Cromalin proofing was something I used to do, daily, as a pre-press lithographer.
After we had produced the 4-colour negatives for a print job, we would coat a piece of gloss white card with a clear photosensitive layer, using a heat roller.
Then we would expose the yellow negative under the same halogen lights that we would use for exposing plates or for compositing film, and in the same vacuum beds.
The clear layer was actually two layers, and after exposure, we would peel off the top layer to reveal a surface that was sticky where the light had hit it.
We then took a high density yellow pigment dust and rubbed it over the sticky areas with a soft chamois cloth. It was imperative to do this in a timely manner, and to make sure that the dust took to all the exposed areas in equal measure. Not to mention keeping other bits of crap from the factory off the surface - they could be very difficult to remove! Typically I would use a surgical scalpel to pry off the interloper, while holding a length of sticky tape in my left hand to catch it. If you dinged the surface with the scalpel, the whole job could be compromised, meaning you start again. Which was not cheap. I once ruined an A0 poster for a major department store with one tiny nick, which made it look as though the young boy in the poster had cut himself shaving!
Then we would repeat the entire process for magenta, then cyan, then black. The whole thing typically took about about 45min/one hour for a single board.
It was as toxic as hell, but the colours were fabulous. If the black had been a little denser, I'd say it was almost the equal of dye-transfer for sheer effect.
I never saw a client hold up a proof-board and not say "wow". Inkjet does not compare, no way no how.
Just as very few painters, these days, work with pure pigment, I think that basically no-one in pre-press today works with pigment proofing as we did back in the eighties and nineties, before computers. A shame, because working on stunningly beautiful things, with beautiful materials, really makes you love your job.
I'm glad I got out of pre-press in '92. Despite the mess and the chaos, it was an industrial process that delivered a lot of satisfaction in sheer beauty and accomplishment. I'm not sure I'd feel that, looking through a computer screen all day, considering how hands-on we were back then.