Kodak HT-2

Every other entry in this formulary is a bath that does something to a picture. This one is a diagnostic reagent that must never touch a picture at all: a drop of it on a print is a black mark that will not come off. Its whole purpose is to run, deliberately and in two minutes on a sheet you intend to throw away, the reaction that a badly washed print performs slowly on its own image over thirty years.

Ingredient	Quantity	Form the source specifies
Acetic acid (glacial)	125 mL of a 28% solution	printed as "KODAK 28% Acetic Acid"; Kodak's footnote makes approximately 28 per cent acid from the glacial acid by diluting 3 parts of glacial acetic acid with 8 parts of water
Silver nitrate	7.5 g	crystals, as Kodak's line specifies
Water	to make 1000 mL	Kodak's first line is 750 mL, which receives the acid and dissolves the silver nitrate, and its last is water to make 1.0 litre. Only the final volume fits this field. No mixing temperature is given for this formula, which is unusual for a J-1 entry - most of the book's solutions carry a starting water temperature - and the course does not supply one.

Purpose

To find out whether the wash worked, by testing the material rather than the water. Kodak’s own sentence under the heading Test for Hypo is the argument for the whole page: the residual hypo content of films and prints can be accurately determined only by actually testing the processed photographic material. And it names why that bites hardest on paper — the paper support retains hypo in its fibre structure, where a wash reaches it far less readily than it reaches a gelatin layer on a plastic base.

What is being looked for is residual thiosulfate, and the reason to look is Ware’s: residual thiosulfate left in the paper after washing slowly attacks the image silver and converts it to silver sulfide, the colour weakening from a rich brown to a pale buff. He adds a finding that makes the case worse — oxidation by air promotes that sulphiding action, so a print that is merely stored is not a print at rest.

This is not a test for residual silver, which is a different failure with a different cause and a different reagent. A sheet can pass HT-2 and still be badly fixed. The Part XII sequence sets out why the two questions have to be asked separately.

Recommended uses

On a sacrificial control sheet carried through the batch. Kodak’s procedure for prints is an unexposed piece of the same paper being used in the batch — same paper, same chemistry, same wash — or, failing that, the extra margin area of one of the prints. Both are Kodak’s own alternatives and the first is much the better: a margin that will later be trimmed is still part of a print you care about, and this reagent stains permanently.

On the clear margin of a film, cut off after washing and immersed. Kodak treats film and paper differently and the difference is not cosmetic; see Behaviour.

As the last check in an archival sequence, after any washing aid and after the final wash, on a sheet that is still wet. The Part XII washing lab is where the course teaches the reading, and the residual-hypo standard operating procedure is what a reader actually follows at the bench.

As a teaching instrument, which may be its best use of all. A row of sheets pulled from one wash at five, ten, twenty, forty and sixty minutes, each carrying one drop, is the most direct picture of a washing curve a home darkroom can make without a densitometer. It turns “wash for an hour” from an instruction into an observation.

When another formula is preferable

• Kodak Limited’s one per cent silver nitrate test, when you have no Hypo Estimator. This is the practical point and it is worth meeting first. HT-2’s published reading is a comparison against the printed tints of the KODAK Hypo Estimator, a card sold by photographic dealers that this course does not hold and cannot reproduce. Without it, HT-2 gives a stain and no scale. Kodak Limited’s 1944 test gives its own comparison instead: a strip immersed in 1 per cent silver nitrate for about three minutes, rinsed, and compared while wet against the untreated portion of the same sheet, no colour difference meaning the hypo has gone and a yellow-brown tint meaning it has not. That is a comparison a reader can actually make, which is why it is what the course’s own procedure runs.
• A residual-silver test, when the question is fixing rather than washing. Kodak prints its residual silver test solution ST-1 immediately above HT-2 on the same page, and ILFORD publishes a sulfide test the course’s procedure uses. Neither answers this question and this one does not answer theirs.
• Kodak Limited’s HT-1a permanganate test, when the material must not be touched at all. It tests the water draining off a film rather than the film itself, which makes it non-destructive and, for that same reason, a weaker piece of evidence. See Variants.
• The methylene blue method, when you need a number. J-1 points at it itself: a quantitative test for residual chemicals in films and papers is in American National Standard PH4.8-1971. The course names that standard and does not hold it, so it can tell you the method exists and no more.
• Nothing at all, on a print you intend to keep. There is no careful way to run this test on a finished print. The stain is silver, and silver stains do not rinse out.

Mixing

Water, then the acid, then the silver nitrate, then make up. That is Kodak’s own printed order and there is a reason to keep it beyond obedience: the silver nitrate goes into a solution that is already acid, which is the condition the finished reagent works in, and it goes in last so that the crystals are the only solid the vessel ever sees.

Glass or plastic throughout, and nothing that has held a fixer. A trace of thiosulfate in the vessel consumes reagent and, worse, tells you nothing is wrong when something is. A vessel that has held a sulfide toner is worse still. Use dedicated glassware and mark it.

Making the 28 per cent acid is the step that carries the hazard, and it is a separate operation. Kodak’s footnote is: to make approximately 28 per cent acetic acid from glacial acetic acid, dilute 3 parts of glacial acetic acid with 8 parts of water. The glacial acid is what its own page classifies — H314, causes severe skin burns and eye damage, in more than 99.9 per cent of over five thousand classifying reports — and the rule is the one Princeton’s guidance states for every acid: the acid goes into the water, never the water into the acid. If a 28 per cent acid is sold to you ready made, buy it and skip this step.

🧮What the two lines actually deliver, and what 28 per cent means

Kodak’s “28 per cent” is a happy accident of two readings agreeing. Three parts of glacial acid to eight of water is 3 ÷ 11 = 27.3 per cent v/v. Taking the glacial acid’s specific gravity of 1.05 from its page, those 3 mL weigh 3.15 g and sit in 11 mL, which is 28.6 per cent w/v. Both round to 28, so for once the ambiguity the course usually insists on resolving does not change the answer.

125 mL × ≈28 % w/v ≈ 35 g of acetic acid per litre, or ≈0.58 mol/L

Acid delivered

7.5 g ÷ 169.873 g/mol = 0.0442 mol/L of Ag⁺, which is 4.76 g/L of silver metal

Silver delivered

The reagent is therefore 0.75 per cent w/v as the salt and 0.48 per cent as silver — about a quarter weaker in silver than Kodak Limited’s 1 per cent test, and a great deal more acid.

The pH, as the course computes it and nobody publishes it. For a weak acid at this concentration, with the IUPAC compilation’s pK_a of 4.76:

[H⁺] ≈ √(K_a × C) = √(1.74 × 10⁻⁵ × 0.58) = 3.2 × 10⁻³ mol/L, so pH ≈ 2.5

Weak-acid approximation

Kodak publishes no pH for HT-2 and this figure is the course’s arithmetic, on the ideal weak-acid approximation and ignoring the silver salt. It is given because the order of magnitude matters to the mechanism below, not because it is a measurement. Do not enter it in a notebook as one.

Behaviour

On paper it is a spot test. Wipe the excess water from the face — the emulsion side — of the processed paper sample. Place one drop of the solution on that face. Let it stand two minutes. Rinse to remove the excess reagent. Then compare the stain with the tints of the Hypo Estimator.

On film it is an immersion test, and the difference is not a preference. Kodak’s instruction is to cut a small strip from the clear margin after washing and immerse a portion of it in a small volume of the solution for about three minutes; well-washed films, including those for record purposes, should show very little or no discoloration. And it states the reason the spot method is not offered: the spot technique should not be used on wet films because of the danger of spreading the reagent. A drop on wet gelatin over a plastic base does not stay where it is put; on paper, the fibre holds it.

Immersion is also the route to a number. Where the spot reading is unreliable, Kodak’s own suggestion is to measure the transmission density of the material after total immersion in the silver nitrate test solution — which is a densitometer measurement of a stain, and is the one published route from this reagent to a figure rather than an impression. The course builds a densitometer in Part XV.

!Kodak's own warning about washing aids, which changes how you read a clean result

J-1 states it plainly and it is easy to skim past. If the spot test is used after washing aids other than KODAK Hypo Clearing Agent, it may give misleading results: the face may show less stain than a print washed only in water, and yet the hypo content of the two prints may be equal.

That warning lands directly on this course. The washing aid the course recommends is the one per cent sulfite bath, which is not Kodak Hypo Clearing Agent. A pale spot on a sheet that went through a sulfite bath is therefore weaker evidence than the same spot on a sheet washed in water alone, and the honest record says which route the sheet took.

Kodak’s own remedy is the immersion-and-density route above. Where you cannot do that, the reading still has value as a comparison between sheets treated identically — the wash series under Experiments is exactly that — and much less value as an absolute verdict.

A negative result is not a certificate. Kodak Limited’s 1944 booklet carries the converse warning for the silver nitrate test: a positive test may also be obtained in the absence of hypo if hydrogen sulfide or wood extracts are present in the water supply. So the reagent can accuse an innocent sheet, and a washing aid can excuse a guilty one. Both failure modes are the manufacturer’s own statements, and a page that printed the procedure without them would be publishing a false precision.

Image characteristics

It has none, and any it acquires is a disaster. This is the only entry in the formulary where the correct answer to this section is that the solution must never reach an image.

Kodak’s storage paragraph is written as a hazard notice for exactly that reason: do not allow the test solution to come in contact with hands, clothing, negatives, prints, or undeveloped photographic material; it will stain them black. Read it as four separate warnings. Hands and clothing, because silver nitrate is photoreduced to metallic silver by light and the mark grows in over hours. Negatives and prints, because the stain is silver in gelatin and there is no clean route back. Undeveloped material, because a silver nitrate aerosol near unexposed paper is fog you will not diagnose for weeks.

On the sacrificial sheet the stain is the datum, and its scale runs from no visible change, through a cream tint, to yellow and then brown as the thiosulfate rises. That is the direction of the scale rather than a calibration: the calibration is the Hypo Estimator’s printed tints, which the course does not have.

And the stain is permanent. It is silver sulfide in a gelatin layer, and the silver sulfide page records the course’s position on removing it: the sources it holds record no reagent that attacks a washed silver sulfide image under darkroom conditions. A tested sheet is a waste item from the moment the drop lands.

The mechanism

What Kodak publishes about the mechanism is nothing at all. J-1 gives the formula, the procedure and the reading, and no 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 worth making explicit.

Established: silver and thiosulfate make two different compounds, and which one you get depends on which is in excess. Kodak’s 1924 primer states it while explaining fixation: two compound sodium silver thiosulfates 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 the fixing complex:

Ag⁺ + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻

Thiosulfate in excess: the soluble complex that fixing depends on

HT-2 inverts that ratio deliberately. A washed sheet holds traces of thiosulfate; the drop that lands on it holds 0.044 mol/L of silver ion. Silver is in enormous excess, so the insoluble compound is what forms, and it forms at the surface where it can be seen.

Established: that insoluble silver–thiosulfate material does not stay as it is. Kodak’s 1928 primer records the everyday version — the silver thiosulfate in a standing, partly exhausted fixing bath reacts with hydrogen sulfide in the air to give a silver sulfide scum. Ware records the version that matters for permanence — residual thiosulfate in a print slowly converts image silver to silver sulfide, and the print fades. Both endpoints are the same compound, and the silver sulfide page gives the reason it is the endpoint: a solubility product of about 10⁻⁴⁹ means that once silver and sulfide meet, essentially nothing takes them apart.

Established: acid pushes thiosulfate towards sulfur, and only sulfite opposes it. The same 1924 primer gives the reaction and the equation, in the passage explaining why an acid fixing bath needs a preservative:

S₂O₃²⁻ + 2 H⁺ → S + H₂SO₃

Acid decomposition of thiosulfate, as Kodak's 1924 primer gives it

There is no sulfite in HT-2, which for a solution containing an acid and destined to meet thiosulfate is a conspicuous absence in a formulary where nearly every other acid solution has one.

The course’s reading, marked as a reading. Put those three together and HT-2 is a small engine for making silver sulfide out of residual thiosulfate as fast as possible: excess silver to precipitate it where it sits, acid to break the thiosulfate down, and no sulfite to protect it. The overall conversion, written as a balanced equation, is:

2 Ag⁺ + S₂O₃²⁻ + H₂O → Ag₂S + 2 H⁺ + SO₄²⁻

The stain, as the course reads it: sulfur to the silver, the rest oxidised away

The stain a reader sees is therefore the fading reaction, run on purpose. That is the sentence this page exists to deliver. The thing HT-2 does in two minutes on a sacrificial margin is the same thing residual thiosulfate does to the picture over decades — and Ware’s note that air promotes the sulphiding is the reason the slow version happens at all in a print sitting in a box.

Two honest limits on that reading. First, the course has not found a source that writes the HT-2 reaction out, so the balanced equation above is its own construction from established chemistry rather than a quotation; a reader who finds Kodak’s own account should trust that instead. Second, the same silver ion is also an oxidant for thiosulfate — the thiosulfate page records Ware naming silver nitrate among the reagents whose oxidation of thiosulfate gives tetrathionate — so more than one reaction is available in that drop, and which dominates in a paper fibre at pH 2.5 is not something this course can settle. What is not in doubt is the observable: more thiosulfate, more stain.

Function of every ingredient

Silver nitrate, 7.5 g of the crystals, giving 0.0442 mol/L of silver ion. The reagent proper, and the only thing in the bottle that finds anything. It is the only soluble silver salt photography uses, and its value here is that it supplies free Ag⁺ in a large, known excess over whatever thiosulfate the sheet is holding — which is what forces the almost-insoluble silver–thiosulfate compound rather than the soluble complex of fixation, and so puts the product where it can be seen instead of carrying it away in solution. More would deepen the stain for a given amount of thiosulfate and compress the useful part of the scale towards the top, besides wasting an expensive salt; less would flatten the low end of the scale, which is precisely the end a well-washed sheet is read at. Neither change is available to a reader who wants to compare a result with anybody else’s, because the Hypo Estimator’s tints are printed for this concentration. It interacts with essentially everything a darkroom holds — see Incompatibilities — and it is what sets this page’s safety level.

Acetic acid, 125 mL of an approximately 28 per cent solution, giving about 35 g and 0.58 mol/L in the litre and a calculated pH near 2.5. Kodak assigns this ingredient no function. The entry prints the line, prints a footnote on how to make the strength, and says nothing about why the acid is there — which is the shortfall docs/FORMULARY-SCHEMA.md records as an open item, since ingredient.function is a required string and cannot say “the source states none”. So the line above is the course describing a quantity, and what follows is the course’s reading rather than Kodak’s statement.

Three things an acid at this strength does here, in the order the course is confident of them. It keeps the silver in solution as the free ion: silver salts precipitate readily from a solution that is not acid, and holding a silver nitrate reagent on the acid side is the ordinary way to keep it clear — general chemistry, not something the course has read about HT-2 in particular. It supplies the condition under which thiosulfate breaks down, per the 1924 primer’s reaction above, with no sulfite present to reverse it — and note that acetic acid is weak, so this is a nudge rather than the strong-acid decomposition the primer describes, which suits a test that must not simply destroy the analyte before it can be counted. And it stops anything alkaline the sheet is carrying from acting in the drop. More acid would push the pH lower and the decomposition harder, and would raise the amount of acetic acid a reader is handling; less would move the solution towards the pH at which silver salts start misbehaving in the bottle. Since Kodak states none of this, none of it is a reason to change the figure — the figure is 125 mL because Kodak printed 125 mL.

Water, 750 mL first and then to make 1000 mL. The split is procedural: the acid and the salt go into the 750 mL and the volume is closed afterwards. That closing line is Kodak’s own convention, stated in its 1928 primer — as a general rule in published formulas the term “cold water to make” is always given at the end of the formula, which ensures dilution to a definite volume and so a known concentration each time. It is not decoration on a reagent whose whole output is a colour read against a chart: a bottle made up short is a bottle that overstates every result it gives.

Not an ingredient, but part of the published method: the KODAK Hypo Estimator. The formula has three lines and a fourth requirement, and Kodak puts it in the procedure rather than the table. Without the Estimator’s printed tints the reagent still stains, and the stain still means what it means, but the reading becomes a comparison between your own sheets rather than a measurement against a scale. The course does not hold it and does not reproduce it.

Interactions

With residual thiosulfate, which is the whole point, and with nothing else on a properly processed sheet. That selectivity is what makes it a test.

With the paper base rather than only the emulsion. Kodak’s reason for testing the material at all is that the fibre holds hypo, so the drop is being asked to interrogate the support as much as the gelatin. This is why the print test is a spot on the face and not an immersion: the face is where the emulsion is and where a comparison against printed tints was calibrated.

With a washing aid, unhelpfully, and Kodak names the effect. See the callout under Behaviour; it is the single most important interaction on this page for a reader following this course, because the course’s recommended washing aid is not the one Kodak’s caveat exempts.

With hydrogen sulfide and organic matter in the water supply, which Kodak Limited records as able to produce a positive silver nitrate test in the absence of hypo. A control sheet that never met a fixer distinguishes the two.

With light, in the bottle and on the sheet. Silver nitrate is reduced by light, and Kodak’s storage instruction — brown bottle, away from strong light — follows from it. On a stained sheet the same photoreduction slowly darkens the mark, which is another reason to read at the stated time rather than later.

With sulfide, destructively, which matters because the sulfide reagent of the residual-silver test is the natural bench companion of this one. They destroy each other on contact and the procedure that runs both separates them by layout rather than by care.

Variants

Kodak prints no variant of HT-2, and the course invents none. What the literature has instead is three other published answers to the same question, and the differences between them are worth more than any variant would be.

Kodak Limited’s one per cent silver nitrate test, 1944. Printed not as a numbered formula but inside the entry for the hypo eliminator HE-1. Process an unexposed white sheet of the batch’s own paper and weight alongside the prints; after the final wash, cut a strip, immerse it in a 1 per cent silver nitrate solution for about three minutes, rinse, and compare it while wet, in subdued daylight or artificial light, with the wet untreated portion of the same sheet. No colour difference means the hypo has been completely removed; a yellow-brown tint indicates its presence. Three differences from HT-2 and each of them is a design decision: it is immersion, not a spot; it carries no acid; and it is read against the sheet’s own untreated half rather than a printed chart. The last is why this is the test the course’s procedure actually runs.

Kodak Limited’s HT-1a, the permanganate hypo test. Potassium permanganate 1.2 g, sodium hydroxide 2.4 g, distilled water to make 1000 c.c. For use, 1 c.c. of that solution is added to 250 c.c. of pure water in a clear glass, and six films or plates of 3¼ × 4¼ in. or the equivalent area are taken from the wash water and drained into the glass. If hypo is present the violet colour turns orange in about 30 seconds, and with more, yellow. Kodak’s 1928 American primer prints the same reagent scaled to a quarter litre — 0.3 g of permanganate and 0.6 g of caustic soda in distilled water to make 250 cc. — so the two printings agree.

It is a different measurement and the difference is the lesson. HT-1a tests the water coming off the material; HT-2 tests the material. That makes HT-1a non-destructive, which is a real advantage, and much weaker evidence, which is why Kodak Limited says of papers that the test is not a completely reliable indication and directs the reader to the residual-hypo test under the HE-1 heading instead. Its own interference is different too: any oxidisable organic matter in the water reacts with permanganate as hypo does, so the 1944 booklet requires a blank test with distilled water and about 10 c.c. of the mains water added.

The methylene blue method. J-1 ends its own hypo section by pointing away from itself: a quantitative test for residual chemicals in films and papers is in American National Standard PH4.8-1971, the methylene blue method for measuring thiosulfate together with a silver densitometric method for residual chemicals. The Library of Congress’s standards list records the same standard in its 1985 revision. The course names it and does not hold it, so it can tell you that the numerical route exists and nothing about what it says.

🕰And one that is not a variant, but is what people did before either

Kodak’s 1928 primer, describing how to follow the progress of washing, offers the hypo test formula and then adds: An even simpler test is to taste the prints since hypo containing silver has a sweet taste.

It is printed without comment, in a manufacturer’s teaching text, in 1928. It works — thiosulfate is detectable by taste — and it is a straightforward route for putting silver salts and whatever else the darkroom carried into a person’s mouth. J-1, fifty years later, opens with the instruction to keep chemicals and solutions out of the mouth and never to start a siphon by mouth.

The course records the sentence because the change between those two paragraphs is the history of darkroom safety in miniature, and because a reader meeting it in a period manual should know it was once ordinary advice and is not now.

Safety

Level B, on the rubric, and silver nitrate is what earns it.

The classification. PubChem’s aggregation of the ECHA notifications and the harmonised CLP entry gives silver nitrate Danger, with H272 (may intensify fire; oxidiser), H314 (causes severe skin burns and eye damage), H318 (causes serious eye damage), H400 (very toxic to aquatic life) and H410 (very toxic to aquatic life with long lasting effects). The glacial acetic acid you may dilute to make the 28 per cent solution carries H226, H314 and H318. The finished reagent is a dilute solution of both, and is still an acid solution of a corrosive oxidising salt.

Controls. Chemical splash goggles that seal against the face rather than spectacles; nitrile gloves, noting that the glove table records no permeation entry for silver nitrate in the guide the course read, so these are splash protection with no breakthrough time behind them; an apron; an eyewash within reach before the bottle is opened, and first aid read beforehand. The silver nitrate handling procedure is the one to follow for weighing, and the spill procedure for what to do when a drop goes where it should not.

The stain is a warning sign, not an injury. Kodak Limited’s caution — silver nitrate solution stains the skin black; avoid direct contact with the solution — describes the visible consequence of a contact that also delivers a corrosive salt. Treat a black mark on a finger as evidence that the controls failed, not as the harm.

What is not a hazard here, and why. There is no vapour to control at the tray: neither silver nitrate nor a 3.5 per cent acetic acid solution produces one at room temperature, and the volumes are drops. Ventilation is a general requirement of the mixing bench rather than a control specific to this reagent, and the acetic-acid vapour J-1’s own safe-handling page names — with formaldehyde, and with the sulfur dioxide a fixing bath may liberate — comes from solutions standing in trays, not from a stoppered 100 mL bottle of test solution. There is no heating, no ultraviolet and no mains. The step that genuinely raises the level is weighing the solid silver nitrate, and where a 0.75 per cent solution can be bought ready made that step disappears.

One combination is forbidden outright. Never bring this reagent near sodium sulfide or a sulfide toner, and never store the two together; the incompatibility matrix carries the pairs.

Storage

Kodak’s instruction, in full: a screw-cap or glass-stoppered brown bottle, away from strong light. Both halves earn their place. Brown glass and darkness because silver nitrate is photoreduced — the silver nitrate page records the datasheets describing it as turning grey or black in light and on contact with organic material — and a reagent that has begun to reduce in its own bottle reports a stain that is not the sheet’s. A screw cap or a ground-glass stopper because a cork or a rubber bung is organic matter in contact with a silver salt.

No keeping figure is published, and none is supplied here. J-1 prints one on the facing page — mixed solutions stored in brown, stoppered glass bottles for one year — and that figure belongs to the stop bath and fixer testing solutions it sits under, not to this formula. Transferring it would be the course inventing a shelf life out of a neighbouring paragraph.

What to do instead of a figure. Make a small volume; 100 mL is a great many drops. Note the mixing date on the bottle, per the labelling procedure. And keep a never-fixed control sheet with the bottle: a reagent that stains that sheet has gone off, and that is a check the reader can run instead of a date.

Keep it away from the fixing bench entirely, on the silver shelf with the other silver salts, and away from food and drink and away from any kitchen container, jug or utensil.

Incompatibilities

Thiosulfate in any form — spent fixer, a wash tray, a bottle that held either. This is the reaction the test is for, and it is a contamination everywhere else. A reagent bottle that has met fixer is finished.

Sulfides, including the residual-silver test reagent and any sulfide toner. Silver ion and sulfide ion meet essentially irreversibly, so cross-contamination destroys both reagents and, if an acid reaches a sulfide, liberates hydrogen sulfide.

Ammonia and ammonium compounds. The HE-1 page and the silver nitrate page both carry the reason: ammoniacal silver solutions can deposit fulminating silver, and the course keeps ammonia and the silver bench apart. There is an ammoniacal silver quench procedure for the case where they meet anyway.

Chlorides and bromides, which precipitate silver halide instantly. Tap water high in chloride will cloud the reagent; Kodak’s advice for other silver-sensitive solutions in the same book is distilled water where chloride is in doubt.

Reducing agents and organic matter — every developer in this formulary, paper dust, a cork, a wooden stirring rod. All of them reduce silver ion to metal.

Strong oxidisers with the acetic acid, which its page lists: chromic acid, sodium peroxide, nitric acid. And metals, which acetic acid corrodes and which reduce silver.

Waste

Two streams meet in a very small volume, and the small volume is not a reason to be casual.

The silver is what governs. Silver nitrate carries H400 and H410 — very toxic to aquatic life, with long lasting effects — and silver is among the parameters most often limited in a discharge consent; Kodak’s own J-52 guidance on small volumes is where the course takes that. A 100 mL bottle of HT-2 holds about half a gram of silver, which is a trivial quantity to a photographer and not a trivial one to a receiving water.

This goes in the silver stream, but not into the spent-fixer bottle. That distinction matters. Spent fixer is held for recovery and is full of thiosulfate; tipping a silver nitrate reagent into it precipitates silver where the recovery process cannot reach it and consumes the reagent’s own evidence. Bottle the residue separately, label it as an acidic silver nitrate solution, and follow the silver-bearing waste procedure alongside the general chemical waste procedure.

The tested sheets are waste too, and they are silver-bearing waste of the ordinary photographic kind. A stained margin is a stained margin; keep it with the batch record if it is evidence, and dispose of it with paper waste rather than pretending it is a print.

The acid contributes little, being weak and dilute, but it is the reason this bottle must not share a container with anything sulfide-bearing.

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. Check your local regulations.

Troubleshooting

A deep stain on a sheet you are confident was well washed. Take Kodak Limited’s alternative explanation first: hydrogen sulfide or wood extracts in the water supply give a positive silver nitrate test in the absence of hypo. Run the drop on a sheet of the same paper that has never been near a fixer. If that stains too, the reagent is reporting your water, not your wash.

No stain at all, on everything, always. Suspect the reagent before the wash. A bottle that has met thiosulfate, sulfide or a reducing agent is exhausted and silent. The never-fixed control sheet under Storage catches this the other way round — it should stay clean — so pair it with a deliberately under-washed scrap, which should stain. A reagent that fails to stain that scrap is finished.

A pale stain after a washing aid. Kodak’s own caveat: after washing aids other than Hypo Clearing Agent the face may show less stain at equal hypo content. Do not upgrade the result. Record which aid the sheet went through, and compare only against sheets treated the same way.

The drop spread and there is no defined spot. On a film, this is Kodak’s stated reason for not using the spot method on wet film; cut a margin strip and immerse it instead. On paper, wipe the excess water off the face first, as the procedure says.

The stain will not photograph or scan consistently. Read it wet and at the stated time, against the same light each time. Kodak Limited’s version of the instruction is explicit — compare while wet, in subdued daylight or artificial light — because a drying gelatin surface changes what a tint looks like.

The bottle has gone grey or thrown a dark deposit. Photoreduction, or organic matter. Discard it into the silver waste, and check that the bottle is brown glass with a screw cap or a ground stopper.

You have no Hypo Estimator and cannot read a scale. This is the ordinary case and it is not a failure. Run the test as a comparison: a series of sheets from the same batch, one drop each, side by side, plus a never-fixed control and a deliberately under-washed scrap as the two ends. Say in the record that the reading is a comparison, not a measurement against Kodak’s tints.

Experiments

Build the washing curve you have been told about. One sheet of paper, processed and fixed with the batch, cut into six. Pull one piece from the wash at 5, 10, 20, 40 and 60 minutes, blot the face and put one drop of HT-2 on each; the sixth piece comes straight from the fixer, drained but unwashed, as the top of the scale. Rinse each after two minutes, lay them out in order and photograph them together, wet, in one light. Draw the drops with a pipette and never return it to the bottle, because the sixth piece is deliberately loaded with the one thing that ruins the reagent. This is the single most instructive hour in Part XII, and the result is yours rather than a manufacturer’s assertion.

Test Kodak’s own warning about washing aids. Two matched sheets, identical to the point of the wash; one washed in water alone, one through the one per cent sulfite bath and then the same total wash time. Drop both. Kodak predicts the aided sheet may show the paler stain at equal hypo content, so a paler stain here does not prove a better wash. Record what you see and record that you cannot resolve it, because you have no independent measure of hypo — which is precisely why Kodak offers the density route instead.

Take the reading off the eye and onto an instrument. Kodak’s own suggestion is transmission density after total immersion. Cut two clear film margins, immerse one in the reagent for three minutes and keep the other as the blank, dry both and read them on the densitometer. The difference is a number. Repeat it across a wash series and the curve above becomes a graph. State the geometry and the spectral response you used, because a stain is a coloured deposit and a densitometer’s answer depends on both.

Compare the three published tests on one batch. HT-2 as a spot; Kodak Limited’s 1 per cent silver nitrate as a three-minute immersion of a strip of the same sheet, read against its own untreated half; and, if you have the reagents and the controls the procedure requires, the permanganate test on the water draining off a film. They will not agree perfectly. Write down where they disagree and which of them your own record will cite in future, and why — that decision, made once with evidence in front of you, is worth more than any of the three results.

Watch the fading reaction on its own timescale. Two sheets from the wash series above, the well washed one and the badly washed one, kept together in the same enclosure for a year and re-examined. HT-2 told you in two minutes what one of them was carrying; this tells you what carrying it costs. It is a long experiment with a short protocol, and the only way a home darkroom can see the failure the whole of Part XII is written to prevent.