Niranjan,
Thank you for your reply and such a very good explanation for me. I follow your explanation which is clear and is, therefore, a sign of a good teacher - not just that you know the answer but can get it across to someone not as informed as yourself.
But to push the question a little further - and away from actual photography, if it is not that necessary for FAC to be present to make the image, why is it there in the first place?
Thanks, Colin for the compliments. I have always believed you haven't truly learned something until you can teach it to someone. Having said that, more than teaching I was simply clearing up some ideas in my own mind as I tried to answer your question.
Before we go into your next question, I wanted to emphasize that although silver nitrate
alone could have given you an image, I don't believe that actually happened in your case. Most likely the clear liquid you decanted leaving sediments back still contained enough FAC to trigger the process. As Serder pointed out it is quite normal for the VDB process to encounter sediments. It might have something to do with solubility equilibrium of the three ingredients involved or may be your FAC is compromised somewhat which would spontaneously precipitate silver as your add the nitrate. To check the latter, you can add a few drops of potassium ferricyanide solution, if you have some, to your Solution A and see if it turns blue. If it does, then you have some ferrous amongst the ferric.
Now to your next question, which is a real good one. To paraphrase: Why did they bother to invent the iron-silver process if plain silver would have done the job? This gets a little into the complicated realm, requiring some hand-waving which may exceed my explaining power. I am still a learner, not an expert as some are on this forum, so please bear with me.
I think the answer is perhaps related to the paper as it was available then and as is now. As I mentioned silver nitrate becomes photosensitive in conjunction with the paper, making what's in the paper important. In particular I noticed that the buffered papers were much more sensitive and gave higher Dmax than the unbuffered paper. Now calcium carbonate is the buffering agent most commonly used in modern papers. So I did an experiment. I took some chalk powder, added a few drops of silver nitrate solution. The white chalk powder seemed to dissolve and a pale yellow precipitate showed up instead after a while, according to the reaction:
CaCO
3 + 2AgNO
3 = Ag
2CO
3 (silver carbonate) + Ca(NO
3)
2
When I left it in the sun for a few minutes, there was instant transformation to black powder signifying that silver had formed. Turns out silver carbonate is quite photosensitive. So if you are using paper that is buffered with calcium carbonate, then this is what might be happening as a side reaction as you coat the VDB sensitizer – making silver carbonate. The presence of tartaric acid will inhibit this reaction as it will compete by reacting itself with the base to form calcium tartrate. I suspect that this is one of the reasons for it to be there in the VDB formula. If you go one step further, calcium tartrate will react with silver nitrate to give silver tartrate which is also photosensitive. (Chemistry is relentless!)
I am not knowledgeable about the history of paper-making and I certainly do not know all the ingredients in the paper of the 1800's. From what I understand, buffering agents were not popular then. The papers that were used for photography were based on more or less pure cotton fibers with or without a sizing agent such as gelatin. So the bare silver nitrate on the paper was probably found not to be that effective. I think Talbot (can't remember where I read that or may be I am imagining) did try to do that in his initial experiments but figured quickly that a better way was to salt the paper so silver chloride can be made in situ before exposure. Another reason for not using the silver nitrate by itself was probably its propensity to be reduced even in darkness in presence of many organics such as proteins (present in gelatin) aldehydes (present in the cellulose fibers) among others, giving rise to what I called “dark” staining in the thread I linked before - making storage of the sensitized paper a problem. Presence of tartaric acid would suppress this too in VDB as would citric acid in salted paper.
Iron chemistry also in the same fashion became an alternate route to get to silver metal in various formulations such as Namias' Sepiaprint, VDB, Kellitype etc. Ferric salts are significantly faster than silver salts, which I presume based on time it takes to expose a pt/pd versus a salt print, so even though the latter can make silver on its own, the route through reduction by ferrous salt would have been deemed more efficient.
So off went the photographic evolution. In fact the first I ever heard about making salt prints without the use of an intermediate salt was in an article by Liam Lawless in the 8th issue of Post-Factory Photography. At the time I was not even a novice (more like an interested by-stander) in the field so I filed it in the mind under: hmm...interesting. Then pdeeh mentioned the fact that when he accidentally forgot to salt the paper first, it still gave him an image.
https://www.photrio.com/forum/index.php?threads/unsalty-salt-print-query.119477/
Finally, when I started to do my own salt prints, I realized that this could be really a way to make an image, if you can understand all the parameters - which is where I am right now.
I am not sure if I cleared up or fogged up your question. Most likely created new ones...
:Niranjan.
