Laughing in the Fern

Alain Manesson Mallet  1719,

"Der Mont. – Lune"

In Chapter 5 of L'Arte Vetraria, Antonio Neri shows how to extract salt for glass from fern plants in an evocative recipe. Fern was and still is widely abundant in Tuscany. It presented a ready source material for glassmakers of the region. Neri directs that harvesting of the plants be done in the spring:

Cut this herb from the ground when it is green, between the end of the month of May and mid June. The moon should be waxing and close to its opposition with the sun, because at this point the plant is in its perfection and gives a lot of salt, more than it would at other times and of better nature, strength and whiteness.

At first, it is tempting to dismiss this lunar influence as the product of a fertile imagination, but let us take a closer look. Even today, grandmothers throughout Italy remember the advice to pick vegetables from the garden and bring them into the kitchen at a half-moon. Can nona be completely off-track? The fact is that tidal forces of the moon do subtly affect plants, fish and animals in ways that can be measured. A closer look at Neri’s advice reveals reasoning that is hard to dismiss as mere astrological superstition. When the moon is waxing, tides rise and so do water tables. According to folklore, this is when sap rises from the roots of plants into stems and leaves. Sap carries the dissolved mineral salts required for glass. Neri also stipulates that harvesting should take place during lunar opposition. When the moon is 'opposed' to the sun, it is on the opposite side of the earth from the sun. In opposition, the moon is near full and rises as the sun sets. Plants see more light at night, leading to increased photosynthesis and growth.

In contrast, violin makers from Cremona valued high alpine spruce called moon wood which contain a minimum of sap. Trees were felled in the wintertime, when lunar tides were low. This minimized the amount of vibration deadening sap in the wood. In his Natural History, Pliny relates Cato’s advice on felling trees in accordance with the lunar cycle. In fact, centuries-old tradition specified lunar conditions for a host of needs from construction timbers to cheese boxes. In this case the advice relates to picking ferns for use in glass.

Fern ash is high in potassium carbonates. If carefully purified it can make an exceedingly clear glass, rivaling or even surpassing Venetian cristallo. It has the additional advantage of being physically tough, making it ideal for engraving or diamond-point work. On the other hand, once out of the furnace it stiffens quickly, giving it a short 'working life' for the hot glass artisan. This limits designs to simple basic forms. While soda-based glass was the norm for the Mediterranean region, throughout the Middle Ages and into the Renaissance, northern Europeans were more likely to be making potash-based glass. They utilized the potassium rich local trees and plants of the northern forests. In France, fern glass is called verre de fougère. In the considered opinion of some connoisseurs, wine tasted better when sipped from verre de fougère cups, hence the delightful expression 'le vin rit dans la fougère' [wine laughs (sparkles) in the fern].

Since the middle ages, fern glass became part of everyday life in northern Europe. It was familiar enough to find its way into literary verse on matters of the heart. There is a nice reference to fern glass by Geoffrey Chaucer, in The Squires Tale:

But notwithstanding, some said that it was

Wondrous to make fern-ashes into glass,

Since glass is nothing like the ash of fern;

But since long since of this thing men did learn,

Chaucer, in turn, borrowed this reference from an epic twenty-two thousand line French poem from the late thirteenth century, when the technique of making glass from ferns was already ancient.

* This post is a mashup of material that first appeared here on 5-7 August 2013.

Alchemy School

Frontispiece woodcut from

 De Chemia Senioris, by Zadith ben Hamuel, 1566

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

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

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

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

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

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

The Dregs of Alchemy

"The struggle of fixed and volatile" 

allegorical illustration from

Splendor solis [detail] 16th C.

To 17th century Italian glassmaker and alchemist Antonio Neri, "Dregs" were otherwise known as terra, gruma, immondita, terrestreità and the evocative sporchezza. It was the "filth" and sediment left in the bottom of vessels after useful material was extracted from a preparation. These often foul-smelling substances were sometimes discarded:

Then filter out the dregs of the vitriol impregnated water; that which is yellowish you should throw away. –L’Arte Vetraria, chap. 38.

Other times, dregs were further refined. A notable example was the potassium rich muck left at the bottom of aged wine barrels. This was Neri's secret ingredient in producing a fine, sparkling cristallo glass. To understand the distinction between the useful and the useless forms of dregs, we must dig deeper into Neri's philosophy.

It might be surprising to some that these lowliest of materials could play an important role in the theory of transmutation—the alchemists' ultimate quest—which was to turn base metals into gold and silver. The idea was that a natural evolutionary process occurred in which  primordial material from the creation of the universe would, over time, mature through the lesser metals into pure gold. It was thought that this maturation was prompted by "seeds" of gold contained in the material. In Neri's view, this could be interrupted by various natural circumstances and could be restarted or sped-up through alchemical manipulation.

If one was to "purify" lesser metals into gold, it was advisable to know what needed to be removed. In his manuscript Discorso, Neri carefully explains five categories of impurity, which he then breaks down further into two sub-groups:

It should be noted in general, that in dealing with the [Aristotelian] elements in accordance with chemical philosophy, we can say that all mixed bodies in this art are discovered to contain five kinds of impure substances, which are completely dead and without any virtue or properties effective to [alchemical] operations. Two are from impure substances and three from pure substances, where all the strength, effectiveness and virtue are located specific to each mixture. Of the two [derived from the impure] one is called 'phlegm', which is to say a watery substance with no odor or taste and the other is called 'dead body' [corpo morto] or 'damned earth' [terra dannata], an earthy substance that is equally tasteless and without virtue.[1]

Indeed, in Neri's chemical philosophy, the above two useless forms of impurity (phlegm and corpo morto) are complemented by three useful forms (salt, oil or true sulfur and spirit or mercury), which are present in so-called pure materials. Researcher Maria Grazzini notes in her annotations to the manuscript that: "The chemical philosophy to which Neri refers is Paracelsian, which in addition to the four Aristotelian elements introduces the principal triad (tria prima) of salt, sulfur, and mercury. References to sulfur and mercury were already present in Arabic alchemy." [2] 

Of the other three [derived from the pure] one is called 'salt' and it is the so-called most fixed substance because it is resistant to the violence of fire; it does not flee or vanish into the air. The second is called 'oil' or 'true sulfur' because of the similarity to it, fatty and viscous. The third is called 'spirit' because it is more spiritual and volatile than all the others and even the slightest heat will cause it to dissipate into the air if it has not been bound to the salt, which is the component fixed by the oil. By its tenacious, slimy nature, [oil] acts to bind the volatile to the fixed. These three types are those of the pure substances, which are called by many other names; 'body', 'soul' [anima], 'spirit'; 'bitter', 'sweet', 'acid'; 'salt', 'sulfur', 'mercury' etc.

In them alone are placed all of the virtue and effectiveness of the minerals, the vegetables and the animals, even if the quantity of pure substance is very small in comparison with the impure in any kind of mixed body. These [three] are found in each mixed quantity of pure substance, in comparison with the ineffectual found in the impure. [3] 

In his view, it is these last three forms of impurity that hold the key to transmutation, which tends to puts dregs in a whole new light.


[1] Grazzini 2012, p. 339.
[2] Ibid, note 45, p. 339.
[3] Ibid, p. 340.

* This post first appeared here on 20August 2014.

Glass, Fire, and Brimstone

The 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.

Tartar Salt

So-called "wine diamonds," (harmless)

Potassium bitartrate deposits which can accumulate

in bottles and barrels of wine

Tartar salt is an example of an alchemist's chemical that is a byproduct of another process, in this case winemaking. In his book L'Arte Vetraria, [1] Antonio Neri uses it in his glassmaking for two very different effects. The first application is to improve the appearance of the glass and the other is to modify colors. In addition to uses in glass, tartar also finds its way into his recipes for red paint made from the dried kermes insect [2] and for cast bronze mirrors, as a flux. (In foundries, a flux keeps the metal bright and shiny while in a molten state). [3]

Neri notes that tartar also went by the name of "gruma" to which we can add the synonyms greppola and argol. He warns readers several times to "leave behind the [dried] powder of the [raw] tartar, which is no good" and that "you should have tartar, from the dregs of red wine, which is better than white wine." [4] Nevertheless, he does specifically use white wine tartar in his recipes for rosichiero, a transparent dark red enamel. [5] In his recipe for producing tartar salt [6] he directs the reader to obtain the raw material from emptied wine barrels, but elsewhere in the book he seems to prefer to use the large crystals of tartar that have “vitrified naturally in bottles of wine.” [7] Chemically, tartar is a potassium compound formed through a reaction with tartaric acid, a major constituent of grape juice. [8] 

The effect of tartar in improving the appearance of glass can be readily explained. Most of the glass that was made in Italy in the seventeenth century used sodium-based additives to lower the melting temperature of finely ground quartz powder; these formulations are known as soda-based glasses. Using potassium compounds can have the same effect and these 'potash' based glasses were predominantly produced in northern Europe where trees and plants rich in potassium were used in glass. Potassium is a heavier element and it produces a denser, more refractive glass, giving it more sparkle, although not as much as lead imparts to crystal. Unfortunately, potassium also makes the glass harder to work for the artisan. Potash glass stiffens more quickly as it cools, whereas soda glass remains workable for a longer time before requiring reheating in the furnace. Many of Neri's recipes blend the two additives, which we can imagine gives some of the advantages of both.

Neri also used tartar to modify color in glass. The effects of tartar are exemplified in a number of passages throughout the book. He uses it as the sole pigment in his recipe for pearl colored glass, but he warns, "Once obtained, you must work the color quickly, because it will dissipate." [9] Conversely, he also uses tartar to produce a black colored enamel, combining it with manganese [oxide], which by itself imparts a magenta color. [10] 

The first step in making Neri's purified tartar salt is to obtain the raw "gruma, from barrels of red wine in which it forms large lumps." Next, he gently roasts it in terracotta pots "until it becomes calcined black and all its sliminess is roasted away. It then will begin to whiten, but do not let it become white, because if you do the salt will be no good." Now, he boils it in water for two hours, evaporating off three-quarters of the liquid. After filtering, he lets the remaining liquid "lye" cool in pans, allowing any sediment to settle to the bottom. He gently pours off  (decants)  the liquid which is further processed on the stove, this time in glass containers. The result, after full evaporation over a slow fire is a “white salt” left in the vessel. He dissolves this in hot water, filters it again and allows more sediment to settle out for a period of two days. Again, the liquid is decanted and evaporated in a glass container. The filtering and evaporation process is repeated four times, resulting in a product that is "whiter than snow."

Neri's final remarks for this chapter are as follows:

“When mixed with sifted polverino, or rocchetta, with its doses of tarso or sand, this salt will make a frit that in crucibles will produce the most beautiful cristallino and common glass, which one cannot make without the accompaniment of tartar salt. Without it, good fine cristallino can be made, nevertheless with it, it will be the absolute most beautiful.” [11]

[1] Neri 1612.
[2] Ibid, ch 116, 117.
[3] Ibid, ch 113. Note that in glassmaking, the term 'flux' has a different meaning than in metallurgy.
[4] Ibid,  ch 41. 
[5] Ibid, ch 125.
[6] Ibid, ch 11.
[7] Ibid. ch 46.
[8] Pure tartar takes the form potassium bitartrate KHC4H4O6.
[9] Neri 1612, ch 60.
[10] Ibid, ch 102.
[11] Ibid, ch 11. Polverino and rocchetta are thought to be forms of dried Salsola Kali plants. Tarso is Neri's term for white quartz river stones. Cristallino was a Venetian style glass that in quality fell between common glass and the premium cristallo, for which Murano became famous.

* This post first appeared here on 5 Sept. 2014.

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.

Sulfur of Saturn

The Roman Goddess Ops, 'sweet'-heart (and wife)

of Saturn, Peter Paul Rubens c. 1630,

“Abundance (Abundantia).” [1]

Antonio Neri’s 1612 book on glassmaking, L'Arte Vetraria, devotes an entire chapter to making artificial gems. These are made with an especially dense, refractive form of lead glass—what today would be called lead crystal. He made it in small batches, in sealed ceramic vessels; each infused with various metal oxide pigments to give characteristic jewel tone colors.  His secret ingredient for making the finest artificial gems was a material known as "sulfur of Saturn." He writes:

You will have jewels of marvelous beauty in every color, which by far surpass those described above, made with ordinary minium. Because with this true sulfur of Saturn, they will surpass all others by far more than I can write here, as I have seen and made many times in Antwerp.[2]

This "ordinary minium" that he speaks of is one of several oxide forms of lead, also known as 'red lead,' it is bright red or orange in color. As a stable red pigment it was well known to ancient Byzantine and Persian illuminators. [3] It was so popular, in fact, that the word used to describe small intricate pictures 'miniature,' is derived directly from 'minium.' [4] The intricate embellishments in manuscripts ultimately took on the name of the scribes' favorite color.  In glass, minium does not impart red or any other color on its own, but does add 'sparkle' and made Neri's jewels highly refractive to light. 

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

In other references "sugar of Saturn" and "sulfur of Saturn" are considered synonyms. This is not the case with Neri, for him they are two different substances; the 'sugar' is a precursor for his final product, sulfur of Saturn:

Left in the bottom will be a salt as white as snow, and as sweet as sugar. Repeat the dissolution, and filtering, and evaporation with common water three times. This is the required sugar of Saturn. 

Keep it to calcine in sand in a glass flask or ball in a furnace over a moderated fire for many days. It will further calcine to a color that is much redder than cinnabar, and more finely impalpable than sifted grain flour. This is the required true sulfur of Saturn; purified from the sediment, foulness and blackness that were upon the lead at first. [6]

It would be a reasonable guess that by heating the acetate for "many days" he is reducing it back to minium, its more basic, bright red oxide state, but this time in a greatly purified form.

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

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

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

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