I came across an interesting article from the Detroit Free Press while browsing t'internet about a US Civil War explosive shell being found in a Michigan recycling plant:
And this got me thinking, a single cannonball doesn't consist of much iron, but if you knew of a battlefield where several thousand solid shot had been fired into a concentrated area, would it be economically worthwhile to dig up spent cannonballs?
Was there an industry for recycling spent ammunition during the 16th Century and onwards? And if so, when did that industry stop being sustainable?
More can always be said on the matter, so if anyone would like to focus on particular shot-salvage operations, please don't hesitate to type up! For the meantime, OP, even this question has arisen before, over here in this thread, with u/The_Alaskan providing an overview of shot recovery.