Fixer Capacity, Exhaustion and Residual Silver

A fixer does not stop working. It gets slower, and then it starts leaving something behind, and the second of those happens before you can see the first. This is the page where the course’s permanence argument begins, so it is also the page that has to be exact about which of its numbers are measurements, which are manufacturers’ recommendations, and which the course is not in a position to give at all.

What accumulates

ILFORD names the three things in one sentence: 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”. Take them in order.

Silver, as the complex, is the one everybody thinks of. It does not stop the reaction directly — the complex is a product and by mass action it pushes back, but the combined constant of 24 is large enough that this is a small effect until very late. What silver does is use up thiosulfate: two ions locked away per silver, permanently, and Part III showed that the crossing point where the free thiosulfate remaining meets the free thiosulfate required is essentially the stoichiometric limit. That crossing is exhaustion, and everything on this page is about the distance to it.

Halide is the one people forget, and it is a genuine product on the right-hand side of the equilibrium.

K = [complex] × [X⁻] ÷ [S₂O₃²⁻]²

The equilibrium, rearranged for what a used bath has in it

K is the combined constant, [complex] the dissolved silver, [X⁻] the accumulated halide and [S₂O₃²⁻] the free thiosulfate. Bromide and silver rise together, mole for mole, so the numerator rises as the square of the work done while the denominator falls — which is why an exhaustion curve turns up sharply rather than sloping. Iodide is the same arithmetic with a very much smaller solubility product behind it, so a little of it goes a long way, which is the mechanism usually offered for the faster exhaustion Kodak warns about on its T-Max sheets.

Carryover dilutes the bath and raises its pH, and the second is worse than the first: an acid fixer that has drifted alkaline is the aluminium sulfite sludge and, in Kodak’s 1928 troubleshooting list, dichroic fog. This is what a stop bath is for, and Part X costed it.

How much silver, per film and per print

🧮Two routes to the same number, which do not quite meet

Route one, from ILFORD’s own two figures. The published capacity at 1+4 is 24 films of 135-36 per litre, and the published ceiling for a film bath is 8 to 10 g of silver per litre. Work the bath to both limits at once and each roll must be shedding

8 to 10 g/L ÷ 24 rolls/L = 0.33 to 0.42 g of silver per roll

Silver per roll, if both published limits are reached together

Route two, from Kodak’s coating weights. Kodak’s J-210 gives a black-and-white 135-36 roll as 592 square feet per thousand, which is 0.055 m². It gives silver coating weights only for colour and radiographic families — 16 to 24 troy ounces per 1000 square feet for the GOLD films, 14.5 for T-MAT RA — which at 31.1035 g per troy ounce and 92.903 m² per 1000 square feet is 4.9 to 8.0 g/m². So a roll’s total silver is 0.27 to 0.44 g, and the fixer removes only the undeveloped part: J-210’s own examples take 30 to 50 per cent for exposed material, and a pictorial negative more.

The two routes do not meet. Route one wants 0.33 to 0.42 g per roll to leave the film; route two says that is the whole of the silver on it. Either black-and-white camera films carry appreciably more silver than the families Kodak tabulates, or — much more likely — ILFORD’s 24 films per litre is set with margin and a bath worked to it does not reach 8 g/L at all.

The course states that as an unresolved cross-check rather than picking one. No published silver coating weight for any black-and-white camera film appears in any source the course holds, which is why route two had to borrow from a colour negative and an X-ray film. What follows practically is worth more than the number would have been: a capacity figure and a silver ceiling are two different statements, they are not derived from one another, and reaching one does not mean you have reached the other.

For paper the arithmetic is easier because ILFORD gives the answer directly: below 2 g/L “approximates to 40, 20.3 × 25.4 cm, FB prints”, and below 0.5 g/L “approximately 10”. That is 50 mg of silver per 8 × 10 fibre print either way, and it is a number a home printer can actually use.

What an exhausted fixer leaves in the paper

This is the part that matters, and it is not slow fixing.

The first lesson set out Kodak’s 1924 statement: two compound sodium silver thiosulfates exist, one almost insoluble and one very soluble, and only the soluble one forms “as long as the fixing bath has any appreciable fixing power”. The corollary is the whole of this section. As the bath loses fixing power, the ladder stops on its lower rung more and more often, and the sparingly soluble species is formed in the material rather than in the tray.

Kodak’s own modern toning manual says what happens next, in a sentence that ought to be printed on the side of every fixer bottle:

An exhausted fixing bath contains insoluble silver compounds that will remain in prints; you can’t remove them completely by washing.

Not “will take longer to wash out”. Cannot be removed. A wash is a diffusion process, and a species that is not in solution does not diffuse.

⚗From a retained complex to a stain: the most insoluble compound in this course

A retained silver thiosulfate complex is a reservoir of silver ion sitting inside gelatin, and sulfide is the one ion that will take it. Silver sulfide’s solubility product, on the table Part IV uses, is 1.6 × 10⁻⁴⁹ — thirty-six orders of magnitude below silver bromide’s, and the smallest number anywhere in this course.

2 [Ag(S₂O₃)₂]³⁻ + S²⁻ → Ag₂S + 4 S₂O₃²⁻

Sulfiding: the retained complex gives its silver up to sulfide and keeps none of it

Nothing pushes that back. It is thermodynamically about as one-way as chemistry in water gets, and the product is a brown-yellow, non-image-forming deposit exactly where the retained complex was — which on a print means the borders and the highlights, because that is where the most undeveloped halide was and so where the most complex was formed. This is sulfiding, and it is behind a large part of what conservators record as yellowing in silver-gelatin prints.

Kodak’s 1928 primer had already seen the same reaction happen in a tray, which is the best evidence that the mechanism is real: leave a partially exhausted fixing bath standing for several days and “the hydrogen sulphide gas usually present in the air reacts with the silver thiosulphate in the bath and forms a metallic appearing scum on the surface of the solution. This scum consists of silver sulphide.” A scum on a standing bath and a yellow stain in a print border are the same chemistry at different scales.

Where does the sulfide come from in a print that never met a toner? From the air, in the trace quantities the primer names; from residual thiosulfate decomposing over years; and, immediately and unmissably, from a sulfide toner — which is why Kodak’s G-23 warns that residual silver compounds meeting a toner “form a dark yellow stain that is especially noticeable in print borders and highlights”. A toner is an accelerated ageing test you did not mean to run.

Capacity figures, and why the paper ones are lower

Three sets of published numbers, and the interesting thing is that they are not the same kind of number. Fixer capacity is a word the trade uses for at least two things, and this table separates them.

Material	Published capacity, per litre at 1+4	Silver ceiling	What sets it
135-36 film	24 films	8 to 10 g/L	The bath’s ability to fix at all. Film base absorbs nothing
RC paper, 8 × 10	80 sheets (4 m²)	4 to 6 g/L	The same, plus a little margin; the polythene keeps the base dry
FB paper, 8 × 10, commercial permanence	40 sheets (2 m²)	below 2 g/L	What the paper base retains, not what the bath can dissolve
FB paper, maximum long-term stability	about 10 sheets	below 0.5 g/L	The same, with the tolerance cut by four

The film and RC rows are capacities. The fibre rows are permanence limits, and they are lower for a reason that has nothing to do with the fixer’s chemistry: fibre base is bare paper, it soaks, and what soaks in has to come out again. ILFORD says exactly this — resin-coated papers tolerate 4 to 6 g/L “as the paper base is protected on both sides by an impervious polythene coating” — and adds the consequence for fibre above 2 g/L: “compounds may remain in the paper base after washing and over time possibly contribute to print staining.”

There is one more sentence in the ILFORD sheet that is easy to skim and worth stopping on: the paper figures “may be exceeded whenever print stability is not critically important”. That is a manufacturer telling you that a capacity figure is not a physical limit but a decision about how long you want the print to last, and that the decision is yours to make.

Two historical figures for scale, both per gallon rather than per litre, and both for sodium baths. Kodak’s 1928 primer gives F-1 seventy-five 8 × 10 films or one hundred 8 × 10 prints per gallon, rising to a hundred and twenty-five prints where an acid rinse bath precedes the fixer — which is Part X’s argument appearing as a 25 per cent capacity gain in somebody else’s table. Kodak Limited’s 1949 handbook gives F-5 a useful life of 120 sheets of 8 × 10 per 160 fluid ounces, which is 26 sheets per litre.

The shape of an exhaustion curve, and where the discard rule cuts it

• Clearing time ÷ fresh clearing time

Show the numbers behind this plot

A curve of clearing time against cumulative films fixed in one litre of a rapid fixer, with clearing time expressed as a multiple of what the same film took in the freshly mixed bath. The curve starts at one and stays almost flat for the first two thirds of the range, reaching only 1.15 after twelve films and 1.28 after sixteen. It then bends upward, passing 1.5 at twenty films and 1.7 at twenty-two, and crosses a horizontal guide drawn at two — the point at which the clearing time has doubled and ILFORD says to discard the bath — at twenty-four films, which is exactly ILFORD's published unreplenished capacity for that bath. Past the crossing the curve rises very steeply, reaching 2.6 at twenty-six films and 3.8 at twenty-eight. The teaching point marked on the plot is that the curve gives almost no warning: two thirds of a bath's life is spent in a region where clearing time has changed by less than a fifth, which is inside the measurement error of a stopwatch and an eye, so the only way to catch the corner is to measure regularly rather than to wait until something looks wrong.

Series	Films of 135-36 fixed, per litre	Clearing time, as a multiple of the fresh-bath clearing time
Clearing time ÷ fresh clearing time	0.00	1.00
Clearing time ÷ fresh clearing time	4.00	1.03
Clearing time ÷ fresh clearing time	8.00	1.08
Clearing time ÷ fresh clearing time	12.00	1.15
Clearing time ÷ fresh clearing time	16.00	1.28
Clearing time ÷ fresh clearing time	20.00	1.50
Clearing time ÷ fresh clearing time	22.00	1.70
Clearing time ÷ fresh clearing time	24.00	2.00
Clearing time ÷ fresh clearing time	26.00	2.60
Clearing time ÷ fresh clearing time	28.00	3.80

The shape is drawn to teach and was not measured from a bath; the two guide lines are ILFORD's published figures and the curve is drawn to pass through their intersection. Measuring this curve for your own fixer is what the experiment in this part does. The curve is drawn to show the shape, not measured from a real material. Your own materials will differ, and measuring them is what the sensitometry part of the course is for.

The course’s own criterion, stated in full

Everything above is a manufacturer’s recommendation, and manufacturers’ recommendations are not the same kind of statement as a standard. The limits that would let anyone certify a print as permanent — how much residual thiosulfate and how much residual silver may remain — are set out in international standards, of which the ones this course names are ISO 18901, on the stability of processed silver-gelatin films, and ISO 18917, which specifies the methods of test.

This course does not hold those standards, does not quote them and does not restate their figures. Purchasing them would not have changed that, because purchase confers no right to reproduce their content. So the course does what it can do honestly instead: it states its own criterion, in public, where you can check it, and it labels every figure it derives as its own.

!The Part XI fixing criterion

A bath is fit for use when a piece of the film being processed clears in no more than twice the time that same film took in that same bath when it was freshly mixed.

Material is fixed for twice the clearing time measured on the day, not twice the clearing time measured when the bath was new.

A bath is retired the moment the first condition fails, regardless of how much work it has done, and for fibre-base prints intended to last, before that: at the point the capacity log says the tenth 8 × 10 print per litre has gone through, which is ILFORD’s own figure for its 0.5 g/L limit.

What this criterion is. A working rule the course can state completely and you can execute with a stopwatch and a film leader. It is modelled on the practice ILFORD and Kodak publish, and the doubling ratio in it is theirs.

What it is not. It is not a residual-silver measurement, it does not certify permanence, and it is not ISO 18901’s criterion or anybody else’s. A figure derived under it is the course’s own measurement under the course’s own criterion, and every page in this part that quotes a capacity the course measured says so in those words.

Where the real tests live. Part XII owns the residual-thiosulfate and residual-silver tests, at Level B, because they need silver nitrate and sodium sulfide. This part deliberately reaches its conclusions without either.

The tests, and what each of them can actually tell you

Clearing time. Free, needs no reagent, works in room light, and is the only one of these the course classifies at Level A. It measures the bath’s ability to complete stage one, which is a proxy for its ability to complete stage two. ILFORD’s method is on the first lesson of this part. It is the instrument this part uses.

Silver estimator strips. A paper strip impregnated with a silver-sensitive material, read against a colour chart. Kodak’s own publication on measuring silver is unusually blunt about the limits: the technique is usable “to estimate silver concentrations greater than 1 gram/litre”, longer soaking “is extremely qualitative and is not reliable to quantify low levels”, and on-site techniques generally “cannot typically be used to demonstrate regulatory compliance”. Set that against the numbers above and the conclusion is stark. The strip’s floor is 1 g/L. The fibre-print permanence limit is 0.5 g/L. ILFORD says the same thing from the other side: silver estimator papers “are usually not sensitive enough to test the very low silver levels suitable for optimum permanence”. A test that cannot see the limit you care about is not a test of that limit.

The iodide hypo check. Add an iodide solution to a sample of fixer; if the fixer carries dissolved silver and has too little free thiosulfate left to hold it, silver iodide precipitates and the sample clouds persistently. The principle follows directly from the course’s own solubility table — silver iodide is the least soluble of the three halides by three orders of magnitude, so iodide is the ion best placed to pull silver back out of a complex — and it is why the test works at all.

But the course could not source a working concentration or an interpretation for it from any document meeting its standard, and therefore publishes neither. No concentration, no drop count, no reading. That is Rule 1 doing its job: a plausible recipe assembled from the general chemistry would be the course inventing a test and then teaching it back to itself. If you use a bought hypo-check product, its own instructions are the authority.

The residual-silver test on the material itself. This is the one that answers the question the others only approximate, and it is a Part XII lab at Level B. ILFORD publishes a version — a sodium sulphide solution, 2 g in 125 mL, diluted 1+9 for use, dropped on a white area of a washed print and compared against a reference spot made on a print known to be well fixed. Kodak’s ST-1 is not merely the classical equivalent of that: it is the same composition, published under the name that got the credit. This page said the course could not give it, which was true of a text search of the corpus and untrue of the document; the J-1 scan was read as page images on 6 September 2026 and ST-1 has been in the formulary since.

Two baths, and the rule for fibre prints

Two-bath fixing is the oldest answer to everything above, and it answers it by changing what the last bath a material meets is like rather than by making any single bath better.

Two-bath fixing, and the rotation that keeps it honest

1. 1Make up two baths of equal volumeSame fixer, same dilution. The second one is the one that matters and it starts fresh.
2. 2Fix for half the recommended time in the first bathThis is where nearly all the silver is dissolved, because the first bath meets the material loaded.
3. 3Then half the time in the secondThe second bath sees a material that has almost no halide left, so it stays close to fresh for a long time. Its job is stage two — converting the retained mono complex and letting it out.
4. 4When bath one reaches its capacity, discard itBy the course criterion above: when the clearing time in it exceeds twice the fresh figure, or when the log says the sheet count is reached.
5. 5Promote bath two to bath one, and mix a fresh bath twoThe material is now always finished in a bath that has done at most one rotation of light work.

Sequence and rotation as ILFORD's RAPID FIXER sheet gives them. The reason it works is Kodak's, from 1924, and is quoted below.

Kodak’s 1924 primer explains the point of the rotation better than any modern sheet, because it names the thing being protected against rather than describing the procedure: two-bath fixing “ensures that no material can be removed from the fixing bath until the first insoluble compound of silver and hyposulphite has been converted into the second soluble compound”, and it adds the sentence that makes the practice non-optional for anything you care about — “this first insoluble compound is invisible”.

For fibre-base prints the rule is different in kind, and it is counter-intuitive: fix briefly, in fresh fixer. ILFORD’s own archival sequence gives one minute at 1+4, followed by five minutes of wash, ten in WASHAID and five more. Kodak’s G-23 approaches from the other side and sets an upper bound: do not fix prints longer than about ten minutes for fibre base, because prolonged fixing drives solution into the paper core where it is difficult to remove.

So a fibre print has a fixing time that is bounded above as well as below, and both bounds are about the paper rather than the emulsion. The instinct to give a doubtful print a bit longer in the fixer is, for fibre base, exactly backwards: the answer to doubt is a fresher bath, not a longer time.

Reuse, replenishment and stopping

Replenishment is what a busy darkroom does instead of discarding: add fresh working-strength fixer at a published rate and the bath holds its activity. ILFORD gives 45 mL per 135-36 film, or 855 mL/m², with 250 mL/m² for RC paper and 500 mL/m² for fibre. Properly replenished, its own advice is to replace the tank solution after twelve months anyway. The United States EPA’s guide for photo-processing businesses records the industrial version of the same move — replenish the strength of a fixer by adding ammonium thiosulfate, and add ammonium thiosulfate to a silver-contaminated bath to extend the allowable build-up of silver — which is the stoichiometric argument of this whole part appearing in a regulator’s waste-minimisation list.

A home darkroom is usually the wrong place for it. Replenishment trades chemistry for record-keeping, and it only pays where the throughput is high enough that discarding a working bath is the dominant cost. At one or two sessions a month the bath will more often die of standing than of work, and ILFORD publishes the keeping figures that decide it: six months in a full tightly capped bottle, two months in a tank or dish with a floating lid, one month in a half-full bottle, and seven days in an open dish. A tray of fixer left out over a fortnight has not been used up; it has expired.

📓The capacity log, which is the actual instrument

Clearing time tells you where you are. A capacity log tells you where you are going, and it costs nothing.

For each bath, one line at mixing — date, product, dilution, volume, and the fresh clearing time measured that day on a scrap of the film you use. Then one line per session: date, what went through it in films or in 8 × 10 equivalents, and the running total. Measure the clearing time again every quarter of the published capacity, and always before anything you care about.

Two columns do most of the work. The running total against the published capacity tells you when to expect the corner; the clearing-time ratio tells you whether it has arrived. When they disagree, believe the clearing time, because it is a measurement of your bath and the other is a number about somebody else’s.

Recap

A fixer accumulates silver, halide and carried-over developer, and the first two enter the equilibrium as products so that the exhaustion curve turns up sharply rather than sloping. Published capacities and published silver ceilings are different statements that do not derive from one another, and the course’s attempt to reconcile ILFORD’s two figures with Kodak’s coating weights does not close — which it says rather than choosing. What an exhausted bath leaves behind is the sparingly soluble low-thiosulfate complex, formed inside the material rather than in the tray, invisible, unwashable, and convertible in one step to silver sulfide, whose solubility product is the smallest number in this course. Fibre-base limits are lower than film limits because the paper base retains what a film base cannot. Two-bath fixing exists to make sure nothing leaves the fixer with the insoluble compound still in it; for fibre prints the rule is short times in fresh fixer, bounded above as well as below. And the criterion by which this part decides a bath is finished is the course’s own, stated in full above, because the standards that would settle it are documents the course can name and cannot quote.

Check your understanding

Question 1. ILFORD publishes 24 films of 135-36 per litre at 1+4 and a tolerable silver level of 8 to 10 g/L for a film bath. A reader multiplies them out to get the silver shed per roll. What is the correct handling of the result?

• It gives 0.33 to 0.42 g per roll, which is a reliable figure because both numbers come from the same sheet
• It gives 0.33 to 0.42 g per roll, but Kodak's published coating weights make that roughly the whole silver content of a roll, so the two figures probably cannot both be reached at once — the capacity is likely set with margin, and the cross-check is left open
• The two numbers cannot be combined at all, because one is per litre and one is per roll
• It gives 3.3 to 4.2 g per roll, which is why fixer exhausts so quickly

Show the answer and why

Answer: It gives 0.33 to 0.42 g per roll, but Kodak's published coating weights make that roughly the whole silver content of a roll, so the two figures probably cannot both be reached at once — the capacity is likely set with margin, and the cross-check is left open

8 to 10 divided by 24 is 0.33 to 0.42 g. Kodak's J-210 gives a black-and-white 135-36 roll as 0.055 m², and coating weights for the colour and radiographic families it does tabulate work out at 4.9 to 8.0 g/m², so a whole roll carries 0.27 to 0.44 g of silver and the fixer removes only the undeveloped part of it. The two routes therefore disagree, and the more likely reading is that a capacity figure is a recommendation with margin in it rather than the point at which the silver ceiling is reached. No coating weight for any black-and-white camera film exists in the course's sources, which is why the check cannot be closed.

Question 2. Why can residual silver compounds left by an exhausted fixer not be washed out, when residual thiosulfate can?

• Because they are bound chemically to the gelatin
• Because they are the sparingly soluble low-thiosulfate complex rather than the soluble one, and a wash removes only what is in solution — Kodak's toning manual says outright that they cannot be removed completely by washing
• Because washing removes only ionic species and the complex is neutral
• Because the wash water is not alkaline enough to redissolve them

Show the answer and why

Answer: Because they are the sparingly soluble low-thiosulfate complex rather than the soluble one, and a wash removes only what is in solution — Kodak's toning manual says outright that they cannot be removed completely by washing

Washing is diffusion, and diffusion moves dissolved species. Kodak's 1924 primer identifies the pair — one compound almost insoluble, one very soluble, and only the soluble one formed while the bath retains fixing power — and its 2005 toning manual states the consequence: an exhausted fixing bath contains insoluble silver compounds that will remain in prints and cannot be removed completely by washing. The fix is another bath of fresh fixer, not more water, which is why refixing rescues an under-fixed negative and rewashing does not.

Question 3. A printer keeps a litre of rapid fixer for fibre-base exhibition prints and wants the best long-term stability ILFORD publishes. How many 8 × 10 prints should go through it, and what silver concentration does that correspond to?

• Forty prints, at below 2 g/L
• About ten prints, at below 0.5 g/L — which works out at roughly 50 mg of silver per print either way
• Eighty prints, at 4 to 6 g/L, which is the paper figure
• Twenty-four prints, since the film and paper figures are the same

Show the answer and why

Answer: About ten prints, at below 0.5 g/L — which works out at roughly 50 mg of silver per print either way

ILFORD gives two fibre-base limits for two different ambitions: below 2 g/L for commercial permanence, about forty prints, and below 0.5 g/L for maximum stability in long-term storage, approximately ten. Both work out at about 50 mg of silver per 8 × 10 sheet, which is a useful sanity check on the pair. Eighty prints at 4 to 6 g/L is the resin-coated figure, and it is higher precisely because the polythene keeps the paper core dry so nothing is retained in the base. Note that ILFORD itself says the paper figures may be exceeded when print stability is not critically important — a capacity is a decision, not a physical limit.

Question 4. Kodak's publication on measuring silver says its estimator strips can estimate concentrations greater than 1 g/L and are not reliable below that. What does that imply for a printer using them to protect fibre prints?

• Nothing much; 1 g/L is a fine working limit for fibre prints
• That the instrument's floor is above the limit that matters — 0.5 g/L for maximum stability, and even the commercial figure of 2 g/L is only twice the floor — so the strip can confirm a bath is badly spent but cannot confirm one is fit for archival work
• That the strips should be soaked for longer, which extends them below 1 g/L reliably
• That the strips measure thiosulfate rather than silver and are therefore the wrong test

Show the answer and why

Answer: That the instrument's floor is above the limit that matters — 0.5 g/L for maximum stability, and even the commercial figure of 2 g/L is only twice the floor — so the strip can confirm a bath is badly spent but cannot confirm one is fit for archival work

This is the general lesson about instruments rather than a fact about strips: a test whose detection floor sits above your acceptance limit cannot decide the question, however convenient it is. ILFORD reaches the same conclusion independently and says silver estimator papers are usually not sensitive enough for the very low silver levels suitable for optimum permanence. Kodak does note that longer soaking gives an indication below 1 g/L, and describes that as extremely qualitative and not reliable for quantifying low levels — an indication is not a measurement. The test that does answer the question is on the material rather than in the bath, and it is a Level B laboratory in Part XII.

Question 5. A fibre print looks under-fixed. The instinct is to give the next batch longer in the fixer. Why is that the wrong move, and what is the right one?

• It is the right move; longer fixing always improves permanence
• Longer fixing drives solution into the paper core where it is hard to remove, and Kodak sets an upper bound of about ten minutes for fibre base; the right move is a fresher bath, and ILFORD's archival sequence is one minute at 1+4
• Longer fixing bleaches the image, so the right move is a weaker fixer
• The print should be washed longer instead, which removes the retained silver compounds

Show the answer and why

Answer: Longer fixing drives solution into the paper core where it is hard to remove, and Kodak sets an upper bound of about ten minutes for fibre base; the right move is a fresher bath, and ILFORD's archival sequence is one minute at 1+4

A fibre print is bounded at both ends. Below, by the need to complete both stages of fixing; above, by Kodak's observation that prolonged fixing expands the paper and lets the solution into the base, where it is difficult to remove and later turns a toned print yellow. Both bounds are about the paper rather than the emulsion, which is why the answer to doubt is a fresher bath rather than a longer time — and why ILFORD's own archival sequence is as short as one minute at 1+4. Washing longer does not help with retained silver compounds at all, because they are not in solution.

Question 6. Under the criterion this page states, when is a bath retired, and what may a figure measured under that criterion be called?

• When the residual silver in a test print falls below the ISO limit; the figure may be called an ISO-compliant capacity
• When a piece of the film being processed takes more than twice as long to clear as it did in the same bath when fresh; a figure obtained that way is the course's own measurement under the course's own criterion, and is not a residual-silver measurement and does not certify permanence
• When the silver estimator strip changes colour; the figure may be called a silver capacity
• After twelve months, whichever comes first; the figure may be called a shelf life

Show the answer and why

Answer: When a piece of the film being processed takes more than twice as long to clear as it did in the same bath when fresh; a figure obtained that way is the course's own measurement under the course's own criterion, and is not a residual-silver measurement and does not certify permanence

The criterion is stated in full on the page so that a reader can check it, and its doubling ratio comes from ILFORD and Kodak. What it deliberately is not is a permanence certification: the residual-silver and residual-thiosulfate limits that would support one live in international standards that this course names by number — ISO 18901 and ISO 18917 — and does not hold, quote or restate. Buying them would not have changed that, since purchase grants no right to reproduce their content. A published criterion the student can execute is worth more than an appeal to a document they cannot read, and it is the only form of the claim the course is entitled to make.

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