[pure-silver] Re: Chemical Fogging in Reversal Processing


Hi Richard,

You're right that the thiourea in this case is not strictly a "foggant" (that renders the silver developable). It's really just a toner, a direct replacement for sodium sulfide (but odourless and non-volatile). I use the thiourea toning as a final step in the reversal process, without a fixing bath (the thiourea is very efficient at converting residual silver halide to silver sulfide).

Thiourea is very convenient if brown-coloured slides are acceptable (e.g., for scanning). As Gerald Koch mentioned, if you want neutral black, you'll have to choose another method (light reversal or SnCl2).

Jordan

Richard Knoppow wrote:

Thiourea is probably being used as a redeveloper in place of
sulfide. The result is a sepia image of sulfide. Because either
Thiourea or Sulfide works directly on the halide it does not need a
fogging exposure or chemical fogging. It also eliminates the need
for a final fixing step because all of the halide is converted to
sulfide whereas there is always a small redidual of halide which is
not made developable by flashing. I am uncertain about the effect
of a chemical fogging agent but would guess its the same as the use
of Sulfide, that is, no final fixing step is necessary.



-- Richard Knoppow dickburk@xxxxxxxxxxxxx Los Angeles, CA, USA =============================================================================================================
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Jordan Wosnick
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