A: This will be easier to do if the bronze piece is annealed first. While steel is annealed by heating to red and being allowed to cool slowly, metals which crystalize in the “Face-centered Cubic” system—which includes the copper alloys as well as gold and silver—will reharden if they are treated this way. The crystals, given time, will form a more organized structure, without the molecular “vacancies” which make the metal malleable. In fact, one can take advantage of this tendency to “heat-harden” non-ferrous metals, holding sterling silver, for instance, at 280° C (536° F) for 2½ hours.

To soften metals of this type, they should be brought to a medium red color, then quenched in water to cool quickly. I would be careful not to overheat it though—some bronzes will suddenly go from solid to falling-apart when hot. See Tim McCreight’s excellent handbook The Complete Metalsmith (1991) Davis Publications, Worcester Mass ISBN 0-87192-240-1, for more details on annealing and hardening, as well as other aspects of the hand metalworker’s craft.