Wednesday, March 29, 2017

Chalcedony Glass

17th century ribbed bottle,Brescia, Italy.
In hopeful anticipation of colorful flowers, the cusp of spring, seems the appropriate time to celebrate Antonio Neri's most colorful creation; chalcedony glass.[1] Through his clever technique, Neri managed to throw every color he knew into one glass pot and come up with, not mud, but the opposite—a swirly rainbow glass that defies verbal description. Somehow, he achieves a balance that blends a full range of colors in a way that seems natural and harmonious. While many glass creations survive from the seventeenth century, none is directly attributable to Neri's glass formulation. But, in my opinion, this piece from Brescia comes close to our alchemist's own description. 

Chalcedony is one of the more exotic varieties of glass described in Antonio Neri's book, L'Arte Vetraria. It is also one of the most labor intensive, exacting recipes and consequently a 'high stakes' risk for losing the entire batch after considerable work. Nevertheless, Neri assures us that the end result is worth the trouble; he describes it as:
Adorned with so many graceful and beautiful areas of undulations and enhanced with the play of diverse, lively, flaming colors.
Chalcedony is a natural mineral, known and admired since antiquity. It occurs in a variety of translucent colors and is most valued when swirls of many different colors are present together in the same piece. In the Roman Empire, it was prized for seals and signet rings; its fine-grained structure allowed intricate carving without fractures. Like many other rare natural materials, it was sometimes supposed to have mystical healing properties. 

Chemically, the mineral chalcedony is identical with quartz or silica, the main ingredient of glass. However, unlike the fabricated substance, the mineral is formed of networks of microscopic interlocking crystals that are responsible for its favorable properties. Small amounts of impurities between the crystal grains cause the swirls of color.

Neri presents three variations of chalcedony glass that span his career as a glassmaker. The first he describes as "the way that I made chalcedony in the year 1601, in Florence at the Casino, in the glass furnace there." The last was made "in the Flemish city of Antwerp, in January of the year 1611" where he presented "His Excellency, the Prince of Orange with two vessels of chalcedony [glass] which delighted him greatly."

His friend Emmanuel Ximenes was anxious to learn the secrets of this glass as early as July of 1603, when he wrote: 

The details of the last chalcedony [glass], which you promised to send to me, did not come in the letter: but I had to recant by the time I got to the end […] I see and understand, that Your Lordship is not at leisure, but in fact busy at work in the service of Christianity... 
Neri advises that in order to bring out the swirls of color, the glassblower must cool and reheat the piece several times, a process that today is known as 'striking'.

Unfortunately, there is also a dark side to this colorful creation.  It is strongly advised to avoid replication of Neri's chalcedony glass as described in his book of recipes; it contains a cocktail of extremely toxic ingredients. While these are relatively harmless once locked inside the glass, in preparation and especially in the hot molten glass melt, vapors of mercury and arsenic can be deadly. Moderate exposure can be expected to cause neurological and liver damage. (The term "mad as a hatter" comes from the unfortunate side effects of inhaled mercury vapors in the formation of felt hats.) In addition, Neri's extensive uses of strong acid reactions in these preparations make sudden eruptions and severe chemical burns a very real danger.

[1] At least in the northern hemisphere, lets say "autumn colors" in the south.
*This post first appeared here in a slightly different form on 27 September 2013.

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