Thermal vs Chemical Black Oxide

A. I think I know what your question is, however, the terms are perhaps different. There are 3 or 4 ways of getting black oxides on steel. Most common is the "Dulite" type process using hot, 280 °F, boiling alkaline bath as called out by MIL-DTL-13924 [on DLA], Class 1. Second is doing likewise except the bath is a molten salt bath at several hundred degrees, same spec but Class 3. Third is a heat treating process wherein the parts are heated to several hundred degrees and then exposed to steam. This oxide is less uniform in appearance and isn't widely used nowadays. Fourth are room temperature blackening processes that contain selenium, a poison, and produce black compounds that actually aren't even oxides but are often passed off as such. So speaking thermally from hottest to coldest: molten salt bath oxide, steam oxide, conventional black oxide, room temperature non-oxide blackening.

Hope this doesn't appear too complicated but now you know!