This Illuminating Objects study is a collaboration with Victoria Druce, who studied at Imperial College London for a postgraduate degree in Science Communication. Her objects, a Venetian or Venetian style 16th century goblet and an 18th century English wine glass, span a fascinating era of European glass making. This project aims to explore the scientific and craft nature of the objects including their chemical compositions and the technique behind the creation of their striking decoration.
Venice to London: Movement of Glass Technology
By the 13th century Venice was famous for its glass. The Venetian lagoon was in prime position for international trade and had a monopoly on luxury glassware. The Venetian Republic guarded their technology by restricting the movements and freedoms of their craftsmen, confining them and their industry to the island of Murano.
Despite strict regulations, many Venetians did leave Murano. By the 16th century, lured by the promise of greater freedoms and emerging markets in new locations, Italians had settled in new glassmaking centres across Europe. Initially craftsmen moved to Antwerp, which was one of the most important ports on the European Continent. Many glassblowers settled there and established glasshouses, unable to return to their homeland for fear of punishment. Some continued further North towards England, including the influential glassmaker Jean Carre in 1567 as well as Jacob Verzeliniho took over Carre’s glasshouse when he died in 1572. The Muranese had brought their skills and knowledge with them to these new destinations where they trained local people in their craft and made glass in the Venetian style, known as façon de venise. The glass made in these European locations was just as delicate and luxurious as the glass produced in Venice.
A New Material: Lead Glass
For centuries the influence of Venice had stretched far and wide. But by the late 1600s Venetian influence in glass technology began to fade as a revolutionary recipe created in England in 1674 became popular. George Ravenscroft is credited with the creation of lead glass (then known as flint glass due to the use of powdered flint as silica) in which lead oxide was added to the molten glass batch. Lead glass was not only stronger and more durable but also clearer than Venetian glass. The large proportions of lead in the glass, up to 40% in some glasses, means it is very stable. The network of molecules in the glass is tightly held together so when the glass is struck the network cannot absorb the shock and instead vibrates in place. This gives lead glass a characteristic ring when tapped. This characteristic ring is absent in Venetian glasses.
Lead glass is less viscous and takes much longer to cool and solidify than the façon de Veniseglass. This dictated the sort of styles possible in lead glass. The fanciful designs of Venice gave way to more practical and straightforward shapes. Individual glasses were now constructed from three components, the foot, the stem and the bowl, made separately and then joined together in a production line typical of the industrialisation taking place at the time. Ornament was added to the glasses by engraving patterns onto them or in the form of decorative stems. Air twist stems were briefly popular around 1750 but were replaced by opaque twist stems (also known as cotton twist). English glass blowers used the filigree technique invented in the 16th century to produce increasingly complex patterns in the stems of glasses. One column of threads was sometimes surrounded by another, creating beautifully intricate interweaving designs.
Craft and Composition
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.
Patterns and Processes
In 1527 brothers Filippo and Bernado Catani (also known as the Serena brothers, after their glassworks) patented a technique known as filigrana. The technique used white glass, known as lattimo from the Italian for milk, in canes to create beautifully intricate patterns. The opaque glass was produced by adding tin and lead to the molten batch. The delicate white designs highlighted the art of the craftsmen and began to replace the gold ornamentation which was popular as decoration on glass previously. The white decoration was fashionable and grew in status amongst refined society.
To produce the complex patterns white canes of glass would be laid in a mould alternating with colourless glass canes. The glassblower would roll a colourless glass bulb over the canes to pick them up and then ‘marver’ them, which means they rolled them over a flat surface to produce a smooth surface with white canes embedded in clear glass. This pattern of simple stripes was called a fili. More complex patterns followed including a retorti where canes were twisted into a spiral.
A variant on this technique that seems to have come slightly later involved creating a cylinder of canes by laying them in a sheet which was heated in the furnace before rolling the canes around the circumference of a clear glass collar. The canes would be fused into a cylindrical shape, which could then be expertly worked by the craftsman.