Now that you have acid etched it, it has the look of cast iron, so my guess would be that it was made out of ductile iron, which is iron and one of copper, nickel, or chromium (15-30%), with a small amount of usually magnesium.
Nickel and chromium acetate's are brightly colored, copper acetate is very dark blue, and copper oxide is black. None of these salts are terribly stable, and you have lots of other things in the soup...
Organic nickel and chromium salts are often carcinogenic btw...