Craft and Composition

Giuseppe Maria Mitelli, Illustration of a Venetian-style glass furnace. From the Latin Version of Neri’s L’Arte vetraria (1668)

Venetian glass is made of two major components: silica and flux. The Venetians used crushed pebbles from the Ticino River as their source of silica. The flux, which helps the silica to melt at a lower temperature, came from the ash of burned coastal plants. The ash also contained impurities which acted as stabilisers in the glass, without which the glass would be fragile and vulnerable to moisture in the air. Typically the stabiliser is lime, known to chemists as calcium oxide. There is little evidence that the Venetians realised the importance of adding stabilisers to their glass, so the presence of naturally occurring stabilisers in the flux was important. However, as well as stabilising impurities the ash contained other compounds which gave the glass a coloured hue instead of the crystal clarity the Venetians desired.

A recipe for cristallo glass, invented in the 1450s by Angelo Barovier, promised better transparency through purifying the ash of compounds which would colour it. The purification process inadvertently also removed the naturally occurring stabilisers meaning the glass was also very fragile and prone to glass diseases. Much academic debate revolves around whether or not cristallo glass has survived. Some experts claim that it is unlikely that large quantities of such fragile glass would have survived unscathed through the centuries. Despite this, many museum labels do refer to their glasses ascristallo simply to describe the very clear and feather-light nature of the glass rather than in reference to the use of Angelo Barovier’s purification technique.

Though much scientific research is currently underway on glass it is still difficult to distinguish Muranese glass from glass made in Northern European glass-making centres. Venetians who moved to Northern Europe brought their recipes with them and continued to use the same raw materials, meaning glasses made in Europe and in Venice have very similar chemical compositions. These recipes remained popular across Europe until the invention of lead glass in the late 17th century.

Venetian soda glass was primarily made from silica and sodium oxide. This representation shows the simplified structure of glass. Silicon atoms bond to oxygen with sodium atoms and impurities also present. Picture credit: Rasmussen S. C. 2012 How Glass Changed the World: The History and Chemistry of Glass from Antiquity to the 13th Century, Springer.

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