Tuesday Times Tables: Glorious Roman Glass

Have you ever wondered how glass was made in the Roman period, or what the components of Roman glass are?

Roman double-handled bulbous vase

Glass has three principal constituents: a former, a flux and a stabiliser. In antiquity, silica, in form of sand, acted as the former; soda was used as flux and calcium in form of lime was used as stabiliser. 

The Roman glassworker could vary the colour of the glass by adding specific metal oxides. Different quantities of these colouring agents, that have different chemical compositions, and manipulation of furnace temperatures produced a very wide range of translucent and opaque colours. 

The addition of copper produced a range of blues, greens and red; cobalt was for dark blue; iron was used for brown and black glass; manganese for yellowish or purple glass; and for the colourless or opaque white glass was used antimony.

Roman melon bead made from glass paste

The ingredients were initially heated together to a temperature of about 600 Celsius degrees to remove the impurities and this process, called “fritting”, produced the material known as frit. The best of the frit was then broken up and heated to 1100 Celsius degrees or more to form glass.  

Roman jar with thick zig-zag pattern from neck to shoulders

The three principal processes used to manufacture Roman glass vessels were casting, mould-blowing and free-blowing.

Tall slightly twisted Roman jug with strap handle

The casting was in widespread use in the early years of Roman Empire, but became rare towards the end of first century AD and occurred only occasionally thereafter. In the casting process the glass objects were cast by directing molten glass into a mould where it solidified. This technique created decorative effects by joining together prefabricated component parts, like handles, feet and rims. The most common cast vessel found in Roman Britain is the pillar-moulded bowl.

Modern glassmakers blowing glass

By the time of the Roman invasion of Britain in the reign of the Emperor Claudius in 43 AD, most Roman glass was produced by the blowing method. The invention of glass-blowing completely transformed the glass industry: it was now possible to produce a wide variety of forms more cheaply. 

It consists of an inflation of a gather (a mass of melted glass) onto the end of an iron rod. The glass could be blown in a mould or with the free-blowing method, but in this last case the glassworker had to shape the vessel by rolling the iron rod across a flat stone or a metal surface and by manipulation with tools. When the vessel was ready it was removed from the iron rod and then handles and other parts were added.

Michela Amato

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