B&W Reversal: thiourea as fogging developer?

pkr1979

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Hi all,

I know that thiourea/toners have been used as a fogging developer in the black and white reversal process. But, when reading about reversal processing in Haist he presents a process that consist of first developer, bleach, and the thiourea fogging developer. No clearing, no re-exposure and no fixing. I've been using dithionite which is convenient as re-exposure and second development is the same step, but its still necessary to clear after bleaching and fix in the end.

So I guess my question is, did anyone use thiourea or some sort of toner as a fogging developer in the reversal process? If so, how as it done and whats the steps of the process?

Cheers
Peter
 

koraks

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Have a look at this:

Film reversal processing with sepia toner.

It’s been a long time since I posted here, maybe going back to the APUG days… Anyway, I stumbled across this article https://www.35mmc.com/02/07/2022/making-4x5-slides-using-a-sepia-toner-kit-by-andrew-smale/ It talks about doing film reversal processing with an off the shelf sepia toner and...
www.photrio.com
 

Raghu Kuvempunagar

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This could also be of interest to some in this context:

B&W Reversal Processing: fogging/nucleating agent for second development

A key step in B&W reversal processing is the formation of the positive image by A) sensitizing the residual silver halides and B) reducing the thus sensitized silver halides to silver. Though it is possible to do both A and B in a single step...
www.photrio.com
 
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pkr1979

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Thanks!

@koraks - reading through the other thread I saw this post: https://tinker.koraks.nl/photography/shades-of-brown-controlling-sepia-toning-hue/ And I think the mix A:1+B:10 looks quite nice - its not an exaggerated sepia by any means. I assume those solutions would work for film as well?

@Raghu Kuvempunagar - reading your post, if I understand you correctly, you used a second developer after using thiourea, why? I was under the impression that a second developer wasnt needed then?

Im also curious to know if the clearing and fix can be emitted when using thiourea. That thiourea will actually clear, reexpose, develop and fix?

Cheers
Peter
 

koraks

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pkr1979 said:
I assume those solutions would work for film as well?
Click to expand...

Yes, but the color range vs. dilution ratios might be different, and you may get slightly different hues to begin with. This has everything to do with the actual grain structure of the silver image, which will be different (generally more finely divided) on film than on (most) paper.

pkr1979 said:
Im also curious to know if the clearing and fix can be emitted when using thiourea. That thiourea will actually clear, reexpose, develop and fix?
Click to expand...
The thiourea will effectively fog everything that's left in terms of silver halides into silver sulfide. At that point, you just have to get rid of the metallic silver (negative) image. You can either do this with a non-rehalogenating bleach (the bleach you're presently using for reversal would do), or you can use a more common Farmer's reducer approach (ferricyanide + fixer).
There's no re-exposure necessary since the thiourea will so aggressively react with the remaining silver halides that their exposure simply doesn't matter.
 
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pkr1979

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But it could serve as a good starting point as any I suppose.

So this would actually mean that the process only consists of the three steps development, bleach, and thiourea? Is there any reason to use Farmer's reducer instead of what I'm using now?
 

Raghu Kuvempunagar

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pkr1979 said:
@Raghu Kuvempunagar - reading your post, if I understand you correctly, you used a second developer after using thiourea, why? I was under the impression that a second developer wasnt needed then?
Click to expand...

The first two paragraphs of the resource I shared provide the required explanation. In reversal processing, Thiourea can be used either as a fogging agent (i.e. it nucleates the halides but doesn't reduce them and needs to followed up with a developer) or as a fogging-redeveloper (i.e. it both nucleates and reduces the halides).

Mildly acidic Thiourea works as a fogging agent and can be used along with regular developers. The resulting positive image is made of metallic Silver and often has warm tone. The positives can be further toned in, say, Selenium.

Alkaline Thiourea works as a fogging redeveloper similar to Dithionite and the resulting positive image is made of Silver Sulphide and has a sepia tone.
 

koraks

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Yes, it's a good starting point.

Indeed, only 3 steps, but in a different order than you stated. But I assume you realized that. Farmer's reducer is less toxic and in Europe it can still be legally traded; dichromate not so much.
 
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pkr1979

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koraks said:
Yes, it's a good starting point.

Indeed, only 3 steps, but in a different order than you stated. But I assume you realized that. Farmer's reducer is less toxic and in Europe it can still be legally traded; dichromate not so much.
Click to expand...

Not in a different order. I should have been clearer as I was referring to this in Haist:



Which describes the process as (at least as I understand it) first developer - bleach - fogging developer.

But does that mean that three steps only works if you do the fogging developer before the bleach? I saw @Raghu Kuvempunagar comments on that and Im not so keen to do that. But I still might want to try thiourea as a fogging developer instead of dithionite. But in a scenario where you go developer - bleach - fogging developer, does that then mean that you cant use Farmers reducer? And that clearing and fixing is necessary?

@Raghu Kuvempunagar - didnt you use thiourea as a fogging developer?
 
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pkr1979

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koraks said:
I see; yes, that can be done, too, but then you need to use the dichromate (or permanganate) bleach and not Farmer's reducer.
Click to expand...

Not clearing and no fixing? The thiourea will take care of that?
 

Raghu Kuvempunagar

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pkr1979 said:
@Raghu Kuvempunagar - didnt you use thiourea as a fogging developer?
Click to expand...

I have done that too to make sepia toned slides but I am not too fond of sepia tone for every slide. As a fogging developer, Thiourea works fine just like your Dithionite. It's also more stable than Dithionite and odourless. I've always used a Sulphite clear after the bleach and I wouldn't advise skipping it. Fixing is not necessary if you let the second development to go to completion. But I have not used the exact fogging
developer formulation that you copy pasted from Haist and can't speak about it. I used substantially less Thiourea and developed at ~25C.
 
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pkr1979

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Ive found that when using dithionite fixing is necessary. What is your thiourea mix and process? I assume first developer time needs to be adjusted if switching from dithionite to thiourea - is there a general rule in increase or decrease percentagewise?
 

Raghu Kuvempunagar

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You can try this formula for fogging redeveloper (reusable):

Thiourea: 2 g
Sodium Carbonate: 100 g
Water to make: 1 l

~10 minutes in the developer @25C should be adequate.

The process is as follows with wash steps added appropriately:
First development
Stop
Bleach
Clear
Second development

I've not needed to adjust FD time but YMMV.
 
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pkr1979

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Thanks! Do you know how reusable it is in terms of shelf life/number of films? I will also try to add sodium hydroxide to control the hue like @koraks did on paper - do you know if that will affect reusability?
 

koraks

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pkr1979 said:
Thanks! Do you know how reusable it is in terms of shelf life/number of films? I will also try to add sodium hydroxide to control the hue like @koraks did on paper - do you know if that will affect reusability?
Click to expand...
I'm not sure whether the solution is stable - and frankly, I never wondered about it, either. The components are so cheap and/or used in such tiny amounts, I just mix it as needed and discard after use.

bernard_L said:
Could thiourea be used to check for complete fixing of paper, for example a test strip of undeveloped paper?
Click to expand...

Yes. It'll stain/turn brown if there's unfixed silver halide in there. It's quite effective, actually.
 
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pkr1979

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@koraks - how come you are not using sodium carbonate in your solution? Is this something thats beneficial for film but not necessary for paper?
 

koraks

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Oh, it's very simple - I took this approach from a web site I found a decade ago or so and that used sodium hydroxide. I stuck with that. I never tried it with carbonate; evidently that would work, too. I couldn't say what the ratios would be for the different hues; you'd have to experimentally determine this.
 

Raghu Kuvempunagar

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Like @koraks I haven't had the need to store the fogging redeveloper after it has been used in a session. A litre of the redeveloper should be good for 3-4 35mm film rolls if used in the same working session. You can always make just enough volume needed for your requirement.

Concentration of Thiourea and pH determine the colour tone of the slides. So, if required, either Sodium Hydroxide or Sodium Bicarbonate can be added in appropriate amounts with consequent shifts in the colour. Old school sepia toners have variants that have some Metol in them and that's another way to modulate the sepia tone. There is a lot of room here to experiment. It is both fun and educative to try various combinations in a single session with a few frames of film and find out what works best for you.
 
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pkr1979

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Thanks for the clarification guys - I'll try the one listed on koraks page with most sodium hydroxide. I'll report back eventually to let you know how that works on film
 

DeletedAcct1

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Thiourea works as a fogging and redeveloping agent all-in-one. I'd not skip the clearing step though.
 
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pkr1979

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Alessandro Serrao said:
Thiourea works as a fogging and redeveloping agent all-in-one. I'd not skip the clearing step though.
Click to expand...

Do you fix when using thiourea as a fogging developer? I found it necessary to fix when using dithionite.
 
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