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. 

Sal Ammoniac

 

Ammoniac crystals, Fan-yagnobskoe coal mine, Tadjikistan,

Photo (c) A. A. Evseev.

Here we will examine "sal ammoniac," a common alchemical ingredient used by Antonio Neri in many of his early seventeenth century preparations. In its pure form, it is a colorless crystalline material and is known to chemists as ammonium chloride. It does occur as a (rare) natural mineral, but it was also manufactured as early as the thirteenth century, as noted by alchemist Albertus Magnus in his De alchymia.[1] Neither he nor Neri provides a recipe for sal ammoniac, but other sources indicate that it was made by allowing urine to putrefy with common salt. French investigators documented another method used in Egypt in the eighteenth century. This scheme involved burning the dung of animals who fed on spring grasses and then sublimating the ammoniac out of the resulting soot. Sublimation occurs when a heated material goes directly from a solid to a gaseous state without ever becoming liquid. Sal ammoniac has this property; when heated it turns to a gas and upon cooling, turns back to a solid.
The usefulness of sal ammoniac in alchemy stems from the fact that when dissolved in water, which it does easily, it immediately dissociates into equal parts of ammonia and hydrochloric acid, which in turn will dissolve some metals, including tin, zinc, iron and (reluctantly) lead. Its most famous use was as an additive to the stronger acid aqua fortis (nitric acid). Together the two formed aqua regis which was strong enough to dissolve gold. At the time that Neri was working, the only known way to dissolve the most 'noble' of metals (gold) was with the 'king' of acids (aqua regis). Neri puts this knowledge to use in his recipe for ruby-red colored glass made with pure gold. His description is light on details, but he does clearly direct the reader to dissolve the precious metal in aqua regis, then gently evaporate away the acid to obtain the red pigment.

Elsewhere in Neri's glassmaking book, L'Arte Vetraria,[2] he uses sal ammoniac in the production of "alemagna blue" paint and in the tinting of natural rock crystal. 

Another of Neri's creations requiring sal ammoniac was Chalcedony glass. It had swirls of every color the glassmaker could produce. He achieved this feat by making extensive use of aqua regis to dissolve a long list of metals. He then gently evaporated off the acid, leaving ultrafine powdered metals, which he added as pigments to the glass melt. 

With this powder, I made a chalcedony in a glass furnace in Antwerp that was then run by a most courteous gentleman; Mr. Filippo Gridolfi. This chalcedony gave rise to work so nice and graceful, that it emulated true oriental agate, and in beauty and delightful colors by far exceeded it.

Today, chemical factories produce vast quantities of the materials used by Neri in his glassmaking exploits and in far higher purities. Having unlimited quantities of every conceivable chemical compound at our fingertips makes it difficult to appreciate the physical labor involved by seventeenth century alchemists, both in the preparation of the glass and in the production of the individual ingredients. The chalcedony glass recipe cited above must have taken workers many, many hours to produce and must have cost a small fortune. 

[1] Magnus 1958.
[2] Neri 1612.
[3] Glauber and others used the term 'sal ammoniac' to describe a related chemical (NH4)2SO4. When mixed with aqua fortis this forms a nitric-sulfuric acid solution, which does not form aqua regis, and does not dissolve gold.

*This post first appeared here 22 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.

Sal Ammoniac

Ammoniac crystals, Fan-yagnobskoe coal mine, Tadjikistan,

Photo (c) A. A. Evseev.

Here we will examine "sal ammoniac," a common alchemical ingredient used by Antonio Neri in many of his early seventeenth century preparations. In its pure form, it is a colorless crystalline material and is known to chemists as ammonium chloride. It does occur as a (rare) natural mineral, but it was also manufactured as early as the thirteenth century, as noted by alchemist Albertus Magnus in his De alchymia.[1] Neither he nor Neri provides a recipe for sal ammoniac, but other sources indicate that it was made by allowing urine to putrefy with common salt. French investigators documented another method used in Egypt in the eighteenth century. This scheme involved burning the dung of animals who fed on spring grasses and then sublimating the ammoniac out of the resulting soot. Sublimation occurs when a heated material goes directly from a solid to a gaseous state without ever becoming liquid. Sal ammoniac has this property; when heated it turns to a gas and upon cooling, turns back to a solid.
The usefulness of sal ammoniac in alchemy stems from the fact that when dissolved in water, which it does easily, it immediately dissociates into equal parts of ammonia and hydrochloric acid, which in turn will dissolve some metals, including tin, zinc, iron and (reluctantly) lead. Its most famous use was as an additive to the stronger acid aqua fortis (nitric acid). Together the two formed aqua regis which was strong enough to dissolve gold. At the time that Neri was working, the only known way to dissolve the most 'noble' of metals (gold) was with the 'king' of acids (aqua regis). Neri puts this knowledge to use in his recipe for ruby-red colored glass made with pure gold. His description is light on details, but he does clearly direct the reader to dissolve the precious metal in aqua regis, then gently evaporate away the acid to obtain the red pigment.

Elsewhere in Neri's glassmaking book, L'Arte Vetraria,[2] he uses sal ammoniac in the production of "alemagna blue" paint and in the tinting of natural rock crystal. 

Another of Neri's creations requiring sal ammoniac was Chalcedony glass. It had swirls of every color the glassmaker could produce. He achieved this feat by making extensive use of aqua regis to dissolve a long list of metals. He then gently evaporated off the acid, leaving ultrafine powdered metals, which he added as pigments to the glass melt. 

With this powder, I made a chalcedony in a glass furnace in Antwerp that was then run by a most courteous gentleman; Mr. Filippo Gridolfi. This chalcedony gave rise to work so nice and graceful, that it emulated true oriental agate, and in beauty and delightful colors by far exceeded it.

Today, chemical factories produce vast quantities of the materials used by Neri in his glassmaking exploits and in far higher purities. Having unlimited quantities of every conceivable chemical compound at our fingertips makes it difficult to appreciate the physical labor involved by seventeenth century alchemists, both in the preparation of the glass and in the production of the individual ingredients. The chalcedony glass recipe cited above must have taken workers many, many hours to produce and must have cost a small fortune. 

[1] Magnus 1958.
[2] Neri 1612.
[3] Glauber and others used the term 'sal ammoniac' to describe a related chemical (NH4)2SO4. When mixed with aqua fortis this forms a nitric-sulfuric acid solution, which does not form aqua regis, and does not dissolve gold.

*This post first appeared here 22 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.

Sal Ammoniac

Ammoniac crystals, Fan-yagnobskoe coal mine, Tadjikistan,

Photo (c) A. A. Evseev.

Here we will examine "sal ammoniac," a common alchemical ingredient used by Antonio Neri in many of his early seventeenth century preparations. In its pure form, it is a colorless crystalline material and is known to chemists as ammonium chloride. It does occur as a (rare) natural mineral, but it was also manufactured as early as the thirteenth century, as noted by alchemist Albertus Magnus in his De alchymia.[1] Neither he nor Neri provides a recipe for sal ammoniac, but other sources indicate that it was made by allowing urine to putrefy with common salt. French investigators documented another method used in Egypt in the eighteenth century. This scheme involved burning the dung of animals who fed on spring grasses and then sublimating the ammoniac out of the resulting soot. Sublimation occurs when a heated material goes directly from a solid to a gaseous state without ever becoming liquid. Sal ammoniac has this property; when heated it turns to a gas and upon cooling, turns back to a solid.
The usefulness of sal ammoniac in alchemy stems from the fact that when dissolved in water, which it does easily, it immediately dissociates into equal parts of ammonia and hydrochloric acid, which in turn will dissolve some metals, including tin, zinc, iron and (reluctantly) lead. Its most famous use was as an additive to the stronger acid aqua fortis (nitric acid). Together the two formed aqua regis which was strong enough to dissolve gold. At the time that Neri was working, the only known way to dissolve the most 'noble' of metals (gold) was with the 'king' of acids (aqua regis). Neri puts this knowledge to use in his recipe for ruby-red colored glass made with pure gold. His description is light on details, but he does clearly direct the reader to dissolve the precious metal in aqua regis, then gently evaporate away the acid to obtain the red pigment.

Elsewhere in Neri's glassmaking book, L'Arte Vetraria,[2] he uses sal ammoniac in the production of "alemagna blue" paint and in the tinting of natural rock crystal. 

Another of Neri's creations requiring sal ammoniac was Chalcedony glass. It had swirls of every color the glassmaker could produce. He achieved this feat by making extensive use of aqua regis to dissolve a long list of metals. He then gently evaporated off the acid, leaving ultrafine powdered metals, which he added as pigments to the glass melt. 

With this powder, I made a chalcedony in a glass furnace in Antwerp that was then run by a most courteous gentleman; Mr. Filippo Gridolfi. This chalcedony gave rise to work so nice and graceful, that it emulated true oriental agate, and in beauty and delightful colors by far exceeded it.

Today, chemical factories produce vast quantities of the materials used by Neri in his glassmaking exploits and in far higher purities. Having unlimited quantities of every conceivable chemical compound at our fingertips makes it difficult to appreciate the physical labor involved by seventeenth century alchemists, both in the preparation of the glass and in the production of the individual ingredients. The chalcedony glass recipe cited above must have taken workers many, many hours to produce and must have cost a small fortune. 

[1] Magnus 1958.
[2] Neri 1612.
[3] Glauber and others used the term 'sal ammoniac' to describe a related chemical (NH4)2SO4. When mixed with aqua fortis this forms a nitric-sulfuric acid solution, which does not form aqua regis, and does not dissolve gold.

*This post first appeared here 22 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.

Sal Ammoniac

Ammoniac crystals, Fan-yagnobskoe coal mine, Tadjikistan,

Photo (c) A. A. Evseev.

Here we will examine "sal ammoniac," a common alchemical ingredient used by Antonio Neri in many of his early seventeenth century preparations. In its pure form, it is a colorless crystalline material and is known to chemists as ammonium chloride. It does occur as a (rare) natural mineral, but it was also manufactured as early as the thirteenth century, as noted by alchemist Albertus Magnus in his De alchymia.[1] Neither he nor Neri provides a recipe for sal ammoniac, but other sources indicate that it was made by allowing urine to putrefy with common salt. French investigators documented another method used in Egypt in the eighteenth century. This scheme involved burning the dung of animals who fed on spring grasses and then sublimating the ammoniac out of the resulting soot. Sublimation occurs when a heated material goes directly from a solid to a gaseous state without ever becoming liquid. Sal ammoniac has this property; when heated it turns to a gas and upon cooling, turns back to a solid.
The usefulness of sal ammoniac in alchemy stems from the fact that when dissolved in water, which it does easily, it immediately dissociates into equal parts of ammonia and hydrochloric acid, which in turn will dissolve some metals, including tin, zinc, iron and (reluctantly) lead. Its most famous use was as an additive to the stronger acid aqua fortis (nitric acid). Together the two formed aqua regis which was strong enough to dissolve gold. At the time that Neri was working, the only known way to dissolve the most 'noble' of metals (gold) was with the 'king' of acids (aqua regis). Neri puts this knowledge to use in his recipe for ruby-red colored glass made with pure gold. His description is light on details, but he does clearly direct the reader to dissolve the precious metal in aqua regis, then gently evaporate away the acid to obtain the red pigment.

Elsewhere in Neri's glassmaking book, L'Arte Vetraria,[2] he uses sal ammoniac in the production of "alemagna blue" paint and in the tinting of natural rock crystal. 

Another of Neri's creations requiring sal ammoniac was Chalcedony glass. It had swirls of every color the glassmaker could produce. He achieved this feat by making extensive use of aqua regis to dissolve a long list of metals. He then gently evaporated off the acid, leaving ultrafine powdered metals, which he added as pigments to the glass melt. 

With this powder, I made a chalcedony in a glass furnace in Antwerp that was then run by a most courteous gentleman; Mr. Filippo Gridolfi. This chalcedony gave rise to work so nice and graceful, that it emulated true oriental agate, and in beauty and delightful colors by far exceeded it.

Today, chemical factories produce vast quantities of the materials used by Neri in his glassmaking exploits and in far higher purities. Having unlimited quantities of every conceivable chemical compound at our fingertips makes it difficult to appreciate the physical labor involved by seventeenth century alchemists, both in the preparation of the glass and in the production of the individual ingredients. The chalcedony glass recipe cited above must have taken workers many, many hours to produce and must have cost a small fortune. 

[1] Magnus 1958.
[2] Neri 1612.
[3] Glauber and others used the term 'sal ammoniac' to describe a related chemical (NH4)2SO4. When mixed with aqua fortis this forms a nitric-sulfuric acid solution, which does not form aqua regis, and does not dissolve gold.

*This post first appeared here 22 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.

Sal Ammoniac

Ammoniac crystals, Fan-yagnobskoe coal mine, Tadjikistan,

Photo (c) A. A. Evseev.

Here we will examine "sal ammoniac," a common alchemical ingredient used by Antonio Neri in many of his early seventeenth century preparations. In its pure form, it is a colorless crystalline material and is known to chemists as ammonium chloride. It does occur as a (rare) natural mineral, but it was also manufactured as early as the thirteenth century, as noted by alchemist Albertus Magnus in his De alchymia.[1] Neither he nor Neri provides a recipe for sal ammoniac, but other sources indicate that it was made by allowing urine to putrefy with common salt. French investigators documented another method used in Egypt in the eighteenth century. This scheme involved burning the dung of animals who fed on spring grasses and then sublimating the ammoniac out of the resulting soot. Sublimation occurs when a heated material goes directly from a solid to a gaseous state without ever becoming liquid. Sal ammoniac has this property; when heated it turns to a gas and upon cooling, turns back to a solid.
The usefulness of sal ammoniac in alchemy stems from the fact that when dissolved in water, which it does easily, it immediately dissociates into equal parts of ammonia and hydrochloric acid, which in turn will dissolve some metals, including tin, zinc, iron and (reluctantly) lead. Its most famous use was as an additive to the stronger acid aqua fortis (nitric acid). Together the two formed aqua regis which was strong enough to dissolve gold. At the time that Neri was working, the only known way to dissolve the most 'noble' of metals (gold) was with the 'king' of acids (aqua regis). Neri puts this knowledge to use in his recipe for ruby-red colored glass made with pure gold. His description is light on details, but he does clearly direct the reader to dissolve the precious metal in aqua regis, then gently evaporate away the acid to obtain the red pigment.

Elsewhere in Neri's glassmaking book, L'Arte Vetraria,[2] he uses sal ammoniac in the production of "alemagna blue" paint and in the tinting of natural rock crystal. 

Another of Neri's creations requiring sal ammoniac was Chalcedony glass. It had swirls of every color the glassmaker could produce. He achieved this feat by making extensive use of aqua regis to dissolve a long list of metals. He then gently evaporated off the acid, leaving ultrafine powdered metals, which he added as pigments to the glass melt. 

With this powder, I made a chalcedony in a glass furnace in Antwerp that was then run by a most courteous gentleman; Mr. Filippo Gridolfi. This chalcedony gave rise to work so nice and graceful, that it emulated true oriental agate, and in beauty and delightful colors by far exceeded it.

Today, chemical factories produce vast quantities of the materials used by Neri in his glassmaking exploits and in far higher purities. Having unlimited quantities of every conceivable chemical compound at our fingertips makes it difficult to appreciate the physical labor involved by seventeenth century alchemists, both in the preparation of the glass and in the production of the individual ingredients. The chalcedony glass recipe cited above must have taken workers many, many hours to produce and must have cost a small fortune. 

[1] Magnus 1958.
[2] Neri 1612.
[3] Glauber and others used the term 'sal ammoniac' to describe a related chemical (NH4)2SO4. When mixed with aqua fortis this forms a nitric-sulfuric acid solution, which does not form aqua regis, and does not dissolve gold.

*This post first appeared here 22 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.