Investigation of coloured lead glass glitter from an early eighteenth century material collection, Cambridge, by electron microscopy/energy dispersive X-ray analysis

Coloured glass glitter is made from thin glass flakes and used for decoration purposes. Published analytical data mainly originates from restoration sites in different countries and is still fragmentary. Samples are often contaminated or corroded, which makes it difficult to evaluate the results, particularly when these were obtained by near-surface techniques. For this investigation, six types of coloured glass glitter from an eighteenth century material collection from Cambridge, England were investigated by optical microscopy and electron microscopy/energy dispersive X-ray analysis in order to obtain a wide range of data, which can be used to examine the production techniques of eighteenth century coloured lead glass glitter. They have been stored in a wooden cabinet for 300 years and were sampled directly from the original small paper boxes. Each sample contained glass flakes of different sizes and thicknesses and even different colours. All flakes were made of lead glass with lead contents in the range between 18 to over 70 wt%. One group showed mixed alkali compositions, probably from potash, lime and nitre, the other contained sodium, potassium and calcium oxides only in trace, if at all. Colouring elements were cobalt, copper, iron, manganese and gold resulting in blue, green, yellow and violet. Although the colouring elements resemble mostly contemporary recipes as known from Neri and his translators, the glass composition is clearly different and differs also from other lead glass compositions from that time. This is a first indication of the production technology of eighteenth century glass glitter. A wide range of analytical data obtained from early eighteenth century glass flakes is presented here and is discussed with regard to the composition of lead glasses, colouring elements, and manufacture details. Lead containing glass glitter is often found in restoration sites, but their manufacture is still waiting to be thoroughly investigated. Almost nothing is known about its historical manufacture. The study provides a wide range of data, which allow to further increase our knowledge on composition and manufacture of these often used decoration materials from the eighteenth century.