Wednesday, January 16, 2019

The Glass of Montpellier

Montpellier, France, in the seventeenth century.
(Attribution unknown)
Montpellier is an old city in southern France. It stands about halfway between Marseille and the Spanish border along the Mediterranean Sea (strategically located slightly inland to avoid pirates). First documented in the tenth century, it is one of very few French cities that developed without the influence of ancient Roman occupation; it is a pure product of the local region. It became a center of intellectual learning and attracted students from throughout Europe. Around it sprung a number of supporting arts not the least of which was glassmaking. Once famous for its fine glassware, today this centuries long heritage is all but forgotten, yet when we dig into the literature, we find a surprise connection to one of the oldest legends in the history of glass.

Since before the Renaissance, Montpellier was an established center for medical and legal education, a strong tradition that continues at the university there; today, it houses the oldest running medical school in the world. This prestigious institution "was founded perhaps by people trained in the Spanish medical schools; it is certain that, as early as 1137, there were excellent physicians at Montpellier University." [1] In 1529, Nostradamus entered to study for a doctorate in medicine, but shortly thereafter, he was expelled when it was discovered that he previously worked as an apothecary; a 'manual' skill that was banned by the school's rules of conduct.

Paradoxically, these impugned 'manual' arts account for some the region's more intriguing activity. Along with medicine, the area became known for the production of paint pigments, for glassmaking and finally for alchemy. All of these turn out to be closely related to each other, but perhaps not obviously so. Throughout the Renaissance, apothecaries were responsible for a wide range of distillations and extracts used by physicians to treat disease. They were also the de facto suppliers of pigments and other fine art supplies and even sourced some of the materials for glassmaking. Glassmakers often relied on painters to embellish their products, painters used ground glass in their pigments and apothecaries needed glassmakers to produce the flasks, beakers and other alchemical equipment required for their profession.

An anonymous Montpellier manuscript of the fourteenth century, called the Liber diversarum arcium [Book of Various Arts], offers us one of the most complete guides to the production of pigments to have survived from that period. [2] Another, later writing of the sixteenth century offers an extensive collection of glassmaking recipies brought to Montpellier from Venice. This one is titled Recette per fare vetri colorati et smalti d’ogni sorte havute in Murano 1536 [Recipes to make colored glasses and enamels of every kind as in Murano, 1536]. [3]

Local history points to the town of Claret just north of Montpellier as the seat of regional glassmaking. Beginning in 1290, the oak forests on the Causse de l'Orthus attracted glassmakers and their families. "So maybe the oaks got used for fuel. (A 'causse' is a geological term for a limestone plateau.) [also a material of glassmaking] At any rate, the glassmakers were ennobled by the King and formed a guild of premium glassmakers whose wares were sold all over Europe from the market at Sommières." [4]

In the early seventeenth century, Pierre-Jean Fabre (1588-1658) studied medicine at Montpellier where he discovered the works of Paracelsus, to which he became a devotee. [5] After securing a medical degree, he returned to the nearby town of his birth, Castelnaudary, to work as a doctor. Eventually, he was awarded the status of "Royal Physician" by Louis XIII, probably for his work on treating victims of the plague with chemical preparations. [6]

Fabre's first book, of a total canon numbering sixteen volumes, was on the subject of alchemy and medicine titled "Palladium spagyricum" 1624. [7] The book, written in Latin, contains advice on the transmutation of metals, turning water into "good wine" and elixirs to cure all disease. It contains one recipe that is of particular interest on the subject of glassmaking; a malleable form of glass, known in legend since the Roman Empire as Vitrum Flexile.

See my previous post to take a closer look at Fabre's specific recipe for a glass that is malleable at room temperature and trace a bit of the legend's history.

[1] Wikipedia, “University of Montpellier” https://en.wikipedia.org/wiki/University_of_Montpellier.
[2] Mark Clarke: Mediaeval Painters' Materials and Techniques: The Montpellier Liber diversarum arcium (London: Archetype Publications, 2011).
[3] Montpellier 1536, MS. H. 486: Recette per fare vetri colorati et smalti d’ogni sorte havute in Murano 1536, Bibliothèque de l'Ecole de Médecine de Montpellier, see also Zecchin 1987, v.1 p 247-276. Although the manuscript is dated 1536 it is probably copied from much earlier Venetian sources.
 [4] Ed Ward, Blog: City on a Hill, 1 Nov. 2010 post “Where the Glass-Blowers Were.” http://wardinfrance.blogspot.com/2010/11/where-glass-blowers-were.html . Also see Halle du Verre regional glass museum website http://www.cc-grandpicsaintloup.fr/-Halle-du-verre-.html .
[5] Paracelsus (1493-1541) was a Swiss born physician and alchemist who looked to nature rather than ancient texts for remedies to disease. He was widely condemned durring his lifetime but became very popular after his manuscripts were printed in the late sixteenth century.
[6] The definitive reference on Fabre is Bernard Joly: La rationalité de l'alchimie au XVIIe siècle (France: Vrin, 1992), pp. 35-50. A good English treatment can be found in Allen George Debus: The French Paracelsians: The Chemical Challenge to Medical and Scientific Tradition in Early Modern France (Cambridge: Cambridge University Press, 2002) pp, 75, 76.
[7] Pierre-Jean Fabre: Palladium spagyricum Petri Ioannis Fabri doctoris medici Monspeliensis ... (Toulouse: Bosc, 1624), p. 276. Later translated into several English editions, notably by William Salmon: Polygraphice: Or the Arts of Drawing, Engraving, Etching, Limming, Painting … (London: T. Passenger & T. Sawbridge, 1685), pp. 598, 599.

Monday, January 14, 2019

Vitrum Flexile

Roman Emperor Tiberius - Glass paste cameo
 c 20ACE 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.

Friday, January 11, 2019

Sonnet for a Barber


Possible portrait of Lodovico Domenichi,
British Museum, inventory #1867,1012.650
This is a post about a 16th century poet who was best friends with alchemist/glassmaker Antonio Neri's grandfather, Jacopo, and may even be the inspiration for the glassmaker's middle name: Lodovico. Neri, born in 1576,  is remembered mostly for his book of glass recipes, L'Arte Vetraria, yet he considered himself first and foremost an alchemist. His father was the personal physician to the grand duke of Tuscany, and his grandfather was a barber surgeon, who probably lived in the family house until his death in 1594, when Antonio was an 18-year-old. There can be little question that our glassmaker/alchemist was steeped in chemistry and medicine from a very early age, but perhaps also literature. 

In November of 1554, poet Lodovico Domenichi wrote a sonnet to his friend, Jacopo Neri. Jacopo was from Dicomano in the upper Arno River valley, then living in Florence with his young family. Domenichi was serving a sentence of house arrest in a wretched paper mill in Pescia, in the hills north of Florence. He had been found guilty by the inquisition on charges of translating the heretical writing of John Calvin into vernacular Italian, a crime for which the poet could have easily been executed. Luckily, he had friends in high places and after some nervous time spent in the stockade in Pisa, his sentence was reduced and later commuted by Grand Duke Cosimo I de' Medici.

Jacopo Neri had taken ill with a grave infirmity and when word reached Domenichi, he took pen to paper and composed seventy stanzas in the style of Petrarch. In so doing he bestowed a precious gift, the only one he could under the circumstances; he immortalized his good friend on paper. The sonnet starts:

      As I have now come to understand your
      Perilous illness and health,
      It is both grief and fondness that I show

      So may merciful God help you,
      Without delay, lest this vile world lose
      So much goodness in you, so much virtue.

In the sonnet, he goes on to extol Jacopo's kindness towards patients, his willingness to forgive and his admiration among scholars. Along the way, Domenichi describes his own predicament; the cruelties of the mill workers, the muddy floors, his desire to flee and trying to sleep on a bed of frozen straw among the work animals. The rain has been falling for weeks and he is miserable. Recalling happier times he evokes the memories of many mutual friends, including a dwarf named Don Gabriello Franceschi, who delivered sermons at the Cestello church, Neri's family church, located a few hundred feet from his front door. Franceschi was from the family into which Jacopo's daughter, Faustina, would later marry. 

      There he is called Don Gabriello 
      Franceschi and I am honored, for good reason,
      A giant of men in a small handsome package.

He goes on to describe his saviors; the men who intervened with the Church on his behalf. One was Pompeo della Barba, educated at Pisa and later called to Rome as the personal physician to Pope Pius IV. In the end, Domenichi was pardoned, allowed to leave the damp mill and return to Florence. Within a short time, he was appointed court historian to Cosimo I. Over his career, he published many volumes; translations of works ranging from Xenophon, to Plutarch, to Pliny's Natural History, to a groundbreaking compilation of poems by contemporary women. At the end of his life, Domenichi suffered devastating debilitation, maybe from a stroke, which robbed him of the ability to speak. Even so, he still received regular visits from his old friend Jacopo Neri the barber surgeon. As we turn away from the subject of Lodovico Domenichi, it is hard to resist speculating that a decade after his death, the poet was remembered with fondness by the Neri family in the christening of our glassmaker, whose full name is Antonio Lodovico Neri.

Lodovico Domenichi, "A Mastro Jacopo di Neri, Cerusico, E Barbiere." in Il Secondo libro dell' Opere Burlesche, di M. Francesco Berni.  (Fiorenza: Apresso li Heredi di Bernardo Giunti, 1555), vv. 2. Reprinted many times. 
*This post first appeared 9 September 2013.

Wednesday, January 9, 2019

Reflections on the Mirror

Jan van Eyck
The Arnolfini Portrait (1434)
L’Arte Vetraria, Antonio Neri's 1612 book, would eventually become the glassmakers' bible throughout Europe. By 1900 it had been translated into five different languages besides the original Italian; English, Latin, German, French, and Spanish (and in this century Japanese). Because of its seminal importance in the spread of glass technology, often overlooked are a few recipes at the back of the book, which have only a tenuous connection to the main subject.

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

Here is Neri's prescription:
A Mixture to Make [Mirror] Spheres:
Have 3 lbs of well-purified tin, and 1 lb of copper also purified. Melt these two metals, first the copper, then the tin. When they fuse thoroughly, throw onto them 6 oz of just singed red wine tartar, and 1½ oz of saltpeter, then ¼ oz of alum, and 2 oz of arsenic. Leave these all to vaporize, and then cast [the metal] into the form of a sphere. You will have good material, which when you burnish and polish, will look most fine. This mixture is called acciaio and is used to make spherical mirrors.
Of note is the fact that the word Neri uses for this alloy, acciaio, translates to "steel." Over the intervening four centuries, the meaning of this term has been refined so that today it denotes not simply a hard white metal, but a specific range of alloys containing iron and carbon. 

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

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

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

Jan van Eyck
The Arnolfini Portrait (detail).

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

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

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

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

* This post first appeared here 17 January 2014.

Monday, January 7, 2019

Casino di San Marco

The Casino di San Marco, Florence
Location of Antonio Neri's first glassmaking job
In 1612, Antonio Neri wrote the first book entirely devoted to making glass from raw materials. It was called L’Arte Vetraria, or in English, 'The Art of Glassmaking'. When Neri put pen to paper for his book, he had already been making glass for over a decade. He had the opportunity to learn his craft from some of the experts in the field. At the beginning, his first known experience in glassmaking was at the laboratory palace of Don Antonio de’ Medici, a prince from the ruling family of Tuscany. 

The Palace was called the Casino di San Marco. “Casino”, we might innocently guess indicated some kind of gambling hall, which it was not.  Instead, "Casino" was Italian parlance for a palace that was informally organized like a small country house with the living quarters on the ground floor. It was built by Don Antonio’s father, the former Grand Duke, as a place where Nature’s secrets would be discovered and new inventions would be made. Neri worked at the Casino for a couple of years before moving to Pisa and then to Antwerp, all the while making glass. He returned to Florence to publish his book, in which he thanked Don Antonio for his long patronage. 

Don Antonio's Casino was as much a grand concept as it was a physical space. Completed to his father’s specifications in 1574, it evolved into a prince's palace par excellence. Within its walls, grand dinners were held, productions were staged and poetry was read. In 1605 Michelangelo Buonarroti the younger staged a play there titled "The Christmas of Hercules." In its chambers, music was performed, philosophy debated and diplomacy conducted. In its laboratories, alchemy was nurtured, and glass was formulated. It was a sort of grand royal conservatory, melding together art, letters, drama, music and science. From its courtyard, hunters set forth into the Tuscan hills in search of unicorns, and within its workshops, artisans explored the territory of new materials and natural secrets.

The Royal Foundry, as it was also called, became a place of pride for Grand Duke Ferdinando. It was a place that visiting dignitaries specifically asked to see and tour. Behind the doors of the Casino di San Marco, Antonio Neri and his associates worked their magic. This is probably where he first learned the secrets of Venetian style glass composition and undoubtedly much more. He assisted the prince in his research, formulated herbal remedies and helped in the production of luxury gifts for visiting dignitaries.


This was the way that I made chalcedony in the year 1601, in Florence at the Casino, in the glass furnace there. At that time, the task of scheduling furnace-work fell to the outstanding Mr. Niccolò Landi, my close friend and a man of rare talent in enamel work at the oil lamp. I made many pots of chalcedony in the furnace there. I never deviated from the method stated above, I always prepared the materials well and it always came out beautifully in all my proofs.[1] 

[1] L'Arte VetrariaAntonio Neri 1612, p. 41.
* This post first appeared here in a shorter form on 12 August 2012.

Friday, January 4, 2019

Reticello Glass

Reticello by Aaron Tate (detail),
Photo by David Lindes.
In fine art of the early twentieth century, ‘cubist’ painters and sculptors caused a sensation by pioneering a style that was fluid and emotional yet rigorously geometric. Earlier still, eighteenth century Baroque musicians thrilled audiences with ‘counterpoint’; music that was melodic and harmonious yet structured with an almost mathematical precision. The recurrence of similar themes in widely differing arts is perhaps not so surprising. When successful, these are themes that resonate deeply within us; they amplify what is common to our nature and remind us that we are in the world together. 

In sixteenth century Venice, a form of glasswork emerged that anticipated these juxtapositions to similar popular acclaim. In fact, the technique of ‘reticello’ went on to become an enduring trademark of Murano technical skill and artistry. To this day, well executed pieces of reticello glass are coveted by collectors and displayed with great pride. 

The form is characterized by a transparent glass base which is embedded with a network of crisscrossing threads of opaque glass, forming a lattice of diamond shaped pockets. Classically, white 'latimo' glass was used in a clear 'cristallo' base.The overall effect is reminiscent of fine lace or of fishing net, both of which are strongly evocative in Venetian culture. When executed in the classical technique of the island’s glassblowers, each diamond in the pattern contains a single bubble of trapped air, perfectly centered and uniformly sized. 

In general, the latticework theme in art traces to much earlier times. It is common in Hellenistic and Islamic art. To an extent it is a natural consequence of mosaic making. However the application of the pattern in glassblowing requires a completely different approach and a complex series of steps. First a ‘filigrana’ bubble is formed. To accomplish this, a series of pencil thin glass rods called ‘canes’ are laid side by side, touching each-other, in a pan ('piera' in Venetian, 'pietra'=stone in Italian) and partially fused together in the furnace.[1] Each cane is made of transparent glass with a core of opaque glass (a thread) running its entire length. Glass artist Emilio Santini writes:
On Murano, the "piera" is coated with a thin layer of clay from the laguna marshes. This is rich in salt and does not stick too much on the glass even if overheated. Then they preheat the, piera (called a 'plate' in the US) to dry the clay. While it is still hot but not scorching they lay down the canes so they are partially warmed before they go in the furnace. Remember that they are not annealed [and could easily shatter from thermal shock]. Then two little square metal pieces are placed at the two ends to hold the cane in place. These are called fereti (V) ferretti (I) . Some of these same terms are also used in the US by skillful glass blowers.[2]

On the end of an iron blowpipe, the fused mat of glass rods is wrapped around into a hollow cylinder. Next the open end is gently worked closed. This forms a sealed bubble of glass that can be manipulated by standard glassblowing techniques. Soft from the heat of the furnace, the glass can be given a twist so the parallel threads form a loose spiral. This piece is stored in a ‘garage’ kiln while a second bubble is formed in the same way but with the spiral running in the opposite direction. The first bubble is opened wide at one end, removed from its iron rod and placed in a cradle on the floor which holds it upright. The second bubble is carefully lowered into the open end of the first bubble. The glass artist stands above, with the second bubble inside the first and blows, inflating the inner glass until it comes in contact with the outer bubble.

Because both glass bubbles are formed with canes, they have a ribbed texture both inside and outside. When the two glass bubbles contact each other, the high-points of the ribs meet first, which is where the threads of the two bubbles cross. The valleys of the ribs are where one cane is fused to the next; these areas cross between the bubbles at the center of each diamond in the pattern. The two bubbles fuse together trapping air in the valleys. As the glass is worked and heated these regions form small, perfectly round air bubbles trapped inside the glass. You can see this for yourself by taking two or three fingers of each hand and crossing them against each other. Imagine the threads of opaque glass running down the center of each finger. The air bubbles are trapped where you can see light between your fingers.
Reticello ("fillacello") style flamework pendant,
by Adam Reetz 2015.

The distinctive diamond pattern of reticello has been successfully achieved with other glassmaking techniques. In flameworking, glass is manipulated using only a torch and handtools. The torch is fixed to a bench where the artist either stands or sits. In general, this is a more accessible technique because it does not require a glass furnace. Here, the reticello pattern is accomplished by starting with glass tubing. The crisscross pattern is ‘painted’ onto the outside of the tubing, one line at a time; with very thin rods of glass known as “stringers.” In one version, evenly spaced straight parallel lines are drawn along the length of the tube. The tube is then reheated in the flame and twisted. Next, a second set of lines are drawn twisting in the other direction, forming the diamonds. The ends of the tube are drawn down and one end can be mounted to a rod or tube of glass and further manipulated in the flame. 

In a testament to the continued popularity of reticello, a new colorful variation of the flamework technique has emerged within the past decade, among American flameworkers. [3] This has been playfully dubbed “fillacello.” After painting the fishnet pattern on tubing, and further working the glass, the individual diamonds are “filled” with various colors using stringers. The resulting effect recalls mosaics and the ancient inspirations of reticello.

[1] The iron tool used to move the 'piera' of filigrana cane in and out of the furnace is called a 'pasorale' (V), 'pastorale' (I) = pastoral, named after the staff carried by the pope. It consists of a straight rod with a U-shaped fork in the end. Thanks to Emilio Santini for his kind assistance with Muranese terminology and knowledge of hotshop technique.
[2] Private correspondence, March 2015. Here is a video of the reticello technique as executed by American glass artist Dante Maroni. http://www.youtube.com/watch?v=WJ3DDon23Lc
[3] My research point to the first examples of this technique emerging on the west coat of the United States around 2005-07. (Further information on its origin would be greatly appreciated).

Wednesday, January 2, 2019

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.