Weights and Measures

 

Ford Madox Brown,  The Manchester Murals: 

"The Proclamation Regarding Weights and Measures, 1556."

In his book L'Arte Vetraria, Antonio Neri's glass recipes depended on precise amounts specified in units as small as the 'grano,' [grain] named after the weight (mass) of a single grain of wheat or barley. In interpreting his formulas, the glassmaker must understand the quantities he used. For us, there are unfamiliar units like the 'fiasco' and the 'dita.' The dita or digit was simply the width of a finger. A fiasco or flask was the volume of a glass wine bottle, about two-and-a-quarter liters in Florence or two-thirds of a US gallon - about half of British imperial gallon. (As an aside, there are many fanciful stories of how the word 'fiasco' came to be synonymous with failure or disaster, perhaps the most believable is that the losers of competitions or bets were expected to buy the next round of drinks.)

In addition to unfamiliar units, there is the problem of standardization; a pound in Florence weighed different from a pound in other areas as close as Massa or Piedmont. Each Italian city maintained its own set of master weights and volumes to which merchants were expected to adhere. In reality, the differences were minor and may have been more attributable to politics than accuracy. Since antiquity, commodity merchants realized that if their own set of weights used in sales were ever so slightly below the norm, over time a savings would be realized, not large but significant. Towns could apply this principle as well; it paid to set standards slightly above or below neighboring towns from which one was buying or selling various goods. In truth, the differences were not great simply because successful commerce demanded that buyers and sellers could agree and strike a deal.

Even in different countries throughout Europe and the Mediterranean, we find close agreement in the various units of measure. Neri's first translator, Christopher Merrett, made an interesting substitution in his 1662 English version of L'Arte Vetraria. In chapter 132, Merrett writes "six pints of water" for Neri's "libre sei di acqua," changing pounds into pints. At first, it seems odd to be converting weight into volume, but this was perfectly valid. At that time in England, the pint was defined as exactly a pound (of wine or beer). Sailors were often each allotted a pint a day; the pint was also one-eighth of a cubic foot. (A cubic foot was equivalent to a gallon.) This system was very convenient for shipping companies who needed to calculate cargo volume and ballast in their trade ships as well as avoid mutiny caused by running out of beer at sea. Later, in 1824 King George IV increased the gallon from eight to ten pounds of water, invalidating Merrett's substitution.

Other conversions were more problematic. As absolute measurements varied from place to place, the size of a batch would be larger or smaller; not a big worry. However, ratios were of critical importance to a recipe. Just as in baking a cake, an entire batch of glass could be ruined by changing the ratio of materials. This sort of difficulty was especially prevalent with the size of an ounce; the troy and apothecaries system were based on a twelve-ounce pound while the avoirdupois system used a sixteen-ounce pound. When Merrett wrote his translation, England had officially been under the avoirdupois system since Henry VIII (although, In 1588, Elizabeth I complicated matters further by raising the weight of a pound by about twenty-one percent.) Meanwhile, Florence and much of Europe continued to use the troy system.

English glassmakers who wished to use Neri's book as a working document would need to know which system to use. Merrett's direct translation added a hurdle that would confuse the unaware. In order to approximate Neri's intended composition under the prevailing avoirdupois system, Merrett's "ingenious" (as he called them) British readers would need to decrease by 1/5 quantities specified in pounds, and increase ounces by 1/15.

This post first appeared on 18 September 2013.

Weights and Measures

Ford Madox Brown,  The Manchester Murals: 

"The Proclamation Regarding Weights and Measures, 1556."

In his book L'Arte Vetraria, Antonio Neri's glass recipes depended on precise amounts specified in units as small as the 'grano,' [grain] named after the weight (mass) of a single grain of wheat or barley. In interpreting his formulas, the glassmaker must understand the quantities he used. For us, there are unfamiliar units like the 'fiasco' and the 'dita.' The dita or digit was simply the width of a finger. A fiasco or flask was the volume of a glass wine bottle, about two-and-a-quarter liters in Florence or two-thirds of a US gallon - about half of British imperial gallon. (As an aside, there are many fanciful stories of how the word 'fiasco' came to be synonymous with failure or disaster, perhaps the most believable is that the losers of competitions or bets were expected to buy the next round of drinks.)

In addition to unfamiliar units, there is the problem of standardization; a pound in Florence weighed different from a pound in other areas as close as Massa or Piedmont. Each Italian city maintained its own set of master weights and volumes to which merchants were expected to adhere. In reality, the differences were minor and may have been more attributable to politics than accuracy. Since antiquity, commodity merchants realized that if their own set of weights used in sales were ever so slightly below the norm, over time a savings would be realized, not large but significant. Towns could apply this principle as well; it paid to set standards slightly above or below neighboring towns from which one was buying or selling various goods. In truth, the differences were not great simply because successful commerce demanded that buyers and sellers could agree and strike a deal.

Even in different countries throughout Europe and the Mediterranean, we find close agreement in the various units of measure. Neri's first translator, Christopher Merrett, made an interesting substitution in his 1662 English version of L'Arte Vetraria. In chapter 132, Merrett writes "six pints of water" for Neri's "libre sei di acqua," changing pounds into pints. At first, it seems odd to be converting weight into volume, but this was perfectly valid. At that time in England, the pint was defined as exactly a pound (of wine or beer). Sailors were often each allotted a pint a day; the pint was also one-eighth of a cubic foot. (A cubic foot was equivalent to a gallon.) This system was very convenient for shipping companies who needed to calculate cargo volume and ballast in their trade ships as well as avoid mutiny caused by running out of beer at sea. Later, in 1824 King George IV increased the gallon from eight to ten pounds of water, invalidating Merrett's substitution.

Other conversions were more problematic. As absolute measurements varied from place to place, the size of a batch would be larger or smaller; not a big worry. However, ratios were of critical importance to a recipe. Just as in baking a cake, an entire batch of glass could be ruined by changing the ratio of materials. This sort of difficulty was especially prevalent with the size of an ounce; the troy and apothecaries system were based on a twelve-ounce pound while the avoirdupois system used a sixteen-ounce pound. When Merrett wrote his translation, England had officially been under the avoirdupois system since Henry VIII (although, In 1588, Elizabeth I complicated matters further by raising the weight of a pound by about twenty-one percent.) Meanwhile, Florence and much of Europe continued to use the troy system.

English glassmakers who wished to use Neri's book as a working document would need to know which system to use. Merrett's direct translation added a hurdle that would confuse the unaware. In order to approximate Neri's intended composition under the prevailing avoirdupois system, Merrett's "ingenious" (as he called them) British readers would need to decrease by 1/5 quantities specified in pounds, and increase ounces by 1/15.

This post first appeared on 18 September 2013.

Weights and Measures

Ford Madox Brown,  The Manchester Murals: 

"The Proclamation Regarding Weights and Measures, 1556."

In his book L'Arte Vetraria, Antonio Neri's glass recipes depended on precise amounts specified in units as small as the 'grano,' [grain] named after the weight (mass) of a single grain of wheat or barley. In interpreting his formulas, the glassmaker must understand the quantities he used. For us, there are unfamiliar units like the 'fiasco' and the 'dita.' The dita or digit was simply the width of a finger. A fiasco or flask was the volume of a glass wine bottle, about two-and-a-quarter liters in Florence or two-thirds of a US gallon - about half of British imperial gallon. (As an aside, there are many fanciful stories of how the word 'fiasco' came to be synonymous with failure or disaster, perhaps the most believable is that the losers of competitions or bets were expected to buy the next round of drinks.)

In addition to unfamiliar units, there is the problem of standardization; a pound in Florence weighed different from a pound in other areas as close as Massa or Piedmont. Each Italian city maintained its own set of master weights and volumes to which merchants were expected to adhere. In reality, the differences were minor and may have been more attributable to politics than accuracy. Since antiquity, commodity merchants realized that if their own set of weights used in sales were ever so slightly below the norm, over time a savings would be realized, not large but significant. Towns could apply this principle as well; it paid to set standards slightly above or below neighboring towns from which one was buying or selling various goods. In truth, the differences were not great simply because successful commerce demanded that buyers and sellers could agree and strike a deal.

Even in different countries throughout Europe and the Mediterranean, we find close agreement in the various units of measure. Neri's first translator, Christopher Merrett, made an interesting substitution in his 1662 English version of L'Arte Vetraria. In chapter 132, Merrett writes "six pints of water" for Neri's "libre sei di acqua," changing pounds into pints. At first, it seems odd to be converting weight into volume, but this was perfectly valid. At that time in England, the pint was defined as exactly a pound (of wine or beer). Sailors were often each allotted a pint a day; the pint was also one-eighth of a cubic foot. (A cubic foot was equivalent to a gallon.) This system was very convenient for shipping companies who needed to calculate cargo volume and ballast in their trade ships as well as avoid mutiny caused by running out of beer at sea. Later, in 1824 King George IV increased the gallon from eight to ten pounds of water, invalidating Merrett's substitution.

Other conversions were more problematic. As absolute measurements varied from place to place, the size of a batch would be larger or smaller; not a big worry. However, ratios were of critical importance to a recipe. Just as in baking a cake, an entire batch of glass could be ruined by changing the ratio of materials. This sort of difficulty was especially prevalent with the size of an ounce; the troy and apothecaries system were based on a twelve-ounce pound while the avoirdupois system used a sixteen-ounce pound. When Merrett wrote his translation, England had officially been under the avoirdupois system since Henry VIII (although, In 1588, Elizabeth I complicated matters further by raising the weight of a pound by about twenty-one percent.) Meanwhile, Florence and much of Europe continued to use the troy system.

English glassmakers who wished to use Neri's book as a working document would need to know which system to use. Merrett's direct translation added a hurdle that would confuse the unaware. In order to approximate Neri's intended composition under the prevailing avoirdupois system, Merrett's "ingenious" (as he called them) British readers would need to decrease by 1/5 quantities specified in pounds, and increase ounces by 1/15.

This post first appeared on 18 September 2013.

Decolorizing Glass

The green tint from iron contamination

is neutralized by magenta from manganese.

For Antonio Neri and his contemporary glassmakers, contaminants, especially metallic contaminants were the bane of producing a crystal clear product. Great care was taken to ensure the purity of each ingredient at each step of the glass making process. The greatest threat of all was iron. Even small amounts will impart a green tint and for Renaissance era glassmakers, nemesis iron was everywhere. It is a common impurity in quartz, the main ingredient of glass, showing up as yellow "rust stains" both in sand and stones. It turned up in the plant salts used for glass flux as a trace-element and finally it was in the tools. Iron was in the mortars and pestles, in the pots and kettles, in the frit rakes, in the ladles, the stirring rods and in the blowpipes. A mistake at any step could easily tint the batch, even at the final stages. Neri admonishes glass workers:

Make sure never to return the neck, where the rod attaches to the glass, into the crucible of cristallo, because there are always remains of the iron that will cause it to become dark ... 

Centuries before Neri, it was discovered that the antidote to iron contaminated glass was manganese or more specifically manganese oxide, a mineral mined throughout Italy. However, Neri cautions: 

… you must always use manganese of Piedmont the way it is made for Murano, because the manganese of Tuscany and Liguria has more rust, which always make the melt dark. 

This color correction technique was known as early as the late 14th century and by Neri’s time it was a widely practiced technique in the glassmaker’s toolbox. Manganese oxide was known as a pigment in antiquity; the Magnesia region in Thessaly, central Greece was an ancient source for the black mineral. Around the year 1450, a clear crystal-like glass suitable for working into thin elaborate forms was developed in Venice. The glass was called cristallo and Murano glassmaker Angelo Barovier is often given credit. He may well be responsible for bringing together several existing techniques—manganese color correction among them — but these techniques individually were all available and utilized by glassmakers at least fifty years earlier. Barovier’s innovation would become a tradition giving Murano the reputation for the finest glass made anywhere, carried on by countless glassmakers and ultimately by our Florentine priest more than a century and a half later.

Removing the green tint of iron contamination with manganese is a clever trick. The manganese imparts a magenta tint to glass. As the complementary color to green, it effectively "cancels out" the green tint. The trade-off is that the glass is slightly darkened, even if appearing as a neutral color to the eye. When light passes through glass tinted by iron, green light is unaffected, which is why it appears green. The other colors that make up white light, notably red and violet are dampened. In effect, the green is enhanced. Now, adding manganese to the glass dampens only green light and brings the spectrum back into closer balance. The overall effect is that all the colors of light are slightly dampened, but by the same amount. In Neri's case, minor contamination from iron would produce only a small green tinge and the problem was corrected with a small dose of manganese. The resulting grey would hardly be noticed, especially in the thin, delicate pieces so popular at the time. To the eye of all but the most experienced expert, this decolorized glass had perfect crystal clarity.

Decolorization of glass is an example of a technique that was developed through experimentation without a deep understanding of the physics involved. Theories of light and color and the way they are perceived by the human eye came much later. In fact, manganese itself was not isolated as a metal until 1774, yet it was used skillfully by glass artisans three or four hundred years earlier. 

This post first appeared here in a slightly shorter form as 'Manganese of Piedmont' on 14 October 2013.

Weights and Measures

Ford Madox Brown,  The Manchester Murals: 

"The Proclamation Regarding Weights and Measures, 1556."

In his book L'Arte Vetraria, Antonio Neri's glass recipes depended on precise amounts specified in units as small as the 'grano,' [grain] named after the weight (mass) of a single grain of wheat or barley. In interpreting his formulas, the glassmaker must understand the quantities he used. For us, there are unfamiliar units like the 'fiasco' and the 'dita.' The dita or digit was simply the width of a finger. A fiasco or flask was the volume of a glass wine bottle, about two-and-a-quarter liters in Florence or two-thirds of a US gallon - about half of British imperial gallon. (As an aside, there are many fanciful stories of how the word 'fiasco' came to be synonymous with failure or disaster, perhaps the most believable is that the losers of competitions or bets were expected to buy the next round of drinks.)

In addition to unfamiliar units, there is the problem of standardization; a pound in Florence weighed different from a pound in other areas as close as Massa or Piedmont. Each Italian city maintained its own set of master weights and volumes to which merchants were expected to adhere. In reality, the differences were minor and may have been more attributable to politics than accuracy. Since antiquity, commodity merchants realized that if their own set of weights used in sales were ever so slightly below the norm, over time a savings would be realized, not large but significant. Towns could apply this principle as well; it paid to set standards slightly above or below neighboring towns from which one was buying or selling various goods. In truth, the differences were not great simply because successful commerce demanded that buyers and sellers could agree and strike a deal.

Even in different countries throughout Europe and the Mediterranean, we find close agreement in the various units of measure. Neri's first translator, Christopher Merrett, made an interesting substitution in his 1662 English version of L'Arte Vetraria. In chapter 132, Merrett writes "six pints of water" for Neri's "libre sei di acqua," changing pounds into pints. At first, it seems odd to be converting weight into volume, but this was perfectly valid. At that time in England, the pint was defined as exactly a pound (of wine or beer). Sailors were often each allotted a pint a day; the pint was also one-eighth of a cubic foot. (A cubic foot was equivalent to a gallon.) This system was very convenient for shipping companies who needed to calculate cargo volume and ballast in their trade ships as well as avoid mutiny caused by running out of beer at sea. Later, in 1824 King George IV increased the gallon from eight to ten pounds of water, invalidating Merrett's substitution.

Other conversions were more problematic. As absolute measurements varied from place to place, the size of a batch would be larger or smaller; not a big worry. However, ratios were of critical importance to a recipe. Just as in baking a cake, an entire batch of glass could be ruined by changing the ratio of materials. This sort of difficulty was especially prevalent with the size of an ounce; the troy and apothecaries system were based on a twelve-ounce pound while the avoirdupois system used a sixteen-ounce pound. When Merrett wrote his translation, England had officially been under the avoirdupois system since Henry VIII (although, In 1588, Elizabeth I complicated matters further by raising the weight of a pound by about twenty-one percent.) Meanwhile, Florence and much of Europe continued to use the troy system.

English glassmakers who wished to use Neri's book as a working document would need to know which system to use. Merrett's direct translation added a hurdle that would confuse the unaware. In order to approximate Neri's intended composition under the prevailing avoirdupois system, Merrett's "ingenious" (as he called them) British readers would need to decrease by 1/5 quantities specified in pounds, and increase ounces by 1/15.

This post first appeared on 18 September 2013.

Decolorizing Glass

The green tint from iron contamination

is neutralized by magenta from manganese.

For Antonio Neri and his contemporary glassmakers, contaminants, especially metallic contaminants were the bane of producing a crystal clear product. Great care was taken to ensure the purity of each ingredient at each step of the glass making process. The greatest threat of all was iron. Even small amounts will impart a green tint and for Renaissance era glassmakers, nemesis iron was everywhere. It is a common impurity in quartz, the main ingredient of glass, showing up as yellow "rust stains" both in sand and stones. It turned up in the plant salts used for glass flux as a trace-element and finally it was in the tools. Iron was in the mortars and pestles, in the pots and kettles, in the frit rakes, in the ladles, the stirring rods and in the blowpipes. A mistake at any step could easily tint the batch, even at the final stages. Neri admonishes glass workers:

Make sure never to return the neck, where the rod attaches to the glass, into the crucible of cristallo, because there are always remains of the iron that will cause it to become dark ... 

Centuries before Neri, it was discovered that the antidote to iron contaminated glass was manganese or more specifically manganese oxide, a mineral mined throughout Italy. However, Neri cautions: 

… you must always use manganese of Piedmont the way it is made for Murano, because the manganese of Tuscany and Liguria has more rust, which always make the melt dark. 

This color correction technique was known as early as the late 14th century and by Neri’s time it was a widely practiced technique in the glassmaker’s toolbox. Manganese oxide was known as a pigment in antiquity; the Magnesia region in Thessaly, central Greece was an ancient source for the black mineral. Around the year 1450, a clear crystal-like glass suitable for working into thin elaborate forms was developed in Venice. The glass was called cristallo and Murano glassmaker Angelo Barovier is often given credit. He may well be responsible for bringing together several existing techniques—manganese color correction among them — but these techniques individually were all available and utilized by glassmakers at least fifty years earlier. Barovier’s innovation would become a tradition giving Murano the reputation for the finest glass made anywhere, carried on by countless glassmakers and ultimately by our Florentine priest more than a century and a half later.

Removing the green tint of iron contamination with manganese is a clever trick. The manganese imparts a magenta tint to glass. As the complementary color to green, it effectively "cancels out" the green tint. The trade-off is that the glass is slightly darkened, even if appearing as a neutral color to the eye. When light passes through glass tinted by iron, green light is unaffected, which is why it appears green. The other colors that make up white light, notably red and violet are dampened. In effect, the green is enhanced. Now, adding manganese to the glass dampens only green light and brings the spectrum back into closer balance. The overall effect is that all the colors of light are slightly dampened, but by the same amount. In Neri's case, minor contamination from iron would produce only a small green tinge and the problem was corrected with a small dose of manganese. The resulting grey would hardly be noticed, especially in the thin, delicate pieces so popular at the time. To the eye of all but the most experienced expert, this decolorized glass had perfect crystal clarity.

Decolorization of glass is an example of a technique that was developed through experimentation without a deep understanding of the physics involved. Theories of light and color and the way they are perceived by the human eye came much later. In fact, manganese itself was not isolated as a metal until 1774, yet it was used skillfully by glass artisans three or four hundred years earlier. 

This post first appeared here in a slightly shorter form as 'Manganese of Piedmont' on 14 October 2013.

The Material of All Enamels

Léonard Limosin, Allegory of Catherine de' Medici as Juno,

French, 1573, Polychrome enamel  on copper and silver.

In L'Arte Vetraria, Antonio Neri's recipe book on glassmaking, he devotes the sixth chapter to making enamel. For Neri, this was a tinted form of opaque glass favored by jewelers and goldsmiths. The material was ground into a fine powder, added to a binder and painted onto glass or metals, usually copper, gold or silver. Once dry, the item was fired over a flame or in a kiln so the enamel would fuse to form a durable glossy coating. Talented workers could use various colors to paint entire scenes.

“We see ornate enameled metals in many colors and they make a pleasant and noble sight; they entice others to look and take notice.”

Neri begins by showing how to produce what he calls “the material to make all enamels.” This is a neutral-white base to which he then adds various metal oxide pigments to produce color.

He starts by mixing together thirty pounds of pure lead and thirty-three pounds of pure tin, both in a finely divided state. He heats them in a low temperature kiln that has a wide, accessible hearth. He roasts or "calcines" the metals, raking the powders around with special iron tools for many hours. This has the effect of oxidizing the metals without melting them.

He sifts the mixture and boils it in a kettle of pure clean water. The kettle is removed from the heat and the contents are allowed to settle for a while. The water is then carefully poured off, carrying with it only the finest particles still in suspension. This liquid is saved, while the sediment at the bottom of the kettle is sent back for reprocessing in the kiln. He repeats the "decanting" procedure many times, and then carefully evaporates the accumulated liquid over a low fire. What remains is an extremely fine powdered tin and lead oxide mixture.

Neri tells us that fifty pounds of this oxide is to be mixed with an equal weight of powdered cristallo frit. Cristallo was the coveted glass perfected by the Venetians on the island of Murano. "Frit" is the granulated glass before it is put into the furnace and melted. The recipe for cristallo had remained a state secret for over a century. It was the clearest, finest glass that money could buy and items made from it commanded top dollar among the richest families in Europe. Even after production methods became known to outsiders, Venice still controlled many of the raw materials through exclusive trade agreements around the Mediterranean. Neri was first to actually publish the recipes for making cristallo, in this very same book. It was made with pure white quartz pebbles from Pavia, mixed with purified salts derived from the Levantine kali plant, and decolorized with manganese from Piedmont.

In order to complete the enamel base material, Neri adds eight ounces of white tartar salt (made from the dregs of wine), sifts the mix and carefully heats it in terracotta pots for ten hours. The result is ground again and stored in a in a dry place, in a sealed container for future use.

The rest of the chapter is devoted to coloring this enamel base material; he always starts with a batch of either four or six pounds. He uses a large furnace pot to melt the enamel because even though the amount is relatively small, some of the pigments cause the batch to froth and swell violently. After adding the pigments, he lets the glass cook for a while, adjusts the color as necessary and then "washes" it several times. In this process, he throws ladlefuls of molten glass from the furnace-pot into large vats of cold clean water. The effect is to remove excess plant salts from the glass, which would ultimately foul the enamel. Finally, he forms the glass into individual dollops of about five ounces each; a size to which the goldsmiths were accustomed.

For green enamel, Neri uses copper and iron oxides in various proportions, for blues he uses "zaffer," which is a cobalt oxide. He mixes green and blue pigments to obtain turquoise colors. Manganese produced a red wine color, for yellow, he adds the unrefined dregs of red wine (potassium) with a pinch of manganese. Violet is a mix of manganese and copper oxides.

Finally, a word to the wise: Do not attempt to duplicate these recipes; fine lead powder is dangerous enough, molten lead enamel will evolve fumes that cause heavy metal poisoning. Remember that Antonio Neri was dead by age thirty-eight.

* This post first appeared here 11 July 2014.

Weights and Measures

Ford Madox Brown,  The Manchester Murals: 

"The Proclamation Regarding Weights and Measures, 1556."

In his book L'Arte Vetraria, Antonio Neri's glass recipes depended on precise amounts specified in units as small as the 'grano,' [grain] named after the weight (mass) of a single grain of wheat or barley. In interpreting his formulas, the glassmaker must understand the quantities he used. For us, there are unfamiliar units like the 'fiasco' and the 'dita.' The dita or digit was simply the width of a finger. A fiasco or flask was the volume of a glass wine bottle, about two-and-a-quarter liters in Florence or two-thirds of a US gallon - about half of British imperial gallon. (As an aside, there are many fanciful stories of how the word 'fiasco' came to be synonymous with failure or disaster, perhaps the most believable is that the losers of competitions or bets were expected to buy the next round of drinks.)

In addition to unfamiliar units, there is the problem of standardization; a pound in Florence weighed different from a pound in other areas as close as Massa or Piedmont. Each Italian city maintained its own set of master weights and volumes to which merchants were expected to adhere. In reality, the differences were minor and may have been more attributable to politics than accuracy. Since antiquity, commodity merchants realized that if their own set of weights used in sales were ever so slightly below the norm, over time a savings would be realized, not large but significant. Towns could apply this principle as well; it paid to set standards slightly above or below neighboring towns from which one was buying or selling various goods. In truth, the differences were not great simply because successful commerce demanded that buyers and sellers could agree and strike a deal.

Even in different countries throughout Europe and the Mediterranean, we find close agreement in the various units of measure. Neri's first translator, Christopher Merrett, made an interesting substitution in his 1662 English version of L'Arte Vetraria. In chapter 132, Merrett writes "six pints of water" for Neri's "libre sei di acqua," changing pounds into pints. At first, it seems odd to be converting weight into volume, but this was perfectly valid. At that time in England, the pint was defined as exactly a pound (of wine or beer). Sailors were often each allotted a pint a day; the pint was also one-eighth of a cubic foot. (A cubic foot was equivalent to a gallon.) This system was very convenient for shipping companies who needed to calculate cargo volume and ballast in their trade ships as well as avoid mutiny caused by running out of beer at sea. Later, in 1824 King George IV increased the gallon from eight to ten pounds of water, invalidating Merrett's substitution.

Other conversions were more problematic. As absolute measurements varied from place to place, the size of a batch would be larger or smaller; not a big worry. However, ratios were of critical importance to a recipe. Just as in baking a cake, an entire batch of glass could be ruined by changing the ratio of materials. This sort of difficulty was especially prevalent with the size of an ounce; the troy and apothecaries system were based on a twelve-ounce pound while the avoirdupois system used a sixteen-ounce pound. When Merrett wrote his translation, England had officially been under the avoirdupois system since Henry VIII (although, In 1588, Elizabeth I complicated matters further by raising the weight of a pound by about twenty-one percent.) Meanwhile, Florence and much of Europe continued to use the troy system.

English glassmakers who wished to use Neri's book as a working document would need to know which system to use. Merrett's direct translation added a hurdle that would confuse the unaware. In order to approximate Neri's intended composition under the prevailing avoirdupois system, Merrett's "ingenious" (as he called them) British readers would need to decrease by 1/5 quantities specified in pounds, and increase ounces by 1/15.

This post first appeared on 18 September 2013.

Decolorization of Glass

The green tint from iron contamination

is neutralized by magenta from manganese.

For Antonio Neri and his contemporary glassmakers, contaminants, especially metallic contaminants were the bane of producing a crystal clear product. Great care was taken to ensure the purity of each ingredient at each step of the glass making process. The greatest threat of all was iron. Even small amounts will impart a green tint and for Renaissance era glassmakers, nemesis iron was everywhere. It is a common impurity in quartz, the main ingredient of glass, showing up as yellow "rust stains" both in sand and stones. It turned up in the plant salts used for glass flux as a trace-element and finally it was in the tools. Iron was in the mortars and pestles, in the pots and kettles, in the frit rakes, in the ladles, the stirring rods and in the blowpipes. A mistake at any step could easily tint the batch, even at the final stages. Neri admonishes glass workers:

Make sure never to return the neck, where the rod attaches to the glass, into the crucible of cristallo, because there are always remains of the iron that will cause it to become dark ... 

Centuries before Neri, it was discovered that the antidote to iron contaminated glass was manganese or more specifically manganese oxide, a mineral mined throughout Italy. However, Neri cautions: 

… you must always use manganese of Piedmont the way it is made for Murano, because the manganese of Tuscany and Liguria has more rust, which always make the melt dark. 

This color correction technique was known as early as the late 14th century and by Neri’s time it was a widely practiced technique in the glassmaker’s toolbox. Manganese oxide was known as a pigment in antiquity; the Magnesia region in Thessaly, central Greece was an ancient source for the black mineral. Around the year 1450, a clear crystal-like glass suitable for working into thin elaborate forms was developed in Venice. The glass was called cristallo and Murano glassmaker Angelo Barovier is often given credit. He may well be responsible for bringing together several existing techniques—manganese color correction among them — but these techniques individually were all available and utilized by glassmakers at least fifty years earlier. Barovier’s innovation would become a tradition giving Murano the reputation for the finest glass made anywhere, carried on by countless glassmakers and ultimately by our Florentine priest more than a century and a half later.

Removing the green tint of iron contamination with manganese is a clever trick. The manganese imparts a magenta tint to glass. As the complementary color to green, it effectively "cancels out" the green tint. The trade-off is that the glass is slightly darkened, even if appearing as a neutral color to the eye. When light passes through glass tinted by iron, green light is unaffected, which is why it appears green. The other colors that make up white light, notably red and violet are dampened. In effect, the green is enhanced. Now, adding manganese to the glass dampens only green light and brings the spectrum back into closer balance. The overall effect is that all the colors of light are slightly dampened, but by the same amount. In Neri's case, minor contamination from iron would produce only a small green tinge and the problem was corrected with a small dose of manganese. The resulting grey would hardly be noticed, especially in the thin, delicate pieces so popular at the time. To the eye of all but the most experienced expert, this decolorized glass had perfect crystal clarity.

Decolorization of glass is an example of a technique that was developed through experimentation without a deep understanding of the physics involved. Theories of light and color and the way they are perceived by the human eye came much later. In fact, manganese itself was not isolated as a metal until 1774, yet it was used skillfully by glass artisans three or four hundred years earlier. 

This post first appeared here in a slightly shorter form as 'Manganese of Piedmont' on 14 October 2013.

The Material of All Enamels

Léonard Limosin, Allegory of Catherine de' Medici as Juno,

French, 1573, Polychrome enamel  on copper and silver.

In L'Arte Vetraria, Antonio Neri's recipe book on glassmaking, he devotes the sixth chapter to making enamel. For Neri, this was a tinted form of opaque glass favored by jewelers and goldsmiths. The material was ground into a fine powder, added to a binder and painted onto glass or metals, usually copper, gold or silver. Once dry, the item was fired over a flame or in a kiln so the enamel would fuse to form a durable glossy coating. Talented workers could use various colors to paint entire scenes.

“We see ornate enameled metals in many colors and they make a pleasant and noble sight; they entice others to look and take notice.”

Neri begins by showing how to produce what he calls “the material to make all enamels.” This is a neutral-white base to which he then adds various metal oxide pigments to produce color.

He starts by mixing together thirty pounds of pure lead and thirty-three pounds of pure tin, both in a finely divided state. He heats them in a low temperature kiln that has a wide, accessible hearth. He roasts or "calcines" the metals, raking the powders around with special iron tools for many hours. This has the effect of oxidizing the metals without melting them.

He sifts the mixture and boils it in a kettle of pure clean water. The kettle is removed from the heat and the contents are allowed to settle for a while. The water is then carefully poured off, carrying with it only the finest particles still in suspension. This liquid is saved, while the sediment at the bottom of the kettle is sent back for reprocessing in the kiln. He repeats the "decanting" procedure many times, and then carefully evaporates the accumulated liquid over a low fire. What remains is an extremely fine powdered tin and lead oxide mixture.

Neri tells us that fifty pounds of this oxide is to be mixed with an equal weight of powdered cristallo frit. Cristallo was the coveted glass perfected by the Venetians on the island of Murano. "Frit" is the granulated glass before it is put into the furnace and melted. The recipe for cristallo had remained a state secret for over a century. It was the clearest, finest glass that money could buy and items made from it commanded top dollar among the richest families in Europe. Even after production methods became known to outsiders, Venice still controlled many of the raw materials through exclusive trade agreements around the Mediterranean. Neri was first to actually publish the recipes for making cristallo, in this very same book. It was made with pure white quartz pebbles from Pavia, mixed with purified salts derived from the Levantine kali plant, and decolorized with manganese from Piedmont.

In order to complete the enamel base material, Neri adds eight ounces of white tartar salt (made from the dregs of wine), sifts the mix and carefully heats it in terracotta pots for ten hours. The result is ground again and stored in a in a dry place, in a sealed container for future use.

The rest of the chapter is devoted to coloring this enamel base material; he always starts with a batch of either four or six pounds. He uses a large furnace pot to melt the enamel because even though the amount is relatively small, some of the pigments cause the batch to froth and swell violently. After adding the pigments, he lets the glass cook for a while, adjusts the color as necessary and then "washes" it several times. In this process, he throws ladlefuls of molten glass from the furnace-pot into large vats of cold clean water. The effect is to remove excess plant salts from the glass, which would ultimately foul the enamel. Finally, he forms the glass into individual dollops of about five ounces each; a size to which the goldsmiths were accustomed.

For green enamel, Neri uses copper and iron oxides in various proportions, for blues he uses "zaffer," which is a cobalt oxide. He mixes green and blue pigments to obtain turquoise colors. Manganese produced a red wine color, for yellow, he adds the unrefined dregs of red wine (potassium) with a pinch of manganese. Violet is a mix of manganese and copper oxides.

Finally, a word to the wise: Do not attempt to duplicate these recipes; fine lead powder is dangerous enough, molten lead enamel will evolve fumes that cause heavy metal poisoning. Remember that Antonio Neri was dead by age thirty-eight.

* This post first appeared here 11 July 2014.

Weights and Measures

Ford Madox Brown,  The Manchester Murals: 

"The Proclamation Regarding Weights and Measures, 1556."

In his book L'Arte Vetraria, Antonio Neri's glass recipes depended on precise amounts specified in units as small as the 'grano,' [grain] named after the weight (mass) of a single grain of wheat or barley. In interpreting his formulas, the glassmaker must understand the quantities he used. For us, there are unfamiliar units like the 'fiasco' and the 'dita.' The dita or digit was simply the width of a finger. A fiasco or flask was the volume of a glass wine bottle, about two-and-a-quarter liters in Florence or two-thirds of a US gallon - about half of British imperial gallon. (As an aside, there are many fanciful stories of how the word 'fiasco' came to be synonymous with failure or disaster, perhaps the most believable is that the losers of competitions or bets were expected to buy the next round of drinks.)

In addition to unfamiliar units, there is the problem of standardization; a pound in Florence weighed different from a pound in other areas as close as Massa or Piedmont. Each Italian city maintained its own set of master weights and volumes to which merchants were expected to adhere. In reality, the differences were minor and may have been more attributable to politics than accuracy. Since antiquity, commodity merchants realized that if their own set of weights used in sales were ever so slightly below the norm, over time a savings would be realized, not large but significant. Towns could apply this principle as well; it paid to set standards slightly above or below neighboring towns from which one was buying or selling various goods. In truth, the differences were not great simply because successful commerce demanded that buyers and sellers could agree and strike a deal.

Even in different countries throughout Europe and the Mediterranean, we find close agreement in the various units of measure. Neri's first translator, Christopher Merrett, made an interesting substitution in his 1662 English version of L'Arte Vetraria. In chapter 132, Merrett writes "six pints of water" for Neri's "libre sei di acqua," changing pounds into pints. At first, it seems odd to be converting weight into volume, but this was perfectly valid. At that time in England, the pint was defined as exactly a pound (of wine or beer). Sailors were often each allotted a pint a day; the pint was also one-eighth of a cubic foot. (A cubic foot was equivalent to a gallon.) This system was very convenient for shipping companies who needed to calculate cargo volume and ballast in their trade ships as well as avoid mutiny by running out of beer at sea. Later, in 1824 King George IV increased the gallon from eight to ten pounds of water, invalidating Merrett's substitution.

Other conversions were more problematic. As absolute measurements varied from place to place, the size of a batch would be larger or smaller; not a big worry. However, ratios were of critical importance to a recipe. Just as in baking a cake, an entire batch of glass could be ruined by changing the ratio of materials. This sort of difficulty was especially prevalent with the size of an ounce; the troy and apothecaries system were based on a twelve-ounce pound while the avoirdupois system used a sixteen-ounce pound. When Merrett wrote his translation, England had officially been under the avoirdupois system since Henry VIII (although, In 1588, Elizabeth I complicated matters further by raising the weight of a pound by about twenty-one percent.) Meanwhile, Florence and much of Europe continued to use the troy system.

English glassmakers who wished to use Neri's book as a working document would need to know which system to use. Merrett's direct translation added a hurdle that would confuse the unaware. In order to approximate Neri's intended composition under the prevailing avoirdupois system, Merrett's "ingenious" (as he called them) British readers would need to decrease by 1/5 quantities specified in pounds, and increase ounces by 1/15.

This post first appeared on 18 September 2013.

Decolorization of Glass

The green tint from iron contamination

is neutralized by magenta from manganese.

For Antonio Neri and his contemporary glassmakers, contaminants, especially metallic contaminants were the bane of producing a crystal clear product. Great care was taken to ensure the purity of each ingredient at each step of the glass making process. The greatest threat of all was iron. Even small amounts will impart a green tint and for Renaissance era glassmakers, nemesis iron was everywhere. It is a common impurity in quartz, the main ingredient of glass, showing up as yellow "rust stains" both in sand and stones. It turned up in the plant salts used for glass flux as a trace-element and finally it was in the tools. Iron was in the mortars and pestles, in the pots and kettles, in the frit rakes, in the ladles, the stirring rods and in the blowpipes. A mistake at any step could easily tint the batch, even at the final stages. Neri admonishes glass workers:

Make sure never to return the neck, where the rod attaches to the glass, into the crucible of cristallo, because there are always remains of the iron that will cause it to become dark ... 

Centuries before Neri, it was discovered that the antidote to iron contaminated glass was manganese or more specifically manganese oxide, a mineral mined throughout Italy. However, Neri cautions: 

… you must always use manganese of Piedmont the way it is made for Murano, because the manganese of Tuscany and Liguria has more rust, which always make the melt dark. 

This color correction technique was known as early as the late 14th century and by Neri’s time it was a widely practiced technique in the glassmaker’s toolbox. Manganese oxide was known as a pigment in antiquity; the Magnesia region in Thessaly, central Greece was an ancient source for the black mineral. Around the year 1450, a clear crystal-like glass suitable for working into thin elaborate forms was developed in Venice. The glass was called cristallo and Murano glassmaker Angelo Barovier is often given credit. He may well be responsible for bringing together several existing techniques—manganese color correction among them — but these techniques individually were all available and utilized by glassmakers at least fifty years earlier. Barovier’s innovation would become a tradition giving Murano the reputation for the finest glass made anywhere, carried on by countless glassmakers and ultimately by our Florentine priest more than a century and a half later.

Removing the green tint of iron contamination with manganese is a clever trick. The manganese imparts a magenta tint to glass. As the complementary color to green, it effectively "cancels out" the green tint. The trade-off is that the glass is slightly darkened, even if neutral in color. When light passes through glass tinted by iron, green light is unaffected, which is why it appears green. The other colors that make up white light, notably red and violet are dampened. In effect, the green is enhanced. Now, adding manganese to the glass dampens only green light and brings the spectrum back into balance. The overall effect is that all the colors of light are slightly dampened, but by the same amount. In Neri's case, minor contamination from iron would produce only a small green tinge and the problem was corrected with a small dose of manganese. The resulting grey would hardly be noticed, especially in the thin, delicate pieces so popular at the time. To the eye of all but the most experienced expert, this decolorized glass had perfect crystal clarity.

Decolorization of glass is an example of a technique that was developed through experimentation without a deep understanding of the physics involved. Theories of light and color and the way they are perceived by the human eye came much later. In fact, manganese itself was not isolated as a metal until 1774, yet it was used skillfully by glass artisans three or four hundred years earlier. 

This post first appeared here in a slightly shorter form as 'Manganese of Piedmont' on 14 October 2013.

The Material of All Enamels

Léonard Limosin, Allegory of Catherine de' Medici as Juno,

French, 1573, Polychrome enamel  on copper and silver.

In L'Arte Vetraria, Antonio Neri's recipe book on glassmaking, he devotes the sixth chapter to making enamel. For Neri, this was a tinted form of opaque glass favored by jewelers and goldsmiths. The material was ground into a fine powder, added to a binder and painted onto glass or metals, usually copper, gold or silver. Once dry, the item was fired over a flame or in a kiln so the enamel would fuse to form a durable glossy coating. Talented workers could use various colors to paint entire scenes.

“We see ornate enameled metals in many colors and they make a pleasant and noble sight; they entice others to look and take notice.”

Neri begins by showing how to produce what he calls “the material to make all enamels.” This is a neutral-white base to which he then adds various metal oxide pigments to produce color.

He starts by mixing together thirty pounds of pure lead and thirty-three pounds of pure tin, both in a finely divided state. He heats them in a low temperature kiln that has a wide, accessible hearth. He roasts or "calcines" the metals, raking the powders around with special iron tools for many hours. This has the effect of oxidizing the metals without melting them.

He sifts the mixture and boils it in a kettle of pure clean water. The kettle is removed from the heat and the contents are allowed to settle for a while. The water is then carefully poured off, carrying with it only the finest particles still in suspension. This liquid is saved, while the sediment at the bottom of the kettle is sent back for reprocessing in the kiln. He repeats the "decanting" procedure many times, and then carefully evaporates the accumulated liquid over a low fire. What remains is an extremely fine powdered tin and lead oxide mixture.

Neri tells us that fifty pounds of this oxide is to be mixed with an equal weight of powdered cristallo frit. Cristallo was the coveted glass perfected by the Venetians on the island of Murano. "Frit" is the granulated glass before it is put into the furnace and melted. The recipe for cristallo had remained a state secret for over a century. It was the clearest, finest glass that money could buy and items made from it commanded top dollar among the richest families in Europe. Even after production methods became known to outsiders, Venice still controlled many of the raw materials through exclusive trade agreements around the Mediterranean. Neri was first to actually publish the recipes for making cristallo, in this very same book. It was made with pure white quartz pebbles from Pavia, mixed with purified salts derived from the Levantine kali plant, and decolorized with manganese from Piedmont.

In order to complete the enamel base material, Neri adds eight ounces of white tartar salt (made from the dregs of wine), sifts the mix and carefully heats it in terracotta pots for ten hours. The result is ground again and stored in a in a dry place, in a sealed container for future use.

The rest of the chapter is devoted to coloring this enamel base material; he always starts with a batch of either four or six pounds. He uses a large furnace pot to melt the enamel because even though the amount is relatively small, some of the pigments cause the batch to froth and swell violently. After adding the pigments, he lets the glass cook for a while, adjusts the color as necessary and then "washes" it several times. In this process, he throws ladlefuls of molten glass from the furnace-pot into large vats of cold clean water. The effect is to remove excess plant salts from the glass, which would ultimately foul the enamel. Finally, he forms the glass into individual dollops of about five ounces each; a size to which the goldsmiths were accustomed.

For green enamel, Neri uses copper and iron oxides in various proportions, for blues he uses "zaffer," which is a cobalt oxide. He mixes green and blue pigments to obtain turquoise colors. Manganese produced a red wine color, for yellow, he adds the unrefined dregs of red wine (potassium) with a pinch of manganese. Violet is a mix of manganese and copper oxides.

Finally, a word to the wise: Do not attempt to duplicate these recipes; fine lead powder is dangerous enough, molten lead enamel will evolve fumes that cause heavy metal poisoning. Remember that Antonio Neri was dead by age thirty-eight.

* This post first appeared here 11 July 2014.

Weights and Measures

Ford Madox Brown,  The Manchester Murals: 
"The Proclamation Regarding Weights and Measures, 1556."

In his book L'Arte Vetraria, Antonio Neri's glass recipes depended on precise amounts specified in units as small as the 'grano,' [grain] named after the weight (mass) of a single grain of wheat or barley. In interpreting his formulas, the glassmaker must understand the quantities he used. For us, there are unfamiliar units like the 'fiasco' and the 'dita.' The dita or digit was simply the width of a finger. A fiasco or flask was the volume of a glass wine bottle, about two-and-a-quarter liters in Florence or two-thirds of a US gallon - about half of British imperial gallon. (As an aside, there are many fanciful stories of how the word 'fiasco' came to be synonymous with failure or disaster, perhaps the most believable is that the losers of competitions or bets were expected to buy the next round of drinks.)

In addition to unfamiliar units, there is the problem of standardization; a pound in Florence weighed different from a pound in other areas as close as Massa or Piedmont. Each Italian city maintained its own set of master weights and volumes to which merchants were expected to adhere. In reality, the differences were minor and may have been more attributable to politics than accuracy. Since antiquity, commodity merchants realized that if their own set of weights used in sales were ever so slightly below the norm, over time a savings would be realized, not large but significant. Towns could apply this principle as well; it paid to set standards slightly above or below neighboring towns from which one was buying or selling various goods. In truth, the differences were not great simply because successful commerce demanded that buyers and sellers could agree and strike a deal.

Even in different countries throughout Europe and the Mediterranean, we find close agreement in the various units of measure. Neri's first translator, Christopher Merrett, made an interesting substitution in his 1662 English version of L'Arte Vetraria. In chapter 132, Merrett writes "six pints of water" for Neri's "libre sei di acqua," changing pounds into pints. At first, it seems odd to be converting weight into volume, but this was perfectly valid. At that time in England, the pint was defined as exactly a pound (of wine or beer). Sailors were often each allotted a pint a day; the pint was also one-eighth of a cubic foot. (A cubic foot was equivalent to a gallon.) This system was very convenient for shipping companies who needed to calculate cargo volume and ballast in their trade ships as well as avoid mutiny by running out of beer at sea. Later, in 1824 King George IV increased the gallon from eight to ten pounds of water, invalidating Merrett's substitution.

Other conversions were more problematic. As absolute measurements varied from place to place, the size of a batch would be larger or smaller; not a big worry. However, ratios were of critical importance to a recipe. Just as in baking a cake, an entire batch of glass could be ruined by changing the ratio of materials. This sort of difficulty was especially prevalent with the size of an ounce; the troy and apothecaries system were based on a twelve-ounce pound while the avoirdupois system used a sixteen-ounce pound. When Merrett wrote his translation, England had officially been under the avoirdupois system since Henry VIII (although, In 1588, Elizabeth I complicated matters further by raising the weight of a pound by about twenty-one percent.) Meanwhile, Florence and much of Europe continued to use the troy system.

English glassmakers who wished to use Neri's book as a working document would need to know which system to use. Merrett's direct translation added a hurdle that would confuse the unaware. In order to approximate Neri's intended composition under the prevailing avoirdupois system, Merrett's "ingenious" (as he called them) British readers would need to decrease by 1/5 quantities specified in pounds, and increase ounces by 1/15.

This post first appeared on 18 September 2013.

Manganese from Piedmont Reprise

The green tint from iron contamination
is neutralized by magenta from manganese.

For Antonio Neri, contaminants, especially metallic contaminants were the bane of producing crystal clear glass. Great care needed to be taken to ensure the purity of each ingredient at each step of the glass making process. The greatest threat of all was iron. Even small amounts will tint glass green and for Renaissance era glassmakers, nemesis iron was everywhere. It is common in quartz, the main ingredient of glass, showing up as yellow "rust stains" both in sand and stones. It turned up in the plant salts as a trace-element and finally it was in the tools. Iron was in the mortars and pestles, in the pots and kettles, in the frit rakes, in the ladles, the stirring rods and in the blowpipes. A mistake at any step could easily tint the batch, even at the final stages. Neri admonishes glass workers:

Make sure never to return the neck, where the rod attaches to the glass, into the crucible of cristallo, because there are always remains of the iron that will cause it to become dark ... 

The antidote to iron is manganese or more specifically manganese oxide, a mineral mined throughout Italy. However, Neri cautions: 

… you must always use manganese of Piedmont the way it is made for Murano, because the manganese of Tuscany and Liguria has more rust, which always make the melt dark. 

This color correction technique was known as early as the late 14th century and by Neri’s time it was a widely practiced technique in the glassmaker’s toolbox. Manganese oxide was known as a pigment in antiquity; the Magnesia region in Thessaly, central Greece was an ancient source for the black mineral. Around the year 1450, a clear crystal-like glass suitable for working into thin elaborate forms was developed in Venice. The glass was called cristallo and Murano glassmaker Angelo Barovier is often given credit. He may well be responsible for bringing together several existing techniques—manganese color correction among them — but these techniques individually were all available and utilized by glassmakers at least fifty years earlier. Barovier’s innovation would become a tradition that would become synonymous with the finest glass made anywhere, carried on by countless glassmakers and ultimately by our Florentine priest more than a century and a half later.

Removing the green tint of iron contamination with manganese is a clever trick. The manganese imparts a magenta tint to glass. As the complementary color to green, it effectively "cancels out" the green tint. The trade-off is that the glass is slightly darkened, even if neutral in color. In terms of what light does, when it passes through glass tinted by iron, green light is unaffected, while red and violet light is dampened. In effect, the green is enhanced. Now, adding manganese to the glass dampens only green light and brings the spectrum back into balance. The overall effect is that all the colors of light are slightly dampened, but by the same amount. In Neri's case, minor contamination from iron would produce only a small green tinge and the problem was corrected with a small dose of manganese. The resulting grey would hardly be noticed, especially in the thin, delicate pieces so popular at the time. To the eye of all but the most experienced expert, this decolorized glass had perfect crystal clarity.

This post first appeared here in a slightly shorter form on 14 October 2013.