Plain sodium thiosulfate fixer for printed-out papers

One salt, one solvent, and one number that looks like a misprint. Every other hypo bath in this formulary carries between 150 and 480 grams of thiosulfate per litre. This one carries fifty. It is not a weak version of a fixer; it is a fixer for a different kind of image, and the difference is that the picture it has to leave behind weighs about three milligrams.

Ingredient	Quantity	Form the source specifies
Sodium thiosulfate pentahydrate	50 g	crystals, as the sheet specifies, and the sheet's own volumetric equivalent is approximately 4 heaping teaspoons
Water	1000 mL, added	Bostick & Sullivan give "per liter of cold tap water", which is water added rather than a make-up volume, and they publish no temperature beyond the word "cold". Fifty grams of a hydrated salt in a litre of water makes a little more than a litre of solution, so the bath is a shade under 5 per cent w/v; at this dilution nothing in the process turns on the difference, but the habit of noticing which of the two a source states is the whole reason the schema keeps them apart. Photographers' Formulary, mixing the same construction at twice the strength, call for water at 52 degrees C because the large crystals dissolve slowly and will not dissolve at all without stirring.

Purpose

To take the unexposed silver chloride off a printed-out sheet without taking the picture with it.

Those are two jobs and they pull against each other. Reilly’s inventory of what is actually in an exposed, unwashed print-out sheet is the place to start: the metallic silver image, the binder, and the unexposed light-sensitive salts — primarily silver chloride, with silver citrate, silver chromate or others present according to what went into the salting and sensitising solutions. Only the first of those is wanted. The rest has to be complexed away, and thiosulfate is what does it — of all the substances with some fixing or stabilising action, Reilly writes, sodium thiosulfate has the fewest drawbacks, and it has been in almost exclusive use for the job since 1839.

The drawback it does have is the second job. Ware’s numbers for a salted paper print are the argument for everything else on this page: the image is about 0.1 g of silver per square metre, some 3.3 mg on a whole-plate print, roughly a tenth of what a modern silver-gelatin print carries, and the particles are perhaps one hundredth the diameter of a modern image’s, so a comparable mass of silver presents about a hundred times the surface area. Thiosulfate attacks that silver too. A bath strong enough to strip a film in three minutes is aimed at the wrong target.

Recommended uses

Salted paper, which is what Bostick & Sullivan publish it for, and the albumen, arrowroot, matte albumen and collodion printing-out papers, whose fixing chemistry is the same. Reilly says so in as many words: his fixing procedure is applicable to all silver printing-out papers.

Anything printed out on hand-coated silver chloride, including photogenic drawings and the print-out end of the calotype family.

As the fixer in a bought printing-out kit, which is where most readers will meet it. Both of the kits this course has read ship plain hypo and nothing else for the purpose.

Not for anything developed out. A print or a negative on a modern bromide or chlorobromide emulsion has fifty to a hundred times the silver per unit area in far larger grains and carries developer alkali into the tray with it. Part XI’s fixers exist for that material and this one does not reach it.

When another formula is preferable

• Reilly’s alkaline bath, whenever permanence is the point. It is the same construction at three times the strength with 2 g of sodium carbonate per litre, and it comes with what a kit sheet does not: a working temperature, a two-bath schedule of four minutes each, and a published capacity. The carbonate is the substantive difference, and Reilly gives two reasons for it — slight alkalinity stops any acid that gets in from decomposing the thiosulfate and liberating sulfur, and it keeps the bath from attacking the finely divided silver of the image, which an acid fixer bleaches out of the highlights and mid-tones.
• The Photographers’ Formulary kit’s own bath, if you are working that kit. It is set out under Variants: the same salt at about twice this concentration, for ten minutes instead of four to five. Follow one sheet or the other; averaging two published procedures produces a third that nobody tested.
• Wall’s plain 400 g/L bath for negatives and developed papers, which is this construction at eight times the strength and is the wrong instrument here.
• F-5, F-52 or TF-2 for silver-gelatin work, and none of them for a printing-out paper. The two acid baths would bleach the image; all three would leave hardener or buffer salts in a paper base that has no baryta layer between the image and the fibres.

Mixing

Weigh the crystals; do not measure them. The sheet offers “approximately 4 heaping teaspoons” as an equivalent for the 50 g, which is a kindness to a reader with no balance and an invitation to a bath of unknown strength. Hypo crystals deliquesce slightly in damp air and cake into a block, so a heaped spoon of a fresh tub and a heaped spoon of a two-year-old one are not the same mass of thiosulfate. The weighing SOP takes ninety seconds and removes the variable.

Cold water is the supplier’s instruction and it is not the easy one. Bostick & Sullivan say cold tap water. Photographers’ Formulary, mixing twice the concentration, call for water at 52 °C and warn that the large crystals dissolve slowly, that the solid will not dissolve unless the solution is stirred, and that you should stir, leave it five minutes and stir again. Both are right about their own bath. Fifty grams in a litre will go into cold water with patience; a hundred grams in half a litre will not, in any reasonable time, without heat.

Dissolving hypo takes heat out of the water. The crystals are two-fifths water of crystallisation by mass and their dissolution is endothermic, so the finished bath is colder than the water you started with. Reilly turns that into an instruction — make the bath with water slightly warmer than the working temperature you want, which for these papers is 18 to 20 °C — and then into a warning that matters for albumen in particular: too great a temperature difference between the fixer and the other trays can blister a heavily albumenised sheet.

There is no mixing order to get wrong, and that is worth saying because it is the only place in the printing-out process where nothing can go wrong. One solid into water. Nothing precipitates, nothing has to be cool before something else goes in, nothing decomposes on contact. The fixer-mixing SOP still applies, mostly for the labelling.

🧮What a 5 per cent bath is, and what it could hold

50 g in 1000 mL of water = a shade under 5 % w/v

As mass concentration, per litre of water added

50 g ÷ 248.19 g/mol = 0.20 mol/L of thiosulfate

As moles

Against the other hypo baths in this formulary: 0.20 mol/L here, 0.60 in Reilly’s alkaline bath, 0.97 in F-5, 1.61 in Wall’s plain bath. Kodak’s 1928 primer puts the optimum for speed of fixation at 30 to 40 per cent. This bath is nowhere near it, and speed is not what it is optimised for.

The ceiling nobody works near. Two thiosulfate ions are consumed per silver ion, so 0.20 mol/L is in principle enough to complex 0.10 mol of silver — 10.9 g — per litre. Now put a print against that. Ware’s 1855 analyses found that only about 2 per cent of the silver taken up by a sensitised sheet remains in the finished image, at an image coating weight of about 0.1 g/m²; so the sheet took up something like 5 g/m², and an 8 × 10 inch print (0.052 m²) started with roughly 0.26 g of silver in total. That is an upper bound on what the fixer must carry, because the wash before fixing has already taken the free silver nitrate off as the milky cloud. Even so, a litre of this bath could stoichiometrically swallow forty such prints.

And Reilly stops at ten to fifteen. His conservative limit is 10 to 15 prints of 8 × 10 per litre of a 15 per cent bath — which offers that bath about 3.9 g of silver against a ceiling of 32.6 g, so he throws it away at roughly an eighth of stoichiometry. That is the whole lesson of fixer capacity in one ratio: the bath does not stop working when the thiosulfate runs out, it stops making the soluble complex long before.

Two inferences, marked as inferences. Scale Reilly’s one-eighth rule to this weaker bath and you get about 1.3 g of silver per litre, or five 8 × 10 prints. Bostick & Sullivan publish no capacity at all, but they ship 250 g of thiosulfate — five litres of bath — with a kit they say makes about fifty prints, which works out at ten prints per litre. Neither figure is published as a capacity by anybody. If they are anywhere near right, a reader who cares about permanence should mix more fixer than the kit’s arithmetic implies, or split it into two baths, and should measure their own capacity with the clearing-time test rather than trusting either number.

Behaviour

It fixes quickly, and the published times are short. Four to five minutes with periodic agitation, in Bostick & Sullivan’s sheet. Ten minutes in the Formulary kit’s stronger bath. Four minutes plus four minutes, with constant agitation, in Reilly’s two-bath schedule. Nothing here is a long fix, and the shortness is deliberate.

Longer is not safer, and on these papers it is specifically dangerous. Reilly is emphatic twice over: fixation is completed fairly rapidly, and prolonged fixation is much more injurious to prints than is generally believed, because thiosulfate that gets inside the paper fibres becomes almost impossible to remove; and, in the washing chapter, that it is very important that immersion is not prolonged beyond the recommended time, or the base paper becomes a reservoir of image-threatening substances. A salted paper print has no baryta layer and no gelatin substratum: the image silver is sitting in intimate contact with the fibres that the thiosulfate is soaking into.

Over-fixing bleaches. Bostick & Sullivan say it in one line — over-fixing can lead to bleaching of the image, while under fixing will cause archival permanence issues — and Ware supplies the mechanism under The mechanism below. The bath that is dissolving your silver chloride is also, slowly, converting your silver to silver sulphide.

It exhausts invisibly, and neither supplier says when. Neither sheet publishes a capacity, and this page prints none, because there is none to print. What is established is the direction: as a thiosulfate bath approaches exhaustion it loses the ability to form the soluble silver–thiosulfate complexes, and the ones it forms instead cannot be washed out. Reilly’s reason for a conservative limit is specific to this material — the silver content of printing-out papers is very high relative to conventional develop-out materials, and it varies from paper to paper.

Which is why the answer is two trays. Reilly’s rule is not an embellishment: because one of the complexes formed during fixation is soluble only in fresh thiosulfate, the first bath does the bulk of the complexing and the second exists to make sure that what the print finally carries is the soluble kind. The two-bath rotation SOP is written for exactly this.

It does not keep. Thiosulfate in solution partially decomposes to sodium sulfite and elemental sulfur, so Reilly’s instruction for these papers is that the fixer be made up just before use. A tray of last week’s hypo is not a saving.

Image characteristics

The print changes a great deal, and almost none of it is the fixer’s fault. Reilly’s account is the one to have in your head before you fix a salt print for the first time, because otherwise you will think you have ruined it. During fixation the original brilliant purple or brown becomes a much yellower and duller brown, with a loss of density. The cause is physical, not chemical staining: in the unfixed sheet the image silver is dispersed through the silver chloride almost as a solid solution, and when the chloride is removed the silver particles pack into aggregates while the refractive index of the layer falls. Covering power drops. The colour shifts.

Then it comes back. The print reddens when it first meets wash water — that is the image layer swelling — and on drying it becomes more neutral and gains slightly in density as the layer contracts. Both suppliers turn this into a printing instruction rather than a piece of theory. Bostick & Sullivan: pull the print from the fixer at about three quarters of the density you want. Photographers’ Formulary, more usefully for calibration: expose a step table and mark the lightest step that shows darkening, because that step and usually four more are lost through washing, toning and fixing, so you print to lose them deliberately.

⁇Does the print darken in the fixer, or lighten?

The two accounts do not agree and the disagreement is worth naming rather than smoothing over.

Reilly describes an unambiguous loss of density during fixation, with the colour going yellower and duller, and gives the physical mechanism for it.

Bostick & Sullivan’s sheet tells you to transfer the print to the fixer and watch as the image darkens, and says salted paper prints darken considerably during washing, fixing, toning and drying.

They can be partly reconciled — a print does end up darker and richer after the whole sequence than it looked when it came out of the frame, and the drying stage is where much of that happens — but Reilly is describing the fixing step alone, from measurements of what fixation does to covering power, and the kit sheet is describing what a beginner sees across a workflow. Where a conservation monograph and a kit instruction conflict on a matter of physical chemistry, this course follows the monograph and tells you the other exists. Watch your own print and record what you see; it is a five-minute observation and it settles the question for your paper.

A faint sepia of the bath’s own. Ware attributes something the practical literature usually calls “the colour coming up” to the fixer itself: pure 10 nm silver appears yellow-orange, and treatment in a hypo bath imparts a thin layer of silver sulphide which deepens the colour of nanoparticle silver to a more satisfying brown. A little sulphiding enriches a print-out image. Complete sulphiding destroys it. The whole of this bath’s design sits between those two sentences.

The mechanism

The reaction you want:

AgCl + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Cl⁻

Silver chloride into the soluble bis(thiosulfato)argentate complex

Silver chloride is insoluble in water because the lattice holds Ag⁺ more tightly than water can pull it away. Thiosulfate binds silver more tightly still, through sulfur, and the resulting anion dissolves freely. This is Herschel’s discovery of January 1839, made possible by his own 1819 work on the hyposulphites; Ware notes that Herschel, who was precise, called it “washing out” and reserved “fixing” for Talbot’s quite different halide treatment, in which excess chloride or iodide leaves the silver salt in place but desensitised.

Why an excess is a condition and not a margin. Reilly: there are probably at least three different kinds of silver–thiosulfate complex formed during fixation, and thiosulfate ions must be present in excess — more than are needed to react with all the silver present — or else insoluble complexes are formed which cannot be washed from the image layer. One of the complexes is soluble only in fresh thiosulfate. That is the entire argument for two baths, for a short fix, and for throwing a bath away long before the arithmetic says it is spent.

The reaction you do not want, in the three steps Ware actually publishes. He starts from the thermodynamics: silver sulphide has one of the smallest solubility products known for any binary salt, about 6 × 10⁻⁵⁰, and the redox potential of the couple is −0.71 V, which tells you that in the presence of sulphide ion metallic silver is quite a powerful reducing agent, comparable with metallic zinc. Thiosulfate does not need to attack the silver directly; it only has to release sulfur, and under acid conditions it disproportionates to do exactly that.

5 S₂O₃²⁻ + 6 H⁺ → 2 S + 2 S₄O₆²⁻ + 3 H₂O

Ware: thiosulfate disproportionating under acid conditions, to sulfur and tetrathionate

2 S₂O₃²⁻ → 2 S + 2 SO₃²⁻

Ware: the other route, to sulfur and sulfite, depending on the pH

2 Ag + S → Ag₂S

Ware: the colloidal sulfur so released is a mild oxidant towards silver

And the damage is one-way. Ware notes that a sulphide-faded image, unlike an iodide-faded one, cannot be brought back by redevelopment in an ordinary photographic developer, whose redox potential is not negative enough; only something as powerful as borohydride will reduce it, which is a conservation laboratory’s problem and not a darkroom’s.

⚗Why this bath is weak, and what is and is not established about it

Three findings, each from a named source, and then the part that is inference.

Established: sulfide attack on the image is real and is favoured by acid. Ware states that silver sulphide can form directly by the reaction of thiosulphate ions with nanoparticle silver, especially under acidic conditions, and that the minute spheres first acquire only a surface coating — like the skin of an apple — while prolonged contact converts them completely. Reilly gives the same fact from the practical end: an acid fixing bath would tend to attack the finely divided metallic silver of the image, causing excessive bleaching of the highlights and middletones.

Established: the consequence is out of all proportion to the amount of silver involved. Complete conversion of nanoparticle silver to silver sulphide drops the optical density by a factor of about 30, in Ware’s photometric comparison. In a modern paper the same conversion is far less visible, because the silver is more concentrated and the particles are larger. The salt print’s tenuity is what makes sulphiding a catastrophe rather than a toning.

Established: time matters. Both authors say so, from different evidence — Ware from the chemistry of the particle surface, Reilly from what prolonged fixation does to the paper base.

Inference, and marked as one: that the weakness of the bath is itself a defence. Neither supplier explains why they publish 50 g/L rather than 150 or 400. The course’s reading is that a weak bath is a defence in the second half of the process rather than the first — it puts less thiosulfate into the paper fibres to be washed out again, which is the failure Reilly and Ware both identify as the one that kills these prints. It is not evidence that a weaker bath sulphides less per minute, and this page does not claim it. If you want the claim tested, the last experiment below tests it.

And what is not established at all: the pH. Neither sheet publishes one, and the course does not supply one. What can be said is structural. There is no acid in this formula, so it is not an acid fixer in the way F-5 is; and there is no alkali in it either, so it is not deliberately alkaline in the way Reilly’s bath is. It is near-neutral by construction, not by measurement, and it sits on the neutral side of a boundary that Reilly says should be crossed on purpose. If you own a pH meter, this is a formula worth measuring; see the calibration SOP first.

Function of every ingredient

Sodium thiosulfate, 50 g of the crystals. The complexing agent, and the whole formula. It supplies a ligand that binds silver more strongly than the chloride lattice does, two thiosulfate ions to each silver, converting insoluble silver chloride into a soluble anion that can be washed out. Reilly’s survey of the alternatives — ammonia, potassium cyanide, strong chloride solutions, thiocyanates, thiourea, sodium sulfite, and the sodium and ammonium thiosulfates — ends with the judgement that thiosulfate has the fewest drawbacks, which is a long way from having none.

More of it — Reilly’s 150 g/L, the Formulary’s 100 g/L, Wall’s 400 g/L — fixes no faster in any way that matters here, since Kodak’s own optimum for speed is far above all of them anyway, and loads the paper with more thiosulfate to be washed back out. Less of it moves the bath towards the region where the insoluble complexes are the product, which is the failure that does not show up for years. Neither direction is free, which is why the honest answer to “how strong?” on this page is “as your sheet says, and measure your own capacity”.

The form is the pentahydrate, and the label may not say so. Photographic hypo is Na₂S₂O₃·5H₂O at 248.19 g/mol; the anhydrous salt is 158.11. Fifty grams of the crystals is 31.9 g of the anhydrous salt. Weigh 50 g of anhydrous by mistake and the bath is 0.32 mol/L instead of 0.20 — half again as strong, and past the Formulary kit’s strength without meaning to be.

Water, 1000 mL, added rather than made up to. Not quite inert. It supplies the heat that dissolving a hydrated salt consumes, which is why the finished bath reads colder than the tap; and its temperature relative to the other trays is a working variable rather than a detail, because Reilly records that too great a differential blisters heavily albumenised paper. Tap water is what the sheet specifies; the Formulary’s kit uses distilled, which costs nothing to prefer if your supply is hard.

Interactions

With the wash that comes before it, decisively. Both sheets insist on it and they are not talking about the same thing as the final wash. Photographers’ Formulary: the free silver nitrate must be washed from the print prior to toning and fixing, in a tray rather than under running water. Bostick & Sullivan: five to seven minutes, agitating every twenty to thirty seconds, until the white milky cloud stops coming off. Silver nitrate carried into a hypo bath is silver that will be complexed at the fixer’s expense — and, per Ware’s list of thiosulfate oxidants, silver nitrate is one of the reagents that oxidises thiosulfate to tetrathionate, which sulfides silver.

With gold and platinum toners, in both directions. A printing-out paper is normally toned before fixing, so the two baths meet through the print. Reilly’s warning about the platinum toner is the sharp one: impurities such as silver nitrate or sodium thiosulfate rob a platinum bath of its activity by altering the platinum to an irreducible condition, so prints must be washed before toning and again after it, so that no acidic toner is carried into the fixer. Photographers’ Formulary take the opposite line for their alkaline gold-borax bath — the print need not be washed between toner and fixer — but add the same prohibition from the other side: do not contaminate the toning bath with fixer. Separate trays, separate tongs, and the fixer tray last.

With the wash and the washing aid, favourably. There is no alum in this bath hardening the layer against diffusion, and there is no gelatin substratum either, so thiosulfate leaves a plain salted sheet readily — until the concentration gets low, where Reilly notes the rate slows down tremendously and water alone stops being able to finish the job. His remedy is the one per cent sodium sulfite washing aid, which displaces absorbed thiosulfate and replaces it with more soluble and less harmful ions; his full sequence is 2 to 4 minutes of running water, 3 to 4 minutes in the sulfite with constant agitation, then 30 minutes of washing. Kodak Hypo Clearing Agent will also serve. Bostick & Sullivan’s 20 to 30 minutes of plain running water is the shorter, kit-scale version of the same idea.

With acid, badly, and there is nothing to restrain it. No sulfite is present. Kodak’s 1928 primer describes what a few drops of a strong acid do to a weak hypo solution: it decomposes, the solution turns milky, and sulfur separates. A dilute bath has less thiosulfate to lose before that matters.

With a used bath, historically and catastrophically. Ware’s section on “old hypo” is the cautionary tale of this whole family of baths. About 1850 Blanquart-Evrard noticed that a much-used fixing bath coloured salt prints a satisfyingly rich brown; photographers adopted the “old hypo colouring bath” enthusiastically, some of them ageing fresh baths artificially with acid, silver nitrate and oxidising agents — and by about 1858 it was widely acknowledged that the prints so treated had faded drastically. An exhausted fixer is an uncontrolled sulphiding bath. That is not an interaction to exploit.

Variants

Photographers’ Formulary, Solution E: the same construction at about twice the strength. Their P.O.P. kit publishes the fixer as 50 g of sodium thiosulfate pentahydrate in 500 mL of distilled water at 52 °C, with a 10-minute immersion “to remove all silver salts” and a final wash of at least an hour. It is a separate published bath, not a correction of this one, and the pair are useful together: two current suppliers of the same process, differing by a factor of two in concentration and by a factor of two in time, in a corner of photography where nobody has published a controlled comparison. Both are below every developed-out fixer in this formulary. Follow whichever sheet came with your chemistry.

Reilly’s alkaline bath: the conservation-grade version. 150 g of the pentahydrate plus 2 g of sodium carbonate to make a litre, two baths of four minutes each, 18 to 20 °C, made fresh for each session, 10 to 15 prints of 8 × 10 per litre. If you are printing anything you want to still be there in fifty years, that page is where to go; this one is what is in the tray beside most people’s kits.

Wall’s plain bath: the same idea for the other material. Four hundred grams per litre, and Wall assigns the plain and alkaline baths to printing-out papers while sending plates, film and developed papers to the acid and acid-alum ones. He does not publish a weak plain bath, and the modern kit sheets do not explain their move away from his strength; the gap between 1924 and now is not documented anywhere this course has been able to read.

Halide “fixing”, which is not this at all. Ware’s section 7.5 draws the distinction that the modern word has blurred. Talbot’s original treatment — a strong solution of common salt, or a weaker one of potassium iodide — leaves the unexposed silver salt in place and merely renders it unresponsive to light; Herschel’s thiosulfate removes it. The first is stabilisation, the second is fixing. Chloride- stabilised images tended to fog to a purplish grey and iodide-stabilised ones to fade to primrose yellow, which is why the thiosulfate bath won even though it brought a new problem with it.

“Old hypo”, which is a variant nobody should reproduce. See Interactions. It is on this page because a reader will meet the phrase in nineteenth-century manuals and should know that its products are the faded ones.

Safety

Level A, the same as the plain bath it is a dilution of, and the least hazardous solution anywhere in the printing-out process. One substance, in water, at 5 per cent.

That classification rests on thin evidence rather than a clean bill of health, and the chemical page is careful to say so: of 281 company reports aggregated by PubChem, 277 state that sodium thiosulfate does not meet GHS criteria, but only about 1.4 per cent of companies supplied any information at all. Nitrile gloves, eye protection, and an apron — dried fixer leaves a white bloom that will mark the next print it touches.

The hazard in this process is in the other trays. Bostick & Sullivan’s own sheet flags the 12 per cent silver nitrate (stains everything black, including you) and Photographers’ Formulary flag the potassium dichromate contrast solution as toxic, an oxidiser, and a compound of a class they describe as potential carcinogens, and the gold chloride as a caustic. Read those pages and the silver nitrate SOP before you read this one. The fixer is the safe end of a process that is not uniformly safe.

Keep acid away from it — see Incompatibilities. That is the only way an ordinary worker turns this bath into a hazard.

Storage

Do not store the bath. Mix it for the session. Reilly is explicit for these papers: thiosulfate in solution is not stable and partially decomposes to sodium sulfite and elemental sulfur, so the fixer should preferably be made up just prior to use. Fifty grams is a small enough quantity that there is no economy in keeping it. His procedure for albumen and salted paper puts both baths freshly made on the day they are used.

Store the dry crystals instead. Dry, cool, dark and tightly closed, in a labelled container that has never held food. They deliquesce slightly and cake; a caked block is still weighable, which is another argument against the teaspoon. Bostick & Sullivan’s five-year shelf-life claim covers the kit’s bottled solutions rather than a bath you mix.

Label the tray and the bottle with the formula, the strength, the date and the count of prints the bath has taken, per the labelling SOP. On a bath with no published capacity the print count is not bookkeeping, it is the only capacity figure that will ever exist for your paper.

Do not leave a print in it while you deal with something else. Storage advice on this page includes the print: an unfixed sheet has a holding procedure, and a fixed one belongs in the wash.

Incompatibilities

Acids, first and worst. There is no sulfite here to restrain the reaction and less thiosulfate to spare than in any other fixer in this formulary. Stop bath, vinegar, descaler, an acidic platinum toner carried over on a print, an acid waste bottle standing next to the fixer waste bottle: all of them out.

H⁺ + S₂O₃²⁻ → S + HSO₃⁻

Ware's form of the acid decomposition: colloidal sulfur, and a bath that is now a sulphiding agent

Oxidising agents. Ware names iron(III) chloride, silver nitrate, iodine and nitric acid: oxidation of thiosulfate gives tetrathionate, which sulfides silver metal and attacks an image. That is the chemistry behind “old hypo” and behind the failure of every hypo eliminator this course declines to recommend.

Silver nitrate, both as an oxidant and because a sensitiser bath and a fixer that meet on a bench, a tong or a tray are the classic way to ruin a printing session. Dedicated trays, labelled, as both suppliers insist.

Potassium alum and alum hardeners. Alum in water is weakly acid, and Kodak’s primer records that alum added to plain hypo with no sulfite present turns the bath turbid and precipitates sulfur. There is no reason to harden a salted paper print and every reason not to.

Heat and long standing, which decompose it in the tray. See incompatibilities.

Waste

Silver-bearing, and disproportionately so. Reilly’s point about capacity is also the point about waste: the silver content of printing-out papers is very high relative to develop-out materials, so a litre of spent salt-print fixer carries more silver than a litre of spent film fixer that has done far more work. So does the wash before it, which carries the free silver nitrate that never became silver chloride.

Collect both. The pre-fixing wash water is a silver stream, not a rinse, and the silver-bearing waste SOP covers it along with the fixer. Keep it away from acid wastes. See silver recovery and waste streams for what recovery actually involves.

A supplier method this course reports and does not adopt. Bostick & Sullivan’s albumen kit sheet describes a home cementation route — two steel wool or copper scouring pads left in a litre of exhausted fixer for 36 to 48 hours — and concludes that the remaining solution is then safe to put down a sewer or septic system. The chemistry of cementation is real and is taught in Part XII. The disposal conclusion is not the course’s to endorse: local regulation decides what may enter a drain, the silver is not the only reason spent fixer is a problem (it is also strongly oxygen-demanding), and a home cementation is not a verified end point. Read the disposal ruling and ask your own authority.

Troubleshooting

The print went pale, flat and buff-coloured in the fixer, and stayed that way. Over-fixed. Ware’s end state: complete conversion of nanoparticle silver to silver sulphide, and a fall in optical density by around a factor of thirty. There is no cure. Cut the time to the sheet’s four to five minutes and check the clock rather than the print.

The print lost density and went yellowish-brown, but looks right again dry. That is not a fault, it is Reilly’s colour change and the drying recovery. Print to lose four or five steps and it will land where you wanted.

Fixing takes noticeably longer than it did. The bath is loading up. Time a clearing test on a scrap of coated, unexposed paper; when the time has roughly doubled against fresh, the bath is past the point where the soluble complex is reliably the product. Mix a fresh one.

The bath went milky and smells of burnt matches. Acid got in. Ventilate, stop, and read incompatibilities. Do not put a print in it.

A white milky cloud is still coming off the print in the fixer. The wash before the fixer was too short. That cloud is free silver, and it should have gone into the wash tray, not into the bath you are about to reuse. Wash for the full five to seven minutes next time, and change the wash water.

Albumen prints blistered. Temperature differential between the trays. Reilly’s fix is to bring the fixer up to the working temperature of the other solutions rather than mixing it cold.

The gold or platinum toner has stopped working. Thiosulfate contamination, per Reilly’s warning about irreducible platinum. New toner, and separate tongs from now on.

Prints yellowed after a year in a box. The invisible failure, and the reason for every procedural rule above: residual thiosulfate in the paper fibres, or insoluble silver–thiosulfate complexes left by a tired bath. It cannot be diagnosed on the day and cannot be reversed afterwards. Test with HT-2 and ST-1 while the print is still wet — the residual hypo and silver SOP has the procedure.

Experiments

Measure the density loss. Coat and expose two identical strips of salted paper, read them with a densitometer or scan them against a step wedge, fix one and dry both. The difference is Reilly’s covering-power argument in numbers, and it is also the number you need to know how far to over-print. Ten minutes of work, and it settles the contested callout above for your paper.

Find your own capacity. Fix a run of prints in a single litre, and after every print clear a scrap of coated, unexposed paper in it and time the clearing. Plot clearing time against prints fixed. The knee in that curve is the only capacity figure that will ever be true for your paper, your silver bath and your water. Compare it with the two inferred numbers in the maths callout — five prints per litre, and ten — and see which the curve supports.

Test the strength question directly. Mix 5, 10 and 15 per cent baths, fix identical strips in each for the same four minutes, wash all three identically, and test each with HT-2 for residual thiosulfate and ST-1 for residual silver. The course’s inference above is that the weak bath wins on the first test and risks losing on the second. That is a prediction, not a finding, and this experiment is what would turn it into one.

Make an “old hypo” print on purpose, and then leave it. Fix one print in fresh bath and one in a bath you have deliberately worked to exhaustion, wash both identically, and put them in the same envelope for a year. Ware’s nineteenth-century photographers thought the second one looked better on the day. It is the slowest experiment in this formulary and the one that makes the strongest argument.

Watch the temperature drop. Take the water’s temperature, dissolve the 50 g, and take it again. It is a small fall at this concentration — try it beside a 400 g litre of Wall’s bath if you want to feel it properly — and it is Reilly’s reason for mixing with water slightly warmer than the temperature you want to work at.

Try the halide route. Fix one photogenic-drawing strip in this bath and stabilise another in a saturated salt solution, as Talbot did, and keep both in the same light for a month. Ware’s account of why thiosulfate won, and of what it cost, becomes a great deal more concrete when the purplish-grey fog appears on your own paper.