Scientific analysis of seventh-century glass fragments from the Crypta Balbi in Rome

Inductively coupled plasma emission spectroscopy, reflectance spectroscopy and X-ray diffraction were used to study seventh-century AD glass fragments from the Crypta Balbi in Rome. All the samples were found to be silica-soda-lime glasses. Iron determines the colour of blue-green, green and yellow-green transparent glasses; chemical composition suggests deliberate addition of iron and/or manganese in about half the samples. Copper was found as the main colourant in red, pale blue and blue-green opaque fragments; elemental copper acts as an opacifier in red glass, and calcium antimonate in white, pale blue and blue-green glasses. Detection of antimony in transparent fragments suggests recycling of opaque mosaic tesserae.