Though uncommon nowadays, owing to the fact that we’ve discovered that the chemical is poisonous, lead glass was once a common material used in the construction of many things from liquid vessels to windows throughout the ages. Empire to empire, lead glass was used in various forms for its decorative properties.
The inclusion of lead oxide in the manufacturing of glass has a tripartite effect on its production.
It raises the refractive index (RI)
This means that the resultant product basically produces a sparkling effect, as seen with Diamonds, which have a very high RI. Through exploiting crystal cutting techniques, manufacturers can create some stunning effects.
It lowers the working temperature
This means that its production requires less time and energy, with its working temperature only around 800°C, compared to some glasses which have a working temp of over 900°C.
It lowers the glass’s viscosity
This means that it’s easier to mould, allowing it to be used easily for enamelling.
Now, this all sounds very scientific, and I suppose it is, but the interesting fact is that lead oxide was first used in the production of glass way in Mesopotamia. A small fragment of the glass, dyed blue, was found in the region of Nippur (an area now in modern day Iraq), which has been dated to 1400BC. There is also mention of a recipe for a lead glaze, which itself dates back to the Babylonian era or around 1700BC. However, before we get too excited about our genius ancestors, there is debate as to whether they were consciously using lead as a primary fluxing agent.
Much later we see that iron oxide is used in glass production in Han era China. Slow on the uptake, it is believed that the Chinese were taught of the uses of iron oxide as colouring agent by Middle Eastern glassworkers travelling along the Silk Road. As time went by, glass was manufactured more and more as the industry developed. Across Europe, it eventually came to be known as vitri Ijudaici or ‘Jewish Glass’ during the Roman period, due to the writings of Heraclius and owing to the fact that it was mostly made by Jewish workers.
Then latterly, in Medieval Europe, we see lead glass being employed extensively as the base for many coloured glasses such as stained glass windows amongst others, where the colouring of the glass was as an imitation of precious stones. It was no longer just a Jewish industry and, although there was still a rich Jewish heritage to the process, its manufacture was relocated to Venice, from which it gained high esteem throughout Europe.
Eventually, a young English businessman called George Ravencroft brought glass manufacture to the industrial scale in the 1600’s. It flourished in England until a harsh taxation was put upon the selling weight of glass, restricting profits and lining the pockets of the British government. This then led to lead glass manufacturers creating far smaller, more intricate products, often with hollow stems – products we know nowadays as Excise Glasses. These techniques were developed throughout the 20th century and into the 21st when Fredrick C. Carder and Thomas G. Hawkes of Corning began manufacturing Steuben Glass. To this day, but to a lesser degree, lead glass is still used in decorative and industrial applications.
Image by justinwkern