Aventurine Glass

 

Small amphora in aventurine glass,

Murano, Salviati.

With all its glitz and sparkle, aventurine (avventurina) stands out as a flamboyant extrovert among the varieties of glass. Developed and perfected on the Venetian island of Murano, also known as 'goldstone', it consists of a transparent base glass with myriad reflective crystalline "spangles" running throughout. The classical version is a deep golden brown with crystallites composed mainly of metallic copper, with a few related compounds as supporting cast. However, numerous colors have been developed, including red, orange, yellow, green, blue, violet, black and white. 

Folklore holds that aventurine was discovered by accident ("a venturi") when unknown monks inadvertently dropped copper or brass shavings into a glass melt as early as the thirteenth century. [1] However, more thorough investigations have recently identified 1620 as a likely date for the first appearance of aventurine glass. [2] No example or written account has been found that dates prior to the seventeenth century. An alternate story accounts for the name aventurine being derived from 'adventure'; referring to the difficulty and uncertainty involved in its production.[3] At first, the formula was a closely held secret among a few glassmakers and subsequently it was lost and then rediscovered not once but  twice.

To complicate matters, natural minerals with a similar appearance were named after the glass, leading to the misconception that they were also discovered after the glass was invented. This is clearly not the case. Early examples of mineral aventurine artifacts date to the Neolithic era [4] and can be found throughout history. First century Roman writer Pliny mentions a type of stone with silvery flecks, a passage that was well known when the glass was developed.  The compositions of these minerals were also identified early; either species of quartz that contain flecks of mica, or a type of feldspar (sunstone).[5] Mineral aventurine turns up as the eyes of Greek statues, in stonework mosaics and later in the 'pietre dure' art perfected by the Medici artisans in Florence around the time Antonio Neri started making glass there. The chances are good that examples of the mineral were known to Neri as well as to the glassmakers on Murano, but a recipe for the glass version does not turn up in Neri’s 1612 book; he was apparently too early by a decade.  

The story of aventurine's accidental discovery by monks may well be apocryphal; nevertheless, it is a great entrée to understanding how the formulation works. First, contrary to what the story implies, aventurine is not the result of dumping metallic confetti into glass. The reflective "spangles" (as early researchers were fond of calling them) are actually uniformly sized, mirror-like crystals that are grown in the glass. In truth, the formula is quite similar to recipes already in use by Neri and others; the difference was in proportions and in how the glass was treated after it was in the furnace. The formula for aventurine calls for the addition of copper, iron and tin oxides, to a base that was a hybrid of soda, potash and lead glass. Neri’s recipe #128 is titled "A Proven Way to Make Rosichiero" [6] and provides for all of these ingredients, albeit in lower concentrations. Rosichiero was a transparent tawny red colored glass that was a staple of furnaces throughout Italy. 

The secret to producing the reflective "spangles" was to mix the glass and heat it in the furnace in a normal way, but then to slowly reduce the heat while creating a low oxygen “reducing” atmosphere. The furnace draught was shut; the glass pot was fitted with a tight lid and then covered with ashes and allowed to cool very slowly.  

Initially, the batch is saturated with copper oxide. This means the glass has dissolved as much copper, iron and tin as it can and any further addition of these powders will simply float to the bottom of the pot.  The exact amount of powdered metals able to dissolve is a function of temperature; the hotter the glass the more that will dissolve and the cooler the glass the less that will dissolve. The key concept here is that as the glass slowly cools, the metals start to come out of solution and crystals start to form. There is some complex chemistry happening at the same time; the reducing atmosphere encourage the metals to stay in a pure un-oxidized form,  Furthermore any oxygen or sulfur  that happens to be present will preferentially combine with the iron, leaving the copper crystals pristine. Once cooled to room temperature, a successful batch would be broken away from the glass pot by workers and divided into smaller pieces. Glass artisans wanting to incorporate the aventurine into their work needed to work quickly. They carefully reheated an appropriate nugget and coated (encase) it in a layer of clear glass; once molten, direct exposure to the air would destroy the glittery effect. 

Over time, it was discovered that various colors could be produced with the addition of different chemicals, but the central principal of growing tiny metallic crystals is the same.


[1]  The earliest instance of this story in print that I can find is fairly late;  Faustino Corsi, Delle pietre antiche: libri quattro (Rome: Salviuccio e figlio, 1828)  pp. 166-167.  
[2] Cesare Moretti (†), Bernard Gratuze and Sandro Hreglich,  “Le verre aventurine (‘ avventurina ‘) : son histoire, les recettes, les analyses, sa fabrication”, ArcheoSciences, 37 | 2013, 135-154.
[3] For instance see  Giulio Salviati, “Venetian Glass” Journal of the Society of Arts (Proceedings), Volume 37 (7 June,1889), p. 630
[4] Neolithic Quartz Aventurine Pendant - 7 Cm/ 2. 76 ", green - 6500 To 2000 Bp – Sahara. Item Id: 106549,  Weight: 83 gm. Sahara - Mauritania - Tagant country.
http://ancientpoint.com/inf/106549-neolithic_quartz_aventurine_pendant___7_cm_2___76____6500_to_2000_bp___sahara.html
[5] Dizionario del cittadino, o sia Ristretto storico, teorico e ..., Volume 1 pp. 38-39.
[6] Antonio Neri, L'Arte Vetraria (Firenze: Giunti, 1612).
[7] Sauzay, A. (1870) Marvels of Glassmaking in All Ages. London, 1870 pp. 173 - 175.

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.

Aventurine Glass

Small amphora in aventurine glass,

Murano, Salviati.

With all its glitz and sparkle, aventurine (avventurina) stands out as a flamboyant extrovert among the varieties of glass. Developed and perfected on the Venetian island of Murano, also known as 'goldstone', it consists of a transparent base glass with myriad reflective crystalline "spangles" running throughout. The classical version is a deep golden brown with crystallites composed mainly of metallic copper, with a few related compounds as supporting cast. However, numerous colors have been developed, including red, orange, yellow, green, blue, violet, black and white. 

Folklore holds that aventurine was discovered by accident ("a venturi") when unknown monks inadvertently dropped copper or brass shavings into a glass melt as early as the thirteenth century. [1] However, more thorough investigations have recently identified 1620 as a likely date for the first appearance of aventurine glass. [2] No example or written account has been found that dates prior to the seventeenth century. An alternate story accounts for the name aventurine being derived from 'adventure'; referring to the difficulty and uncertainty involved in its production.[3] At first, the formula was a closely held secret among a few glassmakers and subsequently it was lost and then rediscovered not once but  twice.

To complicate matters, natural minerals with a similar appearance were named after the glass, leading to the misconception that they were also discovered after the glass was invented. This is clearly not the case. Early examples of mineral aventurine artifacts date to the Neolithic era [4] and can be found throughout history. First century Roman writer Pliny mentions a type of stone with silvery flecks, a passage that was well known when the glass was developed.  The compositions of these minerals were also identified early; either species of quartz that contain flecks of mica, or a type of feldspar (sunstone).[5] Mineral aventurine turns up as the eyes of Greek statues, in stonework mosaics and later in the 'pietre dure' art perfected by the Medici artisans in Florence around the time Antonio Neri started making glass there. The chances are good that examples of the mineral were known to Neri as well as to the glassmakers on Murano, but a recipe for the glass version does not turn up in Neri’s 1612 book; he was apparently too early by a decade.  

The story of aventurine's accidental discovery by monks may well be apocryphal; nevertheless, it is a great entrée to understanding how the formulation works. First, contrary to what the story implies, aventurine is not the result of dumping metallic confetti into glass. The reflective "spangles" (as early researchers were fond of calling them) are actually uniformly sized, mirror-like crystals that are grown in the glass. In truth, the formula is quite similar to recipes already in use by Neri and others; the difference was in proportions and in how the glass was treated after it was in the furnace. The formula for aventurine calls for the addition of copper, iron and tin oxides, to a base that was a hybrid of soda, potash and lead glass. Neri’s recipe #128 is titled "A Proven Way to Make Rosichiero" [6] and provides for all of these ingredients, albeit in lower concentrations. Rosichiero was a transparent tawny red colored glass that was a staple of furnaces throughout Italy. 

The secret to producing the reflective "spangles" was to mix the glass and heat it in the furnace in a normal way, but then to slowly reduce the heat while creating a low oxygen “reducing” atmosphere. The furnace draught was shut; the glass pot was fitted with a tight lid and then covered with ashes and allowed to cool very slowly.  

Initially, the batch is saturated with copper oxide. This means the glass has dissolved as much copper, iron and tin as it can and any further addition of these powders will simply float to the bottom of the pot.  The exact amount of powdered metals able to dissolve is a function of temperature; the hotter the glass the more that will dissolve and the cooler the glass the less that will dissolve. The key concept here is that as the glass slowly cools, the metals start to come out of solution and crystals start to form. There is some complex chemistry happening at the same time; the reducing atmosphere encourage the metals to stay in a pure un-oxidized form,  Furthermore any oxygen or sulfur  that happens to be present will preferentially combine with the iron, leaving the copper crystals pristine. Once cooled to room temperature, a successful batch would be broken away from the glass pot by workers and divided into smaller pieces. Glass artisans wanting to incorporate the aventurine into their work needed to work quickly. They carefully reheated an appropriate nugget and coated (encase) it in a layer of clear glass; once molten, direct exposure to the air would destroy the glittery effect. 

Over time, it was discovered that various colors could be produced with the addition of different chemicals, but the central principal of growing tiny metallic crystals is the same.


[1]  The earliest instance of this story in print that I can find is fairly late;  Faustino Corsi, Delle pietre antiche: libri quattro (Rome: Salviuccio e figlio, 1828)  pp. 166-167.  
[2] Cesare Moretti (†), Bernard Gratuze and Sandro Hreglich,  “Le verre aventurine (‘ avventurina ‘) : son histoire, les recettes, les analyses, sa fabrication”, ArcheoSciences, 37 | 2013, 135-154.
[3] For instance see  Giulio Salviati, “Venetian Glass” Journal of the Society of Arts (Proceedings), Volume 37 (7 June,1889), p. 630
[4] Neolithic Quartz Aventurine Pendant - 7 Cm/ 2. 76 ", green - 6500 To 2000 Bp – Sahara. Item Id: 106549,  Weight: 83 gm. Sahara - Mauritania - Tagant country.
http://ancientpoint.com/inf/106549-neolithic_quartz_aventurine_pendant___7_cm_2___76____6500_to_2000_bp___sahara.html
[5] Dizionario del cittadino, o sia Ristretto storico, teorico e ..., Volume 1 pp. 38-39.
[6] Antonio Neri, L'Arte Vetraria (Firenze: Giunti, 1612).
[7] Sauzay, A. (1870) Marvels of Glassmaking in All Ages. London, 1870 pp. 173 - 175.

Pebbles from Pavia

A Bridge on the River Ticino, near Polleggio,

William Pars (1742‑1782).

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

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

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

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

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

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

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

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

* This post first appeared here on 30 May 2014. 

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.

Pebbles from Pavia

A Bridge on the River Ticino, near Polleggio,

William Pars (1742‑1782).

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

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

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

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

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

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

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

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

* This post first appeared here on 30 May 2014. 

Aventurine Glass

Small amphora in aventurine glass,

Murano, Salviati.

With all its glitz and sparkle, aventurine (avventurina) stands out as a flamboyant extrovert among the varieties of glass. Developed and perfected on the Venetian island of Murano, also known as 'goldstone', it consists of a transparent base glass with myriad reflective crystalline "spangles" running throughout. The classical version is a deep golden brown with crystallites composed mainly of metallic copper, with a few related compounds as supporting cast. However, numerous colors have been developed, including red, orange, yellow, green, blue, violet, black and white. 

Folklore holds that aventurine was discovered by accident ("a venturi") when unknown monks inadvertently dropped copper or brass shavings into a glass melt as early as the thirteenth century. [1] However, more thorough investigations have recently identified 1620 as a likely date for the first appearance of aventurine glass. [2] No example or written account has been found that dates prior to the seventeenth century. An alternate story accounts for the name aventurine being derived from 'adventure'; referring to the difficulty and uncertainty involved in its production.[3] At first, the formula was a closely held secret among a few glassmakers and subsequently it was lost and then rediscovered not once but  twice.

To complicate matters, natural minerals with a similar appearance were named after the glass, leading to the misconception that they were also discovered after the glass was invented. This is clearly not the case. Early examples of mineral aventurine artifacts date to the Neolithic era [4] and can be found throughout history. First century Roman writer Pliny mentions a type of stone with silvery flecks, a passage that was well known when the glass was developed.  The compositions of these minerals were also identified early; either species of quartz that contain flecks of mica, or a type of feldspar (sunstone).[5] Mineral aventurine turns up as the eyes of Greek statues, in stonework mosaics and later in the 'pietre dure' art perfected by the Medici artisans in Florence around the time Antonio Neri started making glass there. The chances are good that examples of the mineral were known to Neri as well as to the glassmakers on Murano, but a recipe for the glass version does not turn up in Neri’s 1612 book; he was apparently too early by a decade.  

The story of aventurine's accidental discovery by monks may well be apocryphal; nevertheless, it is a great entrée to understanding how the formulation works. First, contrary to what the story implies, aventurine is not the result of dumping metallic confetti into glass. The reflective "spangles" (as early researchers were fond of calling them) are actually uniformly sized, mirror-like crystals that are grown in the glass. In truth, the formula is quite similar to recipes already in use by Neri and others; the difference was in proportions and in how the glass was treated after it was in the furnace. The formula for aventurine calls for the addition of copper, iron and tin oxides, to a base that was a hybrid of soda, potash and lead glass. Neri’s recipe #128 is titled "A Proven Way to Make Rosichiero" [6] and provides for all of these ingredients, albeit in lower concentrations. Rosichiero was a transparent tawny red colored glass that was a staple of furnaces throughout Italy. 

The secret to producing the reflective "spangles" was to mix the glass and heat it in the furnace in a normal way, but then to slowly reduce the heat while creating a low oxygen “reducing” atmosphere. The furnace draught was shut; the glass pot was fitted with a tight lid and then covered with ashes and allowed to cool very slowly.  

Initially, the batch is saturated with copper oxide. This means the glass has dissolved as much copper, iron and tin as it can and any further addition of these powders will simply float to the bottom of the pot.  The exact amount of powdered metals able to dissolve is a function of temperature; the hotter the glass the more that will dissolve and the cooler the glass the less that will dissolve. The key concept here is that as the glass slowly cools, the metals start to come out of solution and crystals start to form. There is some complex chemistry happening at the same time; the reducing atmosphere encourage the metals to stay in a pure un-oxidized form,  Furthermore any oxygen or sulfur  that happens to be present will preferentially combine with the iron, leaving the copper crystals pristine. Once cooled to room temperature, a successful batch would be broken away from the glass pot by workers and divided into smaller pieces. Glass artisans wanting to incorporate the aventurine into their work needed to work quickly. They carefully reheated an appropriate nugget and coated (encase) it in a layer of clear glass; once molten, direct exposure to the air would destroy the glittery effect. 

Over time, it was discovered that various colors could be produced with the addition of different chemicals, but the central principal of growing tiny metallic crystals is the same.


[1]  The earliest instance of this story in print that I can find is fairly late;  Faustino Corsi, Delle pietre antiche: libri quattro (Rome: Salviuccio e figlio, 1828)  pp. 166-167.  
[2] Cesare Moretti (†), Bernard Gratuze and Sandro Hreglich,  “Le verre aventurine (‘ avventurina ‘) : son histoire, les recettes, les analyses, sa fabrication”, ArcheoSciences, 37 | 2013, 135-154.
[3] For instance see  Giulio Salviati, “Venetian Glass” Journal of the Society of Arts (Proceedings), Volume 37 (7 June,1889), p. 630
[4] Neolithic Quartz Aventurine Pendant - 7 Cm/ 2. 76 ", green - 6500 To 2000 Bp – Sahara. Item Id: 106549,  Weight: 83 gm. Sahara - Mauritania - Tagant country.
http://ancientpoint.com/inf/106549-neolithic_quartz_aventurine_pendant___7_cm_2___76____6500_to_2000_bp___sahara.html
[5] Dizionario del cittadino, o sia Ristretto storico, teorico e ..., Volume 1 pp. 38-39.
[6] Antonio Neri, L'Arte Vetraria (Firenze: Giunti, 1612).
[7] Sauzay, A. (1870) Marvels of Glassmaking in All Ages. London, 1870 pp. 173 - 175.

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.

Pebbles from Pavia

A Bridge on the River Ticino, near Polleggio,

William Pars (1742‑1782).

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

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

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

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

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

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

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

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

* This post first appeared here on 30 May 2014. 

Aventurine

Small amphora in aventurine glass,

Murano, Salviati.

With all its glitz and sparkle, aventurine (avventurina) stands out as a flamboyant extrovert among the varieties of glass that were developed and perfected on the Venetian island of Murano. Also known as 'goldstone', it consists of a transparent base glass with myriad reflective crystalline "spangles" running throughout. The classical version is a deep golden brown with crystallites composed mainly of metallic copper, with a few related compounds as supporting cast. However, numerous colors have been developed, including red, orange, yellow, green, blue, violet, black and white. 

Folklore holds that aventurine was discovered by accident ("a venturi") when unknown monks inadvertently dropped copper or brass shavings into a glass melt as early as the thirteenth century. [1] However, more thorough investigations have recently identified 1620 as a likely date for the first appearance of aventurine glass. [2] No example or written account has been found that dates prior to the seventeenth century. An alternate story accounts for the name aventurine being derived from 'adventure'; referring to the difficulty and uncertainty involved in its production.[3] At first, the formula was a closely held secret among a few glassmakers and subsequently it was lost and then rediscovered not once but  twice.

To complicate matters, natural minerals with a similar appearance were named after the glass, leading to the misconception that they were also discovered after the glass was invented. This is clearly not the case. Early examples of mineral aventurine artifacts date to the Neolithic era [4] and can be found throughout history. First century Roman writer Pliny mentions a type of stone with silvery flecks, a passage that was well known when the glass was developed.  The compositions of these minerals were also identified early; either species of quartz that contain flecks of mica, or a type of feldspar (sunstone).[5] Mineral aventurine turns up as the eyes of Greek statues, in stonework mosaics and later in the 'pietre dure' art perfected by the Medici artisans in Florence around the time Antonio Neri started making glass there. The chances are good that examples of the mineral were known to Neri as well as to the glassmakers on Murano, but a recipe for the glass version does not turn up in Neri’s 1612 book; he was apparently too early by a decade.  

The story of aventurine's accidental discovery by monks may well be apocryphal; nevertheless, it is a great entrée to understanding how the formulation works. First, contrary to what the story implies, aventurine is not the result of dumping metallic confetti into glass. The reflective "spangles" (as early researchers were fond of calling them) are actually uniformly sized, mirror-like crystals that are grown in the glass. In truth, the formula is quite similar to recipes already in use by Neri and others; the difference was in proportions and in how the glass was treated after it was in the furnace. The formula for aventurine calls for the addition of copper, iron and tin oxides, to a base that was a hybrid of soda, potash and lead glass. Neri’s recipe #128 is titled "A Proven Way to Make Rosichiero" [6] and provides for all of these ingredients, albeit in lower concentrations. Rosichiero was a transparent tawny red colored glass that was a staple of furnaces throughout Italy. 

The secret to producing the reflective "spangles" was to mix the glass and heat it in the furnace in a normal way, but then to slowly reduce the heat while creating a low oxygen “reducing” atmosphere. The furnace draught was shut; the glass pot was fitted with a tight lid and then covered with ashes and allowed to cool very slowly.  

Initially, the batch is saturated with copper oxide. This means the glass has dissolved as much copper, iron and tin as it can and any further addition of these powders will simply float to the bottom of the pot.  The exact amount of powdered metals able to dissolve is a function of temperature; the hotter the glass the more that will dissolve and the cooler the glass the less that will dissolve. The key concept here is that as the glass slowly cools, the metals start to come out of solution and crystals start to form. There is some complex chemistry happening at the same time; the reducing atmosphere encourage the metals to stay in a pure un-oxidized form,  Furthermore any oxygen or sulfur  that happens to be present will preferentially combine with the iron, leaving the copper crystals pristine. Once cooled to room temperature, a successful batch would be broken away from the glass pot by workers and divided into smaller pieces. Glass artisans wanting to incorporate the aventurine into their work needed to work quickly. They carefully reheated an appropriate nugget and coated (encase) it in a layer of clear glass; once molten, direct exposure to the air would destroy the glittery effect. 

Over time, it was discovered that various colors could be produced with the addition of different chemicals, but the central principal of growing tiny metallic crystals is the same.


[1]  The earliest instance of this story in print that I can find is fairly late;  Faustino Corsi, Delle pietre antiche: libri quattro (Rome: Salviuccio e figlio, 1828)  pp. 166-167.  
[2] Cesare Moretti (†), Bernard Gratuze and Sandro Hreglich,  “Le verre aventurine (‘ avventurina ‘) : son histoire, les recettes, les analyses, sa fabrication”, ArcheoSciences, 37 | 2013, 135-154.
[3] For instance see  Giulio Salviati, “Venetian Glass” Journal of the Society of Arts (Proceedings), Volume 37 (7 June,1889), p. 630
[4] Neolithic Quartz Aventurine Pendant - 7 Cm/ 2. 76 ", green - 6500 To 2000 Bp – Sahara. Item Id: 106549,  Weight: 83 gm. Sahara - Mauritania - Tagant country.
http://ancientpoint.com/inf/106549-neolithic_quartz_aventurine_pendant___7_cm_2___76____6500_to_2000_bp___sahara.html
[5] Dizionario del cittadino, o sia Ristretto storico, teorico e ..., Volume 1 pp. 38-39.
[6] Antonio Neri, L'Arte Vetraria (Firenze: Giunti, 1612).
[7] Sauzay, A. (1870) Marvels of Glassmaking in All Ages. London, 1870 pp. 173 - 175.

Glass or Rock

Rock crystal ewer, Egypt (1000-1050)

V&A Museum, #7904-1862

Today, a sharp distinction is made between glass and rock, but in the early seventeenth century, differences in the two materials were not so well defined. One was the product of nature and the other of art, but after that the lines began to blur. Philosophers debated weather glass should be classified as artificial stone, and why not? It is a material that was actually made from crushed up rocks (quartz and calcined limestone) with the addition of plant salts. In a very accurate sense, it is a form of artificial rock. 

In his 1612 book L'Arte Vetraria, Florentine glassmaker Antonio Neri repeatedly illustrates the thinking that glass was an artificial form of rock. The very first part of the book concerns itself with cristallo glass, which was considered to be an imitation of natural rock crystal. He devotes a whole section to the imitation of the colorful stones variously known as chalcedony, jasper and oriental agate. About these he writes: 

It is often said, and it may well seem to be true, that art cannot match nature. However, experience in many things shows, and this is particularly true of colors in glass, that art not only challenges and matches nature, but by far exceeds and surpasses it. Why, if you did not see it for yourself, you would find it hard to believe the beauty and great variety of interplay seen in these particular chalcedonies.

Another entire section of the book is devoted to artificial gemstones: rubies, sapphires, emeralds, topaz, chrysolite; he even uses crushed natural gems to color his glass. He describes a "sky blue even more beautiful, from the garnets of Bohemia." 

An earlier 1540 book that Neri read closely was by Italian metallurgist Vannoccio Biringuccio, called De La Pirotechnia. This was the first book entirely devoted to mining and metal foundry practices. Biringuccio was from Siena, but was considered something of a folk hero in Neri’s nearby Florence because he oversaw the casting of iron cannons for the city's defense in the great siege of 1529–30. His book contains a short six-page chapter devoted to glass where he writes:

[I]t [glass] is one of the effects and real fruits of the art of fire, because every product found in the interior of the earth is either stone, metal or one of the semi-minerals. Glass is seen to resemble all of them, although in all respects it depends on art.

This is an observation that Neri echoes almost verbatim in the introduction of his own book, and in this light, it is not so difficult to see what connects glassmaking to alchemy. Neri was in the business of learning nature's secrets, and then using them to create new materials that were even better than the originals. These are aims quite familiar to any modern materials engineer. Neri and his contemporaries were successful up to a point. They were able to create artificial gems and other items that were impressive in color and clarity, yet they lacked some key properties of their natural counterparts, most notably hardness. 

Based on Neri's earliest known writing Treasure of the World, started in 1598, he was already familiar with mining practices before his glassmaking activities started and over a decade before he would write the book. He devotes this early manuscript to "all of alchemy, its furnaces, instruments and the mining of metals." Around 1600 he started work at Prince Don Antonio de' Medici's Casino di San Marco laboratory. Here, he may have had the opportunity to interact with characters such as Filippo Talducci (1543- c.1615), celebrated Florentine chemist and mining engineer, several of whose relatives worked at the Casino. In his 1613 manuscript Discorso, Neri strongly hints that he has personally been to more than one mine in connection with his alchemical activities. "I would not say this, had I myself not had the good fortune of being in such a mine from which, with much artifice, was extracted a small quantity of real gold liquor, which was the true golden seed. […] To this day I have never found another mine like it, and therefore suitable for this purpose."

Today, we hardly associate glass with the raw materials from which it is composed, just as we hardly ever think of metals in their unrefined state. Antonio Neri was more closely connected to the earth, by virtue of his profession, but also because daily life in the early seventeenth century was filled with such activity; refining raw materials into useful forms had a direct and immediate impact on quality of life. Although glass is now manufactured in highly automated facilities, far away from our daily lives, it is still essentially the same product that Neri made. The next time you come in contact with a piece of glass, to pour a drink or look through a window or read the text on a screen as you are probably doing right now, stop for a minute and think of it as Neri did four centuries ago, as artificial rock.

* This post first appeared here 17 Feb 2014.

Roasting the Frit

Diderot & d'Alembert, L'Encyclopédie (1772)
Raking Out Roasted Frit

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

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

Neri obtained his glass salt from products shipped by traders from the Levant (eastern Mediterranean). It was supplied as the dried, partially charred remains of special plants that grow in arid seaside conditions; 'Kali' and 'Soda'. Shipping them this way cut down on weight and volume, and prevented rotting. These plants contain large amounts of sodium carbonates. This is a white powder, chemically identical to what we know as "washing soda." He advises, 

In buying either of these make sure it is richly salted. This may be determined by touching it with the tongue in order to taste its saltiness; but the surest way of all is to do a test in a crucible and to see if it contains much sand or stones, a thing common in this art and very well known by glass conciatori.

He crushes any large pieces of the product in a stone mortar, and sifts the result through a fine screen, ensuring that most of what remains is salt.  

As the common proverb of the art of glassmaking says: a fine sieve and dry wood bring honor to the furnace. Then with any of these sodas, 100 pounds of soda ordinarily requires 85 to 90 pounds of tarso.

Neri sets up large cauldrons of clean water over brickwork stoves, adds the plant product and boils the water. He strains the insoluble parts out and reduces the liquid by evaporation until crystals of the salt start to form on the surface. He skims these off and continues the process. Finally he carefully dries the product. Our glassmaker describes several variations of this process, including one in which he takes extreme measures to ensure the purity of the salt and clarity of the finished glass. In all, this is a task that could easily take several weeks to perform for the amount of frit to fill a single pot for the gaffer to work.

Not content with the established materials, our glassmaker experimented extensively with other plants: 

[U]se the husks and stalks of fava beans after the farmhands have thrashed and shelled them. With the rules and diligence prescribed for the Levantine polverino salt, extract the salt from this ash, which will be marvelous, and from which a frit can be made using well-sifted white tarso, as is described throughout this work. A very noble frit will result, which in the crucible will make a crystal of all beauty. The same may be made from the ashes of cabbages, or a thorn bush that bears small fruit, called the blackberry, even from millet, rush, marsh reeds, and from many other plants that will relinquish their salt. *

*These other plants produce potassium carbonate salts with similar properties to sodium carbonate.

** This post first appeared here 9 December 2013.

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.

Solid Water

Rock crystal cup , around 1550,

 Museo degli Argenti, Palazzo Pitti, Florence

(click to enlarge)

In the ancient world, certain natural materials commanded a premium and rock crystal was among the most coveted. Large completely transparent crystals of quartz were found in caves and glacial moraines and craftsmen managed to work them into exquisite cups, vessels and ceremonial objects. By the time the Renaissance arrived, unfulfilled demand for rock crystal drove much of the innovation in glassmaking. Indeed, clear glass was very much sought after and thought of as an artificially produced form of rock crystal.

A standard line in the historical narrative tells that the ancient Greeks believed that rock crystal was a form of solidified water, the word “krystallos” meaning ice. Enough written material survives from the likes of Herodotus, Pliny, Seneca and Saint Jerome to know for certain that one version or another of this idea has been around for a few thousand years. It is a line that usually gets attention for all the wrong reasons; when stated so glibly, it does not throw any light on ancient culture or our connection to it and worse, it plays into the fiction that we are somehow very much more astute than our relatives of 150 generations ago, that our progenitors were quaintly uninformed. 

Instead, think of “solidified water” as part of a working model of the world. It was consistent with the prevailing conception of nature in the same way that, for modern physicists, string theory serves; it seems to be correct, to the best of our knowledge, but the chances are good that there is more to the story yet to figure out. Like string theory, solid water is not a concept that would have a strong bearing on the daily lives of most people. For specialized artisans, it did make good sense; to thinking of rock crystal in this way would be a reminder that if treated too roughly, the material would shatter into pieces as if it were ice.

In the fifteenth century, the Venetian island of Murano became the center of the universe for glassmakers, and it stayed that way for the next four centuries. Of all the many forms that glass can take, the one that made Murano most famous was cristallo; named after the mineral it so closely imitates. The genius of cristallo was in bringing together diverse techniques and ingredients to form a product that was clear like water, yet could be worked thinner and in more complex designs than was possible with carved rock crystal.

 The hallmark of cristallo was the specific use of Levantine soda ash and white quartz river stones (tarso) as opposed to sand and it became the pinnacle of the glassmaker’s art. For the first time, artisans combine best practices with premium raw material sources, the extra purification steps, the careful avoidance of iron contamination, the hot glass washing process, the clean burning hardwood fire and the manganese color correction. In Venetian furnaces, even the clay for the crucibles was a special product imported from Constantinople, which did not contaminate the melt.

In 1450 the Christian world was dealt a severe blow by the Byzantine Empire’s fall to the Ottomans and with it went the Venetian’s major trading partners. That same year, the Murano glassmaker Angelo Barovier is given credit for the development of cristallo. He was the man responsible for bringing together the ultimate refinements that elevated glass to a new level of perfection and created a national product for Venice when they sorely needed it. These techniques and materials were all available and utilized by glassmakers individually at least fifty years earlier, but not together in a single product. Barovier had attended lectures by noted alchemist Paola de Pergola, at the School of Rialto. This indicates that he was thinking along the lines of a chemist in putting together a repeatable regimen for an exceptionally clear, bright, workable product. Barovier’s innovation would become a tradition that was carried on by countless glassmakers and ultimately by our Florentine priest Neri more than a century and a half later.

In seventeenth century Florence, Grand Duke Cosimo I de' Medici negotiated for Venetian masters to teach and practice their coveted techniques in Tuscany. Royal architect and polymath Bernardo Buontalenti was credited with the development of an artificial rock crystal, Tuscany’s own version of cristallo. It is an open question whether we see his hand in Neri’s recipe number seventy-six in which actual precious rock crystal is crushed and used as raw material for glass. In any case, Buontalenti’s continued presence at the Casino must have been an influential experience for his former pupil Don Antonio de' Medici, Neri’s sponsor. Our glassmaking priest was impressed enough with this technique to use rock crystal as the basis for his artificial gems. In turn, Neri’s chapter on artificial gems received the most attention, by far, of any other single subject in his groundbreaking book on glassmaking, L’Arte Vetraria. His work on this subject took on a life of its own and was the subject of plagiarism and unattributed derivative works well into the nineteenth century. An argument can be made that his lead glass based gems fueled the revolution pioneered by English glassmaker George Ravenscroft in what has become the current pinnacle of clear glass: lead crystal.