Howard Clark of Morgan Valley Forge answers the question of how to oxidize a new-production tsuba to get the antique look and feel (taken from the public Sword Forum at Bugei):
Howard Clark : "The answer is to rust it on purpose, card the rust off with steel wool, then rust it again, card the rust of, rust it again, card the rust off, rust it again, card the rust off, rust it again, card the rust off. Then, put it in water that is clean and at a rolling boil and boil it for a few minutes (after the rust brown is sufficiently dark to suit you) and it will change it to a more black oxide that is quite durable. If it is then oiled, it will not ever rust again, and it will look quite old, and even look rather like a Japanese antique finish, because that is what it is. All fittings and metal item (except for blades) in fuedal Japan were patinated (read that corroded on puropose). That's because the climate is quite humid, and the temperature fluctuates rather significantly in some parts, and everything metal will corrode anyway. Their answer (and it is a good one) was to pre-rust everything, patinate the non-ferrous materials, and voila, no more corrosion problems, plus a lot of cool colors and shades of colors are made possible, with the bonus of durable finish as well."
A method that works for me:
Tsuba before immersion | Tsuba after 24 hours | Finished Tsuba | finished Tsuba; color variation
Preparation: Fill a platic bucket full of tapwater, add several pieces of iron or simple carbon steel about 1 inch across to the bucket and let it sit in a dark spot until the pieces of steel rust away. Keep adding steel pieces as necessary until half the water has evaporated from your bucket, you have a visible layer of dense rust at least 1/4 inch thick on the bottom of the bucket and the water has an overall orange rust color.
Use:Pieces of steel laid in the bottom of this bucket will rapidly oxidize a dark black. Remove them every day, wire brush any red surface rust off the pieces and place them back in the bucket with the other face down. After a couple of days you will get a nice, hard black oxide layer. Dry the pieces in a warm oven or put them outside on a hot summer day for a while, then oil them and rub gently with a soft cloth. This works well for me and I keep the bucket around all the time because I oxidize a lot of pieces this way.
A quick note: I left a group of tsubas out to dry in the sun and we got a rain shower in Texas in the middle of August (go figure). Needless to say, the tsubas ended up with a nice coat of red rust. Instead of starting over I tried Howard's boiling method on five of them; it worked like a charm. The sixth tsuba got a good rubdown and a nice thick coat of choji oil. Believe it or not folks, I ended up with that rich, deep brown color you find on some antiques. You know the color, it looks like really dark plum wine in a glass. Cool, huh?