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Bronze
Bronze used for the casting of sculpture in the 19th and 20th centuries:
Bronze is essentially copper with combined with other metals to make it stronger and more workable. Ancient bronze was copper with tin or lead. It melted at low temperatures and was fairly soft and generally it was not weld able, what you poured is largely what you got though it could be ground and hammered. Because bronze is mostly copper it conducts heat extremely well making it hard to weld because the whole piece gets so hot it almost melts before the specific area being welded reaches melting temperature. These older low temperature bronzes have had another even more serious problem in modern times, they dissolve in acid rain, acid rain eats through old bronze like it was a bar of soap in the shower. There is a monumental size sculpture in front of the capitol building in Washing DC; it is so dissolved last time I saw it, it looked more like a melting ice sculpture than bronze. Another example of old bronze being washed away by acid rain is the thinker in garden in front of the Rodin Museum in Paris.
Most foundries that cast fine art bronze cast EVEDURE BRONZE;. This is a high temperature bronze, it is cast at a temperature of over 250 F higher than bronzes used in the 19th and early 20th centuries. What makes it so unusual is that it 4% percent silicon (sand) this gives it some unique properties, it does not conduct heat as well and can be more easily welded and it is extremely durable to acid rain, I have yet to see a piece of art produced with EVEDURE BRONZE; to show the pitting and wearing of older low temperature bronze formulations.