The study concerns the chromatic alteration of mosaic glass tesserae in the São João Chapel, São Roque Church in Lisbon. This precious chapel was commissioned by the King of Portugal, João V, in 1742. The polychrome marbles, stucco bas-reliefs, precious silver sculptures and three mosaic altarpieces were sent by ship to Lisbon.

Alessio Mattioli, the glassmaker who produced the glass tesserae for the mosaics, was the famous inventor of new compositions for difficult colours in opaque glass such as red, orange, and the variety of flesh colours. In fact Mattioli succeeded in imitating the technique of painting to produce altarpieces that were “eternal”, or, at least, more durable in appearance than paintings. Our research is based on archive documents discovered in St. Peter's Fabrica in Rome concerning the decoration of the São João Chapel in the São Roque Church in Lisbon and the compositions of Mattioli's glass. The chromatic alterations producing conspicuous spots and shades of grey, brown, and white are similar to the alterations in many mosaic altarpieces in the Basilica di San Pietro in Rome, dating from the same period.

The findings of the chemical analyses on glass and weathering products, are the first attempts to understand the weathering mechanism and to prepare a restoration and conservation plan.