What Goes Around Comes Around

 

The German city of Ulm in the 16th century

Georg Braun, Franz Hogenberg 1570-78

(Click image to enlarge.)

In the spring and summer of 1525, peasants and farmers throughout German speaking Europe staged a popular revolt now called the Deutscher Bauernkrieg. [1] At the heart of the matter was an oppressive system of taxation run by the Roman Catholic Church, in which little or none of the revenue was used to improve life locally. Often, action was lead by Protestant clergy, but to little effect against the mercenary armies hired by the aristocracy. In the end, up to 100,000 of the poorly armed and organized peasants were slaughtered.

Along the banks of the Danube River in southern Germany lies the ancient city of Ulm. Besides being the birthplace of Albert Einstein, Ulm was, in the sixteenth century, near the center of the Peasant’s Revolt, which brings us to a curious story which traces the migration of a technical recipe from Ulm over the alps to Venice, then to Sienna and finally Florence. The recipe is for the metal alloy to make mirrors, and it is told from one friend to another while chatting amiably in Venice.

Among other things, he said that he had made one [concave mirror] almost half a braccio across [about 13 inches], which extended the clear rays of its brightness more than a quarter of a German league when he caught the sun with it. One day, when for amusement he was standing in a window to watch a review of armed men in the city of Ulm, he bore with the sphere of his mirror for a quarter of an hour on the back of the shoulder armor of one of those soldiers. This not only caused so much heat that it became almost unbearable to the soldier, but it inflamed so that it kindled his jacket  underneath and burned it for him, cooking his flesh to his very great torment. Since he did not understand who caused this, he said that God had miraculously sent that fire on him for his great sins. [2]

The story was told to Vannoccio Biringuccio, who recalls it in 1540 in his Pirotechnia, the first printed book devoted to metallurgy. Specifically, the recipe is a variant of what today we would call white bronze, which as Biringuccio states is similar to the metal used to cast bells. He recites ancient formulations that used three parts copper and one part tin. To this was added 1/18th part of antimony and optionally 1/24th part of fine silver to give it a neutral color. Indeed, other ancient formulations for what was known as speculum metal specify a 3:1 ratio of copper to tin. He continues,

But nowadays most of the masters who make them take three parts of tin and one of copper, and melt these together. When they are melted, for every pound of this material, they throw in one ounce of tartar and half an ounce of powdered arsenic, and let them fume and melt and incorporate well. 

Biringuccio’s version reverses the copper and tin ratio from the classical composition. Compare it with Antonio Neri’s prescription which appears half a century later, it is almost identical:

Have 3 lbs of well-purified tin, and 1 lb of copper also purified. Melt these two metals, first the copper, then the tin. When they fuse thoroughly, throw onto them 6 oz of just singed red wine tartar, and 1½ oz of saltpeter, then ¼ oz of alum, and 2 oz of arsenic. Leave these all to vaporize, and then cast [the metal] into the form of a sphere. You will have good material, which when you burnish and polish, will look most fine. This mixture is called acciaio and is used to make spherical mirrors.

To be clear, the tartar, saltpeter and alum act as a surface flux - they form a layer that floats on the liquid metal, preventing oxides from forming, which can foul the melt.  their addition does not change the base alloy composition.

The similarity of the two recipes alone is not enough to draw any conclusions. Biringuccio himself reports that the contemporary artisans favored the tin rich formulation. However, there are other details to consider. The Sienese born Biringuccio was something of a hero in Florence where Antonio Neri was raised. The famous metallurgist helped cast cannons, mortars and guns for the Florentines to defend themselves in the late 1520s, when the city was under siege, just a few years after Biringuccio’s conversation in Venice with his German friend from Ulm.

Neri was definitely familiar with Biringuccio’s book Pirotechnia. In fact, the introduction to Neri’s own book L’Arte Vetraria is patterned after the metallurgist’s survey of glassmaking.  In his chapter 14, book 2 Biringuccio wrote:

… it [glass] is one of the effects and real fruits of the art of fire, because every product found in the interior of the earth is either stone, metal, or one of the semi-minerals.  Glass is seen to resemble all of them, although in all respects it depends on art. [3]

And here is the opening to Neri’s introduction a half century later in 1612,

Without a doubt, glass is a true fruit of the art of fire, as it can so closely resemble all kinds of rocks and minerals, yet it is a compound, and made by art. [4]

Both passages go on to cover much of the same ground, albeit with a change in focus reflective of new thinking about chemistry and nature. In one sense, Neri is paying homage to his distinguished predecessor, and there can be little doubt that he read Biringuccio’s book and its technical recipes closely. 

Lastly, the story of the burning mirror itself mimics a widely known story about the Greek polymath Archimedes. About 200 BCE during the siege of Syracuse, he is said to have set invading Roman ships on fire with a concave mirror, which focused the radiation of the sun.

In fact a depiction of this scene was painted in Florence on the walls of the Uffizi Palace in 1600, when Neri was at the height of his employment for the ruling Medici family. This particular rendering would have been all but impossible for him to miss. 

Uffizi Gallery, Florence, Italy, Wall painting
showing the Greek mathematician Archimedes' mirror 
being used to burn Roman military ships. 
Painted in 1600 by Giulio Parigi.

In the years leading to the publication of Neri’s book, he left his home in Florence and traveled to visit a friend in Antwerp. If he had read the book on metallurgy early, perhaps as part of his education, then he was already familiar with the mirror alloy recipe. If he followed the route suggested by his friend, he would have taken the recipe back to Venice, and then over the Alps and likely through Ulm on his way north to the Low Countries, where he would spend the next seven years before returning to Italy. [5]

[1] For more on the German peasant wars of 1524-25 see https://en.wikipedia.org/wiki/German_Peasants%27_War
[2] Vannoccio Biringuccio, Pirotechnia. Ed., Tr. Cyril Stanley Smith, Martha Teach Gnudi (New York: Basic Books, 1959), pp. 385-390. (Original Italian published in 1540.)
[3] Antonio Neri, L’Arte Vetraria (Florence: Giunti, 1612). p. iv.
[4] Ibid,  p.126 (in original, ff.41r-44v).

[5] Special thanks to Jamie Hall (@PrimitiveMethod) for inspiring this post.

Reflections on the Mirror

 

Jan van Eyck

The Arnolfini Portrait (1434)

L’Arte Vetraria, Antonio Neri's 1612 book, would eventually become the glassmakers' bible throughout Europe. By 1900 it had been translated into five different languages besides the original Italian; English, Latin, German, French, and Spanish (and in this century Japanese). Because of its seminal importance in the spread of glass technology, often overlooked are a few recipes at the back of the book, which have only a tenuous connection to the main subject.

Among these is a metallurgical formula for making convex mirrors. Neri gives instructions for producing what we would now call a "white bronze" that may be cast into a rounded form and polished to take on a highly reflective surface finish. This "spherical" form of mirror was popular throughout the Renaissance. It reflected a wide-angle view of the space in which it was hung, but at the cost of distorting the image. Nevertheless, upon looking into such a mirror, objects are still quite recognizable. 

Here is Neri's prescription:

A Mixture to Make [Mirror] Spheres:
Have 3 lbs of well-purified tin, and 1 lb of copper also purified. Melt these two metals, first the copper, then the tin. When they fuse thoroughly, throw onto them 6 oz of just singed red wine tartar, and 1½ oz of saltpeter, then ¼ oz of alum, and 2 oz of arsenic. Leave these all to vaporize, and then cast [the metal] into the form of a sphere. You will have good material, which when you burnish and polish, will look most fine. This mixture is called acciaio and is used to make spherical mirrors.

Of note is the fact that the word Neri uses for this alloy, acciaio, translates to "steel." Over the intervening four centuries, the meaning of this term has been refined so that today it denotes not simply a hard white metal, but a specific range of alloys containing iron and carbon, which Neri's alloy does not. 

This recipe and a few others in the book show the breadth of Neri's experience in arts other than glassmaking. It is a conclusion greatly amplified by a perusal of his other manuscripts on alchemy and medicine. There is good evidence that our priest was a voracious reader, however he was also quite cautious about repeating techniques only after he had verified them personally. Besides, artisans never wrote down much of this knowledge – only passed in confidence between trusted parties – since, in a very concrete way, superior knowledge represented a competitive advantage over ones rivals. Even if Neri was in the business of divulging secrets, it is safe to assume that many of the artisans and craftsmen he interacted with were decidedly not. Apparently, Neri was not familiar with the process of mirroring glass directly with mercury/tin amalgam; a process for which Venetian glassmakers had already become famous for perfecting. It is an interesting omission from his book, since he almost certainly would have seen examples in Florence and in Antwerp.

Two centuries before Neri, the beginning of the fifteenth century saw the invention of moveable type printing in Germany, but also the mastery of perspective illustration in Italy. The contribution of printing to early modern science is well documented, less obvious is the role playerd by artists and perspective illustration. Moveable type made possible the mass production of books; what did get committed to paper now stood a much better chance of survival and transmission. Perspective illustration played a more nuanced role, one that ultimately brings the convex mirror back into the discussion.

Jan van Eyck
The Arnolfini Portrait (detail).

In Venice and especially in Florence (Neri's hometown), perspective drawing became the rage among artists, largely due to the Italian translation of a book entitled Deli Aspecti, or "Alhazen's Book of Optics." Suddenly, paintings were made to look three-dimensional, with a realistic sense of depth to them. The new techniques were largely kept in Italy, but interest spread across Europe. Patrons placed great value on work depicting scenes in correct perspective, and in excruciatingly accurate detail. 

In Flanders, in 1434, Jan van Eyck produced "The Arnolfini Portrait," (above). Behind the main subjects, hanging on the wall is a convex mirror. The reflection in the mirror shows the backs of the two subjects, but also two other figures further back, one of which is thought to be the artist himself, and beyond him a strong light source. The image in the mirror is distorted exactly as one would experience in real life. 

There is growing speculation that among the secrets of "realist" (or naturalist) painters was a growing arsenal of optical tools and lenses used to map out and understand the attributes of perspective. The mirror, in the Arnolfini Portrait was a sort of boast of the artist's proficiency in recreating reality on the canvas.

The point is that here is a case where art led science into new realms. Painters started to take great pains in reproducing reality "as it is" on canvas. Soon minor experimenters like Neri and major luminaries like Galileo were taking great pains to do the same. They strove to observe nature "as it is," not as was prescribed in ancient texts, or dictated by authority. Once that process started, awareness of the world grew and there was no turning back.

Finally, it is amusing to note that in his many manuscript illustrations, Antonio Neri himself never quite mastered perspective drawing, although he did try.

* This post first appeared here 17 January 2014.

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 shoe-less, 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.

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 endeavor 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.

Emerald Green Glass

 

Jesus' entry into Jerusalem, walking on palm leaves.
 Pietro Lorenzetti 1320

For Western Christians, Easter week begins 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.

Turquoise Glass

  

Turquoise glass stamp

of calif Mustadi  c.1170.

It is estimated that turquoise is among the earliest gems ever mined. With colors that vary from pastel green to a bright sky blue, it has adorned Egyptian sarcophaguses of 5000 years ago, 3000-year-old Chinese art, Aztec death masks and the domes of Persian palaces. 

When traders brought it to Europe from the Mideast, it became known as "turks" or "turquoise" after the old French for "Turkish." While it has never been mined in Turkey, the most highly valued Persian stones were imported there and used extensively for trade. Polished pieces were famously mounted on Turkish equestrian saddles, in the belief that the material conferred sure-footedness and protection from injury during a fall.

As one of the first gems to be collected and traded, turquoise was also one of the first to be imitated. Egyptian faience blue is an early forerunner of glass. It is more porous than glass, but it contains all the same ingredients and could be cast into forms that look just like solid turquoise. In the seventeenth century, the genuine mineral and its imitation continued to hold importance. In Antonio Neri's book L'Arte Vetraria, the subject is mentioned several times; he offers one recipe to restore faded stones by soaking them in almond oil. For turquoise colored enamels he presents two different shades. On the subject of glass, he notes that "Sky Blue, or more properly turquoise, is a principal color in the art of glassmaking" and "I have made this color often, because it is very necessary in beadmaking and is the most esteemed and prized color in the art."

To make his imitation turquoise glass, Neri starts with a batch of high quality transparent aquamarine blue, to which he adds a specially prepared variety of common salt. "Add it little by little, until the aquamarine color loses its transparency and diaphany becoming opaque."

Take the sea salt known as black salt or rather coarse salt, since the ordinary white salt that they make in Volterra would not be good. Put this salt in a frit kiln or oven to calcine, in order to release all moisture and turn white. Next, grind it well into a fine white powder. This salt now calcined should be stored for the use of making sky blue or rather turquoise color as described below.

Sea salt is mostly composed of sodium chloride, which is like table salt that we use for food. However, it can include significant additional minerals, as implied by Neri’s description of it as "black salt." Additional elements can include sulfur, potassium, manganese and more. Regrettably, he leaves us with no further clues to its identity, nor does he explain why the recipe would not work as well with the salt available from Volterra. He goes on to advise that the mix should be used quickly, because if left to sit in the furnace, the glass would start to revert to an ugly transparent color. The remedy for this is to add more salt. He finishes with some practical advice for glassmakers about adding salt to molten glass:

The furnace conciatore should take careful note here, when you add this salt, if it is not well calcined it always bursts. Therefore, you should be cautious and shield your eyes and vision, because there is a danger you could be hurt. Add the doses of salt little by little putting in a bit at a time pausing from one time to the next until you see the desired color. With this, I do not rely on either dose or weight, but only on my eyes. When I see that the glass reaches the desired level of color, I stop adding salt. This all comes with experience. 

* This post first appeared her in a slightly shorter form on 9 April, 2014.

Golden Yellow Glass

Yellow Neon Chandelier, 1995

Dale Chihuly.

(Columbus, Indiana Visitors Center). 

"Very few people know how to make colors like golden yellow and solid red well. These are difficult and troublesome in the art of glassmaking, since in making them you must stick precisely to the doses, the timing, the details and the materials as prescribed. The smallest error will cause everything to be ruined, and the colors to be irreparably spoiled. Therefore, you must be on guard not to make mistakes. [1]

So says Antonio Neri in his groundbreaking 1612 book of glass recipes, L’Arte Vetraria. Elsewhere he warns in several places not to add “tartar” to any glass destined for yellow pigmentation. Tartar was a common additive to boost the ‘sparkle’ of a glass because it contained a high level of potassium carbonates. These converted to potassium oxide in the melt, which has a higher refractive index than the usual glass flux, sodium oxide. However, his actual glass recipes tend to contradict this advice. 

Neri says of his “fern glass,” which is entirely potassium based:

…This frit can be given a wonderful golden yellow color provided there is no tartar salt within, as described in the caution, because then golden yellow will not emerge. This crystal is given to a golden yellow that is far more beautiful and pleasant than can be achieved in cristallo made with Levantine polverino salt and with this crystal unlike the other, every kind of job can be done. [2]

“Polverino” was a sodium based plant product used in many of Neri’s glass recipes, which he says was derived from the Kali plant grown in the Levant. The plot thickens when, for yellow, he recommends substituting ‘rocchetta’ another soda based Kali derivative. 

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

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

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

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

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