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Flashed glass represented the only way to achieve a dense ruby colour and, at the same time, respect the transparency requirement imposed by the very nature of stained glass itself. Owing to the huge absorption of copper nanoparticles, however, the manufacture of transparent red glass proved to be very difficult and just a few workshops were able to find out a suitable procedure until the end of the eighteenth century. Metal copper was extensively used in the production of red mosaic tesserae and ruby stained glass during Roman and Medieval periods. The quest for bright, but transparent, red and yellow coloured glass represents one of the biggest efforts in stained glass production since ancient times. This article is part of the themed issue ‘Raman spectroscopy in art and archaeology’. Raman investigations allowed identifying a different and more pronounced disorder characterizing the oldest glasses, also verified by TEM observations, suggesting a different manufacture. A confirmation of the above-mentioned conclusion is searched by means of transmission electron microscopy (TEM) and local elemental analysis. Moreover, a tentative evaluation of size distribution and crystallinity of the nanoparticles has been performed considering those parameters that are mainly influenced by the disorder of the system, such as Raman band width, surface phonons and the ratio between second and first order band intensities. Zinc substitution is also detected by the shifting of the Raman peak position. As non-invasive conditions are now mandatory when considering objects belonging to cultural heritage, Raman spectroscopy is used to measure the (average) elemental composition of the nanoparticles. Six historical stained glass pieces produced between the late 1920s and modern days have been investigated in order to delineate the average size and the elemental composition of the nanocrystals. The use of cadmium chalchogenide nanoprecipitates to obtain brightly coloured glasses enormously expanded by the beginning of the twentieth century, when the production of cadmium-based pigments was already well established.