Neri's Other Ruby Glass

 

Rhodochrosite, from the Sweet Home Mine, Colorado.

Antonio Neri is widely recognized for publishing a recipe for the coveted and difficult gold ruby glass. "Rubino," as it is sometimes called, achieves a deep ruby red color utilizing only powdered metallic gold as a colorant. Perhaps because of the notoriety of that prescription, Neri’s other transparent red glass is hardly known. His recipe #120 describes a deep red pigment based on manganese. Today manganese takes its place on the periodic table as an elemental metal, but in the early seventeenth century it had not been isolated from its mineral ore. What Neri calls manganese was actually its oxide, which occurs as a black powdery material. Its effects in glass have been known since the early Egyptian dynasties and before that, as a pottery glaze. By itself the oxide produces a tint often likened to violet or amethyst. In small quantities, it is used to neutralize the slight green tint introduced by iron impurities in clear glass. 

To make his ruby red pigment, Neri starts with high quality manganese oxide from Piedmont and processes it through several alchemical operations. I will not be delving into the chemistry in detail here, suffice it to say that he comes close to synthesizing a highly unstable explosive, the likes of which was not "discovered" for another two centuries. It is a striking illustration of how technical ability can be in place long before theory catches up, in this case thankfully so. 

The pigment he did succeed in making is for now a mystery. Manganese carbonate, which can form ruby red crystals might fit the bill, except that it decomposes at the temperatures of molten glass. It occurs in nature as the mineral rhodochrosite as seen above. [1] Below is Neri's recipe for "Transparent Red in Glass" from his 1612 book L’Arte Vetraria. Most of the terminology is straightforward, with the exception of a few terms. 'Porphyry' is a hard granite used for grinding stones. 'Reverberation' is indirect radiation in a furnace, where the heat is reflected from the walls. 'Sublimation' is when certain materials vaporize directly from a solid form and recondense without passing through a liquid phase.

Grind manganese impalpably, then mix it with an equal amount of refined saltpeter and put it into a clay pan set to the fire, reverberating and calcining it for 24 hours. Take it then and wash its saltiness away with warm common water. Once separated from the salt, let it dry. It will be a ruby-red color. With this, mix an equal weight of sal ammoniac and grind them together over porphyry stone with distilled vinegar, which they will soak up. Leave this alone to dry and then put it in a retort with a wide body and a long neck. Heat it in sand for 12 hours to sublimate. 

Then break up the glass. Take all the deposits in the neck and body of the retort and mix it with the residual remains in the bottom. Weigh it and combine it all with as much sal ammoniac as was lost in the first sublimation. Grind everything together over the porphyry stone, with distilled vinegar for it to soak up. Then put it in a retort to sublimate as above. Repeat this sublimation, in this manner, many times until in the end, the manganese will all remain fusible in the bottom. 

This is the medicine that tints crystal and pastes in a diaphanous red color and a ruby red as well. Use 20 oz of this medicine per ounce of cristallo or glass, but more or less may be used accordingly to govern the color. The manganese should be the very best from Piedmont, so that it will have the effect of tinting the glass a beautiful ruby color and be a sight of wonderment.

[1] Manganese carbonate, MnCO3.

Neri's Other Ruby Glass

Rhodochrosite, from the Sweet Home Mine, Colorado.

Antonio Neri is widely recognized for publishing a recipe for the coveted and difficult gold ruby glass. "Rubino," as it is sometimes called, achieves a deep ruby red color utilizing only powdered metallic gold as a colorant. Perhaps because of the notoriety of that prescription, Neri’s other transparent red glass is hardly known. His recipe #120 describes a deep red pigment based on manganese. Today manganese takes its place on the periodic table as an elemental metal, but in the early seventeenth century it had not been isolated from its mineral ore. What Neri calls manganese was actually its oxide, which occurs as a black powdery material. Its effects in glass have been known since the early Egyptian dynasties and before that, as a pottery glaze. By itself the oxide produces a tint often likened to violet or amethyst. In small quantities, it is used to neutralize the slight green tint introduced by iron impurities in clear glass. 

To make his ruby red pigment, Neri starts with high quality manganese oxide from Piedmont and processes it through several alchemical operations. I will not be delving into the chemistry in detail here, suffice it to say that he comes close to synthesizing a highly unstable explosive, the likes of which was not "discovered" for another two centuries. It is a striking illustration of how technical ability can be in place long before theory catches up, in this case thankfully so. 

The pigment he did succeed in making is for now a mystery. Manganese carbonate, which can form ruby red crystals might fit the bill, except that it decomposes at the temperatures of molten glass. It occurs in nature as the mineral rhodochrosite as seen above. [1] Below is Neri's recipe for "Transparent Red in Glass" from his 1612 book L’Arte Vetraria. Most of the terminology is straightforward, with the exception of a few terms. 'Porphyry' is a hard granite used for grinding stones. 'Reverberation' is indirect radiation in a furnace, where the heat is reflected from the walls. 'Sublimation' is when certain materials vaporize directly from a solid form and recondense without passing through a liquid phase.

Grind manganese impalpably, then mix it with an equal amount of refined saltpeter and put it into a clay pan set to the fire, reverberating and calcining it for 24 hours. Take it then and wash its saltiness away with warm common water. Once separated from the salt, let it dry. It will be a ruby-red color. With this, mix an equal weight of sal ammoniac and grind them together over porphyry stone with distilled vinegar, which they will soak up. Leave this alone to dry and then put it in a retort with a wide body and a long neck. Heat it in sand for 12 hours to sublimate. 

Then break up the glass. Take all the deposits in the neck and body of the retort and mix it with the residual remains in the bottom. Weigh it and combine it all with as much sal ammoniac as was lost in the first sublimation. Grind everything together over the porphyry stone, with distilled vinegar for it to soak up. Then put it in a retort to sublimate as above. Repeat this sublimation, in this manner, many times until in the end, the manganese will all remain fusible in the bottom. 

This is the medicine that tints crystal and pastes in a diaphanous red color and a ruby red as well. Use 20 oz of this medicine per ounce of cristallo or glass, but more or less may be used accordingly to govern the color. The manganese should be the very best from Piedmont, so that it will have the effect of tinting the glass a beautiful ruby color and be a sight of wonderment.

[1] Manganese carbonate, MnCO3.

Neri's Other Ruby Glass

Rhodochrosite, from the Sweet Home Mine, Colorado.

Antonio Neri is widely recognized for publishing a recipe for the coveted and difficult gold ruby glass. "Rubino," as it is sometimes called, achieves a deep ruby red color utilizing only powdered metallic gold as a colorant. Perhaps because of the notoriety of that prescription, Neri’s other transparent red glass is hardly known. His recipe #120 describes a deep red pigment based on manganese. Today manganese takes its place on the periodic table as an elemental metal, but in the early seventeenth century it had not been isolated from its mineral ore. What Neri calls manganese was actually its oxide, which occurs as a black powdery material. Its effects in glass have been known since the early Egyptian dynasties and before that, as a pottery glaze. By itself the oxide produces a tint often likened to violet or amethyst. In small quantities, it is used to neutralize the slight green tint introduced by iron impurities in clear glass. 

To make his ruby red pigment, Neri starts with high quality manganese oxide from Piedmont and processes it through several alchemical operations. I will not be delving into the chemistry in detail here, suffice it to say that he comes close to synthesizing a highly unstable explosive, the likes of which was not "discovered" for another two centuries. It is a striking illustration of how technical ability can be in place long before theory catches up, in this case thankfully so. 

The pigment he did succeed in making is for now a mystery. Manganese carbonate, which can form ruby red crystals might fit the bill, except that it decomposes at the temperatures of molten glass. It occurs in nature as the mineral rhodochrosite as seen above. [1] Below is Neri's recipe for "Transparent Red in Glass" from his 1612 book L’Arte Vetraria. Most of the terminology is straightforward, with the exception of a few terms. 'Porphyry' is a hard granite used for grinding stones. 'Reverberation' is indirect radiation in a furnace, where the heat is reflected from the walls. 'Sublimation' is when certain materials vaporize directly from a solid form and recondense without passing through a liquid phase.

Grind manganese impalpably, then mix it with an equal amount of refined saltpeter and put it into a clay pan set to the fire, reverberating and calcining it for 24 hours. Take it then and wash its saltiness away with warm common water. Once separated from the salt, let it dry. It will be a ruby-red color. With this, mix an equal weight of sal ammoniac and grind them together over porphyry stone with distilled vinegar, which they will soak up. Leave this alone to dry and then put it in a retort with a wide body and a long neck. Heat it in sand for 12 hours to sublimate. 

Then break up the glass. Take all the deposits in the neck and body of the retort and mix it with the residual remains in the bottom. Weigh it and combine it all with as much sal ammoniac as was lost in the first sublimation. Grind everything together over the porphyry stone, with distilled vinegar for it to soak up. Then put it in a retort to sublimate as above. Repeat this sublimation, in this manner, many times until in the end, the manganese will all remain fusible in the bottom. 

This is the medicine that tints crystal and pastes in a diaphanous red color and a ruby red as well. Use 20 oz of this medicine per ounce of cristallo or glass, but more or less may be used accordingly to govern the color. The manganese should be the very best from Piedmont, so that it will have the effect of tinting the glass a beautiful ruby color and be a sight of wonderment.

[1] Manganese carbonate, MnCO3.

Neri's Other Ruby Glass

Rhodochrosite, from the Sweet Home Mine, Colorado.

Antonio Neri is widely recognized for publishing a recipe for the coveted and difficult gold ruby glass. "Rubino," as it is sometimes called, achieves a deep ruby red color utilizing only powdered metallic gold as a colorant. Perhaps because of the notoriety of that prescription, Neri’s other transparent red glass is hardly known. His recipe #120 describes a deep red pigment based on manganese. Today manganese takes its place on the periodic table as an elemental metal, but in the early seventeenth century it had not been isolated from its mineral ore. What Neri calls manganese was actually its oxide, which occurs as a black powdery material. Its effects in glass have been known since the early Egyptian dynasties and before that, as a pottery glaze. By itself the oxide produces a tint often likened to violet or amethyst. In small quantities, it is used to neutralize the slight green tint introduced by iron impurities in clear glass. 

To make his ruby red pigment, Neri starts with high quality manganese oxide from Piedmont and processes it through several alchemical operations. I will not be delving into the chemistry in detail here, suffice it to say that he comes close to synthesizing a highly unstable explosive, the likes of which was not "discovered" for another two centuries. It is a striking illustration of how technical ability can be in place long before theory catches up, in this case thankfully so. 

The pigment he did succeed in making is for now a mystery. Manganese carbonate, which can form ruby red crystals might fit the bill, except that it decomposes at the temperatures of molten glass. It occurs in nature as the mineral rhodochrosite as seen above. [1] Below is Neri's recipe for "Transparent Red in Glass" from his 1612 book L’Arte Vetraria. Most of the terminology is straightforward, with the exception of a few terms. 'Porphyry' is a hard granite used for grinding stones. 'Reverberation' is indirect radiation in a furnace, where the heat is reflected from the walls. 'Sublimation' is when certain materials vaporize directly from a solid form and recondense without passing through a liquid phase.

Grind manganese impalpably, then mix it with an equal amount of refined saltpeter and put it into a clay pan set to the fire, reverberating and calcining it for 24 hours. Take it then and wash its saltiness away with warm common water. Once separated from the salt, let it dry. It will be a ruby-red color. With this, mix an equal weight of sal ammoniac and grind them together over porphyry stone with distilled vinegar, which they will soak up. Leave this alone to dry and then put it in a retort with a wide body and a long neck. Heat it in sand for 12 hours to sublimate. 

Then break up the glass. Take all the deposits in the neck and body of the retort and mix it with the residual remains in the bottom. Weigh it and combine it all with as much sal ammoniac as was lost in the first sublimation. Grind everything together over the porphyry stone, with distilled vinegar for it to soak up. Then put it in a retort to sublimate as above. Repeat this sublimation, in this manner, many times until in the end, the manganese will all remain fusible in the bottom. 

This is the medicine that tints crystal and pastes in a diaphanous red color and a ruby red as well. Use 20 oz of this medicine per ounce of cristallo or glass, but more or less may be used accordingly to govern the color. The manganese should be the very best from Piedmont, so that it will have the effect of tinting the glass a beautiful ruby color and be a sight of wonderment.

[1] Manganese carbonate, MnCO3.

Neri's Other Ruby Glass

Rhodochrosite, from the Sweet Home Mine, Colorado.

Antonio Neri is widely recognized for publishing a recipe for the coveted and difficult gold ruby glass. "Rubino," as it is sometimes called, achieves a deep ruby red color utilizing only powdered metallic gold as a colorant. Perhaps because of the notoriety of that prescription, Neri’s other transparent red glass is hardly known. His recipe #120 describes a deep red pigment based on manganese. Today manganese takes its place on the periodic table as an elemental metal, but in the early seventeenth century it had not been isolated from its mineral ore. What Neri calls manganese was actually its oxide, which occurs as a black powdery material. Its effects in glass have been known since the early Egyptian dynasties and before that, as a pottery glaze. By itself the oxide produces a tint often likened to violet or amethyst. In small quantities, it is used to neutralize the slight green tint introduced by iron impurities in clear glass. 

To make his ruby red pigment, Neri starts with high quality manganese oxide from Piedmont and processes it through several alchemical operations. I will not be delving into the chemistry in detail here, suffice it to say that he comes close to synthesizing a highly unstable explosive, the likes of which was not "discovered" for another two centuries. It is a striking illustration of how technical ability can be in place long before theory catches up, in this case thankfully so. 

The pigment he did succeed in making is for now a mystery. Manganese carbonate, which can form ruby red crystals might fit the bill, except that it decomposes at the temperatures of molten glass. It occurs in nature as the mineral rhodochrosite as seen above. [1] Below is Neri's recipe for "Transparent Red in Glass" from his 1612 book L’Arte Vetraria. Most of the terminology is straightforward, with the exception of a few terms. 'Porphyry' is a hard granite used for grinding stones. 'Reverberation' is indirect radiation in a furnace, where the heat is reflected from the walls. 'Sublimation' is when certain materials vaporize directly from a solid form and recondense without passing through a liquid phase.

Grind manganese impalpably, then mix it with an equal amount of refined saltpeter and put it into a clay pan set to the fire, reverberating and calcining it for 24 hours. Take it then and wash its saltiness away with warm common water. Once separated from the salt, let it dry. It will be a ruby-red color. With this, mix an equal weight of sal ammoniac and grind them together over porphyry stone with distilled vinegar, which they will soak up. Leave this alone to dry and then put it in a retort with a wide body and a long neck. Heat it in sand for 12 hours to sublimate. 

Then break up the glass. Take all the deposits in the neck and body of the retort and mix it with the residual remains in the bottom. Weigh it and combine it all with as much sal ammoniac as was lost in the first sublimation. Grind everything together over the porphyry stone, with distilled vinegar for it to soak up. Then put it in a retort to sublimate as above. Repeat this sublimation, in this manner, many times until in the end, the manganese will all remain fusible in the bottom. 

This is the medicine that tints crystal and pastes in a diaphanous red color and a ruby red as well. Use 20 oz of this medicine per ounce of cristallo or glass, but more or less may be used accordingly to govern the color. The manganese should be the very best from Piedmont, so that it will have the effect of tinting the glass a beautiful ruby color and be a sight of wonderment.

[1] Manganese carbonate, MnCO3.

Neri's other Rubino Glass

Rhodochrosite, from the Sweet Home Mine, Colorado.

Antonio Neri is widely recognized for publishing a recipe for the coveted and difficult gold ruby glass. "Rubino", as it is sometimes called, achieves a deep ruby red color utilizing only powdered metallic gold as a colorant. Perhaps because of the notoriety of that prescription, Neri’s other transparent red glass is hardly known. His recipe #120 describes a deep red pigment based on manganese. Today manganese takes its place on the periodic table as an elemental metal, but in the early seventeenth century it had not been isolated from its mineral ore. What Neri calls manganese was actually its oxide, which occurs as a black powdery material. Its effects in glass have been known since the early Egyptian dynasties and before that, as a pottery glaze. By itself the oxide produces a tint often likened to violet or amethyst. In small quantities, it is used to neutralize the slight green tint introduced by iron impurities in clear glass. 

To make his ruby red pigment, Neri starts with high quality manganese oxide from Piedmont and processes it through several alchemical operations. I will not be delving into the chemistry in detail here, but suffice it to say that he comes astoundingly close to synthesizing a highly unstable explosive, the likes of which was not "discovered" for another two centuries. It is a striking illustration of how technical ability can be in place long before theory catches up, in this case thankfully so. 

The pigment he did succeed in making is for now a mystery. Manganese carbonate, which can form ruby red crystals might fit the bill, except that it decomposes at the temperatures of molten glass. It occurs in nature as the mineral rhodochrosite as seen above. [1] Below is Neri's recipe for "Transparent Red in Glass" from his 1612 book L’Arte Vetraria. Most of the terminology is straightforward, with the exception of a few terms. 'Porphyry' is a hard granite used for grinding stones. 'Reverberation' is indirect radiation in a furnace, where the heat is reflected from the walls. 'Sublimation' is when certain materials vaporize directly from a solid form and recondense without passing through a liquid phase.

Grind manganese impalpably, then mix it with an equal amount of refined saltpeter and put it into a clay pan set to the fire, reverberating and calcining it for 24 hours. Take it then and wash its saltiness away with warm common water. Once separated from the salt, let it dry. It will be a ruby-red color. With this, mix an equal weight of sal ammoniac and grind them together over porphyry stone with distilled vinegar, which they will soak up. Leave this alone to dry and then put it in a retort with a wide body and a long neck. Heat it in sand for 12 hours to sublimate. 

Then break up the glass. Take all the deposits in the neck and body of the retort and mix it with the residual remains in the bottom. Weigh it and combine it all with as much sal ammoniac as was lost in the first sublimation. Grind everything together over the porphyry stone, with distilled vinegar for it to soak up. Then put it in a retort to sublimate as above. Repeat this sublimation, in this manner, many times until in the end, the manganese will all remain fusible in the bottom. 

This is the medicine that tints crystal and pastes in a diaphanous red color and a ruby red as well. Use 20 oz of this medicine per ounce of cristallo or glass, but more or less may be used accordingly to govern the color. The manganese should be the very best from Piedmont, so that it will have the effect of tinting the glass a beautiful ruby color and be a sight of wonderment.

[1] Manganese carbonate, MnCO3.

Manganese Overload

The chapel of Santa Cristina, overlooking
the valley near Monte di Voragno, in
Piedmont, Italy where manganese ore was mined
for the Venetian glass industry.

In Antonio Neri's 1612 book on glassmaking he presents a recipe for a transparent red (rubino) glass using manganese oxide as a pigment. The prescription calls for twenty ounces of colorant to each ounce of cristallo or glass. Indeed, it is an almost preposterous ratio, so let us take a closer look.

Checking the translation from Italian is the first order of business. Revisiting the original text is a simple matter, which these days is conveniently accessible online here.  On page 104 (recipe # 120) of Neri’s original 1612 edition of L’Arte Vetraria, (lines 11-12) we find: “…di questa medicina si da venti per oncia di cristallo ò vetro...”  [of this medicine will be twenty per ounce of cristallo or glass]. [1] So it seems the translation is accurate.

The second edition of Neri’s book was printed in 1661, almost fifty years after the glassmaker’s death. [2] The editor claims the correction of a number of errors, but these turn out to be mainly changes in grammatical convention; our passage is identical to the original. Likewise for Christopher Merrett’s 1662 English translation: “…there are used of this medicine 20 ounces, to one of crystall or glass…” and he makes no further comments in his observations section. So it appears for the twenty odd other editions that published over the next two centuries in Latin, German, French and Spanish.

Nevertheless, something is definitely amiss, since in his other recipes Neri always specifies the amount of glass in pounds; the only times he uses ounces are in prescriptions for artificial gems when quite small amounts of material are being prepared. Perhaps we can find a clue elsewhere in the book. In chapter 13 he describes preparing the raw manganese ore, mined in the Piedmont region of Italy. He says, “In Venice, you can always find it in abundance, since on Murano they do not use any other manganese.” For his garnet color (#47) he uses one pound of manganese to one hundred pounds of glass. For a garnet color in lead glass the ratio is 20 pounds of cristallo, 16 pounds of lead oxide, and three ounces of manganese.

Manganese oxide played and still plays a versatile role in glassmaking. In relatively small quantities it performs the role of a color neutralizer, removing the slight greenish tint introduced by iron contamination in glass. On the other extreme, it was used by Neri in conjunction with potassium carbonates and cobalt oxide to make black glass, but never in quantities more than a few percent by weight. Between the two extremes, manganese was used to make violet, wine, garnet, and amethyst colored glass. 

Judging from these other recipes and Neri’s failure to explicitly remark in this one about the outlandish ratio,  we are forced to conclude  that the twenty to one ratio of color to glass is an error that slipped through, apparently unnoticed or at least not commented upon until now. I have a fondness to gravitate to the simplest explanation, which in this case is an inadvertent reversal of the amounts; in other words the ratio should be one part pigment to twenty parts glass. Unfortunately this does not speak to why he is quoting amounts in ounces when pounds are his norm. 

As unsatisfying as it is, a well reasoned answer to this riddle does not present itself. Perhaps Neri left a clue in another of his writings. Sometimes, when we follow the facts, historical research forces us to take the most courageous stand of all: we simply do not know.

[1] Neri 1612.
[2] Neri 1661.
[3] Neri 1662.

* This post first appeared here in a slightly different form on 28 November 2014.

Neri's Other Rubino

Rhodochrosite, from the Sweet Home Mine, Colorado.

Antonio Neri is widely recognized for publishing a recipe for the coveted and difficult gold ruby glass. "Rubino" as it is sometimes called achieves a deep ruby red color utilizing only powdered metallic gold as a colorant. Perhaps because of the notoriety of that prescription, Neri’s other transparent red glass is hardly known. His recipe #120 describes a deep red pigment based on manganese. Today manganese takes its place on the periodic table as an elemental metal, but in the early seventeenth century it had not been isolated from its mineral ore. What Neri calls manganese was actually its oxide, which occurs as a black powdery material. Its effects in glass have been known since the early Egyptian dynasties and before that, as a pottery glaze. By itself the oxide produces a tint often likened to violet or amethyst. In small quantities, it is used to neutralize the slight green tint introduced by iron impurities in clear glass. 

To make his ruby red pigment, Neri starts with high quality manganese oxide from Piedmont and processes it through several alchemical operations. I will not be delving into the chemistry in detail here, but suffice it to say that he comes astoundingly close to synthesizing a highly unstable explosive, the likes of which was not "discovered" for another two centuries. It is a striking illustration of how technical ability can be in place long before theory catches up, in this case thankfully so. 

The pigment he did succeed in making is for now a mystery. Manganese carbonate, which can form ruby red crystals might fit the bill, except that it decomposes at the temperatures of molten glass. It occurs in nature as the mineral rhodochrosite as seen above. [1] Below is Neri's recipe for "Transparent Red in Glass" from his 1612 book L’Arte Vetraria. Most of the terminology is straightforward, with the exception of a few terms. 'Porphyry' is a hard granite used for grinding stones. 'Reverberation' is indirect radiation in a furnace, where the heat is reflected from the walls. 'Sublimation' is when certain materials vaporize directly from a solid form and recondense without passing through a liquid phase.

Grind manganese impalpably, then mix it with an equal amount of refined saltpeter and put it into a clay pan set to the fire, reverberating and calcining it for 24 hours. Take it then and wash its saltiness away with warm common water. Once separated from the salt, let it dry. It will be a ruby-red color. With this, mix an equal weight of sal ammoniac and grind them together over porphyry stone with distilled vinegar, which they will soak up. Leave this alone to dry and then put it in a retort with a wide body and a long neck. Heat it in sand for 12 hours to sublimate. 

Then break up the glass. Take all the deposits in the neck and body of the retort and mix it with the residual remains in the bottom. Weigh it and combine it all with as much sal ammoniac as was lost in the first sublimation. Grind everything together over the porphyry stone, with distilled vinegar for it to soak up. Then put it in a retort to sublimate as above. Repeat this sublimation, in this manner, many times until in the end, the manganese will all remain fusible in the bottom. 

This is the medicine that tints crystal and pastes in a diaphanous red color and a ruby red as well. Use 20 oz of this medicine per ounce of cristallo or glass, but more or less may be used accordingly to govern the color. The manganese should be the very best from Piedmont, so that it will have the effect of tinting the glass a beautiful ruby color and be a sight of wonderment.

[1] Manganese carbonate, MnCO3.

More on Manganese

The chapel of Santa Cristina, overlooking
the valley near Monte di Voragno, in
Piedmont, Italy where manganese ore was mined
for the Venetian glass industry.

Last week, I detailed a recipe for a transparent red (rubino) glass using manganese (oxide) as a pigment. A reader pointed out that Antonio Neri’s prescribed dose seemed a bit excessive; twenty ounces of colorant to each ounce of cristallo or glass. I must admit this particular detail escaped my attention when I was putting the post together. Indeed, it is an almost preposterous ratio, so let us take a closer look.

The translation from Italian in that posting was by yours truly, so the first order of business must be to revisit the original text, which these days, is conveniently accessible online here.  On page 104 (recipe # 120) of Neri’s original 1612 edition of L’Arte Vetraria, (lines 11-12) we find: “…di questa medicina si da venti per oncia di cristallo ò vetro...”  [of this medicine will be twenty per ounce of cristallo or glass]. [1] So it seems my high-school Italian is not to blame after all.

The second edition of Neri’s book was printed in 1661, almost fifty years after the glassmaker’s death. [2] The editor claims the correction of a number of errors, but these turn out to be mainly changes in grammatical convention; our passage is identical to the original. Likewise for Christopher Merrett’s 1662 English translation: “…there are used of this medicine 20 ounces, to one of crystall or glass…” and he makes no further comments in his observations section. So it appears for the twenty odd other editions that published over the next two centuries in Latin, German, French and Spanish.

Nevertheless, something is definitely amiss, since in his other recipes Neri always specifies the amount of glass in pounds; the only times he uses ounces are in prescriptions for artificial gems when quite small amounts of material are being prepared. Perhaps we can find a clue elsewhere in the book. In chapter 13 he describes preparing the raw manganese ore, mined in the Piedmont region of Italy. He says, “In Venice, you can always find it in abundance, since on Murano they do not use any other manganese.” For his garnet color (#47) he uses one pound of manganese to one hundred pounds of glass. For a garnet color in lead glass the ratio is 20 pounds of cristallo, 16 pounds of lead oxide, and three ounces of manganese.

Manganese oxide played and still plays a versatile role in glassmaking. In relatively small quantities it performs the role of a color neutralizer, removing the slight greenish tint introduced by iron contamination in glass. On the other extreme, it was used by Neri in conjunction with potassium carbonates and cobalt oxide to make black glass, but never in quantities more than a few percent by weight. Between the two extremes, manganese was used to make violet, wine, garnet, and amethyst colored glass. 

Judging from these other recipes and Neri’s failure to explicitly remark in this one about the outlandish ratio,  we are forced to conclude  that the twenty to one ratio of color to glass is an error that slipped through, apparently unnoticed or at least not commented upon until this week. I have a fondness to gravitate to the simplest explanation, which in this case is an inadvertent reversal of the amounts; in other words the ratio should be one part pigment to twenty parts glass. Unfortunately this does not speak to why he is quoting amounts in ounces when pounds are his norm. 

As unsatisfying as it is, a well reasoned answer to this riddle does not present itself. Perhaps Neri left a clue in another of his writings. Sometimes, when we follow the facts, historical research forces us to take the most courageous stand of all: we simply do not know.

[1] Neri 1612.
[2] Neri 1661.
[3] Neri 1662.

Overshadowed Rubino

Rhodochrosite, from the Sweet Home Mine, Colorado.

Antonio Neri is widely recognized for publishing a recipe for the coveted and difficult gold ruby glass. "Rubino" as it is sometimes called achieves a deep ruby red color utilizing only powdered metallic gold as a colorant. Perhaps because of the notoriety of that prescription, Neri’s other transparent red glass is hardly known. His recipe #120 describes a deep red pigment based on manganese. Today manganese takes its place on the periodic table as an elemental metal, but in the early seventeenth century it had not been isolated from its mineral ore. What Neri calls manganese was actually its oxide, which occurs as a black powdery material. Its effects in glass have been known since the early Egyptian dynasties and before that, as a pottery glaze. By itself the oxide produces a tint often likened to violet or amethyst. In small quantities, it is used to neutralize the slight green tint introduced by iron impurities in clear glass. 

To make his ruby red pigment, Neri starts with high quality manganese oxide from Piedmont and processes it through several alchemical operations. I will not be delving into the chemistry in detail here, but suffice it to say that he comes astoundingly close to synthesizing a highly unstable explosive, the likes of which was not "discovered" for another two centuries. It is a striking illustration of how technical ability can be in place long before theory catches up, in this case thankfully so. 

The pigment he did succeed in making is for now a mystery. Manganese carbonate, which can form ruby red crystals might fit the bill, except that it decomposes at the temperatures of molten glass. It occurs in nature as the mineral rhodochrosite as seen above. [1] Below is Neri's recipe for "Transparent Red in Glass" from his 1612 book L’Arte Vetraria. Most of the terminology is straightforward, with the exception of a few terms. 'Porphyry' is a hard granite used for grinding stones. 'Reverberation' is indirect radiation in a furnace, where the heat is reflected from the walls. 'Sublimation' is when certain materials vaporize directly from a solid form and recondense without passing through a liquid phase.

Grind manganese impalpably, then mix it with an equal amount of refined saltpeter and put it into a clay pan set to the fire, reverberating and calcining it for 24 hours. Take it then and wash its saltiness away with warm common water. Once separated from the salt, let it dry. It will be a ruby-red color. With this, mix an equal weight of sal ammoniac and grind them together over porphyry stone with distilled vinegar, which they will soak up. Leave this alone to dry and then put it in a retort with a wide body and a long neck. Heat it in sand for 12 hours to sublimate. 

Then break up the glass. Take all the deposits in the neck and body of the retort and mix it with the residual remains in the bottom. Weigh it and combine it all with as much sal ammoniac as was lost in the first sublimation. Grind everything together over the porphyry stone, with distilled vinegar for it to soak up. Then put it in a retort to sublimate as above. Repeat this sublimation, in this manner, many times until in the end, the manganese will all remain fusible in the bottom. 

This is the medicine that tints crystal and pastes in a diaphanous red color and a ruby red as well. Use 20 oz of this medicine per ounce of cristallo or glass, but more or less may be used accordingly to govern the color. The manganese should be the very best from Piedmont, so that it will have the effect of tinting the glass a beautiful ruby color and be a sight of wonderment.

[1] Manganese carbonate, MnCO3.