Re: Reversal

["Paul A. Lehman" <palehman@xxxxxxxxxxxxx>]

For those who find this interesting:
Last week I wrote:

> To summarize (all values per liter final volume):
> 
> 1st developer is D-76 with the following range of
> additives:
>      2-16g  Sodium thiocyanate
>      52-60g Sodium carbonate
>      3-4g   Potassium bromide
> (I have a book somewhere that describes what each one
> of these
>   additives are doing to the silver, I'll try to find
> it this week).

Found the book.

The sodium carbonate is an "accelerator" in that it
enhances the reaction of the reducing agents (eg Metol)
by increasing the alkalinity of the solution. Thus,
more exposed halides will be converted to black silver
within the same developing time. Increased development
should produce cleaner/brighter highlights in the final
reversed image. 

The Potassium bromide is a "restrainer" in that it
inhibits the developer activity where acceleration is
not desired. Although contradictory when used with an
accelerator, it prevents fog in the thinner areas of
the negative (the shadow areas of the final reversed
image).

(suggests that this combinated will improve contrast in
the final reversed image)

The sodium thiosulfate acts in two ways, it dissolves
out excess unexposed halides and some of the developed
silver. It is often used in fine grain developers.
Although little information is available to me on this
chemical when used in developers, I suspect that it
prevents physical development of unexposed silver (by
removing the silver) reducing the size of the final
silver granules/particles.

This information may not be fully correct, so if anyone
knows more, or can clarify, please feel free to set me
straight.

Some speculation: Reversal formulas made for older
silver-rich film probably required more accelerator to
insure more consumption of the silver in the first
developer. If too much silver was left over for the
reversal, the image would end up appearing flat, fogged
and low in contrast. Modern film uses significantly
less silver, so less accelerator should be required. A
film already rated as fine grain would probably require
less Thiosulfate, if needed at all.

So, if one were not satisfied with the appearance of
the final reversed image, one can tinker with the
developer additives (or developer time) to correct the
observed problem.

Paul A. Lehman