From: Sandy King (sanking@clemson.edu)
Date: 12/12/02-07:14:07 PM Z
Scot Wainer wrote:
>Having been primarily a silver gelatin printer and concerned with
>permenance, I began using Ammonium Thiosulfate based fixing baths for prints
>after reading an article on the subject by Ilford. They recommended using a
>film strength fixer for one minute to reduce the amount fixer absorbed by
>fiber paper. Since I now work with a number of alt. processes requiring
>fixing, recent readings have brought several questions to mind:
>
>1. What is the difference (strength wise) between Ammonium and Sodium
>Thiosulfate? I remember reading somewhere (The Darkroom Cookbook?) that the
>ammonium is about 40% stronger - is that about right?
You may get more definitive answers on this but I will offer some
observations based on my expereinces.
Ammonium thiosulfate is widely available as a 40% solution. To make a
fixer folks usually make a fairly concentrated stock and then dilute
it for a working solution. For a liter of stock, for example, I mix
800ml of the 40% ammonium thisulfate with 5g of sodium metaborate and
20g of sodium sulfite, and top off to a liter. I dilute this 1:4 for
a working solution.
Sodium thiosulfate comes in the form of crystals that you mix with
water to form whatever strength solution you need. For example, if
you want a 5% solution, which is about what I have used for salted
paper, VDB and kallitype, you simply mix 50g of sodium thiosfulate
with a liter of water. A 10% solution would consist of 100g of sodium
thiosulfate in a liter of water.
>
>2. Most reference materials list fixers based on Sodium Thiosulfate. Would
>significant processing changes need to be made if I subsituted Ammonium
>Thiosulfate?
Faster fixing times.
>
>3. Would the use of Ammonium Thiosulfate cause prints bleach if the fixer
>was made to the same strength as the one using Sodium Thiosulfate?
Possibly. At the same strength the ammonium thiosulfate fixer works
much more rapidly.
Frankly, I avoid the issue by using both types, an ammonium
thiosulfate fixer for films and modern silver papers, and a sodium
thiosulfate fixer for alternative work.
Sandy King
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