Sweet Lixiviation

Salsola Kali plant,

used in Mediterranean glassmaking

in the 17th century.

A long time ago, perhaps as long ago as the Stone Age, our ancestors discovered that mixing water with the ashes from the previous night's fire makes a good washing-up liquid. Not the solids, but what dissolves in the water forms a mildly caustic lye solution. It cleans dirty hands, and oily hair. The more concentrated the lye, the more pronounced the cleansing effect. When the solids are strained out and the liquid is evaporated, left behind is a crystalline salt known as potash. This process of extracting the soluble components of charred plant material is known as lixiviation. Potash had several applications in the ancient world that played a critical role in the advancement of civilization. Mix potash with tallow (rendered animal fat) and you have soap, spread it on your fields and you have a good fertilizer, put it in a very hot furnace with powdered sand, and you have glass. In simple everyday human activity, we see the roots of modern chemistry. The English word ‘potash’ is derived directly from the process described above: made in a pot with plant ash. It lends its name to the chemical element potassium, a major constituent and a key ingredient for many glass recipes.

Besides potassium, plant ash can also contain significant quantities of sodium, magnesium and calcium. They occur in different amounts depending on the species and on its habitat. Plants are literally composed of the nutrients in the soil from which they grow. Mineral compounds that dissolve in water are brought in through the roots and become part of the plant. In northern European forests, salt from the ash of oak and beech trees and fern plants was used for potassium glass as far back as the middle ages. In the more arid Mediterranean regions, scrub vegetation from the coastal marshes made both sodium and potassium glass, namely with the soda and kali plants. The soda plant lends its name to sodium and the kali plant lends its name to a group of chemical elements, which we now call the alkali group. Of these two plants, salsola soda is high in sodium and very low in potassium, salsola kali is more equal between the two. The high sodium carbonate content of the soda plant and a few other species is unusual, but soda accounts for most glass made, even today. Most other plants are potassium rich. 

To make glass, the material was lixiviated: boiled, filtered, purified and dried. The raw plant material was lightly charred to reduce moisture content; this prevented rotting and reduced volume and weight significantly for shipping. Neri experimented with the ash of many other plants as well and he does not hesitate to mix salt from different sources to achieve a desired effect. He found suitable glass salts can be made from ferns, blackberries, broad bean (fava) husks, cabbage, thorn bushes, millet, rush and marsh reeds. What Neri did not know was that virtually all of these plants produced a salt that was heavily laden with potassium. Potash glass has a fundamentally different character than soda glass. He knew quite well that fern glass had a different workability and appearance from soda glass, but he was over a century too early to understand why. Highly purified, potash glass sparkles in the light more than soda glass. It is denser, giving better refraction of light, yet it is also more difficult to work since it stiffens quickly on the blowpipe.

* This post first appeared here on 29 September 2014.

The Blue Tower

"The Blue Tower" Jozef Linnig 1868.

There are three known facilities where priest Antonio Neri worked as an alchemist formulating glass in the early seventeenth century; in Florence, Pisa and Antwerp. If he did work elsewhere, it must have been for a relatively short period since his time at these three locations accounts well for his entire career. Of the three, he is known best for his work at the Casino di San Marco, on the north side of Florence. It is also the facility about which the most is known, since its owner was Medici prince, Don Antonio. However, a good argument can be made that the facility in Antwerp, about which much less is known, was the one most influential to his career as a glassmaker.

Neri traveled to Antwerp in early 1604 to visit his friend Emmanuel Ximenes (Pronounced Se-men-ez), where he stayed for seven years. Ximenes was an international trader (known then as a 'banker') from one of the wealthiest families in Flanders. At the time, Antwerp stood at the center of the bloody Dutch war for independence from Habsburg Spain. The population of the city was a shadow of its former self, after being sacked and burned by Spanish troops a couple of decades earlier in what has become known as "the Spanish Fury." A Dutch blockade of Antwerp's seaports had strangled commerce, but for the ultra-wealthy, life went on.  

"Antwerpen, het Arsenaal" Jan Wildens, 17th Cent.

 In his book on glassmaking, L'Arte Vetraria, [1] Neri names "the most courteous gentleman Filippo Gridolfi" as the owner of the glass factory in Antwerp. Indeed, records show that Gridolfi was the latest in a long line of who had been granted exclusive rights to make the exalted Venetian style glass known as cristallo. Previously, the facility had been run by Sara Vincx, with Gridolfi her foreman. This is the earliest documented case known to me of a glass furnace run by a woman. After their marriage, the luxury glass business thrived in Antwerp. In the 1590s, they employed seventeen Venetian workers. And later still they hosted Neri, who stayed in the area for seven years.

The most fashionable street in Antwerp was the Meir. This was the address of  Ximene's palace, as well as of his brother in law, Baron Simon Rodrigues d'Evora, who happened to be the most prestigious diamond dealer and jeweler in the region; he was known locally as "the little king." Gridolfi and Vincx had one, and later a second retail space for their glassware here. The factory and furnaces were located a few blocks away, near the fortress wall that ringed the city. Records indicate it was in a district called  the Hopland, near -- or possibly also in -- a huge structure actually built into the defensive wall around Antwerp. Called the "Blauwe Toren" [Blue Tower] for its blue slate roof, this impressive building had at various times functioned as an armory and a storage facility. In this period, a below street level canal led from the basement of the tower directly to the Meir, in later years the canal was filled in. Just on the other side of the city wall was a mote with access to the network of waterways which connected towns and villages throughout the region; this too was eventually turned into usable real estate when the wall was demolished in the nineteenth century. This situation of the glass production facility makes perfect sense. They needed to bring in heavy materials, ship delicate product and occupy a space which was not in danger of burning down the city should disaster strike. 

Blue Tower, 1860. Edmond Fierlants.

Today the foundations of the Blue Tower are preserved just below street level in a busy traffic square. Over the centuries, surrounding structures came and went. A small number of contemporary depictions do exist. Two illustrations that give a flavor of the neighborhood are shown here. One sketch by Jan Wildens  is not in the best of condition, but shows the tower and a few nearby structures from canal level in the seventeenth century. This might well have been typical of the view from a barge making deliveries. The factory would need a steady supply of pure quartz river stones used to make the exceptionally clear cristallo glass. A second view by Jozef Linnig, shows the neighborhood more clearly albeit in 1868. By then a furniture maker was located to the left of the tower, and another structure stood where the canal once was. Also of interest is an early photograph of the tower before it was demolished.

What we know of Neri's experiences in glassmaking come mostly from his book. His activities in Florence included making aquamarine colored glass for beadmaking and chalcedony glass with its multicolored swirls. In Pisa, he made emerald green, pimpernel green and celestial blue glass, he experimented with enamels, constructed a frit kiln and made glass using fern plants. From his early glassmaking activity in Florence, Neri seems to build momentum in Pisa. In these two locations combined he spends at most four years, in Antwerp he spend seven years, and there is no indication that he was slowing down, in fact quite the opposite. There he made artificial gems, a "beautiful aquamarine so nice and marvelous, that you will be astonished." He tinted rock crystal "the colors of balas, ruby, topaz, opal and girasol." He "built a furnace that held twenty glass-pots of various colors" He made ultramarine, the deep blue pigment valued by painters more highly than gold. Finally in 1609, in Antwerp, at Gridolfi's shop he made "the most beautiful chalcedony that I have ever made in my life" and presented two vessels of this glass to the prince of Orange.

[1] Neri 1612.
* This post first appeared here on 1 October 2014.

A Matter of Plagiarism

Francesco Lana Terzi (1631-1687)

Conciatore is pleased to reprise the guest-post of independent researcher Maria Grazzini. Maria studied Antonio Neri under the late Professor Paolo Rossi, philosopher and historian of science at the University of Florence. In 2012, Dr. Grazzini published an annotated English translation of Neri's manuscript in the journal Nuncias. [1] In the course of her research, she discovered a plagiarized version of the manuscript, published by 
a famous Jesuit professor in Brescia. His version matches Neri's handwritten manuscript of 1614 word for word. Here is what Maria had to say on the subject:

The seventeenth century Jesuit scientist Francesco Lana Terzi (1631-1687) is famous for his design of a "flying boat"; he has been immortalized as the father of aeronautical engineering. What is not generally known is that he plagiarized the entire text of Antonio Neri's manuscript Discorso.

The original was never published by Neri, perhaps due to his premature death, but even as a manuscript, it must have circulated widely. It would be interesting to know the history of its diffusion, in order to understand how it became the subject of plagiarism. Lana Terzi, well known in the Italian Academia of the late seventeenth century, published his  in 1670. [2] The entire chapter 20 of his Prodromo is an exact reproduction of Neri's. Lana Terzi was fascinated by experimentation and manual arts. The Jesuit order refused their members permission to write about magic and alchemy; Jesuits with such esoteric interests could never write books directly devoted to these subjects, however, they could write works on the different aspects of natural philosophy. In this broader context chemical philosophy could be admitted.

Title page of Lana Terzi's Prodromo

Neri was popular in his own time for his glassmaking knowledge. His L'Arte Vetraria  was widely read and its reprints and translations appeared over the centuries. [3] Nevertheless, Neri enjoyed a considerable reputation among his contemporaries also for his 'chemical philosophy'. Discorso is a complete treatise on the subjects of chemistry and philosophy, to all appearance not different from many others written during the sixteenth century. It holds a similar structure, with an introduction defining the subject and the description of procedures. The final part lists possible objections raised against the validity of chemistry and gives Neri's timely responses. In this sense Discorso belongs to the alchemical traditions and Neri shows his deep knowledge of the Paracelsian doctrine and literature. Even so, the main features of the new 'scientific' mindset are present in Neri's treatise: the study of "the great book of nature" and the value of experimental practice. The traditional reliance on the authority of ancient wisdom loses its legitimacy. "We should not so easily give credence to all the histories," Neri claims, but we should "prove the possibility of this art of transmutation with certain […] experiences". Knowledge is acquired "with the practice of many experiences." It does not come from a divine revelation or from the study of many books.
There is no contradiction between the alchemist Neri and the glass-conciatore Neri; the will of gaining a deep knowledge of nature, based on the observation and experimentation, is common to both. Neri is always 'the technician' and never 'the philosopher'. Alchemy, the "Great Art," is the result of a deep study of nature and its aim is not to give an imitation of nature, but to make it perfect.

The 'modernity' of Neri can also be understood in his way of talking about chemical philosophy. He does not pretend to teach eternal truths, but only to indicate the way to achieve greater knowledge, by "understanding the modus operandi of nature." Consequently, the writer does not use the form of a dogmatic essay, but that of a conversational chat, or 'discourse'.

It would be interesting to discover how Lana Terzi came into possession of Neri's manuscript. Perhaps he was attracted by the mixture of old and new which was also a predominant theme of his time, when different models of knowledge coexisted and intertwined. Discorso offered him the chance of introducing the topic of alchemy without being accused of magism.

-M. G. Grazzini

[1] Grazzini 2012.
[2] Lana Terzi 1670.
[3] Neri 1612, 1613.
* this post first appeared here on 20 November 2013.

Top Physician

Frontispiece from Ricettario Fiorentino 1597 ed.

In 1580, when Antonio Neri was four years old, just after the birth of his brother Vincenzio, both his father and grandfather were together granted full Florentine citizenship by Grand Duke Francesco I de' Medici. In Tuscany, citizen status was an honor conferred to a small fraction of the population and often through inheritance at around the age of thirty. The fact that Neri Neri gained citizenship at the age of forty and did so together with his father, Jacopo, shows it was not legacy, but perhaps their medical prowess that lead to the award. Antonio's father was a celebrated physician, and his grandfather was a well regarded barber surgeon. This period also corresponds to the first appearance of the unique coat of arms that distinguish this branch of the family. The same arms adorn the central panel of the vestibule ceiling at the Neri's residence. Citizen status bestowed the advantage of direct representation in the government and the right to hold public office. It also carried responsibilities to the city, to its leaders and to the Church. One requirement of citizenship was possession of a domicile within Florence. The baptism register listed Antonio and all of his siblings as residents of San Pier Maggiore parish long before the citizenship grant. However, it is in the 1580s that we see the first reference to Neri Neri's ownership of the palazzo at what is now 27 Borgo Pinti. 

Within a few years, Antonio's father's work on cures for paralysis were published. By the end of the decade he was appointed personal physician to the new grand duke, Ferdinando de' Medici, and to the royal family. The 1590's saw Antonio taking vows, and beginning his career in the Church. The road to priesthood ran on a parallel track to an apprenticeship in a trade. The completion of a trial period led to becoming a novice at the age of sixteen, then deacon and finally priest at around twenty-two. Meanwhile, Antonio's father, along with another doctor and two apothecaries was chosen by the entire Florentine College of Physicians to revise and update the famed Ricettario Fiorentino.[1] This book was the gold standard of doctors and pharmacists in Tuscany and throughout much of Europe. The Ricettario was an official reference for medicinal cures and prescriptions. The law required every apothecary to own a copy and to supply its listed formulas to customers. Many regard this volume as the first European pharmacopoeia. It was a systematic standardization of drug recipes and dosages. Throughout the Medici reign, updated editions appeared as knowledge progressed. The book is a major landmark in the history of medicine. The edition authored by Neri's father and colleagues, in 1597, proved so popular that in 1621 it was reissued without change.

There can be no doubt that Antonio's father and his work had a profound influence on the priest. In the introduction to his book on glassmaking, L'Arte Vetraria,[2] he proclaims his desire to publish his own work on chemical and medical [spagyric] arts, saying "I believe there is no greater thing in nature in the service of humanity." In his recipes he uses the terminology of physicians; adding chemicals in 'doses' and measuring 'ana' (in equal parts). In a 1608 letter to a friend, [3] Antonio describes his great success with medicinal cures of Paracelsus, "to the great wonderment of Antwerp." The priest also references experiments he carried out in Brussels and at the Hospital of Malines. Following in the footsteps of his father and grandfather, medicine would continue to be practiced in the Neri family for generations to come. 

[1] Neri, Benadù, Rosselli, Galletti 1597.
[2]Neri 1612.
[3]Neri 1608.
This post was first published here in a slightly different form on 16 October 2013.

Deadly Fumes

Memento mori, 1605.
Nikolaus Alexander  Mair von Landshut.

17th century glassmaker and alchemist Antonio Neri handled very dangerous materials on a daily basis. He used strong acids, which if splattered could easily burn flesh, or cause blindness. He handled poisonous compounds containing arsenic, mercury and lead. If ingested, or inhaled as fumes these materials caused progressive, irreversible damage to internal organs and especially to the nervous system. There is no question that Neri did take chances with his health, but he was not naive. He knew very well many of the potential dangers and others he could well imagine. In chapter 74 of his book L'Arte Vetraria, he describes a way to tint rock crystal with beautiful colors. He wrote: 

Take orpiment of that really tawny orange-yellow color and pulverize 2 ounces of this along with 2 ounces of powdered crystalline arsenic, 1 ounce of pulverized crude antimony, and 1 ounce of sal ammoniac. […] Perform this entire operation under a large chimney to draw the fumes out of the room. These fumes are not only harmful but also quite deadly. Return to see if the coals have died down, because for the work to come out nicely, they must be burning well and full. For the remainder, leave and let the fire run its course with no one in the room with the work, for it is dangerous; the harsh materials will smoke quite a bit. Leave it to finish all fuming by itself and then extinguish the fire and spread the coals.[1]

In the spring of 1603, Neri was working in Pisa and became seriously ill. The specific cause and symptoms of his ailment are not known. He could have been harmed by one of his own experiments, or just as easily fallen prey to an infection or one of many other maladies prevalent in the early seventeenth century. He postponed his planned visit to Antwerp in order to recuperate. Finally, on 2 May 1603, his friend Emmanuel Ximenes wrote: "Praise God that your indisposition has ended ... if the Pisan air is suited to your recovery then do not change it." [2]

In the winter of 1603-4 Neri embarked on what would become a seven-year-long visit to his friend's palace in Antwerp. There, he would learn many new techniques, and ultimately have special glass vessels made and presented to Philip William, Prince of Orange. Upon his return to Tuscany in 1611, he sat down to write the book for which he is most remembered, L'Arte Vetraria. Two years after publication, in 1614, the priest would be dead. According to the only known account, printed over two centuries later by Francesco Inghirami.

Meanwhile, Neri became gravely ill, so he called the prince to come to him, having the promised the secret [of the philosophers stone]. But the Medici, who was in the countryside, took too much time; the patient died before the prince could be with him. Don Antonio was not quieted and he questioned all of Neri’s friends to see if he could find the information, but his efforts were in vain, as they should be in so groundless a science, although Giacinto Salducci [sic.] said that he had seen great things, specifically a powder that fixed mercury into gold. [3]

[1] Neri 1612, ch. 74.
[2] Ximenes 1601–11, also see Zecchin 1987–89, v. 1, pp. 165–169.
[3] Inghirami 1841–44,v. 13, pp. 457–458 probably based on Targioni-Tozzetti  189.

This post first appeared here in a slightly shorter form on 30 October 2013.

A Band of Alchemists

"The Alchemist" 1558, Pieter Brugle the Elder.

Mention the word 'alchemist' and the images that spring to mind are likely the same ones that have been around for centuries. Perhaps you will imagine something like Pieter Brugle’s 1558 depiction; a fool, whose head is filled with fantasies of conjuring gold. He spends all his earnings on exotic chemicals while his children go shoeless, the cupboard goes bare and his family starves. No? Then perhaps a more classical rendition; a white bearded mystic stirring a cauldron in a deserted castle, summoning unearthly forces, bending the will of nature.    

It is true that outlandish characters like these have existed, but as a fringe element at best.  For every secluded wizard or "get rich quick" schemer there were many more alchemists who lived otherwise unremarkable lives and went to work every day. They interacted with colleagues and used their knowledge to provide valuable services like making painter's pigments or medicines or refining metals. Seventeenth century glassmaker and Catholic priest Antonio Neri fell into the latter category. Another departure from the typical caricature of alchemy is that it was very much a plural endeavor; it was practiced not primarily in isolation but by well connected networks of people, at least in late sixteenth century Florence.

Anibal and Martin

Neri's "Tesoro del Mondo" 1598-1699

Neri's father was the chief physician to the grand duke of Tuscany, and as such probably had something to do with Antonio's education and with the position that he landed in Florence at the renowned "Casino di San Marco," the laboratory of Medici prince Don Antonio, inherited from his father grand duke Francesco. Even before his prestigious appointment, Neri wrote an illustrated manuscript in which he shows a number of young men and some women his own age working at the business of alchemy. A few of them are identified by name and must have been Antonio’s friends: Anibal, Martin, Hiroem and Pietro. [1] 

Female alchemist depicted in Neri's
"Tesoro del Mondo"

The female alchemists depicted in the manuscript are not specifically identified, but a strong possibility is that they were nuns from one of the nearby convents. These facilities often maintained their own pharmacies and ran cottage industries that produced and sold goods to raise funds. Alchemy practiced by women is an area of study which still needs much research, but it is known that convents used alchemical techniques to distill their own medicinal remedies and produced their own paint pigments. The famous painter Suor Plautilla Nelli resided in the Dominican convent across the street from the Medici's Casino laboratory. Sculptor Suor Caterina Eletta was a nun at the same convent around Neri's time and was the daughter of Stefano Rosselli, the royal apothecary, another profession steeped in alchemy. Her uncle Fra Anselmo ran the Dominican's apothecary at San Marco, literally a few steps from the laboratory's front door. Suor Caterina was surrounded by relatives deeply involved in alchemy, how could she not be familiar with the subject?

At Don Antonio's laboratory, the Casino di San Marco, or the Royal Foundry as it also became known, Neri worked closely with Agnolo della Casa, another Florentine of the same age. In fact, all three men, Neri, Della Casa and Don Antonio were all born within a year of each other around 1576. Della Casa took notes on Antonio Neri's experiments in Florence over a period that spanned more than a decade. He filled literally thousands of pages. Much of this material is devoted to transmutation and the philosophers stone, both were subjects dear to Don Antonio de' Medici, their boss. The notebooks also indicate a lively correspondence with other chemical experimenters around Italy and wider Europe. Neri himself carried on a correspondence with his friend Emmanuel Ximenes who lived in Antwerp, a city that would become Antonio's home for seven years. 

The network of alchemy in Florence reached outwards to other experimenters and it also reached forward in time. Knowledge was passed from one generation to the next by schooling children in the art. From another branch of Della Casa's family came two brothers, Ottavio (1596) and Jacinto (Giacinto) Talducci della Casa (1601).  As youngsters they were said to have learned alchemy at the knee of Don Antonio. A century later, historian Giovanni Targioni-Tozzetti chronicled that these boys would go on to serve Grand Duke Ferdinando II de' Medici  and continue the work by directing the Real Fonderia when, after Don Antonio's death, it was moved from the Casino to the Boboli Gardens. Ottavio would become director of the Royal Foundry. [2]  

Jacinto Talducci Della Casa

Jacinto became a parish priest a few kilometers east of Florence, but he was pressed back into service as an alchemist after his brother died. He succeeded Ottavio as director of the Royal Foundry under Francesco Redi. Little is known about Jacinto's contributions to chemistry, but it must have been a remarkable life. He saw the germ of experimentalism really take hold; it would continue to grow and become the basis of our own modern science. Jacinto died in 1700 at the age of 99, he was the last surviving member of Don Antonio's band of alchemists and quite likely the last living soul to have personally met Antonio Neri.

[1] Neri 1598-1600, ff. 22r, 23r, 24r.
[2] Targioni-Tozzetti 1780, p. 127. Don Antonio de' Medici died in 1621.

* This post first appeared here on 26 September 2014.

Glass Discovery Legend

Giovan Maria Butteri,

"The Discovery of Glass"
Studiolo of Francesco I de' Medici

Any self-respecting Roman historian living in the first century could tell you the story that glass was first discovered by Phoenician sailors. They were temporarily grounded at the bay of Haifa, near the Belus River, in the shadow of Mount Carmel, forced ashore by a storm. Needing to eat, they improvised a fire on the beach in order to cook their food. Using natron, a mineral they were carrying as cargo on the ship, they built up a stove. To their amazement, in the heat of the fire, the natron mixed with the beach sand started to melt and liquid glass trickled out.

Actually, starting with Pliny the Elder, the author of most famous version of this story, skepticism abounds about how much of it was true. Nevertheless, if we gently tease apart the loose threads of this yarn we find that it is not without substance. First, there is the location; not just any port in a storm, this region was the site of a thriving glass industry as early as the sixth century BCE, due to the exceptional, pure white sand at the outlet of the Belus river. Archaeologists have excavated ancient glass furnaces at the nearby cities of Tyre and Sidon.

Next, the sodium carbonates in natron do indeed form glass when mixed with fine sand and brought to a high temperature, but this takes strong, concentrated heat, likely more than could be provided by a cook's beach fire. Natron is a hydroscopic mineral – this means it pulls moisture out of the surrounding environment. The water is locked into its solid crystal structure, where it remains until it is released either chemically or through heat. Natron can hold a remarkable amount of water, up to two-thirds of its weight. This is why it was used extensively to preserve mummies in Egypt; it dried out the bodies, quickly preserving them. While the story of the Phoenician sailors deserves a healthy dose of skepticism, it is also easy to see how the decomposition of the natron in the fire, resulting in the release of briny liquor, might be misinterpreted as glass.

In the early 1600s glassmaker and alchemist Antonio Neri published the first printed book of glass recipes, and in his introduction he too recounts the tale. However, in Neri's telling, natron does not make an appearance. Instead, the sailors use 'kali,' a coastal plant that is rich in alkali salts, to fuel their fire. The salts in kali are substantially similar to natron and, according to the story, triggered a similar result. In this period, Kali ash was a well-known ingredient in glass making. Neri used it in his own recipes, so the substitution is not surprising, but in this respect, Neri's version of the story does appear to be unique in the literature. It is interesting to note that Lodovico Domenichi, who was good friends with Neri's grandfather, tells a version of this story in his Italian translation of Pliny's Natural History. Here the sailors use natron, but in the next paragraph, Domenichi describes how local natives later used the plants to make their own glass.

The above depiction of the discovery of glass was painted by Butteri, one of a select group of painters for the Medici court in Florence. The work was commissioned to hang in the secret "studiolo" of Francesco de' Medici, a concealed barrel vaulted room tucked under a staircase in the Palazzo Vecchio in the early 1570's. It was only accessible through secret passages, one leading from Francesco's bed chamber. Another led from the chamber to an unmarked door on the street and a third passage led from the chamber to the secret treasury room once used by his father, Grand Duke Cosimo I. The walls and ceiling were entirely filled with paintings, the lower ones concealing cabinets full of oddities of nature, precious gems, coins, alchemical concoctions, and other treasures. Presumably, the cabinet behind Butteri's "Discovery of glass" would house some of the intricate Venetian glass vessels for which the craftsmen of Murano had become world famous. Shortly before the room was completed, a small number of these glass masters were allowed to teach their secrets in Florence by special arrangement with the Venetian government.

This post first appeared in a shorter form here on 9 October 2013.