1600s Glass Furnace

 

From "De re metallica" 

Agricola (Georg Bauer) 1556.

In the seventeenth century, glass furnaces represented a pinnacle of technology. True, the ability to achieve the high temperatures required to melt glass had been around for centuries – high enough to melt gold, silver and copper as well. What made the glass furnace remarkable was its refinement. It made efficient use of its hardwood fuel and was able to maintain a controlled, even temperature long before any thermometer could measure it. In fact, in the early seventeenth century, Galileo was only just beginning to use glass bulbs and tubes to measure differences in ambient room temperatures.

In Florence, the construction used was typical of the time throughout Europe, called a "beehive" furnace because its shape resembled the classic elongated dome of a beehive. A double wall, built of fire resistant bricks, provided further insulation, trapping heat inside. Vertically, the furnace was divided into three levels, each forming a wide open chamber. The bottom space was used to build the fire, and had one or two openings to the outside, used to add wood fuel, rake the coals, or shovel out ash. The second, central level was where the pots of glass resided. A central hole or "eye" on the floor directly exposed the fire pit below. The space directly next to the eye was the hottest, and temperature could be further controlled by moving the crucibles farther away or closer to the eye. A number of openings in the wall allowed gaffers access to the glass pots, and at least one larger opening was used to place new crucibles, or rearrange the existing ones. The upper chamber was used to control the draft, and sometimes for annealing. Again, a central hole in the floor of this (top) level allowed exhaust gasses to leave the glass chamber and an opening to one side vented the exhaust.

In his 1612 book, L’Arte Vetraria, Neri is careful to stress that only dry oak or other hardwood should be used because it burns cleanly, and will not deposit ash or creosote in the glass.

The furnace should have dry wood, hard wood of oak because soft wood tinges the furnace and does no good. Stoke it steadily and continuously so that the flame is always clear, and there is never any smoke, which is very important in order to make a beautiful cristallo.

Once a finished piece of glassware is made, it must be allowed to cool slowly, over a period of many hours. This was often accomplished by building a long enclosed horizontal trough that connected to the furnace. A draft opening at the far end allowed heat from the furnace to be drawn in, and finished pieces were placed in a pan at the furnace end and then slowly pulled by a chain further and further down the trough toward the cooler end. This "annealing" process ensured the glass would not develop stresses and crack as it cooled.

Although Neri does not concern himself with the vagaries of furnace construction in the book, it is clear that he did possess considerable knowledge on the subject. Several of his unconventional methods for making pigments for glass involve taking bricks out of the furnace wall to stash chemicals for long term exposure to the heat.

Take small pieces of copper and put them inside the arches of the furnace. In that place, they will be within the walls. Leave them that way until each piece of copper is well calcined, using a simple fire.

While it is true that artisans of the early seventeenth century did not possess the same understanding of nature that we now enjoy, they did have a working knowledge that served them very well. It was backed by a theoretical framework that was quite sophisticated and was consistent with what could be observed and measured at the time. This is no different from our own modern understanding of nature: sophisticated and consistent with what we can observe and measure.

*This post first appeared here 24 January 2014

Early Modern Glass Furnace

From "De re metallica" 

Agricola (Georg Bauer) 1556.

In the seventeenth century, glass furnaces represented a pinnacle of technology. True, the ability to achieve the high temperatures required to melt glass had been around for centuries – high enough to melt gold, silver and copper as well. What made the glass furnace remarkable was its refinement. It made efficient use of its hardwood fuel and was able to maintain a controlled, even temperature long before any thermometer could measure it. In fact, in the early seventeenth century, Galileo was only just beginning to use glass bulbs and tubes to measure differences in ambient room temperatures.

In Florence, the construction used was typical of the time throughout Europe, called a "beehive" furnace because its shape resembled the classic elongated dome of a beehive. A double wall, built of fire resistant bricks, provided further insulation, trapping heat inside. Vertically, the furnace was divided into three levels, each forming a wide open chamber. The bottom space was used to build the fire, and had one or two openings to the outside, used to add wood fuel, rake the coals, or shovel out ash. The second, central level was where the pots of glass resided. A central hole or "eye" on the floor directly exposed the fire pit below. The space directly next to the eye was the hottest, and temperature could be further controlled by moving the crucibles farther away or closer to the eye. A number of openings in the wall allowed gaffers access to the glass pots, and at least one larger opening was used to place new crucibles, or rearrange the existing ones. The upper chamber was used to control the draft, and sometimes for annealing. Again, a central hole in the floor of this (top) level allowed exhaust gasses to leave the glass chamber and an opening to one side vented the exhaust.

In his 1612 book, L’Arte Vetraria, Neri is careful to stress that only dry oak or other hardwood should be used because it burns cleanly, and will not deposit ash or creosote in the glass.

The furnace should have dry wood, hard wood of oak because soft wood tinges the furnace and does no good. Stoke it steadily and continuously so that the flame is always clear, and there is never any smoke, which is very important in order to make a beautiful cristallo.

Once a finished piece of glassware is made, it must be allowed to cool slowly, over a period of many hours. This was often accomplished by building a long enclosed horizontal trough that connected to the furnace. A draft opening at the far end allowed heat from the furnace to be drawn in, and finished pieces were placed in a pan at the furnace end and then slowly pulled by a chain further and further down the trough toward the cooler end. This "annealing" process ensured the glass would not develop stresses and crack as it cooled.

Although Neri does not concern himself with the vagaries of furnace construction in the book, it is clear that he did possess considerable knowledge on the subject. Several of his unconventional methods for making pigments for glass involve taking bricks out of the furnace wall to stash chemicals for long term exposure to the heat.

Take small pieces of copper and put them inside the arches of the furnace. In that place, they will be within the walls. Leave them that way until each piece of copper is well calcined, using a simple fire.

While it is true that artisans of the early seventeenth century did not possess the same understanding of nature that we now enjoy, they did have a working knowledge that served them very well. It was backed by a theoretical framework that was quite sophisticated and was consistent with what could be observed and measured at the time. This is no different from our own modern understanding of nature: sophisticated and consistent with what we can observe and measure.

*This post first appeared here 24 January 2014

1600s Glass Furnace

From "De re metallica" 

Agricola (Georg Bauer) 1556.

In the seventeenth century, glass furnaces represented a pinnacle of technology. True, the ability to achieve the high temperatures required to melt glass had been around for centuries – high enough to melt gold, silver and copper as well. What made the glass furnace remarkable was its refinement. It made efficient use of its hardwood fuel and was able to maintain a controlled, even temperature long before any thermometer could measure it. In fact, in the early seventeenth century, Galileo was only just beginning to use glass bulbs and tubes to measure differences in ambient room temperatures.

In Florence, the construction used was typical of the time throughout Europe, called a "beehive" furnace because its shape resembled the classic elongated dome of a beehive. A double wall, built of fire resistant bricks, provided further insulation, trapping heat inside. Vertically, the furnace was divided into three levels, each forming a wide open chamber. The bottom space was used to build the fire, and had one or two openings to the outside, used to add wood fuel, rake the coals, or shovel out ash. The second, central level was where the pots of glass resided. A central hole or "eye" on the floor directly exposed the fire pit below. The space directly next to the eye was the hottest, and temperature could be further controlled by moving the crucibles farther away or closer to the eye. A number of openings in the wall allowed gaffers access to the glass pots, and at least one larger opening was used to place new crucibles, or rearrange the existing ones. The upper chamber was used to control the draft, and sometimes for annealing. Again, a central hole in the floor of this (top) level allowed exhaust gasses to leave the glass chamber and an opening to one side vented the exhaust.

In his 1612 book, L’Arte Vetraria, Neri is careful to stress that only dry oak or other hardwood should be used because it burns cleanly, and will not deposit ash or creosote in the glass.

The furnace should have dry wood, hard wood of oak because soft wood tinges the furnace and does no good. Stoke it steadily and continuously so that the flame is always clear, and there is never any smoke, which is very important in order to make a beautiful cristallo.

Once a finished piece of glassware is made, it must be allowed to cool slowly, over a period of many hours. This was often accomplished by building a long enclosed horizontal trough that connected to the furnace. A draft opening at the far end allowed heat from the furnace to be drawn in, and finished pieces were placed in a pan at the furnace end and then slowly pulled by a chain further and further down the trough toward the cooler end. This "annealing" process ensured the glass would not develop stresses and crack as it cooled.

Although Neri does not concern himself with the vagaries of furnace construction in the book, it is clear that he did possess considerable knowledge on the subject. Several of his unconventional methods for making pigments for glass involve taking bricks out of the furnace wall to stash chemicals for long term exposure to the heat.

Take small pieces of copper and put them inside the arches of the furnace. In that place, they will be within the walls. Leave them that way until each piece of copper is well calcined, using a simple fire.

While it is true that artisans of the early seventeenth century did not possess the same understanding of nature that we now enjoy, they did have a working knowledge that served them very well. It was backed by a theoretical framework that was quite sophisticated and was consistent with what could be observed and measured at the time. This is no different from our own modern understanding of nature: sophisticated and consistent with what we can observe and measure.

*This post first appeared here 24 January 2014

Neri's Glass Furnace

From "De re metallica" 

Agricola (Georg Bauer) 1556.

In the seventeenth century, glass furnaces represented a pinnacle of technology. True, the ability to achieve the high temperatures required to melt glass had been around for centuries – high enough to melt gold, silver and copper as well. What made the glass furnace remarkable was its refinement. It made efficient use of its hardwood fuel and was able to maintain a controlled, even temperature long before any thermometer could measure it. In fact, in the early seventeenth century, Galileo was only just beginning to use glass bulbs and tubes to measure differences in ambient room temperatures.

In Florence, the construction used was typical of the time throughout Europe, called a "beehive" furnace because its shape resembled the classic elongated dome of a beehive. A double wall, built of fire resistant bricks, provided further insulation, trapping heat inside. Vertically, the furnace was divided into three levels, each forming a wide open chamber. The bottom space was used to build the fire, and had one or two openings to the outside, used to add wood fuel, rake the coals, or shovel out ash. The second, central level was where the pots of glass resided. A central hole or "eye" on the floor directly exposed the fire pit below. The space directly next to the eye was the hottest, and temperature could be further controlled by moving the crucibles farther away or closer to the eye. A number of openings in the wall allowed gaffers access to the glass pots, and at least one larger opening was used to place new crucibles, or rearrange the existing ones. The upper chamber was used to control the draft, and sometimes for annealing. Again, a central hole in the floor of this (top) level allowed exhaust gasses to leave the glass chamber and an opening to one side vented the exhaust.

In his 1612 book, L’Arte Vetraria, Neri is careful to stress that only dry oak or other hardwood should be used because it burns cleanly, and will not deposit ash or creosote in the glass.

The furnace should have dry wood, hard wood of oak because soft wood tinges the furnace and does no good. Stoke it steadily and continuously so that the flame is always clear, and there is never any smoke, which is very important in order to make a beautiful cristallo.

Once a finished piece of glassware is made, it must be allowed to cool slowly, over a period of many hours. This was often accomplished by building a long enclosed horizontal trough that connected to the furnace. A draft opening at the far end allowed heat from the furnace to be drawn in, and finished pieces were placed in a pan at the furnace end and then slowly pulled by a chain further and further down the trough toward the cooler end. This "annealing" process ensured the glass would not develop stresses and crack as it cooled.

Although Neri does not concern himself with the vagaries of furnace construction in the book, it is clear that he did possess considerable knowledge on the subject. Several of his unconventional methods for making pigments for glass involve taking bricks out of the furnace wall to stash chemicals for long term exposure to the heat.

Take small pieces of copper and put them inside the arches of the furnace. In that place, they will be within the walls. Leave them that way until each piece of copper is well calcined, using a simple fire.

While it is true that artisans of the early seventeenth century did not possess the same understanding of nature that we now enjoy, they did have a working knowledge that served them very well. It was backed by a theoretical framework that was quite sophisticated and was consistent with what could be observed and measured at the time. This is no different from our own modern understanding of nature: sophisticated and consistent with what we can observe and measure.

*This post first appeared here 24 January 2014

Early Modern Glass Furnace

From "De re metallica" 

Agricola (Georg Bauer) 1556.

In the seventeenth century, glass furnaces represented a pinnacle of technology. True, the ability to achieve the high temperatures required to melt glass had been around for centuries – high enough to melt gold, silver and copper as well. What made the glass furnace remarkable was its refinement. It made efficient use of its hardwood fuel and was able to maintain a controlled, even temperature long before any thermometer could measure it. In fact, in the early seventeenth century, Galileo was only just beginning to use glass bulbs and tubes to measure differences in ambient room temperatures.

In Florence, the construction used was typical of the time throughout Europe, called a "beehive" furnace because its shape resembled the elongated dome of a beehive. A double wall, built of fire resistant bricks, provided further insulation, trapping heat inside. Vertically, the furnace was divided into three levels, each forming a wide open chamber. The bottom space was used to build the fire, and had one or two openings to the outside, used to add wood fuel, rake the coals, or shovel out ash. The second, central level was where the pots of glass resided. A central hole or "eye" on the floor directly exposed the fire pit below. The space directly next to the eye was the hottest, and temperature could be further controlled by moving the crucibles farther away or closer to the eye. A number of openings in the wall allowed gaffers access to the glass pots, and at least one larger opening was used to place new crucibles, or rearrange the existing ones. The upper chamber was used to control the draft, and sometimes for annealing. Again, a central hole in the floor of this (top) level allowed exhaust gasses to leave the glass chamber and an opening to one side vented the exhaust.

In his book, L’Arte Vetraria, Neri is careful to stress that only dry oak or other hardwood should be used because it burns cleanly, and will not deposit ash or creosote in the glass.

The furnace should have dry wood, hard wood of oak because soft wood tinges the furnace and does no good. Stoke it steadily and continuously so that the flame is always clear, and there is never any smoke, which is very important in order to make a beautiful cristallo.

Once a finished piece of glassware is made, it must be allowed to cool slowly, over a period of many hours. This was often accomplished by building a long enclosed horizontal trough that connected to the furnace. A draft opening at the far end allowed heat from the furnace to be drawn in, and finished pieces were placed in a pan at the furnace end and then slowly pulled by a chain further and further down the trough toward the cooler end. This "annealing" process ensured the glass would not develop stresses and crack as it cooled.

Although Neri does not concern himself with the vagaries of furnace construction in the book, it is clear that he did possess considerable knowledge on the subject. Several of his unconventional methods for making pigments for glass involve taking bricks out of the furnace wall to stash chemicals for long term exposure to the heat.

Take small pieces of copper and put them inside the arches of the furnace. In that place, they will be within the walls. Leave them that way until each piece of copper is well calcined, using a simple fire.

While it is true that artisans of the early seventeenth century did not possess the same understanding of nature that we now enjoy, they did have a working knowledge that served them very well. It was backed by a theoretical framework that was quite sophisticated and was consistent with what could be observed and measured at the time. This is no different from our own modern understanding of nature: sophisticated and consistent with what we can observe and measure.

*This post first appeared here 24 January 2014

The Art of Metals

Fig. 1: Antonio Neri, "Ars Preparatio Metallor[um]"

in Tesoro del Mondo, f. 8r.

Recently we examined another of a set of four illustrations drawn by priest Antonio Neri in a manuscript started in 1598.[1] They show various "arts" being practiced involving items found in nature; plants, animals, stone and today's subject: metals. 

Metals and their chemical manipulation concern Neri's work both as a glassmaker and as an alchemist. Most of the colorful pigments he uses in his glassmaking derive from metals; white from tin, green from iron, red from copper and gold, blue from cobalt, although Neri knew this last one only as a mineral called "zaffer." As an alchemist, metals were the subject of medicinal cures and of course were the focus of transmutation—the quest to turn one metal into another. In another manuscript written towards the end of his life, he indicates that he spent time in mines, where it was thought that metals "grew" underground as "veins" which nourished the earth. [2]

After an examination of Neri's "Art of Preparing Metals" we will take a look at another picture on the same subject, this time a painting titled the "Goldsmith's Workshop" by Alessandro Fei. It was executed a few years before Antonio was born, but bears some remarkable similarities in subject matter.

First, the manuscript illustration (fig. 1): At center left, we see a special furnace construction which the label tells us is for "reverberation and calcination." Two crucibles sit inside, exposed to the heat. These are both procedures with which Neri was quite familiar. In the recipes of his glassmaking book, L'Arte Vetraria, [3] he speaks numerous times about what was called "calcination"—the conversion of a metal into what is usually a powdered oxide form. [4] This was a fundamental operation in producing the pigments for glass. "Reverberation" was exposing a material to secondary heat reflected from the inner walls of the furnace chamber; it was a more even heat than that produced by a direct flame.

Above the reverberation furnace is a foundry furnace, used for melting metals. Because of the high temperatures that needed to be achieved, it has a large bellows attached to force air into the fire causing a "blast" effect, which causes the fire to burn faster and hotter. Resting on the hearth is a long pair of pincers used to insert and remove crucibles of molten metal.

To the right, we see a curious arrangement of two glass retorts, each feeding into the other. They sit on purpose built stoves. The chimneys are capped by dome shaped dampers to control the draft and paddle mechanisms in front regulate heat under the glassware. A worker tends the apparatus, and the caption below reads "for extraction of the quintessence." Neri's other writing makes plain that this "quintessence" was a material that exhibits such a harmonious blending of the four essences: air, water, fire and earth that a new fifth essence (supposedly) emerged with extraordinary properties. To the right is a similar stove set-up with a single retort which is labeled "distilling." This station might have been used in preparation of the acids needed to purify metals.

At lower right, a blacksmith is hammering a piece of iron into a sword on an anvil. High-carbon "steel" was being made and used in the Medici court at the time, and that may well be what the smith is working with. Around him are several examples of his art, including two finished swords, a large copper bowl or cauldron, and a covered drinking stein made of tin.

To his left, we find a goldsmith, also hammering on an anvil, fashioning a silver platter. Displayed behind him are a variety of his handiwork, including more platters, gold necklaces and crosses and other ornamentation. A number of tools hang along the bottom of the anvil station, and behind it a small dog is curled up on the floor. 

Fig 2: Alessandro Fei (Barbieri)
"Goldsmith's Workshop" c. 1572.

About twenty-five years before Neri wrote his manuscript, Alessandro Fei painted a similar picture (fig. 2). Here we are treated to a view of a royal foundry housed in the yet to be completed Uffizi palace. [5] There is a remarkable concordance between this painting and Neri's illustration, right down to a small dog, hanging around the goldsmith's bench. 

The central figure of the painting is  Francesco de' Medici, seated in the foreground, inspecting his father's gold crown at a workbench. The dog is playfully engaging with the future Grand Duke, while on the opposite side of the bench, two other men are amiably discussing their work, perhaps silver chasing.To the left of Francesco, a younger man examines a gold ewer in the company of an older goldsmith seated across the table. The artisan is wearing spectacles and (although it is hard to see) he holds a small hammer in his hand. A worker to Francesco’s right is also intently engaged in his work. In the right mid-ground we find a small furnace with a tall chimney leading upward. On the mantle are displayed a number of gold and silver items, including platters, vessels and chains or necklaces. To the left, four men are busy seated around a large workbench. Behind that group, toward the center of the picture, are two men working at an anvil and three more at another bench on the right, accompanied by a second small dog. In an enclosed area in the background we see two large open furnace hearths, with their fireboxes glowing from underneath and men working in close proximity. In the far left background is a group entering the work area, including a figure in a yellow cape accompanied by a third dog.

The one element present in Neri's illustration that is missing in the painting is the glassware. Metalwork was a venerated and mature art long before Neri's era, which is underlined by the similarity of the two scenes, even though they are a quarter century apart, the tools, equipment and work methods are identical for all intents and purposes. There is no doubt that chemistry and distillation apparatus were used by the earlier metals artisans, the fact that Neri chose to emphasize this aspect of the work is a clear indication of his own interest and a foreshadowing of his future involvement in the production of the glass itself.

[1] Neri 1598-1600.
[2] Neri 1613, Grazzini 2012. 
[3] Neri 1612.
[4] In several of Neri's glass recipes the result of "calcination" is a sulfide.
[5] Beretta, 2014.

(Bibliography)
*This post first appeared here on 17 September 2014.

Early Modern Glass Furnace

From "De re metallica" 

Agricola (Georg Bauer) 1556.

In the seventeenth century, glass furnaces represented a pinnacle of technology. True, the ability to achieve the high temperatures required to melt glass had been around for centuries – high enough to melt gold, silver and copper as well. What made the glass furnace remarkable was its refinement. It made efficient use of its hardwood fuel and was able to maintain a controlled, even temperature long before any thermometer could measure it. In fact, in the early seventeenth century, Galileo was only just beginning to use glass bulbs and tubes to measure differences in ambient room temperatures.

In Florence, the construction used was typical of the time throughout Europe, called a "beehive" furnace because its shape resembled the elongated dome of a beehive. A double wall, built of fire resistant bricks, provided further insulation, trapping heat inside. Vertically, the furnace was divided into three levels, each forming a wide open chamber. The bottom space was used to build the fire, and had one or two openings to the outside, used to add wood fuel, rake the coals, or shovel out ash. The second, central level was where the pots of glass resided. A central hole or "eye" on the floor directly exposed the fire pit below. The space directly next to the eye was the hottest, and temperature could be further controlled by moving the crucibles farther away or closer to the eye. A number of openings in the wall allowed gaffers access to the glass pots, and at least one larger opening was used to place new crucibles, or rearrange the existing ones. The upper chamber was used to control the draft, and sometimes for annealing. Again, a central hole in the floor of this (top) level allowed exhaust gasses to leave the glass chamber and an opening to one side vented the exhaust.

In his book, L’Arte Vetraria, Neri is careful to stress that only dry oak or other hardwood should be used because it burns cleanly, and will not deposit ash or creosote in the glass.

The furnace should have dry wood, hard wood of oak because soft wood tinges the furnace and does no good. Stoke it steadily and continuously so that the flame is always clear, and there is never any smoke, which is very important in order to make a beautiful cristallo.

Once a finished piece of glassware is made, it must be allowed to cool slowly, over a period of many hours. This was often accomplished by building a long enclosed horizontal trough that connected to the furnace. A draft opening at the far end allowed heat from the furnace to be drawn in, and finished pieces were placed in a pan at the furnace end and then slowly pulled by a chain further and further down the trough toward the cooler end. This "annealing" process ensured the glass would not develop stresses and crack as it cooled.

Although Neri does not concern himself with the vagaries of furnace construction in the book, it is clear that he did possess considerable knowledge on the subject. Several of his unconventional methods for making pigments for glass involve taking bricks out of the furnace wall to stash chemicals for long term exposure to the heat.

Take small pieces of copper and put them inside the arches of the furnace. In that place, they will be within the walls. Leave them that way until each piece of copper is well calcined, using a simple fire.

While it is true that artisans of the early seventeenth century did not possess the same understanding of nature that we now enjoy, they did have a working knowledge that served them very well. It was backed by a theoretical framework that was quite sophisticated and was consistent with what could be observed and measured at the time. This is no different from our own modern understanding of nature: sophisticated and consistent with what we can observe and measure.

*This post first appeared here 24 January 2014

The Art of Metals

Fig. 1: Antonio Neri, "Ars Preparatio Metallor[um]"

in Tesoro del Mondo, f. 8r.
(click to enlarge)

Recently we examined another of a set of four illustrations drawn by priest Antonio Neri in a manuscript started in 1598.[1] They show various "arts" being practiced involving items found in nature; plants, animals, stone and today's subject: metals. 

Metals and their chemical manipulation concern Neri's work both as a glassmaker and as an alchemist. Most of the colorful pigments he uses in his glassmaking derive from metals; white from tin, green from iron, red from copper and gold, blue from cobalt, although Neri knew this last one only as a mineral called "zaffer." As an alchemist, metals were the subject of medicinal cures and of course were the focus of transmutation—the quest to turn one metal into another. In another manuscript written towards the end of his life, he indicates that he spent time in mines, where it was thought that metals "grew" underground as "veins" which nourished the earth. [2]

After an examination of Neri's "Art of Preparing Metals" we will take a look at another picture on the same subject, this time a painting titled the "Goldsmith's Workshop" by Alessandro Fei. It was executed a few years before Antonio was born, but bears some remarkable similarities in subject matter.

First, the manuscript illustration (fig. 1): At center left, we see a special furnace construction which the label tells us is for "reverberation and calcination." Two crucibles sit inside, exposed to the heat. These are both procedures with which Neri was quite familiar. In the recipes of his glassmaking book, L'Arte Vetraria, [3] he speaks numerous times about what was called "calcination"—the reduction of a metal into what is usually a powdered oxide form. [4] This was a fundamental operation in producing the pigments for glass. "Reverberation" was exposing a material to secondary heat reflected from the inner walls of the furnace chamber; it was a more even heat than that produced by a direct flame.

Above the reverberation furnace is a foundry furnace, used for melting metals. Because of the high temperatures that needed to be achieved, it has a large bellows attached to force air into the fire causing a "blast" effect, which causes the fire to burn faster and hotter. Resting on the hearth is a long pair of pincers used to insert and remove crucibles of molten metal.

To the right, we see a curious arrangement of two glass retorts, each feeding into the other. They sit on purpose built stoves. The chimneys are capped by dome shaped dampers to control the draft and paddle mechanisms in front regulate heat under the glassware. A worker tends the apparatus, and the caption below reads "for extraction of the quintessence." Neri's other writing makes plain that this "quintessence" was a material that exhibits such a harmonious blending of the four essences: air, water, fire and earth that a new fifth essence (supposedly) emerged with extraordinary properties. To the right is a similar stove set-up with a single retort which is labeled "distilling." This station might have been used in preparation of the acids needed to purify metals.

At lower right, a blacksmith is hammering a piece of iron into a sword on an anvil. High-carbon "steel" was being made and used in the Medici court at the time, and that may well be what the smith is working with. Around him are several examples of his art, including two finished swords, a large copper bowl or cauldron, and a covered drinking stein made of tin.

To his left, we find a goldsmith, also hammering on an anvil, fashioning a silver platter. Displayed behind him are a variety of his handiwork, including more platters, gold necklaces and crosses and other ornamentation. A number of tools hang along the bottom of the anvil station, and behind it a small dog is curled up on the floor. 

Fig 2: Alessandro Fei (Barbieri)
"Goldsmith's Workshop" c. 1572.

About twenty-five years before Neri wrote his manuscript, Alessandro Fei painted a similar picture (fig. 2). Here we are treated to a view of a royal foundry housed in the yet to be completed Uffizi palace. [5] There is a remarkable concordance between this painting and Neri's illustration, right down to a small dog, hanging around the goldsmith's bench. 

The central figure of the painting is  Francesco de' Medici, seated in the foreground, inspecting his father's gold crown at a workbench. The dog is playfully engaging with the future Grand Duke, while on the opposite side of the bench, two other men are amiably discussing their work, perhaps silver chasing.To the left of Francesco, a younger man examines a gold ewer in the company of an older goldsmith seated across the table. The artisan is wearing spectacles and (although it is hard to see) he holds a small hammer in his hand. A worker to Francesco’s right is also intently engaged in his work. In the right mid-ground we find a small furnace with a tall chimney leading upward. On the mantle are displayed a number of gold and silver items, including platters, vessels and chains or necklaces. To the left, four men are busy seated around a large workbench. Behind that group, toward the center of the picture, are two men working at an anvil and three more at another bench on the right, accompanied by a second small dog. In an enclosed area in the background we see two large open furnace hearths, with their fireboxes glowing from underneath and men working in close proximity. In the far left background is a group entering the work area, including a figure in a yellow cape accompanied by a third dog.

The one element present in Neri's illustration that is missing in the painting is the glassware. Metalwork was a venerated and mature art long before Neri's era, which is underlined by the similarity of the two scenes, even though they are a quarter century apart, the tools, equipment and work methods are identical for all intents and purposes. There is no doubt that chemistry and distillation apparatus were used by the earlier metals artisans, the fact that Neri chose to emphasize this aspect of the work is a clear indication of his own interest and a foreshadowing of his future involvement in the production of the glass itself.

[1] Neri 1598-1600.
[2] Neri 1613, Grazzini 2012. 
[3] Neri 1612.
[4] In several of Neri's glass recipes the result of "calcination" is a sulfide.
[5] Beretta, 2014.

(Bibliography)
*This post first appeared here on 17 September 2014.

Early Modern Glass Furnace

From "De re metallica" 

Agricola (Georg Bauer) 1556.

In the seventeenth century, glass furnaces represented a pinnacle of technology. True, the ability to achieve the high temperatures required to melt glass had been around for centuries – high enough to melt gold, silver and copper as well. What made the glass furnace remarkable was its refinement. It made efficient use of its hardwood fuel and was able to maintain a controlled, even temperature long before any thermometer could measure it. In fact, in the early seventeenth century, Galileo was only just beginning to use glass bulbs and tubes to measure differences in ambient room temperatures.

In Florence, the construction used was typical of the time throughout Europe, called a "beehive" furnace because its shape resembled the elongated dome of a beehive. A double wall, built of fire resistant bricks, provided further insulation, trapping heat inside. Vertically, the furnace was divided into three levels, each forming a wide open chamber. The bottom space was used to build the fire, and had one or two openings to the outside, used to add wood fuel, rake the coals, or shovel out ash. The second, central level was where the pots of glass resided. A central hole or "eye" on the floor directly exposed the fire pit below. The space directly next to the eye was the hottest, and temperature could be further controlled by moving the crucibles farther away or closer to the eye. A number of openings in the wall allowed gaffers access to the glass pots, and at least one larger opening was used to place new crucibles, or rearrange the existing ones. The upper chamber was used to control the draft, and sometimes for annealing. Again, a central hole in the floor of this (top) level allowed exhaust gasses to leave the glass chamber and an opening to one side vented the exhaust.

In his book, L’Arte Vetraria, Neri is careful to stress that only dry oak or other hardwood should be used because it burns cleanly, and will not deposit ash or creosote in the glass.

The furnace should have dry wood, hard wood of oak because soft wood tinges the furnace and does no good. Stoke it steadily and continuously so that the flame is always clear, and there is never any smoke, which is very important in order to make a beautiful cristallo.

Once a finished piece of glassware is made, it must be allowed to cool slowly, over a period of many hours. This was often accomplished by building a long enclosed horizontal trough that connected to the furnace. A draft opening at the far end allowed heat from the furnace to be drawn in, and finished pieces were placed in a pan at the furnace end and then slowly pulled by a chain further and further down the trough toward the cooler end. This "annealing" process ensured the glass would not develop stresses and crack as it cooled.

Although Neri does not concern himself with the vagaries of furnace construction in the book, it is clear that he did possess considerable knowledge on the subject. Several of his unconventional methods for making pigments for glass involve taking bricks out of the furnace wall to stash chemicals for long term exposure to the heat.

Take small pieces of copper and put them inside the arches of the furnace. In that place, they will be within the walls. Leave them that way until each piece of copper is well calcined, using a simple fire.

While it is true that artisans of the early seventeenth century did not possess the same understanding of nature that we now enjoy, they did have a working knowledge that served them very well. It was backed by a theoretical framework that was quite sophisticated and was consistent with what could be observed and measured at the time. This is no different from our own modern understanding of nature: sophisticated and consistent with what we can observe and measure.

*This post first appeared here 24 January 2014

The Art of Metals

Fig. 1: Antonio Neri, "Ars Preparatio Metallor[um]"

in Tesoro del Mondo, f. 8r.
(click to enlarge)

Recently we examined another of a set of four illustrations drawn by priest Antonio Neri in a manuscript started in 1598.[1] They show various "arts" being practiced involving items found in nature; plants, animals, stone and today's subject: metals. 

Metals and their chemical manipulation concern Neri's work both as a glassmaker and as an alchemist. Most of the colorful pigments he uses in his glassmaking derive from metals; white from tin, green from iron, red from copper and gold, blue from cobalt, although Neri knew this last one only as a mineral called "zaffer." As an alchemist, metals were the subject of medicinal cures and of course were the focus of transmutation—the quest to turn one metal into another. In another manuscript written towards the end of his life, he indicates that he spent time in mines, where it was thought that metals "grew" underground as "veins" which nourished the earth. [2]

After an examination of Neri's "Art of Preparing Metals" we will take a look at another picture on the same subject, this time a painting titled the "Goldsmith's Workshop" by Alessandro Fei. It was executed a few years before Antonio was born, but bears some remarkable similarities in subject matter.

First, the manuscript illustration (fig. 1): At center left, we see a special furnace construction which the label tells us is for "reverberation and calcination." Two crucibles sit inside, exposed to the heat. These are both procedures with which Neri was quite familiar. In the recipes of his glassmaking book, L'Arte Vetraria, [3] he speaks numerous times about what was called "calcination"—the reduction of a metal into what is usually a powdered oxide form. [4] This was a fundamental operation in producing the pigments for glass. "Reverberation" was exposing a material to secondary heat reflected from the inner walls of the furnace chamber; it was a more even heat than that produced by a direct flame.

Above the reverberation furnace is a foundry furnace, used for melting metals. Because of the high temperatures that needed to be achieved, it has a large bellows attached to force air into the fire causing a "blast" effect, which causes the fire to burn faster and hotter. Resting on the hearth is a long pair of pincers used to insert and remove crucibles of molten metal.

To the right, we see a curious arrangement of two glass retorts, each feeding into the other. They sit on purpose built stoves. The chimneys are capped by dome shaped dampers to control the draft and paddle mechanisms in front regulate heat under the glassware. A worker tends the apparatus, and the caption below reads "for extraction of the quintessence." Neri's other writing makes plain that this "quintessence" was a material that exhibits such a harmonious blending of the four essences: air, water, fire and earth that a new fifth essence (supposedly) emerged with extraordinary properties. To the right is a similar stove set-up with a single retort which is labeled "distilling." This station might have been used in preparation of the acids needed to purify metals.

At lower right, a blacksmith is hammering a piece of iron into a sword on an anvil. High-carbon "steel" was being made and used in the Medici court at the time, and that may well be what the smith is working with. Around him are several examples of his art, including two finished swords, a large copper bowl or cauldron, and a covered drinking stein made of tin.

To his left, we find a goldsmith, also hammering on an anvil, fashioning a silver platter. Displayed behind him are a variety of his handiwork, including more platters, gold necklaces and crosses and other ornamentation. A number of tools hang along the bottom of the anvil station, and behind it a small dog is curled up on the floor. 

Fig 2: Alessandro Fei (Barbieri)
"Goldsmith's Workshop" c. 1572.

About twenty-five years before Neri wrote his manuscript, Alessandro Fei painted a similar picture (fig. 2). Here we are treated to a view of a royal foundry housed in the yet to be completed Uffizi palace. [5] There is a remarkable concordance between this painting and Neri's illustration, right down to a small dog, hanging around the goldsmith's bench. 

The central figure of the painting is  Francesco de' Medici, seated in the foreground, inspecting his father's gold crown at a workbench. The dog is playfully engaging with the future Grand Duke, while on the opposite side of the bench, two other men are amiably discussing their work, perhaps silver chasing.To the left of Francesco, a younger man examines a gold ewer in the company of an older goldsmith seated across the table. The artisan is wearing spectacles and (although it is hard to see) he holds a small hammer in his hand. A worker to Francesco’s right is also intently engaged in his work. In the right mid-ground we find a small furnace with a tall chimney leading upward. On the mantle are displayed a number of gold and silver items, including platters, vessels and chains or necklaces. To the left, four men are busy seated around a large workbench. Behind that group, toward the center of the picture, are two men working at an anvil and three more at another bench on the right, accompanied by a second small dog. In an enclosed area in the background we see two large open furnace hearths, with their fireboxes glowing from underneath and men working in close proximity. In the far left background is a group entering the work area, including a figure in a yellow cape accompanied by a third dog.

The one element present in Neri's illustration that is missing in the painting is the glassware. Metalwork was a venerated and mature art long before Neri's era, which is underlined by the similarity of the two scenes, even though they are a quarter century apart, the tools, equipment and work methods are identical for all intents and purposes. There is no doubt that chemistry and distillation apparatus were used by the earlier metals artisans, the fact that Neri chose to emphasize this aspect of the work is a clear indication of his own interest and a foreshadowing of his future involvement in the production of the glass itself.

[1] Neri 1598-1600.
[2] Neri 1613, Grazzini 2012. 
[3] Neri 1612.
[4] In several of Neri's glass recipes the result of "calcination" is a sulfide.
[5] Beretta, 2014.

(Bibliography)
*This post first appeared here on 17 September 2014.

16-17th Century Glass Furnace

From "De re metallica" 
Agricola (Georg Bauer) 1556.

In the seventeenth century, glass furnaces represented a pinnacle of technology. True, the ability to achieve the high temperatures required to melt glass had been around for centuries – high enough to melt gold, silver and copper as well. What made the glass furnace remarkable was its refinement. It made efficient use of its hardwood fuel and was able to maintain a controlled, even temperature long before any thermometer could measure it. In fact, Galileo was only just beginning to use glass bulbs and tubes to measure differences in ambient room temperatures.

In Florence, the construction used was typical of the time throughout Europe, called a "beehive" furnace because its shape resembled the elongated dome of a beehive. A double wall, built of fire resistant bricks, provided further insulation, trapping heat inside. Vertically, the furnace was divided into three levels, each forming a wide open chamber. The bottom space was used to build the fire, and had one or two openings to the outside, used to add wood fuel, rake the coals, or shovel out ash. The second, central level was where the pots of glass resided. A central hole or "eye" on the floor directly exposed the fire pit below. The space directly next to the eye was the hottest, and temperature could be further controlled by moving the crucibles farther away or closer to the eye. A number of openings in the wall allowed gaffers access to the glass pots, and at least one larger opening was used to place new crucibles, or rearrange the existing ones. The upper chamber was used to control the draft, and sometimes for annealing. Again, a central hole in the floor of this (top) level allowed exhaust gasses to leave the glass chamber and an opening to one side vented the exhaust.

In his book, L’Arte Vetraria, Neri is careful to stress that only dry oak or other hardwood should be used because it burns cleanly, and will not deposit ash or creosote in the glass.

The furnace should have dry wood, hard wood of oak because soft wood tinges the furnace and does no good. Stoke it steadily and continuously so that the flame is always clear, and there is never any smoke, which is very important in order to make a beautiful cristallo.

Once a finished piece of glassware is made, it must be allowed to cool slowly, over a period of many hours. This was often accomplished by building a long enclosed horizontal trough that connected to the furnace. A draft opening at the far end allowed heat from the furnace to be drawn in, and finished pieces were placed in a pan at the furnace end and then slowly pulled by a chain further and further down the trough toward the cooler end. This "annealing" process ensured the glass would not develop stresses and crack as it cooled.

Although Neri does not concern himself with the vagaries of furnace construction in the book, it is clear that he did possess considerable knowledge on the subject. Several of his unconventional methods for making pigments for glass involve taking bricks out of the furnace wall to stash chemicals for long term exposure to the heat.

Take small pieces of copper and put them inside the arches of the furnace. In that place, they will be within the walls. Leave them that way until each piece of copper is well calcined, using a simple fire.

While it is true that artisans of the early seventeenth century did not possess the same understanding of nature that we now enjoy, they did have a working knowledge that served them very well. It was backed by a theoretical framework that was quite sophisticated and was consistent with what could be observed and measured at the time. This is no different from our own modern understanding of nature: sophisticated and consistent with what we can observe and measure.

*This post first appeared here 24 January 2014

The Art of Metals

Fig. 1: Antonio Neri, "Ars Preparatio Metallor[um]"
in Tesoro del Mondo, f. 8r.

Recently we examined three of a set of four illustrations drawn by priest Antonio Neri in a manuscript started in 1598.[1] They show various "arts" being practiced involving items found in nature; plants, animals, stone and today's subject: metals. 

Metals and their chemical manipulation concern Neri's work both as a glassmaker and as an alchemist. Most of the colorful pigments he uses in his glassmaking derive from metals; white from tin, green from iron, red from copper and gold, blue from cobalt, although Neri knew this last one only as a mineral called "zaffer." As an alchemist, metals were the subject of medicinal cures and of course were the focus of transmutation—the quest to turn one metal into another. In another manuscript written towards the end of his life, he indicates that he spent time in mines, where it was thought that metals "grew" underground as "veins" which nourished the earth. [2]

After an examination of Neri's "Art of Preparing Metals" we will take a look at another picture on the same subject, this time a painting titled the "Goldsmith's Workshop" by Alessandro Fei. It was executed a few years before Antonio was born, but bears some remarkable similarities in subject matter.

First, the manuscript illustration (fig. 1, click to enlarge): At center left, we see a special furnace construction which the label tells us is for "reverberation and calcination." Two crucibles sit inside, exposed to the heat. These are both procedures with which Neri was quite familiar. In the recipes of his glassmaking book, L'Arte Vetraria, [3] he speaks numerous times about what was called "calcination"—the reduction of a metal into what is usually a powdered oxide form. [4] This was a fundamental operation in producing the pigments for glass. "Reverberation" was exposing a material to secondary heat reflected from the inner walls of the furnace chamber; it was a more even heat than that produced by a direct flame.

Above the reverberation furnace is a foundry furnace, used for melting metals. Because of the high temperatures that needed to be achieved, it has a large bellows attached to force air into the fire causing a "blast" effect, which causes the fire to burn faster and hotter. Resting on the hearth is a long pair of pincers used to insert and remove crucibles of molten metal.

To the right, we see a curious arrangement of two glass retorts, each feeding into the other. They sit on purpose built stoves. The chimneys are capped by dome shaped dampers to control the draft and paddle mechanisms in front regulate heat under the glassware. A worker tends the apparatus, and the caption below reads "for extraction of the quintessence." Neri's other writing makes plain that this "quintessence" was a material that exhibits such a harmonious blending of the four essences: air, water, fire and earth that a new fifth essence (supposedly) emerged with extraordinary properties. To the right is a similar stove set-up with a single retort which is labeled "distilling." This station might have been used in preparation of the acids needed to purify metals.

At lower right, a blacksmith is hammering a piece of iron into a sword on an anvil. High-carbon "steel" was being made and used in the Medici court at the time, and that may well be what the smith is working with. Around him are several examples of his art, including two finished swords, a large copper bowl or cauldron, and a covered drinking stein made of tin.

To his left, we find a goldsmith, also hammering on an anvil, fashioning a silver platter. Displayed behind him are a variety of his handiwork, including more platters, gold necklaces and crosses and other ornamentation. A number of tools hang along the bottom of the anvil station, and behind it a small dog is curled up on the floor. 

Fig 2: Alessandro Fei (Barbieri)
"Goldsmith's Workshop" c. 1572.

About twenty-five years before Neri wrote his manuscript, Alessandro Fei painted a similar picture (fig. 2). Here we are treated to a view of a royal foundry housed in the yet to be completed Uffizi palace. [5] There is a remarkable concordance between this painting and Neri's illustration, right down to a small dog, hanging around the goldsmith's bench. 

The central figure of the painting is  Francesco de' Medici, seated in the foreground, inspecting his father's gold crown at a workbench. The dog is playfully engaging with the future Grand Duke, while on the opposite side of the bench, two other men are amiably discussing their work, perhaps silver chasing.To the left of Francesco, a younger man examines a gold ewer in the company of an older goldsmith seated across the table. The artisan is wearing spectacles and (although it is hard to see) he holds a small hammer in his hand. A worker to Francesco’s right is also intently engaged in his work. In the right mid-ground we find a small furnace with a tall chimney leading upward. On the mantle are displayed a number of gold and silver items, including platters, vessels and chains or necklaces. To the left, four men are busy seated around a large workbench. Behind that group, toward the center of the picture, are two men working at an anvil and three more at another bench on the right, accompanied by a second small dog. In an enclosed area in the background we see two large open furnace hearths, with their fireboxes glowing from underneath and men working in close proximity. In the far left background is a group entering the work area, including a figure in a yellow cape accompanied by a third dog.

The one element present in Neri's illustration that is missing in the painting is the glassware. Metalwork was a venerated and mature art long before Neri's era, which is underlined by the similarity of the two scenes, even though they are a quarter century apart, the tools, equipment and work methods are identical for all intents and purposes. There is no doubt that chemistry and distillation apparatus were used by the earlier metals artisans, the fact that Neri chose to emphasize this aspect of the work is a clear indication of his own interest and a foreshadowing of his future involvement in the production of the glass itself.

[1] Neri 1598-1600.
[2] Neri 1613, Grazzini 2012. 
[3] Neri 1612.
[4] In several of Neri's glass recipes the result of "calcination" is a sulfide.
[5] Beretta, 2014.

(Bibliography)