Re: haunted GUM (update)
thanks again,
actually it's still not certain whether it's the dichromate or something
else. but it is the most likely reason, since all the other ingredients
worked for me earlier. it's also possible that the pot-di is somehow bad
quality, since this is from a new batch, which i bought a few months ago.
it's also possible that my gum is rotten (smells/ looks fine though). i
just mixed up a new mix, i will print with it as soon as it's completely
dissolved. next step will be trying a different paper.
regards
phritz
Katharine Thayer schrieb:
Okay, I'm going to backpedal a little here:
On Oct 7, 2009, at 7:32 AM, Katharine Thayer wrote:
phritz, there's no reason why changing to potassium dichromate from
ammonium dichromate should cause tonal inversion, as long as you're
adjusting the exposure to accommodate the difference in sensitivity.
It's the dichromate ion that participates in the reaction that
hardens the gum, and that doesn't change;
In general and for most practical purposes, the dichromates are
considered roughly equivalent at somewhat equal concentrations, with
some wiggle room; the exactly equivalent concentrations aren't exactly
the same concentration in the two dichromates, but close enough that
it doesn't matter for this discussion. However, I was technically
incorrect in saying that only the dichromate iion is involved in the
crosslinking and so the reaction is the same whichever dichromate you
use. Fairly recent research published research on the crosslinking of
PVA, validated by a chemist I was working with a few years ago,
demonstrated that the crosslinking is actually different for the two
compounds, at least for PVA; the crosslinking for the ammonium
dichromate involves the N in the ammonium ion as well as the Cr in the
dichromate ion.
I don't have any idea whether, much less how, that difference in
crosslinking might affect the production of a tonal inversion; all
I'm saying here is that my first statement that there isn't any reason
why changing dichromates would produce tonal inversions was stated
with more confidence than was warranted. The fact is that I really
can't say with any certainty that the dichromate you use does or
doesn't affect tonal inversion.
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