Scraping the Barrel

4th century BCE philosopher Diogenes
(supposedly lived in a wine barrel)
by Gaetano Gandolfi (1792)

To seventeenth century glassmaker Antonio Neri, "tartar" was a well-known byproduct of the winemaking process. If we chill wine or grape juice to below 50 degrees (10 deg. C.) crystals of tartar start to form and once they do, then tend not to dissolve back, even at room temperatures. Today, these crystals are commonly found in a powdered form, in kitchen cupboards as "cream of tartar." Bakers and cooks use it to stabilize whipped egg whites, and it has a number of other applications. 

Neri used it in his glass to add sparkle, a trick known to Venetian glassmakers as early as the 1400s. It was obtained from the dregs at the bottom of wine barrels. To understand how this works, it is useful to know that Florentines, Venetians and most southern Europeans made glass from crushed up quartz pebbles or sand mixed with a specific flux known as "glass salt." This salt was rich in sodium carbonates, which greatly reduced the melting point of the quartz. It allowed artisans to work the material at the temperatures easily achieved in their furnaces. Tartar turns out to be very similar, except that it is rich in potassium rather than sodium. 

Potassium atoms are bigger and heavier than sodium atoms and when light passes through a piece of potassium-fluxed glass, it bends and refracts more. This effect is not as pronounced as when adding the even heavier lead to form fine crystal, but it still adds noticeable sparkle to finished pieces. Using all tartar as a flux has the undesirable effect of reducing the workability of the hot glass. Outside the furnace, it becomes stiff quicker and artists have less time to create fancy shapes and forms. The solution to this dilemma is to use a mix of sodium and potassium fluxes together, which is exactly what Neri did. 

In his 1612 book L'Arte Vetraria, he shows how to prepare tartar and then adds it to a number of his glass recipes saying, "The tartar is the secret way to produce more salt and to make cristallo which is whiter and of rare beauty." Here is Neri’s prescription:

To make Purified Tartar Salt you should obtain tartar, which is also called gruma, from barrels of red wine in which it forms large lumps, however do not use powder. Roast it in earthenware pots amongst hot coals 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.  

Calcine tartar this way: put it in large earthenware pans full of hot common water, or better yet in glazed earthenware pans then made to boil on a slow fire. You should do it in such a way that in two hours the level of the water will slowly decrease to one-quarter, at this point lift it from the fire and leave it to cool and to clarify. Now decant off the liquid, which will be strong lye and refill the pans containing the remains of the tartar with new common water. In the way stated above, boil as before and repeat the procedure until saltiness no longer charges the water. 

At this point, the [decanted] water is impregnated with all the salt. Filter the lye clear and put it in glass chamber pots to evaporate in the ash of the stove over a slow fire. In the bottom, white salt will remain. Dissolve this salt in new hot common water and leave it in the pans, letting it settle for two days. Then filter it and return it to chamber pots to evaporate over a slow fire. In the bottom, a much whiter salt will be left than the previous time. Now dissolve this salt in fresh hot common water and leave it to settle for two days. Evaporate, filter and repeat everything as before. Overall, repeat this procedure four times to dissolve, filter and evaporate the salt of tartar. This will make the salt whiter than snow and purified from the vast majority of its sediment.

When mixed with sifted polverino, or rocchetta, with its doses of tarso [quartz] or sand, this salt will make a frit that in crucibles will produce the most beautiful crystallino and common glass, which one cannot make without the accompaniment of tartar salt. Without it [tartar], good fine crystallino can be made, nevertheless with it, it will be the absolute most beautiful.

The Importance of Being Diligent

Fennec fox pup, 2008
Everland Zoo, South Korea. 

In his 1612 landmark book about making glass from raw materials, there is a specific term that Antonio Neri uses repeatedly: "diligence." I count forty-two distinct instances spread throughout the book. Even so, in all of these occurrences, not once does our priest-alchemist uses this term in a casual manner; each appearance is in a critical step, in which he urges his readers to pay extremely close attention to what they are doing. 

If you want to have fine crystal, then in this you must exercise great diligence; when the frit is made with this careful attention, it will be white and pure like snow from heaven.

And

Then you must stir the glass with a paddle, but when the tinsel is calcined well and as directed, it swells so much that it could make all the glass go out of a large crucible. So use diligence in this.

And

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.

One might well ask, why so much attention to diligence? To be sure, there are many important aspects to the successful outcome of a batch of glass. Yet Neri singles out the seemingly simple act of paying attention. One reason is to avoid disasters. The last two quotes above imply that he has seen his share of these.

There is no question that glassmaking is a technically demanding process, where many things can go wrong. But there is more to it than that; Neri’s admonitions may have more to do with developing the proper attitude in a glassmaker. At first, it might seem that a recipe is a recipe is a recipe and as long as one follows it, mental state has no bearing on the situation. Yet as surprising as some may find it, this is definitely not the case. Materials and conditions vary in ways that cannot always be measured. What a recipe specifies is strictly limited by our perceptions and to quote the bard, "There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy."

Unlike some more intellectual pursuits, glassmaking falls into a category of activity in which there is no room for self-indulgence. It does not work to throw the ingredients into a furnace and walk away, assuming that they will react the way we anticipate. They will react, rather, according to the physical laws of nature. Diligence could make the difference between a minor correction and a disaster. For the headstrong, this can be a rude awakening. A would-be glassmaker is forced into a confrontation with stark, unforgiving reality. What one wants or expects has no bearing on the situation, only what one does, and when.

Neri makes clear that this philosophy of diligence is not only required by the novice, but also by the seasoned professional conciatore.

This entire exercise depends on the practice of being a worthy and diligent glass conciatore, because neither sure weight nor measure can be given.

And

The diligent furnace conciatore will meticulously remove any lead that has returned to its metallic state from inside the pot.

And

When working this glass, use the same diligence that the skilled masters use and in so doing you will make material that is perfectly true to the jasper, agate and chalcedony of the orient.

In the early seventeenth century, to be a glassmaker meant becoming a careful observer, ever mindful that Nature does not care a whit what you think. 

Don Antonio de’ Medici

Don Antonio de' Medici
Frontispiece from Pierfilippo Covoni 1892

In 1612, Priest Antonio Neri published his book of glassmaking recipes. L’Arte Vetraria went on to become a primary reference for glass artisans throughout Europe. He dedicated his book to Prince Don Antonio de' Medici the son of Grand Duke Francesco I:

In all consideration, it is my proud duty to dedicate this book to none other than you, most Illustrious Excellency; for you have always been my outstanding patron. You are a gifted leader in this and in all other noble and worthy developments made continually in all the arts.

As an eleven year-old, Don Antonio was slated to succeed his father as the next Grand Duke of Tuscany, but that situation changed quickly. In the autumn of 1587 the young prince lost both of his parents in the space of a few days. They both fell extremely ill and died within a short time of each other. Rumors flew that they had been poisoned, but forensic investigators have found pernicious malaria pathogens in Francesco’s remains, a disease with symptoms consistent with the reports of physicians on the scene. Historians trace their infection to an outing in the damp forest a few days earlier, where they had probably been bitten by mosquitoes carrying the disease. 

The boy’s uncle, Cardinal Ferdinando took charge, consolidated power and excluded Don Antonio from the royal succession, although he was given a prominent place at court as a diplomat. As part of a deal that he would never marry, he was allowed to keep the title of Prince of Capestrano, to which was added Grand Prior of Pisa, in the Knights of Malta. The deal also gave him possession of the laboratory facility that his father had built and several other properties. That laboratory, the Casino di San Marco would become the prince’s residence and the place where Antonio Neri would learn about glass formulation.

Poor health attended Don Antonio from his first months through the end of his life. Doctors and medical examinations were to become a regular part of his routine; they may well have inspired his later pursuit of medicinal cures, as well as his foray into alchemy, which also involved Antonio Neri. At some point, probably as a teenager, Don Antonio contracted syphilis, a condition that may well have been treated by Antonio Neri’s father who was physician to the royal family.

The prince had played a major part in Neri's life, elevating him into the upper stratum of Florentine craftsmen and to the forefront of alchemical research in Europe. However, in another manuscript, Discorso, we see a different side of Neri. On the subject of turning base metals into gold, the priest was less forthcoming:

I would add that God's providence over human affairs must not easily allow many to acquire this art, particularly not the great princes. It should not be made clear and common to the vulgar, because in this way, gold and silver and consequently coins lose their value, so that the good order of human trade will be disrupted and we should go back to the ancient barter of things that are necessary to a civil life, creating great disruption and confusion.

Although never allowed to marry, over his lifetime Don Antonio managed to have a number of children; his last three sons were ultimately legitimized by the pope as Medici heirs. In the end, it was the slow, progressive ravages of syphilis that brought him down. He died in 1621, at the age of forty-five, unable to leave his bed. He was given a proper funeral, and interred at the Medici chapel of princes in Florence.

Washing Molten Glass

Washing, sorting and carrying cullet
Denis Diderot 1772

One of the continuing frustrations with the study of glassmaker Antonio Neri, is that there is no known example of his glass to be found anywhere. It is very possible that pieces do survive, but so far, none has been tied to him or his recipes. At first it might seem to be a straightforward task of analyzing the composition of likely candidates and comparing the results to his formulas. Unfortunately, this plan does not hold water. Even if a recipe for glass was followed exactly, the result will have a different composition from the starting materials. One reason is that before the hot glass was crafted by artisans, a new batch was typically "washed" by flinging ladlefuls of molten glass into great vats of cold, clean water. In his 1612 book L'Arte Vetraria Neri wrote:

After a while, when the glass is well fused, take it out of the crucibles and throw it into large earthenware pans or clean sturdy wooden tubs filled with fresh water. This step of throwing the glass into water has the effect of causing the water to remove a kind of salt called Alkali salt [glass gall], which ruins the cristallo and makes it dark and cloudy. So while it is still being worked let the glass spit out this salt, a substance quite foul, then return it to clean crucibles. Carry out this flinging into water repeatedly as necessary. In order to separate the cristallo from all its [alkali] salt, this should be repeated to the satisfaction of the furnace conciatore [glassmaker].

This step, he assures us, is absolutely necessary for the finest glass, but also helps improve the most common glass:

If you throw it into water at least one time, what you will have will be beautiful and clear. The same is true for common glass, which once brought to perfection you should return to the crucibles for use. It will be bright, fine and quite satisfactory to work in those jobs that require it. […] when a more than ordinary fine glass is desired it is necessary. Beyond becoming very white[clear], it calcines and clarifies nicely with few impurities.

This technique becomes even more critical for Neri's lead crystal, in fact, any glassmaker who ignored this step for a leaded glass did so at risk of a major disaster.

In a few hours everything will have clarified, now purify it by throwing it in water. Inspect the glass carefully before returning it to the crucible. 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.

In addition to washing the glass, sometimes the top layer of a melt was skimmed off and discarded because it contained contaminants that floated to the surface. To complicate matters further, molten glass can stratify in the crucible, meaning the composition might vary from top to bottom and from the center of the pot to the edges. 

Scientists and historians have collaborated to see what can be learned from period samples of glass. When attention is focused on the composition of a single type of glass, like Venetian style cristallo for example, one might expect a wide variation. The opposite turns out to be true. Even with all of these factors conspiring to change the glass composition, remarkably the analysis shows it is quite difficult to tell apart glass that was known to be made in Florence from that of Antwerp or Venice. Recent efforts have centered on identifying miniscule amounts of trace materials in the old glass that were unique to the raw ingredients of a specific region. Meanwhile, Antonio Neri's glass continues to elude us, even though it might be sitting on the shelves of museums around the world, right in front of our eyes.

The Neri Chapel

The Vision of St Bernard, by Pietro Perugino (1448–1523)

The c. 1598 altarpiece that was commissioned 

for Cestello by Antonio Neri’s father

On a narrow unassuming street called Borgo Pinti on the northeast side of Florence, there are two structures in particular which are of great interest in the study of seventeenth century glassmaker Antonio Neri. The first is the family’s residence at number 27, now a hotel, but largely intact and the second is the church located a few steps further north, now known as Santa Maria Maddalena dei Pazzi. 

In the sixteenth and early seventeenth century, the church was part of a Cistercian monastery known locally as “Cestello” It was attended by Antonio’s royal physician father Neri Neri, his grandfather, Jacopo and it is safe to assume, the rest of the family. This is where the medical doctor would later be buried. In 1598, he was laid to rest in the chapel that Poccetti and Passignano reworked to his specifications; both were well-respected fine artists of the time. One author would later refer to the space as “the famous Neri chapel.” The royal physician asked, in his will, that his remains never be moved outside of his beloved homeland. He provided money for what amounts to a complete restoration of the entire property. The work in the chapel and the main church was a top to bottom renovation and must have taken several years to complete; today it is considered Poccetti’s crowning achievement. In earlier times, the monks of Cestello had played a trusted role in pre-Medicean Florence, collecting and counting the beans used in votes of the Signoria (town council), and holding the keys to the Palazzo Vecchio.

The chapel and the church still stand, but the occupants changed in 1628-29. Pope Urban VIII was seeking better accommodations for two of his nieces who were Carmelite nuns on the other side of the Arno River at San Frediano Monastery. He asked that the Cistercian brothers, against their protestations, exchange properties with the nuns, who brought with them the remains of Sister Maria Maddalena de’ Pazzi, the influential mystic who died in 1607. The church was renamed in her honor after canonization in 1669. 

The Cistercian’s archives record that:

In 1598 Doctor Neri remodeled in more honorable form the chapel which was originally built by Tommaso del Giglio, in the year 1505, which had since become occupied by the cobblers guild, who had begun to meet there and which he feared they wanted to usurp. Maestro Neri approached the Grand Duke [Ferdinando I] in order to have authority to free it from the hands of the cobblers and enlarge and redo it from the foundation and have it all painted in frescos by the hand of Bernardino Poccetti and dedicated to the honor of Saints Nereus and Achilleus.

Far more than the mere renovation of the chapel, the royal physician played a central role in the rehabilitation of an ancient and venerated church, a church that today continues as a testament to Poccetti’s great talent. Tommaso del Giglio, the former owner of Neri’s chapel and successful apothecary supplied Cestello as well as many Florentine hospitals with medicines and other staples. That the cobblers had occupied the chapel might seem strange. What possible claim could the band of shoe-makers have to this space? The simple answer is that Del Giglio was himself a member of their guild, perhaps the cobbler’s most famous member. About 1462 he moved his family to Florence from Montevarchi, where he was trained as an apothecary. For whatever reason he could not gain membership to the guild in Florence, so instead he joined the subordinate cobblers’ guild but retained his practice as an apothecary. He was still subject to the  rules, regulations and periodic inspections of the authorities. Eventually he attained the honor of apothecary to the ruling Medici family. A century after his death, it seems the chapel and the church itself was falling into disrepair; the cobblers, with the family’s blessing met there on a regular basis.

The Cestello monastery, of which the church was a part, served as an early home to the famed Accademia del Disegno (Florentine Academy of Design). The city’s greatest artists, sculptors and architects were all members, Giambologna not least among them. The rooms of the monastery were used to store artistic works and teach classes. For the members it served as a place to meet and plan projects for the city and royal family. A young Galileo Galilei would study mathematics here under Ostilio Ricci. 

It is unknown if the Neri chapel serves as the last resting place for the children of royal physician Neri Neri, among them his most famous son, glassmaker Antonio Neri, who died in 1614 at the age of thirty-eight.

Eyes of a Lynx Reprise

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 famous 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. Here is what I wrote about it last year on 1 August:

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. Within a few months of Neri's death, his book was already on its way to making history.

Book of Secrets

Title page, L'Arte Vetraria,
by Antonio Neri (1576-1614)

Today, we recognize L'Arte Vetraria as the first printed book devoted to the art of glass formulation. It is a work committed to the subject of refining raw materials and combining them into a variety of glasses, over a rainbow of colors. 'First in print' is a notable distinction, but one that Neri surpassed with ease through a deeper accomplishment. His book provides a rare glimpse of skilled practical knowledge. This was an era when prized techniques were often lost to subsequent generations because artisans so often spared the pen. Their precious knowledge was kept far away from paper, passing in strict confidence from master to apprentice working side by side. Trade secrets were guarded possessions. Recipes committed to writing were an invitation for prying eyes—in the case of virtuoso glass, an invitation to be compromised by individual competitors and by rival states. This is what makes Neri’s book of glassmaking recipes extraordinary. In an age of secrecy, L'Arte Vetraria was a deliberate exposition, a lucid guide to the art by a seasoned professional. He intended it for the enlightenment of those with no other qualification than a "kind and curious spirit." He assures us that "given a bit of experience and practice, as long as you do not foul-up on purpose, it will be impossible to fail." 

Books of secrets have been a part of our culture since history began. More than the mysticism they imply, they chronicle the development of various technologies and show us in a literal sense how we got to where we are today. They play a unique role in preserving types of knowledge taught by doing. For one practiced in a particular art, a book of secrets was a roadmap, sequencing new steps on a landscape that was already familiar. Neri's L'Arte Vetraria is a prime example of this genre; it documents the chemistry of glassmaking in the late Renaissance and it highlights the link between alchemy and the arts. But Neri takes on an increased challenge by inviting the novice to partake in his craft. He created a document that spoke to the uninitiated of his own time and still speaks to us today. Neri and the other masters of his day may be long gone, yet his painstaking, jargon-free instructions tell us with exact language what those master glassmakers were doing. 

Be warned in particular to give careful consideration to the colors for which exact and determined amounts cannot be given. Indeed, with experience and due practice learn and with the eye and judgment know when a glass is colored sufficiently and appropriately for the work at hand. 

These books have unique characteristics that distinguish them from other forms of writing. The author must have one foot in the learned world of letters and the other foot in the intuitive world of craft. Working at the Casino di San Marco and elsewhere, Neri and his fellow artisans think in images, smells and textures. They have a first-hand detailed experience that does not lend itself to written words with ease. The essence of an artisan's work defies accurate capture between pages, yet Neri verbalizes these procedures in a clear narrative. When he cannot give exact amounts, he explains with care the decision making process that must take place.

"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." From the contents of his glass recipes, we can see his bottom-up thinking. He starts with details gained from hands-on experience and forms his conclusions based on that knowledge. In sharp contrast to other authors of the period, Neri presents no grand theories of nature. Nor does he struggle to fit glass into a larger abstraction. Many contemporaries debated at length over the true essence of glass—whether it was more like metal or mineral and therefore possessed one set of essences over another. Perhaps more significant, Neri does not impose extranatural explanations on the results of his chemistry; instead he follows nature. The top-down approach is not his way, he interacts with his materials, he observes how they behave and he encourages the desired result by optimizing favorable conditions. 

On a practical level, alchemists interacted with nature to transform materials in ways other arts could not. Sculptors could use physical tools to transform a block of marble into a statue, but the substance of the marble was unchanged. Ironsmiths could forge a suit of armor and weavers could transform fibers into cloth. However, it was alchemists who made the fabric dyes permanent. It was alchemists who could dissolve a block of marble with reagents before one's eyes. To common men and women, the alchemist's world of transmutation must have seemed magical. Books like Neri’s gave a glimpse into the real-world mechanics of these mysterious arts.