Blackening Steel Jewelry

A. Don't quite catch you on the "even from the inside" line. If you mean turn steel black throughout its entire thickness...its not possible. For surface try.. blueing salts, paint, powder coat, or a titanium PVD coating (which won't bother allergies and lasts much longer than most methods) PVD you have to ship out to have done, no way to do it at home. If you just have a few pieces you need done or the profit margin is good you can use niobium in place of steel and heat color it with a torch...gets a great grey-ish black (sort of looks like dark hematite)this coating lasts quite some time and doesn't fade with UV rays.

A. Brown to black for steel:
50 gm sulfuric acid
200 gm potassium bichromate
1 lit water,temp. 30 °C