Silver and color

[Murray Kelly]

This is terribly basic but what does the silver image actually do? PPD and its derivatives don't feature strongly in B&W and my experience is that of poor acutance with B&W. So, it is the color developer but is the process tuned to the silver image developing qualities of CD4 or ???

Direct me to a thread if that is better if you know one, I just don't know.

What is the role of silver in C-41?

 

[Rudeofus]

When PPD (or any other developer) reacts with developable silver ions, the silver ions get reduced to metallic silver, while the developer becomes oxidized. That oxidized developer can react with various compounds:

1. oxidized primary developer can be restored to reduced form by secondary developer, c.f. superadditivity.
   
2. together with Sulfite ion they can form a sulfonate. This is the typical reaction happening in B&W developers.
3. a special group of compounds called couplers can be split by these oxidized developer molecules to form other molecules. This is the reaction typically happening in color photographic products (C-22/C-41, E4/E-6, ECN-2, RA-4, Kodachrome).

Reaction type 3 can form stable dye molecules from certain PPD derivatives (CD-3, CD-4, CD-6) and the right couplers, whereas other oxidized developers either don't react that way at all, or form unstable dyes.

This is a very brief summary, you should try to get Grant Haist's Modern Photographic Processing for a very detailed description of what's going on in color processes.

 

[Mr Bill]

What is the role of silver in C-41?

Hi, "silver" is still the light-sensitive part of C-41 (color neg) film. And development of the silver image is still a crucial step in the process.

I think that Rudi's explanation works best if you already understand how things work. Let me try to explain things at a simpler (?) level.

Here's basically how the magic of C-41 color works: the film is different from B&W in two ways, 1) it has essentially three different emulsion layers, each layer treated to be sensitive to a different color of light. 2) Each color-sensitive layer is loaded with so-called color couplers, which are roughly one-half of a colored dye molecule. Meaning that they are not actually colored dyes, but rather the first stage of making a colored dye. Note that each layer has its own specific color couplers which can only be "completed" into a specific color of dye.

Now here's the magic part. The color developing agent is special; as each molecule - the tiniest little bit possible - develops a bit of silver, the developing agent becomes oxidized. And this oxidized developing agent can fill in as the missing part of the colored dye molecule. Said differently, the color couplers in the film can combine with the oxidized color developer to form an actual color dye. So as each tiny bit of silver is developed, a tiny bit of color developer gets oxidized, and that tiny bit of oxidized developer almost immediately reaches a dye-coupler (in its own color layer) and the two combine to form a tiny bit of actual colored dye (which is the correct color for its layer).

After all this is done, the silver image is no longer needed, so it can be "bleached out" and removed. Hopefully you can follow what I've said. To me, the thing that is even more amazing than this process is that people actually figured out how to do this whole thing.

 

[Murray Kelly]

Thank you Rudi and Mr Bill. It was the connection between the silver and dye couplers that I wanted cleared up.

The rest I know. I admit I thought the Kodachrome dyes were added during processing unlike regular reversal film.

 

[Rudeofus]

It's not dyes which are added in Kodachrome, it's dye couplers. The underlying principle is identical between C-41/E-6/RA-4 and Kodachrome, as far as the reaction is concerned, it doesn't matter whether the coupler comes with the color developer (Kodachrome) or is already embedded in the emulsion (C-41/E-6/RA-4).

 

[Murray Kelly]

Ah! Thank you again, Rudi, for the clarification. It's sorta academic now I guess. I was close, but no cigar. I have read the rumor about starting up again but put little credit to it.

Murray