Crocus Martis

The many different alchemical symbols used to denote crocus martis.

In order to understand the seventeenth century glass recipes of Antonio Neri and for that matter, any alchemical recipes, it is first necessary to have a grasp of the chemical repertoire; no degree in chemistry required. The ingredients referenced by Neri were not especially exotic, but over the intervening four centuries since he wrote, the common names of materials have changed considerably. In the current lexicon, we name materials by their chemical formulas, so iron oxide names its two constituent elements, iron and oxygen. But in the seventeenth century, the periodic table of elements was a concept yet to be invented; hence "crocus Martis." Martis refers to Mars, the Roman god of war, the red planet, and the ancient alchemical name for iron. The origins of crocus are lost, but it may refer to the similar looking spice saffron, (crocus sativus) which has been highly valued since antiquity. Poetically, 'saffron of Mars' alludes to the orange-red color of rust. 

A first encounter with the technical recipes of sixteenth and seventeenth century alchemy can be confusing, frustrating and more than a little disorienting. There is no shortage of records, letters, manuscripts and recipe books that have been preserved from that period, however, the challenge lays in making sense of them in a way that relates to our current view of the world. It is understandable that alchemical materials and compounds had unfamiliar names; but even larger difficulties arise with the realization that any given name might describe several different chemicals, and in fact, there may be several different interchangeable synonyms/symbols for any given name. As it happens, there were good reasons for this state of affairs. The challenge of deciphering alchemy is not insurmountable. Taking the time to understand really opens up a window onto a strange and wonderful landscape of history. 

Antonio Neri considered himself an alchemist first and glassmaker second. His purely alchemical works are somewhat cryptic, but in the glass book he bends over backwards to be accessible to novices. Because of this crossover, his work can help us to navigate details in both areas that might otherwise go without explanation. His writings broadly divide into two categories: one intended for generally curious readers and another intended only for those familiar with the arcane coded language of alchemy. We have examples of both styles by Neri and all written within the period of about fifteen years at the beginning of the seventeenth century. As a result, we can use his works intended for a general audience to decode some of his more arcane passages elsewhere. With the glass recipes, his careful explanations lead us to gain considerable confidence in his technical abilities. In the introduction to his famous book on glassmaking, L'Arte Vetraria, he wrote, "I have described every last detail clearly and distinctly in this work, I am sure that if you do not purposely foul up, it will be impossible to fail, after having acquired experience and practice." Even in Neri's more obscure works, he earned the respect of later chemists; in 1870, in the journal Nature, George Rodwell, the first science master of Marlborough College, pronouncing him a "sensible chemist." Rodwell went on to note that Neri had used "no less than thirty-five different names, and twenty-two symbols" to denote a single material, the metal mercury. 

In still other writings, it becomes clear that Neri was deeply concerned that some alchemical operations were inappropriate for general consumption and were better kept secret among true practitioners of the art. In particular, he worried that if the transmutation of base metals into gold and silver were practiced widely, the result would be a collapse of the economy and would ultimately plunge civilization into disarray.

We will start with his glass book, where his materials and methods are detailed with special consideration for those not familiar with the art. Then perhaps we can build on that and make sense of his more esoteric material.

In L'Arte Vetraria, Neri gives four different methods to make crocus Martis. He explains: 

Crocus Martis is nothing other that a refinement and calcination of iron. A means by which its pigment, that in glass is a deep rutty red, is opened and imparted to the glass. It not only manifests itself but makes all the other metallic colors as well, which ordinarily hide and are dead in the glass, dance in resplendent apparition. Since this is the way to make the hidden metallic colors appear, I have put down four ways to make it.

The term calcination means only 'to roast' in a hot oven. It derives from the ancient practice of cooking seashells into powdered quicklime, which is a prime ingredient of cement; now called calcium oxide.  In his first method for crocus Martis, Neri mixes iron filings with sulfur and then heats the mixture in the furnace for a long period. We can guess that the result is a mix of iron oxide and iron sulfide. These are the constituents of a popular red pigment of the same name (Crocus Martis) used in pottery glazes. In the second method, he takes the iron and sulfur mixture and sprinkles it with vinegar and leaves it in the sun for many days. The third method uses aqua fortis (nitric acid) for better effect. In the fourth and final method, Neri uses aqua regis, an even stronger acid. 

In all four cases the predominant result will be for the acids to chemically react with the metal to form iron oxide and sulfide, but alas, chemistry is not that simple; there will be minor concentrations of other compounds depending on the acid used, which may or may not affect the final product. We must consider that vinegar and the other acids in the seventeenth century were significantly different from those products today. They were made by different methods and contained a variety of impurities that would never be found in the current products. As Neri notes, each method produces different effects in the glass and therefore we must conclude that each, to some extent, differs in chemistry as well. Today, we know that iron forms several different oxides, each responsible for different color effects in glass. In a number of green glass recipes Neri uses crocus made with vinegar, yet in chapter 71 he uses it to diminish the green in yellow lead crystal. In chapter 124 he uses crocus made with aqua fortis for red glass. The lesson here is that the name of a material tells us only the basics, how it was made is at least as important if not more so. 

* this post first appeared in a somewhat different form on 15 August 2014. 

Crocus Martis

The many different alchemical symbols used to denote crocus martis.

In order to understand the seventeenth century glass recipes of Antonio Neri and for that matter, any alchemical recipes, it is first necessary to have a grasp of the chemical repertoire; no degree in chemistry required. The ingredients referenced by Neri were not especially exotic, but over the intervening four centuries since he wrote, the common names of materials have changed considerably. In the current lexicon, we name materials by their chemical formulas, so iron oxide names its two constituent elements, iron and oxygen. But in the seventeenth century, the periodic table of elements was a concept yet to be invented; hence "crocus Martis." Martis refers to Mars, the Roman god of war, the red planet, and the ancient alchemical name for iron. The origins of crocus are lost, but it may refer to the similar looking spice saffron, (crocus sativus) which has been highly valued since antiquity. Poetically, 'saffron of Mars' alludes to the orange-red color of rust. 

A first encounter with the technical recipes of sixteenth and seventeenth century alchemy can be confusing, frustrating and more than a little disorienting. There is no shortage of records, letters, manuscripts and recipe books that have been preserved from that period, however, the challenge lays in making sense of them in a way that relates to our current view of the world. It is understandable that alchemical materials and compounds had unfamiliar names; but even larger difficulties arise with the realization that any given name might describe several different chemicals, and in fact, there may be several different interchangeable synonyms/symbols for any given name. As it happens, there were good reasons for this state of affairs. The challenge of deciphering alchemy is not insurmountable. Taking the time to understand really opens up a window onto a strange and wonderful landscape of history. 

Antonio Neri considered himself an alchemist first and glassmaker second. His purely alchemical works are somewhat cryptic, but in the glass book he bends over backwards to be accessible to novices. Because of this crossover, his work can help us to navigate details in both areas that might otherwise go without explanation. His writings broadly divide into two categories: one intended for generally curious readers and another intended only for those familiar with the arcane coded language of alchemy. We have examples of both styles by Neri and all written within the period of about fifteen years at the beginning of the seventeenth century. As a result, we can use his works intended for a general audience to decode some of his more arcane passages elsewhere. With the glass recipes, his careful explanations lead us to gain considerable confidence in his technical abilities. In the introduction to his famous book on glassmaking, L'Arte Vetraria, he wrote, "I have described every last detail clearly and distinctly in this work, I am sure that if you do not purposely foul up, it will be impossible to fail, after having acquired experience and practice." Even in Neri's more obscure works, he earned the respect of later chemists; in 1870, in the journal Nature, George Rodwell, the first science master of Marlborough College, pronouncing him a "sensible chemist." Rodwell went on to note that Neri had used "no less than thirty-five different names, and twenty-two symbols" to denote a single material, the metal mercury. 

In still other writings, it becomes clear that Neri was deeply concerned that some alchemical operations were inappropriate for general consumption and were better kept secret among true practitioners of the art. In particular, he worried that if the transmutation of base metals into gold and silver were practiced widely, the result would be a collapse of the economy and would ultimately plunge civilization into disarray.

We will start with his glass book, where his materials and methods are detailed with special consideration for those not familiar with the art. Then perhaps we can build on that and make sense of his more esoteric material.

In L'Arte Vetraria, Neri gives four different methods to make crocus Martis. He explains: 

Crocus Martis is nothing other that a refinement and calcination of iron. A means by which its pigment, that in glass is a deep rutty red, is opened and imparted to the glass. It not only manifests itself but makes all the other metallic colors as well, which ordinarily hide and are dead in the glass, dance in resplendent apparition. Since this is the way to make the hidden metallic colors appear, I have put down four ways to make it.

The term calcination means only 'to roast' in a hot oven. It derives from the ancient practice of cooking seashells into powdered quicklime, which is a prime ingredient of cement; now called calcium oxide.  In his first method for crocus Martis, Neri mixes iron filings with sulfur and then heats the mixture in the furnace for a long period. We can guess that the result is a mix of iron oxide and iron sulfide. These are the constituents of a popular red pigment of the same name (Crocus Martis) used in pottery glazes. In the second method, he takes the iron and sulfur mixture and sprinkles it with vinegar and leaves it in the sun for many days. The third method uses aqua fortis (nitric acid) for better effect. In the fourth and final method, Neri uses aqua regis, an even stronger acid. 

In all four cases the predominant result will be for the acids to chemically react with the metal to form iron oxide and sulfide, but alas, chemistry is not that simple; there will be minor concentrations of other compounds depending on the acid used, which may or may not affect the final product. We must consider that vinegar and the other acids in the seventeenth century were significantly different from those products today. They were made by different methods and contained a variety of impurities that would never be found in the current products. As Neri notes, each method produces different effects in the glass and therefore we must conclude that each, to some extent, differs in chemistry as well. Today, we know that iron forms several different oxides, each responsible for different color effects in glass. In a number of green glass recipes Neri uses crocus made with vinegar, yet in chapter 71 he uses it to diminish the green in yellow lead crystal. In chapter 124 he uses crocus made with aqua fortis for red glass. The lesson here is that the name of a material tells us only the basics, how it was made is at least as important if not more so. 

* this post first appeared in a somewhat different form on 15 August 2014.

Crocus Martis

The many different alchemical symbols used to denote crocus martis.

In order to understand the seventeenth century glass recipes of Antonio Neri and for that matter, any alchemical recipes, it is first necessary to have a grasp of the chemical repertoire. The ingredients referenced by Neri were not especially exotic, but over the intervening four centuries since he wrote, the common names of materials have changed considerably. In the current lexicon, we name materials by their chemical formulas, so iron oxide names its two constituent elements, iron and oxygen. But in the seventeenth century, the periodic table of elements was a concept yet to be invented; hence "crocus Martis." Martis refers to Mars, the Roman god of war, the red planet, and the ancient alchemical name for iron. The origins of crocus are lost, but it may refer to the similar looking spice saffron, (crocus sativus) which has been highly valued since antiquity. Poetically, 'saffron of Mars' alludes to the orange-red color of rust. 

A first encounter with the technical recipes of sixteenth and seventeenth century alchemy can be confusing, frustrating and more than a little disorienting. There is no shortage of records, letters, manuscripts and recipe books that have been preserved from that period, however, the challenge lays in making sense of them in a way that relates to our current view of the world. It is understandable that alchemical materials and compounds had unfamiliar names; but even larger difficulties arise with the realization that any given name might describe several different chemicals, and in fact, there may be several different interchangeable synonyms/symbols for any given name. As it happens, there were good reasons for this state of affairs. The challenge of deciphering alchemy is not insurmountable. Taking the time to understand really opens up a window onto a strange and wonderful landscape of history. 

Antonio Neri considered himself an alchemist first and glassmaker second. His purely alchemical works are somewhat cryptic, but in the glass book he bends over backwards to be accessible to novices. Because of this crossover, his work can help us to navigate details in both areas that might otherwise go without explanation. His writings broadly divide into two categories: one intended for generally curious readers and another intended only for those familiar with the arcane coded language of alchemy. We have examples of both styles by Neri and all written within the period of about fifteen years at the beginning of the seventeenth century. As a result, we can use his works intended for a general audience to decode some of his more arcane passages elsewhere. With the glass recipes, his careful explanations lead us to gain considerable confidence in his technical abilities. In the introduction to his famous book on glassmaking, L'Arte Vetraria, he wrote, "I have described every last detail clearly and distinctly in this work, I am sure that if you do not purposely foul up, it will be impossible to fail, after having acquired experience and practice." Even in Neri's more obscure works, he earned the respect of later chemists; in 1870, in the journal Nature, George Rodwell, the first science master of Marlborough College, pronouncing him a "sensible chemist." Rodwell went on to note that Neri had used "no less than thirty-five different names, and twenty-two symbols" to denote a single material, the metal mercury. 

In still other writings, it becomes clear that Neri was deeply concerned that some alchemical operations were inappropriate for general consumption and were better kept secret among true practitioners of the art. In particular, he worried that if the transmutation of base metals into gold and silver were practiced widely, the result would be a collapse of the economy and would ultimately plunge civilization into disarray.

We will start with his glass book, where his materials and methods are detailed with special consideration for those not familiar with the art. Then perhaps we can build on that and make sense of his more esoteric material.

In L'Arte Vetraria, Neri gives four different methods to make crocus Martis. He explains: 

Crocus Martis is nothing other that a refinement and calcination of iron. A means by which its pigment, that in glass is a deep rutty red, is opened and imparted to the glass. It not only manifests itself but makes all the other metallic colors as well, which ordinarily hide and are dead in the glass, dance in resplendent apparition. Since this is the way to make the hidden metallic colors appear, I have put down four ways to make it.

The term calcination means only 'to roast' in a hot oven. It derives from the ancient practice of cooking seashells into powdered quicklime, which is a prime ingredient of cement; now called calcium oxide.  In his first method for crocus Martis, Neri mixes iron filings with sulfur and then heats the mixture in the furnace for a long period. We can guess that the result is a mix of iron oxide and iron sulfide. These are the constituents of a popular red pigment of the same name (crocus Martis) used in pottery glazes. In the second method, he takes the iron and sulfur mixture and sprinkles it with vinegar and leaves it in the sun for many days. The third method uses aqua fortis (nitric acid) for better effect. In the fourth and final method, Neri uses aqua regis, an even stronger acid. 

In all four cases the predominant result will be for the acids to chemically react with the metal to form iron oxide and sulfide, but alas, chemistry is not that simple; there will be minor concentrations of other compounds depending on the acid used, which may or may not affect the final product. We must consider that vinegar and the other acids in the seventeenth century were significantly different from those products today. They were made by different methods and contained a variety of impurities that would never be found in the current products. As Neri notes, each method produces different effects in the glass and therefore we must conclude that each, to some extent, differs in chemistry as well. Today, we know that iron forms several different oxides, each responsible for different color effects in glass. In a number of green glass recipes Neri uses crocus made with vinegar, yet in chapter 71 he uses it to diminish the green in yellow lead crystal. In chapter 124 he uses crocus made with aqua fortis for red glass. The lesson here is that the name of a material tells us only the basics, how it was made is at least as important if not more so. 

* this post first appeared in a somewhat different form on 15 August 2014.

Crocus Martis

The many different alchemical symbols used to denote crocus martis.

In order to understand the seventeenth century glass recipes of Antonio Neri and for that matter, any alchemical recipes, it is first necessary to have a grasp of the chemical repertoire. The ingredients referenced by Neri were not especially exotic, but over the intervening four centuries since he wrote, the common names of materials have changed considerably. In the current lexicon, we name materials by their chemical formulas, so iron oxide names its two constituent elements, iron and oxygen. But in the seventeenth century, the periodic table of elements was a concept yet to be invented; hence "crocus Martis." Martis refers to Mars, the Roman god of war, the red planet, and the ancient alchemical name for iron. The origins of crocus are lost, but it may refer to the similar looking spice saffron, (crocus sativus) which has been highly valued since antiquity. Poetically, 'saffron of Mars' alludes to the orange-red color of rust. 

A first encounter with the technical recipes of sixteenth and seventeenth century alchemy can be confusing, frustrating and more than a little disorienting. There is no shortage of records, letters, manuscripts and recipe books that have been preserved from that period, however, the challenge lays in making sense of them in a way that relates to our current view of the world. It is understandable that alchemical materials and compounds had unfamiliar names; but even larger difficulties arise with the realization that any given name might describe several different chemicals, and in fact, there may be several different interchangeable synonyms/symbols for any given name. As it happens, there were good reasons for this state of affairs. The challenge of deciphering alchemy is not insurmountable. Taking the time to understand really opens up a window onto a strange and wonderful landscape of history. 

Antonio Neri considered himself an alchemist first and glassmaker second. His purely alchemical works are somewhat cryptic, but in the glass book he bends over backwards to be accessible to novices. Because of this crossover, his work can help us to navigate details in both areas that might otherwise go without explanation. His writings broadly divide into two categories: one intended for generally curious readers and another intended only for those familiar with the arcane coded language of alchemy. We have examples of both styles by Neri and all written within the period of about fifteen years at the beginning of the seventeenth century. As a result, we can use his works intended for a general audience to decode some of his more arcane passages elsewhere. With the glass recipes, his careful explanations lead us to gain considerable confidence in his technical abilities. In the introduction to his famous book on glassmaking, L'Arte Vetraria, he wrote, "I have described every last detail clearly and distinctly in this work, I am sure that if you do not purposely foul up, it will be impossible to fail, after having acquired experience and practice." Even in Neri's more obscure works, he earned the respect of later chemists; in 1870, in the journal Nature, George Rodwell, the first science master of Marlborough College, pronouncing him a "sensible chemist." Rodwell went on to note that Neri had used "no less than thirty-five different names, and twenty-two symbols" to denote a single material, the metal mercury. 

In still other writings, it becomes clear that Neri was deeply concerned that some alchemical operations were inappropriate for general consumption and were better kept secret among true practitioners of the art. In particular, he worried that if the transmutation of base metals into gold and silver were practiced widely, the result would be a collapse of the economy and would ultimately plunge civilization into disarray.

We will start with his glass book, where his materials and methods are detailed with special consideration for those not familiar with the art. Then perhaps we can build on that and make sense of his more esoteric material.

In L'Arte Vetraria, Neri gives four different methods to make crocus Martis. He explains: 

Crocus Martis is nothing other that a refinement and calcination of iron. A means by which its pigment, that in glass is a deep rutty red, is opened and imparted to the glass. It not only manifests itself but makes all the other metallic colors as well, which ordinarily hide and are dead in the glass, dance in resplendent apparition. Since this is the way to make the hidden metallic colors appear, I have put down four ways to make it.

The term calcination means only 'to roast' in a hot oven. It derives from the ancient practice of cooking seashells into powdered quicklime, which is a prime ingredient of cement; now called calcium oxide.  In his first method for crocus Martis, Neri mixes iron filings with sulfur and then heats the mixture in the furnace for a long period. We can guess that the result is a mix of iron oxide and iron sulfide. These are the constituents of a popular red pigment of the same name (crocus Martis) used in pottery glazes. In the second method, he takes the iron and sulfur mixture and sprinkles it with vinegar and leaves it in the sun for many days. The third method uses aqua fortis (nitric acid) for better effect. In the fourth and final method, Neri uses aqua regis, an even stronger acid. 

In all four cases the predominant result will be for the acids to chemically react with the metal to form iron oxide and sulfide, but alas, chemistry is not that simple; there will be minor concentrations of other compounds depending on the acid used, which may or may not affect the final product. We must consider that vinegar and the other acids in the seventeenth century were significantly different from those products today. They were made by different methods and contained a variety of impurities that would never be found in the current products. As Neri notes, each method produces different effects in the glass and therefore we must conclude that each, to some extent, differs in chemistry as well. Today, we know that iron forms several different oxides, each responsible for different color effects in glass. In a number of green glass recipes Neri uses crocus made with vinegar, yet in chapter 71 he uses it to diminish the green in yellow lead crystal. In chapter 124 he uses crocus made with aqua fortis for red glass. The lesson here is that the name of a material tells us only the basics, how it was made is at least as important if not more so. 

* this post first appeared in a somewhat different form on 15 August 2014.

Crocus Martis

The many different alchemical symbols used to denote crocus martis.

In order to understand the seventeenth century glass recipes of Antonio Neri and for that matter, any alchemical recipes, it is first necessary to have a grasp of the chemical repertoire. The ingredients referenced by Neri were not especially exotic, but over the intervening four centuries since he wrote, the common names of materials have changed considerably. In the current lexicon, we name materials by their chemical formulas, so iron oxide names its two constituent elements, iron and oxygen. But in the seventeenth century, the periodic table of elements was a concept yet to be invented; hence "crocus Martis." Martis refers to Mars, the Roman god of war, the red planet, and the ancient alchemical name for iron. The origins of crocus are lost, but it may refer to the similar looking spice saffron, (crocus sativus) which has been highly valued since antiquity. Poetically, 'saffron of Mars' alludes to the orange-red color of rust. 

A first encounter with the technical recipes of sixteenth and seventeenth century alchemy can be confusing, frustrating and more than a little disorienting. There is no shortage of records, letters, manuscripts and recipe books that have been preserved from that period, however, the challenge lays in making sense of them in a way that relates to our current view of the world. It is understandable that alchemical materials and compounds had unfamiliar names; but even larger difficulties arise with the realization that any given name might describe several different chemicals, and in fact, there may be several different interchangeable synonyms/symbols for any given name. As it happens, there were good reasons for this state of affairs. The challenge of deciphering alchemy is not insurmountable. Taking the time to understand really opens up a window onto a strange and wonderful landscape of history. 

Antonio Neri considered himself an alchemist first and glassmaker second. His purely alchemical works are somewhat cryptic, but in the glass book he bends over backwards to be accessible to novices. Because of this crossover, his work can help us to navigate details in both areas that might otherwise go without explanation. His writings broadly divide into two categories: one intended for generally curious readers and another intended only for those familiar with the arcane coded language of alchemy. We have examples of both styles by Neri and all written within the period of about fifteen years at the beginning of the seventeenth century. As a result, we can use his works intended for a general audience to decode some of his more arcane passages elsewhere. With the glass recipes, his careful explanations lead us to gain considerable confidence in his technical abilities. In the introduction to his famous book on glassmaking, L'Arte Vetraria, he wrote, "I have described every last detail clearly and distinctly in this work, I am sure that if you do not purposely foul up, it will be impossible to fail, after having acquired experience and practice." Even in Neri's more obscure works, he earned the respect of later chemists; in 1870, in the journal Nature, George Rodwell, the first science master of Marlborough College, pronouncing him a "sensible chemist." Rodwell went on to note that Neri had used "no less than thirty-five different names, and twenty-two symbols" to denote a single material, the metal mercury. 

In still other writings, it becomes clear that Neri was deeply concerned that some alchemical operations were inappropriate for general consumption and were better kept secret among true practitioners of the art. In particular, he worried that if the transmutation of base metals into gold and silver were practiced widely, the result would be a collapse of the economy and would ultimately plunge civilization into disarray.

We will start with his glass book, where his materials and methods are detailed with special consideration for those not familiar with the art. Then perhaps we can build on that and make sense of his more esoteric material.

In L'Arte Vetraria, Neri gives four different methods to make crocus Martis. He explains: 

Crocus Martis is nothing other that a refinement and calcination of iron. A means by which its pigment, that in glass is a deep rutty red, is opened and imparted to the glass. It not only manifests itself but makes all the other metallic colors as well, which ordinarily hide and are dead in the glass, dance in resplendent apparition. Since this is the way to make the hidden metallic colors appear, I have put down four ways to make it.

The term calcination means only 'to roast' in a hot oven. It derives from the ancient practice of cooking seashells into powdered quicklime, which is a prime ingredient of cement; now called calcium oxide.  In his first method for crocus Martis, Neri mixes iron filings with sulfur and then heats the mixture in the furnace for a long period. We can guess that the result is a mix of iron oxide and iron sulfide. These are the constituents of a popular red pigment of the same name (crocus Martis) used in pottery glazes. In the second method, he takes the iron and sulfur mixture and sprinkles it with vinegar and leaves it in the sun for many days. The third method uses aqua fortis (nitric acid) for better effect. In the fourth and final method, Neri uses aqua regis, an even stronger acid. 

In all four cases the predominant result will be for the acids to chemically react with the metal to form iron oxide and sulfide, but alas, chemistry is not that simple; there will be minor concentrations of other compounds depending on the acid used, which may or may not affect the final product. We must consider that vinegar and the other acids in the seventeenth century were significantly different from those products today. They were made by different methods and contained a variety of impurities that would never be found in the current products. As Neri notes, each method produces different effects in the glass and therefore we must conclude that each, to some extent, differs in chemistry as well. Today, we know that iron forms several different oxides, each responsible for different color effects in glass. In a number of green glass recipes Neri uses crocus made with vinegar, yet in chapter 71 he uses it to diminish the green in yellow lead crystal. In chapter 124 he uses crocus made with aqua fortis for red glass. The lesson here is that the name of a material tells us only the basics, how it was made is at least as important if not more so. 

* this post first appeared in a somewhat different form on 15 August 2014.

Crocus Martis

The many different alchemical symbols used to denote crocus martis.

In order to understand the seventeenth century glass recipes of Antonio Neri and for that matter, any alchemical recipes, it is first necessary to have a grasp of the chemical repertoire. The ingredients referenced by Neri were not especially exotic, but over the intervening four centuries since he wrote, the common names of materials have changed considerably. In the current lexicon, we name materials by their chemical formulas, so iron oxide names its two constituent elements, iron and oxygen. But in the seventeenth century, the periodic table of elements was a concept yet to be invented; hence "crocus Martis." Martis refers to Mars, the Roman god of war, the red planet, and the ancient alchemical name for iron. The origins of crocus are lost, but it may refer to the similar looking spice saffron, (crocus sativus) which has been highly valued since antiquity. Poetically, 'saffron of Mars' alludes to the orange-red color of rust. 

A first encounter with the technical recipes of sixteenth and seventeenth century alchemy can be confusing, frustrating and more than a little disorienting. There is no shortage of records, letters, manuscripts and recipe books that have been preserved from that period, however, the challenge lays in making sense of them in a way that relates to our current view of the world. It is understandable that alchemical materials and compounds had unfamiliar names; but even larger difficulties arise with the realization that any given name might describe several different chemicals, and in fact, there may be several different interchangeable synonyms/symbols for any given name. As it happens, there were good reasons for this state of affairs. The challenge of deciphering alchemy is not insurmountable. Taking the time to understand really opens up a window onto a strange and wonderful landscape of history. 

Antonio Neri considered himself an alchemist first and glassmaker second. His purely alchemical works are somewhat cryptic, but in the glass book he bends over backwards to be accessible to novices. Because of this crossover, his work can help us to navigate details in both areas that might otherwise go without explanation. His writings broadly divide into two categories: one intended for generally curious readers and another intended only for those familiar with the arcane coded language of alchemy. We have examples of both styles by Neri and all written within the period of about fifteen years at the beginning of the seventeenth century. As a result, we can use his works intended for a general audience to decode some of his more arcane passages elsewhere. With the glass recipes, his careful explanations lead us to gain considerable confidence in his technical abilities. In the introduction to his famous book on glassmaking, L'Arte Vetraria, he wrote, "I have described every last detail clearly and distinctly in this work, I am sure that if you do not purposely foul up, it will be impossible to fail, after having acquired experience and practice." Even in Neri's more obscure works, he earned the respect of later chemists; in 1870, in the journal Nature, George Rodwell, the first science master of Marlborough College, pronouncing him a "sensible chemist." Rodwell went on to note that Neri had used "no less than thirty-five different names, and twenty-two symbols" to denote a single material, the metal mercury. 

In still other writings, it becomes clear that Neri was deeply concerned that some alchemical operations were inappropriate for general consumption and were better kept secret among true practitioners of the art. In particular, he worried that if the transmutation of base metals into gold and silver were practiced widely, the result would be a collapse of the economy and would ultimately plunge civilization into disarray.

We will start with his glass book, where his materials and methods are detailed with special consideration for those not familiar with the art. Then perhaps we can build on that and make sense of his more esoteric material.

In L'Arte Vetraria, Neri gives four different methods to make crocus Martis. He explains: 

Crocus Martis is nothing other that a refinement and calcination of iron. A means by which its pigment, that in glass is a deep rutty red, is opened and imparted to the glass. It not only manifests itself but makes all the other metallic colors as well, which ordinarily hide and are dead in the glass, dance in resplendent apparition. Since this is the way to make the hidden metallic colors appear, I have put down four ways to make it.

The term calcination means only 'to roast' in a hot oven. It derives from the ancient practice of cooking seashells into powdered quicklime, which is a prime ingredient of cement; now called calcium oxide.  In his first method for crocus Martis, Neri mixes iron filings with sulfur and then heats the mixture in the furnace for a long period. We can guess that the result is a mix of iron oxide and iron sulfide. These are the constituents of a popular red pigment of the same name (crocus Martis) used in pottery glazes. In the second method, he takes the iron and sulfur mixture and sprinkles it with vinegar and leaves it in the sun for many days. The third method uses aqua fortis (nitric acid) for better effect. In the fourth and final method, Neri uses aqua regis, an even stronger acid. 

In all four cases the predominant result will be for the acids to chemically react with the metal to form iron oxide and sulfide, but alas, chemistry is not that simple; there will be minor concentrations of other compounds depending on the acid used, which may or may not affect the final product. We must consider that vinegar and the other acids in the seventeenth century were significantly different from those products today. They were made by different methods and contained a variety of impurities that would never be found in the current products. As Neri notes, each method produces different effects in the glass and therefore we must conclude that each, to some extent, differs in chemistry as well. Today, we know that iron forms several different oxides, each responsible for different color effects in glass. In a number of green glass recipes Neri uses crocus made with vinegar, yet in chapter 71 he uses it to diminish the green in yellow lead crystal. In chapter 124 he uses crocus made with aqua fortis for red glass. The lesson here is that the name of a material tells us only the basics, how it was made is at least as important if not more so. 

* this post first appeared in a somewhat different form on 15 August 2014.

Crocus Martis

The many different alchemical symbols used to denote crocus martis.

In order to understand the seventeenth century glass recipes of Antonio Neri and for that matter, any alchemical recipes, it is first necessary to have a grasp of the chemical repertoire. The ingredients referenced by Neri were not especially exotic, but over the intervening four centuries since he wrote, the common names of materials have changed considerably. In the current lexicon, we name materials by their chemical formulas, so iron oxide names its two constituent elements, iron and oxygen. But in the seventeenth century, the periodic table of elements was a concept yet to be invented; hence "crocus Martis." Martis refers to Mars, the Roman god of war, the red planet, and the ancient alchemical name for iron. The origins of crocus are lost, but it may refer to the similar looking spice saffron, (crocus sativus) which has been highly valued since antiquity; poetically, 'saffron of Mars'. 

A first encounter with the technical recipes of sixteenth and seventeenth century alchemy can be confusing, frustrating and more than a little disorienting. There is no shortage of records, letters, manuscripts and recipe books that have been preserved from that period, however, the challenge lays in making sense of them in a way that relates to our current view of the world. It is understandable that alchemical materials and compounds had unfamiliar names; but even larger difficulties arise with the realization that any given name might describe several different chemicals, and in fact, there may be several different interchangeable synonyms/symbols for any given name. As it happens, there were good reasons for this state of affairs and taking the time to understand them really opens up a window onto a strange and wonderful landscape of history. 

Antonio Neri considered himself an alchemist first and glassmaker second. His purely alchemical works are somewhat cryptic, but in the glass book he bends over backwards to be accessible to novices. Because of this crossover, his work can help us to navigate details in both areas that might otherwise go without explanation. His writings broadly divide into two categories: one intended for generally curious readers and another intended only for those familiar with the arcane coded language of alchemy. We have examples of both styles by Neri and all written within the period of about fifteen years at the beginning of the seventeenth century. As a result, we can use his works intended for a general audience to decode some of his more arcane passages. With the glass recipes, his careful explanations lead us to gain considerable confidence in his technical abilities. In the introduction to his famous book on glassmaking, L'Arte Vetraria, he wrote, "I have described every last detail clearly and distinctly in this work, I am sure that if you do not purposely foul up, it will be impossible to fail, after having acquired experience and practice." Even in Neri's more obscure works, he earned the respect of later chemists; in 1870, in the journal Nature, George Rodwell, the first science master of Marlborough College, pronouncing him a "sensible chemist." Rodwell went on to note that Neri had used "no less than thirty-five different names, and twenty-two symbols" to denote a single material, the metal mercury. 

In still other writings, it becomes clear that Neri was deeply concerned that some alchemical operations were inappropriate for general consumption and were better kept secret among true practitioners of the art. In particular, he worried that if the transmutation of base metals into gold and silver were practiced widely, the result would be a collapse of the economy and would ultimately plunge civilization into disarray.

We will start with his glass book, where his materials and methods are detailed with special consideration for those not familiar with the art. Then perhaps we can build on that and make sense of his more esoteric material.

In L'Arte Vetraria, Neri gives four different methods to make crocus Martis. He explains: 

Crocus Martis is nothing other that a refinement and calcination of iron. A means by which its pigment, that in glass is a deep rutty red, is opened and imparted to the glass. It not only manifests itself but makes all the other metallic colors as well, which ordinarily hide and are dead in the glass, dance in resplendent apparition. Since this is the way to make the hidden metallic colors appear, I have put down four ways to make it.

The term calcination means only 'to roast' in a hot oven. It derives from the ancient practice of cooking seashells into powdered quicklime, which is a prime ingredient of cement; now called calcium oxide.  In his first method for crocus Martis, Neri mixes iron filings with sulfur and then heats the mixture in the furnace for a long period. We can guess that the result is a mix of iron oxide and iron sulfide. These are the constituents of a popular red pigment of the same name (crocus Martis) used in pottery glazes. In the second method, he takes the iron and sulfur mixture and sprinkles it with vinegar and leaves it in the sun for many days. The third method uses aqua fortis (nitric acid) for better effect. In the fourth and final method, Neri uses aqua regis, an even stronger acid. 

In all four cases the predominant result will be for the acids to chemically react with the metal to form iron oxide and sulfide, but alas, chemistry is not that simple; there will be minor concentrations of other compounds depending on the acid used, which may or may not affect the final product. We must consider that vinegar and the other acids in the seventeenth century were significantly different from those products today. They were made by different methods and contained a variety of impurities that would never be found in the current products. As Neri notes, each method produces different effects in the glass and therefore we must conclude that each, to some extent, differs in chemistry as well. Today, we know that iron forms several different oxides, each responsible for different color effects in glass. In a number of green glass recipes Neri uses crocus made with vinegar, yet in chapter 71 he uses it to diminish the green in yellow lead crystal. In chapter 124 he uses crocus made with aqua fortis for red glass. The lesson here is that the name of a material tells us only the basics, how it was made is at least as important if not more so. 

* this post first appeared in a somewhat different form on 15 August 2014.

Crocus Martis

The many alchemical symbols used to denote crocus martis.

In order to understand the seventeenth century glass recipes of Antonio Neri and for that matter, any alchemical recipes, it is first necessary to have a grasp of the chemical repertoire. The ingredients referenced by Neri were not especially exotic, but over the intervening four centuries the common names of materials have changed considerably. In the current lexicon, we name materials by their chemical formulas, so iron oxide names its two constituent elements, iron and oxygen. But in the seventeenth century, the periodic table of elements was a concept yet to be invented; hence "crocus Martis." Martis refers to Mars, the red planet. The origins of crocus are lost, but it may refer to the similar looking spice saffron, (crocus sativus) which has been highly valued since antiquity; poetically, 'saffron of Mars'. 

A first encounter with the technical recipes of sixteenth and seventeenth century alchemy can be confusing, frustrating and more than a little disorienting. There is no shortage of records, letters, manuscripts and recipe books; the challenge lays in making sense of them in a way that relates to our current view of the world. It is understandable that alchemical materials and compounds had unfamiliar names; but even larger difficulties arise with the realization that any given name might describe several different chemicals, and in fact, there may be several different interchangeable synonyms/symbols for any given name. As it happens, there were good reasons for this state of affairs and taking the time to understand them really opens up a window onto a strange and wonderful landscape of history. 

Antonio Neri considered himself an alchemist first and glassmaker second. His purely alchemical works are somewhat cryptic, but in the glass book he bends over backwards to be accessible to novices. Because of this crossover, his work can help us to navigate details in both fields that might otherwise go without explanation. His writings broadly divide into two categories: one intended for generally curious readers and another intended only for those familiar with the arcane coded language of alchemy. We have examples of both styles by Neri and all within the period of about fifteen years. As a result, we can start to decode some of his more arcane passages, which we will save for another time. With the glass recipes, his careful explanations lead us to gain considerable confidence in his technical abilities. In the introduction to his famous book on glassmaking, L'Arte Vetraria, he wrote, "I have described every last detail clearly and distinctly in this work, I am sure that if you do not purposely foul up, it will be impossible to fail, after having acquired experience and practice." Yet, in 1870, in the journal Nature, chemist George Rodwell commented on one of Neri's more obscure works, pronouncing him a "sensible chemist." Rodwell went on to note that Neri had used "no less than thirty-five different names, and twenty-two symbols" to denote a single material, the metal mercury. 

We will start with his glass book, where his materials and methods are detailed with special consideration for those not familiar with the art. Then perhaps we can build on that and make sense of his more esoteric material.

In L'Arte Vetraria, Neri gives four different methods to make crocus Martis. He explains: 

Crocus Martis is nothing other that a refinement and calcination of iron. A means by which its pigment, that in glass is a deep rutty red, is opened and imparted to the glass. It not only manifests itself but makes all the other metallic colors as well, which ordinarily hide and are dead in the glass, dance in resplendent apparition. Since this is the way to make the hidden metallic colors appear, I have put down four ways to make it.

The term calcination means nothing more than 'to roast' in a hot oven. It derives from the ancient practice of roasting seashells into powdered quicklime, which is a prime ingredient of cement; now called calcium oxide.  In his first method for crocus Martis, Neri mixes iron filings with sulfur and then heats the mixture in the furnace for a long period. We can guess that the result is a mix of iron oxide and iron sulfide. These are the constituents of a popular red pigment of the same name (crocus Martis) used in pottery glazes. In the second method, he takes the iron and sulfur mixture and sprinkles it with vinegar and leaves it in the sun for many days. The third method uses aqua fortis (nitric acid) for better effect. In the fourth and final method, Neri uses aqua regis, an even stronger acid. 

In all four cases the predominant result will be iron oxide and sulfide, but alas, chemistry is not that simple; there will be minor concentrations of other compounds depending on the acid used. We must consider that vinegar and the other acids in the seventeenth century were significantly different from those products today. They were made by different methods and contained a variety of impurities that would never be found in the current products. As Neri notes, each method produces different effects in the glass and therefore we must conclude that each, to some extent, differs in chemistry as well. In a number of green glass recipes he uses crocus made with vinegar, yet in chapter 71 he uses it to diminish the green in yellow lead crystal. In chapter 124 he uses crocus made with aqua fortis for red glass. The lesson here is that the name of a material tells us only the basics, how it was made is at least as important if not more so. 

* this post first appeared in a somewhat different form on 15 August 2014.

Neri’s Cabinet, Part 1

The many alchemical symbols used to denote crocus martis.

In addition to the usual eclectic mix of Antonio Neri related material that I bring to this space, I have decided to start a regular series of posts dedicated specifically to his chemical repertoire. 

A first encounter with the technical aspects of sixteenth and seventeenth century alchemy can be confusing, frustrating and more than a little disorienting. There is no shortage of records, letters, manuscripts and recipe books; the challenge lays in making sense of them in a way that relates to our current view of the world. It is understandable that alchemical materials and compounds had unfamiliar names; but the real difficulties arise with the realization that any given name might describe several different chemicals, and in fact, there may be several different interchangeable synonyms/symbols for any given name. As it happens, there were good reasons for this state of affairs and taking the time to understand them really opens up a window onto a strange and wonderful landscape of history. 

Antonio Neri considered himself an alchemist first and glassmaker second. Because of this "crossover," his work can help us to navigate details in both fields that might otherwise go without explanation. His writing broadly divides into two categories: one intended for generally curious readers and another intended only for those familiar with the arcane coded language of alchemy. We have examples of both styles by Neri and all within the period of about fifteen years. By comparing one group against the other, we can start to decode some of his more arcane passages and at the same time gain considerable confidence in his technical abilities. In the introduction to his famous book on glassmaking, L'Arte Vetraria, he wrote, "I have described every last detail clearly and distinctly in this work, I am sure that if you do not purposely foul up, it will be impossible to fail, after having acquired experience and practice.” Yet, in 1870, in the journal Nature, George Rodwell commented on another of his works, pronouncing him a “sensible chemist” on one hand, but on the other, noting that Neri had used “no less than thirty-five different names, and twenty-two symbols" to denote a single material, the metal mercury. 

We will start with his glass book, where his materials and methods are detailed with special consideration for those not familiar with the art. Then perhaps we can build on that and make sense of his more esoteric material.

In L'Arte Vetraria, Neri gives four different methods to make crocus martis. He explains: 

Crocus martis is nothing other that a refinement and calcination of iron. A means by which its pigment, which in glass is a deep rutty red, is opened and imparted to the glass. It not only manifests itself but makes all the other metallic colors as well, which ordinarily hide and are dead in the glass, dance in resplendent apparition. Since this is the way to make the hidden metallic colors appear, I have put down four ways to make it.

In the first method, Neri mixes iron filings with sulfur and then heats the mixture in the furnace for a long period. We can guess that the result is a mix of iron oxide and iron sulfide. These are the constituents of a popular red pigment of the same name (crocus martis) used in pottery glazes. In the second method, he takes the iron and sulfur mixture and sprinkles it with vinegar and leaves it in the sun for many days. The third method uses aqua fortis (nitric acid) for better effect. In the fourth and final method, Neri uses aqua regis, an even stronger acid. 

In all four cases the predominant result will be iron oxide and sulfide, but alas, chemistry is not that simple; there will be minor concentrations of other compounds depending on the acid used. We must consider that vinegar and the other acids in the seventeenth century were significantly different from those products today. They were made by different methods and contained a variety of impurities that would never be found in the current products. As Neri notes, each method produces different effects in the glass and therefore we must conclude that each, to some extent, differs in chemistry as well. In a number of green glass recipes he uses crocus made with vinegar, yet in chapter 71 he uses it to diminish the green in yellow lead crystal. In 124 he uses crocus made with aqua fortis for red glass. The lesson here is that the name of a material tells us only the basics, how it was made is at least as important if not more so. 

Red Like Blood

Dionysos in a ship, sailing among dolphins,
(iron oxide red and black glazes)
bowl discovered at Vulci, Viterbo.
Artist: Exekias (c. 530 BCE)

The use of iron oxide, or "rust" as it is commonly known, as a pigment is ancient. In fact, it is one of the oldest colorants in human history. It is found in the earliest cave paintings and in shells and cups as a residue of body paint, used by peoples who lived tens of thousands of years ago. It is found as solid minerals and also as iron rich clay, occurring in many variations, taking the colors of red, yellow, orange, green, grey and black. In the Iron Age, workers learned to produce some of these oxides directly, as byproducts of the forging process. Of course, until relatively recently it was not known that these were "oxides," only that they were colorant materials associated with iron.

The three most common forms all occur as dark grey or black mineral, but each possesses distinctive properties. "Wüstite" (FeO) is often found in meteorites, "hematite" (Fe2O3) forms a red powder when scraped, appearing to bleed, hence the name and finally, "magnetite" (Fe3O4), which is magnetic. These oxides can occur chemically bound with water, as mixtures and in different crystal structures, forming a family of compounds. The iconic red and black pottery of the Greeks was colored with iron oxide; the red (or orange) was hematite and the black was magnetite. Chinese "celadon" glaze is a green-blue that resembles jade; it is derived from an iron oxide (FeO), which does occur in nature as wüstite, but in this case forms chemically in the potters kiln.

Iron, as a pigment in glass, can be responsible for several different colors, notably green and according to Antonio Neri also red. Today red from iron is common in pottery glazes, but not in glass. In his 1612 book, L’Arte Vetraria, Neri does not offer a specific recipe for iron red, but in chapter seventeen he teases us with this passage:

This second way to make crocus martis although very easy should not be disparaged, but rather highly regarded, since crocus produced this way causes the glass to appear a rather bright blood red.

Crocus martis was the alchemical name for the red oxide of iron (Fe2O3). Neri’s iron red may depend on making a suspension of the finely ground powder in the glass. It is questionable how stable it would be in the furnace; eventually the powder would dissolve in the glass turning it transparent green. But as mentioned above, this oxide is successfully used in red pottery glazes. For glassmakers in the seventeenth century, not iron but copper was the most common pigment for producing red, yet here he presents us with an intriguing alternative. However basic crocus martis might be, Neri holds it in great esteem, using his alchemical reasoning to explain that it has the power to expose colors which are usually hidden in the glass. He continues:

The way to make it is like this: Obtain some iron filings, although steel is better if you are able to get it. Mix it well in a terracotta oven-pan with strong vinegar, which is sprinkled only until it is moist throughout. Now spread it out in the pans, and put it in the sun to bake. When the sun is clouded, leave it in the open air to dry. Now turn it into powder. If it leaves any hard lumps, sprinkle and moisten them with new vinegar, leave it to dry again and then pulverize, as above. This work should be repeated eight times, then grind and sift it through a fine sieve, which will make a very fine powder the color of brick dust. Store this in well sealed vessels for use in the coloring of glass.

From this prescription, we can guess that he was producing a mixture of hematite and iron acetate. The fact that iron does not appear as a red pigment in other glassmaking references makes his statement something of a mystery, perhaps to be solved by engaging our glassmaker's own favorite activity: experimentation.