Fox Talbot and salt fixing ?

Clive, you wrote that once before relating to VDB. I guarantee what Pdeeh wrote is without any doubt true for Namias' process. It's not subtle at all!
The primary difference between VDB and Namias' process is that VDB uses tartaric acid and Namias' process uses citric acid. Namias' process also uses somewhat stronger solutions of FAC and AgNO3.

By the way, I'm pretty sure a new thread will start soon about variations of brown/sepia/VDB prints... just have a hunch.

pdeeh said:

{my emphasis}

The discussion really has reached beyond the limits of my understanding of most of the chemistry involved, but I would like to make one observation on the matter of colour & density change: In Namias' Papier sepia process (an Iron-Silver process, with which Ned is familiar, and upon which I won't enlarge (as there is a running thread started by Ned on the subject)), an exposed print is first washed (significant increase in contrast and density) and then fixed in hypo. As soon as the print hits the hypo, contrast and density increase again.

Rodolfo Namias himself speculated "I am inclined to attribute this to a slight sulphurization, facilitated probably by by traces of iron present in the image."

So this would perhaps be an effect limited to Iron-Silver processes, rather than the Silver halide only processes.

As I say, just an observation to refine O'Reilly's statement "a characteristic phenomenon in all silver printing-out paper"

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Just a quick comment on the Papier Sepia (as well as other similar ammonium ferric citrate based iron-silvers like Van Dyke): Even though there is an image formed at the outset on exposure, it is a hybrid POP/DOP process where the water is the developer that carries the ferrous ion over to the the available silver nitrate, precipitating silver metal as a result. The moisture in the paper does a partial a job of this on exposure, the rest being finished in the wash step. Presumably it might be carried over to the fixing step if not all ferric ions have been oxidized in the first step and there is still some AgNO3 available.

cliveh said:

Not in my experience.

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I didn't know you have used the process I am referring to Clive.

nmp said:

Just a quick comment ...

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No need to be quick, though what you wrote was helpful and illuminating. thanks.

pdeeh said:

No need to be quick, though what you wrote was helpful and illuminating. thanks.

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I guess I was thinking out loud a bit there...had to go someplace yesterday.

NedL said:

Niranjan,

Also, what about highlights? If you print until there are only faint highlight details, the details can "bleach away" to paper white in the fixation step. This happens even after toning. In fact it was common practice in the early days to use extended hypo bath ( even to many hours ) to brighten up highlights. It seems to me the particles themselves must be getting reduced in size (to the point of becoming invisible!). If it was just a matter of packing, wouldn't the highlights remain, but just shift color?

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Hi, Ned:

I am kind of intrigued about this observation. Can you please point out a reference or two about this extended fixing procedure. I would like to know what exact composition of fixer was - was it acidic or alkaline or straight hypo. This hypo, sodium thiosulfate, is a very complicated business - being highly unstable particularly in acidic environment, giving rise to all kinds of side reactions. I am trying to get a handle on these because of my own observation that under some circumstances my POP prints come out significantly more neutral than the rest using seemingly identical process. Compare #4 (Worn and Faded) with the rest of the images on this page:

http://niranjanpatel.zenfolio.com/p790410975

The only thing I did not control (that I know of) was the season (hot and humid to dry and cold) and the age of the fixer and the selenium toner (I did not know to.) The image in question was most certainly the one where I used about 4 month old left-over fixer and toner. Within days, I made the #7 (Empty Warehouse) with fresh fixer and toner. Note the significantly more warmth in that image. Although the toner could be guilty until proven otherwise, I am focusing on fixer chemistry more. My fixer is 15% hypo + 2% sodium carbonate. (I actually like the more neutral image so all the more reason to figure out why.)

Taken in their pure forms, non-ionic silver does not react with sodium thiosulfate, as per my reading of the silver chemistry. In order for the highlights to be bleached away, silver metal has to be converted to some other form which is either non-reflective like a salt (a la silver ferricyanide in the classic bleach step) and/or gets taken out subsequently by action of the hypo. (Another possibility would be if the small silver particles n the highlights simply get physically dragged out if they are not adhering strongly in the matrix. But then same effect should occur in plain water washing. And it does not, to my knowledge.)

Some more food for thought....

:Niranjan.