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


-----Original Message-----
>From: John Banister <jbanister@xxxxxxxxxxx>
>Sent: Aug 14, 2006 10:46 PM
>To: pure-silver@xxxxxxxxxxxxx
>Subject: [pure-silver] Re: Chemical Fogging in Reversal Processing
>
>
>Thanks very much for your words.  Thiourea sounds like an interesting avenue
>to explore.  It also sounds like I can hope to approach B&W reversal 
>without strange
>unexpected pitfalls if I do a little studying and experimentation, and that's
>comforting to know.
>
>John
>
> > Hi John,
> >
> > I don't think that the science of chemical fogging for B&W 
>reversal has advanced too
> > much since Grant Haist wrote his book, but there may be 
>"variations on a theme".
> >
> > For what it's worth, I do chemical fogging with an alkaline 
>thiourea bath (the "odourless"
> > version of an alkaline sodium sulfide bath). It works quite well. 
>The image tone depends
> > (to some extent) on the alkalinity of the toning bath as well as 
>on the grain structure of
> > the film (in my hands, TMX gives a sickly yellow colour, while 
>Pan-F+ gives nice
> > chocolate brown tones).
> >
> > Jordan

   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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