Here's something from the Michael Caine "not a lot of people know that" school of photography. At least, I don't think a lot of people will. Simply stated, it is:
- If a black and white, fibre-based print can't be thoroughly washed then it's better if you leave it to dry soaked in fixer.*
Kind of goes against what I'd imagine most of us would think, doesn't it? For a print to be thought of as archival, it must have almost all the fixer washed from it. But if the fixing bath has been over-used, it may well contain a relatively high concentration of a soluble ion (monoargentodithiosulphate) that is heavier than the normal thiosulphate ion. Being larger and heavier, these ions diffuse less quickly out of the emulsion.
Under these circumstances, washing may initially reduce the concentration of thiosulphate ions more rapidly than the heavier ions. If that happens, insoluble ions can form and, since these can't be removed by washing, they are bound to remain in the emulsion. The danger is that they may decompose and give off other sulphur compounds which attack and discolour the silver of the image.
Also, if any of the heavier ions are left in the emulsion due to insufficient washing, free silver ions are subsequently formed and these may react with atmospheric impurities in various ways to cause stains and discolouration of the image.
The upshot, as I said near the start, is that an image that is not washed at all and remains saturated with thiosulphate is more permanent than one that is completely washed because the excess of thiosulphate ions prevents the liberation of silver ions. I've already used the phrase "who'da thunk it" in the recent past in relation to Ralph Gibson and now here were go again. Who'da thunk that!
* The information for this comes from a book called Basic Photo Science (1977 edition) so if you'd like to dispute it, you'd have to take it up with one or both of the authors, H.J. Walls and G.G. Attridge. The book is an interesting read but it's not exactly a page turner. :)