Giovan Maria Butteri,
"The Discovery of Glass"
Studiolo of Francesco I de' Medici
Actually, starting with Pliny the Elder, the author of the most famous version of this story, skepticism abounds about how much of it was true. Nevertheless, if we gently tease apart the loose threads of this yarn we find that it is not without substance. First, there is the location; not just any port in a storm, this region was the site of a thriving glass industry as early as the sixth century BCE, due to the exceptional, pure white sand at the outlet of the Belus river. Archaeologists have excavated ancient glass furnaces at the nearby cities of Tyre and Sidon.
Next, the sodium carbonates in natron do indeed form glass when mixed with fine sand and brought to a high temperature, but this takes strong, concentrated heat, likely more than could be provided by a cook's beach fire. However, there is another possible explanation; natron is a hydroscopic mineral – this means it pulls moisture out of the surrounding environment. The water is locked into its solid crystal structure, where it remains until it is released either chemically or through heat. Natron can hold a remarkable amount of water, up to two-thirds of its weight. This is why it was used extensively to preserve mummies in Egypt; it dried out the bodies, soaking up moisture, quickly preserving them. While the story of the Phoenician sailors deserves a healthy dose of skepticism, it is also easy to see how the decomposition of the natron in the fire, resulting in the release of briny liquor, might be misinterpreted as glass, which then disappeared into the sandy beach.
In the early 1600s glassmaker and alchemist Antonio Neri published the very first printed book of glass recipes, and in his introduction he too recounts the tale. However, in Neri's telling, natron does not make an appearance. Instead, the sailors use 'kali,' a coastal plant that is rich in alkali salts, to fuel their fire. The salts in kali are substantially similar to natron and, according to the story, triggered a similar result. In this period, Kali ash was a well-known ingredient in glass making. Neri used it in his own recipes, so the substitution is not surprising, but in this respect, Neri's version of the story with kali does appear to be unique in the literature. It is interesting to note that Lodovico Domenichi, who was good friends with Neri's grandfather, tells a version of this story in his Italian translation of Pliny's Natural History. Here the sailors use natron, but in the next paragraph, Domenichi describes how local natives later used kali plants to make their own glass.
The above depiction of the discovery of glass was painted by Butteri, one of a select group of painters for the Medici court in Florence. The work was commissioned to hang in the secret "studiolo" laboratory of Francesco de' Medici, a concealed barrel vaulted room tucked under a staircase in the Palazzo Vecchio in the early 1570's. It was only accessible through secret passages, one leading from Francesco's bed chamber. Another led from the chamber to an unmarked door on the street and a third passage led from the chamber to the secret treasury room once used by his father, Grand Duke Cosimo I. The walls and ceiling were entirely filled with paintings, the lower ones concealing cabinets full of oddities of nature, precious gems, coins, alchemical concoctions, and other treasures. Presumably, the cabinet behind Butteri's "Discovery of glass" would house some of the intricate Venetian glass vessels for which the craftsmen of Murano had become world famous. Shortly before the room was completed, a small number of these glass masters were allowed to teach their secrets in Florence by special arrangement with the Venetian government.
This post first appeared in a shorter form here on 9 October 2013.