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

Re: Dichromate concentration question

From the chart (which at first I couldn't believe it said that 50g of pot di was soluble in 100ml water at room temp or whatnot until I realized it was centigrade :)) it looks like at freezing pot di goes down to about 5%.  Since very few of us work in the darkroom at freezing, except Keith, and mostly at maybe a range of 50-80, I wonder if it would be best to just mix a 5% pot di always and then never worry about solubility.  Keith, when you find out how much different 5% pot di and 10% are in practice exposurewise, let the list know--is it a full stop, for instance? OH, Keith is BELOW freezing....that's right. Like way below. Then maybe your solution isn't even 5%?  Is it an ice cube, Keith? Did you have to microwave it before use :)?
Christina Z. Anderson
----- Original Message -----
Sent: Thursday, January 15, 2009 10:55 AM
Subject: Re: Dichromate concentration question

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.....


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.

Christina Z. Anderson
----- 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?