In my experience, for gelatine to increase the "luminosity" of a print,
the pigment has to be suspended *in* it (as in Drtikol's carbon prints)
not *under* it as you suggest here.
>
> Here's another one: To a finished print, you could add a coat of
> dichromated gelatin (or gum), *sans* pigment and once dry, expose the print
> to strong light through the back. Develop the unexposed gelatin in hot
> water (40C) or the gum in room temperature water. Areas of the print that
> were not protected by the dense pigment will retain the colloid for a
> different effect.
>
This also would probably be counterproductive. Exposing through the back
would more or less grease the paper and highlights, while doing nothing
for the parts with the pigment in them. The exposures are also
VERRY long: a half hour through a paper as light as typing paper just 1
1/2 inches from the bulbs, an hour for anything to happen on a thicker
paper. As noted this does nothing for the pigment areas.
If you do want to add something to the paper only,not the whole print,
rather than struggling through the back, which is fine in theory but
problematic in practice (repeating the paper texture as often as not), I'd
suggest exposing under a positive.
A gelatine coat over the *pigment areas* might possibly add something
beyond a greasy look (hasn't for me, but others might do better). However,
gum arabic in dichromate exposed then "developed" is shiny enough when
"fresh" but once exposed and soaked it dulls out -- unless it's over quite
a few previous layers, which it wouldn't be if you're just doing the
highlights. I've also found the general effect unpleasant more often than
not. I don't make a generalization since every size/paper/emulsion
combination has its own character, just note that so far I've found it
problematic.
Judy