Fixer Faults, Washing Faults and the Stains They Leave

Most of the faults on this page do not appear in the session that caused them. The negative looked right coming off the line; the print looked right on the drying rack. Six weeks or six years later the highlights have gone yellow, or a metallic sheen has crept in from the edges, and by then the evidence that would have identified the cause has been thrown away.

That is what makes this page different from the four before it. There, you look at the material and read what happened. Here, you mostly cannot — so the work is done by tests run at the time, and by a small number of appearances that are diagnostic when you know what to look for.

Cleared is not fixed

Fixing removes silver halide by turning it into soluble complexes with thiosulfate. It does not do that in one step, and the difference between the steps is this whole page.

⚗The sequence, and where it can stall

Thiosulfate attacks silver bromide progressively, building complexes with more and more thiosulfate ligands per silver atom. The first complexes formed are sparingly soluble and cling to the gelatin; the later ones are soluble and wash out.

AgBr + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Br⁻

The washable complex, which is the point of fixing

Clearing is the disappearance of the visible halide. The milky look goes because the crystals that scattered light are gone. That happens when the first complexes form.

Fixing is carrying the sequence far enough that what remains can be washed out. It takes longer, and there is nothing to watch, because the film looks exactly the same throughout the second half.

That is why the course’s rule — from Part XI — is fix for twice the clearing time, measured on the bath you are actually using. The first half gets you to clear; the second gets you to washable. A film pulled at clearing looks perfect and is not fixed.

The reason paper is worse than film, and needs a different rule, is that paper carries a fibre base that holds solution as well as an emulsion that holds it. On film, everything that has to leave is in one thin gelatin layer. On fibre paper, thiosulfate and complexes soak into the paper itself, from which they leave slowly.

Under-fixing

The signatures on film, in order of how early you can catch them:

• A milky or violet cast in what should be clear film. Kodak’s Z-133 lists it as “Milkiness (most apparent in low-density areas) — pale, white translucence”, against “retained silver halide due to inadequate fixing”, with the corrective action of checking fixer dilution, time and temperature. Undissolved halide crystals scatter light, so the effect is strongest where there is least silver to mask it: the rebate.
• A magenta or pink cast on films that carry sensitising dyes. Z-133 lists “severe magenta (pink) stain” against “retained sensitising dye due to inadequate fixing and/or washing”. Note the and/or — the dye needs both a working fixer and an adequate wash to leave, so this stain does not by itself say which failed. Some films clear their dye slowly by design and a faint residual cast on a properly fixed film is not a fault; a strong, persistent one is.
• A residual-halide test that finds the halide still there.

The signature on paper, and it is the cruel one: a print that yellows within months. Retained halide and retained silver complexes convert over time to yellow-brown silver sulfide, in the highlights first because the highlights are where the halide had least image silver to compete with. The print left the darkroom looking correct.

Why the fault so often appears long after the session. Because nothing about a freshly under-fixed print looks wrong. Fixing removes something invisible and leaves behind something invisible; the consequences are slow chemistry. This is the single strongest argument in the course for running the wash and fixing tests on a schedule rather than on suspicion, because suspicion arrives two years late.

Over-fixing, and why paper is the vulnerable one

Fixer dissolves silver. It is meant to dissolve silver halide, and it is far better at that than at dissolving metallic silver — but “far better” is not “exclusively”, and given enough time it attacks the image.

The signature. Highlight detail lost first, and maximum density falling. On a print, the delicate tones go and the deepest black gets slightly less deep.

Why the highlights go first, and it is a matter of size. In the highlights of a print — the lightest tones — the developed silver is present as the smallest and fewest deposits. A given rate of dissolution takes a larger fraction of a small deposit than of a large one, and a deposit small enough disappears entirely. The blacks, where silver is piled deep, lose a proportionally trivial amount over the same time.

ILFORD name the fault and give it a name of their own. The MULTIGRADE FB CLASSIC sheet states that “there is no benefit in extending fixation beyond the recommended time” and that “some loss of print quality might be seen when long fixing times are given due to image etching”. The optimum permanence section repeats the warning from the other direction: do not exceed the fixer’s capacity and do not extend the fixing time, because both make washing more difficult.

Why film and paper have different times, and they are not close. ILFORD’s own table for RAPID FIXER at 20 °C with fresh fixer:

Material	Dilution	Time
General-purpose film	1+4	2–5 minutes
RC paper	1+4	30 seconds
RC paper	1+9	1 minute
FB paper	1+4	1 minute
FB paper	1+9	2 minutes

Film gets two to five minutes; fibre paper at the same dilution gets one. A worker who fixes prints for the film time is over-fixing them by a factor of two to five, and this is one of the commonest avoidable faults in a home darkroom, because it feels like being careful.

Exhausted fixer

What exhaustion actually is. Not “the fixer is used up” in the sense of a reagent consumed to nothing. ILFORD’s own account: an unreplenished bath “is eventually exhausted by the build up of silver and halides in it and the action of solutions carried over from the preceding baths that can cause some dilution and the pH to be raised”. Three mechanisms at once — product accumulation, dilution, and pH drift — and the first is the one that matters, because dissolved silver is the reaction’s own product and it slows the reaction down.

Why doubling the time is not a repair. Two reasons, and the second is the important one. First, the bath’s chemistry has changed, not merely its concentration. Second, and worse: an overworked bath loads the material with the sparingly soluble complexes, and those are exactly what a wash struggles to remove — so extending time in a tired bath makes the washing problem worse while pretending to solve the fixing one. Part XII’s own two-by-two of test results makes this concrete: a sample that fails the residual-silver test but passes the thiosulfate test has a fixing failure, and “longer washing will not help”.

The practical measure is the clearing time, and the course’s rule is settled in Part XI: measure the clearing time of the bath on a scrap of the same film, discard when it exceeds twice the fresh clearing time, and fix for twice the clearing time of the bath you are using. That rule requires you to have measured the fresh clearing time, which costs thirty seconds once per bottle and cannot be recovered afterwards.

Capacities, from the same sheet, per litre of working-strength ILFORD RAPID FIXER at 1+4: 24 films of 135-36; 80 sheets of 8 × 10 inch RC paper; 40 sheets of 8 × 10 inch fibre. Note the sting in the tail of the paper figures — ILFORD add that they “may be exceeded whenever print stability is not critically important”, which is a manufacturer saying, politely, that the published capacity is a permanence figure rather than a works-at-all figure.

Two-bath fixing is the manufacturer’s own answer, not a refinement invented by printers. ILFORD describe it as “an extremely efficient method”: two baths of equal volume, half the recommended time in each, and when the first reaches capacity it is discarded, the second is promoted, and a fresh second bath is made. The material is therefore always finished in relatively fresh fixer, which is exactly where the second half of the fixing sequence — the half that produces washable complexes — needs to happen.

Dichroic fog

It belongs here rather than with the fogs because it is neither a light event nor a development event. It is colloidal silver deposited in the emulsion out of a solution.

What you see. Kodak’s 1928 primer: a film stained this way “appears yellowish-green” in reflected light and “looks reddish-pink” by transmitted light. Wall’s 1912 dictionary, thirteen years earlier and across an ocean, describes the same thing in the same terms — red or violet by transmission, greenish or yellowish by reflection.

The name describes the two colours, not a cause. Dichroic means two-coloured. It says nothing about where the silver came from, which is why the name has to be paired with the mechanism to be useful.

The conditions. The primer names two: a fixing bath that does not contain acid, or one that “is old and exhausted and contains an excess of dissolved silver salts”. It adds the conditions under which it never occurs — a fresh acid bath, or a film rinsed before fixing, with the bath at 18–21 °C. Wall adds that Lumière and Seyewetz concluded it is due to silver, its formation favoured by a silver-bromide solvent in the developer and by developer present in the fixing bath, “hence the desirability of washing the plate between developing and fixing”.

The mechanism follows from those conditions. A fixer holds dissolved silver as a complex. Carried-over developer is a reducing agent. Where the two meet inside the gelatin, some of that dissolved silver is reduced back to metal before it can diffuse out — physical development, happening in the fixer, in the wrong place. Colloidal silver of that particle size scatters and transmits differently, which is where the two colours come from.

!The discriminator, and it works by naming a treatment that fails

Wall’s sentence is the one to remember: the plate “looks foggy and suggests that it has not been properly fixed, but prolonged immersion in hypo solution does not mend matters”.

Of course it does not. Metallic silver is not silver halide, and a fixer has nothing to say to it.

So: if refixing helps, it was under-fixing. If refixing does nothing, it was dichroic fog. Ten seconds of looking at the film and then through it identifies it; a refix confirms it by failing.

The atlas entry carries the historical removal treatments — the primer’s 0.25 per cent plain permanganate, which attacks finely divided silver and has “no appreciable action on the silver of the image”, and Lumière and Seyewetz’s persulphate route — and records the course’s position, which is that it prescribes neither, because potassium permanganate has no hazard record in this course’s corpus and the persulphate route needs a concentrated mineral acid. The answer here is a thirty-second rinse before the fixer.

Residual thiosulfate and residual silver: two faults, two tests

These are the two invisible residues, they have opposite causes, and telling them apart is the single most useful measurement in this part of the process.

Residual thiosulfate is fixer that did not wash out. Over time it attacks the image silver, producing silver sulfide — the yellowing and the loss of the delicate tones. It is a washing failure.

Residual silver is silver complexes that never became washable, because the bath was tired or the time was short. They convert to yellow-brown silver sulfide too, by a slightly different route and usually faster. It is a fixing failure.

Why the effect is worse on fibre paper than on film or RC. Distance and absorbency. Film has a gelatin layer on an impermeable base; RC paper has a gelatin layer between two impermeable polyethylene skins; fibre paper has a gelatin layer on an absorbent cellulose base that soaks up solution and gives it back reluctantly. ILFORD’s own wash times say it plainly: 2 minutes for RC paper, 60 minutes for fibre, in fresh running water. A factor of thirty.

A washing aid is an ion-exchange step, not a longer wash. ILFORD’s WASHAID is described as “a hypo-eliminator formulated to aid the efficient removal of the thiosulphate by-products of fixation by ion exchange”, and their optimum-permanence sequence for fibre paper is 1 minute fixing, 5 minutes wash, 10 minutes in WASHAID at 1+4, 5 minutes final wash — twenty-one minutes in total against sixty of running water, on far less water. Part XII’s washing lab puts that sequence against sixty minutes of water and measures both.

The two tests are Part XII’s and are not repeated here. Test A, for residual thiosulfate, treats half of each strip in 1 per cent silver nitrate for three minutes and compares it wet against its own untreated half; a yellow-brown tint is thiosulfate. Test B, for residual silver, puts a drop of diluted sodium sulfide on a white area, blots the excess, and compares against a reference spot. Read together they give the diagnosis:

	Test B passes	Test B fails
Test A passes	Properly fixed and properly washed	Fixing failed — longer washing will not help
Test A fails	Washing failed — extend the wash or add a wash aid	Both failed; fix the fixing first, because a tired bath makes washing harder

The lower-right box is the one people misdiagnose, and it is why the order matters: an overworked or over-long fixation produces a washing fault, so lengthening the wash and declaring victory leaves the actual cause running.

On limits: the course does not state numerical residual-chemistry limits here, because Part XII’s tests are comparisons against a reference rather than measurements, and a limit expressed in grams per square metre cannot be read off a colour comparison. Part XII says the same thing in its own words — the tests detect failure, not degree.

Silver mirroring

What you see. A bluish metallic sheen, visible by reflected light and not by transmission, starting from the edges and sometimes spreading over the whole surface. The conservation catalogue for gelatin dry plates calls it “a very common deterioration” and gives exactly that description.

Why it is at the surface. The Image Permanence Institute’s definition is the mechanism in one sentence: an oxidation of black-and-white images “in which the image silver migrates to the surface, creating a mirror-like appearance”. Oxidising gases convert image silver to mobile silver ions, those ions migrate — towards the surface, where the oxidant is — and are reduced back to metal there, in a finely divided form that reflects.

Why the edges first. The edges are where the atmosphere reaches the image most freely, and, in a framed or mounted print, where poor mount and mat materials are in contact.

It is a storage fault as much as a processing one, and the distinction is worth being precise about. The oxidative attack that drives it comes from the environment, not from the darkroom. But residual chemistry left in the material makes the material more vulnerable to it, so a badly washed print in a good enclosure and a well washed print in a bad one can arrive at the same place.

The discriminator against a processing stain is clean and costs nothing. The dry-plate catalogue sets mirroring — reflected light, metallic, from the edges — against improperly washed processing chemistry, which “appears instead as yellow or brown stains in transmitted light”. Look at the plate, then look through it. One shows in reflection, the other in transmission.

One handling warning that follows from the mechanism: the same catalogue records that areas with silver mirroring are extremely susceptible to abrasion, because the silver sitting at the surface is not protected by the gelatin above it. Do not attempt to polish or clean a mirrored area.

The sludges and the scums

Four different deposits, three of which the 1928 primer distinguishes by colour and behaviour.

Aluminium sulphite sludge — white, gelatinous. It forms in an alum hardening fixer when there is too little acid: either because too little was put in, or because carried-over developer has neutralised what was there. The primer is specific about printing, where the carry-over is worst: “when fixing prints, a relatively large proportion of the developer is carried over to the fixing bath (unless a water or acid rinse bath has been used) which soon neutralizes the acid, and therefore increases the tendency for precipitation of aluminium sulphite”. An exhausted bath does it too, because it “still contains alum and sulphite but no acid, and these combine to form a sludge”.

Its counterpart on the material is a white scum, and the primer gives three causes — insufficient rinsing after development, too low an acid concentration in the bath, and insufficient agitation of the film on first immersion — together with the one removal treatment on this page that the course can report as published: because aluminium sulphite is soluble in alkali, the scum “may be removed by swabbing the film or print with a 10 % solution of sodium carbonate and then washing thoroughly”.

Sulfur — pale yellow, settling slowly. A different milkiness with a different meaning. The primer’s three causes are too much acid in the hardener, too little or impure sulphite, and high temperature, with a figure attached: above 29 °C (85 °F) an acid fixing bath “will not remain clear longer than a few days even when mixed correctly”. Its consequence is not cosmetic — the sulphur “is apt to penetrate the gelatin, and later may cause fading of the image” — and the primer’s only remedy is to throw the bath away. Kodak’s modern Z-133 says the same in modern words, listing “graininess or mottle — white, grainy particles” against a sulfurised fixer, with the instruction to check for sources of oxidation such as excessive aeration or agitation, and replace it.

Silver sulphide — a metallic scum on the bath’s surface. When a partially exhausted bath stands unused for days, hydrogen sulphide in the air reacts with the dissolved silver and forms a metallic-looking film on the surface. The primer’s remedy is mechanical: draw the edge of a sheet of blotting paper across the surface. It is worth knowing chiefly because it is evidence — a bath with a skin on it is a bath carrying dissolved silver and standing idle.

Calcium scum — from the water rather than from the bath. Hard water carries calcium and magnesium, and what is dissolved in the last of the water is left behind on the material as it dries. Two Kodak publications give the same practical answer eighty years apart: the 1944 formulary instructs that where the water is very hard and tends to deposit a scum on drying, “give the final rinse in distilled water containing this addition of wetting agent” — and adds the counter-intuitive refinement that for hard water a smaller amount of wetting agent should be used. The modern processing sheets compress it to one line: to reduce drying scum, mix the wetting-agent solution with distilled water in hard-water areas.

✕A deposit on the surface and a change in the emulsion are different problems

The whole of the section above is about material sitting on the material — and that is why some of it can be swabbed or rewashed away.

Everything else on this page is a change in the emulsion: silver dissolved out by over-fixing, colloidal silver deposited inside the layer by a tired fixer, image silver converted to sulfide by residual chemistry, image silver migrated to the surface and reduced there.

The test that separates them is mechanical, not chemical. Look at the material at a grazing angle under a strong light. A deposit sits proud of the surface and catches the light as a separate plane; a change in the emulsion is in the plane of the layer. If you are unsure, a swab of clean water on a corner that will be trimmed settles it — a deposit softens and moves, a change does not.

Developer and oxidation stains

Dried developer on an edge. A print lifted from the developer and laid down before it went into the stop, or a sheet whose edge was above the solution, leaves developer to oxidise in air. The stain is brown, it is at an edge or in a drip shape, and it is a handling fault rather than a chemical one.

Oxidised developer’s brown stain over a whole sheet. A developer whose agents have oxidised carries coloured products, and those products stain the gelatin. This is the appearance half of a fault whose cause lives in the bottle, and it belongs to the chemistry in the bottle; its atlas entry is oxidation stain.

Uneven drainage marks. Streaks running down a hanging film or print in the direction it drained, from solution that concentrated as it ran. Their direction is their signature, exactly as it is for drying marks.

!The stain a staining developer leaves is not a fault, and must not be diagnosed as one

Pyrogallol and catechol developers produce a coloured oxidation product on purpose, and it forms in proportion to the silver developed — an image-wise stain that adds printing density where the silver is and adds none where it is not.

A negative developed in a staining developer therefore comes out of the process yellow-brown, and it is supposed to. Diagnosing that as oxidation, contamination or a washing failure and “correcting” it destroys the effect the developer was chosen for.

Two things separate a wanted stain from an unwanted one. A staining developer’s stain is image-wise: it follows the picture, it is absent from the rebate, and it is strongest where the density is greatest. An oxidation or contamination stain is general: it covers the rebate as thoroughly as the frame.

And there is a related trap in the reduction chapter worth knowing: Kodak’s 1928 primer records that ferricyanide removes the silver of a pyro-developed negative but not the stain, so such a negative appears to grow yellower as it is reduced. The reducer is not making the colour; it is uncovering it.

Locating the fault from where the stain is

Position is evidence, and it is often the only evidence left.

Where the mark is, and what step that implicates

1. 1At an edge, or in a drip shapeSolution that stayed on the material when the rest had gone: dried developer before the stop, drainage down a hanging sheet, a print laid on a wet bench. A handling fault at a transfer between baths.
2. 2In the highlights, spreadingResidual chemistry converting to sulfide, or an over-fixed print losing its smallest deposits. The two are separated by whether density has been lost or colour gained — and by Part XII's two tests.
3. 3Over the whole sheet including the bordersSomething the whole sheet met equally: a stained developer, a contaminated bath, a general fog, or a wash that failed on every print in the session.
4. 4Only where a tong or a finger touchedContact chemistry — fixer on a tong used in two trays, or a hand that had been in the fixer touching dry paper. Not a bath fault at all.
5. 5Only on the sheets from the end of a sessionCapacity. Something ran out while you worked, and the running total in your log will name it.
6. 6Only under a mount, or only at the edges of a framed print, years laterStorage: mirroring, or an enclosure attacking the image. Not a darkroom fault, and the fix is the framing rather than the process.

What is reversible, and the honest position on the rest

Refixing and rewashing works in one case and one only: a print that was under-fixed but has not yet stained. The complexes are still there, they are still soluble, and putting the print through a fresh fixer at the correct time and a proper wash removes them. Do it as soon as the test says to.

It does not work once the print has yellowed, because by then the residues have converted the image silver into silver sulfide, and that is a different compound in place of the picture. Nothing removes it, because there is nothing left to remove — the yellow is the image.

Nor does it work on dichroic fog, by the diagnostic argument above.

Nor on an over-fixed print, because the missing silver has left the building.

Aluminium sulphite scum is the one published exception on this page — swabbing with 10 per cent sodium carbonate and washing thoroughly, per the 1928 primer, because the deposit is alkali-soluble. Note that this is a treatment for a deposit, which is exactly the class the callout above says can sometimes be removed.

Mirroring should be left alone. The silver is at the surface and is extremely susceptible to abrasion, so attempts to clean it remove image.

And the general position, stated once: the great majority of the faults on this page are preventable and not repairable, and the prevention costs a rinse between developer and fixer, a clearing-time test on a schedule, the manufacturer’s fixing time rather than a longer one, a two-bath sequence for paper, and a wash designed rather than guessed. That is perhaps five minutes of work per session against a fault that arrives two years late and takes the print with it.

Atlas entries this page owns

Eleven entries in the troubleshooting atlas belong here: under-fixing, exhausted fixer, over-fixing, dichroic fog, residual thiosulfate, residual silver and yellow staining, silver mirroring, hardener sludge, calcium scum, developer stain and oxidation stain.

The mechanism of fixing is Part XI’s, the physics of washing is Part XII’s, and the two residue tests are Part XII’s lab. This page owns the signatures and the diagnoses only.

Recap

Clearing is the disappearance of visible halide and fixing is carrying the complexes far enough to be washable, so a film pulled at clearing looks perfect and is not fixed; the rule is twice the clearing time of the bath in use. Under-fixing shows as milkiness on film and as yellowing months later on paper. Over-fixing takes the highlights first because the deposits there are smallest, and paper’s fixing times are a fraction of film’s, so fixing prints for the film time is over-fixing them. Exhaustion is product accumulation rather than reagent depletion, doubling the time makes the washing worse rather than better, and two-bath fixing is the manufacturer’s own answer. Dichroic fog is colloidal silver from a solution, identified by two colours and confirmed by a refix that fails. Residual thiosulfate is a washing failure and residual silver a fixing failure, and Part XII’s two tests read together say which. Mirroring shows by reflection and processing stains by transmission. Sludges and scums sit on the material and can sometimes be removed; everything that is a change in the emulsion cannot.

Check your understanding

Question 1. A print looked correct when dry and has yellowed six months later. List the two candidate causes, the test that separates them, and the change that would have prevented each.

• Fog and over-development; a densitometer separates them; more restrainer prevents both
• Residual thiosulfate, a washing failure, and residual silver, a fixing failure. Part XII's two tests separate them: Test A, silver nitrate on half a strip, finds thiosulfate; Test B, sodium sulfide blotted on a white area, finds residual silver. A longer or better-designed wash prevents the first; a fresh bath, the correct time and a two-bath sequence prevent the second.
• Safelight fog and dichroic fog; look at the print by reflection and transmission
• Over-fixing and bronzing; measure the maximum black

Show the answer and why

Answer: Residual thiosulfate, a washing failure, and residual silver, a fixing failure. Part XII's two tests separate them: Test A, silver nitrate on half a strip, finds thiosulfate; Test B, sodium sulfide blotted on a white area, finds residual silver. A longer or better-designed wash prevents the first; a fresh bath, the correct time and a two-bath sequence prevent the second.

Both faults end in the same compound — yellow-brown silver sulfide — which is why the appearance alone cannot separate them and why the tests exist. The order of correction matters: an overworked or over-long fixation leaves complexes that a wash cannot shift, so a print failing both tests has a fixing problem first, and lengthening the wash without fixing the fixer leaves the cause running. Note also that neither fault is repairable once the yellowing has appeared; the tests are worth running on a schedule for this reason.

Question 2. Explain why paper over-fixes more readily than film, in terms of the size of the silver deposits involved.

• Because paper is thinner than film
• Because paper fixer is more concentrated
• Because the print's highlights carry the smallest and fewest silver deposits, and a given rate of dissolution removes a much larger fraction of a small deposit than of a large one — so the delicate tones disappear first while the blacks, where silver is piled deep, lose a proportionally trivial amount. ILFORD call the result image etching and give fibre paper one minute at 1+4 against two to five minutes for film.
• Because paper has a baryta layer that dissolves

Show the answer and why

Answer: Because the print's highlights carry the smallest and fewest silver deposits, and a given rate of dissolution removes a much larger fraction of a small deposit than of a large one — so the delicate tones disappear first while the blacks, where silver is piled deep, lose a proportionally trivial amount. ILFORD call the result image etching and give fibre paper one minute at 1+4 against two to five minutes for film.

The argument is about fractions rather than about rates. A fixer attacks metallic silver slowly compared with silver halide, but it attacks it everywhere at roughly the same rate per unit of exposed surface, so the loss is measured against how much was there. The practical consequence is a real and common home-darkroom fault: a printer who fixes prints for the film time, out of caution, is systematically etching the highlights out of every print they make.

Question 3. A negative has a milky, foggy look. You refix it in fresh fixer for the full time and nothing changes. What have you learned?

• That the fixer is exhausted and a longer time is needed
• That the fault is not retained silver halide — a fixer acts on halide and has nothing to say to metallic silver — so the diagnosis is dichroic fog, which is colloidal silver deposited from the solution; the confirming observation is that it looks greenish by reflected light and reddish by transmitted light
• That the film was fogged by light in the camera
• That the negative was under-developed

Show the answer and why

Answer: That the fault is not retained silver halide — a fixer acts on halide and has nothing to say to metallic silver — so the diagnosis is dichroic fog, which is colloidal silver deposited from the solution; the confirming observation is that it looks greenish by reflected light and reddish by transmitted light

This is a diagnosis made by a treatment that fails, which is a legitimate and underused form of evidence — Wall recorded exactly this in 1912. Under-fixing and dichroic fog produce a similar milky appearance and have completely different causes and remedies, and the refix separates them in one step. The two-colour appearance confirms it in ten seconds without any chemistry at all, which is why looking at the film and then through it is the first thing to do.

Question 4. Why does ILFORD warn that exceeding the fixer's capacity and extending the fixing time both "make washing more difficult"?

• Because a tired bath is more viscous
• Because both leave the material carrying the sparingly soluble intermediate complexes rather than the fully complexed soluble ones — an overworked bath because its chemistry has changed and its own product inhibits it, and a long fixation because the material has absorbed more thiosulfate and more silver to be removed
• Because both raise the pH of the wash water
• Because both harden the gelatin

Show the answer and why

Answer: Because both leave the material carrying the sparingly soluble intermediate complexes rather than the fully complexed soluble ones — an overworked bath because its chemistry has changed and its own product inhibits it, and a long fixation because the material has absorbed more thiosulfate and more silver to be removed

It is the clearest illustration on the page that fixing and washing are one problem rather than two consecutive ones. The purpose of adequate fixing in a fresh bath is not only to remove halide but to leave behind residues that a wash can actually shift, which is why the wash-test two-by-two puts "fixing failed" in a box where longer washing is explicitly no help. The corollary is the two-bath sequence: finishing in relatively fresh fixer is what gets the sequence to the washable end.

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