[LargeFormat] Removing black spots from prints

[Richard Knoppow]

----- Original Message ----- 
From: "Wilkes, Don MSER:EX" <Don.Wilkes@gems9.gov.bc.ca>
To: <largeformat@f32.net>
Sent: Thursday, October 16, 2003 3:39 PM
Subject: RE: [LargeFormat] Removing black spots from prints


>
> >   For removing black spots from prints the best bleach
is an
> > Iodine bleach.
>
> Thanks for the formula, Richard -- I may give this a shot,
even if just to
> put another trick in my bag for future reference.  A
couple of questions,
> though, as photochemistry is far from my strong suit:
>
> - Is isopropyl alcohol the same as methyl?
> - Iodine in crystal form?
> - Is thiocarbamide hard to come by? Would my local (large)
pharmacy have it?
>
>
>
>
> > The makers of SpotTone make a product called Spot Off
which
> > is evidently an Iodine bleach.
>
> I haven't noticed this on the shelf, but will certainly
have a look!
>
> Cheers,
> \donw
>
  I think any kind of alcohol will do. Crystaline Iodine
will do, I think you can still get it at drugstores. For
that matter tincture of iodine would work provided its just
iodine and alcohol. Since the stuff is sold for medicinal
purposes it must say what is in it.
  I don't think Thiocarbamide is too readily available. You
would probably have to get it from a photo chemical
supplier.
  In any case, I think the Iodine will work with rapid
fixer. The problem with Iodine bleach is that it converts
the silver to silver iodide, which is difficult to fix out.
The silver iodide in films like T-Max and Tri-X is what
makes them so hard to fix.
  The advantage of Iodine is that it bleaches very quickly
and completely. Farmer's will do the same job but tends to
be very slow. Farmer's has a reputation for leaving yellow
stains but I think that is because people leave out the step
of refixing the film or paper after finishing bleaching. The
weak hypo used with Farmer's may not remove all the halide
left by the bleach. Eventually it will decompose and stain.
  Iodine needs good fixing afterward too but the old Iodine
bleach used Potassium cyanide as the fixer. Cyanide is a
very effective fixer, thats why it was used for the old
wet-plate process, which had a sensitive coating of mostly
silver iodide. It was discontinued there for the same reason
its not popular for bleaching, namely its damned dangerous
to handle. In any case, ammonium thiosulfate fixer (rapid
fixer) works fine so you don't need the cyanide.
  It seems to me that there was a long thread about Iodine
bleach in the Pure-silver list not long ago. Unfortunately,
there is no searchable archive for it. I'll see what I can
find.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com