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Dichroic fog
From Silvergrain Labs


Contents
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    1 How it is formed
    2 How to prevent it
        2.1 Adjust silver-halide solvency
        2.2 Antistaining agents
            2.2.1 Chlororesorcinol
            2.2.2 2,4-dihydroxybenzophenone
            2.2.3 poly(vinyl pyrrolidone)
            2.2.4 2-mercapto-4-hydroxypyrimidine
            2.2.5 2-mercaptobenzoimidazole
        2.3 One-shot processing
    3 How to remove it

How it is formed
How to prevent it
Adjust silver-halide solvency

If the silver halide solvency of the developer is too high, dichroic fog may occur. Therefore, the amount of such agents as thiocyanate, thiosulfate, thioethers, ammonia, amines (especially primary and secondary) and other strong silver-complexing agents must be minimized or omitted from the developer altogether.

Sometimes, sodium sulfite may be sufficient to cause dichroic fog in replenished or reused developers.
Antistaining agents
Chlororesorcinol

Henn (1964), US Patent 3161514 discloses 1,3-dihydroxy-4-chlorobenzene (chlororesorcinol) as the antistaining agent. Examples show this agent's antistaining effect in fine grain film developers such as DK-20 or D-25 replenished with DK-25R, in a range of 0.5 to 1 g/L of this agent in working developer. (Preferred concentration is 0.1 to 5 g/L.)

Henn notes that resorcinol and chlorohydroquinone have some antistaining effects but they are much inferior to chlororesorcinol.

Several 1,3-dihydroxy carbocyclic compounds are disclosed in Kitze and Rosecrants (1968) US Patent 3380828. This patent is mostly concerned about unhardened paper products incorporating developing agents, and reducing dichroic staining in rapid processing of such materials. Some of the effective agents are 1,3-dihydroxy-4-chlorobenzene (4-chlororesorcinol), 1,3,5-trihydroxybenzene
2,4-dihydroxybenzophenone

These compounds are effective in concentrations from 0.1 to 1g per liter of working developer, when silver complexing agent is sulfite .

Henn (1964), US Patent 3161513.
poly(vinyl pyrrolidone)

This agent is effective when silver complexing agent is organic amine compounds. In HC-110 type developer concentrate (to be diluted 1+31), 0.25 to 0.5 g/L of PVP of MW 10kD to 40kD is effective. Larger polymers are said to be as effective but they are more difficult to dissolve in water.

Henn, King and Surash (1971), US Patent 3552969

PVP was used as an antistaining agent in quantities 3 to 15 g/l in monobath developers containing a large amount of thiosulfate, which is a very strong silver halide solvent. See Levy (1965) US Patent 3167429.

PVP is used for a wide range of non-photographic applications, such as:

    clarification of beer and white wines (removal of excessive polyphenols)

See poly(vinyl pyrrolidone) for more info.
2-mercapto-4-hydroxypyrimidine

In claims 3 and 6 of US Patent 5503965 (Okutsu 1996), a family of compounds are claimed as an antistaining agent: 2-mercapto-4-hydroxypyrimidine and its derivatives where 5 and/or 6 positions are substituted with usual hydrophobic groups.
2-mercaptobenzoimidazole

In claims 3 and 6 of US Patent 5503965 (Okutsu 1996), another family of compounds are claimed as an antistaining agent: 2-mercaptobenzimidazole and its derivatives, wherein the benzene ring is sulfonated at at least one place.
One-shot processing

Sometimes, dichroic fog may result because of silver complex present in the developer from previous film processing. This may be prevented by formulating a developer incorporating one or more of the suitable antistaining agent. However, it is also easily prevented by using a diluted developer one-shot.
How to remove it

A very slow acting and gentle bleach is useful in removing dichroic fog. A good example is below:
Ammonium thiosulfate reducer
citric acid 	15g
sodium metabisulfite 	10 g
ammonium thiosulfate 	40g
(or 60\% solution & 65 ml)
water to make 	1.0 l

The negative or print should be immersed with intermittent agitation until desired effect is obtained. This processing may require many minutes, and because the solution is colorless, it is not ideal for brush application. This reducer reduces fine grain emulsions faster. Therefore, dichroic fog and silver stains will be reduced first, followed by small grains and larger grains. Removal of dichroic fog takes 2 to 5 minutes at room temperature.