Many ways to do this, but here is the process that yielded the best results for me. Other methods were also tried with varying success before this one, including using Palmolive, placing in bags with eggs, just using vinegar or salt + vinegar, etc.
Process:
- Wash the watch with soap and water, then dry. Make sure not to touch / get oils from your fingers on the case.
- Submerge 1hr+ in vinegar-salt mixture (just covering, not deeply submerged). The mix should be as much salt as you can dissolve into the vinegar, and be thoroughly mixed.
- Allow to air dry. You will see lots of patina forming but it will look very uneven at this stage. Rinse with water. This yields the base patina.
Then to darken use sulfur: - Place in airtight container next to cup of red wine / vinegar / salt mix (mostly wine, just a few ml of vinegar) and a hard-boiled egg yolk. The watch should be placed so it does not touch liquid (I used an upside-down sauce cup that was just as wide as the watch bottom to prevent pooling of liquid under the watch)
- Leave for 4-6 hours, removing 2-3x to blot dry (droplets will form on the case and discolor if left there). Do not rub as this will rub off the patina at this stage and give you uneven coloration.
- Remove watch, rinse with water and allow to air dry, then lightly brush with soapy water and rinse, blot dry. It shoukd look something like this:
- Let sit overnight and then rub with butcher block oil and dry
Also of note, the same process works well on bronze (did on my Nethuns diver) - brass comes out with more red hues, bronze more buttery yellows and browns. Here the comparison: