as he made the outward journey for the fair last September. I shall send him a letter by means of my brother to give him the money on my account that would be necessary. But you should decide and advise me immediately, in order to go to Venice in time to find a group. I will wait for your decision, asking God to inspire the best . . .  [4]


[1] “E fatto con prestezza fagotto, la mattina all'aprir della porta uscì el di Firenze e se n'andò in Inghilterra. Girò la Spagna, Olanda, Francia e Germania…” [He packed in haste and in the morning opened his door, left Florence, and went to England. He toured Spain, Holland, France and Germany…] Targioni-Tozzetti 189, p. 149.
[2] See Rodwell 1870.
[3] Neri 1608.
[4] Ximenes 1601–11, 5 December 1602.
* This post first appeared here on 19 Dec. 2014.

Monday, December 7, 2015

Francesco's Studiolo

The Studiolo of Francesco de' Medici,
Palazzo Vecchio, Florence.
(click to enlarge)
In all likelihood, Antonio Neri started his glassmaking career around 1601 in the laboratory palace of Medici prince Don Antonio. The Casino di San Marco is located on the north side of Florence, not far from where the old city walls once stood. It was purpose-built as a laboratory by Don Antonio’s father, Grand Duke Francesco and stands on the former location of the sculptural school that Michelangelo attended as a boy. The Casino is a remarkable structure in the history of science, however, our subject today is not the laboratory but one of its primary inspirations, located across town in the Palazzo Vecchio; The Studiolo of Francesco de’ Medici.

Books of secrets, like Neri’s L’Arte Vetraria exposed methods to transform nature. Cabinets of curiosities, on the other hand, celebrated the finished products as well as nature’s ready-made treasures. These so-called cabinets were a sort of physical counterpart to books of secrets. Starting as small collections of exotic objects, princes and nobles strove to out-do each other and the largest examples encompassed entire rooms. In the early 1570s, at around age thirty, Francesco de’ Medici initiated a special project; he constructed a secret room in the Palazzo Vecchio. [1] Accessible through a concealed staircase in his bedroom, this small, but opulent, study chamber was devoted to natural curiosities and secrets. The “Studiolo” contained his collection of rare gems, exotic seashells, animal horns, chemicals, potions, scientific instruments and other strange and wonderful treasures collected from around the world. 

From floor to barrel-vaulted ceiling, paintings and niched sculptures covered the chamber walls. Celebrated artist Giorgio Vasari designed and constructed the secret room in partnership with Vincenzo Borghini, a Benedictine priest and close Medici advisor. [2]  In all, thirty-two of the city’s artists contributed to the project, although most had no idea where their work was destined to hang. Francesco organized paintings such that each wall was themed by one of the four Aristotelian elements: air, earth, water and fire. Behind nineteen of the lower paintings, cabinets housed the treasures of Francesco’s collection. From within the Studiolo two other secret passages were accessible from behind concealed panels. One leads to a smaller private treasury once used by Francesco’s father, Cosimo and another leads down a stairs to an unmarked outside door on the street. [3]

Part study and part museum, Francesco used the Studiolo to escape public life and explore the secrets of nature. This menagerie and ones like it were an outgrowth of the wunderkammer or “cabinet of curiosities.” Early in their evolution, they took the form of single pieces of furniture for the display of collections. Monarchs and nobles throughout Europe boasted collections of ever-increasing size and diversity. In a way, the Casino di San Marco was the next evolutionary step; from a cabinet of curiosities, to a study room, to an entire facility devoted to nature’s secrets. 

The paintings in Francesco’s Studiolo depict various religious, mythological, historical and industrial scenes. [4] Some of them show various royal workshops documenting activities as diverse as goldsmithing and wool dying. A 1571 painting by Giovanni Stradano [5] is entitled the Alchemy Studio. It shows Francesco I in the Uffizi surrounded by laboratory equipment and workers. 

[1] Constructed between 1567 and 1675, cf. Feinberg 2002, Edwards 2007.
[2] Giorgio Vasari (1511–1574), Vincenzo Borghini (1515–1580).
[3] Via della Nina.
[4] Feinberg 2002.


[5] Giovanni Stradano, also called Jan van der Straet (1523–1605).

Friday, December 4, 2015

Royal Apothecary

Fresco, early 16th century speziale,Castello di  Issogne, lower Aosta Valley, Italy.
As physician to the Medici royal family, Antonio Neri’s father (Neri Neri) worked closely with the other medical professionals in Florence. Among them was Stefano Rosselli, the royal pharmacist, who owned the Speziale al Giglio in the center of town.  We would readily recognize this shop today; rows of labeled jars lined the walls holding exotic remedies and ingredients from around the world. Freshly made sweets, cakes and other confections sat on the counter to entice customers. 

One of several such shops located around the city, ‘speziali’ dealt with medicines and herbal remedies, but also a wide variety of other needs, from pigments for artists, to the distillation of spirits, to the rental of funerary equipment, to raw materials for alchemists. Each shop had its own specialty to distinguish itself from the others. To some extent, they also competed with private operations run by convents and monasteries. Rosselli’s shop was famous for catering to the Medici family and courtiers, and as such he could command premium prices. But the function of these shops was not all business; they also proved popular meeting places for the local intelligentsia. Poetry, literary and other groups with various interests often congregated in the back rooms of the speziali.

Stefano Rosselli came from a family that was a mixture of artists and pharmacists. His father Romollo, with a degree from the university at Pisa, was a specialist in herbal remedies (simples). His brother Antonio (Fra Anslemo) worked at the pharmacy of San Marco, directly across the street from the Casino where Antonio Neri would later make glass. Their grandfather Bernardo Rosselli was a painter, a favorite of Michelangelo. Stefano’s daughter Fiametta joined the Dominicans as a nun, and became the famous sculptor Suor Caterina Eletta. A distant cousin was Cosimo Rosselli, who returned from Rome with a small fortune, earned painting for the pope. Biographer Giorgio Vasari noted that Cosimo could have lived comfortably on those earnings had the passion for alchemy not overtaken him. 

Stefano proudly maintained a proprietary list of treasured recipes gathered over a lifetime of experience. Some were given to him directly by Grand Duke Cosimo de’ Medici, his son Francesco and daughter Isabella. Between 1589 and 1593 when Rosselli was in his late sixties, he started to compile these into three notebooks, which he eventually passed down to his sons and today have survived the ravages of time. The three books are devoted to medicines, perfumes and confections. Antonio Neri was a teenager in this period, and it is easy to imagine an occasional visit to the Speziale al Giglio for a piece of something sweet, perhaps one of Rosselli’s excellent pistachio calissons.

For a transcription and French translation of the recipes see: Rodrigo de Zayas, Mes Secrets, à Florence au temps des Médicis 1593 (Paris: Jean-Michel Place, 1996).

This post first appeared here on 4 December 2013.

Wednesday, December 2, 2015

Yellow Glass

Yellow Neon Chandelier, 1995
Dale Chihuly.
(Columbus, Indiana Visitors Center). 
"Very few people know how to make colors like golden yellow and solid red well. These are difficult and troublesome in the art of glassmaking, since in making them you must stick precisely to the doses, the timing, the details and the materials as prescribed. The smallest error will cause everything to be ruined, and the colors to be irreparably spoiled. Therefore, you must be on guard not to make mistakes. [1]
So says Antonio Neri in his groundbreaking 1612 book of glass recipes, L’Arte Vetraria. Elsewhere he warns in several places not to add “tartar” to any glass destined for yellow pigmentation. Tartar was a common additive to boost the ‘sparkle’ of a glass because it contained a high level of potassium carbonates. These converted to potassium oxide in the melt, which has a higher refractive index than the usual glass flux, sodium oxide. 

However, the reason for the mismatch is not clear, since Neri also says of his “fern glass,” which is entirely potassium based:
…This frit can be given a wonderful golden yellow color provided there is no tartar salt within, as described in the caution, because then golden yellow will not emerge. This crystal is given to a golden yellow that is far more beautiful and pleasant than can be achieved in cristallo made with Levantine polverino salt and with this crystal unlike the other, every kind of job can be done. [2]
“Polverino” was a sodium based plant product used in many of Neri’s glass recipes, which he says was derived from the Kali plant grown in the Levant. The plot thickens when, for yellow, he recommends substituting ‘rocchetta’ another soda based Kali derivative. 

His primary recipe for golden yellow is #46, in which he reveals two ingredients responsible for the color: “For every 100 pounds of [glass], add 1 pound of tartar from the dregs of red wine. Use large pieces well vitrified naturally in bottles of wine, because the powder is no good. Crush these raw dregs well, and pass them through a fine sieve. For every 1 pound of these dregs, add 1 pound of prepared Piedmont manganese…” [3] To this he adds the advice that “the powder is always given in parts and given [to the frit], not to the fused glass, because then it will not tint.”

He also offers advice to add more or less pigment depending on the intended use of the glass: more for thin items, less for heavier ones. “For larger [thick] spit beads, it is said that at Murano they reduce the dose of [wine] dregs and manganese by nearly half.”

For Neri’s lead glass, he uses a different combination, this time pairing copper sulfate with iron oxide: “Take 16 pounds of cristallo frit and 16 pounds of lead calx. Mix them well and pass them through a sieve. To this material, add 6 ounces of thrice cooked copper, made with flakes of the kettle-smiths [chapter 28], and 2 pennyweight of iron crocus made with vinegar [chapter 17].” He goes on to advise, “If it leans toward greenishness, add a little iron crocus, which will remove the greenishness and will bring out a yellow color of the most beautiful gold.

Yellow is one of several colors that iron oxide can form in glass, and is used frequently in low-fire pottery glazes. In that realm, it has a reputation as a difficult, unstable color, as Neri alludes to in his warnings. But in modern, higher temperature borosilicate glass, iron oxide is relied on for a nice yellow. In modern soda-lime glass, cadmium, titanium or the exotic praseodymium are more likely choices. They produced bright reliable color that is stable at the higher temperatures of modern operations. In lead glass, selenium is the modern favorite for yellow.

[1] Neri 1612, ch. 45.
[2] ibid, ch 5.
[3] ibid, ch 46.