Te - it's a long article in Judy Seigal's Post Factory Journal #3, so I'll just outline it. There are two methods - (1) toning a fresh unfixed print which still contains residual silver and (2) toning a fixed print by bleaching it back to silver halide. The first silver-plates the whites, the second silver plates the darks. both to a greater or lesser degree depending on how long they are in the toning bath and how much a print has been bleached.
A copper bleach is used, such as 10g of salt (kosher or pure: ie, uniodized) + 10g copper sulfate in 300ml of water, dilute this 1:1 with tap water for use. Other bleaches can be used with varying effects. Acid (eg 5ml of hydrochloric or 10ml glacial acetic) may also be added to the undiluted bleach mix.
There are a few different toning solutions. One is 1g hydroxylamine hydrochloride, 2g hyDrazine sulfate, 12g sodium hydroxide, 50ml ammonium hydroxide (25% strength) and 150ml distilled water. NB: note the D in hyDrazine - I quoted another source before that spelled it wrong). The ingredients are added to the water in the order given, stirring until each is dissolved. The ammonium generates a strong, unpleasant smell - use a fume hood if you can. Toner keeps indefinitely and is diluted 1:10 with tap water for use.
For the first method you expose and dev the print, rinse well, then put it in the toner until plated - lights off. In the second method, a finished fixed print is bleached with the copper toner, rinsed well, then put in the plating mixture - lights can be on. The author didn't re-fix the prints. some have lasted 20 years, some have had subtle shifts, some have faded.
I haven't used the above methods, but have used a product called Halo-Chrome which is said to contain the same chemicals. It can be used for either of the two methods outlined above. The effects can be very mundane or very exciting. The silver coating seems to last fine (I have some that are 5 years old or more) but it is very fragile and prone to marking, veiling, staining and scratching when wet.
Regards,
Neil.