Most are a mixture of Nitric Acid, Hydrochloric Acid and Ferric Chloride a couple like Laurel Mountain Forge contain Copper Sulfate and at least one that I know of contains Mercuric Chloride. I dont like playing with Mercuric Chloride.

Personally I like the stuff produced by Mark Lee

Bottom line is that they all cause rusting, and the rust that the produce can all be converted to black oxide by steaming.