Mellow yellow: an experiment in amber

Amber natron glasses were produced from at least the Hellenistic period and continued to be produced into the early second century CE. However, as with other strong colours used for Roman vessel production, this colour gradually declined in popularity as colourless and blue-green glass came to dominate. Whilst the colouring mechanisms for blue-green glasses are relatively well understood, the cause of the distinctive amber colour is more complex and can be attributed to the iron sulphur chromophore. This paper demonstrates, using analytical data and model glasses, that the amber colour develops during primary production, and that the sulphate-rich natron is key. The analytical data show that most natron amber glass was probably produced in the Levant alongside the more common blue-green glasses, however, its composition is different. Whilst many glass colours were made in a secondary stage, by adding colourants and opacifiers to a blue-green or colourless glass base, amber glass was not made this way since it required a slightly different set of raw materials and melting technologies. These findings suggest that the production of the glass required specialist knowledge, and particularly skilled furnace operation, in order to produce repeatable results. Skilled specialists would also be required to work amber glass whilst retaining the same clear amber hue, especially for complex wares, such as mosaic vessels, where the glass would be reheated more than once.