Thiourea is not strong enough as a reducing agent to convert AgX to Ag0 to any appreciable degree. Usually those sulfur compounds that can reduce AgX contain S-S bond. Instead, thiourea has C=S and this sulfur can convert AgX to Ag2S. Jordan's reversal process replaces fogging and second developer with alkaline thiourea bath to convert undeveloped, unbleached grains to stable Ag2S grains, which are also opaque to visible light.
Originally Posted by Photo Engineer
Dithionite contains S-S bond that gives it a rather strong reducing property.
The commercial "Iron Out" is hydrosulfite (dithionate) mixed with bisulfite to either neutralize it or acidify it. It does not say dithionate on the package, so if you are looking for dithionate among the cleaning supplies in the supermarket, it helps to know they are synonymous. Maybe nobody but Gainer looks there. I do a lot of that. Many years ago I needed a tiny spring to fix an ancient flash synchronizer. I recodnized it as the kind of spring one found in cigarette lighters to hold the flint against the wheel, so I made the rounds of the stores to find the cheapest lighter that fit the bill. I turned down a few after examining the spring to the amazement of the clerk.
I'm old enough to tell such stories, aren't I?
Mees describes experiments which leave this in doubt.
Originally Posted by Ryuji
Work by Sheppard and Ballard tend to give equal weight to the formation of Ag and Ag-thiourea complexes which then form the sulfide on alkaline treatment. With hindsight, I tend to agree with you, Ryuji, but then the oxidation tests to determine the contribution of silver metal to this process showed that it was being formed to a significant degree in the presence of both thiourea and allyl thiourea.
It appears that both silver metal and silver sulfides are formed in the presence of thiourea, and by further treatment with developer, you get both silver metal from the 'fogged' silver halide and silver sulfide from the silver halide-thiourea complex making silver sulfides in the presence of alkali.
This is complex chemistry with many paths that we are talking about here. There is no simple cut and dry example to this.
That is why photographic engineering is such an art.
Modern day chemical sensitzations are as complex or more so than the average emulsion precipitation, just for this very reason. It takes advantage of this type of sulfur chemistry. And, very few of them are in the public domain such as in patents or other literature. The nearest approaches are in the descriptions of toners like we are having here, but even so are a far cry from the actual chemistry involved.
I might use this analogy. Sulfur toning of images or making sulfur toned images or making fogged emulsions via sulfur treatment is on a macro scale what is done with chemical sensitzation on a micro scale when sulfur and gold are added to 'finish' an emulsion. This is a gross oversimplification, but the rough analogy can be made, nevertheless.
I wonder how much Ag(0) I'm producing when I immerse my bleached-and-cleared film in alkaline thiourea. Probably not too much, as the "sulfiding" is very rapid (seems complete within seconds). Complexes of thiourea where sulfur is an electron-pair donor are not stable at all in basic solution, IIRC. This is used in organic chemistry to selectively generate thiols from organic halides. Urea is another product.
I agree with your reasoning. Ron is talking about thiourea concentration on the order of one million times lower than your case, at an elevated temp, and pH of 5-8 range. At such conditions, effect of gelatin is not negligible, and there could be a number of possibilities where amino acid residues in gelatin and thiourea could interact.
Originally Posted by Jordan
Reactions of thiourea are complex and interesting because of tautomerism between thiourea and isothiourea. In alkaline thiourea bath used as "odorless sepia" it is probably fair to say that the thiol group in the isothiourea form is involved. This is very different from acid thiourea solution, which would dissolve metallic silver as well as a lot of silver compounds that are usually insoluble. (This latter solution is used as developer tank/tray cleaner. It's also known as the last trick to revive badly treated pH electrode whose reference junction bridge is clogged with silver sulfide deposit.)
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Ryuji is correct. Actual practice goes beyond what he has stated.
I was merely pointing out that there is a complete continum of conditions you meet when you add thiols and thiones to silver and silver halides. All of those concentration conditions can be met in the micro/macro environment that is the coating or emulsion, and all of the interfaces between the chemicals present are possible.
It is a real gemisch in there, and you will probably find every possible reaction taking place on the micro and macro scale whether toning, developing or senstizing.
Adding a drop of sensitizing solution of allyl thiourea, even though at ppm in the final emulsion, implies a local concentration gradient as the dropwise addition of the sensizer is added. The same is true when you adujst pH or when you carry out any of the other operations. And so, in actual practice the fact that workers find that the real answer is "all of the above" does not surprise me in the least.
To address just one comment made above, chemical sensitzation and ripening are often carried out on the alkaline side, and are often quenched with acid. This is a fact little known outside of the closed darkrooms of the manufacturers although I have alluded to it elsewhere in posts.
As in all chemical reactions, you don't add a + b and get c. You get d, e, f, g etc. Being a chemist myself, I find this sort of result is normal. There is nothing that goes 100% in chemistry. The bottom line is that more often than not, side reactions are often of as great importance in photography as are the main reactions. In some cases, equillibria do not enter fully into the picture, as some reactions, once completed cannot be reversed, as you can see for yourself. Done is done. Formation of silver sulfide is one and formation of silver metal is another. Once there, silver metal can become a catalyst for formation of more metal amplifying the effect. That is the nature of development taking place at the site of the latent image, isn't it?
Ron, aren't you tired after going flat out bringing up irrelevant issues and proceeding to lecture on trivial things that don't matter in understanding what Jordan is doing, just because you made one mistake and someone pointed that out? Do you feel good? People spending effort in wrong places is exactly why a lot of internet forums are waste land.
Ryuji, please note that I said I agreed with you for the most part in a previous post.
I have said elsewhere that you are doing good, but complex work and I have said nothing to contravene here what you or Jordan have said in this thread. Rather, I have tried to extend it and place some what you have said in more simple terms.
So, what is your problem? I am trying to show that a complex series of reactions take place on the micro and macro scale both, and that we cannot easily make a positive statement about what is going on. The proof, as cited by Mees, is that even experts disagree as to what is going on in some of these reactions.
You have made statements that even far greater experts than we disagree over. I'm sorry if that makes you uncomfortable, but there are things in photographic science and engineering that we just do not know with certainty. This area is one of them. It yields a mix of reactions that without chemical analysis we cannot be certain about. And even there, there is an uncertainty factor due to the very nature of the chemistry involved.
That is my point.
It comes from 30 years of hard lab and desk work, 8 hours / day, 5 days / week with experiments and modeling. How much photo engineering time have you spent? My experience comes with working side by side with the people you sometimes cite in your work, knowing what they think and what they are doing.
I see no mistake above. I see no reason for me to resort to a personal attack on you either, for any reason whatsoever.
As a matter of fact, I reported my comments as derived from Mees, but I am not sure I agree with him myself as I pointed out and that I tend to agree with you, a point you seem to overlook. This covers the entire field of thiourea reactions with silver halide whatever the scale, pH, concentration etc.
Permanganate-based b&w reversal reconsidered
Originally Posted by Jordan
Hi Jordan, hi all,
I'm using this thread to post updates about my eternal journey towards the proper b&w reversal process.
Tonight I've developed a scrap of T-Max 100 (@100) using the following chemicals and the following steps:
1) first developer: Tetenal eukobrom 1+4, 12 minutes, agitation: 10 inversions every 30 seconds;
2) stop bath: citric acid 1,5% for 1 minute, agitation: 10 inversions every 30 seconds;
3) bleach: 2g potassium permanganate + 50g sodium bisulfate in 500ml for 5 minutes, agitation: 10 inversions every 30 seconds;
4) wash: 1 minute;
5) clearing: 30g/l potassium metabisulfite + 1 teaspoon of sodium hypo, agitation: 10 inversions every 30 seconds;
6) wash: 1 minute;
7) redeveloper: sodium sulfide (10g/1l) for 3 minutes, agitation: 10 inversions every 30 seconds;
8) wash: 1 minute;
9) hardening: Tetenal harter (potassium alum based)
10) wash: 1 minute;
11) wash: 1 minute;
12) wash: 1 minute.
First and foremost: NO EMULSION DAMAGE!!! even with the permanganate bleach. I've found the CULPRIT: every solution must be at the EXACT SAME TEMPERATURE (mine, the ambient one). As simple as that.
Second: I've got brown-yellow slides, slightly dark, but with HUGE grain: why?
Alessandro, none of my clearing bath formulas for reversal B&W contain hypo. I suspect that this can damage the remaining positive image. I would suggest you remove it.
The use of Sodium Sulfide as the redeveloper will give a brownish image. Sodium Sulfide is used as the toner in sepia or brown toners.
It is best to use a conventional high contrast developer. Many of my processes use something like D19.
As for the grain, IDK. Perhaps it is the hypo or the sulfide or it could be something else. You will have to play with the problem areas you mention first, before you can sort out the grain problem. I suspect that it is the Sodium Sulfide, as reversal developers are generally not of that type.
Best of luck to you.