Like Snow From Heaven

28th December 2005 in Florence

Photo by Marco De La Pierre

In its simplest incarnation, glass is nothing more than crushed quartz or sand mixed with 'glass salt'. This salt is the alkali carbonates of sodium or potassium. Essentially, the ash of certain plants that reduces to oxide in the furnace and dramatically lowers the melting point of the sand.

In Pliny's ancient account of the discovery of glass, the process takes place in a single step. The ingredients came together accidentally in a fire and "glass trickled out." The problem is that pure quartz does not react with the salt until it gets very hot and even then, it does so reluctantly. In a wood-fired furnace, quartz stones, even small pebbles or sand would melt with excruciating slowness. As Neri advises, "... this would only succeed after a protracted period of time and a great amount of trouble." 

'Fritting' is an intermediate step that speeds the process considerably. Glassmakers reduce the quartz to a fine powder and then mix it with the alkali salt. In the heat of a kiln, the entire content of each stone is thus exposed to the salt right from the beginning. This roasting process starts a chemical reaction between the ingredients. In the late Renaissance, the combination was then cooled and 'aged' for several months before use. When made from pure quartz river stones and the best Levantine ash, the result was what Neri calls 'bollito,' "white and pure like snow from heaven."

A third ingredient of glass, critical to its long-term stability is lime, or calcium oxide. Without the lime, glass is susceptible to attack by water. The water actually dissolves the glass, or less dramatically makes it subject to 'glass disease' or 'crizzling', a condition where the glass slowly decomposes due to humidity in the air. Waterglass is a product made without the lime. On Murano and elsewhere, it was dissolved in water and painted onto the shells of eggs to seal and preserve them. In Neri's era, lime was produced by roasting seashells. It was a key ingredient of cement, and as such was readily available. It had been a major commodity throughout the Mediterranean since the Roman Empire. Neri advises to add lime to all his glass recipes, but it is not so clear that he himself understood why it was so important.

*This post first appeared here on 11 October 2013.

Like Snow from Heaven

28th December 2005 in Florence
Photo by Marco De La Pierre

In its simplest incarnation, glass is nothing more than crushed quartz or sand mixed with 'glass salt'. This salt is the alkali carbonates of sodium or potassium. Essentially, the ash of certain plants that reduces to oxide in the furnace and dramatically lowers the melting point of the sand.

In Pliny's ancient account of the discovery of glass, the process takes place in a single step. The ingredients came together accidentally in a fire and "glass trickled out." The problem is that pure quartz does not react with the salt until it gets very hot and even then, it does so reluctantly. In a wood-fired furnace, quartz stones, even small pebbles or sand would melt with excruciating slowness. As Neri advises, "... this would only succeed after a protracted period of time and a great amount of trouble." 

'Fritting' is an intermediate step that speeds the process considerably. Glassmakers reduce the quartz to a fine powder and then mix it with the alkali salt. In the heat of a kiln, the entire content of each stone is thus exposed to the salt right from the beginning. This roasting process starts a chemical reaction between the ingredients. In the late Renaissance, the combination was then cooled and 'aged' for several months before use. When made from pure quartz river stones and the best Levantine ash, the result was what Neri calls 'bollito,' "white and pure like snow from heaven."

A third ingredient of glass, critical to its long-term stability is lime, or calcium oxide. Without the lime, glass is susceptible to attack by water. The water actually dissolves the glass, or less dramatically makes it subject to 'glass disease' or 'crizzling', a condition where the glass slowly decomposes due to humidity in the air. Waterglass is a product made without the lime. On Murano and elsewhere, it was dissolved in water and painted onto the shells of eggs to seal and preserve them. In Neri's era, lime was produced by roasting seashells. It was a key ingredient of cement, and as such was readily available. It had been a major commodity throughout the Mediterranean since the Roman Empire. Neri advises to add lime to all his glass recipes, but it is not so clear that he himself understood why it was so important.

*This post first appeared here on 11 October 2013.

Like Snow From Heaven

28th December 2005 in Florence
Photo by Marco De La Pierre

In its simplest incarnation, glass is nothing more than crushed quartz or sand mixed with 'glass salt'. This salt is the alkali carbonates of sodium or potassium. Essentially, the ash of certain plants that reduces to oxide in the furnace and dramatically lowers the melting point of the sand.

In Pliny's ancient account of the discovery of glass, the process takes place in a single step. The ingredients came together accidentally in a fire and "glass trickled out." The problem is that pure quartz does not react with the salt until it gets very hot and even then, it does so reluctantly. In a wood-fired furnace, quartz stones, even small pebbles or sand would melt with excruciating slowness. As Neri advises, "... this would only succeed after a protracted period of time and a great amount of trouble." 

'Fritting' is an intermediate step that speeds the process considerably. Glassmakers reduce the quartz to a fine powder and then mix it with the alkali salt. In the heat of a kiln, the entire content of each stone is thus exposed to the salt right from the beginning. This roasting process starts a chemical reaction between the ingredients. In the late Renaissance, the combination was then cooled and 'aged' for several months before use. When made from pure quartz river stones and the best Levantine ash, the result was what Neri calls 'bollito,' "white and pure like snow from heaven."

A third ingredient of glass, critical to its long-term stability is lime, or calcium oxide. Without the lime, glass is susceptible to attack by water. The water actually dissolves the glass, or less dramatically makes it subject to 'glass disease' or 'crizzling', a condition where the glass slowly decomposes due to humidity in the air. Waterglass is a product made without the lime. On Murano and elsewhere, it was dissolved in water and painted onto the shells of eggs to seal and preserve them. In Neri's era, lime was produced by roasting seashells. It was a key ingredient of cement, and as such was readily available. It had been a major commodity throughout the Mediterranean since the Roman Empire. Neri advises to add lime to all his glass recipes, but it is not so clear that he himself understood why it was so important.

*This post first appeared here on 11 October 2013.

Scraping the Barrel

4th century BCE philosopher Diogenes
(supposedly lived in a wine barrel)
by Gaetano Gandolfi (1792)

To seventeenth century glassmaker Antonio Neri, "tartar" was a well-known byproduct of the winemaking process. If we chill wine or grape juice to below 50 degrees (10 deg. C.) crystals of tartar start to form and once they do, then tend not to dissolve back, even at room temperatures. Today, these crystals are commonly found in a powdered form, in kitchen cupboards as "cream of tartar." Bakers and cooks use it to stabilize whipped egg whites, and it has a number of other applications. 

Neri used it in his glass to add sparkle, a trick known to Venetian glassmakers as early as the 1400s. It was obtained from the dregs at the bottom of wine barrels. To understand how this works, it is useful to know that Florentines, Venetians and most southern Europeans made glass from crushed up quartz pebbles or sand mixed with a specific flux known as "glass salt." This salt was rich in sodium carbonates, which greatly reduced the melting point of the quartz. It allowed artisans to work the material at the temperatures easily achieved in their furnaces. Tartar turns out to be very similar, except that it is rich in potassium rather than sodium. 

Potassium atoms are bigger and heavier than sodium atoms and when light passes through a piece of potassium-fluxed glass, it bends and refracts more. This effect is not as pronounced as when adding the even heavier lead to form fine crystal, but it still adds noticeable sparkle to finished pieces. Using all tartar as a flux has the undesirable effect of reducing the workability of the hot glass. Outside the furnace, it becomes stiff quicker and artists have less time to create fancy shapes and forms. The solution to this dilemma is to use a mix of sodium and potassium fluxes together, which is exactly what Neri did. 

In his 1612 book L'Arte Vetraria, he shows how to prepare tartar and then adds it to a number of his glass recipes saying, "The tartar is the secret way to produce more salt and to make cristallo which is whiter and of rare beauty." Here is Neri’s prescription:

To make Purified Tartar Salt you should obtain tartar, which is also called gruma, from barrels of red wine in which it forms large lumps, however do not use powder. Roast it in earthenware pots amongst hot coals until it becomes calcined black and all its sliminess is roasted away. It then will begin to whiten, but do not let it become white, because if you do the salt will be no good.  

Calcine tartar this way: put it in large earthenware pans full of hot common water, or better yet in glazed earthenware pans then made to boil on a slow fire. You should do it in such a way that in two hours the level of the water will slowly decrease to one-quarter, at this point lift it from the fire and leave it to cool and to clarify. Now decant off the liquid, which will be strong lye and refill the pans containing the remains of the tartar with new common water. In the way stated above, boil as before and repeat the procedure until saltiness no longer charges the water. 

At this point, the [decanted] water is impregnated with all the salt. Filter the lye clear and put it in glass chamber pots to evaporate in the ash of the stove over a slow fire. In the bottom, white salt will remain. Dissolve this salt in new hot common water and leave it in the pans, letting it settle for two days. Then filter it and return it to chamber pots to evaporate over a slow fire. In the bottom, a much whiter salt will be left than the previous time. Now dissolve this salt in fresh hot common water and leave it to settle for two days. Evaporate, filter and repeat everything as before. Overall, repeat this procedure four times to dissolve, filter and evaporate the salt of tartar. This will make the salt whiter than snow and purified from the vast majority of its sediment.

When mixed with sifted polverino, or rocchetta, with its doses of tarso [quartz] or sand, this salt will make a frit that in crucibles will produce the most beautiful crystallino and common glass, which one cannot make without the accompaniment of tartar salt. Without it [tartar], good fine crystallino can be made, nevertheless with it, it will be the absolute most beautiful.

*This post first appeared here on 16 May 2014.

Like Snow From Heaven

28th December 2005 in Florence
Photo by Marco De La Pierre

In its simplest incarnation, glass is nothing more than crushed quartz or sand mixed with glass salt. This salt is the carbonates of sodium or potassium. Essentially, plant ash oxidizes in the furnace and dramatically lowers the melting point of the sand.

In Pliny's account of the discovery of glass, the process takes place in a single step. The ingredients came together accidentally in a fire and "glass trickled out." The problem is that pure quartz does not react with the salt until it gets very hot and even then, it does so reluctantly. In a wood-fired furnace, quartz stones, even small pebbles or sand would melt with excruciating slowness. As Neri advises, "... this would only succeed after a protracted period of time and a great amount of trouble." 

'Fritting' is an intermediate step that speeds the process considerably. Glassmakers reduce the quartz to a fine powder and then mix it with alkali salt. In the heat of a kiln, the entire content of each stone is thus exposed to the salt right from the beginning. This roasting process starts a chemical reaction between the ingredients. The combination is then cooled and 'aged' for several months before use. When made from pure quartz river stones and the best Levantine ash, the result is what Neri calls 'bollito,' "white and pure like snow from heaven."

A third ingredient of glass, critical to its long-term stability is lime, or calcium oxide. Without the lime, glass is susceptible to attack by water. The water actually dissolves the glass, or less dramatically makes it subject to 'glass disease' or 'crizzling', a condition where the glass slowly decomposes due to humidity in the air. Waterglass is a product made without the lime. On Murano and elsewhere, it was painted onto the shells of eggs to seal and preserve them. In Neri's era, lime was produced by roasting seashells. It was a key ingredient of cement, and as such was readily available. It had been a major commodity throughout the Mediterranean since the Roman Empire. Neri advises to add lime to all his glass recipes, but it is not so clear that he himself understood why it was so important.

*This post first appeared here on 11 October 2013.

Scraping the Barrel

4th century BCE philosopher Diogenes
(supposedly lived in a wine barrel)
by Gaetano Gandolfi (1792)

To seventeenth century glassmaker Antonio Neri, "tartar" was a well-known byproduct of the winemaking process. If we chill wine or grape juice to below 50 degrees (10 deg. C.) crystals of tartar start to form and once they do, then tend not to dissolve back, even at room temperatures. Today, these crystals are commonly found in a powdered form, in kitchen cupboards as "cream of tartar." Bakers and cooks use it to stabilize whipped egg whites, and it has a number of other applications. 

Neri used it in his glass to add sparkle, a trick known to Venetian glassmakers as early as the 1400s. It was obtained from the dregs at the bottom of wine barrels. To understand how this works, it is useful to know that Florentines, Venetians and most southern Europeans made glass from crushed up quartz pebbles or sand mixed with a specific flux known as "glass salt." This salt was rich in sodium carbonates, which greatly reduced the melting point of the quartz. It allowed artisans to work the material at the temperatures easily achieved in their furnaces. Tartar turns out to be very similar, except that it is rich in potassium rather than sodium. 

Potassium atoms are bigger and heavier than sodium atoms and when light passes through a piece of potassium-fluxed glass, it bends and refracts more. This effect is not as pronounced as when adding the even heavier lead to form fine crystal, but it still adds noticeable sparkle to finished pieces. Using all tartar as a flux has the undesirable effect of reducing the workability of the hot glass. Outside the furnace, it becomes stiff quicker and artists have less time to create fancy shapes and forms. The solution to this dilemma is to use a mix of sodium and potassium fluxes together, which is exactly what Neri did. 

In his 1612 book L'Arte Vetraria, he shows how to prepare tartar and then adds it to a number of his glass recipes saying, "The tartar is the secret way to produce more salt and to make cristallo which is whiter and of rare beauty." Here is Neri’s prescription:

To make Purified Tartar Salt you should obtain tartar, which is also called gruma, from barrels of red wine in which it forms large lumps, however do not use powder. Roast it in earthenware pots amongst hot coals until it becomes calcined black and all its sliminess is roasted away. It then will begin to whiten, but do not let it become white, because if you do the salt will be no good.  

Calcine tartar this way: put it in large earthenware pans full of hot common water, or better yet in glazed earthenware pans then made to boil on a slow fire. You should do it in such a way that in two hours the level of the water will slowly decrease to one-quarter, at this point lift it from the fire and leave it to cool and to clarify. Now decant off the liquid, which will be strong lye and refill the pans containing the remains of the tartar with new common water. In the way stated above, boil as before and repeat the procedure until saltiness no longer charges the water. 

At this point, the [decanted] water is impregnated with all the salt. Filter the lye clear and put it in glass chamber pots to evaporate in the ash of the stove over a slow fire. In the bottom, white salt will remain. Dissolve this salt in new hot common water and leave it in the pans, letting it settle for two days. Then filter it and return it to chamber pots to evaporate over a slow fire. In the bottom, a much whiter salt will be left than the previous time. Now dissolve this salt in fresh hot common water and leave it to settle for two days. Evaporate, filter and repeat everything as before. Overall, repeat this procedure four times to dissolve, filter and evaporate the salt of tartar. This will make the salt whiter than snow and purified from the vast majority of its sediment.

When mixed with sifted polverino, or rocchetta, with its doses of tarso [quartz] or sand, this salt will make a frit that in crucibles will produce the most beautiful crystallino and common glass, which one cannot make without the accompaniment of tartar salt. Without it [tartar], good fine crystallino can be made, nevertheless with it, it will be the absolute most beautiful.

Like Snow from Heaven

In its simplest incarnation, glass is nothing more than crushed quartz or sand mixed with glass salt. This salt is the carbonates of sodium or potassium. Essentially, plant ash oxidizes in the furnace and dramatically lowers the melting point of the sand.

In Pliny's account of the discovery of glass, the process takes place in a single step. The ingredients came together accidentally in a fire and "glass trickled out." The problem is that pure quartz does not react with the salt until it gets very hot and even then, it does so reluctantly. In a wood-fired furnace, quartz stones, even small pebbles or sand would melt with excruciating slowness. As Neri advises, "... this would only succeed after a protracted period of time and a great amount of trouble." 

'Fritting' is an intermediate step that speeds the process considerably. Glassmakers reduce the quartz to a fine powder and then mix it with alkali salt. In the heat of a kiln, the entire content of each stone is thus exposed to the salt right from the beginning. This roasting process starts a chemical reaction between the ingredients. The combination is then cooled and 'aged' for several months before use. When made from pure quartz river stones and the best Levantine ash, the result is what Neri calls 'bollito,' "white and pure like snow from heaven."

A third ingredient of glass, critical to its long-term stability is lime, or calcium oxide. Without the lime, glass is susceptible to attack by water. The water actually dissolves the glass, or less dramatically makes it subject to 'glass disease' or 'crizzling', a condition where the glass slowly decomposes due to humidity in the air. Waterglass is a product made without the lime. On Murano and elsewhere, it was painted onto the shells of eggs to seal and preserve them. In Neri's era, lime was produced by roasting seashells. It was a key ingredient of cement, and as such was readily available. It had been a major commodity throughout the Mediterranean since the Roman Empire. Neri advises to add lime to all his glass recipes, but it is not so clear that he himself understood why it was so important.