Glass was often the preferred material for storing expensive oils, perfumes, and medicines because it was not porous. By the 1st century AD, the technique of glass-blowing had revolutionised the art of glass-making, allowing for the production of small medicine, incense, and perfume containers in new forms. These small glass bottles, known as unguentaria, are found frequently at Hellenistic and Roman sites, especially in cemeteries, and the liquids which filled them would have been gathered from all corners of the expansive Roman Empire. The natural colour of Roman glass was tinted an aqua colour. Through the use of additional chemicals and minerals, Roman craftsmen were able to alter the colour of glass. This clear glass was created through the addition of antimony or manganese oxide, which oxidised the natural aqua Iron II oxide to Iron III oxide. The new iron III oxide, although slightly yellow in colouring, was much weaker and thus gives the illusion the glass is colourless. The iridescence on ancient Roman glass was unintentional, and was caused by weathering on its surface. The extent to which a glass object weathers depends mainly on the burial conditions; the humidity, heat, and type of soil in which the glass was buried all affect its preservation.
To find out more about Roman glass please see our relevant blog post: Ancient Roman Glass and Collecting Roman Glass.
