Re: My own PVA glue sizing experience became: dark reaction in gumemulsion
phritz, the dark reaction is very dependent on humidity; the more
ambient humidity, the faster the dark reaction proceeds. In drier
climates/environments the dark reaction is less of a problem. The
relationship is a very steep one, so a relatively small difference in
humidity can make a large difference in how long the paper can be
kept without being affected. This is, again, only for dichromated
colloid coated into a film; dichromated colloid in solution undergoes
different reactions and forms different reaction products, so what
happens with dichromated colloid in solution can't be compared to
what happens to coated paper, nor can any conclusions be drawn from
one to the other.
kt
On Oct 28, 2009, at 6:36 AM, phritz phantom wrote:
hi all,
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.
regards
phritz
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.
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