I have maxed Furnace, its description says it gives me 25% fire and physical penetration but it's giving me 29%, I have nothing that alters those values in any way.
Btw, the back iron steamgun paperdoll gfx has the wrong name for every race, I think. It's "leftt_steamgun_back_iron.png" now, with two t's. It should be "left_steamgun_back_iron.png", I've seen the code.