Originally Posted by
Smiles
From the book Home Machinist's Handbook: "Black Finish for Steel Parts: The following method can be used to provide a jet black, rust-inhibiting finish on any steel part. This finish looks especially nice on nuts, bolts, and screws used in small mechanical assemblies. To obtain the finish, simply coat the part with motor oil and place it in an oven or a hot plate. Heat it to approximately 350ºF. The oil will bake into the part in about 10 minutes, leaving a hard, durable black finish."