A Metallurgical Investigation of the formation of Silver Beads during Cupellation of a 925 0/00 Silver Alloy

Abstract

In the late 15th century and fi rst half of 16th century, the moneyer in the Archbishop’s Palace in Nidaros (present-day Trondheim) struck coins for the last three Norwegian archbishops of Nidaros in the period AD 1475–1537. In his profession he would have to master how to make bullion with the correct silver content for the coins. Th is raises the question of to what degree of accuracy could be achieved in the measurement of the silver content with the use of bone-ash cupels at that time. To address this question the authors made replica bone-ash cupels from scratch and performed cupellation experiments on a silver alloy with known composition. Both the annealing time at 900°C and the cooling rate were varied. Th e results of the experiments were used to create a model of the metallurgical processes that took place during annealing and cooling. One important result was that the silver bead, which resulted from the cupellation process, was not pure silver once the ‘fl ash’ had been observed. However, the authors suggest that with prolonged annealing and slow cooling, the bead may become close to having a content of pure silver, depending upon type of impurities in the silver.