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, August 31, 2015

Glass Salt

Diderot & d'Alembert, L'Encyclopédie (1772)Raking Out Roasted Frit
Making glass from raw materials involves several steps. In his 1612 book on glassmaking, L'Arte Vetraria, Antonio Neri breaks the process down into parts so that, "given a bit of experience and practice, as long as you do not purposely foul-up, it will be impossible to fail." Pure white sand, or preferably quartz river stones which Neri calls "tarso" is broken up and pulverized into a fine powder. The initial work can be done by heating the stones in a furnace, then dropping them into a vat of clean cold water, where they will fracture due to the thermal shock. The process was often repeated multiple times. From there, the pieces are pulverized in a stone mortar and pestle. Stone, because metal tools would contaminate the quartz, and in the end tint the glass. Finally, a powder is obtained by grinding with a stone tool on a flat granite "porphyry stone." This powdered quartz is the main ingredient of glass.

The second critical ingredient is the flux, what Neri calls "glass salt" or "soda." This can be obtained from mineral sources, but European glassmakers in the seventeenth century extracted all their salt from certain plants. The powdered quartz was mixed with the salt and a third ingredient, which is critical, lime. Lime is simply calcium oxide used by builders to make cement. It is nothing more than pulverized seashells roasted to a high temperature. Neri advises using two pounds of lime for every hundred pounds of salt. He specifies that it should be added to all his frit recipes, but it is not clear that he understood its critical importance; without lime, the glass would be subject to attack by mere water, eventually decomposing. This mixture of soda, lime and silica when heated in a kiln would chemically react forming "frit." The combined materials were raked around in a kiln for a long period (many hours) and finally formed nut sized pieces. It was cooled and heaped into piles in dry cellars where it was aged for a time. This is where some real "magic" in glassmaking takes place. The glass salt or soda dramatically lowers the melting temperature of the quartz, all the way down to a point that was easily achieved in a wood fired furnace. When a batch of glass was made, the aged frit was then melted in furnace crucibles and skimmed to remove excess salt, which floated on the surface; it could foul the glass, and smelled terrible. The melted glass, now ready to work, was sometimes colored and finally made into objects by gaffers. 

Neri obtained his glass salt from products shipped by traders from the Levant (eastern Mediterranean). It was supplied as the dried, partially charred remains of special plants that grow in arid seaside conditions. Shipping them this way cut down on weight and volume, and prevented rotting. These Soda and Kali plants contain large amounts of sodium carbonates. This is a white powder, chemically identical to what we know as "washing soda." He advises, 
In buying either of these make sure it is richly salted. This may be determined by touching it with the tongue in order to taste its saltiness; but the surest way of all is to do a test in a crucible and to see if it contains much sand or stones, a thing common in this art and very well known by glass conciatori.
He crushes any large pieces of the product in a stone mortar, and sifts the result through a fine screen, ensuring that most of what remains is salt.  
As the common proverb of the art of glassmaking says: a fine sieve and dry wood bring honor to the furnace. Then with any of these sodas, 100 pounds of soda ordinarily requires 85 to 90 pounds of tarso.
Neri sets up large cauldrons of clean water over brickwork stoves, adds the plant product and boils the water. He strains the insoluble parts out and reduces the liquid by evaporation until crystals of the salt start to form on the surface. He skims these off and continues the process. Finally he carefully dries the product. Our glassmaker describes several variations of this process, including one in which he takes extreme measures to ensure the purity of the salt and clarity of the finished glass. In all, this is a task that could easily take several weeks to perform for the amount of frit to fill a single pot for the gaffer to work.

Not content with the established materials, our glassmaker experimented extensively with other plants: 
[U]se the husks and stalks of fava beans after the farmhands have thrashed and shelled them. With the rules and diligence prescribed for the Levantine polverino salt, extract the salt from this ash, which will be marvelous, and from which a frit can be made using well-sifted white tarso, as is described throughout this work. A very noble frit will result, which in the crucible will make a crystal of all beauty. The same may be made from the ashes of cabbages, or a thorn bush that bears small fruit, called the blackberry, even from millet, rush, marsh reeds, and from many other plants that will relinquish their salt. *
*These other plants produce potassium carbonate salts with similar properties to sodium carbonate.
** This post first appeared here 9 December 2013.

Friday, August 28, 2015

Sulfur

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

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

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

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

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


[1] Neri 1612, p.114.
[2] See this post for a more detailed description  http://www.conciatore.org/2014/01/turning-iron-into-copper.html
[3] Grazzini 2102.
[4] The effect had previously been described Georgius Agricola, in book 5 [9] of Nature Fossilium. See edition, transl. from the first Latin edition of 1546 by Mark Chance Bandy, Jean A. Bandy (New York: Mineralogical Society of America, 1955), p. 188.
*This post first appeared here on 7 November 2014.

Wednesday, August 26, 2015

Lime

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

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

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

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

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

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

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

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

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

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

Monday, August 24, 2015

Saltpeter

Saltpeter beds, 1683 English ed.
Beschreibung allerfurnemisten mineralischen Ertzt 
by Lazarus Ercker. (
Prague, 1574).
In the era of glassmaker and alchemist Antonio Neri (the early seventeenth century), it was commonly assumed there was a direct causal connection between the "macrocosm" (stars, planets etc.) and the "microcosm" (the very small), with human life on earth standing in the balance, in-between the two. This philosophy forms the basis for astrology, which in Neri's time was utilized for all sorts of decisions and the planning of events. Even so, there was much debate about what the exact connections were and how they worked. This cosmic theory of the very large being directly connected to the very small is an ancient idea that precedes even the discovery of saltpeter, today's topic of discussion. Saltpeter was the critical ingredient of gunpowder, fireworks, fertilizers, medicines and as we will see, even the preparation of materials for glassmaking. It undoubtedly held an important place in Antonio Neri's alchemical cabinet. 

From our vantage point on Earth, Sirius is the brightest star in the heavens. As such, it has a rich history in almost every ancient culture. The Egyptians used it to base their calendar; when the star made its last appearance for the year, rising just ahead of the sun, it foreshadowed the coming season of Nile flooding (and abundant crops). To the ancient Greeks, this event signaled a hot, dry summer ahead. In the constellation Canis Major, the main star is Sirius, which  is also commonly known as the "dog star," hence the expression, "dog days of summer." [1]


Next Month in September, Sirius makes its reappearance in the evening sky. Just after dusk, you can find it following the sun toward the horizon to the west. Since it is the brightest star in the night sky, it tends to be visible before other stars. In Renaissance Italy, this evening event heralded the end of summer’s heat and the start of cool autumn rains. Which, among other things, meant it was time to harvest the saltpeter, before it washed away. 


Chemically, saltpeter is potassium nitrate, [2] although several similar compounds have traditionally been referred to by the same name. The term derives not from the name Peter, but from the Latin word for rock "petra"; it often forms on the walls of caves as mineral deposits, hence "salt from rocks." 


In China, where saltpeter was first put to use in gunpowder, it was widely collected from caves. Eventually it was discovered to be more prevalent where bat droppings were to be found. As a result, a new  "organic" open air method of production soon gained favor.


You may have noticed this; when animal dung sits in the barnyard for a while and there has not been any rain, a powdery white deposit starts to form on what is exposed to the air. That white powder is saltpeter. In France, saltpeter production turned into a cottage industry, with large beds being maintained, covered from the rain. Beds were prepared several ways, but the most straightforward was to mix dung with potassium rich wood ash and straw and let it sit for up to a year, then flush out the saltpeter with water. The liquid was carefully collected and further purified.


As far as I know, Antonio Neri does not speculate in any of his writing about the natural process by which saltpeter forms, but one of his contemporaries does. Polish born alchemist and medical doctor Michele Sendivogius made a name for himself at the court of Holy Roman Emperor Rudolf II, in Prague. Sendivogius was convinced that saltpeter was a product of the air around us. He thought it was the congealed spirits of air and considered it "the food of life." He supported his argument with the fact that saltpeter makes a wonderful fertilizer for plants. He outlines his theories in A New Chemical Light which published in Frankfort in 1604, the same year Neri would have attended the spring fair there, assuming he followed the travel advice of his friend Emmanuel Ximenes. It was the year Neri embarked on what would become a seven year visit to Antwerp. [3]


While he was visiting in Antwerp, Neri makes particular use of saltpeter in his recipes for his colorful chalcedony glass "adorned with so many graceful and beautiful areas of undulations and enhanced with the play of diverse, lively, flaming colors." [4] He presented "His Excellency, the Prince of Orange, with two vessels of this chalcedony, which delighted him greatly." [5] In the recipe for this exotic glass, he makes extensive use of aqua fortis, acids for which saltpeter was an essential ingredient. These were the strongest acids known and the only way to dissolve precious metals like silver and gold. Neri uses a few gallons of these acids in the preparation of the ingredients for a single batch of his chalcedony glass. These acids were themselves thought to be composed of common water that had been infused with "powerful spirits." If Neri shared Sendivogius' reasoning, he might have thought that the process of making the acid was one of transferring the spirits of the air into water.


[1] Due to precession of the equinoxes, the date when Sirius rises and sets with the sun has shifted later by  about 25 days. In the Egyptian calendar of 3000 BCE the start of "dog days" coincided with the summer solstice.
[2] It is also called niter, with the chemical formula KNO3, composed of potassium, nitrogen and oxygen. 
[3] Again, there is no direct evidence of Neri's route, or that he was familiar with Sendivogius or the book. In Discorso (Neri 1613), he does mention alchemist Alessandro Scotus, who was released from prison by Sendivogius in 1590, after a conviction of practicing alchemy in public. See Grazzini 2012 p.346, n.55. 
[4] Neri 1612, p. 34.
[5] Ibid, p. 48.
* This post first appeared here on 19 September 2014.

Friday, August 21, 2015

Sulfur of Saturn

The Roman Goddess Ops, 'sweet'-heart (and wife)
of Saturn, Peter Paul Rubens c. 1630,
“Abundance (Abundantia).” [1]
Antonio Neri’s 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.

Wednesday, August 19, 2015

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. 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. 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, August 17, 2015

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.

Friday, August 14, 2015

Sal Ammoniac

Alchemical symbols used to denote sal ammoniac.
Today we will examine "sal ammoniac," a common alchemical ingredient used by Antonio Neri in many of his preparations. In its pure form, it is a colorless crystalline material and is known to chemists as ammonium chloride. It does occur as a (rare) natural mineral, but it was also manufactured as early as the thirteenth century, as noted by alchemist Albertus Magnus in his De alchymia.[1] Neither he nor Neri provides a recipe for sal ammoniac, but other sources indicate that it was made by allowing urine to putrefy with common salt. French investigators documented another method used in Egypt in the eighteenth century. This scheme involved burning the dung of animals who fed on spring grasses and then sublimating the ammoniac out of the resulting soot. Sublimation occurs when a heated material goes directly from a solid to a gaseous state without ever becoming liquid. Sal ammoniac has this property; when heated it turns to a gas and upon cooling, turns back to a solid.

The usefulness of sal ammoniac in alchemy stems from the fact that when dissolved in water, which it does easily, it immediately dissociates into equal parts of ammonia and hydrochloric acid, which in turn will dissolve some metals, including tin, zinc, iron and (reluctantly) lead. Its most famous use was as an additive to the stronger acid aqua fortis (nitric acid). Together the two formed aqua regis which was strong enough to dissolve gold. At the time that Neri was working, the only known way to dissolve the most 'noble' of metals (gold) was with the 'king' of acids (aqua regis). Neri puts this knowledge to use in his recipe for ruby-red colored glass made with pure gold. His description is light on details, but he does clearly direct the reader to dissolve the precious metal in aqua regis, then gently evaporate away the acid to obtain the red pigment.

Elsewhere in Neri's glassmaking book, L'Arte Vetraria,[2] he uses sal ammoniac in the production of "alemagna blue" paint and in the tinting of natural rock crystal. 

Another of Neri's creations requiring sal ammoniac was Chalcedony glass. It had swirls of every color the glassmaker could produce. He achieved this feat by making extensive use of aqua regis to dissolve each of a long list of metals. He then gently evaporated off the acid, leaving ultrafine powdered metals, which he added as pigments to the glass melt. 
With this powder, I made a chalcedony in a glass furnace in Antwerp that was then run by a most courteous gentleman; Mr. Filippo Gridolfi. This chalcedony gave rise to work so nice and graceful, that it emulated true oriental agate, and in beauty and delightful colors by far exceeded it.
Today, chemical factories produce vast quantities of the materials used by Neri in his glassmaking exploits and in far higher purities. Having unlimited quantities of every conceivable chemical compound at our fingertips makes it difficult to appreciate the physical labor involved by seventeenth century alchemists, both in the preparation of the glass and in the production of the individual ingredients. The chalcedony glass recipe cited above must have taken workers many, many hours to produce and must have cost a small fortune. 

[1] Magnus 1958.
[2] Neri 1612.
[3] Glauber and others used the term 'sal ammoniac' to describe a related chemical (NH4)2SO4. When mixed with aqua fortis this forms a nitric-sulfuric acid solution, which does not form aqua regis, and does not dissolve gold.
* This post first appeared here on 22 August 2014.

Wednesday, August 12, 2015

Crocus Martis

The many 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 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 red planet. 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'. 

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; 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 and taking the time to understand them 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 fields 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 within the period of about fifteen years. As a result, we can start to decode some of his more arcane passages, which we will save for another time. 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." Yet, in 1870, in the journal Nature, chemist George Rodwell commented on one of Neri's more obscure works, 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. 

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 nothing more than 'to roast' in a hot oven. It derives from the ancient practice of roasting 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 iron oxide and sulfide, but alas, chemistry is not that simple; there will be minor concentrations of other compounds depending on the acid used. 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. In a number of green glass recipes he 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.

Monday, August 10, 2015

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. 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, August 7, 2015

Filippo Sassetti

Goa, India 1509
In the Florentine baptism records, the entry for Antonio Neri 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, 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, August 5, 2015

Decolorization of Glass

The green tint from iron contamination
is neutralized by magenta from manganese.
For Antonio Neri and his contemporary glassmakers, contaminants, especially metallic contaminants were the bane of producing a crystal clear product. Great care was taken to ensure the purity of each ingredient at each step of the glass making process. The greatest threat of all was iron. Even small amounts will impart a green tint and for Renaissance era glassmakers, nemesis iron was everywhere. It is a common impurity in quartz, the main ingredient of glass, showing up as yellow "rust stains" both in sand and stones. It turned up in the plant salts used for glass flux as a trace-element and finally it was in the tools. Iron was in the mortars and pestles, in the pots and kettles, in the frit rakes, in the ladles, the stirring rods and in the blowpipes. A mistake at any step could easily tint the batch, even at the final stages. Neri admonishes glass workers:
Make sure never to return the neck, where the rod attaches to the glass, into the crucible of cristallo, because there are always remains of the iron that will cause it to become dark ... 
Centuries before Neri, it was discovered that the antidote to iron contaminated glass was manganese or more specifically manganese oxide, a mineral mined throughout Italy. However, Neri cautions: 
… you must always use manganese of Piedmont the way it is made for Murano, because the manganese of Tuscany and Liguria has more rust, which always make the melt dark. 
This color correction technique was known as early as the late 14th century and by Neri’s time it was a widely practiced technique in the glassmaker’s toolbox. Manganese oxide was known as a pigment in antiquity; the Magnesia region in Thessaly, central Greece was an ancient source for the black mineral. Around the year 1450, a clear crystal-like glass suitable for working into thin elaborate forms was developed in Venice. The glass was called cristallo and Murano glassmaker Angelo Barovier is often given credit. He may well be responsible for bringing together several existing techniques—manganese color correction among them — but these techniques individually were all available and utilized by glassmakers at least fifty years earlier. Barovier’s innovation would become a tradition giving Murano the reputation for the finest glass made anywhere, carried on by countless glassmakers and ultimately by our Florentine priest more than a century and a half later.

Removing the green tint of iron contamination with manganese is a clever trick. The manganese imparts a magenta tint to glass. As the complementary color to green, it effectively "cancels out" the green tint. The trade-off is that the glass is slightly darkened, even if neutral in color. When light passes through glass tinted by iron, green light is unaffected, which is why it appears green. The other colors that make up white light, notably red and violet are dampened. In effect, the green is enhanced. Now, adding manganese to the glass dampens only green light and brings the spectrum back into balance. The overall effect is that all the colors of light are slightly dampened, but by the same amount. In Neri's case, minor contamination from iron would produce only a small green tinge and the problem was corrected with a small dose of manganese. The resulting grey would hardly be noticed, especially in the thin, delicate pieces so popular at the time. To the eye of all but the most experienced expert, this decolorized glass had perfect crystal clarity.

Decolorization of glass is an example of a technique that was developed through experimentation without a deep understanding of the physics involved. Theories of light and color and the way they are perceived by the human eye came much later. In fact, manganese itself was not isolated as a metal until 1774, yet it was used skillfully by glass artisans three or four hundred years earlier. 

This post first appeared here in a slightly shorter form as 'Manganese of Piedmont' on 14 October 2013.

Monday, August 3, 2015

Reports from Parnassus

Rafael - El Parnaso (Vatican, Rome, 1511)
Apollo on Parnassus, (fresco detail). 
In the spring of 1612, Antonio Neri finished writing L’Arte Vetraria, and the Holy Office of the Inquisition approved it for publication. The Glassmaking book passed the Church’s censors despite it containing a large number of alchemy related methods. Contrary to what we might imagine, they did not have any problems with Neri’s work; in fact, one bureaucrat commented that the book was full of useful information. Remember, this was at the same time that so-called sorcerers and witches were being tortured and executed around Europe. One big difference in this case was intent, or rather the perceived intent of Neri's writing. Another factor was his personal connections. The truth is that alchemy was something of a fad among the wealthy nobility, who used the equipment for everything from making rosewater, to distilling liquor, to quietly trying their hand at transmutation. 

To form a better sense of the public image of the chemical arts in the early seventeenth century, we can turn to the satirical critic Traiano Boccalini, who published a book of his own the same year as Neri. Ragguagli di Parnaso [Reports from Parnassus] was an immediate hit. In fact, it became so popular that he and his sharp tongue were forced to leave Rome for the comparative safety of Venice soon after its completion, and soon after that friends found him dead under suspicious circumstances. Initially, Boccalini had been on good terms with Church officials, but perhaps he had seen a bit too much of the inner workings of the ChurchEventually, he became bitterly disillusioned and wrote Ragguagli as a collection of fictitious news-sheets. These were patterned after the letters that circulated widely as the forerunners of modern newspapers. His reports took place in the mythical state of “Parnassus,” which struck an uncanny resemblance to contemporary Rome. Its monarch, Apollo, struck an uncanny resemblance to the pope, as the princes of Parnassus did to cardinals, bishops and curia officials. Report LXXXIX is illustrative of what were the more practical fears over alchemy, in this case, some creative wealth building among the clergy. It gives a sense of alchemy’s public persona. Apparently, in real life, there was a crackdown by the Church on chemical apparatus, under the guise of concern for public health. Our author slyly suggests, in the very last line, that it may have had more to do with putting an end to clergy lining their own pockets. 
Apollo [the pope] Prohibits the Princes from the Use of Distillers or Alembics At Home: 
In the past few months here, various ailments have emerged in this state of Parnassus, which have caused in some an extraordinary fatigue with frequent agitation: in many a tenacious fever, a faint pulse and a monstrous appetite: in others an intense stomach ache with an ardent thirst.  
The doctors cannot find a single remedy. However, the true cause of these maladies, by decree of Apollo, was revealed before a recent meeting of the grand Asclepius [Society] of prominent Greek, Latin and Arabic doctors, where it was the subject of long and erudite debate. Because neither the enemies of the grand princes nor other eminent gentlemen were spared, it was doubted whether the sickness was caused artificially by powerful poisons. Furthermore, it is clear these troubles were not only happening nearby.  
And we see several modern princes put great study in their most excellent facilities to prepare in their alembics things other than rose water. They conceal subjects dangerous and heinous with their hidden machinations of poison. This cannot be allowed to be covered up; such a scandal must be exposed with the violence of a dagger.  
His Majesty, concurring with the opinion of the congregation, yesterday morning made a public speech, to issue a strict edict, which forbids princes of any color, from ever keeping distillation or alembic operations at home or outside. However, he allowed similar exercises in the hands of experimenters and herbalists. A thing being extremely foul is the minting of counterfeit money in the night and then by day covering so treacherous a crime by running shops openly making medals for the crown. 

In this piece, Boccalini crystallizes the complex social stigma carried by alchemy. Of all the practical tasks that could be performed in this art, the most notoriety by far was derived from the concept of turning base metals into gold. In his writing Neri openly expressed his fear that true transmutation, in the hands of the masses was likely to collapse the economy. The Vatican’s concerns were more immediate: financial gluttony within the Church. Anyone with enough alchemical knowledge could produce convincing counterfeit money. Turning lead into gold was a theoretical issue; dealing with rogue counterfeiters was a real and immediate one. 

*This post first appeared here on 8 August 2014.