U of S | Mailing List Archive | alt-photo-process-l | Re: Dichromate concentration question

Re: Dichromate concentration question



Keith, I'm glad someone knew where a chart was (glad to know about
that myself) but I'm curious about something. You didn't mention
having a lot of the dichromate settle out of solution, which would be
what would happen as the solubility decreased with the temperature.
Did you start out with a saturated solution?

Also, I'm curious about your experience printing at such low
temperatures, because I found last winter that I simply couldn't work
in the cold; the gum simply wouldn't coat the way it should and
prints were ruined because of the streaking and weirdness of the
gum. And this was at much warmer temperatures than you're talking
about (above freezing). It sounds like you've had no trouble with
the gum itself, only with the exposures. I don't think I had to
adjust temperatures for the cold, but then it was only 30 degrees
below normal room temperature.
Katharine



On Jan 15, 2009, at 12:46 PM, Keith Gerling wrote:

OK, so I was exaggerating.  A tad.  Now that the sun is warming the
studio, it is 10 C and according to the chart, I'm getting a little
more that half the amount of dichromate that I would at a more typical
20C.   That is significant.

Thanks for the link.  That chart is fascinating and I'm doing really
well on the test.

On Thu, Jan 15, 2009 at 11:55 AM, Dirk-Jan Treffers
<dirkjan.treffers@gmail.com> wrote:

Hi Keith,

just for fun, check
http://www.sciencegeek.net/Chemistry/taters/solubility.htm
The line for K2Cr2O7 (pot dichrom) drops dramatically when
reaching 0 C
(32 F).... Although it looks like the lower the temp gets, the more
asymptotic the line becomes....
Try printing the graph on a bigger piece of paper, and draw the x-
axis
further to the left, and see where the graph would more or less be
(concentration-wise) at -30C (-22 F)....

My guess would be 3-4%. That seems not nearly enough to really become
light-sensitive.... Try Chris's suggestion to use Am-dichrom.
Maybe at -22F
that would result in a higher concentration of dichromates....


Good luck with the icy temperatures.... Here in Holland we just
experienced
-10 to -15 (night time, 5-14F). At those temperatures, everybody
is hoping
for an 'elfstedentocht' (eleven-city tour, a 200 km ice-skating
tour trough
eleven cities, something 90% of the Dutch wants from time to
time.. See
http://en.wikipedia.org/wiki/Elfstedentocht )... Ok, enough off-
topic
nonsense.....


deejay




2009/1/15 Christina Z. Anderson <zphoto@montana.net>


LOL poor you, Keith--apparently this cold snap missed Montana,
but is in
MN big time. It is in the 30's and 40's outside so it feels like
spring to
me here, but my son is experiencing -38.

You're not SERIOUS when you say your studio is only a "tad"
warmer than
-23, are you????? Is your dichromate solution an ice cube???

As far as pot di, this is, again, a reason I prefer am di because
down to
32 degrees am di is still 15% soluble, and that is what I use it
at anyway.
But if pot di starts at 10% solubility max, I've read it goes
down to about
5% at colder temps (not -23 though!!). BUT I have no idea how this
correlates to exposure so am not answering your question, only
guessing that
a stop more exposure might be it.
Chris
__________________

Christina Z. Anderson
http://christinaZanderson.com/
__________________
----- Original Message ----- From: "Keith Gerling"
<keith.gerling@gmail.com>
To: <alt-photo-process-l@usask.ca>
Sent: Thursday, January 15, 2009 5:53 AM
Subject: Dichromate concentration question



As I sit typing the temperature outside is -23F. My studio is a
tad
warmer, but I have noticed a dramatic change in what I need for
printing times for gum. We all know that "saturated" solutions
change
with temperature, and the cautious printer will weigh out the
chemicals. But is there a multiplier I can apply to my printing
speeds that will take into account the actual amount of ingredients
contained in my "saturated" solution of Potassium Dichromate at
different temperatures?