If I remember correctly, recent test at IPI Rochester have shown that a
polysulfide toning is the best one can do for permanence. Selenium
wasn't very good in their tests. They sell or have others sell "IPI
Silverlock", which shall be used for film, but can also be used for
papers. I haven't used it yet so I can't tell how big the colour shift
(towards brown) will be with paper.
For a long time AGFA recommended their "VIRADON" toner for enhanced
permanence, which, I think, is a thiocarbamide toner.
What old P.O.P. prints tell us, it seems as if gold toning plus
subsequent platinum toning gives perfectly neutral black image tone of
great permanence. Most unfaded portraits with neutral, deep black image
tone in albums from the turn of the century probably have been produced
this way, i. e. silver chloride gelatin (or collodion) printing-out
paper + gold + platinum. Most manuals from 1890 to 1940... will provide
formulas for these toners.
To test the effectiveness of your toning, you may try the bleaching bath
described in an ANSI or ISO test (ISO 12206:1995; Photography - Methods
for the evaluation of the effectiveness of chemical conversion of silver
images against oxidation) for the permanence of silver images. I don't
have it at hand here, so I can't give exact details. They use an
acidified dichromate bleach and measure the density (around 1.0 logD, I
guess) before and after the bleaching. If more than 60% (I hope I
remember correctly?) of the initial density are still there afterwards,
the toning should be sufficient for long term storage.
Klaus Pollmeier