A quick search of the board didn't show anything about this exploit, but it seems like it would be fairly easy to discover so it might already be known.
If your put your tribal arms station or whatever it is called near your meat/bone industry, and tell it to make rock weapons, your Orcs will make clubs and mauls out of Black Smoke, Rotting Smell, Bacteria, Ect. This alone isn't too bad, as such weapons seem too light to be of much use (although the are nice trade goods, and would likely be helpful for the justice system), but I discovered the true exploit after buying a bunch of weapons of the caravan, and telling my molten pit to melt them down on repeat.
Several minutes later, I check in on the pit, and notice the Orc there is working with a Black Smoke Maul, which he successfully turns into a bar of Rusty Iron.
I had built the outdoor section of my fort on top of a volcano, and the surrounding lands, so my metal and butchery/refuse industries were right next to each other. I had made, completely on accident, a very efficient metal factory; Things were cremated into ash, which becomes Ironbone/Bloodsteel as usual, and black smoke, which becomes weapons and then rusty iron (and other potentially useful byproducts) via the molten pit.