[pure-silver] Re: Purple-Tinted Negs


----- Original Message ----- From: "David Foy" <dfoy@xxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Monday, May 29, 2006 12:26 PM
Subject: [pure-silver] Re: Purple-Tinted Negs


Assuming it's not T-Max, possibly it's not a sensitizing dye, but rather an anti-halation dye between the emulsion and the base. It should decolorize in the fixer, and failing that it should wash out with water. Sometimes it doesn't.

David Foy

T-Max and Delta films are not the only ones with persistent sensitizing dyes.
Anti-halation dye, in most B&W films, is in the back coating and is decolorized by the sulfite in both developer and fixing bath. Color films typically have an anti-halation layer as you describe, between the emulsion and subbing layer. This is actually a more effective place to have it since it eliminates reflection from the top surface of the support as well as the back surface. In addition, it prevents fogging from light conducted along the support.
Residual dye in the emulsion can come from a small amount of residual silver halide due to inadequate fixing, or, sometimes the dye seems simply to bind to the gelatin. While the tint on T-Max is often blamed on incomplete fixing I have found that extended fixing times do not seem to cure it. Rather treating with a sulfite wash aid, and probably any alkaline bath, will cause the bond to break and the stuff is washed out. This may be due to the change in electrical charge in the emulsion from going to an alkaline pH.
Since wash aid is nearly all sulfite it should also decolorize anti-halation dye but I would be suspicious about access of processing solutions to the back of the film when the anti-halation dye is present.
Inadequate fixing can also result in a "stain" or fog on the film. This will usually be removed by refixing in _fresh_ fixer. The residual silver halides and fixer reaction products from incomplete fixing begin to change wtih time so refixing should be done as soon after the original fixing as possible. After a few weeks there may be reaction products that have changed so that they can no longer be made soluble.
Fixing is a complex process that takes place in stages although the transformation is continuous from stage to stage. The insoluble silver halide of the emulsion is eventually made soluble so it will wash out in the fixer and subsequent wash. A certain number of free thiosulfate ions are necessary to completely react the halide into a soluble form. If the fixer is partially exhausted there may not be enough free ions available to carry out this reaction no matter how long fixing is continued.
The capacity of a single bath to fix out to archival levels is quite limited. It is strongly recommended that a two bath fixing system be adopted. Film and paper are fixed for half the total time in each bath. The bulk of silver comes out in the first bath leaving the second one fairly fresh. This method will usually result in complete fixing and is significantly more economical than a single bath because its capacity is from 4 to 10 times as great.
A sulfite wash aid will also aid in releasing some reaction products which will otherwise be bound to the gelatin or to the image silver. Since sensitizing dye can be bound to these materials the treatment will also get rid of the dye.
Ammonium thiosulfate fixer has a greater capacity than Sodium Thiosulfate fixer but is still limited in capacity so even when using it a two bath system is wise. Ammonium thiosulfate (rapid fixer) has a higher tollerance for Iodide ions from film than Sodium fixer but complete, archival, fixing can be done using either type.
The old rule of thumb still holds, measure the clearing time and fix for double this time. The old rule is also that the fixer should be discarded when clearing time doubles. This might be OK for a two bath system but IMO is well beyond the safe limit for a single bath.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 

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