U of S | Mailing List Archive | alt-photo-process-l | Re: Paper negatives- Ink Selection

Re: Paper negatives- Ink Selection

15 Ekim 2008, Çarşamba, 7:07 pm tarihinde, Christina Z. Anderson yazmış:
> ...
> Loris,
> Not to butt in on this dialogue you all are having, but I thought I'd
> re-remind you that a 10% am di is not equivalent to a 10% pot di--it is
> faster.

10% AD is about equivalent to saturated PD, which is something like 13% at
room temperature. PD solubility in 20C / 68F is 12% (according to MSDS),
at 22C / 72F (room temperature) should be slightly more (about 13%,

AD molar mass = 252.07g, 100 (10% in a liter) / 252.07 = 0.39M
PD molar mass = 294.19g, 130 (13% in a liter @ 22C) / 294.19 = 0.44M
and 120g PD (12% in a liter @ 20C) / 294.19 = 0.40M

Accepting there isn't any factor affecting speed (chemically) other than
molarity of dichromate ion in solution(!?), then in theory, due to
slightly stronger concentration (0.44M > 0.39M), 13% PD should be slightly
faster than (or about equivalent to) 10% AD.

I assumed Keith's workspace / chemistry storage isn't colder than 20C /

Thanks for reminding though! :)

BTW, I think it's better to use solutions slightly below saturation levels
(for instance I never mix more than 10% PD and 25% AD) because I think it
aids in consistency.

> ... but it was posted to the list a while back by Sandy King,
> and then I think even by you and Alberto haggling out the
> percentage.  That can account for some of the difference.

There can be a slight difference, but 1.3 stop seems way too much for
attributing it solely to minute difference in dichromate concentration.

> Also, is it more humid in Turkey?

Not necessarily, Chicago's humidity (and temperature levels) should be
close to Istanbul's in this season. Core winter or summer time can be more
different I presume...

> Also, thick vs. thin layers, etc. etc. I find that the
> way
> one brushes on a gum coat is probably the biggest reason for success and
> failure.  The thinner the better.

Very true, that's what I'm getting through -> the more proficient(!) I
become in coating, the better gets my results. I think, along with
choosing paints/colors (which is also very important), coating is the part
which you need the most finesse in gum printing.

> But all of that is determined with
> overlapping the step wedge with the substrate of choice so it might be
> helpful to send a small strip of your substrate of choice to each other or
> I
> probably forgot it was the same stuff didn't I....

Yes you did ;)

> ...
> I have 1 1/2 days next week to teach a class in Missoula MT how to do
> perfect gums so we'll see if it is possible :) But how the heck I am going
> to do that when I am  in a full leg brace is a totally other story.

What a pity! For how long it will stay? Good luck in rehabilitation...