Here is a cheap way to patina steel without having to spend money on expensive patinas and without having to wait for something to rust.
you pee on it.
Step 1: Clean Steel With Acetone
First you clean the metal with acetone to remove any oils. Oils protect the steel and will keep the process from working.
Step 2: Roughen Up the Surface to Get Rid of the Mill Scale
exposing the raw steel and removing mill scale will also aid in the process
Step 3: Using Gloves, Urinate Into a Disposable Cup
Clean your hands afterwards
Step 4: Fill Plastic Bag With Urine and Place Steel Inside
Remember that if the steel is fully submerged and does not contact oxygen, then it will not oxidize. A technique could be to use a pan filled with gravel, urinating into the pan, then covering with a plastic bag to seal in the moisture but keeping the steel out of the urine with the gravel.
Step 5: Wait a Day or Two and Voila!
wash the steel afterward.
warning: it will smell fairly rancid... like 5th and Market in San Francisco on a warm muggy day...