I have gone over exclusively to the following formula which came from a variety of sources but I traced it back to Angier:
Ferrous sulphate 1.6 gms
Sol. ferric nitrate 28% 5.7 gms
Sol. ferric chloride 29% 9.0 gms
Made up to 100ml with distilled water
It seems to work pretty well on all types of twist and damascus, is quite gentle and keeps well.
Humidity in my workshop is low except in high summer so I use a tented arrangement with a tray of water laid underneath. I allow 24 hours rusting between passes but if the action is very slow (low humidity and/or temp) I find a second coat applied 1 hour after the first helps things along a great deal.
Most barrels take about 15 to 20 passes to get a good intense brown colour but one has to be careful not to continue on until the contrast becomes muddy, an easy thing to do.
Washing off with a very dilute solution of Sulphuric acid (10-12 drops 7M concentration in 25ml) can recover the situation but you must neutralize this with weak caustic soda solution and maybe apply a couple more passes afterwards to re-intensify the colour.
The browning solution does not seem to after rust at all but to play safe, I soak the finished barrels in 1oz:12lts caustic soda solution at 50-60 deg C for one hour before scolding, drying and oiling.
