B&W Reversal: thiourea as fogging developer?

[Alessandro Serrao]

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

Theoretically no, as long as the fogging+redeveloper is done to completion. But you safely use a 1+9 diluited rapid fixer for a couple of minutes just to be safe. It does no harm at all.

 

[Raghu Kuvempunagar]

What exactly do you mean when you say "I found it necessary to fix when using dithionite"? Did you measure and find out any difference in density before and after fixing?

 

[pkr1979]

What exactly do you mean when you say "I found it necessary to fix when using dithionite"? Did you measure and find out any difference in density before and after fixing?

On the 2 last frames of the roll I shoot the exact same photo, with the same settings, in the same light. Before fixing cut off one of them while fixing the other. They were quite different:



IronOut (dithionite) for 6 minutes. 4 initial inversions and 4 more half way (about 20 degrees). The one at the left is fixed.

 

[pkr1979]

PS:
30 grams of IronOut: https://summitbrands.com/wp-content/uploads/2018/03/Iron-OUT-Powder-English-GHS-Rev-02-12-2018.pdf
In 300 mm of water.
For one film.

 

[Raghu Kuvempunagar]

IronOut (dithionite) for 6 minutes. 4 initial inversions and 4 more half way (about 20 degrees). The one at the left is fixed.

Any reason why you went with minimal agitation? Even Kodak 's documentation for using FD-70 second developer, Dithionite based, recommends you to "agitate continuously during the first 30 seconds in each solution and for 5 seconds every minute thereafter."

 

[pkr1979]

Any reason why you went with minimal agitation? Even Kodak 's documentation for using FD-70 second developer, Dithionite based, recommends you to "agitate continuously during the first 30 seconds in each solution and for 5 seconds every minute thereafter."

Thats what Ive been told was the way to do it by others who use Iron Out.

 

[pkr1979]

Thats what Ive been told was the way to do it by others who use Iron Out.

Whats your thoughts on this @Raghu Kuvempunagar ?

 

[Raghu Kuvempunagar]

Minimal agitation looks too weak for me, but you can always try what Kodak recommends (intermittent agitation and longer second development time) and find out if it makes a difference to your slides.

 

[pkr1979]

Minimal agitation looks too weak for me, but you can always try what Kodak recommends (intermittent agitation and longer second development time) and find out if it makes a difference to your slides.

I also once fixed a 120 slide after getting a tip on that it should be done, even when using dithionite. That also changed density. Back then I tid agitate 10 seconds every minute with Iron Out too.

 

[Raghu Kuvempunagar]

Back then I tid agitate 10 seconds every minute with Iron Out too.

How long was the second development?

 

[pkr1979]

How long was the second development?

5 or 6 minutes. Cant remember, but I am assuming 5.

 

[Raghu Kuvempunagar]

Dithionite in alkaline medium is a very powerful reducing agent, supposedly stronger than both Thiourea and Sodium Sulphide. It is unlikely that a treatment in alkaline Dithionite will leave any silver halide untouched. So, what the fixer is removing, I suspect, is not the residual silver halide but some of the metallic silver itself. Note that Dithionite reduces silver halide to metallic silver and the silver specks are very small compared to the negative image as all the big grains have been reduced already by the first developer and removed by the bleach. And these very fine grained silver specks are very likely removed by the relatively strong fixer and thereby lightening the slide in some instances.

Interestingly, even the purest Dithionite available has substantial Thiosulphate as impurity and one of the degradation products of Dithionite in aqueoues media is also Thiosulphate. So, if Dithionite couldn't reduce some halides for any reason, Thiosulphate in the same solution would have removed them even before the fixer touched the positive image.

 

[pkr1979]

Dithionite in alkaline medium is a very powerful reducing agent, supposedly stronger than both Thiourea and Sodium Sulphide. It is unlikely that a treatment in alkaline Dithionite will leave any silver halide untouched. So, what the fixer is removing, I suspect, is not the residual silver halide but some of the metallic silver itself. Note that Dithionite reduces silver halide to metallic silver and the silver specks are very small compared to the negative image as all the big grains have been reduced already by the first developer and removed by the bleach. And these very fine grained silver specks are very likely removed by the relatively strong fixer and thereby lightening the slide in some instances.

Interestingly, even the purest Dithionite available has substantial Thiosulphate as impurity and one of the degradation products of Dithionite in aqueoues media is also Thiosulphate. So, if Dithionite couldn't reduce some halides for any reason, Thiosulphate in the same solution would have removed them even before the fixer touched the positive image.

How does this differ from a 'regular' reversal process where you use light and a regular second developer? I will say that fixing after Iron Out does improve the overall image. And the one example here, as you can see, also changed tone in addition to density.

 

[pkr1979]

I mean, wouldnt we want the fixer to remove whatever it is removing (ie residual silver halide) when using dithionite?

 

[pkr1979]

And, is what you are describing likely to occur also with thiourea?

 

[Raghu Kuvempunagar]

I will say that fixing after Iron Out does improve the overall image. And the one example here, as you can see, also changed tone in addition to density.

Very fine silver grain can impart colour to the image. But I am not sure that's what is happening in your case. Though the slide looks slightly better due to lightening, it appears that the density in the unexposed areas also reduced after fixing and thereby reducing DMax substantialy. Only you can tell for certain as observations made from a phone snapshot might not be accurate.

I have not had problems getting very clean highlights with Dithionite as the second developer unlike the results you shared above. But the films could be different and Dithionite I used was not Iron Out.