Lead Crystal

 

Roemer type drinking glass c. 1677,

George Ravenscroft.

The entire fourth part of Antonio Neri's 1612 book L'Arte Vetraria is devoted to the preparation of lead glass, a forerunner of what is now commonly known as lead crystal. This section is unique in the book in that it contains the only instance of the author giving direct advice to glass artists themselves:

"To work lead glass into various drinking glasses or other vessels, or even to draw cane for beadmaking, it is necessary to raise the punty [out of the melt], and to make a gather of glass by turning. Take it out, let it cool somewhat and then work it on a well-cleaned marble [marver]. The marble should be somewhat cool, and well bathed with water before use."

He goes on to describe what might be termed a kind of dance with the glass. As with a human partner, gentle patience is required in learning the boundaries of what can and cannot be done. Ultimately, an artist must come to understand the material's behavior and personality in order to result in a great partnership. For the artist who makes unrealistic demands, glass can be a heartbreaker.  

"This sort of glass, lead glass, is so runny that were it not cooled, and taken up by turning [the punty] to wind a gather, it would be impossible to work. It is so runny that it would not even hold onto the punty, because it is as loose as soup. This arises out of [the fact that] the lead calx causes it to become very fluid."

"Namely, gather the glass little by little, allow it to cool, and work it over marble frequently bathed in water. Furthermore, make sure to keep the pot of glass rather calm, and in a place in the furnace where it will not see too much heat, otherwise it will not be possible to work this glass at all."

The formulation of lead crystal as we know it is a relatively recent development. This is a composition of crushed silica (sand or quartz), potash (potassium carbonates) and lead oxide substituting for calcium to stabilize the composition. It is also true that lead has been added to glass since its invention a few thousand years ago. It is not clear that this addition was always intentional, but a Babylonian tablet of 1700 BCE gives a recipe for pottery glaze that explicitly contains lead. At some point, a discovery showed that small amounts of lead and pigment smeared on glass and fired made stained glass paintings possible. The earliest known examples of colored stained glass windows date to 800-820 (San Vicenzo Abbey excavation in Volturno, Italy.)  In medieval Europe, leading up to Antonio Neri's time, lead glass was used in mosaic tesserae and in artificial gems.

Finally, it is worth noting that Neri's childhood church in Florence, Cestello (now called Santa Maria Maddalena dei Pazzi), was then run by Cistercian monks. It was the Cistercian luminary St. Bernard of Clairvaux who, in the twelfth century, built the first church with large windows, urging, "The soul shall seek the light by following the light."

This post first appeared here 15 November 2013.

Fabergé and Purpurine

 

Fabergé c.1900. Purpurine cherries,

nephrite leaves, gold stalk, rock crystal pot.

Peter Carl Fabergé is known the world over for producing elaborate jeweled fantasy eggs for the Russian royal family in the late nineteenth and early twentieth centuries. [1] The artisans of his firm made use of a wide variety of exotic and precious materials in the execution of their commissions and later in items made available to the general public. Among the most exotic and sought after were objects made with an opaque bright red stone-like material known as ‘purpurine’. This was a glass based concoction whose composition was kept a tightly guarded secret. In fact, it was so tightly guarded that the formula was subsequently lost. Purpurine was typically cast into blocks which were then sawed and carved using traditional lapidary and gem carving techniques. The final appearance was of an unknown exotic mineral.

Red Glass Beads, 1st cent. BCE, Tissamaharama, Sri Lanka

The origin story of purpurine begins much earlier than Fabregé, in fact, not hundreds but thousands of years earlier. “The art of making this type of glass seems to have originated in India; glass beads of a similar material have been found in the Indus Valley and were dated to the late 2nd millennium BCE.” [2] In southern Sri Lanka deep red opaque glass beads have been found dating to the first millennium BCE; these turn out to be closely related to purpurine, through a long glassmaking tradition. [3] A version of the bright red glass was made in the Egyptian- Roman era. The first century CE historian Pliny the Elder noted that in Greek it was called ‘haematinon’ or blood-red ware. [4] He implied that this specialty glass, was routinely produced in Rome and indeed archaeologists have recovered numerous examples. The glass was used in a wide range of applications from dinner plates to pieces of elaborate mosaics. Eventually, though, the method of making haematinon was forgotten and remained so for several hundred years.

A small (1cm) Medusa's head in
opaque red glass c.1st. cent. CE.

The Renaissance era had been marked by a strong motivation to recover lost knowledge of the ancient world, but many challenges were beyond the technology of the time. However, attempts were made that eventually led to success.  In the second half of the 16th century, Pope Gregory XIII instituted the Vatican mosaic studio to decorate the new Saint Peter’s Basilica, begun by Pope Julius II in 1506. As an aside, this workshop continues today, repairing and conserving the ceilings of St. Peter’s. [5] Having quickly exhausted local talent in Rome, Gregory brought in Venetian masters to teach the art. With the mandate to make the mosaics appear as if painted, the studio developed many new formulations for the glass tesserae – the individual tiles used to form mosaics. It was in this environment that the deep red purpurine [Ital: porporino] was eventually rediscovered.

C. 1st cent. BCE/CE Roman bowl (patella cup) in
red opaque glass (haematinon).

It is still an open question whether the secret was discovered in Rome or brought there. There is evidence that the fabled red glass was being produced in Venice in the eighteenth century and possibly earlier.[6] One (Swedish) source credits Vatican studio employee Alessio Matteoli, in the 1700s, when he oversaw the development of many new colors. In the early 1800s, interest rekindled in the ancient material and by then, analytical methods were up to the task of finding their composition. German chemist, Martin Heinrich Klaproth, analyzed haematinon from the Villa Jovis, a first century palace built by Roman emperor Tiberius on the island of Capri in southern Italy. [7] He correctly found copper, but incorrectly assumed the material was glassy slag, a byproduct of the smelting process. Later, in 1844, Schubarth did further work indicating haematinon was, in fact, a true glass. [8] King Ludwig I of Bavaria intended to build a reconstruction of a Pompeian villa for educational purposes. He assigned Max Joseph von Pettenkofer to the task of rediscovering the method of manufacturing the antique “blood glass,” and the young chemist reported success in 1853. [9] His process fused standard alkali-lead glass with copper oxide and magnetite in the presence of small amounts of magnesium oxide and carbon, followed by very slow cooling of the resultant mass, which would then take on a deep red color.

Roman Mosaicist
Michelangelo Barberi, 1809.

Other sources name one of two students of the famous Roman mosaicist Michelangelo Barberi (1787–1867). Barberi had a long standing relationship with the Russian royal family and accepted Russian pupils at his studio in Rome, he even set up a mosaic shop in St. Petersburg at the request of Tsar Nicholas I. [10] In 1846, these two pupils of Barberi, brothers Giustiniano and Leopold Bonafede were called to St. Petersburg by the tsar to work for the royal court. Giustiniano (1825-66) had served as head chemist at the Vatican studio and both would attain that post for the tsar at the Russian Imperial Glassworks. It is Leopold (1833-78) who is now most often credited with the invention of purpurine as a recreation of the fabled haematinon. His formulation was based on a standard potash lead crystal.

Purpurine taza made at the Russian
Imperial Glassworks, c.1867.
(Shown at Paris Exposition)

The first documented uses in objects of art were five entries of the Imperial Glassworks at the Paris Exposition Universelle in 1867, for which the glassworks was awarded gold medal status. [11] “After Bonafede's death in 1878, purpurine continued to be made at the factory under the direction of the chief chemist, S. P. Petuchov.” [12]

In 1882, after considerable training and apprentice work, which began when he was a teenager, a 46-year-old Peter Carl Fabergé fully assumed control of his father’s small jewelry shop in St. Petersburg. Within a short time, he was supplying the royal family with his exquisite eggs and many other items made by a growing assemblage of master craftsmen. The first use of purpurine by the Fabergé shop occurs early in Carl’s tenure, perhaps as early as 1880. Initially, they use material supplied by Petuchov at the Imperial Glassworks. Over a period of years, though, the Fabergé shop developed its own recipe based on soda lead glass, more similar in composition to the ancient samples of haematinon.[13]  Other isolated examples of purpurine are known to exist made by competitive jewelers of the time, but no documented recipe has been found. [14] Apparently, Petuchov took the Imperial Glassworks formula for purpurine to his grave. As fame grew for Fabergé, their version is the one that became familiar to a growing clientele in Great Britain and in the United States. When the February Revolution of 1917 brought an end to the Romanov dynasty in Russia, Carl Fabergé fled the country, his company disbanded. In the west, the Fabergé name only multiplied in prestige among the elite and wealthy and items made with purpurine continue to command stratospheric prices.

Significant analytical work has been done on the ancient haematinon as well as purpurines of the Imperial Glassworks and of Fabergé. [3,5] The technical differences could easily be the subject of a separate treatment; suffice it to say that knowing the composition of a glass is not the same as knowing the recipe. (Just as knowing the composition of a cake does not mean that one can bake it.) The exact method for making the glass must have involved a long period in which snowflake-like crystals of cuprous oxide (Cu2O) would be encouraged to form, grow and spread throughout the glass forming a tightly interlocking network in the glass. One interesting point is that unlike many other opaque glasses, the ingredients of purpurine do not include a discrete opacifier; it is a clear glass base, which is so loaded with deep red cuprous oxide crystals that light does not pass through even small or thin pieces of the material. Another point is that this glass was not suitable for blowing on a blowpipe and therefore did not take forms typically expected for glass. Perhaps because of this, it has been largely overlooked.

The history of purpurine is a reminder of the fragility of human knowledge; it was discovered in ancient times, lost, rediscovered and lost again in modern times.


[1] Peter Carl Fabergé =Карл Густавович Фаберже. For more, see Abraham Kenneth Snowman, The Art of Carl Fabergé, Faber & Faber, 1974.(original ed 1953). Also see
 https://en.wikipedia.org/wiki/Peter_Carl_Faberg%C3%A9
[2] Gowlett, J.A.J.: High Definition Archaeology: Threads Through the Past, Routledge, 1997, pp. 276–277. Quoted in https://en.wikipedia.org/wiki/Purpurin_(glass)
[3] Rösch, Cordelia; Hock, Rainer; Schüssler, Ulrich; Yule, Paul; Hannibal, Anne. “Electron Microprobe Analysis and X-ray Diffraction Methods in Archaeometry: Investigations on Pre-Islamic Beads from the Sultanate of Oman” in: European Journal of Mineralogy, 9 (1997), 763–783. (Specifically, beads found at Tissamaharama, pp. 771,772). http://archiv.ub.uni-heidelberg.de/propylaeumdok/volltexte/2009/305
[4] Natural History, xxxvi, LXVII, 198.
[5] For more, see http://www.30giorni.it/articoli_id_10283_l3.htm
[6] RR Harding, S Hornytzkyj, A. R. Date. “The composition of an opaque red glass used by Fabergé”in the Journal of Gemmology, 1989. No.5, pp. 275-287.
[7] Klaproth M.H., Beiträge zur chemischen Kenntnis der Mineralkörper Vol. VI (1815), p. 136
[8] Schubarth. "Einige Notizen über rothes und blaues Glas." Journal für Praktische Chemie Vol. 3 (1844), pp. 300-316
[9] Pettenkofer, M. "Ueber einen antiken rothen Glasfluss (Haematinon) und über Aventurin-Glas." Abhandlungen der naturw.-techn. Commission der k. b. Akad. der Wissensch. I. Bd. München, literar.-artist. Anstalt, 1856. Also see https://en.wikipedia.org/wiki/Purpurin_(glass)
[10] Alessio Matteoli https://nononsensejewellery.wordpress.com/tag/purpurin-faberge/ , for more on Matteoli see http://www.aiellomosaics.com/about-mosaics/techniques-and-materials/roman-or-byzantineglass-or-marble-tilesmicromosaic-or-glass-enamels/ .  On Michelangelo Barberi, see Renata Battaglini Di Stasio, “Michelangelo Barberi” in Dizionario Biografico degli Italiani – v. 6 (1964)  http://www.treccani.it/enciclopedia/michelangelo-barberi_(Dizionario_Biografico)/
[11] Catalogue Special de la Section Russe a l'Exposition Universelle de Paris en 1867, p. 44, Classe 16, no.111.
[12] See http://art.thewalters.org/detail/77444/pair-of-tazzas/
[13] Op cit. RR Harding, S Hornytzkyj, A. R. Date, 1989.
[14] For more on competitive jeweler’s purpurine, see: Géza von Hapsburg: “Some of Fabergé’s Other Russian Competitors” in Fabergé, Imperial Craftsman and His World, London: Booth-Clibborn, 2000, pp. 323-325.

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. Watch master Dante Maroni at work in this video  http://www.youtube.com/watch?v=WJ3DDon23Lc

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).

Lead Crystal

Roemer type drinking glass c. 1677,

George Ravenscroft.

The entire fourth part of Antonio Neri's 1612 book L'Arte Vetraria is devoted to the preparation of lead glass, a forerunner of what is now commonly known as lead crystal. This section is unique in the book in that it contains the only instance of the author giving direct advice to glass artists themselves:

"To work lead glass into various drinking glasses or other vessels, or even to draw cane for beadmaking, it is necessary to raise the punty [out of the melt], and to make a gather of glass by turning. Take it out, let it cool somewhat and then work it on a well-cleaned marble [marver]. The marble should be somewhat cool, and well bathed with water before use."

He goes on to describe what might be termed a kind of dance with the glass. As with a human partner, gentle patience is required in learning the boundaries of what can and cannot be done. Ultimately, an artist must come to understand the material's behavior and personality in order to result in a great partnership. For the artist who makes unrealistic demands, glass can be a heartbreaker.  

"This sort of glass, lead glass, is so runny that were it not cooled, and taken up by turning [the punty] to wind a gather, it would be impossible to work. It is so runny that it would not even hold onto the punty, because it is as loose as soup. This arises out of [the fact that] the lead calx causes it to become very fluid."

"Namely, gather the glass little by little, allow it to cool, and work it over marble frequently bathed in water. Furthermore, make sure to keep the pot of glass rather calm, and in a place in the furnace where it will not see too much heat, otherwise it will not be possible to work this glass at all."

The formulation of lead crystal as we know it is a relatively recent development. This is a composition of crushed silica (sand or quartz), potash (potassium carbonates) and lead oxide substituting for calcium to stabilize the composition. It is also true that lead has been added to glass since its invention a few thousand years ago. It is not clear that this addition was always intentional, but a Babylonian tablet of 1700 BCE gives a recipe for pottery glaze that explicitly contains lead. At some point, a discovery showed that small amounts of lead and pigment smeared on glass and fired made stained glass paintings possible. The earliest known examples of colored stained glass windows date to 800-820 (San Vicenzo Abbey excavation in Volturno, Italy.)  In medieval Europe, leading up to Antonio Neri's time, lead glass was used in mosaic tesserae and in artificial gems.

Finally, it is worth noting that Neri's childhood church in Florence, Cestello (now called Santa Maria Maddalena dei Pazzi), was then run by Cistercian monks. It was the Cistercian luminary St. Bernard of Clairvaux who, in the twelfth century, built the first church with large windows, urging, "The soul shall seek the light by following the light."

This post first appeared here 15 November 2013.

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).

Lead Crystal

Roemer type drinking glass c. 1677,

George Ravenscroft.

The entire fourth part of Antonio Neri's 1612 book L'Arte Vetraria is devoted to the preparation of lead glass, a forerunner of what is now commonly known as lead crystal. This section is unique in the book in that it contains the only instance of the author giving direct advice to glass artists themselves:

"To work lead glass into various drinking glasses or other vessels, or even to draw cane for beadmaking, it is necessary to raise the punty [out of the melt], and to make a gather of glass by turning. Take it out, let it cool somewhat and then work it on a well-cleaned marble [marver]. The marble should be somewhat cool, and well bathed with water before use."

He goes on to describe what might be termed a kind of dance with the glass. As with a human partner, gentle patience is required in learning the boundaries of what can and cannot be done. Ultimately, an artist must come to understand the material's behavior and personality in order to result in a great partnership. For the artist who makes unrealistic demands, glass can be a heartbreaker.  

"This sort of glass, lead glass, is so runny that were it not cooled, and taken up by turning [the punty] to wind a gather, it would be impossible to work. It is so runny that it would not even hold onto the punty, because it is as loose as soup. This arises out of [the fact that] the lead calx causes it to become very fluid."

"Namely, gather the glass little by little, allow it to cool, and work it over marble frequently bathed in water. Furthermore, make sure to keep the pot of glass rather calm, and in a place in the furnace where it will not see too much heat, otherwise it will not be possible to work this glass at all."

The formulation of lead crystal as we know it is a relatively recent development. This is a composition of crushed silica (sand or quartz), potash (potassium carbonates) and lead oxide substituting for calcium to stabilize the composition. It is also true that lead has been added to glass since its invention a few thousand years ago. It is not clear that this addition was always intentional, but a Babylonian tablet of 1700 BCE gives a recipe for pottery glaze that explicitly contains lead. At some point, a discovery showed that small amounts of lead and pigment smeared on glass and fired made stained glass paintings possible. The earliest known examples of colored stained glass windows date to 800-820 (San Vicenzo Abbey excavation in Volturno, Italy.)  In medieval Europe, leading up to Antonio Neri's time, lead glass was used in mosaic tesserae and in artificial gems.

Finally, it is worth noting that Neri's childhood church in Florence, Cestello (now called Santa Maria Maddalena dei Pazzi), was then run by Cistercian monks. It was the Cistercian luminary St. Bernard of Clairvaux who, in the twelfth century, built the first church with large windows, urging, "The soul shall seek the light by following the light."

This post first appeared here 15 November 2013.

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).

Fabergé and Purpurine

Fabergé c.1900. Purpurine cherries,

nephrite leaves, gold stalk, rock crystal pot.

Peter Carl Fabergé is known the world over for producing elaborate jeweled fantasy eggs for the Russian royal family in the late nineteenth and early twentieth centuries. [1] The artisans of his firm made use of a wide variety of exotic and precious materials in the execution of their commissions and later in items made available to the general public. Among the most exotic and sought after were objects made with an opaque bright red stone-like material known as ‘purpurine’. This was a glass based concoction whose composition was kept a tightly guarded secret. In fact, it was so tightly guarded that the formula was subsequently lost. Purpurine was typically cast into blocks which were then sawed and carved using traditional lapidary and gem carving techniques. The final appearance was of an unknown exotic mineral.

Red Glass Beads, 1st cent. BCE, Tissamaharama, Sri Lanka

The origin story of purpurine begins much earlier than Fabregé, in fact, not hundreds but thousands of years earlier. “The art of making this type of glass seems to have originated in India; glass beads of a similar material have been found in the Indus Valley and were dated to the late 2nd millennium BCE.” [2] In southern Sri Lanka deep red opaque glass beads have been found dating to the first millennium BCE; these turn out to be closely related to purpurine, through a long glassmaking tradition. [3] A version of the bright red glass was made in the Egyptian- Roman era. The first century CE historian Pliny the Elder noted that in Greek it was called ‘haematinon’ or blood-red ware. [4] He implied that this specialty glass, was routinely produced in Rome and indeed archaeologists have recovered numerous examples. The glass was used in a wide range of applications from dinner plates to pieces of elaborate mosaics. Eventually, though, the method of making haematinon was forgotten and remained so for several hundred years.

A small (1cm) Medusa's head in
opaque red glass c.1st. cent. CE.

The Renaissance era had been marked by a strong motivation to recover lost knowledge of the ancient world, but many challenges were beyond the technology of the time. However, attempts were made that eventually led to success.  In the second half of the 16th century, Pope Gregory XIII instituted the Vatican mosaic studio to decorate the new Saint Peter’s Basilica, begun by Pope Julius II in 1506. As an aside, this workshop continues today, repairing and conserving the ceilings of St. Peter’s. [5] Having quickly exhausted local talent in Rome, Gregory brought in Venetian masters to teach the art. With the mandate to make the mosaics appear as if painted, the studio developed many new formulations for the glass tesserae – the individual tiles used to form mosaics. It was in this environment that the deep red purpurine [Ital: porporino] was eventually rediscovered.

C. 1st cent. BCE/CE Roman bowl (patella cup) in
red opaque glass (haematinon).

It is still an open question whether the secret was discovered in Rome or brought there. There is evidence that the fabled red glass was being produced in Venice in the eighteenth century and possibly earlier.[6] One (Swedish) source credits Vatican studio employee Alessio Matteoli, in the 1700s, when he oversaw the development of many new colors. In the early 1800s, interest rekindled in the ancient material and by then, analytical methods were up to the task of finding their composition. German chemist, Martin Heinrich Klaproth, analyzed haematinon from the Villa Jovis, a first century palace built by Roman emperor Tiberius on the island of Capri in southern Italy. [7] He correctly found copper, but incorrectly assumed the material was glassy slag, a byproduct of the smelting process. Later, in 1844, Schubarth did further work indicating haematinon was, in fact, a true glass. [8] King Ludwig I of Bavaria intended to build a reconstruction of a Pompeian villa for educational purposes. He assigned Max Joseph von Pettenkofer to the task of rediscovering the method of manufacturing the antique “blood glass,” and the young chemist reported success in 1853. [9] His process fused standard alkali-lead glass with copper oxide and magnetite in the presence of small amounts of magnesium oxide and carbon, followed by very slow cooling of the resultant mass, which would then take on a deep red color.

Roman Mosaicist
Michelangelo Barberi, 1809.

Other sources name one of two students of the famous Roman mosaicist Michelangelo Barberi (1787–1867). Barberi had a long standing relationship with the Russian royal family and accepted Russian pupils at his studio in Rome, he even set up a mosaic shop in St. Petersburg at the request of Tsar Nicholas I. [10] In 1846, these two pupils of Barberi, brothers Giustiniano and Leopold Bonafede were called to St. Petersburg by the tsar to work for the royal court. Giustiniano (1825-66) had served as head chemist at the Vatican studio and both would attain that post for the tsar at the Russian Imperial Glassworks. It is Leopold (1833-78) who is now most often credited with the invention of purpurine as a recreation of the fabled haematinon. His formulation was based on a standard potash lead crystal.

Purpurine taza made at the Russian
Imperial Glassworks, c.1867.
(Shown at Paris Exposition)

The first documented uses in objects of art were five entries of the Imperial Glassworks at the Paris Exposition Universelle in 1867, for which the glassworks was awarded gold medal status. [11] “After Bonafede's death in 1878, purpurine continued to be made at the factory under the direction of the chief chemist, S. P. Petuchov.” [12]

In 1882, after considerable training and apprentice work, which began when he was a teenager, a 46-year-old Peter Carl Fabergé fully assumed control of his father’s small jewelry shop in St. Petersburg. Within a short time, he was supplying the royal family with his exquisite eggs and many other items made by a growing assemblage of master craftsmen. The first use of purpurine by the Fabergé shop occurs early in Carl’s tenure, perhaps as early as 1880. Initially, they use material supplied by Petuchov at the Imperial Glassworks. Over a period of years, though, the Fabergé shop developed its own recipe based on soda lead glass, more similar in composition to the ancient samples of haematinon.[13]  Other isolated examples of purpurine are known to exist made by competitive jewelers of the time, but no documented recipe has been found. [14] Apparently, Petuchov took the Imperial Glassworks formula for purpurine to his grave. As fame grew for Fabergé, their version is the one that became familiar to a growing clientele in Great Britain and in the United States. When the February Revolution of 1917 brought an end to the Romanov dynasty in Russia, Carl Fabergé fled the country, his company disbanded. In the west, the Fabergé name only multiplied in prestige among the elite and wealthy and items made with purpurine continue to command stratospheric prices.

Significant analytical work has been done on the ancient haematinon as well as purpurines of the Imperial Glassworks and of Fabergé. [3,5] The technical differences could easily be the subject of a separate treatment; suffice it to say that knowing the composition of a glass is not the same as knowing the recipe. (Just as knowing the composition of a cake does not mean that one can bake it.) The exact method for making the glass must have involved a long period in which snowflake-like crystals of cuprous oxide (Cu2O) would be encouraged to form, grow and spread throughout the glass forming a tightly interlocking network in the glass. One interesting point is that unlike many other opaque glasses, the ingredients of purpurine do not include a discrete opacifier; it is a clear glass base, which is so loaded with deep red cuprous oxide crystals that light does not pass through even small or thin pieces of the material. Another point is that this glass was not suitable for blowing on a blowpipe and therefore did not take forms typically expected for glass. Perhaps because of this, it has been largely overlooked.

The history of purpurine is a reminder of the fragility of human knowledge; it was discovered in ancient times, lost, rediscovered and lost again in modern times.


[1] Peter Carl Fabergé =Карл Густавович Фаберже. For more, see Abraham Kenneth Snowman, The Art of Carl Fabergé, Faber & Faber, 1974.(original ed 1953). Also see
 https://en.wikipedia.org/wiki/Peter_Carl_Faberg%C3%A9
[2] Gowlett, J.A.J.: High Definition Archaeology: Threads Through the Past, Routledge, 1997, pp. 276–277. Quoted in https://en.wikipedia.org/wiki/Purpurin_(glass)
[3] Rösch, Cordelia; Hock, Rainer; Schüssler, Ulrich; Yule, Paul; Hannibal, Anne. “Electron Microprobe Analysis and X-ray Diffraction Methods in Archaeometry: Investigations on Pre-Islamic Beads from the Sultanate of Oman” in: European Journal of Mineralogy, 9 (1997), 763–783. (Specifically, beads found at Tissamaharama, pp. 771,772). http://archiv.ub.uni-heidelberg.de/propylaeumdok/volltexte/2009/305
[4] Natural History, xxxvi, LXVII, 198.
[5] For more, see http://www.30giorni.it/articoli_id_10283_l3.htm
[6] RR Harding, S Hornytzkyj, A. R. Date. “The composition of an opaque red glass used by Fabergé”in the Journal of Gemmology, 1989. No.5, pp. 275-287.
[7] Klaproth M.H., Beiträge zur chemischen Kenntnis der Mineralkörper Vol. VI (1815), p. 136
[8] Schubarth. "Einige Notizen über rothes und blaues Glas." Journal für Praktische Chemie Vol. 3 (1844), pp. 300-316
[9] Pettenkofer, M. "Ueber einen antiken rothen Glasfluss (Haematinon) und über Aventurin-Glas." Abhandlungen der naturw.-techn. Commission der k. b. Akad. der Wissensch. I. Bd. München, literar.-artist. Anstalt, 1856. Also see https://en.wikipedia.org/wiki/Purpurin_(glass)
[10] Alessio Matteoli https://nononsensejewellery.wordpress.com/tag/purpurin-faberge/ , for more on Matteoli see http://www.aiellomosaics.com/about-mosaics/techniques-and-materials/roman-or-byzantineglass-or-marble-tilesmicromosaic-or-glass-enamels/ .  On Michelangelo Barberi, see Renata Battaglini Di Stasio, “Michelangelo Barberi” in Dizionario Biografico degli Italiani – v. 6 (1964)  http://www.treccani.it/enciclopedia/michelangelo-barberi_(Dizionario_Biografico)/
[11] Catalogue Special de la Section Russe a l'Exposition Universelle de Paris en 1867, p. 44, Classe 16, no.111.
[12] See http://art.thewalters.org/detail/77444/pair-of-tazzas/
[13] Op cit. RR Harding, S Hornytzkyj, A. R. Date, 1989.
[14] For more on competitive jeweler’s purpurine, see: Géza von Hapsburg: “Some of Fabergé’s Other Russian Competitors” in Fabergé, Imperial Craftsman and His World, London: Booth-Clibborn, 2000, pp. 323-325.

Lead Crystal

Roemer type drinking glass c. 1677,

George Ravenscroft.

The entire fourth part of Antonio Neri's book L'Arte Vetraria is devoted to the preparation of lead glass, a forerunner of what is now commonly known as lead crystal. This section is unique in the book in that it contains the only instance of the author giving direct advice to glass artists themselves:

"To work lead glass into various drinking glasses or other vessels, or even to draw cane for beadmaking, it is necessary to raise the punty [out of the melt], and to make a gather of glass by turning. Take it out, let it cool somewhat and then work it on a well-cleaned marble [marver]. The marble should be somewhat cool, and well bathed with water before use."

He goes on to describe what might be termed a kind of dance with the glass. As with a human partner, gentle patience is required in learning the boundaries of what can and cannot be done. Ultimately, an artist must come to understand the material's behavior and personality in order to result in a great partnership. For the artist who makes unrealistic demands, glass can be a heartbreaker.  

"This sort of glass, lead glass, is so runny that were it not cooled, and taken up by turning [the punty] to wind a gather, it would be impossible to work. It is so runny that it would not even hold onto the punty, because it is as loose as soup. This arises out of [the fact that] the lead calx causes it to become very fluid."

"Namely, gather the glass little by little, allow it to cool, and work it over marble frequently bathed in water. Furthermore, make sure to keep the pot of glass rather calm, and in a place in the furnace where it will not see too much heat, otherwise it will not be possible to work this glass at all."

The formulation of lead crystal as we know it is a relatively recent development. This is a composition of crushed silica (sand or quartz), potash (potassium carbonates) and lead oxide substituting for calcium to stabilize the composition. It is also true that lead has been added to glass since its invention a few thousand years ago. It is not clear that this addition was always intentional, but a Babylonian tablet of 1700 BCE gives a recipe for pottery glaze that explicitly contains lead. At some point, a discovery showed that small amounts of lead and pigment smeared on glass and fired made stained glass paintings possible. The earliest known examples of colored stained glass windows date to 800-820 (San Vicenzo Abbey excavation in Volturno, Italy.)  In medieval Europe, leading up to Antonio Neri's time, lead glass was used in mosaic tesserae and in artificial gems.

Finally, it is worth noting that Neri's childhood church in Florence, Cestello (now called Santa Maria Maddalena dei Pazzi), was then run by Cistercian monks. It was the Cistercian luminary St. Bernard of Clairvaux who, in the twelfth century, built the first church with large windows, urging, "The soul shall seek the light by following the light."

This post first appeared here 15 November 2013.

Baptism of an Alchemist

"Florence - Church of San Giovanni, the Baptistry",

Photo: Giacomo Brogi (1822-1881).

The San Giovanni Baptistery stands in the heart of Florence, directly in front of the city's most famous structure, Santa Maria del Fiore Cathedral, also known as The Duomo. The church and its dome are so large that they dwarf the baptistery by comparison. Nevertheless, a simple look inside its cavernous, octagonal space easily shatters this illusion. In Antonio Neri's time, this place was a civic hub; a space where masons and tailors rubbed shoulders with nobles and princes. One of the oldest structures in the region, it was also one of most cherished, clad in polychrome marble, with imposing bronze doors intricately sculpted by masters. Michelangelo dubbed them "the gates of heaven." It was here, in this ancient basilica that the future of Tuscany could be seen plainly, in the eyes of its youth. New generations of Florentines were welcomed into the world by their neighbors, anointed and christened, as they had been for centuries. The current structure dates to the eleventh century, but it replaces octagonal baptisteries built on the same spot as early as the fourth century, originally surrounded by a cemetery.

Glass Mosaic ceiling of San Giovanni
(Click to enlarge)

Antonio Neri was baptized here, as were all his brothers and sisters, his parents, some grandparents and quite possibly some much earlier relatives. His sponsor, Prince Don Antonio and his Medici ancestors were also given rights at San Giovanni, in the same octagonal marble bath at its center. The unusual font was a sister to the one still standing in the identically named baptistery in Pisa. Along each side, there was a dry-well. By standing in it, a priest could perform the ceremony and avoid jostling by the crowd. Legend tells that one of the stations showed the repair made after poet Dante Alighieri took an axe to a well in order to free a child who had become entangled and was at risk to drown. The font was replaced later in the same year of Antonio Neri’s birth, 1576, on orders of the Grand Duke, in preparation for a royal baptism.

The ancient mosaic tile floor, although repaired many times, still shows its original signs of the zodiac and other early Christian iconography. As a newborn infant, tightly wrapped in swaddling, Antonio Neri would have been carried across that floor, busy with families and children, and then gently handed to the priest, perhaps by his father, already a famous physician. Joining them would be his grandfather the barber surgeon, his other grandfather the lawyer to Michelangelo, his godparents, other family, friends and perhaps a wet nurse. His mother stayed at home with her close friends to recuperate from the ordeal of birth, and to prepare for a neighborhood celebration. Even in the dead of winter, if Antonio Neri's eyes were open, the sparkling glass mosaics covering the entire domed ceiling of San Giovanni could not have failed to catch the wandering eyes of this future glassmaker.

*This post first appeared here in a slightly different form on 11 Nov 2013.