I thought the baking process would remove everything but rust. Try giving the machine shop a call and ask them what the oven bake DOES remove, or ask if they have a cold tank.

my machine shop has both hot tank and cold tank. I don't know if the cleaning solvent is the same in both, and one is just heated up or if they use different solutions, but the cold tank was about half the cost of a hot tank dip, but it came out looking like a fresh cast block....so I don't really know WHAT the need for the more expensive "hot tank" would be!