[pure-silver] Re: alkaline stop bath

From: "Garry Lewis" <garrylewis747@xxxxxxxxxxxxx>
Subject: [pure-silver] alkaline stop bath
Date: Fri, 31 Dec 2004 15:28:22 -0600

> Several lists have this topic going.

I'm glad it's not going here.

Despite the name "stop bath" the most important role of the stop bath
is to prevent developer carryover to the fixing bath, where dichroic
fog may form, especially in alkaline fixer. In color processing,
dichroic fog is not a problem at all (will be removed together with
other silver), so alkaline fixer is not a problem. In machine
processing of b&w material, stop stage is often replaced with
mechanical squeegee, and the material go straight to acid fixing
bath. That would do. Water rinse is also fine. Acid stop is fine,
though I prefer pH of 5.0 to 5.2.

If the material is not very well hardened like infrared films, the
emulsion can be hardened before development, or stop bath can contain
sulfate as an antiswelling agent. Or gently use cold water rise.

For fixing bath, I prefer pH of 7 to 8, where pH is on the favorable
side for rapid washing, yet the pH is too low for significant
development by the carryover developer.

Other than that, I don't think there's any need or benefit to change
the processing procedure.

Incidentally, isoelectric point of lime treated ossein gelatin is
typically pH of 4.8 to 5.0. Before mid 1950's, this was probably the
only kind of gelatin used in commercial material. HOWEVER, there are
chemically modified gelatins in common photographic use. Some highly
purified acid or enzyme treated pigskin gelatin is also said to be
used in commercial products. Moreover, commercial products blend
synthetic polymers such as polyacrylamides, latex polymers, colloidal
silica, etc. and I am not so sure of giving so much meaning to the
value 5.0 any more. Indeed, the gelatin I use for my plate emulsion is
a mixture of two gelatins, one of which has isoelectric point near pH
of 3.

--
Ryuji Suzuki
"Keep a good head and always carry a light camera."
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