Kodak ST-1

Two grams of one salt in 125 millilitres of water is the shortest formula in this book, and it answers the question that the longest wash in the world cannot. A print that fails ST-1 does not need more washing. It needs fixing again, in fresh fixer, because the silver in it was never dissolved in the first place — and the reagent that reveals it is doing, in three minutes on a margin, exactly what a sepia toner does to a whole picture on purpose.

Ingredient	Quantity	Form the source specifies
Sodium sulfide	2 g	anhydrous, as Kodak's line specifies
Water	125 mL, added	Kodak's line is "Water 125 milliliters" and there is no "Water to make" line beneath the salt, which by Kodak's own published convention means water added rather than a make-up volume. The HT-2 entry in the next column of the same page does carry the make-up line, so the distinction is Kodak's typography on one sheet of paper rather than a reading of it. ILFORD prints the same thing in words - "dissolving 2 g of sodium sulphide in 125 ml of water" - which settles it. No mixing temperature is published.

Purpose

To find out whether the fixing worked, by testing the material rather than the bath. Kodak’s own paragraph under the heading Tests for Silver states the failure this reagent exists to catch: an overworked fixing bath contains complex silver thiosulfate compounds that are retained by the films or prints and cannot be removed completely by washing, and those salts lead to stains which may not become evident for a period of time. Every clause of that sentence matters. The compounds are retained. Washing does not take them out. And the evidence arrives years late.

What is being looked for is residual silver. It is not residual thiosulfate, which is a different failure with a different cause, a different remedy and a different reagent — HT-2, printed in the next column of the same page. A sheet can pass HT-2 and fail this one. Kodak’s 1928 primer puts the relationship between them in a single sentence: it should not be necessary to wash out silver compounds but only the chemicals of the fixing bath, and if an exhausted fixing bath is used, silver compounds will be present during washing and must be removed very completely. Residual silver is therefore a washing problem you gave yourself in the fixing tray.

And the reason to test the sheet rather than the bath is that the bath cannot be read finely enough. ILFORD states it directly: silver estimator papers are usually not sensitive enough to test the very low silver levels suitable for optimum permanence, so for important prints the paper itself is tested. That is the whole argument for this page. A bath-side instrument tells you about the bath; only the material tells you about the material.

Recommended uses

On a sacrificial sheet carried through the batch, which is Kodak’s own second alternative and much the better of the two: an unexposed piece of the same type of paper being processed, treated in the same chemicals. Same paper, same fixer, same washing. It is the control that makes the result mean something, because a stain on it cannot be blamed on an image.

On the margin of a squeegeed film or print, which is Kodak’s first alternative. It is offered here because Kodak offers it and because on a sheet film the clear margin is genuinely spare. On a print it means marking a print you intend to keep, permanently, and the Image characteristics section says why that is a worse bargain than it looks.

As ILFORD’s reference-spot comparison, which is the version this course actually recommends. Establish a permanent reference for one type of paper by putting a drop of the diluted solution on a white area of a print known to be well fixed and thoroughly washed by the two-bath method; blot the excess; the barely visible cream tint left behind is the reference colour for that paper. Every later test on that paper is read against it. This turns a described tint into a physical standard you hold in your hand, which is the same move that makes Kodak Limited’s silver nitrate hypo test — set out under Variants on the HT-2 page — more usable than HT-2 itself in a darkroom that does not own Kodak’s printed estimator.

As the acceptance test at the end of the archival processing sequence, paired with a residual-thiosulfate test and never instead of one. The Part XII wash-testing lab teaches the reading; the residual hypo and residual silver procedure is what a reader follows at the bench, and it is the page that says which steps happen where.

Early in the archival sequence rather than late, which is the scheduling point most readers get wrong. The permanence treatments J-1 prints around this formula all come after fixing and washing — the hypo eliminator HE-1, and then the gold protective solution GP-1 — and every one of them changes the sheet in a way this test cannot see through. GP-1 puts gold on the image; a sulfide or selenium toner converts the retained silver along with the picture and, in Kodak’s words, the stain so formed is permanent. So the order is: fix, wash, test for silver, test for hypo, and only then treat. A print that fails after toning has no remedy left.

As the honest end of a fixer capacity log. The Part XI capacity argument retires a bath on clearing time and on a sheet count. ST-1 is how you find out whether that criterion was right for your paper, your dilution and your throughput, rather than trusting a number transcribed from a datasheet.

When another formula is preferable

• A residual-hypo test, when the question is washing rather than fixing. HT-2 is the one printed beside this formula, and Kodak Limited’s one per cent silver nitrate immersion is the one the course’s own procedure runs. Neither answers this question, and this one does not answer theirs. Run both or say in the record that you ran one.
• Kodak’s FT-1, when you want to test the bath instead of the print. J-1 prints it on the facing page: potassium iodide 190 g and water to make one litre, of which five drops are mixed with five drops of the fixing bath under test and five drops of water, the fixer to be discarded if a yellow-white precipitate forms instantly. It is faster, it wastes no paper, and it is a coarser instrument — it tells you the bath is loaded, not whether this print got out clean. ILFORD’s sentence about estimator papers is the general form of the same limitation.
• Kodak’s own selenium-toner alternative, when the sulfide bottle will not keep. J-1 offers it in as many words: if you wish to use a more stable reagent than KODAK Residual Silver Test Solution ST-1, dilute one part of Rapid Selenium Toner with nine parts of water, the proportions not critical, and follow the ST-1 directions. See Variants for what the course can and cannot say about that bottle.
• A quantitative method, when you need a number rather than a verdict. J-1’s hypo section points at the American national standard for measuring residual chemicals in films, plates and papers, whose 1985 revision the Library of Congress’s standards list also records. It specifies a silver densitometric method alongside the methylene blue thiosulfate method. The course names the standard and does not hold it, so it can tell you the numerical route exists and nothing about what it says.
• Nothing at all, on a print that has already been toned. Kodak is explicit and the reason is chemical rather than procedural: prints toned in a sulfide or selenium toner will not yield to refixing, because the residual silver has been toned along with the image. Test before you tone. After toning there is no remedy to act on and the test spot is simply another permanent mark.
• Nothing at all, on a print straight from the fixer. ILFORD states the boundary twice: prints must be well washed before the test, and it is not effective on prints direct from the fixer bath. Kodak states the same limit from the other end in its NOTE — the test fails where a very large excess of hypo is present. A result obtained on an unwashed sheet is not a lenient result, it is not a result.

Mixing

Water first, then the salt, and 125 millilitres is 125 millilitres of water. This is one of the places where the difference between water added and water to make is not pedantry, and Kodak demonstrates it on a single sheet of paper: ST-1’s lines are “Water 125 milliliters” and “Sodium Sulfide (Anhydrous) 2 grams” with nothing beneath them, while HT-2 in the next column ends with “Water to make 1.0 liter”. Kodak’s 1928 primer states the convention that governs both — as a general rule in published formulas the term “cold water to make” is always given at the end of the formula, which insures dilution to a definite volume and thus a known concentration each time. No such line appears here, and ILFORD’s wording removes the last doubt: dissolving 2 g of sodium sulphide in 125 ml of water.

Weigh the solid once, and treat that operation as a different job from the test. This is the step that sets the safety level of the page, and it is the reason the course’s bench procedure makes the stock a separate operation performed under separate controls. Inside the darkroom the reagent should exist only as a dilute solution in a dropper bottle.

Make a small volume and say so on the label. Kodak’s own storage line is a small stoppered bottle, and the arithmetic is why: 125 mL of stock makes 1.25 litres of working solution, which at roughly a twentieth of a millilitre per drop is tens of thousands of tests. Date it, per the labelling procedure, because the keeping figures below are the only thing standing between a reader and a reagent that has quietly stopped working.

Dedicated glassware, and nothing that has ever held an acid. Not a rinsed stop-bath measure, not a graduate that lives on the fixing bench. The residue of an acid bath in a vessel that receives a sulfide solution is the reaction under Safety, performed at the scale of whatever was left in the vessel.

🧮What the two lines deliver, and why no molarity is given

The stock. Two grams in the 125 mL of water Kodak names is 1.6 g per 100 mL, or nominally 1.6 per cent w/v.

2 g ÷ 125 mL = 0.016 g/mL = 1.6 % w/v, or 16 g/L

Stock, as published

That figure is a nominal one and the reason is the water line itself. Kodak states the water added, not a final volume, so the finished stock is slightly more than 125 mL and its true strength is slightly under 1.6 per cent. Nothing in the procedure turns on the difference, which is presumably why neither maker bothers to close the volume — but a page that quoted 1.6 per cent as a measured concentration would be overstating what two published lines support.

The working solution.

1.6 % ÷ 10 = 0.16 % w/v, or about 1.6 g/L

Diluted 1 + 9

And no molarity, deliberately. Every other quantity page in this course converts a weight into moles per litre where it helps the mechanism. Here it would be a fabrication. The sodium sulfide page records that the course has no sourced molar mass it can stand behind for this substance: the PubChem record the encyclopaedia cites is for the hydrated material of commerce carrying not less than 30 per cent water, its stated relative mass of 79.06 corresponds to a hydrogen sulfide anion with two sodium ions rather than to Na₂S, and what a supplier actually ships is a nonahydrate or a fused flake of variable composition. Dividing 2 g by an assumed 78.05 g/mol would produce a confident number resting on an assumption the course has already declined to make. The concentration is therefore given as a percentage and a mass per litre, which is how both makers give it.

Behaviour

One drop, two or three minutes, blotted off. Kodak’s procedure is a spot test and the steps are exact: the film or print is squeegeed first, so that surface water cannot spread or dilute the drop; the drop goes on the margin; and at the end of the period the solution is removed with a clean white blotter. White, because the blotter is where you first see what colour came off.

ILFORD’s version differs in what you compare against, not in what happens. Its drop goes on a white area of the print, the excess comes off with clean blotting paper or absorbent tissue, and it gives no standing time at all — because the answer is a comparison against a reference spot made the same way on a print known to be well fixed, and both spots have had the same treatment. Kodak’s fixed two-or-three minutes exists because its reading is against a described tint rather than against a sheet of your own.

The reading, in Kodak’s words: any yellowing of the test spot, other than a barely visible cream tint, indicates the presence of silver. The negative result is therefore not a clean white. It is a faint cream, and ILFORD says the same thing in the same words from the other direction — a barely visible cream tint should be left on a well-fixed, well-washed print, and that tint is the reference. A reader expecting no mark at all will condemn every print they make.

The scale is a direction, not a calibration. Cream, then yellow, then brown, as the retained silver rises. Neither maker publishes tints for it the way Kodak published a Hypo Estimator for HT-2, which is why ILFORD’s own-reference method is the one that gives a reader a standard they actually possess.

The remedy exists, and it is the one that makes this test worth running. If the test is positive, Kodak’s instruction is to refix the print or negative in fresh hypo and rewash for the recommended time. This is the sharpest practical difference between the two tests on that page: a failed hypo test means a longer wash, and a failed silver test means the sheet goes back through fixing — a remedy that only exists while the print is still in your hands and still untoned.

!Two ways to get a result that is not a result

Kodak’s NOTE, in full: the test fails where a very large excess of hypo is present, as in stabilized prints. ILFORD prints the operational form of the same limit: prints must be well washed before using the test, and it is not effective on prints direct from the fixer bath.

Two Tier 1 manufacturers, thirty years apart, agreeing on a boundary. Neither of them says why, and the course has found no source that does. What can be said is what follows from it at the bench, which is unambiguous: wash the sheet first. A drop of this reagent on a sheet still loaded with fixer is not a lenient test or a rough one. It is a test whose makers state it does not work.

The reading below, under The mechanism, is the course’s own and is marked as such.

Image characteristics

It has none, and every mark it makes is permanent. ST-1 is never applied to a photograph you are keeping, and where Kodak allows the margin of a real print it is allowing a permanent mark in a place you expect to trim.

The stain is silver sulfide in gelatin, and it is not removable. That page records the course’s position and the number behind it: a solubility product of 1.6 × 10⁻⁴⁹ against 5.0 × 10⁻¹³ for silver bromide, which is thirty-six orders of magnitude, and no reagent in the course’s sources attacks a washed silver sulfide image under darkroom conditions. Kodak says the same thing in the plainest terms about a toned print: the yellow stain so formed is permanent.

A tested sheet is a waste item from the moment the drop lands, and that is the argument for the sacrificial control sheet over the margin of a print you love.

The mechanism

Neither maker publishes one. Both print the formula, the dilution, the procedure and the reading, and say nothing about the chemistry. What follows separates what the course’s sources establish from what the course infers, because Rule 7 asks for that separation and because the inference here is short and worth making.

Established: fixation makes two silver–thiosulfate compounds, and which one you get depends on how much free thiosulfate there is. Kodak’s 1924 primer states it while explaining fixing: two compound sodium silver thiosulphates exist, one of them almost insoluble in water, while the other is very soluble, and as long as the fixing bath has any appreciable fixing power the soluble compound only is formed. The soluble one is what fixing depends on:

AgBr + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Br⁻

Fixing, with thiosulfate in excess: the freely soluble complex that washes out

Established: when the bath runs short, the other compound is what stays behind. J-1’s own sentence is that an overworked bath leaves complex silver thiosulfate compounds retained by the films or prints that washing cannot remove, and Kodak’s 1928 primer states the practical consequence — silver compounds present during washing must be removed very completely, and where the wash has to be cut short it is fixing that must be complete. Two-bath fixing is offered in the same paragraph as the best way of insuring that.

The course’s reading, marked as a reading. The two published compounds are the two rungs of a thiosulfate ladder, and the sparingly soluble one is the first rung — the mono-thiosulfato species, formed where there was not enough free thiosulfate left to climb to the second. Neither Kodak nor ILFORD writes a formula for it, so the species below is the course’s identification and not a quotation.

Established: sulfide takes silver away from every other ligand, irreversibly. The solubility product above is the reason. Whatever form the retained silver is in, sulfide ion converts it:

2 Ag⁺ + S²⁻ → Ag₂S

The reaction the test runs, in its simplest form

2 [Ag(S₂O₃)]⁻ + S²⁻ → Ag₂S + 2 S₂O₃²⁻

The same reaction on the retained complex, as the course reads it

And the reagent is alkaline for free, which is why it works without anything else in the bottle. Sodium sulfide hydrolyses in water, and the sodium sulfide page gives the equilibrium:

S²⁻ + H₂O ⇌ HS⁻ + OH⁻

Hydrolysis: why a sulfide solution is alkaline, and why an acid near it is dangerous

That single line explains three separate things on this page. It is why the solution needs no alkali added. It is why it must never meet an acid. And it is why the reagent perishes: the sulfide is oxidised by air, which is what a working solution that keeps a week against a stock that keeps three months is telling you.

So the test is a sepia toner, run deliberately on the wrong silver. The reaction above is exactly the redevelopment half of T-7a, where a bleached image is converted to silver sulfide on purpose because — as Kodak’s 1928 primer puts it — the sulfide is very insoluble and therefore permanent. Here the same conversion is applied to silver that should not be in the sheet at all, and the permanence that makes it a good toner is what makes it a good indicator: the mark cannot un-form while you are looking at it.

And it is the same endpoint as the slow failure the whole of Part XII is written to prevent. Ware records residual thiosulfate converting image silver to silver sulfide over years, the colour weakening from a rich brown to a pale buff, with oxidation by air promoting the conversion; the Getty atlas records sulfur toning as the same chemistry performed on purpose. Sulfiding from residual thiosulfate stains everywhere, because the thiosulfate is everywhere; sulfiding from residual silver stains the highlights and the borders, because that is where the unexposed halide was. ST-1 pre-empts the second of those, in three minutes, with the same reagent nature would eventually supply.

⁇Why a large excess of hypo defeats the test — a reading, not a finding

Both makers state the limit and neither explains it, and the course has found no published mechanism. Two readings are available and it cannot separate them.

The first: the comparison is destroyed rather than the reaction. A sheet saturated with fixer holds mobile silver–thiosulfate complexes everywhere rather than retained ones in the unexposed areas, so there is no clean region left to read a local spot against. On that reading the reagent still makes silver sulfide; what it has lost is the contrast that made the spot mean something.

The second: the silver moves before it can be caught. In a sheet that wet and that loaded, the complexes diffuse — into the drop, into the blotter — so the stain that forms is diffuse and weak rather than a defined spot.

Both readings predict the same fix, which is ILFORD’s: wash the sheet properly first. Neither is offered as chemistry the course can stand behind, and a reader who finds either maker’s own account should trust that instead.

Function of every ingredient

Sodium sulfide, 2 g of the anhydrous grade, giving a nominal 1.6 per cent stock and 0.16 per cent in use. It is the entire reagent, and unusually for this formulary there is nothing else in the bottle to share the work with — no preservative, no buffer, no restrainer, no alkali. Each of those absences is worth a sentence, because in a one-ingredient formula what is not there is most of the design.

What it is. The sodium salt of hydrogen sulfide, supplied as a deliquescent crystal, flake or fused lump, alkaline in its own right and, as CAMEO’s datasheet records, mildly corrosive to most metals. It is the redeveloper of classical sepia toning, which is the same chemistry this page uses as a test.

Why it is here. Because sulfide ion beats every other ligand a photographic sheet can offer. The solubility product of silver sulfide, 1.6 × 10⁻⁴⁹, sits about thirty-six orders of magnitude below silver bromide’s, so the conversion of retained silver to a visible sulfide is quantitative, essentially instantaneous and effectively irreversible. That is what turns an invisible fault into a mark on a blotter.

What it does chemically. It supplies S²⁻ and, through the hydrolysis above, HS⁻ and OH⁻. The sulfide takes the silver; the hydroxide is why the solution is alkaline without an added alkali; and the same hydrolysis is the reason an acid anywhere near this bottle produces hydrogen sulfide gas.

What follows on the sheet. A yellow to brown stain in proportion to the silver retained — and, crucially, no reaction at all with residual thiosulfate, which is why a sheet can pass this test and fail HT-2, and why the two tests are not interchangeable.

More, and less. More sulfide would deepen every spot including the reference tint, compressing the useful low end of the scale — which is the end a properly fixed print is read at — while raising the alkalinity, the odour and the hazard of the bottle in the same proportion. Less would flatten the same low end and, worse, make the reagent’s ordinary decay indistinguishable from a good result: a weak solution and a well-fixed print both give a pale spot. Neither change is available to anyone who wants to compare a result with anyone else’s, and neither maker offers a range. The number is 2 g because two manufacturers thirty years apart both printed 2 g.

The grade, which is a real practical problem. Kodak specifies anhydrous. What is generally sold is the hydrated material — CAMEO’s datasheet is written for the grade carrying not less than 30 per cent water — and 2 g of that is meaningfully less sulfide than 2 g of the anhydrous salt. The course gives no conversion, because the composition of the material of commerce is variable and the encyclopaedia has no molar mass it can stand behind for it. The honest resolution is ILFORD’s protocol rather than arithmetic: establish your own reference spot, with your own reagent, on your own paper, and the reference absorbs a reagent that is somewhat weaker than Kodak’s. Remake the stock and remake the reference together.

What it interacts with. Acids, catastrophically; unexposed photographic material, ruinously; silver in every form, which is the point. All three are under Incompatibilities.

Water, 125 mL, added rather than made up to. Not a make-up volume, on the evidence set out under Mixing, so the finished stock is a little over 125 mL and the strength a little under 1.6 per cent. It is worth naming as an ingredient anyway for one reason: this reagent is diluted 1 + 9 before use, so the water in the bottle and the water in the dropper together set the working strength, and a reader who makes the stock up to 125 mL instead of adding 125 mL has made a slightly stronger reagent than either manufacturer prints. Where the mains water is heavily chlorinated or sulfurous, use distilled — not because either source says so, but because the reference-spot method assumes the reagent is constant between makings.

Interactions

With retained silver, which is the whole point, and with essentially nothing else on a properly processed sheet. That selectivity is what makes it a test rather than a stain.

With residual thiosulfate, not at all — and that is the design. The most useful thing about this reagent is the question it cannot answer. Run it beside HT-2 and the two between them separate a fixing failure from a washing failure, which no single spot test can do.

With a large excess of hypo in the sheet, destructively, as both makers state and neither explains. See the callout under Behaviour. This is the one interaction that turns a positive procedure into no procedure at all.

With a toned image, terminally. Kodak’s statement is that residual silver in a print toned in a sulfide or selenium toner has been toned together with the image, so refixing will not remove it and the stain is permanent. The retained silver has already become silver sulfide or silver selenide, and hypo does not dissolve either. The practical rule is a sequence rule: test, then tone.

With silver nitrate, mutually destructively. The residual-hypo reagent on the facing page of J-1 is a silver nitrate solution, and silver ion and sulfide ion meet irreversibly. Two bottles that must both be on the bench and must never touch is a layout problem, not a care problem, and the procedure that runs both solves it by layout.

With unexposed photographic material, ruinously and at a distance. Kodak’s 1928 primer states that a very small quantity of hydrogen sulfide converts enough silver halide to sulfide to produce severe fog, and that no photographic material should be stored in a room where sulfides are kept. This reagent lives in a darkroom full of paper. Treat that as the reason for a stoppered bottle and a closed box rather than as a remote possibility.

With air, slowly and continuously. The published keeping figures — three months for the stock, one week for the diluted solution — are the visible form of that interaction, and the ratio between them is the evidence that surface and dilution matter more than the calendar.

Variants

Neither maker publishes a variant of the quantities, and the course invents none. What the sources hold instead are two different reagents for the same question and one procedural variant that changes what the answer is read against.

Kodak’s Rapid Selenium Toner at 1 + 9, offered by Kodak itself. J-1’s words are that if you wish to use a more stable reagent than ST-1 you may dilute one part of the toner with nine parts of water — these proportions are not critical — and follow the ST-1 directions. Two things are worth saying about it and they point in opposite directions.

The first is that Kodak’s stated reason is stability, not safety. A sulfide solution oxidises; a selenium toner is sold as a stable working liquid; and the phrase “not critical” tells you that this is a comparison method throughout, where the sulfide version at least has published quantities.

The second is that the course cannot tell you what is in the bottle. Kodak describes the active ingredient of Rapid Selenium Toner only as a sulfite salt at a concentration of less than 2 per cent, with working solutions containing less than half a per cent of “selenium sulfite” — a phrase with no CAS number behind it. The sodium selenite page sets out how far three different manufacturers’ sheets disagree about which selenium species is actually present. So this is a Kodak-published procedure using an undisclosed product, and it is not a safer route: the course classifies sodium selenite at Level C and selenium itself at Level D. It is offered here because Kodak offers it, with the boundary stated.

ILFORD’s reference-spot procedure, which is a variant of the reading rather than of the formula. Same 2 g, same 125 mL, same 1 + 9. What changes is that the negative result is defined empirically — the tint left by a print you know is well fixed, on the paper you are actually using — instead of by a description. The course recommends it for the same reason it recommends Kodak Limited’s silver nitrate hypo test over HT-2: a comparison a reader can physically make beats a comparison against a tint they have only read about. It also quietly absorbs the anhydrous-versus-hydrated grade problem, since your reference is made with your reagent.

A remedy that is part of the published method, and differs between the two sources. Kodak: refix in fresh hypo and rewash for the recommended time. ILFORD: soak the prints in water for five minutes, then repeat the recommended fixing and washing sequence using fresh fixer. ILFORD’s five-minute soak is the addition and it is a sensible one — a dry or part-dried print put straight into fixer takes up solution unevenly. Neither maker explains the difference, so the course records both rather than merging them.

🕰Why there is no British ST-1

Kodak Limited’s London handbooks of 1944 and 1949 — which supply a good deal of this formulary — carry a test for hypo and no test for silver at all. The material-side residual-silver test in the course’s corpus enters through the American J-1 and survives into ILFORD’s modern fixer datasheets, where it appears without a designation, without attribution and with a reading procedure of ILFORD’s own.

That is worth noticing rather than smoothing over. For most of the twentieth century the amateur literature treated washing as the archival question and fixing as a step with a time. The test that says otherwise is the younger of the two, and the reason it survived when so many J-1 formulas did not is that it is the only one of them a print manufacturer still needs.

Safety

Level C, on the rubric, and it is sodium sulfide that earns it. It is the highest classification carried by any test in this formulary, and the course states plainly what the letter means: a home darkroom does not have the controls Level C assumes.

The classification. PubChem’s aggregation of 24 notifications to the ECHA inventory gives sodium sulfide Danger, with H314 (causes severe skin burns and eye damage) and H400 (very toxic to aquatic life) in 100 per cent of reports, H311 (toxic in contact with skin) in 96.2 per cent, H301 (toxic if swallowed) in 65.3 per cent, H318 in 62.9 per cent and H290 (may be corrosive to metals) in 14.7 per cent. No notifier reports that it meets no criteria. That is unusually strong agreement for a notified rather than harmonised classification.

The step that sets the level is weighing the solid, and the course has already ruled on it: the residual hypo and silver procedure makes the stock once, as a separate operation, under the controls the encyclopaedia entry requires, so that inside the test itself the reagent exists only as a dilute solution dosed in drops. If you do not have those controls, that procedure’s instruction stands — run the thiosulfate test alone and write in the record that the fixing was not tested, which is a decision rather than a blank.

The hazard that is specific to this bottle rather than to the substance is hydrogen sulfide. The Environment Agency’s waste guidance writes the reaction out and assigns the substance the hazard statement for liberating a toxic gas on contact with acid:

Na₂S + 2 H⁺ → H₂S + 2 Na⁺

What an acid does to this reagent, as the Environment Agency's guidance writes it

Every number that matters here is a manufacturer’s or a regulator’s. NIOSH gives hydrogen sulfide an IDLH of 100 ppm and a recommended ceiling of 10 ppm over ten minutes, and carries the note that decides how the substance is handled: the sense of smell becomes rapidly fatigued and cannot be relied upon to warn of continuous presence. HSE’s EH40 sets a workplace exposure limit of 5 ppm over eight hours and 10 ppm over fifteen minutes. A darkroom has none of the instruments those numbers assume, and the rotten-egg smell everyone treats as a warning stops being one within minutes of exposure.

Controls. Chemical splash goggles that seal against the face, not spectacles, because the severe eye damage statement is carried by every notifier. Chemical-resistant gloves, because the acute dermal toxicity statement is carried by 96 per cent of them; the glove table is where the course records what published permeation data actually exist. An apron and long sleeves. Engineered extraction for the weighing, not an open window. Eyewash within reach before a bottle is opened, and first aid read beforehand rather than during.

The layout control that no PPE substitutes for: no acid comes to this bench. Not a stop bath, not an acid fixer, not the acetic acid of HT-2, not a vinegar bottle. Princeton’s photography guidance requires a print to be rinsed thoroughly after an acid bath before it enters a sulfide bath, and the same reasoning applies to a print carrying acid fixer into a sulfide drop. The incompatibility matrix carries the pairs.

What is not a hazard here, and why. There is no heating, no ultraviolet, no mains and no concentrated acid. The volumes are drops of a 0.16 per cent solution. What raises the level is the solid in the weighing boat and the gas that any acid would liberate from the bottle — which is precisely why separating the mixing from the testing is the control that does the most work.

Storage

Kodak’s instruction, in full: a small stoppered bottle, for not more than three months. Both halves carry information. Stoppered, because the sulfide oxidises in air and because hydrogen sulfide is what a leaking bottle offers a room full of photographic paper. Small, because a part-full bottle is a bottle of air in contact with sulfide, which is the same reasoning that governs every developer in this book and bites harder here.

Three months for the stock; one week for the diluted solution. Both are published, on the same page, for the same substance at one tenth the strength — a ratio of roughly thirteen to one. It is the most useful keeping datum in this part of the formulary, because it says the enemy is oxidation and surface rather than the calendar. Mix the working solution in small amounts and date the dropper bottle as well as the stock.

Kodak’s one-year figure on the facing page is not this formula’s. J-1 states that mixed solutions may be stored in brown, stoppered glass bottles for one year — and that sentence belongs to the stop bath and fixer testing solutions, SBT-1 and FT-1. Transferring it here would quadruple a published figure by borrowing from a neighbouring paragraph. The contrast is itself informative: Kodak gives an iodide solution a year and this one three months.

Away from every acid, away from unexposed material, and away from food. Sealed and dry, labelled, kept apart from acids, from oxidising agents and from metals, and out of the room where sensitive materials are stored — the encyclopaedia entry’s storage rule, which for a darkroom means a closed cupboard outside the paper safe rather than a shelf above the trays.

Keep the never-fixed control with the bottle. A scrap of unfixed, unexposed paper of the paper you use, stained deliberately, is a positive control that tells you the reagent still works. Since a weak reagent and a well-fixed print produce the same pale spot, that control is the only thing that distinguishes them, and it costs a corner of a sheet.

Incompatibilities

Acids of any kind, which liberate hydrogen sulfide: stop baths, acid fixers, acetic acid, citric acid, the acid hardener stocks. CAMEO states it plainly and the Environment Agency’s guidance writes the equation. This is the one absolute on the page.

Stop baths and fixing baths as waste, as well as at the bench. Kodak’s G-23 sheet directs that sulfide-type toners must not be discarded with stop baths or fixing baths, for the same reason.

Silver in any form — silver nitrate, a spent fixer bottle, a silver recovery cartridge. Sulfide and silver meet irreversibly, so contamination destroys the reagent and seeds an untraceable stain wherever it happens.

Unexposed photographic materials, which is not a chemical incompatibility but a ruinous one all the same: the 1928 primer’s severe fog from a very small quantity of hydrogen sulfide.

Strong oxidisers, carbon, charcoal and diazonium salts, with all of which CAMEO records violent reaction, and metals, which it records the substance as mildly corroding with possible production of flammable hydrogen. Heat and shock are on the same datasheet: the material can explode on rapid heating or when shocked, and heating to decomposition emits oxides of sodium and sulfur.

Anything you would drink from. Dedicated glassware, dedicated dropper, labelled, and never a kitchen container.

Waste

Two questions here, and the smaller volume is the more regulated one.

The sulfide. The Environment Agency’s waste classification assigns sodium sulfide the hazard statement for liberating a toxic gas on contact with acid, and calculates a threshold of 0.4 per cent sodium sulphide for a waste to be hazardous on that ground alone. The course’s arithmetic against that published threshold is worth stating because it is unusually clean: the stock at a nominal 1.6 per cent is four times the threshold, and the 1 + 9 working solution at 0.16 per cent is below it. That is not permission to pour the working solution away — every other route and every local rule still applies — but it does say which bottle is the serious one. Treat the stock as hazardous waste and do not attempt to neutralise it: the ordinary darkroom move of acidifying an alkaline waste is, here, the reaction under Safety performed into a sink.

The silver. It is a trivial mass — a drop of a 0.16 per cent solution, and whatever it lifted off a margin — but silver is among the parameters most often limited in a discharge consent, and Kodak’s own J-52 guidance on small volumes is where the course takes that. Bottle the residue separately from the spent-fixer stream, which is held for recovery and is full of thiosulfate: sulfide tipped into it precipitates silver where the recovery process cannot reach it.

The tested sheets are waste too, and they are silver-bearing paper waste of the ordinary kind. Keep one with the batch record if it is evidence; do not treat a stained margin as a print.

Follow the silver-bearing waste procedure alongside the general chemical waste procedure. Everything above is chemistry and general practice: local regulation decides what you may actually do, and the disposal ruling says why this course cannot tell you what it says where you are.

Troubleshooting

Every spot comes out cream, on every print, always. Suspect the reagent before you congratulate the fixing. A sulfide solution that has oxidised gives a pale spot, and so does a properly fixed print. The never-fixed control under Storage is what separates them: it should stain strongly. If it does not, the bottle is finished, whatever the date on it says.

Every spot comes out yellow, including on a print you know is well fixed. Take the grade question first — a fresh, strongly anhydrous salt gives a deeper reference tint than the hydrated material — then remake the reference spot. ILFORD’s whole method exists because the negative result is a cream tint and not a blank, and a reader who calls that cream a failure will refix everything they make.

The spot spread and there is no defined mark. The sheet was not squeegeed, or it was not washed. Kodak’s procedure begins with a squeegeed film or print for the first reason; ILFORD’s condition — well washed, not straight from the fixer — covers the second.

A strong yellow on a sheet straight out of the fixer. That is not a result. Both makers exclude it: Kodak’s NOTE about a very large excess of hypo, ILFORD’s statement that the test is not effective on prints direct from the fixer bath. Wash the sheet and test again.

A positive result on a print that has been toned. There is no remedy, and Kodak says so: the residual silver was toned with the image and refixing will not touch it. Record it, and move the test earlier in the sequence for everything that follows.

A positive result that refixing does not clear. Check that the fixer really was fresh and that the rewash matched the recommended time, then look at ILFORD’s remedy instead of Kodak’s — five minutes in water first, then the full fixing and washing sequence. A part-dried fibre print put straight into fixer takes up solution unevenly.

A smell of rotten eggs at the bench. Stop, ventilate, and find the acid. Then remember the NIOSH note: the smell fades long before the hazard does, so the absence of the smell a few minutes later is not evidence of anything.

The bottle has gone cloudy or thrown a deposit. Oxidation, or metal contact, or both. Discard it as hazardous waste and remake it — and remake the reference spot at the same time, because the reference is only valid for the reagent that made it.

Experiments

Make the fixer exhaustion curve visible. Fix five identical sheets of the same paper in one bath, one after another, deliberately continuing well past the sheet count your capacity log allows — the sixth in a bath you have loaded on purpose with an earlier batch. Wash all of them properly, then one drop on each, blotted, laid out in order. This is the single most direct demonstration in Part XI that a fixer does not stop working but starts leaving something behind, and the point at which the tint departs from the reference is your own number rather than a manufacturer’s.

Prove that the two tests ask different questions. Two sheets. One fixed properly in fresh fixer and then deliberately under-washed; one fixed in an exhausted bath and then washed for an hour. Run HT-2 and ST-1 on both. The results should cross: the under-washed sheet fails the hypo test and passes this one, the under-fixed sheet the reverse. If they do not cross, one of the two reagents is telling you about itself.

Build your reference set and keep it. One drop on a well-fixed, well-washed print of each paper you use, blotted, dried, dated and filed with the batch records. It takes an afternoon and it converts every future test from a judgement into a comparison. Repeat it whenever you remake the stock, and keep the old card: the drift between two references is a direct measurement of your reagent’s decay.

Measure the keeping figure instead of trusting it. Make one stock and split it: half in a small full bottle, half in a large half-empty one. Test both against the same never-fixed control at one week, one month and three months, reading against a fresh dilution each time. Kodak’s three months is stated for a small stoppered bottle, and this is the experiment that shows what the word “small” is doing in that sentence.

Take the reading off the eye and onto an instrument. ST-1’s spot is a coloured deposit, and Kodak suggests exactly this route for its companion reagent: measure the density of the stain rather than describing it. Immerse two clear film margins, one from a properly fixed strip and one from a deliberately under-fixed one, and read them on the densitometer against an untreated blank. State the geometry and the spectral response you used, because a yellow-brown stain read through different filtration gives different numbers.

Watch the failure this test predicts. Keep two prints from the exhaustion series above — one that passed and one that clearly failed — in the same enclosure for a year, then look at the borders and the highlights rather than the image. ST-1 told you in three minutes what one of them was carrying; this is what carrying it costs, and it is the only way a home darkroom can see the difference between a print that will last and one that will not.