Dan, there are several classes of silver photographic bleaches, with different actions on the silver in the film. (**) For a reversal process, a bleach is needed that converts the exposed silver to a form that is soluble in the bleach. For potassium dichromate + sulfuric acid, the silver is oxidized to silver sulfate.
So in the reversal processing sequence, we have:
1) Unexposed film, no image -- unexposed silver halogen salts.
2) Exposed film, latent image -- exposed and unexposed silver salts.
3) Developed film, silver positive image -- metallic silver and unexposed silver salts.
In normal processing, we'd fix now, removing the unexposed silver salts, leaving behind only a metallic silver image. Reversal, however, continues on like so:
Potassium ferricyanide is used as a re-halogenating bleach, where the silver is converted back to a halogen form (e.g. with bromide to achieve Ag -> AgBr), which must then be removed by fixing. This creates a problem during steps 6 and 7 above, since the silver salts from the bleaching would result in nasty image fog.
(**) I'm going from memory on the bleach info, hopefully someone will correct if I've misstated anything. Mean library () took back Mason's Photographic Processing Chemistry, and my copy of Haist's Modern Photographic Processing hasn't arrived yet.
For potassium dichromate + sulfuric acid, the silver is
oxidized to silver sulfate.
Potassium ferricyanide is used as a re-halogenating bleach,
...hopefully someone will correct if I've misstated anything.
I mentioned only potassium ferricyanide. No mention of
re-halogenation was made. Likely ferricyanide is not used
because silver ferrocyanide is highly insoluble while silver
sulfate is very slightly soluble.
After posing the question I pondered some. So the
reversal process produces silver sulfate. Dan
Right; I'm rather new to photographic chemistry, so I kind of ran off with the common use of potassium ferricyanide as in the formulation for sepia toner... blasting right past using it alone as a bleach. I note that this article on blue toning by Liam Lawless confirms that the silver ferrocyanide can be removed by fixing, something I'd only just assumed earlier.
pot ferri IS a re-halogenating bleach- a completely different type of chemical action than what is needed for this process- that's what he was trying to explain to you my man-
I note that
this article on blue toning by Liam Lawless confirms that the
silver ferrocyanide can be removed by fixing, something I'd only
just assumed earlier.
You've made clear the two bleach methods. Bleaching for removal
of the silver image does call for the silver's conversion to a soluble
form. Checking for solubilities I find the permanganate to be
9 grams per liter and the sulfate 8.3 grams.
A post fix image bleaching with potassium ferricyanide ALONE
leaves an image of silver ferrocyanide. Following toner or
redevelopment results will likely differ from those of a
halogenated image. I'm going to check on that. Dan
Adv Reply
Originally Posted by payral
) took back Mason's Photographic Processing Chemistry, and my copy of Haist's Modern Photographic Processing hasn't arrived yet.
