Subject: Why two step fixing is a Good Thing
While looking at the thread entitled Again
a purple tri-x question
I was led to an interesting article about two stage
fixing ...And I Highly Recommend reading it.
"taco*boy" <ses_2@snet.net> wrote: >Is fixer
OK to use over and over and over? I've heard that> fixer will turn
fixer purple when depleted. I have never seen this. >Am I
wasting fixer I throw it away and it's still clear?
The best option is to use two fixer bath fixation. The rationale
is outlined below, taken from one of my old posts.
Post Development Processing
©Copyright 1998 by Dr. Michael J. Gudzinowicz
The basis of fixation and accompanying problems aren't treated in
depth in most texts. This oversight often leads to postponed
"accidents" whenever people are tempted by a sense of false
economy to save time or materials. An introduction to the underlying
chemistry should help to define a more critical approach to film and
paper preservation, which doesn't rely on rumor and the advertising
literature. The following notes were taken from Grant Haist's
"Modern Photographic Processing, Vol.1" (Wiley, 1979),
"The Theory of the Photographic Process" edited by T. H.
James (3rd & 4th ed., 1st & 2nd edited by C. E. K. Mees;
Macmillan, 1966 (3rd)), "Ilford Monochrome Darkroom
Practice" by Jack Coote, and the research and technical
literature.
Fixation:
The common notion is that the fixer removes undeveloped silver halide
by a simple reaction involving the replacement of the halide by
thiosulfate to form a soluble silver complex, and then if the film or
paper looks or tests "clear", the only problem is
fixer removal. Unfortunately, this is not the case. When a film is
"fixed", a number of complexes are formed between silver
and thiosulfate, and all are in dynamic equilibrium. In addition, the
accumulation of halide during fixation reduces fixer capacity
with use when free silver and halide levels approach their limits of
free, non-complexed solubility.
A simple table outlining the dissolution of silver in fixer, and
equilibria with fixer is outlined below.
The silver halide may dissociate to a very small degree in aqueous
solutions, and the thiosulfate anion will form a 1:1 complex with the
silver cation (Rxn 1) or the thiosulfate may react directly with the
solid silver halide crystal (Rxn 1). In either case, the first
complex (I) is >very insoluble< and remains tightly adsorbed to
the surface of the solid silver halide.
A second thiosulfate anion may react with the first complex (I), to
form a soluble product (II) with a silver to thiosulfate ratio of 1:2
(Rxn 2); and then if "free" thiosulfate concentrations are
high, a third thiosulfate anion may react with the soluble second
complex (II), creating a third complex (III) with one atom of silver
and three molecules of thiosulfate which is quite soluble (Rxn 3).
Sequence of Complex Formation:
Note: Charge of ions is in () brackets; the # of kinds molecules
[kind of molecule]# in the complex follows brackets; TS is
thiosulfate (hypo) anion; Ag is silver; Br is bromide. <->
shows equilibrium reactions.
Rxn 1) Ag (+) + TS (-2) <-> AgTS (-)
AgTS (-) is the first complex (I) called monoargentomonothiosulate
since it contains one silver cation and one hypo anion; it is
insoluble and remains adsorbed to the crystal as it forms.
Rxn 2) AgTS (-) + TS (-2) <-> Ag[TS]2 (-3) (aq)
Ag[TS]2 (-3) (aq) is the second complex (II) complex formed by the
addition of another thiosulfate anion to monoargentomonothiosulate to
form monoargentodithiosulfate. The second complex is soluble in
aqueous solutions and is removed from the emulsion by diffusion.
Rxn 3) Ag[TS]2 (-3) (aq) + TS (-2) <-> Ag[TS]3 (-5) (aq)
Ag[TS]3 (-5) (aq) is the third complex (III) called
monoargentotrithiosulfate since it has three thiosulfate anions
complexed with one silver cation. It very soluble in aqueous
solutions.
In solution, these reactions are reversible, so all complexes are
present, and a small amount of Ag+ cation is not complexed in
solution.
The following equilibria also occur:
Rxn 4) Ag (+) (aq) + TS (-2) <-> AgTS(-) (aq)
where all components are in solution (aq) and adsorption doesn't
occur.
Rxn 5) AgTS (-) (aq) + TS (-2) <-> Ag[TS]2 (-3) (aq)
where the monoargentomonothiosulfate is in solution and not adsorbed.
However, in solution the concentration of monoargentomonothiosulfate
in this and the preceeding aqueous reactions are very low since it's
nearly insoluble.
Rxn 6) Ag[TS]2 (-3) (aq) + TS (-2) <-> Ag[TS]3 (-5) (aq)
where both the monoargentodithiosulfate and monoargentotrithiosulfate
complexes are in solution.
As more silver is put into solution with fixer use, more
complexes II & III are formed, and the level of the less soluble
1:1 complex (I) and free silver ion are also increased. After a few
uses of fresh fixer , the less soluble complex (I) and silver
halide are left in the paper or film at low, but destructive levels,
although the film appears to clear.
Also, thiosulfate is adsorbed to developed silver grains in papers
(iodide tends to displace it from films). Residual complex I and
residual thiosulfate adsorbed to developed silver grains are
converted to trithionite and higher thionites in a few days, and then
degrade and react with silver giving stains (sulfiding) and fog.
(Brown silver sulfide is seen after bleaching the silver grains, and
is proportional to the developed silver.)
With progressive use of the fixer, levels of bromide rise, as well as
chloride from papers and iodide from films. Silver halides have very
low solubility, and as the level of bromide or iodide rises, it forms
silver halide crystals in solution and the fixer will no longer
dissolve silver halide. A number of complexes and equilibria occur
with each halide and mixtures. On a relative basis, silver chloride
is more soluble than bromide and has little effect on fixer capacity;
silver bromide is less soluble and determines fixer activity to a
significant degree, unless films containing low levels of iodide are
fixed, in which case fixer capacity is reduced significantly
due to silver iodide insolubility (a problem with T-Max films,
treated later). In instances where silver is removed to
"regenerate" fixers, iodide accumulation may interfere.
Also, in two - bath fixation which follows, carry-over ocurs,
which requires periodic replacement of both baths.
The only way to ensure that little silver bromide (AgBr) or the
insoluble first complex is left in the paper, is to use fresh
fixer with little accumulated silver and halide, and an excess of
non-complexed free thiosulfate to remove it. This approach to
archival fixing when used with one fixer bath is fairly wasteful,
though effective.
Rather than using one fixer bath, the same results can be obtained
with two baths, and the capacity of the fixer is far greater.
Essentially, the first bath removes the bulk of the silver and
halide, and leaves traces of silver halides and the first insoluble
complex in the emulsion and paper. The amount when carried over to a
second fixer bath is insignificant in comparison to the amount of
free thiosulfate, so the second bath always acts as
"fresh" fixer with high non-complexed thiosulfate levels
to react with the small amounts of silver halide and less soluble
complexes to speed their complete removal from the emulsion.
More on Fixing - One and Two fixer Bath Fixation:
Grant Haist, the former director of research at Kodak, cites the
following maximal permissible values for one-fixerbath film and
paper fixers for commercial and archival processing:
One-fixer bath fixation: Commercial Archival
Film:
Max. Ag conc.: 1.5 g/l 0.2 g/l
Max rolls/gal: 25 rolls/gal 2 rolls/gal
Non-image Ag in film: 0.01 mg/in^2 0
Paper:
Max. Ag conc.: 0.3 g/l 0.05 g/l
Max. sheets/gal: 30 8x10 5 8x10
Non-image Ag in paper: 0.005 mg/in^2 0
Essentially, as fixer total silver (free and complexed) and halide
concentrations rise, the fixer's ability to remove all of the
silver from the paper diminishes markedly, as indicated by the very
limited capacity of one-fixer bath to remove silver to archival
levels.
The solution to the limited capacity is to use a fresh second
fixer bath to maintain a very low total silver level, with a
water rinse between the first and second baths to minimize
fixer/silver carry-over. Some older texts even suggest a fresh third
fixer bath .
Two bath fixation: Commercial Archival
Film:
fixer Bath 1:
Max. Ag conc.: 6 g/l 3.5 g/l
Max. rolls/gal: 60-70 40
fixer Bath 2:
Max. Ag conc.: 0 .5-1.5 g/l 0.02 g/l
after 60-70 after 40
Non-image Ag in film: 0.01 mg/in^2 0
Paper:
fixer Bath 1:
Max. Ag conc.: 2 g/l 0.8 g/l
Max. sheets/gal: 200 8x10 70 8x10
fixer Bath 2:
Max. Ag conc.: 0.3 g/l 0.05 g/l
after 200 after 70
Non-image Ag in paper: 0.005 mg/in^2 0
The first fixer gets rid of most of the silver, and the second
maintains a very low silver concentration and relatively high free
thiosulfate concentration to remove the remainder of the insoluble
complexes and non-image silver present in the emulsion after the
first fixation.
The first fixer bath is used for the maximum number of sheets or
rolls indicated, and then discarded after silver recovery.
The second fixer bath is substituted for the first, and a fresh
second bath is prepared.
After 5 cycles (substitutions), or one week if continuously exposed
to air in tanks, both baths are replaced. Compare the capacity for
commercial or archival standards using fixer two baths to that for
one. Two bath fixation is far more economical than using
one fixer bath , and avoids the temptation to over-use fixer which
results in under-fixation and difficult removal of insoluble
complexes which destroy prints and film.
Films:
With films, the fixation time in the first fixer should be at
least twice the clearing time... likewise for the second bath .
The clearing time should be checked often if that approach is used,
however, Kodak recommends 5-10 minute fixation with non-rapid fixers
and most films.
Since there is _no_ danger in longer fixing times, incorporating a
five minute minimum fix in each fixer bath into a
"normal" development procedure may avoid problems and
provide some security.
Agitation should be constant to remove fixer from the surface
of the film to facilitate diffusion, however, increased agitation
never can replace adequate fixing time or counteract the cumulative
effects of re-using fixer .
With rapid fixers, there is little "danger" of bleaching
film with 5-10 minute fixation. Also, if standard procedures are
used, any minimal bleaching would never be noticed, since it would be
incorporated into tests for contrast and development time.
With T-Max films, Kodak recommends longer times. For instance, they
suggest that it is "safe" to check clearing at five minutes
with standard fixers or three minutes with rapid fixers, and that
total fixing time should be twice the clearing time. (Kodak's
"advice" on T-Max varies from simplistic on 35 mm film
boxes, to warnings in detailed technical literature, not only on
times, but also on fixer replenishment rates for processors.)
T-Max Films:
With some films, such as Kodak's T-Max series, fixer capacity
is reduced to one-half of what one normally expect, and fixing times
are extended to twice the usual time, since silver iodide present in
the "high tech" emulsions is resistant to fixation, and
exceedingly insoluble.
In Kodak publication F-32 on T-Max films, Kodak indicates that a
magenta stain may be left in the emulsion with inadequate fixing, and
recommends further fixing with fresh fixer to remedy the problem. The
magenta sensitizing dye is adsorbed to the silver halide (EKC
statement - not speculation) and when the halide is fully dissolved,
the dye is removed.
In some instances, the dye can be removed by treatment with hypo-
clear, which usually contains sulfite or high salt concentrations
which can act as weak fixers in addition to displacing hypo, or with
prolonged water washes.
The "stain" problem isn't whether it will interfere with
variable contrast paper filtration or not, but its indication that
the film isn't fixed properly.
Papers:
For paper fixation, do not use fixer which has been used for film. It
is difficult to track capacity accurately (see table above... silver
capacity differs for film and paper), fixer dilutions vary between
paper and film fixers, and the "sudden" accumulation of
iodide after developing films may greatly prolong paper fixation or
leave insoluble silver iodide behind.
The clearing time for papers may be determined experimentally or by
manufacturer recommendation (for Ilford, see below). Fixing times for
most fiber papers is on the order of five minutes for each bath ,
with an intervening water rinse and storage in water. To save time,
prints can be fixed in the first fixer bath, rinsed and held in
water, then fixed in the second fixer bath at the end of a session.
Long contact with fixer can cause problems if fixer enters the
paper fibers (not between them). Papers and fixers vary, and it is
best to use at least the minimum time recommended by the paper
manufacturer.
Kodak recommends 10 min for fiber base and 2 min for RC in one
bath , or half that time for each of fixer two baths. The RC time
is optimistic, though five minutes per fixer bath is reasonable for
fiber papers. Prolonged contact with rapid fixers will slowly bleach
an image or cause uneven bleaching if prints remain in rapid fix
without agitation for prolonged times (hours).
In any case, paper and film should be promptly removed from the
second fixer , rinsed, and placed in a water fixer bath until
treated with a hypo clearing solution to displace free thiosulfate.
Rapid fixer:
Rapid fix has the advantage of a shorter contact time, and that may
minimize the penetration or degradation of fixer in the paper's
fibers.
Also, the useful capacity of rapid fixers is fairly high... 10-15 g/l
silver vs. 6g/l for films or 2 g/l for papers using regular fixers
(James; Haist table above for fixer bath 1 of a fixer two fixer
bath sequence).
However, there is little data to extrapolate those numbers into
increased capacity _without risk_ of problems. In that regard,
Kodak's recommendation for capacity of rapid fix and other fixers is
nearly the same (100-120 sheets or rolls), which is optimistic for
one fixer bath commercial processing. The only advantage of rapid
fix with film is decreased processing time and perhaps, decreased
rinse time.
Hardeners:
For film, a hardening fixer is often preferred to minimize any
emulsion damage in handling and to avoid reticulation. Very alkaline
developers can remove the manufacturer's hardeners. If the emulsion
is loaded with salts such as fixer , and placed in plain water, the
emulsion can swell markedly due to water uptake in the emulsion due
to osmotic pressure. If the water is warm, the emulsion may ripple on
the surface, giving the alligator pattern associated with
reticulation.
Non-hardening fixers are often preferred for development of the
stain with pyro developers.
For paper, rapid fix without hardener is often preferred, and gives
better results with toning. Paper curl seems to be minimized and
there is less danger of "breaking" the emulsion when prints
are flattened or mounted. Also, the avoidance of alum may reduce
silver complexes bound in the emulsion which can speed wash times.
If one wishes to remove hardener for toning, the following treatments
may be used: household ammonia diluted 1:10 (0.3%) for 2 min with 45
min wash or 5 min in 2% solution of Kodalk or sodium carbonate, then
wash.
An exception to the rapid fix recomendation is Agfa Portriga paper
which has a soft emulsion. If it is sepia toned (basic toner removes
hardeners), emulsion damage may occur if the paper is heat dried.
Therefore it should be hardened after toning. If fibers from a canvas
mat drier or blotters stick to the emulsion, you may have that
problem even with other papers. Kodak makes a separate hardener,
however, I find the hardener offered by Sprint to be effective and
economical. I also use it with their rapid fixer .
Common Fixer Tests:
Tests for fixer exhaustion which rely on precipitation of silver
iodide aren't sensitive enough to determine whether a fixer is in the
"archival" range or "commercial" range, and in
some cases, whether the fixer is near exhaustion. Relying on that
type of test with one-fixer bath fixation invites future disaster.
Likewise, tests of wash water for fixer can't detect insoluble
complexes of fixer in the paper or unfixed silver halide in the
emulsion. Sulfide or selenium toner tests for silver in paper don't
measure the insoluble complex (I) or degradation complexes, nor does
silver nitrate react with those complexes. Some tests may be better
than none and any warnings should be heeded, but in this instance,
they may give a false sense of security if the results are false
negatives.
Follow the tables given by Haist (above), and reduce capacities by
1/2 for TMax and other high tech emulsions.
Hypo-clear and Eliminator:
Usually, the removal of fixer and its complexes from film is fairly
straightforward. With or without hypo-clear, the hypo and complexes
diffuse out of the emulsion with washing, and aren't tightly bound.
The potassium alum used as a hardener may complex small amounts of
hypo and silver complexes, but that doesn't seem to occur with chrome
alum. However, chrome alum isn't used in commercial products, and
probably should be avoided for environmental reasons and staining
problems.
With papers, additional problems can arise due to the nature of the
support. Some of the hypo and complexes are adsorbed to the baryta
layer, fixer always penetrates the interstices between fibers of
fiber-base papers, and with prolonged fixing (over 15-30 minutes),
hypo and complexes can enter the fiber "cells", from which
it is very difficult to remove. However, this does not occur with
reasonable fixing times of 5 to 10 minutes.
The hypoclearing properties of saline solutions was discovered by Dr.
Bannow in 1889, but he used a 10% sodium chloride solution (100,000
ppm) with rinses with moderate success. In 1903, Dr. Bayssellanee
found that sea water was more effective, and used 30,000 ppm sea salt
with 1 hour soaks followed by washes to remove salt (so much for the
"US Navy discovery" myth).
Although it was noted that film and paper washed in sea water (3%
salts of which 2.6% is sodium chloride) lost fixer much more rapidly
than washing in tap water (65% faster for film; 80-90% faster for
paper), using table salt or sea salt as a clearing agent isn't a good
idea. Removal of chloride was required, since chloride resulted in
faster degradation of any residual hypo in the emulsion (note: the
seawater use was for rapid processing and conservation of fresh
water, not archival stability).
Subsequently other hypo-clearing agents were examined, and polyvalent
anions were found to be most effective in displacing silver. Of the
group, 2% sodium sulfite buffered to pH 7.0 was found to be most
effective. EDTA or other chelating agents may be included to remove
calcium sulfite which can precipitate in/on emulsions.
Although some suppliers indicate that a short soak in hypo-clear (1
min) after fixing followed by a short wash time is adequate, rinsing
films and papers before a 10 minute hypoclearing agent treatment
works better, and prolongs hypo-clear life.
Hypo-Eliminators
Hypo-eliminators rely on the use of an oxidant such as peroxide to
rapidly oxidize any residual hypo complexes in the film, preventing
the reaction with image silver which would occur if they were
permitted to degrade. Kodak HE-1 is a dilute mixture of peroxide and
ammonia made up when used (never kept in an enclosed container) which
oxidizes such complexes. However, it has been noted that oxidation is
incomplete unless bromide is added to speed the reactions.
In the "Craft of Photography" Vestal mentions that some
studies indicated that HE-1 treatment wasn't as "archival"
as supposed, and that a small amount of thiosulfate might stabilize
the image. The point was clarified at a subsequent conference
reported by Vestal. The topic is considered below (RC papers and
stability).
The current concensus seems to indicate that hypo-eliminators should
not be used unless the image is subsequently toned with selenium or
sulfur (sepia), or treated with Agfa's Sistan (thiocyanate).
The Ilford Story:
Coinciding with the introduction of Galerie paper, Ilford decided to
introduce a quick 20 minute archival processing procedure. After
development and stop, paper was to be fixed for 30 seconds in film
strength rapid fixer , followed by a five minute wash, 10 minute soak
in a wash aid, and another 5 minute wash.
Later, the recommended fixing time was extended to 1 minute with
little fanfare. If the wash aid isn't used, Ilford recommends a wash
time of at least one hour. The precautions mentioned include good
agitation, and use of fixer which hasn't approached its capacity. The
"theory" is that silver removal from the emulsion is faster
than accumulation in paper interstices, so supposedly little
accumulation occurs.
There are some problems.
The procedure does _not_ work with Kodak papers and others which
require longer fixing times. (Elite is a prime example.)
Also, the retention of complex I in the paper isn't addressed or
tested for, and complete non-image silver removal isn't checked.
Ilford recommends one fixer bath rapid fixation. A capacity of 40
sheets of 8x10 paper per qt (160 per gal) is suggested when a wash
aid is used with a single fixing fixer bath or when a fixer two
fixer bath system is used (which negates the short fixing time
rationale). However, the capacity is reduced to 10 sheets per qt (40
per gal) using a regular wash and single fixer bath fix.
That disparity in capacities suggests that Ilford is relying on the
wash aid to extend so-called fixer capacity when a single fixer
bath is used. The implication is that for the 30 sheet difference
between use and non-use of wash aid, significant insoluble complexes
are carried over (see the Haist table). Note that Ilford's capacities
for single fixer bath fixing are greater than Haist's
recommendation for commercial processing (Ilford uses 2 g/l with wash
aid or 0.5 g/l without). Their rapid fix might have a slightly
greater capacity, but it is unlikely that silver levels are as low as
Haist's _archival_ standard when silver levels higher than Haist's
limit for commercial processing are tolerated.
In the current Ilford tech sheet on Galerie, they mention that the
archival treatment with a 20 min wash results in 1/4 the level of
hypo in the paper as a 5-10 minute fix with "normal"
washing. Note that a wash aid wasn't used with the paper fixed 5 to
10 minutes in the "comparison". Remember, wash aids can
increase rate of fixer removal by 80-90% with papers. The comparison
really isn't valid, and it appears that Ilford's only standard for
archival processing is residual reactive (free) hypo levels, and not
the target of absence of insoluble monoargentothiosulfate and silver
halide.
Toning
I don't intend to cover this in any detail, other than to say that
selenium or sepia toning is required to ensure image permance,
especially if prints are displayed. Gelatin always retains some water
which can dissolve atomospheric oxidizing gases such as ozone and
nitrous oxides, which can bleach the image and permit silver
migration.
Toning in selenium (1:3 for color chage to 1:20 for permance with
minimal tone effect), or sepia prevents the problem. Also, it is
claimed that treatment with Agfa's Sistan protects the image, though
I can't find data to support their contention.
There are a number of arcane approaches to selenium toning. If Rapid
Selenium Toner is exposed to hypo in an acidic environment (acid
fixer ), the selenite will be reduced to colloidal or metallic
selenium, and a red stain will result. To avoid that problem, rinse
the paper after fixing, and dilute the Rapid Selenium Toner in a
solution of 2% Kodalk (20 g/l) rather than water or hypoclearing
agent. The dilute toner may be stored in a glass container until
exhausted through use.
Water Spots
Water spots are caused by high salt or particulate concentrations in
wash water, which dry onto/in the emulsion. If you have problems with
water spots, then soak the negatives or RC prints in the following
solution for a few minutes before hanging to dry (don't use a
squeegee - water will run off):
1 gal distilled water
10 ml Photoflo
100 ml 70% isopropanol (rubbing alcohol from a pharmacy - be sure
that it doesn't contain anything else)
The solution can be reused if it's filtered before returning it to
the storage container.
, Mar 08, 2004; 12:50 p.m.
