Lime

Vintage 1920's Water Glass Label.

The temperature of 1500 degrees (825C) was comfortably within reach of seventeenth century glassmakers like Antonio Neri. It was also achievable in much earlier times and was used in one of the earliest chemical reactions of  human industry; the production of "lime" by heating seashells or limestone to the above mentioned temperature. This is the point at which these materials give up the carbon dioxide gas bound into their chemical structure, leaving behind calcium oxide, otherwise known as lime.

What makes lime so useful is a property early Roman engineers knew well; that when mixed with water, lime undergoes a chemical reaction that releases heat and quickly solidifies into a rock-hard mass that is impervious to the elements. The Romans used it as a construction material; they mixed it with ash, clay, sand and stones to form mortar, cement and concrete. It has played a critical role ever since in the construction of buildings, roads and monuments.

Today it is perhaps not an obvious choice in imagining the supplies of a seventeenth century alchemist’s cabinet; lime does not require exotic methods to produce, it was made in industrial quantities and readily available as a construction material. Nevertheless, lime played an important part in many diverse chemical preparations of the day. Antonio Neri mentions it numerous times in his writing. He used it in his lute recipe; a hard, cement like coating which protected alchemical glassware in direct flame and sealed joints. He also used it to extract the color from flowers in the production of paints. Most notably, he used lime as an ingredient in the formulation of glass itself. 

Lime turns out to be a rather critical ingredient in glassmaking and the central player in a mystery about the history of glass that to this day has not been satisfactorily explained. Neri obtained the white powder from suppliers as "lime cake." Early in his book, L'Arte Vetraria, he states it should be used in all his glass recipes. 

Take lime salt, which is used for building. Purify this salt and mix it with ordinary Levantine polverino salt in the proportion of 2 pounds per 100, which is 2 pounds of lime salt, to every 100 pounds of polverino salt, purified and well made, as previously described. With this salt mixture, you can make yourself ordinary frit, and put it in the crucible to clarify. I will refer to this frit from now on in the recipes for cristallino, cristallo, and common glass. This way you will have cristallo quite subtle and beautiful.[1]

The above excerpt is the full extent of Neri's discussion on lime in glassmaking. An interesting point here is that he does not say why lime is so necessary, but it is. [2] He must have known that adding too much lime will cause glass to become cloudy, the exact opposite effect that he was after. What is not so clear is if Neri (or anyone else in that period) knew that too little lime allows glass to dissolve slowly when exposed to water. 

One of the great enduring mysteries in the history of glassmaking is the question of exactly when it was clearly understood that lime was so important. The subject became a matter of deliberate chemical investigation in the eighteenth century, but the use of lime extends much earlier. Some of the earliest glass fragments known contain healthy amounts of lime. Neri's ambiguous endorsement is the first mention in print, but even among much earlier manuscripts, its role is unclear. In the first century, Roman author Pliny made a cryptic reference to adding seashells to the glass melt, but again, without further explanation. [3] Analysis of ancient glass often shows a significant lime content; seemingly more than could be reasonably accounted for by happy accident or contamination. The ingredient is never mentioned in most surviving early recipes. Of course, it may be that  lime is present in many ancient glass artifacts precisely because those artifacts are the ones that have survived without dissolving.

Some have speculated that calcium could have been introduced inadvertently as seashell fragments when sand was crushed into powder for glassmaking. This hypothesis is certainly possible, but it seems suspect considering just how much seashell would be required and how fussy the glassmakers were in obtaining pure white sand from particular locations. However, a distinction must be made between knowing that sand from a particular location makes good glass, and knowing what is in that sand. 

Perhaps the most famous such site for good glassmaking sand is the "Belus" [Na'aman] River outlet in what is now northern Israel. This is where the story of the discovery of glass takes place, as told by Pliny and other early writers. Sailors were driven here by a storm, they used natron (glassmaking salt) [4] from their cargo to hold their cooking pots up over a beach fire, and when the natron mixed with the sand in the fire, glass was formed. Or so the story tells. The nearby Phoenician cities of Tyre and Sidon are known to have hosted a thriving glass industry. The sand in this area is composed of exceptionally pure quartz, with a healthy amount of calcium from shells ground down from wave action. Together these make an ideal mix for stable glass. 

There is some evidence that experimenters such as Basil Valentine were aware that glass without lime would dissolve as early as 1520. [5] For what it is worth, I can contribute some anecdotal evidence from the twentieth century. There is a well-known product called water-glass that is essentially composed of dissolved glass without the lime. [6] My glass artist friend Emilio tells me that growing up on Murano in Venice, his mother regularly preserved fresh eggs by dipping them in water-glass, which when dry, formed a hard seal over the existing shell. Presumably, she learned the technique from her own mother. I do not know of any early references, but the use of water-glass clearly indicates a knowledge of the role lime played.

There are other explanations possible for the addition of lime to glass. Lime was a commodity that was made since ancient times, in furnaces very similar to the ones producing glass. In fact, Neri specifies the use of a “limekiln” for the production of his enamels. The archaeological remains of Tyre and Sidon show prolific use of lime based plaster and mortar in their construction practices and there are indications of lime making facilities. It seems only good business sense that the same furnaces making glass may have also been making lime. Cross contamination, and even experimentation would be a natural outcome. 

[1] Neri 1612, ch 7.
[2] As an aside, magnesia (MgO) can also serve as a stabilizer in glass.
[3] Pliny, Natural History v.36, ch. 66.192.
[4] Natron is a mix of sodium carbonates. 
[5] Basil Valentine, et al. see footnote 6 of the Wikipedia article on sodium silicate for a discussion.
[6] Sodium Silicate.
* This post first appeared here in a slightly different form on 10 October 2014.

Lime

Vintage 1920's Water Glass Label.

The temperature of 1500 degrees (825C) was comfortably within reach of seventeenth century glassmakers like Antonio Neri. It was also achievable in much earlier times and was used in one of the earliest chemical reactions of  human industry; the production of "lime" by heating seashells or limestone to the above mentioned temperature. This is the point at which these materials give up the carbon dioxide gas bound into their chemical structure, leaving behind calcium oxide, otherwise known as lime.

What makes lime so useful is a property early Roman engineers knew well; that when mixed with water, lime undergoes a chemical reaction that releases heat and quickly solidifies into a rock-hard mass that is impervious to the elements. The Romans used it as a construction material; they mixed it with ash, clay, sand and stones to form mortar, cement and concrete. It has played a critical role ever since in the construction of buildings, roads and monuments.

Today it is perhaps not an obvious choice in imagining the supplies of a seventeenth century alchemist’s cabinet; lime does not require exotic methods to produce, it was made in industrial quantities and readily available as a construction material. Nevertheless, lime played an important part in many diverse chemical preparations of the day. Antonio Neri mentions it numerous times in his writing. He used it in his lute recipe; a hard, cement like coating which protected alchemical glassware in direct flame and sealed joints. He also used it to extract the color from flowers in the production of paints. Most notably, he used lime as an ingredient in the formulation of glass itself. 

Lime turns out to be a rather critical ingredient in glassmaking and the central player in a mystery about the history of glass that to this day has not been satisfactorily explained. Neri obtained the white powder from suppliers as "lime cake." Early in his book, L'Arte Vetraria, he states it should be used in all his glass recipes. 

Take lime salt, which is used for building. Purify this salt and mix it with ordinary Levantine polverino salt in the proportion of 2 pounds per 100, which is 2 pounds of lime salt, to every 100 pounds of polverino salt, purified and well made, as previously described. With this salt mixture, you can make yourself ordinary frit, and put it in the crucible to clarify. I will refer to this frit from now on in the recipes for cristallino, cristallo, and common glass. This way you will have cristallo quite subtle and beautiful.[1]

The above excerpt is the full extent of Neri's discussion on lime in glassmaking. An interesting point here is that he does not say why lime is so necessary, but it is. [2] He must have known that adding too much lime will cause glass to become cloudy, the exact opposite effect that he was after. What is not so clear is if Neri (or anyone else in that period) knew that too little lime allows glass to dissolve slowly when exposed to water. 

One of the great enduring mysteries in the history of glassmaking is the question of exactly when it was clearly understood that lime was so important. The subject became a matter of deliberate chemical investigation in the eighteenth century, but the use of lime extends much earlier. Some of the earliest glass fragments known contain healthy amounts of lime. Neri's ambiguous endorsement is the first mention in print, but even among much earlier manuscripts, its role is unclear. In the first century, Roman author Pliny made a cryptic reference to adding seashells to the glass melt, but again, without further explanation. [3] Analysis of ancient glass often shows a significant lime content; seemingly more than could be reasonably accounted for by happy accident or contamination. The ingredient is never mentioned in most surviving early recipes. Of course, it may be that  lime is present in many ancient glass artifacts precisely because those artifacts are the ones that have survived without dissolving.

Some have speculated that calcium could have been introduced inadvertently as seashell fragments when sand was crushed into powder for glassmaking. This hypothesis is certainly possible, but it seems suspect considering just how much seashell would be required and how fussy the glassmakers were in obtaining pure white sand from particular locations. However, a distinction must be made between knowing that sand from a particular location makes good glass, and knowing what is in that sand. 

Perhaps the most famous such site for good glassmaking sand is the "Belus" [Na'aman] River outlet in what is now northern Israel. This is where the story of the discovery of glass takes place, as told by Pliny and other early writers. Sailors were driven here by a storm, they used natron (glassmaking salt) [4] from their cargo to hold their cooking pots up over a beach fire, and when the natron mixed with the sand in the fire, glass was formed. Or so the story tells. The nearby Phoenician cities of Tyre and Sidon are known to have hosted a thriving glass industry. The sand in this area is composed of exceptionally pure quartz, with a healthy amount of calcium from shells ground down from wave action. Together these make an ideal mix for stable glass. 

There is some evidence that experimenters such as Basil Valentine were aware that glass without lime would dissolve as early as 1520. [5] For what it is worth, I can contribute some anecdotal evidence from the twentieth century. There is a well-known product called water-glass that is essentially composed of dissolved glass without the lime. [6] My glass artist friend Emilio tells me that growing up on Murano in Venice, his mother regularly preserved fresh eggs by dipping them in water-glass, which when dry, formed a hard seal over the existing shell. Presumably, she learned the technique from her own mother. I do not know of any early references, but the use of water-glass clearly indicates a knowledge of the role lime played.

There are other explanations possible for the addition of lime to glass. Lime was a commodity that was made since ancient times, in furnaces very similar to the ones producing glass. In fact, Neri specifies the use of a “limekiln” for the production of his enamels. The archaeological remains of Tyre and Sidon show prolific use of lime based plaster and mortar in their construction practices and there are indications of lime making facilities. It seems only good business sense that the same furnaces making glass may have also been making lime. Cross contamination, and even experimentation would be a natural outcome. 

[1] Neri 1612, ch 7.
[2] As an aside, magnesia (MgO) can also serve as a stabilizer in glass.
[3] Pliny, Natural History v.36, ch. 66.192.
[4] Natron is a mix of sodium carbonates. 
[5] Basil Valentine, et al. see footnote 6 of the Wikipedia article on sodium silicate for a discussion.
[6] Sodium Silicate.
* This post first appeared here in a slightly different form on 10 October 2014.

Lime

Vintage 1920's Water Glass Label.

The temperature of 1500 degrees (825 C) was comfortably within reach of seventeenth century glassmakers like Antonio Neri. It was also achievable in much earlier times and was used in one of the earliest chemical reactions of  human industry; the production of "lime" by heating seashells or limestone to the above mentioned temperature. This is the point at which these materials give up the carbon dioxide gas bound into their chemical structure, leaving behind calcium oxide, otherwise known as lime.

What makes lime so useful is a property early Roman engineers knew well; that when mixed with water, lime undergoes a chemical reaction that releases heat and quickly solidifies into a rock-hard mass that is impervious to the elements. The Romans used it as a construction material; they mixed it with ash, clay, sand and stones to form mortar, cement and concrete. It has played a critical role ever since in the construction of buildings, roads and monuments.

Today it is perhaps not an obvious choice in imagining the supplies of a seventeenth century alchemist’s cabinet; lime does not require exotic methods to produce, it was made in industrial quantities and readily available as a construction material. Nevertheless, lime played an important part in many diverse chemical preparations of the day. Antonio Neri mentions it numerous times in his writing. He used it in his lute recipe; a hard, cement like coating which protected alchemical glassware in direct flame and sealed joints. He also used it to extract the color from flowers in the production of paints. Most notably, he used lime as an ingredient in the formulation of glass itself. 

Lime turns out to be a rather critical ingredient in glassmaking and the central player in a mystery about the history of glass that to this day has not been satisfactorily explained. Neri obtained the white powder from suppliers as "lime cake." Early in his book, L'Arte Vetraria, he states it should be used in all his glass recipes. 

Take lime salt, which is used for building. Purify this salt and mix it with ordinary Levantine polverino salt in the proportion of 2 pounds per 100, which is 2 pounds of lime salt, to every 100 pounds of polverino salt, purified and well made, as previously described. With this salt mixture, you can make yourself ordinary frit, and put it in the crucible to clarify. I will refer to this frit from now on in the recipes for cristallino, cristallo, and common glass. This way you will have cristallo quite subtle and beautiful.[1]

The above excerpt is the full extent of Neri's discussion on lime in glassmaking. An interesting point here is that he does not say why lime is so necessary, but it is. [2] He must have known that adding too much lime will cause glass to become cloudy, the exact opposite effect that he was after. What is not so clear is if Neri (or anyone else in that period) knew that too little lime allows glass to dissolve slowly when exposed to water. 

One of the great enduring mysteries in the history of glassmaking is the question of exactly when it was clearly understood that lime was so important. The subject became a matter of deliberate chemical investigation in the eighteenth century, but the use of lime extends much earlier. Some of the earliest glass fragments known contain healthy amounts of lime. Neri's ambiguous endorsement is the first mention in print, but even among much earlier manuscripts, its role is unclear. In the first century, Roman author Pliny made a cryptic reference to adding seashells to the glass melt, but again, without further explanation. [3] Analysis of ancient glass often shows a significant lime content; seemingly more than could be reasonably accounted for by happy accident or contamination. The ingredient is never mentioned in most surviving early recipes. Of course, it may be that  lime is present in many ancient glass artifacts precisely because those artifacts are the ones that have survived without dissolving.

Some have speculated that calcium could have been introduced inadvertently as seashell fragments when sand was crushed into powder for glassmaking. This hypothesis is certainly possible, but it seems suspect considering just how much seashell would be required and how fussy the glassmakers were in obtaining pure white sand from particular locations. However, a distinction must be made between knowing that sand from a particular location makes good glass, and knowing what is in that sand. 

Perhaps the most famous such site for good glassmaking sand is the "Belus" [Na'aman] River outlet in what is now northern Israel. This is where the story of the discovery of glass takes place, as told by Pliny and other early writers. Sailors were driven here by a storm, they used natron (glassmaking salt) [4] from their cargo to hold their cooking pots up over a beach fire, and when the natron mixed with the sand in the fire, glass was formed. Or so the story tells. The nearby Phoenician cities of Tyre and Sidon are known to have hosted a thriving glass industry. The sand in this area is composed of exceptionally pure quartz, with a healthy amount of calcium from shells ground down from wave action. Together these make an ideal mix for stable glass. 

There is some evidence that experimenters such as Basil Valentine were aware that glass without lime would dissolve as early as 1520. [5] For what it is worth, I can contribute some anecdotal evidence from the twentieth century. There is a well-known product called water-glass that is essentially composed of dissolved glass without the lime. [6] My glass artist friend Emilio tells me that growing up on Murano in Venice, his mother regularly preserved fresh eggs by dipping them in water-glass, which when dry, formed a hard seal over the existing shell. Presumably, she learned the technique from her own mother. I do not know of any early references, but the use of water-glass clearly indicates a knowledge of the role lime played.

There are other explanations possible for the addition of lime to glass. Lime was a commodity that was made since ancient times, in furnaces very similar to the ones producing glass. In fact, Neri specifies the use of a “limekiln” for the production of his enamels. The archaeological remains of Tyre and Sidon show prolific use of lime based plaster and mortar in their construction practices and there are indications of lime making facilities. It seems only good business sense that the same furnaces making glass may have also been making lime. Cross contamination, and even experimentation would be a natural outcome. 

[1] Neri 1612, ch 7.
[2] As an aside, magnesia (MgO) can also serve as a stabilizer in glass.
[3] Pliny, Natural History v.36, ch. 66.192.
[4] Natron is a mix of sodium carbonates. 
[5] Basil Valentine, et al. see footnote 6 of the Wikipedia article on sodium silicate for a discussion.
[6] Sodium Silicate.
* This post first appeared here in a slightly different form on 10 October 2014.

Lime

Vintage 1920's Water Glass Label.

The temperature of 1500 degrees (825 C) was comfortably within reach of seventeenth century glassmakers like Antonio Neri. It was also achievable in much earlier times and was used in one of the earliest chemical reactions of  human industry; the production of "lime" by heating seashells or limestone to the above mentioned temperature. This is the point at which these materials give up the carbon dioxide gas bound into their chemical structure, leaving behind calcium oxide, otherwise known as lime.

What makes lime so useful is a property early Roman engineers knew well; that when mixed with water, lime undergoes a chemical reaction that releases heat and quickly solidifies into a rock-hard mass that is impervious to the elements. The Romans used it as a construction material; they mixed it with ash, clay, sand and stones to form mortar, cement and concrete. It has played a critical role ever since in the construction of buildings, roads and monuments.

Today it is perhaps not an obvious choice in imagining the supplies of a seventeenth century alchemist’s cabinet; lime does not require exotic methods to produce, it was made in industrial quantities and readily available as a construction material. Nevertheless, lime played an important part in many diverse chemical preparations of the day. Antonio Neri mentions it numerous times in his writing. He used it in his lute recipe; a hard, cement like coating which protected alchemical glassware in direct flame and sealed joints. He also used it to extract the color from flowers in the production of paints. Most notably, he used lime as an ingredient in the formulation of glass itself. 

Lime turns out to be a rather critical ingredient in glassmaking and the central player in a mystery about the history of glass that to this day has not been satisfactorily explained. Neri obtained the white powder from suppliers as "lime cake." Early in his book, L'Arte Vetraria, he states it should be used in all his glass recipes. 

Take lime salt, which is used for building. Purify this salt and mix it with ordinary Levantine polverino salt in the proportion of 2 pounds per 100, which is 2 pounds of lime salt, to every 100 pounds of polverino salt, purified and well made, as previously described. With this salt mixture, you can make yourself ordinary frit, and put it in the crucible to clarify. I will refer to this frit from now on in the recipes for cristallino, cristallo, and common glass. This way you will have cristallo quite subtle and beautiful.[1]

The above excerpt is the full extent of Neri's discussion on lime in glassmaking. An interesting point here is that he does not say why lime is so necessary. [2] He must have known that adding too much lime will cause glass to become cloudy, the exact opposite effect that he is after. What is not so clear is if Neri (or anyone else) understood that too little lime allows glass to dissolve slowly when exposed to water. 

One of the great enduring mysteries in the history of glassmaking is the question of exactly when it was understood that lime was so important. The subject became a matter of deliberate chemical investigation in the eighteenth century, but the use of lime extends much earlier. Some of the earliest glass fragments known contain healthy amounts of lime. Neri's ambiguous endorsement is the first mention in print, but even among much earlier manuscripts, its role is unclear. In the first century, Roman author Pliny made a cryptic reference to adding seashells to the glass melt, but again, without further explanation. [3] Analysis of ancient glass often shows a significant lime content; seemingly more than could be reasonably accounted for by happy accident or contamination. The ingredient is never mentioned in most surviving early recipes. Of course, it may be that  lime is present in many ancient glass artifacts precisely because those artifacts are the ones that have survived without dissolving.

Some have speculated that calcium could have been introduced inadvertently as seashell fragments when sand was crushed into powder for glassmaking. This hypothesis is certainly possible, but it seems suspect considering just how much seashell would be required and how fussy the glassmakers were in obtaining pure white sand from particular locations. However, a distinction must be made between knowing that sand from a particular location makes good glass, and knowing what is in that sand. 

Perhaps the most famous such site for good glassmaking sand is the "Belus" [Na'aman] River outlet in what is now northern Israel. This is where the story of the discovery of glass takes place, as told by Pliny and other early writers. Sailors were driven here by a storm, they used natron (glassmaking salt) [4] from their cargo to hold their cooking pots up over a beach fire, and when the natron mixed with the sand in the fire, glass was formed. Or so the story tells. The nearby Phoenician cities of Tyre and Sidon are known to have hosted a thriving glass industry. The sand in this area is composed of exceptionally pure quartz, with a healthy amount of calcium from shells ground down from wave action. Together these make an ideal mix for stable glass. 

There is some evidence that experimenters such as Basil Valentine were aware that glass without lime would dissolve as early as 1520. [5] For what it is worth, I can contribute some anecdotal evidence from the twentieth century. There is a well-known product called water-glass that is essentially composed of dissolved glass without the lime. [6] My glass artist friend Emilio tells me that growing up on Murano in Venice, his mother regularly preserved fresh eggs by dipping them in water-glass, which when dry, formed a hard seal over the existing shell. Presumably, she learned the technique from her own mother. I do not know of any early references, but the use of water-glass clearly indicates a knowledge of the role lime played.

There are other explanations possible for the addition of lime to glass. Lime was a commodity that was made since ancient times, in furnaces very similar to the ones producing glass. In fact, Neri specifies the use of a “limekiln” for the production of his enamels. The archaeological remains of Tyre and Sidon show prolific use of lime based plaster and mortar in their construction practices and there are indications of lime making facilities. It seems only good business sense that the same furnaces making glass may have also been making lime. Cross contamination, and even experimentation would be a natural outcome. 

[1] Neri 1612, ch 7.
[2] As an aside, magnesia (MgO) can also serve as a stabilizer in glass.
[3] Pliny, Natural History v.36, ch. 66.192.
[4] Natron is a mix of sodium carbonates. 
[5] Basil Valentine, et al. see footnote 6 of the Wikipedia article on sodium silicate for a discussion.
[6] Sodium Silicate.
* This post first appeared here in a slightly different form on 10 October 2014.

Le Fritte

Diderot & d'Alembert, L'Encyclopédie (1772)
Raking Out Roasted Frit

In the opening chapters of Antonio Neri’s 1612 book, L’Arte Vetraria, he details the production of a little appreciated invention, which had a tremendous impact on the technical art of glassmaking. The development of “frit” or as Neri termed it “le fritte” is nothing less than the product that makes glass possible. [1] 

The legend about how glass was first discovered centers around the chance combination of the critical ingredients plus the all-important intense heat necessary to melt them. The only problem is that if you throw the ingredients of glass into a pot and heat them, what you get is a really hot pot of ingredients that have no intention of forming glass. Even two-thousand years ago, we could not resist the allure of a story line in which a sudden discovery changed history. The truth about glass is probably a lot less dramatic and a lot more gradual. Therein lays the widely neglected and underappreciated technical innovation of frit. To form glass, the ingredients were first pulverized to a fine powder, thoroughly mixed, heated and raked together continuously for a long period of time. The result was then allowed to cool and sit in heaps for several weeks or months where the ingredients continued to chemically react. They formed a solid mass that was then cracked apart and often reground into a granular form again; this is “frit” and when this was heated in a crucible it did readily form glass.

Frit is an intermediate product between raw materials and finished glass. It allows transparent glass to form quickly, efficiently and without being saturated with gas bubbles. It also has close ties to the formulation of ancient precursors to glass; to Egyptian faience as well as to pottery glazes and even some fired clay formulations. [2] The implication is that glass as we know it today is the result of a gradual evolution and refinement that started with clay pottery, but there simply is not enough evidence to draw such a conclusion. Frit falls into a grey area of technical history where we do not have enough information to form a narrative and therefore there is no ‘story’ to latch onto. As a result, frit has taken a place on the sidelines. All we have is tantalizing clues from the chemical analysis of archaeological finds. Nevertheless, by the time Antonio Neri came along in the early seventeenth century, fritting was a standard intermediate step in making glass, a necessary step, and Neri left us a detailed, coherent account.  

It is important that the tarso is ground as finely as flour, so that it will all pass through a fine sieve. Now take 200 pounds of the best finely sifted tarso [crushed quartz], as before, and about 130 pounds of polverino salt [soda] ground and sifted, as before, and stir everything thoroughly and blend it together in its entirety. Once fully mixed, put it in a kiln that has been well heated in advance, because if it were put into a cold kiln the frit would never form. I begin with a moderate fire for an hour, always stirring with the rake and mixing the frit until it is well incorporated and calcined [roasted], then I stoke the fire. Continuously mix the frit well with the rake, which is very important. Keep on doing this without fail for 5 hours, maintaining the strong fire. […] 

Keep this crystal frit, as said before, in a dry place where it is not exposed to moisture, because in regions and places that are humid it will suffer greatly. Soon the salt would soak up water and would dissolve away; all that would remain is the tarso alone which would not vitrify. Furthermore, this frit should not be washed the way others are. When it is allowed to rest for 3 or 4 months it is in much better shape to put in crucibles [in the furnace], and will clarify sooner. This is the way to make crystal frit with its dosages and circumstances as I have made many times. [3]

Neri is recounting long tradition here, possibly extending back to the Roman Empire. Frit is a necessary and deliberate step to the reliable production of glass. It speaks to a significant specialization within the glass industries of antiquity. When we look for similar activity we find occupations like the making of cement by roasting seashells or limestone for long periods. This is perhaps a clue to the introduction of lime, the third ingredient necessary for glass. 

[1] In modern parlance “frit” is most often used to describe finished glass that has been crushed for various artistic purposes.
[2] Wikipedia has a good synopsis, http://en.wikipedia.org/wiki/Frit
[3] Neri 1612, chap. 2.

Neri's Cabinet #7: Lime

Vintage 1920's Water Glass Label.

The temperature of 1500 degrees (825 C) was comfortably within reach of seventeenth glassmakers like Antonio Neri. It was also achievable in much earlier times and was used in one of the earliest chemical reactions known; the production of "lime" by heating seashells or limestone to the above mentioned temperature. This is the point at which these materials give up the carbon dioxide gas bound into their chemical structure, leaving behind calcium oxide, otherwise known as lime.

What makes lime so useful is a property early Roman engineers knew well; that when mixed with water, lime undergoes a chemical reaction that releases heat and quickly solidifies into a rock-hard mass. The Romans used it as a construction material; they mixed it with ash, clay, sand and stones to form mortar, cement and concrete. It has played a critical role ever since in the construction of buildings, roads and monuments.

Today it is perhaps not an obvious choice in imagining the supplies of a seventeenth century alchemist’s cabinet; lime does not require exotic methods to produce, it was made in industrial quantities and readily available as a construction material. Nevertheless, lime played an important part in many diverse chemical preparations of the day. Antonio Neri mentions it numerous times in his writing. He used it in his lute recipe; a hard, cement like coating which protected alchemical glassware in direct flame and sealed joints. He also used it to extract the color from flowers in the production of paints. Most notably, he used lime as an ingredient in the formulation of glass itself. 

Lime turns out to be a rather critical ingredient in glassmaking and the central player in a mystery about the history of glass that to this day has not been satisfactorily explained. He obtained the white powder from suppliers as "lime cake." Early in his book, L'Arte Vetraria, he states it should be used in all his glass recipes. 

Take lime salt, which is used for building. Purify this salt and mix it with ordinary Levantine polverino salt in the proportion of 2 pounds per 100, which is 2 pounds of lime salt, to every 100 pounds of polverino salt, purified and well made, as previously described. With this salt mixture, you can make yourself ordinary frit, and put it in the crucible to clarify. I will refer to this frit from now on in the recipes for cristallino, cristallo, and common glass. This way you will have cristallo quite subtle and beautiful.[1]

The above excerpt is the full extent of Neri's discussion on lime in glassmaking. An interesting point here is that he does not say why lime is so necessary. [2] He must have known that adding too much lime will cause glass to become cloudy, the exact opposite effect that he is after. What is not so clear is if Neri (or anyone else) understood that too little lime allows glass to dissolve slowly when exposed to water. 

One of the great enduring mysteries in the history of glassmaking is the question of exactly when it was understood that lime was so important. The subject became a matter of deliberate chemical investigation in the eighteenth century, but the use of lime extends much earlier. Neri's ambiguous endorsement is the first mention in print, but even among much earlier manuscripts, its role is unclear. In the first century, Roman author Pliny made a cryptic reference to adding seashells to the glass melt, but again, without further explanation. [3] Analysis of ancient glass often shows a healthy lime content, but the ingredient is never mentioned in most surviving early recipes. Of course, it may be that  lime is present in many ancient glass artifacts precisely because those artifacts are the ones that have survived without dissolving.

Some have speculated that calcium could have been introduced inadvertently as seashell fragments when sand was crushed into powder for glassmaking. This hypothesis is certainly possible, but it seems suspect considering just how much seashell would be required and how fussy the glassmakers were in obtaining pure white sand from particular locations. However, a distinction must be made between knowing that sand from a particular location makes good glass, and knowing what is in that sand. 

Perhaps the most famous such site for good glassmaking sand is the "Belus" [Na'aman] River outlet in what is now northern Israel. This is where the story of the discovery of glass takes place, as told by Pliny and other early writers. Sailors were driven here by a storm, they used natron (glassmaking salt) [4] from their cargo to hold their cooking pots up over a beach fire, and when the natron mixed with the sand in the fire, glass was formed. Or so the story tells. The nearby Phoenician cities of Tyre and Sidon are known to have hosted a thriving glass industry. The sand in this area is composed of exceptionally pure quartz, with a healthy amount of calcium from shells ground down from wave action. Together these make an ideal mix for stable glass. 

There is some evidence that experimenters such as Basil Valentine were aware that glass without lime would dissolve as early as 1520. [5] For what it is worth, I can contribute some anecdotal evidence from the twentieth century. There is a well-known product called water-glass that is essentially composed of dissolved glass without the lime. [6] My glass artist friend Emilio tells me that growing up on Murano in Venice, his mother regularly preserved fresh eggs by dipping them in water-glass, which when dry formed a hard seal over the existing shell. Presumably, she learned the technique from her own mother, but I do not know of any early references.

I would like to propose an additional explanation. Lime was a commodity that was made since ancient times, in furnaces very similar to the ones producing glass. In fact, Neri specifies the use of a “limekiln” for the production of his enamels. The archaeological remains of Tyre and Sidon show prolific use of lime based plaster and mortar in their construction practices and there are indications of lime making facilities. It seems to me that good business sense dictates that the same furnaces making glass may have also made lime. Cross contamination, and even experimentation would be a natural outcome. 

[1] Neri 1612, ch 7.
[2] As an aside, magnesia (MgO) can also serve as a stabilizer in glass.
[3] Pliny, Natural History v.36, ch. 66.192.
[4] Natron is a mix of sodium carbonates. 
[5] Basil Valentine, et al. see footnote 6 of the Wikipedia article on sodium silicate for a discussion.
[6] Sodium Silicate.