An investigation of the mechanical and physical properties of copper-silver alloys and the use of these alloys in Pre-Columbian America

Abstract

In both the Andean zone of South America and in Mesoamerica, copper-silver alloys were important in the production of thin, silver-colored sheet metal artifacts. This thesis examines the mechanical and physical properties of the copper-silver alloy system that are important to understanding why copper-silver alloys became central to the metallurgies that developed among prehistoric societies of the Andean zone and Western Mexico. These properties include their range of malleability, the microstructures behind their toughness, and the recrystallization and annealing behaviors that led to their development of silver-enriched surfaces. To determine these properties, a series of cold rolling, cold hammering, and annealing experiments were performed on five Cu-Ag alloys and pure copper. Results of the cold rolling and cold hammering experiments reported here indicate that over the copper-silver alloy compositional range studied, the alloys can be cold rolled without annealing to over 90% reduction in thickness. Similar reductions in thickness were also achieved in two alloys (95 wt% Cu - 5 wt% Ag and 30 wt% Cu - 70 wt% Ag) by cold hammering. The rate of work hardening and the Vickers Hardness Number, as functions of the percent reduction in thickness, are similar for alloy compositions containing between 30 wt% and 80 wt% Cu. This suggests that ancient metalsmiths likely annealed the copper-silver alloy artifacts intentionally to produce the desired silver surface color rather than for any improvement in malleability. The silver surface colors were important for their cultural associations. The recrystallization temperature for the copper-silver alloys tested (70 wt% Cu - 30 wt% Ag and 30 wt% Cu - 70 wt% Ag) is determined to be 500°C given a 30 minute anneal time.