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

Monday, April 29, 2019

Alchemy School

Frontispiece woodcut from
 De Chemia Senioris, by Zadith ben Hamuel, 1566
A common notion holds that alchemists were eccentrics, lone practitioners working in dingy basements, cut off from the rest of the world. This was a myth already well established in glassmaker Antonio Neri's time, but far from the whole truth. In the early seventeenth century, alchemy was practiced in medicine and pharmacy, in precious metals refining and even in the preparation of artists' supplies. The glassmaker was in good company. Where Antonio Neri received his training is unknown, but there are intriguing clues. 

We turn first to Antonio's own father, Neri Neri, the royal physician to the Medici family. According to historian Giulio Negri, he received his medical degree at the prestigious Studio Fiorentino, the forerunner of today's University of Florence. At the time, it had already been in operation for two centuries, having been granted a charter in 1348 by Holy Roman Emperor Charles IV. The charter was a response to a personal appeal by Archbishop Piero Corsini. Antonio Neri's own distant relative, Ser Giovanni Neri, later became Corsini's secretary, so a family legacy of attendance is not out of the question. That our priest might attend his father's alma mater is pure speculation, but it seems foolish to doubt that he started his alchemy education at his father's knee, in his own home.

The manuscript entitled Treasure of the World that Antonio devoted to "all of alchemy" was completed in 1600, but started two years earlier. On a page near the beginning, dated 1598, the twenty-two year-old clearly identifies himself as a "priest." Church rules forbade anyone from becoming a novice before the age of sixteen, which for Neri would have been in the spring of 1592. Full ordination as a Catholic priest typically took six years, meaning in 1598 Neri only recently underwent the 'laying on of hands' ceremony by the archbishop, confirming his title. The inescapable conclusion is that Neri learned his craft while in seminary and the Church sponsored his education. 

The identity of Neri’s order is a mystery, but the few scattered pieces of the puzzle that we do have allow for some interesting speculation. The list of candidates having some connection to the priest and the means to support, if not supply an education is not long. The five names that stand out are the Dominicans, the Carmelites, the Augustinians, the Knights of Malta and the Benedictines. The Dominicans were noted for their scholarship and ran two apothecaries in Florence. The Carmelites are named in the deposition notes taken by Agnolo della Casa, which identify Neri’s confessor as such. The Augustinians counted a Francesco Neri as abbot of their San Clemente monastery who worked for Don Antonio de’ Medici at the Casino in 1619. Antonio’s aunt, Faustina, apparently entered an Augustinian convent after the death of her husband.  Also, though Neri’s confessor was a Carmelite, he also served as the parish priest of an abbey run by the Canons Regular of the Lateran—an independent Augustinian congregation. The Knights of Malta ran two churches in Florence and Neri can be connected to both.  The knights followed the rule of Augustine and enjoyed a close relationship with the Augustinians. The order traces its roots to the crusades  and has various associations with alchemy such as George Ripley. Their main presence was on Malta, though in 1565 they suffered a devastating defeat there to the Ottomans. Neri was not a knight, but he could have occupied a place in their clergy. In Florence, any resources for schooling in alchemy by the knights would have been overseen by its most prominent local official; that official was Neri’s benefactor Don Antonio de’ Medici, Grand Prior of Pisa. The Benedictines were also associated with Neri’s family and had the means to provide him with an education in alchemy. He was born in the parish district of Benedictine church, San Pier Maggiore. His father was buried in the Cistercian (reformed Benedictine) church of Cestello. His sister  committed to a Benedictine order (the Camaldolese). After his ordination, in 1601, Priest Antonio Neri lived across the street from the Vallombrosan (Benedictine) mother church, Santa Trinita.

In any case, as royal physician Neri's father was an esteemed member of the court and Antonio would have enjoyed rare access to its inner sanctum. The grand duke's own laboratory at the Galleria dei Lavori certainly would have been a familiar haunt for our alchemist-priest.

This post first appeared here in a shorter form on 2 October, 2013.

Friday, April 26, 2019

Filippo Sassetti

Goa, India 1509
Later distinguished as a renowned glassmaker and alchemist, Antonio Neri was born into a patrician household. In the Florentine baptism records, his entry was made on a Thursday, the first of March, 1576. He was born the previous evening, to Dianora Parenti and Neri Neri. His godmother is listed in the document as Ginevra Sassetti. Not a great amount is known about her; she was from a prominent family, at the time in her late fifties. However, there are indications that other members of her family interacted with the Neri's. Her nephew Filippo mentions Antonio's father favorably several times in letters, providing a fascinating glimpse into the way disease was diagnosed and treated.

When Antonio was born, his father Neri Neri was in his early thirties, and already a highly regarded physician. Baccio Valori was director of the famed Laurentian Library in Florence and steward of the Medici's simples (medicinal herbs) garden. He was friend to Neri Neri and godfather to Antonio's oldest sister Lessandra. Between 1583 and 1588, Valori received letters from a mutual friend, Filippo Sassetti, who was living in Goa and Cochin – trading settlements in India. Filippo was a native Florentine; he attended university in Pisa with Valori and they became lifelong friends. After Sassetti's father was forced to sell the family home to pay off a debt. Filippo moved to Lisbon and became a spice trader. Not suited for a desk job, he soon set sail seeking adventure in the orient. 

In a 1586 letter to his old friend Valori, Sassetti discusses an Indian remedy against the plague, with a substance called bezoar. The bezoar stone is a mass that develops and becomes trapped in the digestive systems of certain animals. It often resembles a smooth rock. Some thought ground bezoar to be a universal antidote to any poison. Sassetti was puzzled about how the grindings of bezoar could work to cure the plague. Its Aristotelian elemental properties would not be a match for correcting the imbalance of humors in the body. "This is a principle," he explains, taught to him by Neri Neri. "I have thought about it and I can not understand how it works, because the plague is of the same corruption and this is a lack of heat inherent in the humidity. And the stones, if I recall correctly, they have a cold and dry complexion, hence may not precede the restoration of heat. Messer Neri one time did me the favor of telling me." 

In another letter to Baccio Valori, Sassetti notes that he has collected rare varieties of cinnamon in his travels along the Malibar coast in India. His intention was to rediscover the species thought to be a powerful cure of disease by the ancients. He planned to send a parcel of seeds of these and other medicinal plants. "If it pleases God, in the coming year, I will send this to you, so that you may see it all, together with our Messer Neri Neri, who graces my memories." Later he writes that he is sending Baccio the discourse on cinnamon, which he has compiled, along with some plants. These, it later turned out, were water damaged in the journey. He had hoped for some help from Neri Neri on the question of whether the cinnamon he collected from the island of Zeilan [Ceylon], is the same thing as the curative cinnamon of Mantua described by the ancients. Valori was an authority on these matters in his own right. As librarian for the Medici's imposing collection of books and manuscripts, he had vast academic access. As keeper of the simples garden, he had first hand experience in horticulture and its derivative medicinal cures. 

The principles of "humorism" were passed down from celebrated physicians of the ancient world, like Galen, Hippocrates and Dioscorides. It was thought that the cure of disease was dependent on the restoration of balance between four substances in the body: blood, phlegm, black bile and yellow bile. In turn, each of these was associated with one of the Aristotelian elements: air, water, earth and fire respectively. Each was further associated with specific symptoms and characteristic traits of the patient, even their psychological outlook and physical complexion. This system formed the foundation of Western medicine and was taught and practiced well into the nineteenth century. Although, within Antonio Neri's lifetime newer experimentally based methods did start to take hold. A decade after his father's death, in a 1609 letter, Antonio boasts about his success in curing disease in Antwerp using the methods of the medical upstart Paracelsus. It is unlikely that his father would have approved.

*This post first appeared here 13 August 2014.

Wednesday, April 24, 2019

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.

Monday, April 22, 2019

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.

Friday, April 19, 2019

Carries the Palm

Jesus' entry into Jerusalem, walking on palm leaves.
 Pietro Lorenzetti 1320
For Western Christians, this week began with "Palm Sunday," a feast day that falls on the Sunday before Easter and celebrates Jesus' entry into Jerusalem. His procession is said to have included his followers laying palm tree leaves before him along his path. The connection to seventeenth century priest and glassmaker Antonio Neri is this: In his book L'Arte Vetraria, Neri describes his very best green glass with a colloquial expression; saying the recipe "carries the palm" for all other greens.   


Saint Justina of Padua with a palm frond,
Bartolo Montagna 1490s
In his book, Neri presents a string of recipes for variations of green glass. Finally, in chapter 35, he presents his ultimate green, which he titles: "Another Green, Which 'Carries the Palm' for All Other Greens, Invented by Me." The phrase "carries the palm" alludes to the biblical story of Jesus entering Jerusalem, in which the people welcomed him by laying down cloth and palm branches on the ground in his path. Even before that, the palm branch served as a symbol of victory; in ancient Greece, palm fronds were awarded to victorious athletes. Later in history, Roman lawyers who won a case decorated their doors with palm leaves.


Copper Sulfate (vitriol of copper)
Cristallino was a mid-grade glass made with a soda based plant ash from the Levant which Neri called "rocchetta." For this recipe, he blends it with common glass, and adds red lead oxide to the mix, in effect forming an early version of what we now call lead crystal. He "cleans" the glass by using the well-established technique of flinging the molten glass into a large tub of clean water. This had the effect of "washing out" excess glass salt (flux). In addition, it provided the opportunity to sort through the fragments to remove any undissolved metallic lead. Lead that did not go into the glass had the tendency to collect at the bottom of the clay crucible as lumps of molten metal. It could then eat a hole in the vessel, resulting in a glass-shop disaster, as Neri warns: 
All lead precipitating out of the glass must be removed with diligence, throwing it away, so that it does not make the bottom of the crucible break out, as can happen. Return the glass that was thrown in water to the crucible and leave it to clarify for a day. Then add the color using the powder, made chemically by the dry distillation of vitriol of copper [chapter 31]. Also, add a little crocus of iron, but very little. The result will be a most marvelous beautiful green, the best that I ever made. It will seem just like an emerald of ancient oriental rock, and you can use it in every sort of job.
The "crocus of iron" mentioned above is simply iron oxide or 'rust' as it is more commonly known. The "vitriol of copper" he refers to is copper sulfate. Neri forms it in a laborious process that involves cutting copper sheet into small, coin-sized pieces, mixing it with sulfur, heating in the furnace and then reprocessing it several times. The result is then added to water and the soluble part is further processed, filtered and evaporated. The final product is a pure blue crystalline material that has uses for our alchemist that go far beyond glassmaking, as he alludes to in the final sentence of 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.
*This post first appeared her in a slightly different form on 25 October 2013.

Wednesday, April 17, 2019

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.

Monday, April 15, 2019

Sulfur of Saturn

The Roman Goddess Ops, 'sweet'-heart (and wife)
of Saturn, Peter Paul Rubens c. 1630,
“Abundance (Abundantia).” [1]
Antonio Neri’s 1612 book on glassmaking, L'Arte Vetraria, devotes an entire chapter to making artificial gems. These are made with an especially dense, refractive form of lead glass—what today would be called lead crystal. He made it in small batches, in sealed ceramic vessels; each infused with various metal oxide pigments to give characteristic jewel tone colors.  His secret ingredient for making the finest artificial gems was a material known as "sulfur of Saturn." He writes:
You will have jewels of marvelous beauty in every color, which by far surpass those described above, made with ordinary minium. Because with this true sulfur of Saturn, they will surpass all others by far more than I can write here, as I have seen and made many times in Antwerp.[2]
This "ordinary minium" that he speaks of is one of several oxide forms of lead, also known as 'red lead,' it is bright red or orange in color. As a stable red pigment it was well known to ancient Byzantine and Persian illuminators. [3] It was so popular, in fact, that the word used to describe small intricate pictures 'miniature,' is derived directly from 'minium.' [4] The intricate embellishments in manuscripts ultimately took on the name of the scribes' favorite color.  In glass, minium does not impart red or any other color on its own, but does add 'sparkle' and made Neri's jewels highly refractive to light. 

In the recipe, he uses white vinegar and he reacts it with finely ground minium through a laborious process. The acetic acid in vinegar chemically combines with the minium to form lead acetate, which alchemists called "sugar of Saturn" because it had a distinctively sweet taste (more on that later). [5] Lead (Saturn) could be added to the melt in a number of different forms including acetate, carbonate and various oxides. The heat of the furnace reduces them all and the result is essentially identical glass from any of these sources. The lead acetate has one major advantage that set it apart from the others. Lead acetate is soluble in water and therefore it can easily be purified of contaminants and to a much higher degree. It can be filtered, allowed to stand and decanted after any insoluble impurities settle out.

In other references "sugar of Saturn" and "sulfur of Saturn" are considered synonyms. This is not the case with Neri, for him they are two different substances; the 'sugar' is a precursor for his final product, sulfur of Saturn:
Left in the bottom will be a salt as white as snow, and as sweet as sugar. Repeat the dissolution, and filtering, and evaporation with common water three times. This is the required sugar of Saturn. 
Keep it to calcine in sand in a glass flask or ball in a furnace over a moderated fire for many days. It will further calcine to a color that is much redder than cinnabar, and more finely impalpable than sifted grain flour. This is the required true sulfur of Saturn; purified from the sediment, foulness and blackness that were upon the lead at first. [6]
It would be a reasonable guess that by heating the acetate for "many days" he is reducing it back to minium, its more basic, bright red oxide state, but this time in a greatly purified form.

Unfortunately for his health, in this recipe for sulfur of Saturn, Antonio makes extensive use of heating and evaporation. Even more unfortunate is that in a number of steps he judges the potency of his product by taste. Lead acetate is highly toxic; because it is soluble in water, it enters the bloodstream easily. It attacks the nervous system, accumulates in the bones and can cause organ failure. It is quite likely that by breathing the fumes and tasting crystals of sugar of Saturn, Neri was contributing to his own demise at the relatively young age of thirty-eight. Lead acetate was not the only toxic substance he handled regularly, but it was certainly one of the worst for his health. 

 It does not follow that he was totally ignorant of the risks he was taking. The dangers of heavy metals were recognized from early times. Pliny speaks of the noxious fumes from lead furnaces, and Plutarch opined that lead and mercury mines were "unwholesome and pestilent places." Neri’s own father followed the work of Dioscorides, who wrote that ceruse (lead carbonate), taken internally, could be fatal and that certain sweet wines could adversely affect the abdomen and the nerves.

Ancient Romans discovered that when wine started to turn to vinegar, it could be boiled down in lead lined pots to produce a highly sweet syrup called sapa. A late Roman cookbook made extensive use of sapa, which presumably contained considerable lead acetate. [7] It has been conjectured that  lead laced sapa contributed to poisoning among the Roman aristocracy. While lead acetate is a deadly poison, it is a step too far to imply that it contributed to the fall of the Roman Empire.

[1] Alchemical tradition does provide for an association between the metal lead and the Roman god Saturn. However, there is no such association between 'sugar of Saturn' and either of his consorts Ops or Lua, although Ops, the goddess of abundance (opulence) does rather nicely represent the chemical that made Neri's most opulent artificial gems possible.
[2] Neri 1612, ch. 91.
[3] Chemically, this is Pb3O4, also known as Lead (II,IV) oxide, triplumbic tetroxide. Historically, it was called minium and red lead.
[4] The accepted etymology of ‘miniature’ is from ‘minium,’ but may have been influenced by similar Latin terms such as  minor, minimus, minutus, etc.
[5] Lead acetate comes in two forms; Pb (C2H3O2)4 and Pb (C2H3O2)2, both are toxic; the later variety is soluble in water.
[6] Neri 1612, ch. 91.
[7] “Saba” endures today as a popular grape syrup, albeit without the lead.
* This post forst appeared here on 12 September 2014.

Friday, April 12, 2019

Crocus Martis

The many different alchemical symbols used to denote crocus martis.
In order to understand the seventeenth century glass recipes of Antonio Neri and for that matter, any alchemical recipes, it is first necessary to have a grasp of the chemical repertoire. The ingredients referenced by Neri were not especially exotic, but over the intervening four centuries since he wrote, the common names of materials have changed considerably. In the current lexicon, we name materials by their chemical formulas, so iron oxide names its two constituent elements, iron and oxygen. But in the seventeenth century, the periodic table of elements was a concept yet to be invented; hence "crocus Martis." Martis refers to Mars, the Roman god of war, the red planet, and the ancient alchemical name for iron. The origins of crocus are lost, but it may refer to the similar looking spice saffron, (crocus sativus) which has been highly valued since antiquity. Poetically, 'saffron of Mars' alludes to the orange-red color of rust. 

A first encounter with the technical recipes of sixteenth and seventeenth century alchemy can be confusing, frustrating and more than a little disorienting. There is no shortage of records, letters, manuscripts and recipe books that have been preserved from that period, however, the challenge lays in making sense of them in a way that relates to our current view of the world. It is understandable that alchemical materials and compounds had unfamiliar names; but even larger difficulties arise with the realization that any given name might describe several different chemicals, and in fact, there may be several different interchangeable synonyms/symbols for any given name. As it happens, there were good reasons for this state of affairs. The challenge of deciphering alchemy is not insurmountable. Taking the time to understand really opens up a window onto a strange and wonderful landscape of history. 

Antonio Neri considered himself an alchemist first and glassmaker second. His purely alchemical works are somewhat cryptic, but in the glass book he bends over backwards to be accessible to novices. Because of this crossover, his work can help us to navigate details in both areas that might otherwise go without explanation. His writings broadly divide into two categories: one intended for generally curious readers and another intended only for those familiar with the arcane coded language of alchemy. We have examples of both styles by Neri and all written within the period of about fifteen years at the beginning of the seventeenth century. As a result, we can use his works intended for a general audience to decode some of his more arcane passages elsewhere. With the glass recipes, his careful explanations lead us to gain considerable confidence in his technical abilities. In the introduction to his famous book on glassmaking, L'Arte Vetraria, he wrote, "I have described every last detail clearly and distinctly in this work, I am sure that if you do not purposely foul up, it will be impossible to fail, after having acquired experience and practice." Even in Neri's more obscure works, he earned the respect of later chemists; in 1870, in the journal Nature, George Rodwell, the first science master of Marlborough College, pronouncing him a "sensible chemist." Rodwell went on to note that Neri had used "no less than thirty-five different names, and twenty-two symbols" to denote a single material, the metal mercury. 

In still other writings, it becomes clear that Neri was deeply concerned that some alchemical operations were inappropriate for general consumption and were better kept secret among true practitioners of the art. In particular, he worried that if the transmutation of base metals into gold and silver were practiced widely, the result would be a collapse of the economy and would ultimately plunge civilization into disarray.

We will start with his glass book, where his materials and methods are detailed with special consideration for those not familiar with the art. Then perhaps we can build on that and make sense of his more esoteric material.

In L'Arte Vetraria, Neri gives four different methods to make crocus Martis. He explains: 
Crocus Martis is nothing other that a refinement and calcination of iron. A means by which its pigment, that in glass is a deep rutty red, is opened and imparted to the glass. It not only manifests itself but makes all the other metallic colors as well, which ordinarily hide and are dead in the glass, dance in resplendent apparition. Since this is the way to make the hidden metallic colors appear, I have put down four ways to make it.
The term calcination means only 'to roast' in a hot oven. It derives from the ancient practice of cooking seashells into powdered quicklime, which is a prime ingredient of cement; now called calcium oxide.  In his first method for crocus Martis, Neri mixes iron filings with sulfur and then heats the mixture in the furnace for a long period. We can guess that the result is a mix of iron oxide and iron sulfide. These are the constituents of a popular red pigment of the same name (crocus Martis) used in pottery glazes. In the second method, he takes the iron and sulfur mixture and sprinkles it with vinegar and leaves it in the sun for many days. The third method uses aqua fortis (nitric acid) for better effect. In the fourth and final method, Neri uses aqua regis, an even stronger acid. 

In all four cases the predominant result will be for the acids to chemically react with the metal to form iron oxide and sulfide, but alas, chemistry is not that simple; there will be minor concentrations of other compounds depending on the acid used, which may or may not affect the final product. We must consider that vinegar and the other acids in the seventeenth century were significantly different from those products today. They were made by different methods and contained a variety of impurities that would never be found in the current products. As Neri notes, each method produces different effects in the glass and therefore we must conclude that each, to some extent, differs in chemistry as well. Today, we know that iron forms several different oxides, each responsible for different color effects in glass. In a number of green glass recipes Neri uses crocus made with vinegar, yet in chapter 71 he uses it to diminish the green in yellow lead crystal. In chapter 124 he uses crocus made with aqua fortis for red glass. The lesson here is that the name of a material tells us only the basics, how it was made is at least as important if not more so. 

* this post first appeared in a somewhat different form on 15 August 2014.

Wednesday, April 10, 2019

Adam Olearius

Adam Olearius
In the autumn of 1633, a trade mission heavily laden with gifts headed east from northern Germany. The great duke of Holstein was sending an ambassador to Moscow to request the czar’s permission for travel rights along the Volga River. Holstein was the northernmost tip of the Holy Roman Empire, in many ways more  closely tied to Scandinavia than to their Habsburg overlords. This expedition was part of an attempt to establish an inland silk-trade route between Europe and the Orient. Such a route would eliminate the circumnavigation of Africa, shortening the trip and reducing risk. After several years and initial high hopes, it became clear that the effort was doomed to failure. Czar Michael I of Russia was enthusiastic about the project, but at the southern end of the route, the king of Persia was not so receptive. Nevertheless, the expedition did become famous for a different reason: a written account of the adventures of the diplomats.

Upon the delegation’s return to Holstein, their secretary Adam Olearius (1599-1671) published a book that chronicled their travels. [1] As a mathematician, astronomer and general polymath, Olearius provides an uncommon perspective of the various cultures, practices and technologies encountered. Ultimately, he was appointed royal librarian and keeper of the duke’s cabinet of curiosities. The book proved so popular that more comprehensive editions soon followed, adding the contemporary accounts of another traveler, Johann Albrecht von Mandelslo. [2]

Glass is not the central theme of his adventures, yet this and related subjects are discussed in a number of different passages, giving us a unique insight into the material’s place in those societies.

In August of 1634, the group arrived in Moscow. Having been granted audience with the czar, Olearius describes the procession of the entourage and enumerates a long list of the gifts they brought. Among them was “a great looking-glass, being an ell and a quarter high and half an ell broad, in an ebony frame, with boughs and fruits carv'd thereon in silver, carried by two Muscovites.” [3] An ‘ell’ was the northern european equivalent of a cubit or about 25 inches, so the mirror measured about 12 inches wide by 30 inches tall, not enormous by current standards, but quite an achievement in the seventeenth century. Glass workers would have to blow a large cylindrical bubble, cut it open and lay it flat on a polished marble surface. To be usable, the sheet of glass would have to be made without waves or defects and it would have to be cooled slowly, over a period of many hours in order not to form cracks. Silvering the back was a whole other ordeal performed by an artisan schooled in alchemy.

Speaking of the chemical arts, gifts from the duke of Holstein to the czar of Russia also included “an ebony cabinet, garnish'd with gold, like a little apothecaries shop, with its boxes and vials of gold, enrich'd with precious stones, full of several excellent chymical extractions, carried by two Muscovites” [4]

While in Moscow, Olearius put on a demonstration of optics for some locals. “I shew’d them upon a wall of an obscure chamber, through a little hole I had made in the shutter of the window, by means of a piece of glass polish’d and cut for optics, all was done in the street, and men walking upon their heads: This wrought such an effect in them, that they could never after be otherwise persuaded than that I held a correspondence with the devil.” [5] Here he is describing a ‘camera obscura’ in which scenes from outside are projected upside-down onto the wall of a darkened room.

A couple of years later, after a return to Holstein to ratify a treaty with the czar, the delegation arrived in Persia. They were treated to a sumptuous meal by the king’s chancellor. “The walls were all set about with looking-glasses, to the number of above two hundred, of all sizes. So that when a man stood in the midst of the hall, he might see himself of all sides. We were told that in the king’s palace, in the apartment of his wives, there is also a hall done all about with looking-glasses, but far greater and much fairer than this.” [6]

“For the teaching of astronomy they have neither sphere nor globe, insomuch that they were not little astonished to see in my hands a thing which is so common in Europe. I asked them whether they had ever seen any such before. They told me they had not, but said that there was heretofore in Persia a very fair globe which they call ‘felek’, but that it was lost during the wars between them and the Turks. They haply meant that which Sapor, king of Persia, had caused to be made of glass, so large, that he could sit in the center of it, and observe the motions of the stars and must no doubt be like that of Archimedes, where of Claudian speaks in the Epigram which begins thus: Jupiter in parvo cum cerneret aehera vitro.” [7]

Olearius goes on to describe the Persian army in some detail, including this account of an early form of chemical warfare. “At the siege of Iran, in the year 1633, they had the invention of casting into the place with their arrows, small glasses full of poison, which so infected the air that the garrison was extremely incommodated thereby and made incapable of handling their arms for the defense of the place.” [8]

In 1637, near Tehran, Iran, he describes a royal tomb “adorn’d all about with glass of all sorts of colors, which are preserved by iron grates.” [9] And in the same area,

“At Kimas, in the province of Kilan, there was one of these mountebanks, who having found out the trick of setting cotton on fire by means of a crystal cut in half-round and held in the sun like a burning-glass, would have people persuaded that by operation, which he affirmed to be supernatural, that he was of the kindred of Mohammed. After our return to Holstein, I shew’d the Persians, whom Schach-Sefi [king of Persia] sent thither, that it was the easiest thing in the world to get fire from the sun, and I lighted paper in the very depth of winter by means of a crystal full of cold water, or a piece of ice, which I had made half round in a pewter dish. They were astonish’d at it, and said, that if I had done as much in Persia, I should have pass’d there for either a great saint, or a sorcerer.” [10]

As an addendum, in later editions of Olearius’ book, the recollections of Johan Albrecht de Mandelslo (1616–1644) were added. He accompanied an unrelated trade mission to Isfahan,Persia, and then split from his group to continue touring the region. He traveled through India, and then down the African coast. In 1639, the German traveler passed through Madagascar. He commented on the inferior quality of European glass trade-beads, compared to those of India, which he acquired earlier in the trip.

“The glass-bracelets, beads and agates, we had brought from the Indies [India] were incomparably beyond what they were laden with out of Europe; so that it was resolved ours should not be produced, till the others were sold. By this means, we bought every day four oxen for forty pair of glass bracelets, which the inhabitants call ‘rangus’; a sheep for two, and a calf for three ‘rangus’; and for a brass ring, ten or twelve inches about, a man might have an ox worth here six or seven pound.” [11]

Much has been made of the use of glass trade-beads by Europeans around the world.
In Madagascar and along the trade routes of the Indian Ocean, European traders were fairly late to the party. For a thousand years earlier, [12] beads had been used as the currency of choice among disparate cultures from Indonesia and China to India to Africa who did business with each other.[13] The above quote from Mandelslo’s diary provides a fascinating firsthand account of transactions with beads. The passage also hints at the superior quality of Indian glass beads. Today, glass beadmaking continues on an industrial scale there, and the glass bracelet industry still survives, notably in Firozabad, northern India.

[1] Adam Olearius: Beschreibung der muscowitischen und persischen Reise, (Schleswig: 1647).
[2] Adam Olearius, John Davies, Johann Albrecht von Mandelslo, Philipp Crusius, Otto Brüggemann: The Voyages and Travells of the Ambassadors Sent by Frederick Duke of Holstein, to the Great Duke of Muscovy, and the King of Persia... (London: John Starkey, and Thomas Basset, 1669).
[3] Ibid, p. 11.
[4] Ibid.
[5] Ibid, p. 58.
[6] Ibid, p. 212-213.
[7] Ibid, p.252. Jupiter in parvo quum cerneret æthera vitro [When Jove a heav’n of small glass did behold,] see Henry Vaughan: Silex scintillan, Hermetical physick, Thalia redivava, Translations, Pious thoughts and ejaculations. (Oxford: Clarendon Press, 1914) v. 2, p.635.
[8] Ibid, p. 271.
[9] Ibid, p 182.
[10] Ibid, p. 280-281.
[11] Ibid, p.204.
[12] For example warring states beads in China, see https://en.wikipedia.org/wiki/Ancient_Chinese_glass
[13] Carla Klehm has a nice post on the subject of trade beads used around the Indian Ocean; see “Trade Tales and Tiny Trails: Glass Beads in the Kalahari Desert” in The Appendix, Jan. 2014, v. 2, n. 1. http://theappendix.net/issues/2014/1/trade-tales-and-tiny-trails-glass-beads-in-the-kalahari-desert

Monday, April 8, 2019

The Neighbors

"Portrait of Giambologna"by Hendrick Goltzius
In Florence, directly across the street from the Palazzo Neri, where glassmaker Antonio Neri spent his youth (now the Marzichi-Lenzi), was the residence and workshop of famed sculptor Giambologna. This two-building compound was a 'gift' to the artist from the newly crowned Grand Duke Ferdinando de' Medici, in 1587. It was intended to settle debts incurred for the artist's work by the previous grand duke, Francesco. The dwelling, at 26 Borgo Pinti, was located on the same street that was formerly inhabited by the likes of Michelangelo, Perugino, Pontormo and Cellini.
Neri's father, the royal physician, had collected art. According to historian Giovanni Cinelli, in 1677, when Antonio Neri's nephew owned the property, among the pieces in the house were:
Two small bronze horses by Giambologna, many works of [Simone] Pignoni and others, among which are two marvelous holdings; a waist-up Ecce Homo by Titian and a Satyr of beautiful ancient bronze which is wonderfully captivating; it is of the Greek manner and expresses an attitude of prompt movement that recalls liveliness, the muscles are very well prepared. Finally, a statue of Cupid flanked in marble in the best Greek style.
Giambologna had a strong influence on Florentine art and his work was to be found throughout the city, from the courtyard of the Palazzo Vecchio, to the gardens of the Casino di San Marco, where Neri made glass. The sculptor was well known for the fine surface finishes he achieved on marble and for his ability to resolve the technical challenges of portraying multiple figures, especially those involving a complex intertwining of limbs and bodies. The Rape of the Sabine Women, completed when Neri was a boy, is considered his crowning achievement.

Giambologna was born in Douai, Flanders, now in France, as Jean Boulogne. He landed in Florence in 1553, after a period working in Rome for Pope Pius IV among others. The Medici never allowed him to leave Tuscany for fear that once out of their reach, he would be enticed to go to work for another of the European sovereigns, never to return. It is reasonable to speculate that before his own trip to Flanders in 1604, Antonio Neri offered to relay messages or other effects to the family of his seventy-five year old former neighbor. Neri spent seven years with his friend Emmanuel Ximenes in Antwerp. Before his return to Tuscany, in 1611, both Grand Duke Ferdinando and Giambologna had already gone to meet their maker.


Neri had left Florence in his twenties, and returned in his thirties to a very different city. He settled down to write the book for which he will forever be remembered, L'Arte Vetraria, the first printed book devoted to the formulation of glass.

This post first appeared here on 20 Sept. 2013.

Friday, April 5, 2019

Glass Blowers

Berkshire Glass Works cane from 1878 –
Charles Flint collection. These were novelty items 
made by glassworkers after hours.
(hollow, filled with the fine quality sand of the area)
Since early days, the technical aspects of making and manipulating hot glass have remained a closed and secretive business. Down through the ages, those with inside knowledge of this art have been highly valued and highly sought after. Even with the circulation of technical manuscripts and the ultimate publication of the technology in Antonio Neri’s 1612 book L’Arte Vetraria. The beating heart of the craft remained at the furnace among those with hands-on experience. [1] These artisans could and often did make the difference between success and failure in a business that was notorious for sending substantial fortunes up in smoke.  


In some places, strict laws forbade glass workers from leaving their employ and forbade outsiders from attempts to lure them away. Recruiting seasoned talent could be a delicate if not clandestine undertaking. This was as true in the ancient world as it was through the Renaissance and it also played a significant role in the 18th and 19th century for the nascent American glass industry.

Many have heard of the legendary prohibition forbidding Renaissance Venetian glass workers from travel abroad. The fear was that they might divulge secrets of their craft to outsiders, thereby compromising the virtual monopoly on high quality luxury glassware made on the fabled island of Murano. Many also have heard that the penalty for violating these rules was death at the public gallows located between the columns at Saint Mark's square.  While there were laws on the books that listed possible penalties as severe as death, the records show that no glass worker was ever executed for leaving and many did leave. In fact, glass workers were sometimes part of state brokered deals to exchange technology.

These Venetian laws have also led many to the incorrect conclusion that the glassmakers were held captive and treated like prisoners. Nothing could be farther from the truth; glass workers were treated like rock-stars of their time. The daughters of glassmakers were allowed to marry into the nobility, a privilege granted to no other Venetian guild. [2]

Early American efforts to recruit experienced glass workers provide some amusing stories. In 1894, William G. Harding, a principal in the Berkshire Glass Works in Massachusetts wrote a detailed history of glassmaking in the region.  Harding’s research was meticulous, drawn from factory records, official legal filings and personal correspondence. Two stories in particular relate the great lengths pursued in recruiting experienced European glass workers to New England.


The first concerns Robert Hew[e]s, who despite repeated failures in starting a glassworks in the late 1700s, finally partnered in Boston with one Charles F. Kupfer newly arrived from the Duchy of Brunswick in Germany. Together they formed Boston Crown Works on Essex street.  Kupfer immediately returned to Germany to recruit workers, as harding records,
“Mr. Kupfer upon his arrival at his old home, found it no easy matter to get his blowers. The works belonged to the Duke of Brunswick, and it was a penal offense either for the men to leave, or for him to entice them away. He was obliged to conceal his designs and operate in the dark, but succeeded in escaping in the night with a set of workmen and sailed from a German port before being overtaken. After a long voyage they landed in Boston and met with a Royal reception. So much interest in the new enterprise had been awakened in the citizens of Boston, that they turned out en masse…” [3]


The other story of clandestine recruiting related by Harding concerns a venture started in Sand Lake, about 10 miles east of Albany, New York around 1806.
“They had to import their skilled workmen. Mr. William Richmond, a Scotchman, was the Superintendent of their works. He went abroad to procure workmen. Disguised as a mendicant [monk/priest], with a patch upon one eye and playing upon a bag-pipe, he wandered through the glass district of Dumbarton in Scotland and engaged his blowers to cross the Sea. With great difficulty they secreted their tools on Ship-board, for it was a penal offense for glass-workers to leave Scotland as well as Germany.” [4]


[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’Illvst.mo et eccell.mo Sig., Il Sig, Don Antonio Medici (Florence: Giunti 1612).
[2] A daugher of a glassmaker could marry into the nobility, which was a great honour for a family, but strategically, the son of a glassmaker had no such privilege, meaning that there were no inheritance rights for the family.
[3] Collections of the Berkshire Historical and Scientific Society, (Pittsfield: Sun Printing, 1894),  p. 37-39
Note that Robert Hews is also known as 'Hewes' in other sources.
[4] op cit. p. 39-40.

Wednesday, April 3, 2019

Eyes of a Lynx

The seal of the Accademia dei Lincei.
In the spring of 1612, Florentine priest Antonio Neri published his book on glassmaking. L'Arte Vetraria was the first printed book devoted to the formulation of glass from raw materials, but unfortunately for him it did not exactly take the world by storm, at least not at first. Sales were such that a number of copies still exist from the initial printing; they remain in pristine condition, never bound. 

Initially, the book received scant attention, but it was noticed. In fact, within a couple of years word had reached Rome, where Prince Federico Cesi, the founder of a scientific society, asked a Pisan member of his group to obtain a copy. That other member would go on to become one of the most recognized scientists in history. Meanwhile, L'Arte Vetraria gained prestige and readers, slowly but steadily.  By the end of the century, Neri’s book would be translated into English, Latin, German, French and then back into English from the French. It became the bible of glassmakers throughout Europe. 

In 1614, the year of Antonio Neri's death, naturalist Prince Federico Cesi wrote to his good friend Galileo. He complained of the difficulties in getting material from the Roman libraries, urging the astronomer to send him a copy of Antonio Neri's book.
The poor management of these libraries in Rome makes me feel continually thirsty for good books that come to light, which I can use for my study of compositions. They are scarcely giving me the titles, and after a long wait, only a tenth of what I asked. […] now I hear that printed in Florence is L'Arte Vetraria by Priest Antonio Neri, and I think there is some good in it. Please, your lordship, send me a copy, and believe me that I will gladly give them trouble.
 Shortly after, having received the book the prince wrote,
I thank your lordship for the book on glass, which I find very rich in experiments and beautiful artistry.
In 1603, Cesi founded the Accademia dei Lincei (Society of the Lynxes), an early scientific society whose members (with eyes as sharp as a lynx's) eventually included both Galileo Galilei and Giambattista della Porta.[1] Within a few months of Neri's death, his book was already on its way to making history.

[1] In classical Greek mythology Lynceus was the grandson of Perseus, and had preternaturally keen eyesight. See Apollodorus, Bibliotheke I, viii, 2 & ix, 16; III, x, 3 & ix, 2.

* This post first appeared here in a shorter form on 1 August 2013.