Incalmo Glass

Incalmo vessels by Tapio Wirkkala for Venini.

In this post, we will explore one of the classical techniques of glass art. Along with filigrana, latticino, reticello and pezzato, incalmo is a classical Venetian technique well established in the art, even if poorly understood by many outsiders.

‘Incalmo’, in Venetian dialect literally means “graft” as in joining two plants. That is a pretty good description of how this effect is achieved; think of a vase whose bottom-half is one color and top is another. The glass artist blows two separate bubbles of glass, opens them and joins them together to form a single bubble. It is a difficult operation because the two open lips must be exactly the same size to join properly. The process can be continued to add more colors; virtuoso pieces may include several sections, each a different color. In addition, the position of the iron rod that the artist uses to hold the bubble can be changed while the piece is under construction, leading to asymmetrical effects.

16th century incalmo plate,
unknown artist.

The above description is the classical way of achieving incalmo, but modern materials and equipment allow artists to achieve a similar effect with considerably less skill. For instance, precise diameter glass tubing is now available in a wide variety of colors. This can be cut into rings with a saw, then stacked in a kiln and fused together. From there, this “prefabricated incalmo tube can be worked by traditional methods. Whether or not this meets the definition of true incalmo depends entirely on whether one focuses on the method or on the end result.

9-10th century incalmo vase,
Syria or Iraq.

The name ‘incalmo’, was applied to glass in the first half of the twentieth century by the Venini factory on Murano, in Venice. [1] However, both the word and the method are much older. The Venini artisans revived the technique to great acclaim, but Venetian examples date from the sixteenth century and Islamic examples from ninth century Syria have also survived. It is not hard to imagine that this joining technique was experimented with shortly after glassblowing became common around the first century BCE. However, what is truly amazing is that any of these early examples survived to be sold to customers without breaking in the cooling process. The reason for this is a technical issue that we have not discussed yet.

All glass expands a little when it is heated and shrinks when it cools. Different formulations of glass generally expand by differing amounts. When a single piece incorporates more than one type of glass, and the thermal expansions differ significantly, the result is disaster. After the piece is finished it is placed in a kiln where it slowly cools back to room temperature. Because of the mismatch, one area wants to shrink more than the adjacent area and the glass cracks along the join. The expansion and contraction is microscopic, but it is enough to ruin hours and hours of work, leading to much gnashing of teeth the morning after, when the finished work is inspected. 

The Venini glass masters had the benefit of this knowledge, but for earlier artisans, trial and error must have played a big role in determining which formulas were compatible. Different colors mean different metallic additives and to match expansion other ingredients would need to be adjusted. Today, manufacturers produce glass in a series based on expansion; artists can be relatively sure that two different colors from the same series can be “grafted” and not self-destruct when cooled.

[1] I have not absolutely confirmed this, but authoritative secondary references credit Venini, and I can find no mention to "incalmo" as a glass technique prior to the twentieth century.

Primordial Matter

16th century Mining practices, 

from Agricola, De Re Metallica

In the early seventeenth century, Florentine priest Antonio Neri wrote the first printed book devoted to formulating glass from raw materials. His work is called L'Arte Vetraria, which translates to "the art of glassmaking." The book became quite famous and this is what he is remembered for today, yet he considered himself first and foremost an alchemist. In previous posts, we have explored the commonalities between glassmaking, medicine and the apothecary's trade. Another field closely connected to alchemy was mining. 

In the seventeenth century, the earth was considered a living entity; metals were found to occur in "veins" which were thought to grow and once mined, regenerated over time. The metals themselves were thought to undergo a maturation process. The thinking was that primordial material left over from the creation of the world exerted its influence deep in the ground. Nurtured by the earth, under the influence of the suns rays, a process took place that eventually turned base metals into the more noble silver and gold. As far as Neri was concerned, alchemy was the art of imitating and enhancing natural processes that were already at work. In his manuscript Discorso, he writes:

I feel that the more perfect the art the most simple it is; so the authors [of alchemy] most unanimously agree that the ‘primordial material’ [prima materia] of the [philosopher’s] stone is something vile [base] and not bought with money, but easy to find. Moreover, the manner of work must imitate nature, which in order to produce gold makes use of the singular or simple material, which is the seed of gold, of a single vessel, which is the ‘womb of the earth’ [seno della terra] and of a single natural and vital fire, which is the sun.*

Elsewhere in the manuscript, Neri discusses several specific mines. He discusses the use of "vitriol" water that flowed in certain mines and how it could be used to transmute iron into copper. He discusses an unidentified mine "some distance from Leiden" (possibly in Limburg) and another in Slovakia in the town of Smolnik. It is reasonable to think that Neri visited these places himself. A third location, which he purposely keeps under wraps, is where he obtained "immature" gold that he was able to "multiply" through alchemical manipulation. With a certain disappointment, he writes "To this day I have never found another mine like it, and therefore suitable for this purpose." Clearly, he spent a significant portion of his time looking. He advises:

The gold mines are not all in the same condition, which is well understood for those of silver and all the other [metals]. Some are already perfect, in which nature has done what it could do and reduced the gold to its maturity, while other [mines] are still imperfect and in their infancy*

In his work for Medici prince Don Antonio in Florence, Neri's assistant/disciple was Agnolo della Casa. Della Casa took copious notes of Neri's experiments, and literally filled thousands of pages in notebooks that are today held by the National Library in Florence. Much of this material dealt with the transmutation of metals, and as we have seen Neri was not only concerned with materials, but with their specific place of origin. His first manuscript was titled "Treasure of the World, By Priest Antonio Neri – which [covers] the whole of alchemy with various illustrations, not only of the furnaces, vessels and chemical instruments but with other illustrations concerning the mining of all the metals." For he and his colleagues, mining and alchemy shared theoretical connections but also familial ones. Della Casa had a relative named Filippo Talducci della Casa (1543- c.1615), who was a celebrated alchemist and mining engineer, working in Prague and Krakow for the Holy Roman Emperor. Last but not least, there was also a practical connection. Mining provided many of the raw materials used in Antonio Neri’s glassmaking activities.  

* For a full discussion of Neri’s Discorso, see M. G. Grazzini, “Discorso sopra la Chimica: The Paracelsian Philosophy of Antonio Neri”, Nuncius 27, pp. 411-467.

** This post first appeared here 30 April 2014.

Washing Molten Glass

Washing, sorting and carrying cullet

Denis Diderot 1772

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

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

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

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

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

In a few hours everything will have clarified, now purify it by throwing it in water. Inspect the glass carefully before returning it to the crucible. All lead precipitating out of the glass must be removed with diligence, throwing it away, so that it does not make the bottom of the crucible break out, as can happen. Return the glass that was thrown in water to the crucible and leave it to clarify for a day.

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

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

* This post first appeared here on 9 May 2014.

Pebbles from Pavia

A Bridge on the River Ticino, near Polleggio,

William Pars (1742‑1782).

In the sixteenth and seventeenth century, a type of glass known as 'cristallo' was the absolute pinnacle of the art. Its recipe was invented in Venice and guarded there as a state secret. Its name derived from the mineral it was designed to mimic: rock crystal. As clear as water, rock crystal was valued since ancient times for carving into cups, vessels and other objects of art. Today we know it as a form of quartz, but in Roman times it was thought to be a type of frozen or coagulated water.

In the early 1600s, when Antonio Neri started making glass in Florence, the grand duke's craftsmen were routinely carving this hard and brittle rock crystal into complex thin shapes, a process that took great skill and effort. Due to the expense involved in producing a piece, this art was the exclusive province of extremely wealthy individuals. Thus, objects made from rock crystal were considered markers of status. The recipe for cristallo glass was a very great secret indeed, but its real value lay in the specific materials used. Even if the recipe found its way out of Murano, which it inevitably did, the Venetian's tight trade network ensured a monopoly on many of the ingredients. It is said that even the furnace crucibles for cristallo were made from a specific clay gathered in Constantinople.

Cristallo was not only exceptionally clear, but for the artist it had working properties like no other glass. Thin, complex shapes were possible in cristallo that could never be duplicated in common glass. The secret for making cristallo came to Florence in the late 1560's, only a few years before the birth of Antonio Neri, who would learn the techniques and go on to publish the recipe for the first time anywhere.

 After protracted overtures, which involved diplomats, spies and the archbishop of Florence, Grand Duke Cosimo I managed to negotiate with the Venetian Doge and Senate for a Muranese master and two assistants to come to Florence and teach the way to make cristallo. It is likely that the raw materials were all purchased through the Venetians, at least initially. By the time Neri wrote his book, L'Arte Vetraria, in 1612, the Florentines were already finding alternate sources. In Venice, the ingredients of cristallo were prescribed and controlled by strict laws. The Florentines did not have this constraint and were free to experiment.

In the second recipe of Neri's book, he spills the beans on where the Venetians procured the single most important ingredient for cristallo, the pure quartz stones which account for the material's clarity. Notice in the following excerpt that Neri mistakenly thinks that the white river stones are a form of marble and also notice the alchemical language he uses to describe the process in which the stone is "transmuted" into glass.

When you want to a make cristallo that is beautiful and fully perfect, see that you have the very whitest tarso. At Murano they use pebbles from Ticino [Pavia], a stone abundant in the Ticino River. Tarso, then is a species of very white hard marble [quartz]. 

In Tuscany, it is found at the foot of Mount Veruca in Pisa, at Seravezza, at Massa near Carrara, and in the Arno River both above and below Florence. In other places as well, common stone is often recognized, which is seen to have the same qualities as tarso; it is very white and does not have dark veins, or the yellowish appearance of rust, but is spotless and pure. Take note that any stones that will spark with a piece of steel or strike plate, are apt to vitrify and will make glass and cristallo. All those stones that do not make sparks with a piece of steel or striker as above will never vitrify. This serves as advice for being able to distinguish stones that have the ability to transmute their form, from those that cannot be transmuted. 

Start with this same tarso, as fair and as white as possible. Grind it finely into powder in stone mortars. Do not use bronze or any other metal for this purpose or the stone will take in the color of the metal, which then would tinge the glass or cristallo, and make it imperfect. The pestle must be iron by necessity but at least the other materials will not have the possibility of causing any effect. Pulverize the tarso well and sift with a fine sieve. It is important that the tarso is ground as finely as flour, so that it will all pass through a fine sieve.

* This post first appeared here on 30 May 2014. 

Scraping the Barrel

4th century BCE philosopher Diogenes

(supposedly lived in a wine barrel)

by Gaetano Gandolfi (1792)

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

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

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

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

To make Purified Tartar Salt you should obtain tartar, which is also called gruma, from barrels of red wine in which it forms large lumps, however do not use powder. Roast it in earthenware pots amongst hot coals until it becomes calcined black and all its sliminess is roasted away. It then will begin to whiten, but do not let it become white, because if you do the salt will be no good.  

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

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

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

*This post first appeared here on 16 May 2014.

Zaffer

Antique Apothecary jars,

cobalt blue glass.

As best as I can remember, the first time that I really noticed glass was at four or five years old, at my grandmother's house in Queens, New York. The sunlight filtering through a low window caught my eye with a brilliant blue glint through a small cobalt glass bottle.  My grandmother held it up to the light for me and I was enchanted; transported into a realm of exquisite pure color.

Little did I know that the spell cast on me at such a young age had been cast on Egyptian pharaohs of the eighteenth dynasty and on Persian princesses, two thousands years ago, by their cobalt glass jewelry. In all three cases, the deep rich blue of cobalt oxide glass was responsible. Today, the source of ancient Middle Eastern cobalt is unknown, possibly West Africa, but more recently, in the Renaissance; it was mined in Hungary, in Bohemia and in German Saxony, where it was called "zaffer," after its sapphire color.

Legend tells that sixteenth century silver miners in Germany amassed a hoard of smaltite thinking it was silver ore. When they tried to smelt it, the arsenic which cobalt ores always have, evolved highly toxic fumes that made them sick. Discouraged and maligned, they said the product of their labors was cursed by goblins; they named it "kobald" (cobalt) after the evil spirits. The theme of 'evil spirits' (toxic fumes) was common in mining circles. A mythology persisted from ancient times up until the eighteenth century that divided the spirits into two groups; mischievous, and malevolent. The mischievous spirits played tricks on the miner's perception and equipment, the malevolent spirits could stop a man dead in his tracks, literally. 

Regardless of the difficulties both terestrial and otherwise, a strong market developed for the newly found "kobald" among artists for paint, potters for glazes, and glassmakers. The Saxon miners gained a reputation for producing the finest zaffer.

In his glassmaking book L'Arte Vetraria, Antonio Neri describes his method for purifying and preparing zaffer for use in glass. It is a recipe that would stand the test of time, still quoted by authors into the nineteenth and twentieth century.

To Prepare Zaffer, Which Serves for Many Colors in the Art of Glassmaking

You should get zaffer in large pieces and put it in earthenware oven-pans holding it in the furnace chamber for half a day. Then put it into iron ladles to inflame it in the furnace. Heat it well, then take and sprinkle it with strong vinegar. When cold, grind it finely over a porphyry stone into glazed earthen pots with hot water. Then wash more water over it always leaving the zaffer to settle in the bottom.

Now gently decant, to carry away the sediment and impurities of the zaffer. The good part and pigment of the zaffer will remain in the bottom. The pigment remains are now prepared and purified to be far better than it was at first, which will make clear and limpid pigment. This zaffer should be dried and kept in sealed vessels for use, which will be much improved over the original.

Until the mid 1700s zaffer had been associated with silver and copper mines, and was commonly thought to be a derivative of copper. It was Swedish chemist Georg Brandt who finally isolated the new metal, and gave it the name which honors the miners and the subterranean spirits which still can cast a spell on us through its deep pure blue color in glass.

* This post first appeared here in a slightly different form on 4 April 2014.

Flexible Glass

Roman Emperor Tiberius - Glass paste cameo

 c 20CE by "Herophilos, Son of Dioskurides"

In the first century CE, references appear in the literature for a malleable form of glass --that is to say flexible-- the method for which is reported as lost. This 'vitrum flexile' was a material that supposedly could be worked with a hammer like metal; not brittle but plastic, yet retaining the other favorable properties of glass. In the ancient world, historians Strabo (c. 63 BCE–24 CE), Pliny (c. 23–79 CE), Petronius (c. 27–66 CE) and others recount the story of a hapless artisan who brought his great discovery to Roman emperor Tiberius. Fearing the devaluation of his wealth, the emperor had the glassmaker executed on the spot and his workshop destroyed. The implication being that the inestimable value of a malleable glass would crash the markets for gold and silver. [1]

Since then, the legend has resurfaced in various forms, notably at periods in history of technological upheaval; times when innovative knowledge threatened to 'disrupt' the established order. One incarnation has the sophy of Persia gifting a set of malleable drinking glasses to the king of Spain, Philip III, around 1610. Just then, new trade deals with the Middle East rattled the European economy and Venetian glass craftsmen fanned out through Europe, disrupting local glass furnaces and guilds with their superior techniques. [2] Another version tells of a French sculptor bringing his malleable glass-work to Cardinal Richelieu (1585-1642), of Dumas' Three Musketeers fame. According to the story, that particular artisan’s reward was life in prison. The tale takes place in a tumultuous political period in French history and appears in print at the dawn of the so-called industrial revolution. [3]

Chronologically, this last story aligns nicely with the publication, in southern France, of a Latin book of alchemical recipes by royal physician Pierre-Jean Fabre (1588-1658). Contained in his volume is a prescription for a malleable form of glass, presumably the fabled vitrum flexile. Fabre's book was titled Palladium Spagyricum, 1624. [4] Spagyricum is a reference to spagyrics, the specific brand of chemistry practiced by sixteenth century alchemist-physician Paracelsus. Palladium translates to 'protector' or 'savior'. As an aside, it is interesting to note that Fabre developed an elaborate philosophy which integrated chemistry as a 'sacrament' to Catholic theology, but that is a story for another day.

In 1685, Fabre's recipes were nicely translated into English and tucked into the end of a book otherwise devoted to the art of drawing, by William Salmon [5]. Here is the full recipe "To Make Malleable Glass" as rendered in Salmon's book:

I.        Take oyl of Luna, twenty drachms: oyl of Mercury, or its water seven times rectified, one pound: mix them together and distill them.

II.      Repeat this distillation till the oleum Lunae rises with the water of Mercury in distillation. 

III.     Distill this water again until it is fixed, and converted into a fixed oyl, and this repeat four times. 

IV.     In the fourth time the oyl of Luna is fixed with the oyl of Mercury, so that they render glass malleable; for so great is the viscosity in your oyls, that it removes the brittleness of the glass, and so leaves it of a malleable temper. 

V.      The reason is, because that the radical moisture of the glass is multiplied by the radical moisture of the metals. Which is plentiful and turgent or swelling in the oyls of Luna and Mercury. 

VI.     And if [in] this oyl made volatile, diamonds should be dissolved, and then digested into a fixt oyl, it would transmute all glass into diamonds, only by projecting this oyl onto melted glass. 

VII.   There are also other precious stones comprehended within this oyl, when it is made volatile, and digested, and fixed again by digestion continually for the space of a year. 

VIII.  Also this oyl can turn glass into precious stones of any kind whatsoever, if therein (being made volatile) precious stones of the same kind have been dissolved, and digested with it into a fixed oyl. 

IX.     For as metals are included in their fixed oyls: so are precious stones in theirs, as Raymundus Lullius doth witness in many places; the which thing we shall teach you in the following chapter. 

 Elsewhere in the book, it is explained that 'oyl of Luna' is silver dissolved in acid, and 'oyl of Mercury' is a sublimation of mercury and saltpeter. [6] From a purely technical standpoint, the formula would have been regarded credible by 17th century and earlier practitioners in that both mercury and silver were successfully used as additives to glass and they do integrate into the matrix. Notably, Antonio Neri used both of them in his chalcedony glass. In Neri's case he was using these metals to produce color, although he does not attribute specific tints to the ingredients. Under some circumstances, silver is known to produce an attractive blue. It should also be noted that Renaissance glassmakers used similar silver-mercury formulas to produce the reflective layer on finished glass mirrors, in a process that resembled gilding. They formed what was known as "mercury glass." It is not beyond the pale to speculate that experiments would have been conducted by alchemists to add such concoctions directly to the glass melt. In the end, though, there is no indication that these additives would produce a malleable glass. Last, we should note that prolonged exposure to mercury vapor causes irreversable neurological and organ damage. It undoubetly contributed to the demise of alchemists, gilders, milliners and many others who worked with it throughout history. 

As far as I am aware, this is the first example of a specific recipe for malleable glass uncovered in the literature. If nothing else, it is an important marker for further research into the history of glassmaking.

[1] Strabo: Geography, v. 8; Pliny: Naturalis Historia XXXVI.lxvi.195; Petronius: Satyricon 50.7; Also recounted by Casius Dio (c.150–235 CE): Historia Romana 57.21.7; Isidore of Seville (c. 560–636 CE): Etymologiae XVI.16.6, ‘De vitro’; Suetonius; Ibn Abd Alhokin; John of Sailsbury.

[2] Knolles, Grimstone, Johnson: Richard Knolles' The General Historie of the Turkes (London: Adam Islip, 1621).

[3] Neri 1697 (Introduction). A French translation of Florentine glassmaker Antonio Neri’s 1612 book L’Arte Vetraria [The art of making glass] by Jean Haudicquer de Blancourt. Also see my earlier post here http://www.conciatore.org/2014/07/flexible-glass-reprise.html .

[4] Pierre-Jean Fabre: Palladium spagyricum Petri Ioannis Fabri doctoris medici Monspeliensis ... (Toulouse: Bosc, 1624), p. 276. Later translated into several English editions, see note [5].

[5] William Salmon: Polygraphice: Or the Arts of Drawing, Engraving, Etching, Limming, Painting … (London: T. Passenger & T. Sawbridge, 1685), pp. 598, 599.

[6] To more adventurous readers: DO NOT TRY THIS AT HOME, or anywhere else, ever. Vaporized mercury is a powerful neurotoxin. Small amounts can cause permanent brain damage and multiple organ failure. Furthermore, this recipe uses powerful acids and nitrates, which are extremely dangerous even in a controlled laboratory setting. Even if you have little regard for your own health and safety, consider those around you; this includes loved ones, family, children, pets, neighbors and the emergency workers who will inevitably be left to clean up your mess.