[alt-photo] Re: gum contrast...

[Loris Medici]

Hi Paul,

2010/5/18 Paul Viapiano <viapiano at pacbell.net>:
> ...
> Riddle me this, sil vous plait: Knowing that decresing dichromate increases
> contrast, what would be the difference between a mixture of 2:1
> gum/dichromate and a mixture of 1:1 where the dichromate is 1/2 strength,
> both at the same exposure time?

Isn't the dilutions / strengths mentioned below identical (except final volume)?

Let me se:

2:1 -> 20ml gum : 10 ml 20% AD -> 20ml gum : 2g dichromate ->
gum:dichromate 10:1 (water evaporates)
1:1 -> 10ml gum : 10 ml 10% AD -> 10ml gum : 1g dichromate ->
gum:dichromate 10:1

Gum:dichromate proportions don't change, therefore - common sense says
that - these two emulsion are identical in terms of exposure scale and
density range. Of course, consistency will definitely differ between
them, therefore they may behave differently at time of application -
which may lead to different results at the end... ??? Also, they have
to be exposed at the same dryness level, the #2 emulsion with more
water may act faster in certain (short) drying times... ???

> Katherine mentions in her website that halving dichromate reduces speed by
> approx one stop. If you halved the dichromate, and increased the exposure by
> two, would you effectively get the same print as a 1:1? I'm trying to see
> relationships in theory here...

Less dichromate = more contrast = less distinct steps. Therefore they
aren't going to act the same way... (Says common sense again.) OTOH,
again, in your examples the emulsions are identical in terms of
gum:dichromate proportion, therefore you'll get the same step counts -
keeping everything else constant (and giving enough exposure)...

Regards,
Loris.