From the Machinery's Handbook 12th edition.
Clean the grease off well then wash the piece in a bath of one part carbonate of soda or potash to ten parts water. after boiling wash in clean water. Do not touch the surfaces with fingers, but handle by wire or tongs.
For rich gold color boil in a solution composed of 2 parts by weight of saltpeter, 1part common salt, 1part alum, 24 parts water and 1 part hydrochloric acid. wash and dry.