This is very similar to rust blueing and both of these methods could come under this heading.This type of finish will work on 1075 and pure nickel but not on 1075 and L6. To use this method, finish your damascus to the desired grit. Take a plastic container with a lid (sealable) big enough to take your damascus plus a bit of spare room. With your damascus in the container also place 2 small containers (plastic is fine) with 5 drops of nitric acid in one and 5 drops of hydrochloric acid in the other. Close the lid on the main container but leave the acid containers open. The fumes from both the acids combine to create a rusting effect on your article in there with them. The rest of the process is the same as rust blueing. When a coating of rust has built up - the best indication I can give you timewise is 1/2 a day to a day (maybe). Open the container - don't shove your nose in it, the fumes are quite strong - take out the item and boil it for 15 minutes. Then card it with steelwool, clean under cold running water, dry and put back in the container for another rust session. The temperature and humidity for rusting are important. At one time I put a damp cloth in the container as well to create a bit more humidity. It is a fairly judgemental process, a bit of a 'suck it and see' situation. If possible, it's probably worth trying on a scrap of material first. The whole deal from start to finish might take about 4 days but this depends on how often you are boiling and carding.