Re: Rethinking pigment stain

Christina, I have also read that some pigments react with the dichromate
producing insolubilization. Pigments based on chrome salts, I would
think. But I have never used such pigments and I have never seen this
effect either.

As to 'dark reaction' and 'continuing action', perhaps there is a difference,
but this is more of a hunch than product of serious reflection :-)

Both produce fog, to be sure. 'Dark reaction' would be the spontaneous
hardening that happens since the dichromate and the gum have been
mixed together but without having been exposed to light.

'Continuing action' instead would be the hardening that happens
_after_ the emulsion has been exposed.

I don't know at this point whether these two processes are the same
or not. It is conceivable that the spontaneous cross-linking
proceeds at a different rate (faster?) after being exposed than before,
but who knows.

How about an experiment to verify this?

1. Coat a paper with a gum/dichromate/pigment mixture and cut it
in 2 halves.

2. Expose one of the halves under some calibration tablet. Let the
other half without exposing. Store both the papers in darkness but
not in a too cold place, so as not to inhibit any reaction.

3. After a few days, expose the other half under the same conditions
than the first and develop both.

If there is a difference between both kinds of dark reactions, it
should be visible.

Or not :-) What do you think?

Tom Sobota
Madrid, Spain

At 16:08 14/12/2005, you wrote:
>Right on, Tom,
>
>fog = global insolubilization of the gum layer...
>
>I'll add to your list one thing they used to say but I have never found to
>be true is they used to say certain pigments produced as they termed it
>"spontaneous insolubilization of the gum".
>
>I could say that when I was doing 8 large gums at once down in South
>Carolina where the humidity was higher, the last print to go into the water
>would take longer development to remove nonimage "fog". Fog, up to a
>certain point in gum practice, usually doesn't make too much difference
>because it is removable, unlike silver gelatin.
>
>Does this fog, then, form an umbrella over the two terms of "dark reaction"
>and "continuing action"? Because both result in global, non-image
>insolubilization. In other words, back in "the day" (1800's-1900's) they
>didn't term it fog in the first place but used these two terms to describe
>this global insolubilization.
>chris
>
>>It all depends on how you define 'fog'. To me, fog is a global
>>hardening or insolubilization of the gum-dichromate layer which
>>does not contribute to the formation of an image.
>>
>>It can be produced by any (or a sum of) several causes, among which
>>accidental exposure to light too, of course. Others are heat, exposure
>>to chemicals, and the simple passing of time. Surely others.
>>
>>The normal exposure to light of a layer of dichromated gum under
>>a negative produces a hardening which is not physically different
>>from a fog-producing exposure to light, but it produces an usable image
>>since it is not global but rather differential. So we don't call it fog.
>>
>>This loose definition of fog could be applied to silver-gelatin emulsions
>>too, actually.
>>
>>Pigment stain is different, and perhaps unique to gum, carbon and
>>other processes using finely ground particles of pigment.
>>
>>But dichromate stain has a sort-of analogue in silver-gelatin emulsions
>>when you develop with pyrogallol or pyrocatechol, for example, which
>>also produce stain. This stain contributes to the image, however, because
>>it is in the negative and not in the positive.
>>
>>If you want to see fog in gum, coat a paper and keep it in the dark for a
>>week or two in a warm place. Then develop side by side with a
>>just-prepared emulsion. The relative darkening of the older coat should
>>be evident.
>>
>>Tom Sobota
>>Madrid, Spain
Received on Wed Dec 14 10:35:32 2005