Re: My own PVA glue sizing experience became: dark reaction in gumemulsion
as i've already said, about a year ago when i was first beginning printing color gums, i printed with a way more gum-heavy emulsion (3gum + 1dichro). because of the size of my smallest measuring spoon and big amount of gum, i could only mix rather much emulsion at once. i way also expecting a lot of failures, so i kept the used emulsion (closed with a lid, in the dark) for a few (3 or 4) days. and i was able to print with it. after a while the pigment sinks down with the fluid on top and it needs to be stirred up again.
i'm not saying this is acceptable technique, nor that there isn't any deterioration in the resulting image quality (i never checked with step wedges), but the emulsion prints and doesn't go off with a bang after one or two hours.
the comment on humidity is very interesting to me, katharine.
with an ambient humidity higher (or lower?? i can never wrap my brain around which way the > symbols go) than 85% your paper only keeps for four hours?
i'd expect a low ambient humidity to result in more dark reaction than in high humidity. just because of the wet emulsion has no or little senxitivity and in humid climate the dried emulsion would be "wetter" than in a dry climate.
Katharine Thayer schrieb:
Loris can speak for himself, but I have never seen a dark reaction in a mixed emulsion, and like Keith, I am skeptical. Dichromated colloids in solution behave very differently chemically from dichromated colloids coated into a film. Three or four years ago, when I was studying the chemistry of dichromated colloids in depth, I could have given the details from memory, but now I'd have to look it up, but if there's a "dark reaction" in dichromated gum solution, it's not anything like the dark reaction in coated paper. I've kept a mixed emulsion for as long as eight hours without any problem, printing from the same emulsion for a long day; that's the longest I can say from observation. This is in the same conditions (ambient humidity >85%) where a coated paper would become entirely insoluble from the dark reaction in less than four hours. I mix my emulsion in very shallow dishes; if I leave a mixed emulsion overnight, it just dries up in the dish.