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Although the tin-bronze artisans' tools found at Machu Picchu appear to have been cast to their final shape, examination of their microstructures shows that they were forged and annealed. As found, the tools were broken or cracked due to poor ductility of the bronze. Rolling and annealing of samples of bronze formulated with sulphur and iron additions to replicate the metal used at Machu Picchu show that the minimum amount of deformation needed to induce the observed recrystallization is a 12% reduction in thickness. The sulphur and iron impurities retard nucleation of new grains during annealing, but do not inhibit subsequent grain growth. Tensile tests show that while up to 10% porosity in cast bronze has no effect on hardness or initial strain-hardening rate, it reduces the tensile strength and ductility enough to embrittle the metal. Hardness is an inadequate indicator of the strength properties of cast bronze intended for use in tools or weapons. The results show that when the metalsmiths at Machu Picchu started making bronze tools for use by stone and woodworkers, they had not mastered the art of making metal with adequate strength properties for this service. The forging and annealing procedure they used did not enhance the mechanical properties of their tools, and may reflect an inherited metallurgical tradition. |
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