In the late 15th century the Murano craftsmen sought new technical solutions: typical of this period is ‘chalcedony’ glass (so called because of its likeness to the hardstone of that name). This glass is dull red in colour with polychrome venation obtained by adding metallic compounds to the vitreous mixture. Numerous contemporary paintings provide ample pictorial evidence of the presence and use of such glass objects in Venice and elsewhere during this period. Since glass is a material that does not change significantly with age (which means the date of manufacture cannot be deduced from the state of the material) and the techniques and even the designs used by the Murano glassmakers remained almost unchanged over the centuries, such iconographic references assist with the identification and dating of glass articles.