The San Giovanni altar from Baptistery of Florence: the goldsmith's workshop through the fourteenth and fifteenth centuries

During restoration work supervised by the Opificio delle Pietre Dure (OPD), the San Giovanni altar, located in the Opera del Duomo Museum in Florence, was extensively investigated. The altar is made of 200 kg of silver and more than 1000 enamelled plaques. It was manufactured in three different periods, mainly centred in the Renaissance. The work described here focused on the scientific investigation of both enamels and metal alloys. The composition of the base glass and colourants used for "basse taille" enamels of the three periods of production was examined. The results show a high content of sodium and potassium and a low content of calcium, magnesium and aluminium in the base glass. The blue enamels have high cobalt content, with different trace elements, related to impurities from the metal ores used in each production period. Analyses of the alloys show differences in their copper content that relate to the working technique applied to the metal components: cast statues or cold-worked panels. A finer alloy was found in the enamelled plaques, which was needed to fulfil the requirements of enamelwork. Trace elements in the alloy, such as lead, bismuth, gold and mercury, were found to differ between the various workshops and production periods.