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.

Cristallo

Dragon-stem goblet, Venice, Italy, 
(or in the Venetian style), 17th cent.
Corning Museum of Glass 51.3.118.

Venetian cristallo is quite literally the glass that made Murano famous. Ultimately, royal courts and wealthy families throughout Europe coveted glassware of this type. It had the clarity of its namesake mineral, rock crystal, and its working properties were such that one could make extremely thin, yet complex pieces. At a time when glass made from dirty sand and fireplace ash was churned out cheaply and quickly around Europe, cristallo was the absolute pinnacle of the glassmaker's art.

Murano glassmaker Angelo Barovier gets credit for the development of cristallo around the year 1450. However, it is a mistake to think that suddenly, he exclaimed "eureka" and burst out of his workshop with an entirely new invention. The predecessor of cristallo was what Antonio Neri called cristallino, which already incorporated some of the advantages. Barovier may well be responsible for bringing together the ultimate refinements that elevated cristallino to a new level of perfection, but these techniques individually were all available and utilized by glassmakers at least fifty years earlier. 

Barovier had attended lectures by noted alchemist Paola de Pergola, at the School of Rialto. This indicates that he was thinking along the lines of a chemist in putting together a repeatable regimen for an exceptionally clear, bright, workable product. The real genius in cristallo was in controlling the purity and quality of all the materials through Venice's extensive trade network in the Mediterranean. Even if the recipe leaked out, exclusive trade agreements blocked access to the raw materials. Local ordinances tightly controlled source materials used in the furnaces to ensure consistency, from the quartz river pebbles crushed for frit, to the plants used to make the flux, to the manganese which eliminated any slight tinting. Requirements even stipulated the making of the furnace crucibles from a specific variety of clay from Constantinople.

Barovier's innovation would become a tradition that would be carried on by countless glassmakers and ultimately by our Florentine priest, Antonio Neri, more than a century and a half later. Neri describes the Venetian methods, but then goes on to innovate with variations of his own. Numerous family manuscripts in Venice documented the method to make cristallo; what Neri did was to publish these relatively secret techniques and make them available to anyone who could read his book, and many did. 

For silica, the main ingredient in glass, Neri says the craftsmen on Murano specifically used quartz river pebbles (tarso) from the Ticino River in Pavia. He describes special procedures to pulverize the stones without contamination, into a powder "ground as finely as flour." Next, he makes the flux (glass salt), which allows the quartz to melt at lower temperatures. He says to use the ash of the kali plant from Syria (polverino and rocchetta). This plant ash is very rich in sodium carbonates; he extracts and purifies it in a process called lixiviation. Neri purchased the starting material in a charred state, which reduces it weight and volume for shipping. From 300 pounds of ash, he usually produced 80-90 pounds of salt. The ash was boiled in cauldrons full of water, strained, filtered, evaporated and dried leaving the all important glass salt. Into each cauldron, he mixes ten pounds of tartar—the dried dregs scraped from the inside of red wine barrels—a material high in potassium carbonates. Today, we know the soda component is responsible for the favorable property of long working times for artisans before the hot glass stiffens, the potassium adds sparkle. The final ingredient was two pounds of lime (calcium oxide) per one hundred pounds of salt, which is added to the dry salt because it is not water soluble. This stabilized the glass against future degradation by moisture over time. 

200 pounds of powdered quartz mixed thoroughly with 130 pounds of the salt forms the basis of the frit. He raked the powder around in a hot kiln for about five hours, until "nut sized" pieces began to form. Now this cristallo frit,—which Neri calls bollito—was carefully packed to remain dry and allowed to age for three or four months, when it was ready to melt in the furnace. If the glass should have a slight green tint from iron tools, it was corrected with manganese (oxide), specifically mined in the Piedmont region. The cristallo was boiled for up to six days and repeatedly washed by ladeling the hot glass into cold water and then remelting.

The result was a glass that was a pleasure for artisans to work and make finished pieces over which royalty clamored. Cristallo was uniform in consistency, it could be blown into molds or worked freehand into complex shapes, like the iconic "dragon goblet" form, which became a hallmark of the Venetian masters.

In the Cathedral of Santa Maria and Donato on Murano, behind the altar, there is an unusual display of four very large bones found buried under the ancient church. Local folklore tells that the bones belong to a dragon slayed by Saint Donatus of Arezzo, the patron saint of the island.

* This post first appeared here in a slightly shorter form on 12 February 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.

The Curious Reader

An unknown early modern reader with spectacles.
Church of San Pedro y San Pablo Teposcolula,
High Mixteca Region, Oaxaca, Mexico, 16th century fresco. 

In my last post ("The Art of Fire Reprise"), I discussed Priest Antonio Neri’s familiarity with Biringuccio’s Pirotechnia and the allusions he makes to it in the introduction of his own book on glassmaking, L’Arte Vetraria. The entire introduction is relatively short and well worth reading for a fascinating glimpse into the state of the art four hundred years ago. It also outlines Neri’s thoughts on the earliest origins of glass; he recounts the prevailing stories of the day and adds his observations, but as a careful historian, he lets us decide for ourselves how much is truth and how much is myth. Here is the introduction to L’Arte Vetraria in its entirety, addressed to "The Curious Reader":

Without a doubt, glass is a true fruit of the art of fire, as it can so closely resemble all kinds of rocks and minerals, yet it is a compound and made by art. In the fire, it fuses and becomes imperishable. Indeed, like the perfect shining metal gold, the fire refines it, polishes it and makes it beautiful. 

Clearly, its use in drinking vessels and in other utilitarian objects is far more graceful, appealing and noble than any metal or stone suited for such creations. Beyond the ease and low cost with which it is made, and the fact that it can be made anywhere, glass is more delicate, clean and attractive than any material currently known to the world. It is so useful to the distillation and spagyric arts, not to mention indispensable to the preparation of medicines for man that would be nearly impossible to make without glass. Furthermore, many kinds of vessels and instruments are produced with it; boccie [cucurbits,] cappelli [alembics,] recipienti [receivers,] pellicani [pelicans,] leuti [lenses,] storte [retorts,] antenitorij [antenitors,] serpentine [coils,] fiale [vials,] nasse [nasse,] quadretti [tesserae,] ampolle [ampules,] oui filisofici [philosophic eggs]  and palle [balls].*

Countless other types of vessels are invented every day to compose and produce elixirs, secret potions, quintessences, salts, sulfurs, vitriols, mercuries, tinctures, elemental separations, all metallic things and many others that are discovered daily. Also, glass containers are made for acque forti and acque regie,** which are so essential for refiners and partitori [masters in royal mint] to purify gold and silver and to bring them to perfection. So many benefits for the service of humanity come from glass, which seem nearly impossible to make without it. We can see in it and in any other thing the great providence of God that has put glassmaking materials so abundantly in every place and region, so that something of such great use to humanity can be made with such facility for all. Glass is also a great ornament to God's churches since, among other things, many beautiful windows are made adorned with graceful paintings in which the metallic colors are so intense and vivid that they seem like so many oriental gems. In the furnaces, glasses of many colors are formed with so much beauty and perfection that it seems no material like it can be found on earth. 

We can suppose the invention of glass is very ancient since the Holy Scripture says in The Book of Job [28:17], "gold and glass shall not be equal to it [wisdom]…", which gives clear evidence that glass was invented in antiquity. Saint Jerome held that Job descended from Abraham, and was the son of Zanech, who descended from Esau and therefore was only fifth from Abraham. There are many who attribute the invention of glass, and perhaps with some reason, to the alchemists; wanting to imitate jewels, they discovered glass. This may not be too far from the truth; I show clearly in book five of the present work, methods to imitate all the jewels, in which we see the vitrification of stones that never could fuse and vitrify by themselves. Pliny indicates that glass was discovered by chance, in Syria [now Israel], at the mouth of the Bellus [now Na'aman] river by certain merchants pushed off course at the whim of the sea. They were forced to stop there and set up camp. In order to eat they built fires on the beach, where there was a large amount of that kind of herb that many call kali, whose ashes make soda and rocchetta. This burned in the fire, its ashes and salt reacted with the sand and with the stones suitable to vitrify; these materials changed into glass. This event illuminated the mind of man on the means and manner to make not only glass, but cristallo, cristallino and many other beautiful things made with glass. Also, it is taught that in the time of the Roman Emperor Tiberius, a way of making glass malleable was invented, a thing that was subsequently lost and today is hidden to all. Indeed, if such a thing were to be known today, without any doubt it would be more valued than silver or gold for its beauty and incorruptibility, since glass does not give rise to rust or taste or smell or any other [adverse] quality. Furthermore, it provides great value to humanity in the use of reading spectacles and [reflecting] spheres. Even though the former may also be made of natural rock crystal, also known as mountain crystal and the latter with the alloy known as steel [bronze], a composition made of copper and tin, nevertheless, in the one as well as the other, glass is better suited, not nearly as expensive, more desirable and more effective, especially with [reflecting] spheres. Beyond the difficulty and expense in making them, they do not image as vividly as the glass and what is worse, in a brief time they fade, not reflecting anything at all.

For these and many other reasons, we can easily conclude, glass is one of the most noble things on earth that humanity has today for its use. Since I have labored in the craft of glassmaking for a long time and seen many things, I am moved to report to the world part of that which I have seen and done in it. Although the methods to make salts, bollitos [salts for cristallo] and frits are well known to many, it still seems to me that this is a matter requiring clear and distinct treatment, which I do with observations and diligence. Moreover, if my work is well considered it will not be judged altogether useless, indeed, perhaps even necessary and enlightening. Besides, with my particular method of extracting salts to make the noblest cristallo, the artisan who is diligent in doing things the way I, with clear demonstrations, reveal and teach, will make things as worthy, appealing and noble perhaps as anything made today or that can be made in any other way. In this and every other matter that I deal with in the present work, the diligent and careful craftsman will find that I have written and shown the truth, not as I was told or persuaded by any person whatsoever, but as I actually did and experienced many times with my own hands.

I have always possessed the resolve to write and to speak the truth. If anyone trying my recipes and methods to make the colors, pastes and tinctures does not succeed in making what I have described, do not reject my work or think I have written lies, but reflect upon where he may have erred. Especially those who have never before handled such things, because it is impossible on the first try to have the ability to be a master. That being the case, repeat the work so that it will continuously improve and ultimately be perfect as I describe it. Be warned in particular to give careful consideration to the colors for which exact and determined amounts cannot be given. Indeed, with experience and due practice learn and with the eye and judgment know when a glass is colored sufficiently and appropriately for the work at hand. With paste made for the imitation of jewels, in determining the size that you want to make, bear in mind that those to be set in gold with foils as for rings and other similar applications should always be more transparent. Those that are also set in gold, but are intended to hang in the air, as earrings and similar items, require a greater charge of color. All of this is nearly impossible to teach, but must be left to the judgment of the careful artisan. Keep in mind also to be meticulous so the materials and colors that you make are well prepared and well ground. To ensure that materials slated for exquisite work are beyond question, prepare and make the colors personally the same way as I teach and to that end you can be confident of good work that will come to fruition happily. 

The fire in this art is of notable importance. Indeed, this is what perfects everything and without which nothing can be done, therefore give it in a proportionate way. In particular, use hard dry wood, being careful of harmful smoke, which always causes damage, especially in the furnaces, where pots and pans stand open and the glass could become imbedded with specks causing imperfections and notable ugliness. 

In closing, I say that the artisan who is or becomes diligent, practical and works step by step, as I describe, will find truth in the present work. He will find that I have published and given to the world only as much as I have proven and personally verified through experiment. 

If the world acknowledges my arduous efforts, as I hope, I will perhaps also be encouraged to publish the experience of my endeavors over many years, working in various parts of the world, in the chemical and spagyric arts. Known and perfected in the ancient times, I believe there is no greater thing in nature in the service of humanity. Men who became experts in these arts were thought to be gods and were esteemed and revered. 

I will not enlarge on this matter [glass] since 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. Therefore take what I have to offer in good course, as I have candidly written this work, first for the glory of God and then for the delight, benefit and utility of all.

* A more detailed treatment of the translation of these terms can be found in the post dated 27 December 2013.

** 'Aqua fortis' and 'aqua regis' Neri uses the plural forms, so I have kept his italian.

Cristallo

Dragon-stem goblet, Venice, Italy,
(or in the Venetian style), 17th cent.
Corning Museum of Glass 51.3.118.

Venetian cristallo is quite literally the glass that made Murano famous. Ultimately, royal courts and wealthy families throughout Europe coveted glassware of this type. It had the clarity of its namesake mineral, rock crystal, and its working properties were such that one could make extremely thin, yet complex pieces. At a time when glass made from dirty sand and fireplace ash was churned out cheaply and quickly around Europe, cristallo was the absolute pinnacle of the glassmaker's art.

Murano glassmaker Angelo Barovier gets credit for the development of cristallo around the year 1450. However, it is a mistake to think that suddenly, he exclaimed "eureka" and burst out of his workshop with an entirely new invention. The predecessor of cristallo was what Antonio Neri called cristallino, which already incorporated some of the advantages. Barovier may well be responsible for bringing together the ultimate refinements that elevated cristallino to a new level of perfection, but these techniques individually were all available and utilized by glassmakers at least fifty years earlier. 

Barovier had attended lectures by noted alchemist Paola de Pergola, at the School of Rialto. This indicates that he was thinking along the lines of a chemist in putting together a repeatable regimen for an exceptionally clear, bright, workable product. The real genius in cristallo was in controlling the purity and quality of all the materials through Venice's extensive trade network in the Mediterranean. Even if the recipe leaked out, exclusive trade agreements blocked access to the raw materials. Local ordinances tightly controlled source materials used in the furnaces to ensure consistency, from the quartz river pebbles crushed for frit, to the plants used to make the flux, to the manganese which eliminated any slight tinting. Requirements even stipulated the making of the furnace crucibles from a specific variety of clay from Constantinople.

Barovier's innovation would become a tradition that would be carried on by countless glassmakers and ultimately by our Florentine priest, Antonio Neri, more than a century and a half later. Neri describes the Venetian methods, but then goes on to innovate with variations of his own. Numerous family manuscripts in Venice documented the method to make cristallo; what Neri did was to publish these relatively secret techniques and make them available to anyone who could read his book, and many did. 

For silica, the main ingredient in glass, Neri says the craftsmen on Murano specifically used quartz river pebbles (tarso) from the Ticino River in Pavia. He describes special procedures to pulverize the stones without contamination, into a powder "ground as finely as flour." Next, he makes the flux (glass salt), which allows the quartz to melt at lower temperatures. He says to use the ash of the kali plant from Syria (polverino and rocchetta). This plant ash is very rich in sodium carbonates; he extracts and purifies it in a process called lixiviation. Neri purchased the starting material in a charred state, which reduces it weight and volume for shipping. From 300 pounds of ash, he usually produced 80-90 pounds of salt. The ash was boiled in cauldrons full of water, strained, filtered, evaporated and dried leaving the all important glass salt. Into each cauldron, he mixes ten pounds of tartar—the dried dregs scraped from the inside of red wine barrels—a material high in potassium carbonates. Today, we know the soda component is responsible for the favorable property of long working times for artisans before the hot glass stiffens, the potassium adds sparkle. The final ingredient was two pounds of lime (calcium oxide) per one hundred pounds of salt, which is added to the dry salt because it is not water soluble. This stabilized the glass against future degradation by moisture over time. 

200 pounds of powdered quartz mixed thoroughly with 130 pounds of the salt forms the basis of the frit. He raked the powder around in a hot kiln for about five hours, until "nut sized" pieces began to form. Now this cristallo frit,—which Neri calls bollito—was carefully packed to remain dry and allowed to age for three or four months, when it was ready to melt in the furnace. If the glass should have a slight green tint from iron tools, it was corrected with manganese (oxide), specifically mined in the Piedmont region. The cristallo was boiled for up to six days and repeatedly washed by ladeling the hot glass into cold water and then remelting.

The result was a glass that was a pleasure for gaffers to work and make finished pieces over which royalty clamored. Cristallo was uniform in consistency, it could be blown into molds or worked freehand into complex shapes, like the iconic "dragon goblet" form, which became a hallmark of the Venetian masters.

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.