There is more difference. Modern emulsions, even paper
emulsions, contain significant amounts of silver iodide. This is
especially true of high speed film. Silver iodide is notoriously
difficult to fix out. Ammonium thiosulfate is much more effective
for silver iodide. Secondly the bleaching effect is partly the
result of the acid in acid fixer. Sodium thiosulfate has the same
effect but considerably slower. If a non hardening fixer, which
does not need to be acid or is only slightly acid, the bleaching
effect does not take place.
In my opinion alkaline fixing baths are not necessary and can
cause staining from carried over developer. Thiosulfate of either
kind will fix regardless of the pH of the bath. Acid fixer are
acid partly so that the hardening agents will be effective and
partly to neutralize carried over developer to prevent continuing
development in the fixing bath. Staining from carried over
developer can be reduced by including enough sulfite in the
fixing bath. In acid fixers the sulfite acts to prevent
decomposition of the thiosulfate by the acid and also to prevent
stainging. Of course, the acid immediately inactivates the
developer. An acid stop bath can be used with a non-hardening
fixing bath. A non hardening fixing bath can be made much less
acid than a hardening bath because there is not hardener to
require it. A mild acidity will be sufficient to stop any
continuation of developing in the fixer. For that reason a fixing
bath containing sulfite in about the same amount as in a acid
hardening bath is a good idea, the slight acidity can be achieved
with sodium metabisulfite
Note that the pH range that causes electronic binding is
anything more acid than the characteristic of the gelatin. No
binding takes place near neutral and that is where a buffered
sulfite wash aid is targeted. The sulfite is the main ingredient
for accelerating washing. It acts as an ion exchange medium for
the thiosulfate and fixer reaction products and helps to make
incompletely fixed silver complexes soluble. An alkaline bath,
say in mild sodium carbonate, will eliminate the electronic
binding but does not have the ion exchange property.
Ilford recommended a strong solution of ammonium thiosulfate
because it fixed out fast enough to prevent much absorption of
the fixer by the fibers of uncoated paper. The problem with the
Ilford method is that many papers will not fix out in such a
brief time. For those a conventional fixation followed by a
sulfite wash aid and a fairly long wash is necessary.
Unfortunately, once anything is absorbed by the fibers of
uncoated paper it is bound by mechanical forces so continuing
washing in running water is about the only way to remove it
although some methods of mechanical squeezing have been tried.
On 11/19/2019 5:47 PM, Randy Stewart wrote:
For all practical purposes, the only difference between the two types of fixer is that sodium fixer acts more slowly, and for a given volume, it probably has less capacity. On the other hand, ammonium fixer acts quickly enough that you can start bleaching your film or print if you seriously over fix with it. I have mixed all of my own chemicals from reagents for about 40 years, and I have used both types of fixer. Sodium fixer is simpler chemically and cheap to mix from scratch if you are so inclined. When I started into DIY in the darkroom, the formula for fixer was "throw a handful of sodium thiosulphate in a tray of water". A 10 lb. paper bag of the stuff cost $5. (not so today!) The basic difference between fixers is "hardening" versus "non-hardening". In my distant youth, film emulsions were soft enough that they could be easily stratched if mishandled, so one component of most fixers would harden the emulsion to resist such. In the 1960s, film was improved to not require this treatment, and non-hardening fixers became popular, as they wash out of film or paper much faster. (I like to forget the one hour tray washes of double weight fiber-based prints with continuous running water.) If you go with a sodium fixer, the best practice is to use two tray baths for prints, usually 5 minutes in the first and 5 minutes in the second. The first bath does most of the chemical work and the 2nd bath insures a more fresh solution for complete results. After the first bath is exhausted, you toss it, move bath #2 up to #1, and make a fresh #2, and so on. Good luck.
----- Original Message -----
*From:* Miha Golobic <mailto:miha.golobic@xxxxxxxxx>
*To:* pure-silver@xxxxxxxxxxxxx
<mailto:pure-silver@xxxxxxxxxxxxx>
*Sent:* Wednesday, November 06, 2019 3:27 AM
*Subject:* [pure-silver] Introduction and Sodium fixer question
Greetings from Slovenia. I have been reading pure-silver
archives for quite some years now and I never cease to be
amazed by the wealth of knowledge available over there.
As for me, I've developing film since I was 15, which is over
25 years now, setting up my first darkroom in the bathroom
when I was 17, now having a permanent one in the basement of
my house. For the last 5 years or so I've been practicing LF
(4x5) photography as well which meant I had to source out an
appropriate enlarger. I was lucky enough to find a mint Durst
locally from a private seller. It sits happily next to my
Kaiser which can only go up to 6x6.
Recently I have been playing with the idea of using powdered
chemicals only. Developer is of no concern to me, however
fixer is another matter (so they say!). Fot too many times a
message came across from many experts and "experts"
advocating that with today's materials ammonium based fixer
is a must and that sodium fixer can produce less than
permanent images. Is that really the case? Kodak, Foma,
Tetenal (as well as Formulary, etc) offer such without
reservations I would really like to know if anyone has tested
modern papers fixed with sodium fixer for residual silver?
Cheers! Miha
