Monday, August 28, 2017

Lapidaries in 1698

Antonio Neri, Tesoro del Mondo, 1598-1600
f. 7v, "Ars Preparatio Lapidum"
In 1598, in his early twenties, before his glassmaking career began, Antonio Neri completed an extraordinary manuscript. Tesoro del Mondo or 'Treasure of the World' was devoted to all aspects of alchemy and was intended for publication, but it never saw a printer's ink. By a minor miracle of providence, the manuscript survives today, in the special collections department of the University of Glasgow Library. The pages include a set of fascinating images, this one (left) among them.   Labeled "The Art of Preparing Stone," the picture shows five men working with various pieces of equipment related to the art. [1]

In the upper-left a lapidary works at a polishing wheel. It turns at low speed, driven by a belt which is powered from below, possibly by a foot pedal. The artisan holds two polishing fixtures against the surface of the disk, while four more stand at the end of the table. Under this workstation is an inscription in Italianate Latin that reads "Accontiare et lustrare pietre praciose" [preparing/dressing and polishing precious stones]. Behind the lapidary are shelves holding finished pieces. They range from small objects that appear to be rings, to cups, vessels and large bowls, presumably all made from stones, gems or minerals. 

Proceeding clockwise to the upper-right side of the image, we arrive at a worker tending a furnace with iron tools. Inside is a crucible sitting in the flames. Below is the firebox, and underneath that is the word "Calcinare" [calcination], which in Neri's parlance refers to the process of breaking down a material into a powder usually through the use of heat. Neri uses this method extensively in his glass formulations; in his book L'Arte Vetraria, [2] almost every colorant discussed is a metal which requires "calcination" before use in the glass melt. In the case of stone, Neri uses the furnace to make the main ingredient of glass. He breaks down quartz stones into powder by repeatedly heating them and then quenching in cold clean water. The rocks fracture into coarse granules which are then ground into a fine powder.

The middle-right of the illustration shows a worker checking on a distilling apparatus. This consists of a small stove and three pieces of glassware.  The "body" holding the raw material to be evaporated or sublimated is capped by a "head" that sports a long snout leading to a "receiver" vessel which collects the finished product.  Stills were useful in producing everything from alcoholic spirits like grappa to acids and reagents. Their specific use in stonework is not clear, possibly in dissolving precious metals from the constituent minerals in stone.

The lower-right portion of the image depicts two men seated at a low bench, each holding specialized tools used to shape "alabaster, marble and porphyry." Finally, in the lower-left we see a specialized mortar and pestle used to grind stone and minerals into a fine powder.

The two benches and the distilling stove all bear a distinctive diamond shaped insignia with a small circle at its center. The two men in the lower portion of the illustration appear to be working on a stone inlay version of this same pattern.  The implication is of an identifying symbol, but a specific affiliation is elusive.

 This technique of creating designs entirely in colorful gems and minerals (pietre dure) is an ancient one revived by the Medici family, specifically by its first three grand dukes.  In the 1560s Cosimo de' Medici employed two such artisans (commessi) in the courtyard of the Palazzo Vecchio. By the 1570s a larger group was working out of Francesco de' Medici's new palace, the Casino di San Marco. In 1588 Ferdinando de' Medici moved them into the Galleria dei Lavori [Gallery of Works] at the Uffizi Palace where they were named the "Opificio delle Pietre Dure." [workshop of hard-stone]  There the organization thrived and refined the art of creating inlaid stonework to the point of producing realistic life-like scenes. Their work graces the most opulent spaces in Florence, including the Chapel of Princes and the interior of Santa Maria del Fiore. The Opificio delle Pietre Dure continues to operate to this day. The organization has been charged with the maintenance and conservation of many of Italy's great works of art. They maintain a worldwide reputation for excellence. [3]

It seems well within the realm of possibility that Neri's illustration indeed depicts an early incarnation of the Opificio. The artisans working in Pietre dure were handling precious materials and as such might not be readily accessible to the general public. The fact that Antonio Neri's father was a prestigious member of the Medici royal court all but ensured his entree to the royal workshops.

[1] Neri 1598-1600, f. 7v.
[2] Neri 1612.
[3] Official website:
* This post first appeared here in a slightly different form on 3 Sept 2014.

Friday, August 25, 2017

Alchemy in the Kitchen

Tesoro del Mondo, "Ars Preparatio Animalium"
Antonio Neri 1598-1600, f. 10r (MS Ferguson 67).
Between 1598 and 1600, Antonio Neri wrote a manuscript filled with alchemical recipes. He entitled it Tesoro del Mondo or "Treasure of the World" and stated that it was a book in draft form, intended for publication. It never did see the ink of a printer's press, but Neri's original did survive intact and today occupies a place of pride in the Ferguson Collection of the University of Glasgow Library. [1]

Tesoro is an ambitious work, devoted to "all of alchemy," containing numerous hand-drawn ink and watercolor illustrations; some of the pictures are allegorical, many others document practitioners working with equipment. Only passing reference is made in this manuscript to Neri's later claim to fame: glass—its most notable appearance in Tesoro  is in illustrations of experimental vessels. 1598 was two or three years before Antonio's glassmaking career is thought to have started. His life was busy on other fronts. He had just been ordained as a Catholic Priest, undergoing the laying on of hands ceremony, probably by his neighbor on Borgo Pinti, the Florentine archbishop Alessandro Ottaviano de' Medici. 1598 was also a year of tragedy for the Neri family; by the summer, his father Neri Neri, personal physician to the grand duke, died of an unknown illness, but not before making arrangements for his family, which included a will. A court magistrate would oversee the now orphaned children's education, inheritance and dowries. They would soon suffer further tragedy in the unexpected death of a brother; sixteen year-old Emilio would leave them on Christmas day of the following year, in Castello outside Florence. 

The losses must have been devastating. In 1598, five of the nine children were still under the age of twenty. Without a mother or father, great responsibility must have fallen on the shoulders of the matriarch of the house, the children's elderly paternal grandmother Maddalena. [2] To outsiders, the Neri's were a wealthy and quite well respected family, but internally, the tragic events undoubtedly threw their lives into turmoil. It is in these circumstances that Antonio started his manuscript. The events allude to the reasoning behind a cryptic Latin quote scribbled at the top of a page above the first recipe of Tesoro: "fuimus troes." It means, "We are Trojans no more" From Virgil's Aeneid, referring to the fall of Troy. [3] 

Early in the manuscript there is a series of four illustrations, each showing a different activity, In order, they are titled The Art of Preparation of Stones, Metals, Plants and Animals. [4] Each is filled with multiple workers engaged in various activities pertinent to the specific art. Each highlights interactions between the Aristotelian elements: air, water, earth and fire. There is no specific indication of where any of the four scenes take place, although some educated guesses can be taken. For instance, the stone workers, or lapidaries, almost certainly are an early incarnation of the famed 'Opificio delle pietre dure' working in the Uffizi's Galleria dei Lavori. But we will leave that discussion for another time. 

Of the four illustrations, "Preparing Animals" focuses on activity within a kitchen. (See above, click to enlarge). The scene is intriguing in that it appears to take place in a domestic setting. Labels call out the four Aristotelian elements; birds hanging from the rafters represent air, fish on a grilling rack represent water, a whole carcass on the spit represents earth and fire appears as itself. The scene suggest that this setting is none other than the Neri family kitchen and that the practitioners of the art are three of Antonio's siblings. In the middle right, a young boy is engaged in turning the spit (in green). The best candidate would be then eleven year-old Allesandro, the eventual heir to the family, whose own son named Neri would carry on the family practice as a physician. Of the two young women pictured, the three family choices are fourteen year-old Lucretia, Sixteen year-old Maria and twenty-six year-old Lessandra.

In one sense, the methodology of alchemy lives-on today in kitchens around the world. The process of combining raw ingredients and cooking them together, of experimentation and of iteratively refining a recipe to perfection, this is not so different from what Antonio Neri and his siblings were doing four hundred years ago.

[1] Neri 1598-1600.
[2] Maddalena di Bartolomeo di Niccolò Bartoloz[z]i, married Jacopo Neri, and they gave birth to Antonio’s father Neri Neri. (ASF 599).
[3] Fuimus Troes, fuit Ilium, et ingens Gloria Teucrorum. [We Trojans are at an end, Illium has ended and the vast glory of the Trojans], The Aeneid: Book 2, Line 325.  See also the post in this blog ( dated 13 June 2014.
[4] Neri 1598-1600, ff. 7r, 8r, 9r, 10r.
* This post first appeared here on 27 August 2014,

Wednesday, August 23, 2017

A Very Good Run

The title page of Antonio Neri's 1612 book
L'Arte Vetraria.
For most of the past five thousand years, the techniques of glassmaking were passed only in strict confidence from master to apprentice. When artisans did commit methods to writing, they were held close as precious possessions, often passed down within families. Inevitably though, some glass recipe compilations did become public, a few were even purposefully shared. But before the advent of printed books and some time after, a manuscript was typically propagated through the laborious and often error-prone process of writing copies out by hand. Even at the beginning of the seventeenth century, a mere four hundred years ago, glassmaking techniques and materials were passed primarily through the apprentice system. 

All of that changed forever, in 1612, when a Florentine priest named Antonio Neri published the first printed book devoted to the ‘art of glassmaking’. In fact, the title he chose was exactly that: L’Arte Vetraria. [1] Neri’s volume was noticed almost immediately by technical types; Galileo owned a copy and supplied one to his friend Federico Cesi, founder of an early scientific society called the Lincei [lynxes]. In his introduction, Neri specifically invites anyone curious and willing to apply themselves to give glassmaking a try, saying, “Unless you purposely foul-up, it will be impossible to fail”. However, the book did not exactly catch fire with the general public. Slowly but surely, copies found their way to the hands of early scientific investigators and also to the hands of glassmakers throughout Europe. As Italian artisans migrated to northern Europe, Neri’s book came with them.

In 1661, a reprint of L’Arte Vetraria was issued in the priest’s native Florence, the next year an English translation was published in London for the Royal Society and the year after that a Venetian edition appeared. Within ten years, illustrated Latin and German translations were published and French and Spanish versions were not far behind. By the year 1800 over two dozen editions circulated around Europe. It had become the de facto bible of glassmakers throughout. There is precious little personal information in the book about Neri, but it does make clear that he started his career at the Medici court, in the laboratory of prince Don Antonio de’ Medici. He went on to work in a glass house in Pisa – one that supplied fine glassware to the Vatican – and then spent the bulk of his career, seven years, in Antwerp

Neri’s book, L’Arte Vetraria, shined brightest in the hands of an artisan. Neri has the rare ability to translate non-verbal skills into written words. Where exact amounts could not be given, he urges the glassmaker to develop an eye for the right color and to take the final intended purpose into account. He warns against the pitfalls of roasting a chemical too much or not purifying an ingredient enough. His book became a platform upon which later glass experimenters added their own findings and it became a kind of working document. This started with the 1662 English translation by London physician Christopher Merrett. Not particularly familiar with glassmaking himself, Merrett canvassed experienced artisans in England and made extensive notes that he appended to Neri’s original. Merrett also rearranged Neri’s text in a format that he felt more appropriate. The Latin edition by Frisius, in Amsterdam, restored Neri’s original format, but also retained Merrett’s observations. In 1679, Johann Kunckel made a German translation that added his own extensive knowledge, producing what is perhaps the most authoritative edition that the book attained.

In 1697, Jean Haudicquer de Blancourt translated Merrett’s English edition into French, but he gave no acknowledgment to Neri or Merrett. Blancourt plagiarized the work, putting his own name as the sole author. He greatly expanded the page count, but added little or no new material. Where Neri stipulated, for instance, that artificial gems of all colors could be made by adding previously discussed pigments to a basic lead glass formula, Blancourt turned each of these into a separate recipe. In doing this, that one particular section grew larger than the entirety of Neri’s original book. Two years later, in 1699, Daniel Brown translated Blancourt’s English-to-French rendition back into English. From there, the formulas – especially the artificial gem recipes – started to appear un-attributed in popular craft encyclopedias; the Neri provenance of these recipes was now erased, but they continued to influence artisans through the nineteenth century.

Meanwhile, new translations with correct attribution were published and the book flourished in popularity. Authors built on each other’s annotations; from Neri to Merrett to Kunckel. In 1752, Paul Thiry d’Holbach published a proper French edition that included the accumulated comments of the three annotators. In 1780, Suárez and Núñez completed their Spanish edition, based on Holbach.[2] And the tradition continues today; in 2007, Holbach’s edition was used as the basis for the first ever Japanese translation by Sakata and Ikeda. [3]  

On one hand, this particular lineage of the book is remarkable for the tour of languages; Italian, English, German, French, Spanish and Japanese. On the other hand, it spans a remarkable period in history, starting with alchemy and progressing through the volumes to modern science. With each new set of annotations Neri's book starts with a purely empirical set of recipes, using classical alchemy. By Kunckel’s edition, experimentation was more formalized and rigorous; the chemistry behind pigments for coloring glass became an intense subject of scrutiny. By Holbach’s edition, the physical and mechanical properties of glass were being investigated and in turn, glass played an ever-critical role in instrumentation like thermometers, barometers, microscopes, telescopes and a newly invented electrical device called a Leiden jar.

Antoine Lavoisier is considered by many to be the ‘father of modern chemistry’. For all practical purposes, his isolation of oxygen as a discrete element, in 1778, rang the death knell for classical alchemy. About 1799, Lavoisier’s good friend Pierre Loysel wrote what was to be the successor to Neri’s book on glassmaking, Essai sur l’art de la verrerie [Essays on the art of Glassmaking]. [4] While it never attained the fame or currency of Neri’s contribution, it did mark the passing of the mantle to new techniques and a better understanding of the materials. [5] For nearly two centuries before Loysel, Neri’s book and its derivatives held the floor as the authoritative reference for glassmakers throughout Europe and beyond. 187 years is a remarkable run for any book, even more so for a volume devoted to technical advice and recipes. Antonio Neri would have probably preferred to be remembered for his work on alchemical transmutation and medicinal cures, but in the end it was his sensible book on glass formulation that continues to endear him to anyone interested in the art of glass.

[1] Antonio Neri, L’Arte vetraria, distinta in libri sette, del R.[everendo] P.[rete/ padre] Antonio Neri fiorentino. Ne quali si scoprono, effetti maravigliosi, & insegnano segreti bellissimi, del vetro nel fuoco & altre cose curiose. All’ et Sig., Il Sig, Don Antonio Medici (Florence: Giunti 1612).
[2] “Sobre el Vidrio Y Los Esmaltes” and “Continuacion Del Arte De Vidrieria” in Memorias instructivas, y curiosas sobre agricultura, comercio, industria, economía, chymica, botanica, historia natural, &c, ... Miguel Gerónimo Suárez y Núñez, ed., tr. (Madrid: Dom Pedro Marin, 1780) v. 4., pp. 185–224 (Mem. 50, prolog); 225–470
[3] L’Arte Vetraria by Antonio Neri, Japanese Translation, Hironobu Sakata, Mayumi Ikeda eds., tr. (Yokohama: Shunpusha, 2007), a translation of Neri 1759 (Holbach).
[4]Pierre Loysel, Essai sur l’art de la verrerie (Paris, 1799/1800).
[5] For more on Loysel and his book see Marco Beretta, “Unveiling Glass’s Mysteries Lavoisier, Loysel and the First Chemical Treatise on Glass (1765–1799),” in Objects of Chemical Inquiry ed. by Ursula Klein and Carsten Reinhardt (Sagamore Beach: Science History Publications/USA, 2014)

Monday, August 21, 2017


Vintage 1920's Water Glass Label.
The temperature of 1500 degrees (825C) was comfortably within reach of seventeenth century glassmakers like Antonio Neri. It was also achievable in much earlier times and was used in one of the earliest chemical reactions of  human industry; the production of "lime" by heating seashells or limestone to the above mentioned temperature. This is the point at which these materials give up the carbon dioxide gas bound into their chemical structure, leaving behind calcium oxide, otherwise known as lime.

What makes lime so useful is a property early Roman engineers knew well; that when mixed with water, lime undergoes a chemical reaction that releases heat and quickly solidifies into a rock-hard mass that is impervious to the elements. The Romans used it as a construction material; they mixed it with ash, clay, sand and stones to form mortar, cement and concrete. It has played a critical role ever since in the construction of buildings, roads and monuments.

Today it is perhaps not an obvious choice in imagining the supplies of a seventeenth century alchemist’s cabinet; lime does not require exotic methods to produce, it was made in industrial quantities and readily available as a construction material. Nevertheless, lime played an important part in many diverse chemical preparations of the day. Antonio Neri mentions it numerous times in his writing. He used it in his lute recipe; a hard, cement like coating which protected alchemical glassware in direct flame and sealed joints. He also used it to extract the color from flowers in the production of paints. Most notably, he used lime as an ingredient in the formulation of glass itself. 

Lime turns out to be a rather critical ingredient in glassmaking and the central player in a mystery about the history of glass that to this day has not been satisfactorily explained. Neri obtained the white powder from suppliers as "lime cake." Early in his book, L'Arte Vetraria, he states it should be used in all his glass recipes. 
Take lime salt, which is used for building. Purify this salt and mix it with ordinary Levantine polverino salt in the proportion of 2 pounds per 100, which is 2 pounds of lime salt, to every 100 pounds of polverino salt, purified and well made, as previously described. With this salt mixture, you can make yourself ordinary frit, and put it in the crucible to clarify. I will refer to this frit from now on in the recipes for cristallino, cristallo, and common glass. This way you will have cristallo quite subtle and beautiful.[1]
The above excerpt is the full extent of Neri's discussion on lime in glassmaking. An interesting point here is that he does not say why lime is so necessary, but it is. [2] He must have known that adding too much lime will cause glass to become cloudy, the exact opposite effect that he was after. What is not so clear is if Neri (or anyone else in that period) knew that too little lime allows glass to dissolve slowly when exposed to water. 

One of the great enduring mysteries in the history of glassmaking is the question of exactly when it was clearly understood that lime was so important. The subject became a matter of deliberate chemical investigation in the eighteenth century, but the use of lime extends much earlier. Some of the earliest glass fragments known contain healthy amounts of lime. Neri's ambiguous endorsement is the first mention in print, but even among much earlier manuscripts, its role is unclear. In the first century, Roman author Pliny made a cryptic reference to adding seashells to the glass melt, but again, without further explanation. [3] Analysis of ancient glass often shows a significant lime content; seemingly more than could be reasonably accounted for by happy accident or contamination. The ingredient is never mentioned in most surviving early recipes. Of course, it may be that  lime is present in many ancient glass artifacts precisely because those artifacts are the ones that have survived without dissolving.

Some have speculated that calcium could have been introduced inadvertently as seashell fragments when sand was crushed into powder for glassmaking. This hypothesis is certainly possible, but it seems suspect considering just how much seashell would be required and how fussy the glassmakers were in obtaining pure white sand from particular locations. However, a distinction must be made between knowing that sand from a particular location makes good glass, and knowing what is in that sand. 

Perhaps the most famous such site for good glassmaking sand is the "Belus" [Na'aman] River outlet in what is now northern Israel. This is where the story of the discovery of glass takes place, as told by Pliny and other early writers. Sailors were driven here by a storm, they used natron (glassmaking salt) [4] from their cargo to hold their cooking pots up over a beach fire, and when the natron mixed with the sand in the fire, glass was formed. Or so the story tells. The nearby Phoenician cities of Tyre and Sidon are known to have hosted a thriving glass industry. The sand in this area is composed of exceptionally pure quartz, with a healthy amount of calcium from shells ground down from wave action. Together these make an ideal mix for stable glass. 

There is some evidence that experimenters such as Basil Valentine were aware that glass without lime would dissolve as early as 1520. [5] For what it is worth, I can contribute some anecdotal evidence from the twentieth century. There is a well-known product called water-glass that is essentially composed of dissolved glass without the lime. [6] My glass artist friend Emilio tells me that growing up on Murano in Venice, his mother regularly preserved fresh eggs by dipping them in water-glass, which when dry, formed a hard seal over the existing shell. Presumably, she learned the technique from her own mother. I do not know of any early references, but the use of water-glass clearly indicates a knowledge of the role lime played.

There are other explanations possible for the addition of lime to glass. Lime was a commodity that was made since ancient times, in furnaces very similar to the ones producing glass. In fact, Neri specifies the use of a “limekiln” for the production of his enamels. The archaeological remains of Tyre and Sidon show prolific use of lime based plaster and mortar in their construction practices and there are indications of lime making facilities. It seems only good business sense that the same furnaces making glass may have also been making lime. Cross contamination, and even experimentation would be a natural outcome. 

[1] Neri 1612, ch 7.
[2] As an aside, magnesia (MgO) can also serve as a stabilizer in glass.
[3] Pliny, Natural History v.36, ch. 66.192.
[4] Natron is a mix of sodium carbonates. 
[5] Basil Valentine, et al. see footnote 6 of the Wikipedia article on sodium silicate for a discussion.
[6] Sodium Silicate.
* This post first appeared here in a slightly different form on 10 October 2014.

Friday, August 18, 2017

Fire and Brimstone

The Alchemical Symbol for Sulfur
Bright yellow elemental sulfur or “brimstone” as it was often called, occupied a central place in the cabinets of seventeenth century alchemists. Antonio Neri used it in many of his preparations and specifically in pigments for glass. When sulfur is heated with thin sheets or shavings of metal, foul smelling chemical reactions can take place that reduce the metal to a powdered compound and some of these turn out to be effective glass colorants. Neri’s 1612 book, L’Arte Vetraria, offers a variety of recipes, which specifically prepare iron and copper using sulfur to form pigments. In reality, the resultant chemicals were mixtures of oxides and sulfur compounds. Since these also chemically interact with each other in the glass melt, many different effects are possible. Modern glass artists sometimes specifically use both oxide and sulfide pigmented glass side by side in the same piece; a striking effect can be the spontaneous formation of a third color along the boundary. As Neri says in the closing line of his book:
Although I have placed here the way to make this powder with much clarity, do not presuppose that I have described a way to make something ordinary, but rather a true treasure of nature, and this for the delight of kind and curious spirits.[1]
Keep in mind that the thinking of alchemist Neri was that the sulfur acted upon the metal, but did not necessarily combine with it. From his point of view, the exposure resulted in the metal’s infusion with new properties. The Aristotelian conception of the world was that everything under the sun contained various amounts of four elemental essences: air, water, fire and earth. Sulfur was seen to be dominated by the latter two, ‘fire’ because it burned easily and ‘earth’ because it occurs as a solid.

In the sixteenth century, a Swiss physician named Paracelsus developed an extension of the four-element system. After his death, his writings enjoyed a new popularity among chemical experimenters in the period that Neri came of age. Since his teenage years, the work of Paracelsus was a strong influence on both Neri and separately on his benefactor, Florentine prince Don Antonio de’ Medici. According to Paracelsus, sulfur was one of a triad of “principles” consisting of salt, sulfur and mercury. These three had philosophical as well as physical interpretations attached to them. Besides other applications, like in medicine, the three physical materials figured prominently in efforts to transmute one metal into another. 

In fact, sulfur in particular played a starring role in a very convincing demonstration that purported to turn iron into copper. Mining operations often utilized water to clean or separate ore from tailings. Other times, water was used to keep dust down, or simply flowed naturally through underground springs. When sulfur-bearing earth is exposed to air and moisture, the result can be the formation of dilute sulfuric acid. This “vitriol” was an irritant to the eyes and skin, and very unpopular with the miners. However, in at least one location, it seemed to have a miraculous property. When this “vitriolated water” flowed out of the mine, it seemed to transform bits of iron into copper. [2]

Chemically, copper had already been dissolved in the acid, forming a copper sulfate solution. But sulfuric acid shows a preference for iron. When the copper solution flowed over iron tools, it took up the iron and dropped the copper, depositing it in a thin layer. The effect appeared to be a transmutation of iron into copper. Further testing and scrutiny confirmed that pure iron, when exposed to the mine fluids resulted in real copper. Neri for one was well aware that the vitriolated water might have arrived containing copper, as he explains in his manuscript Discorso. [3] But apparently, it did not occur to him that the water leaving the scene might have contained the iron. If he had made the connection, the observation would have advanced the understanding of both ion-exchange chemistry and the principal of conservation of matter; these were two ideas that would not be explored seriously for another hundred years.

Well into the eighteenth century, the mine at Smolnik, (now in Slovakia), was a highly touted tourist destination for chemical experimenters. [4] For some, it was considered among the strongest evidence that transmutation could and did take place in the natural world. I like this demonstration so much because it works the same way as a parlor trick; while we are so intently focused on the metal changing before our eyes, Mother Nature quietly slips the copper in with one hand and takes the iron away with the other, no one the wiser.

[1] Neri 1612, p.114.
[2] See this post for a more detailed description
[3] Grazzini 2102.
[4] The effect had previously been described Georgius Agricola, in book 5 [9] of Nature Fossilium. See edition, transl. from the first Latin edition of 1546 by Mark Chance Bandy, Jean A. Bandy (New York: Mineralogical Society of America, 1955), p. 188.
*This post first appeared here on 7 November 2014.

Wednesday, August 16, 2017

The Dregs of Alchemy

"The struggle of fixed and volatile" 
allegorical illustration from
Splendor solis [detail] 16th C.
To 17th century Italian glassmaker and alchemist Antonio Neri, "Dregs" were otherwise known as terra, gruma, immondita, terrestreità and the evocative sporchezza. It was the "filth" and sediment left in the bottom of vessels after useful material was extracted from a preparation. These often foul-smelling substances were sometimes discarded:
Then filter out the dregs of the vitriol impregnated water; that which is yellowish you should throw away. –L’Arte Vetraria, chap. 38.
Other times, dregs were further refined. A notable example was the potassium rich muck left at the bottom of aged wine barrels. This was Neri's secret ingredient in producing a fine, sparkling cristallo glass. To understand the distinction between the useful and the useless forms of dregs, we must dig deeper into Neri's philosophy.

It might be surprising to some that these lowliest of materials could play an important role in the theory of transmutation—the alchemists' ultimate quest—which was to turn base metals into gold and silver. The idea was that a natural evolutionary process occurred in which  primordial material from the creation of the universe would, over time, mature through the lesser metals into pure gold. It was thought that this maturation was prompted by "seeds" of gold contained in the material. In Neri's view, this could be interrupted by various natural circumstances and could be restarted or sped-up through alchemical manipulation.

If one was to "purify" lesser metals into gold, it was advisable to know what needed to be removed. In his manuscript Discorso, Neri carefully explains five categories of impurity, which he then breaks down further into two sub-groups:
It should be noted in general, that in dealing with the [Aristotelian] elements in accordance with chemical philosophy, we can say that all mixed bodies in this art are discovered to contain five kinds of impure substances, which are completely dead and without any virtue or properties effective to [alchemical] operations. Two are from impure substances and three from pure substances, where all the strength, effectiveness and virtue are located specific to each mixture. Of the two [derived from the impure] one is called 'phlegm', which is to say a watery substance with no odor or taste and the other is called 'dead body' [corpo morto] or 'damned earth' [terra dannata], an earthy substance that is equally tasteless and without virtue.[1]
Indeed, in Neri's chemical philosophy, the above two useless forms of impurity (phlegm and corpo morto) are complemented by three useful forms (salt, oil or true sulfur and spirit or mercury), which are present in so-called pure materials. Researcher Maria Grazzini notes in her annotations to the manuscript that: "The chemical philosophy to which Neri refers is Paracelsian, which in addition to the four Aristotelian elements introduces the principal triad (tria prima) of salt, sulfur, and mercury. References to sulfur and mercury were already present in Arabic alchemy." [2] 
Of the other three [derived from the pure] one is called 'salt' and it is the so-called most fixed substance because it is resistant to the violence of fire; it does not flee or vanish into the air. The second is called 'oil' or 'true sulfur' because of the similarity to it, fatty and viscous. The third is called 'spirit' because it is more spiritual and volatile than all the others and even the slightest heat will cause it to dissipate into the air if it has not been bound to the salt, which is the component fixed by the oil. By its tenacious, slimy nature, [oil] acts to bind the volatile to the fixed. These three types are those of the pure substances, which are called by many other names; 'body', 'soul' [anima], 'spirit'; 'bitter', 'sweet', 'acid'; 'salt', 'sulfur', 'mercury' etc.
In them alone are placed all of the virtue and effectiveness of the minerals, the vegetables and the animals, even if the quantity of pure substance is very small in comparison with the impure in any kind of mixed body. These [three] are found in each mixed quantity of pure substance, in comparison with the ineffectual found in the impure. [3] 
In his view, it is these last three forms of impurity that hold the key to transmutation, which tends to puts dregs in a whole new light.

[1] Grazzini 2012, p. 339.
[2] Ibid, note 45, p. 339.
[3] Ibid, p. 340.
* This post first appeared here on 20August 2014.

Monday, August 14, 2017

Glass Salt

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 real "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. Shipping them this way cut down on weight and volume, and prevented rotting. These Soda and Kali 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.

Friday, August 11, 2017

Art and Science

Jacopo Ligozzi,1518,  fanciful glass vessels,
ink and watercolor on paper.
Antonio Neri's writing on glassmaking and alchemy was distinguished from that of many contemporary authors in that his work was all deeply rooted in hands-on experience. He worked in the early 17th century, when art and science were different sides of the same endeavour to understand the world. His contemporaries were often content to repeat century's old teachings about the four Aristotelian elements; that chemical interactions could be explained through an analysis of the balance between hot and cold, dry and wet. But more and more, these notions were being discarded and replaced. It is common to cite the invention of instruments, and other technical developments; these factors certainly did contribute to advancement. But many different forces worked toward the emergence of early modern science, and one in particular is so obvious that it is easily overlooked: artists.

Working with hot glass was a profession in which attention to nature was essential: artists did not have the luxury of fanciful explanations of physical processes. They were obliged by their work to learn the ways glass mixed, moved and behaved in the furnace, not as they imagined it should, but as it actually did. The only way to achieve the complex forms and vessels for which master glassblowers were renowned was through long experience. Failure to understand the glass and predict its properties accurately resulted in failure of the piece.

Neri was immersed in this environment and the same principles applied to his own work in formulating the glass. Ancient theories had little value if they did not accurately predict nature. Like the glass artists, the way forward for Neri was careful attention and hands-on experience. He learned the value of starting with highly purified ingredients for his glass melts. He learned that too much glass salt resulted in a putrid 'gall' that would need to be skimmed off the molten surface. Substituting salts made from fern plants, for the Kali based ones from the Levant, produced a more lustrous glass, yet it stiffened more quickly for the glassblowers.

A glass artist's work also serves as a kind of narrative. For those familiar with the techniques, a finished piece of glass work can be 'read' like a story: The handles were put on last, before that, perhaps a thin bead of color was applied to the lip of the vessel. And the work started as a blown bubble of glass, shaped and opened with special tools. Each step is an insight into the artist's technique, but also into the way nature itself operates. Each motion was a well practiced negotiation between the artist and the properties of the material.

On one hand, an artist's job was to produce objects contemplated for their physical beauty and cultural significance. On the other hand, the act of producing these objects created an environment where accurate reasoning flourished. By collecting artists and employing them together, the Medici rulers of Tuscany were creating a cauldron effect where experiences collected, stewed and nature's secrets unraveled.

* This post first appeared here on 23 October 2013.

Wednesday, August 9, 2017

Benjamin Franklin and Glass

Note: This is a shorter version of a piece appearing in the Spring/Summer 2016 issue of the NAGC Bulletin. Many thanks for their permission to share it here. A copy of the complete article is available through interlibrary loan from the numerous public and art museum libraries which subscribe to this journal(including the Rakow Library at The Corning Museum of Glass). The Bulletin can also be obtained directly from its publisher, the National American Glass Club.

Benjamin Franklin and His Gathering of Glassmakers

Portrait of Benjamin Franklin, lampworked glass murrina, 
2016, by Stephen BoehmeRivertonUtah
Benjamin Franklin (1706-1790) became famous in his own lifetime as a printer, author, inventor, statesman, diplomat and scientific investigator. The man gracing the hundred-dollar bill has been celebrated for his work in a formidable range of fields, so perhaps we should not be surprised to learn of one that has largely escaped notice. In fact, there is an extraordinary further chapter to be told. Franklin fostered a lifelong fascination with glass and spent considerable energy in efforts to attract talent from Europe to work in America.

Glass and glassmaking garnered not only Franklin’s own enthusiasm, but also that of his family and friends. He worked closely with artisans on two continents and applied his considerable knowledge of glass to areas ranging from music to optics to electrical experimentation. On his European diplomatic missions, he tirelessly encouraged foreign glass workers to set up shop in the colonies. In the decades before America gained independence, he recognized and promoted the vital importance of glassmaking. Unbeknownst to many, Benjamin Franklin played a sustained and influential role in the formation of the American glassmaking industry. 

His connections to the field were wide ranging; his older, favorite brother, John Franklin, became co-founder of a glass factory in Braintree Massachusetts from the late 1740s to early 1750s. [1] In Philadelphia, Benjamin befriended Thomas Godfrey (1704–1749), a glazier, optician, glass and instrument maker who rented space from Franklin to work on new inventions. Lambert Emerson was another glass related acquaintance and fellow Freemason; an émigré from Dublin who advertised in Franklin’s newspaper The Pennsylvania Gazette as a “looking glass maker at the Sign of the looking Glass in Front Street, Philadelphia.” [2] Franklin’s neighbor, Caspar Wistar, was a German glassmaker from Cologne. He owned the nearby glassworks at Alloway Creek in Salem County, New Jersey, just twenty miles south of Philadelphia. Besides making windows and bottles, Wistar manufactured special “philosophical” glassware for Franklin, used in his electrical investigations. 

But Franklin had much more than a passing familiarity with glassmaking. In 1746, he advised Connecticut businessman Thomas Darling on the particulars of running a glass foundry, referring to Wistar’s New Jersey operation. In his correspondence, Franklin consistently referred to his neighbor’s ‘Wistarburgh Glass Manufactory’ as “our glasshouse,” strongly implying a business relationship with Wistar. He is credited with several of his own glass inventions. Two popular items that particularly employed glass were bifocal spectacles and his musical armonica. 

Over his lifetime, Franklin traveled to Europe four times between 1725-75. In London, Paris and on side excursions throughout Europe, he was in frequent contact with glassmakers and he spent considerable effort encouraging them to immigrate to America. 
He understood the strong potential of a glass industry as an economic driver and as a window to groundbreaking scientific discoveries. Critically, he also understood the value of inviting foreign workers to participate in the American dream. In London, he frequented the Royal Society (of scientific investigators) and was correspondent and houseguest of such luminaries as Joseph Priestly, David Hume and Erasmus Darwin. Glass was a hot topic of discussion in these circles due to the recent development of electrostatic generators and Leyden jars, which all had critical components made of glass. 

Franklin continued his efforts to woo glassmakers to America, but plainly, it was not always easy. In a correspondence of 1771, we see a rare pessimism rear its head. In Philadelphia, Joseph Leacock, a cousin of his wife, partnered with a local tanner to start a glass factory on what is now Richmond Street. They wrote to Franklin in London hoping to find workers, but his reply was not encouraging, “It is always a Difficulty here to meet with good Workmen and sober that are willing to go abroad. I heartily wish you Success in your laudable Undertaking to supply your Country with so useful a Manufacture…” [3] He had already seen a deal fall apart a couple of months earlier. Dutch glassworker Jacob Schaub borrowed money from Franklin to book passage across the Atlantic bound for the Stiegel Glassworks in Lancaster, Pennsylvania. Unfortunately, Schaub then failed to appear for work. [4]

After nearly four decades championing an American glass industry, word seems to have gotten around. Although his responses are not available, glassmakers wrote to Franklin from around Europe. In 1778, he received a letter from French master glassmaker Müller de la Piolotte, who explained his family’s long history in the art, starting with his ancestors in the Black Forest of Germany. The 43-year-old bachelor had worked in Champagny, Burgundy and in the Alsace-Lorrain regions of France. 

By 1783, the war was over; Britain conceded and signed an armistice with the United States. Franklin received official permission to entertain applications from those willing to immigrate to the former colonies to work; the floodgates opened and finally he received the interest among glassmakers that he had sought all along. Among the letters from many diverse tradesmen around Europe, in July he received one from Bremen, Germany. The family of Herman Heyman thanked Franklin for letters of introduction. They asked for the ambassador’s consideration of “a Plan which we lately received from one of our principal Glass Manufacturers in upper Germany, who intend to establish a Glass Manufactory in Nord [sic] America.” [5]

In October, he received a letter from a Paris glassmaker who worked for the famed Brossard family of Normandy. This man, Sutter, had heard rumors that Franklin was looking for master glassmakers to work in Philadelphia. He offered his services and suggested that he could convince several other glass workers to accompany him. [6]

In January of 1784, Franklin again heard from the Heyman family, in Germany, who wrote,
… to inform you that three other Gentlemen with me Considered most Earnestly … to Erect a Glass-Manufactory in some part of the United states, and we Chused Maryland to be the properest Country for it. … One of my three Friends Mr. John Fried Amelong who had the Mánage of a Glass Manufactory here in Germany will go himself in the spring by the first Vessell [sic] over to Baltimore and take the Direction of the intended Establishing Glas[s] Manufactory, he Carries besides him 80 more Families all Experiented to our Purpose in the Vessell for Baltimore [sic]. [7]

In February of 1784, Caquery de Mezancy wrote to Franklin, on behalf of five other French glassworkers. He had come from a well-known glassmaking family and a month earlier, he discussed with Franklin his desire to establish a glassworks in America. Franklin assured him that once there, they would have no trouble finding a partner who would furnish all necessary funds; he also indicated that they would receive free passage on an American ship. [8]

In 1785, Franklin himself boarded one of those ships and returned to America for good. Even into his eighties, as a senior statesman-scientist he was engaged and still thinking about glass. In 1787 his good friend, astronomer and mathematician David Rittenhouse wrote him this note, perhaps about a sample of copper ruby or gold ruby glass, in which the color develops upon reheating, a so-called 'striking glass':
I broke a little bit off the colourless end of the Glass Tube and placed it in the focus of the Burning Glass leaving it there several minutes, but no change was produced in its Colour. After examining it I exposed it again to the collected rays of the Sun without observing the least change in its colour, but touching it with the end of a small splinter of Cedar wood the wood took fire and the Glass immediately became a fire red. [9]
Franklin and Rittenhouse enjoyed a regular Wednesday appointment, when they met with others and talked about their interests. Thomas Jefferson once commented that he would happily trade a week in Paris for a single evening shooting the breeze with Franklin and Rittenhouse at one of their gatherings. [10]

Franklin’s America was driven by the promise of fresh talent flooding in from abroad. In the realm of glassmaking, he spent a lifetime courting workers from foreign lands to settle here and become his trusted friends and neighbors. Perhaps we should end with a light-hearted but poignant piece of advice written in his Poor Richard’s Almanac, in August of 1736 when he was just thirty-years-old, “Don't throw stones at your neighbors, if your own windows are glass.” [11]

 [1] The glass factory in Braintree Massachusetts was founded by Joseph Crell, John Franklin and Peter Etter in the late 1740s. In 1752, others assumed management. See Carla J.Mulford, Benjamin Franklin and the Ends of Empire, Oxford: Oxford Univ. Press, 2015, p. 156. Thanks to Gail Bardhan of the Rakow Research Library of the Corning Museum of Glass for her assistance on this point and many others throughout the article.
[2] The Knight of Glin, James Peill, John Rogers, Paul Mellon Centre for Studies in British Art., et al (2007), Irish Furniture, New Haven: Yale Univ. Press, p. 37. For the Freemason reference, see The Pennsylvania Gazette, June 25, 1741. Franklin was the chapter’s grand master by 1734.
[3] Letter to Joseph Leacock and Robert Towers from BF, London dated 22 Aug 1772. See The Papers of Benjamin Franklin (1959–) New Haven: Yale Univ. Press, v. 19, p. 282. Also, see 
[4] Henry William Stiegel (1729-83) owned three glasshouses, in Lancaster County Pa. See letter from Richard Bache to Franklin, dated 16 May 1772 (Bache was Franklin’s son-in-law).
[5] Letter from sons of Herman Heyman, Bremen to BF, Passay, dated 31 July 1783. Op. cit. Papers of BF, v. 40, p. 143.
[6] Letter from Mr. Sutter to BF, Passay, dated 29 October 1783. Op. cit., Papers of BF, v. 41, p. 548.
[7] Letter from Herman Heyman Jr., Bremen to BF, Passay, dated 19 January 1784. Op. cit., Papers of BF, v. 41, p. 489–90.
[8] Op. Cit., Papers of BF, v. 41, p. 552.
[9] Note to Dr. Franklin, from David Rittenhouse, Monday noon [c. 1787]. Op. cit. Papers of BF, (forthcoming).
[10] Kevin Keim, Peter Keim, (2007). A Grand Old Flag, a History of the United States through Its Flags. New York, New York: Dorling Kindersley Ltd. p. 43.

[11] BF (1736), Poor Richard’s Almanack, Philadelphia, August 1736, (v. 2, p. 141), see

Monday, August 7, 2017

Tartar Salt

So-called "wine diamonds," (harmless)
Potassium bitartrate deposits which can accumulate
in bottles and barrels of wine
Tartar salt is an example of an alchemist's chemical that is a byproduct of another process, in this case winemaking. In his book L'Arte Vetraria, [1] Antonio Neri uses it in his glassmaking for two very different effects. The first application is to improve the appearance of the glass and the other is to modify colors. In addition to uses in glass, tartar also finds its way into his recipes for red paint made from the dried kermes insect [2] and for cast bronze mirrors, as a flux. (In foundries, a flux keeps the metal bright and shiny while in a molten state). [3]

Neri notes that tartar also went by the name of "gruma" to which we can add the synonyms greppola and argol. He warns readers several times to "leave behind the [dried] powder of the [raw] tartar, which is no good" and that "you should have tartar, from the dregs of red wine, which is better than white wine." [4] Nevertheless, he does specifically use white wine tartar in his recipes for rosichiero, a transparent dark red enamel. [5] In his recipe for producing tartar salt [6] he directs the reader to obtain the raw material from emptied wine barrels, but elsewhere in the book he seems to prefer to use the large crystals of tartar that have “vitrified naturally in bottles of wine.” [7] Chemically, tartar is a potassium compound formed through a reaction with tartaric acid, a major constituent of grape juice. [8] 

The effect of tartar in improving the appearance of glass can be readily explained. Most of the glass that was made in Italy in the seventeenth century used sodium-based additives to lower the melting temperature of finely ground quartz powder; these formulations are known as soda-based glasses. Using potassium compounds can have the same effect and these 'potash' based glasses were predominantly produced in northern Europe where trees and plants rich in potassium were used in glass. Potassium is a heavier element and it produces a denser, more refractive glass, giving it more sparkle, although not as much as lead imparts to crystal. Unfortunately, potassium also makes the glass harder to work for the artisan. Potash glass stiffens more quickly as it cools, whereas soda glass remains workable for a longer time before requiring reheating in the furnace. Many of Neri's recipes blend the two additives, which we can imagine gives some of the advantages of both.

Neri also used tartar to modify color in glass. The effects of tartar are exemplified in a number of passages throughout the book. He uses it as the sole pigment in his recipe for pearl colored glass, but he warns, "Once obtained, you must work the color quickly, because it will dissipate." [9] Conversely, he also uses tartar to produce a black colored enamel, combining it with manganese [oxide], which by itself imparts a magenta color. [10] 

The first step in making Neri's purified tartar salt is to obtain the raw "gruma, from barrels of red wine in which it forms large lumps." Next, he gently roasts it in terracotta pots "until it becomes calcined black and all its sliminess is roasted away. It then will begin to whiten, but do not let it become white, because if you do the salt will be no good." Now, he boils it in water for two hours, evaporating off three-quarters of the liquid. After filtering, he lets the remaining liquid "lye" cool in pans, allowing any sediment to settle to the bottom. He gently pours off  (decants)  the liquid which is further processed on the stove, this time in glass containers. The result, after full evaporation over a slow fire is a “white salt” left in the vessel. He dissolves this in hot water, filters it again and allows more sediment to settle out for a period of two days. Again, the liquid is decanted and evaporated in a glass container. The filtering and evaporation process is repeated four times, resulting in a product that is "whiter than snow."

Neri's final remarks for this chapter are as follows:
“When mixed with sifted polverino, or rocchetta, with its doses of tarso or sand, this salt will make a frit that in crucibles will produce the most beautiful cristallino and common glass, which one cannot make without the accompaniment of tartar salt. Without it, good fine cristallino can be made, nevertheless with it, it will be the absolute most beautiful.” [11]

[1] Neri 1612.
[2] Ibid, ch 116, 117.
[3] Ibid, ch 113. Note that in glassmaking, the term 'flux' has a different meaning than in metallurgy.
[4] Ibid,  ch 41. 
[5] Ibid, ch 125.
[6] Ibid, ch 11.
[7] Ibid. ch 46.
[8] Pure tartar takes the form potassium bitartrate KHC4H4O6.
[9] Neri 1612, ch 60.
[10] Ibid, ch 102.
[11] Ibid, ch 11. Polverino and rocchetta are thought to be forms of dried Salsola Kali plants. Tarso is Neri's term for white quartz river stones. Cristallino was a Venetian style glass that in quality fell between common glass and the premium cristallo, for which Murano became famous.
* This post first appeared here on 5 Sept. 2014.

Friday, August 4, 2017

Vitriol of Venus

Crystals of Copper Sulfate Pentahydrate
(Vitriol of Venus)
Vitriol of Venus was one of the most cherished items in Antonio Neri’s chemical library. In his book, L'Arte Vetraria, he describes its effect in glass this way:
To your great contentment, you will be astonished at what you see. I do not know of anybody else who has tried it this way and I Priest Antonio Neri trying it found it most marvelous, as said above, and it is of my own invention. [1]
To be clear, Neri is claiming a novel preparation technique for a chemical substance that was known since antiquity. I think it is quite reasonable to say that a particular personality trait led him down this path of discovery; his almost maniacal drive for purification. For a seventeenth century alchemist, it is a trait that served him well. Where other practitioners were content to use contaminated or substitute ingredients in their formulations, Neri always goes the extra mile in verifying his ingredients and using any extra filtering steps that might be warranted, no matter how time consuming. More than anything else this is what led him to such success in glass formulation, the assurance of exceptionally clear product and bright colors.

He is so proud of his creation that he spreads the description of his method over four full chapters of the book, going into a level of meticulous detail that is extreme, even for Neri. Rest assured, dear readers, that I have taken the liberty of distilling said description down to a more manageable form for your reading pleasure. Nevertheless, our priest-alchemist clearly puts great stock in this preparation, going so far as to drop hints that this material has uses that go far beyond glassmaking: "Many things could be said here, which are omitted as not being pertinent to the art of glassmaking, which perhaps upon another occasion you will be able to judge." [2]

Before starting, he gives some general advice:
You should make the sulfurs, vitriols, ammoniac salts, and similar materials slowly, over a low fire, so they are well prepared and well opened, because a violent fire will cause great damage to them.[3]
To begin, Neri cuts thin copper sheet into small pieces half the size of a small coin. filling a crucible, he layers the copper pieces with common sulfur (known as brimstone).  He cements the vessel shut with a lid and then buries it in the hot coals of a drafted furnace for two hours.

The dark purple contents are then ground and sifted through a fine screen, mixed with six ounces of pulverized sulfur per pound and then heated in a round terracotta pan, which is sitting in the hot coals. When the sulfur starts to burn, he stirs the mixture, rolling it into balls with an iron hook so it does not stick to the pan, continuing until it stops smoking. He removes the mixture from the heat, grinds it finely, adds more sulfur and repeats the entire process three times.

Neri grinds the resulting reddish tawny colored material into powder, putting a pound of it into a large glass vessel containing six pounds of clean water and gently evaporate away a third of the water. The liquid is carefully poured off and saved. The residual solids are dried and recycled in the process. Now more solids are allowed to settle out of the "beautiful blue" liquid over a two-day period and then the liquid is filtered through a felt cone.

He heats the liquid again, this time evaporating two thirds and then puts the remaining third into glazed terracotta pans, and leaves them in a cold damp place overnight. "You will find the vitriol of copper has formed into crystalline points that mimic true oriental emeralds." The crystals are removed, dried and the liquid is further evaporated in order to obtain more crystals. To the chemist, this material is copper sulfate pentahydrate [4]; today it is sold inexpensively as a fungicide for swimming pools. One reason it was so valuable to alchemists is that when gently heated or added to water this chemical forms a sulfuric acid solution. 
This is the true flaming azure blue [tincture], with which marvelous things are made. It is most potent, and as sharp as anything known in nature today, as can easily be perceived from its odor.
However important this was in other areas of alchemy, those applications do not have any particular relevance to the blue-green tint it imparts to glass, which he does make use of throughout the book. The full recipe was so long that he continued it several times and finished as the final chapter of L'Arte Vetraria. Here are the closing words to the book:
Although I have placed here the way to make this powder with much clarity, do not presuppose that I have described a way to make something ordinary, but rather a true treasure of nature, and this for the delight of kind and curious spirits.[5]
[1] Neri 1612, ch. 31.
[2] Ibid, ch. 133.
[3] Ibid, ch. 37.
[4] CuSO4•5H2O
[5] Ibid, ch 133.
* This post first appeared here on 29 Aug 2014.