Alkaline plain-hypo fixing bath

Two grams of carbonate in a litre is the smallest deliberate quantity in this formulary, and it is the whole difference between a bath that fixes a salted-paper print and one that eats it. Everything else here is plain hypo at a lower strength.

Ingredient	Quantity	Form the source specifies
Sodium thiosulfate	150 g	pentahydrate
Sodium carbonate	2 g	Reilly names no hydrate
Water	to make 1000 mL	slightly warmer than the 18 to 20 °C working temperature

Purpose

To remove the unreduced silver chloride and the other light-sensitive silver salts — silver citrate among them — from a printing-out image, without attacking the image itself. On a developing-out material those two aims do not conflict. On a printing-out paper they do, and the carbonate is how Reilly resolves them.

Reilly gives two reasons for the alkalinity and they are different in kind. The first is chemical insurance: the slight alkalinity prevents any acids which might be inadvertently introduced into the fixing bath from decomposing the sodium thiosulfate and liberating sulfur. The second is the one that decides the formula: an acid fixing bath would tend to attack the finely divided metallic silver of the image, causing excessive bleaching of the highlights and middletones, and that attack is minimised when the pH is kept on the alkaline side. A third, from his albumen chapter, is mechanical — the alkali promotes more effective fixation and helps to prevent the blistering of the albumen layer on heavily albumenised papers.

Recommended uses

All silver printing-out papers. Reilly says so directly: the alkaline 15 per cent bath is useful for all albumen and salted papers, and the procedure he gives in his summary chapter is applicable to every printing-out paper.

Two baths, four minutes in each, constantly agitated, with at least a five-second drain between them. Eight to ten minutes in total.

Made fresh for the session and thrown away after it. This is not a bath that is kept.

After the print has been washed free of anything acidic — particularly a platinum or gold toning bath. Reilly is explicit that the prints must be washed free of other substances before they are placed in the fixer, and gives 3 to 5 minutes of running water between toning and fixing.

When another formula is preferable

• For film, plates and enlarging papers, F-5 or F-52. A developed-out image is coarse-grained metallic silver in a hardened gelatin layer and is not troubled by a mildly acid bath; it is troubled by development continuing in the fixer, which is what an unbuffered bath allows.
• Where you want to see fixing with nothing else in the tray, plain hypo, which is this formula without the carbonate and at nearly three times the strength.
• Where an acid non-hardening bath is genuinely wanted for paper, F-52 — but not for a printing-out process, for the reason above.
• There is no case for a hardening fixer here at all. Reilly’s formula is specified as one to which no hardeners have been added, and hardening an albumen or salted-paper layer makes the washing that follows harder in a process whose permanence already depends on that wash.

Mixing

Water first, warm. Reilly’s instruction is unusual and worth reading twice: make the bath with water slightly warmer than the working temperature of 18 to 20 °C, since some heat is always consumed in the formation of the solution. Dissolving 150 g of a hydrated salt cools the water measurably, so a bath mixed at working temperature arrives below it.

Then the hypo, then the carbonate. There is nothing to precipitate and no order that can be got dangerously wrong. Adding the carbonate last simply means you can see it dissolve in a solution that is already clear.

Get the temperature right before the print goes in. This is the one procedural hazard in the formula and it is not a chemical one: Reilly warns that too great a temperature differential between the fixing bath and the other processing solutions might cause blistering of albumen paper, and that changes of temperature or pH are the main causes of blisters. Large blisters, or blisters that break, usually ruin the print.

Two trays, both fresh. The second bath is not a rinse and it is not last session’s first bath. Both are made up on the day.

🧮Fifteen per cent, two grams, and why neither number is arbitrary

150 g ÷ 248.19 g/mol = 0.60 mol/L, a 15 % w/v bath

Thiosulfate

2 g ÷ 105.99 g/mol = 0.019 mol/L

Carbonate, read as the anhydrous salt

The hypo is less than half the strength of Wall’s plain bath and under two-thirds of F-5. That is not a compromise: a bath discarded after ten prints never reaches the strength-limited regime, so the extra hypo would buy nothing and would have to be washed out of the paper afterwards.

The carbonate is about thirty times less than a developer carries and its job is correspondingly small — to sit on the alkaline side of neutral, not to make an alkaline solution. That is why the hydrate question in the provenance note matters less than it looks: read as the decahydrate, 2 g is 0.74 g of anhydrous carbonate and 0.007 mol/L, which is still carbonate in a bath that would otherwise have none. Both readings do the stated job; they differ in how much accidental acid the bath could absorb before it stopped doing it.

Behaviour

It exhausts fast, and Reilly’s numbers say how fast. The literature figure he quotes is no more than 150 prints of 8 × 10 inches per litre of 15 per cent hypo. His own recommendation is no more than 10 to 15. That is a factor of ten, and he explains it rather than asserting it: the silver content of printing-out papers is very high relative to developing-out materials and varies between rawstocks, and the colloidal silver image is vulnerable. Compared with the other costs of albumen and salted-paper printing, he writes, the price of fixer is cheap.

It is difficult to know when it is spent, and that is stated rather than hidden. Reilly: the value of the maximum solubility of silver chloride in sodium thiosulfate is known, but it does not establish the practical limit, because excess thiosulfate is required for permanence. There is no clearing test here of the kind a negative gives you, because a printing-out print has no milky opalescence to lose. What there is instead is a print count and a discipline.

Unused solution breaks down. Reilly’s summary chapter says it plainly — even unused fixer solutions break down very rapidly — which is the reason both baths are made on the day rather than the day before.

The prints change colour in it, dramatically, and that is not a fault. See Image characteristics.

Fixing longer is a mistake, not insurance. Reilly: the fixing process is completed fairly rapidly, and prolonged fixation is much more injurious to prints than is generally believed, because thiosulfate that penetrates the paper fibres becomes almost impossible to remove. Four minutes means four minutes.

Image characteristics

The print will look worse coming out of the fixer than it did going in, and that is expected. Reilly describes the change: the brilliant purple or brown of the exposed print becomes a much yellower, duller brown, with a loss of density. The reasons are physical rather than chemical damage.

In an exposed but unfixed print the metallic silver particles exist in a highly dispersed state, forming a kind of solid solution of silver in silver chloride, and the colour and density of that system depend on the particle size and on the combined refractive indices of the silver chloride and the binder. Fixing removes the silver chloride. The silver particles then pack into aggregates and the refractive index of the system falls, so covering power drops and the colour shifts towards yellow.

Two corollaries a beginner needs. The print reddens again in the first wash, because the image layer swells; and it regains a little density and neutrality as it dries, because the layer contracts. Judge nothing wet.

What the bath must never contribute is bleaching of the highlights and middle tones. If that appears, the bath has gone acid — from carry-over, from a contaminated tray, or from a carbonate that was never added.

The mechanism

AgCl + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Cl⁻

Fixing a printing-out image: silver chloride into the soluble complex

The fixing chemistry is the same as every other bath in this section: thiosulfate binds silver more strongly than the halide lattice does, two ligands to each silver, and the resulting anion is soluble. What is different here is the material the bath is working on and the second thing in the tray that thiosulfate can attack.

Why acid is the enemy of this particular image. The image silver of a printing-out print is far more finely divided than the filamentary silver of a developed image — it is what gives printing-out papers their long scale and their colour — and finely divided silver has a great deal of surface. An acid bath attacks it, and Reilly’s word for the result is bleaching of highlights and middle tones, because the thinnest deposits go first.

Why alkali also protects the bath from itself. Thiosulfate in acid decomposes:

H⁺ + S₂O₃²⁻ → S + HSO₃⁻

What two grams of carbonate is insurance against

A print carrying traces of an acid toner, a tray that last held stop bath, water that is naturally acidic: each of these can push an unbuffered plain bath over. Two grams of carbonate does not make the bath resistant to a real dose of acid — it is a very small reserve — but it moves the starting point to the right side of neutral and keeps it there through the small insults.

Why two baths, and not one for eight minutes. Thiosulfate must be present in excess or complexes with fewer thiosulfate ligands per silver form, and those are insoluble and cannot be washed from the image layer. The first bath does the bulk of the complexing and becomes silver-rich; the second, still nearly fresh, ensures that what the print carries out is the soluble complex. It is the same argument that Kodak’s 1924 primer makes for negatives, applied to a material carrying several times as much silver.

⚗The silver this bath removes has to go somewhere

This is the formula behind open question 1.3. Salted paper, albumen and Van Dyke shed far more silver than the darkroom parts of this course, and most of it lands in the first wash rather than in the fixer — which makes the first wash a silver-bearing stream and not water.

The course could not source a measured silver concentration for a printing-out first wash, and it refuses to borrow Kodak’s figure for a developing-out one. What it does instead is collect the whole stream and say why. The disposal ruling carries the reasoning and the standing jurisdictional caveat; the silver-bearing waste SOP carries the container.

Function of every ingredient

Sodium thiosulfate, 150 g of the pentahydrate. The fixing agent. It complexes silver out of the residual silver chloride and out of the other silver salts a salted-paper sensitiser leaves behind — Reilly names silver citrate among them — two thiosulfate ions to each silver, giving a soluble anion that the wash can remove. More than 15 per cent gains nothing here, because the bath is discarded on a print count long before its thiosulfate is used up, and every extra gram is another gram to wash out of the paper afterwards. Less moves the bath towards the region where the insoluble complex is the product, which on a printing-out paper is the failure mode that matters. Note the form: Reilly specifies the pentahydrate, and 150 g of the anhydrous salt would be a 24 per cent bath.

Sodium carbonate (anhydrous), 2 g. The alkali, and it is present in an almost homeopathic quantity by the standards of the rest of this formulary. Its job is not to make the bath alkaline in any strong sense but to put it on the alkaline side of neutral and keep it there. Three consequences follow, all Reilly’s: the finely divided image silver is not attacked, so the highlights and middle tones are not bleached; small acid contaminations do not decompose the thiosulfate to sulfur; and heavily albumenised paper is less likely to blister. More carbonate is not obviously harmful but is not what he specifies, and a strongly alkaline bath is a different proposition — an alkalised ammonium thiosulfate bath releases ammonia, which is why the ammonium thiosulfate page records a manufacturer’s ceiling on pH for those. Less, or none, and you have plain hypo with a printing-out paper in it.

The form of the carbonate is not stated by the source and the course does not supply it. Reilly writes “sodium carbonate”. The course reads an unqualified name as the anhydrous salt because that is the identity its own chemical registry assigns; if the decahydrate was meant, the bath carries 0.74 g of anhydrous equivalent instead of 2 g. Both are small, both do the stated job, and the difference is in how much accidental acid the bath tolerates before it stops doing it. Reilly’s own sulfite alternative for the same purpose — he notes that dry sodium sulfite may be substituted in the older formulas of this type — would be a third reading again.

Water, to make 1000 mL, slightly warm. The temperature is a specification here rather than a convenience, at both ends. Warm enough that the endothermic dissolution leaves the bath at working temperature; close enough to the other trays that albumen does not blister.

Interactions

With the toner before it, badly if the rinse is skipped. Reilly’s warning is specific: interaction between the fixer and the toner may cause changes in the fixing solution that would damage the colour and permanence of the prints, and an acidic platinum or gold toning bath carried into an alkaline fixer is exactly the accidental acid the carbonate is insurance against. Three to five minutes of running water between the two trays.

With the wash after it, favourably. No alum, no hardening, so the layer stays permeable and the thiosulfate diffuses out. The sequence Reilly specifies is a 2-to-4-minute running-water wash, then the 1 per cent sulfite washing aid for 3 to 4 minutes, then 30 minutes of final washing. The short wash first exists to avoid overloading the ion exchange in the sulfite bath.

With the print’s own silver, at the heart of the design. The image is the thing the bath must not dissolve, and the only reason it does not is that metallic silver is not a silver halide. Push the pH down and that distinction starts to erode.

With acid, and there is nothing else in the bath to help. Two grams of carbonate is a small reserve. Treat the bath as an unprotected thiosulfate solution that has been nudged the right way, not as a buffered one.

Variants

Plain hypo at 40 per cent, from Wall, which is what this formula is a refinement of. Wall assigns plain and alkaline baths alike to printing-out papers, so the two entries answer the same question a century apart, and Reilly’s is the one with the procedure, the exhaustion figure and the reasoning attached.

Wall’s own alkaline bath cannot be published. It is on the same page of his fixing chapter — hypo 125 g, dry sodium carbonate 7 g, salt, water to 1000 ccm, with a note that dry sodium sulfite may replace the carbonate — and the quantity against “salt” is illegible in the copy the course holds. Its metric column gives nothing and its avoirdupois column cannot supply the figure without inference, because the two columns in that table are not scaled to the same volume. A formula with a quantity the course would have to guess is not published here, in outline or otherwise. What survives of it is the fact that a period alkaline fixer carried a chloride as well as a carbonate, which is a difference worth knowing about and not a formula.

A sulfite-alkalised version is implied by both sources — Wall’s substitution note, and Reilly’s use of sulfite as the washing aid — but neither prints one as a fixer formula with a quantity, so the course does not either.

No hardening variant exists and none should. Reilly specifies a bath to which no hardeners have been added, and gives the reason in the washing chapter rather than the fixing one: everything that slows the removal of thiosulfate from a printing-out paper works against the permanence the process is being run for.

Safety

Level A. Two substances, both of which carry aggregated GHS classifications with no hazard statements — sodium thiosulfate on a thin evidence base, and sodium carbonate as its own page records — in warm water.

Gloves, eye protection and an apron, as for any solution. The genuine hazards of a printing-out session are elsewhere: the silver nitrate at the sensitising end and, if the print is toned, whatever the toner contains.

What is not a hazard here, and why. There is no acid in the formula and no acid should reach it, so the sulfur dioxide that dominates the safety section of every other fixer in this formulary has no route to form. There is no alum, so no aluminium and no dust with a workplace exposure limit. There is no hardener of any kind. This is the simplest fixing bath the course publishes and its safety section is short for a real reason rather than an evasive one.

The consequence is that the discipline here is about permanence rather than about people. Ten prints per litre, two baths, four minutes each, made fresh. Those are the numbers that protect the print, and nothing in the tray will punish you for ignoring them until years later.

Storage

It is not stored. Reilly requires a fresh batch for each printing session, and records that even unused fixer solutions break down very rapidly. Both baths are made up on the day they are used and discarded afterwards.

The dry chemicals keep well. Hypo crystals deliquesce slightly in moist air and cake; anhydrous sodium carbonate takes up water and, on long standing in a damp cupboard, turns into a partly hydrated solid that is no longer the mass you weighed. Both in labelled containers that have never held food, per the labelling SOP.

Incompatibilities

Acids of every kind, including the ones that arrive on a print. Two grams of carbonate is protection against traces, not against a splash of stop bath. See incompatibilities.

Acidic toning baths, specifically. The rinse between toner and fixer is a chemical requirement rather than housekeeping.

Oxidising agents. Thiosulfate oxidises to tetrathionate, which sulfides silver and attacks an image — a general reason to keep peroxide, persulfate and hypochlorite out of a print darkroom, and a particular reason not to use a hypo eliminator on prints whose image silver is this vulnerable.

Silver nitrate and the sensitising bench. A trace of fixer on a coating rod or a brush ruins the next sheet before it is exposed. Separate trays, separate tongs, separate towels.

Alum and hardeners, which have no place in this bath and which would turn an unbuffered thiosulfate solution turbid if they arrived without sulfite.

Waste

This bath and, more importantly, the wash that follows it are the silver stream of the printing-out processes, and they are more concentrated than anything the film side of the course produces. A printing-out paper carries several times the silver of a developing-out one, and Reilly’s ten-prints-per-litre discipline means that a session generates a small volume of a comparatively rich solution.

The first wash is part of the waste, not part of the plumbing. That is the finding behind open question 1.3: most of the silver a printing-out process sheds lands in the first wash rather than in the fixer. The course collects the first wash into the same container as the spent fixer and routes both as photographic chemistry.

No measured concentration is published here, because the course could not source one for a printing-out first wash and will not transfer a figure measured on a developing-out process. What would close that gap is a measurement — the course’s own, or a reader’s — of a first wash from a named sensitiser at a stated coating weight.

Collect it, label it, keep it away from acid wastes, and follow the silver-bearing waste SOP. Local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

Highlights and middle tones bleaching during fixing. The bath is acid. Either the carbonate was omitted, or an acidic toner was carried in, or the tray last held something acid. Discard and remake; the density is not recoverable.

Blisters on an albumen print. Reilly’s first two suspects are a temperature difference between trays and a pH change between them, and his third is the rawstock itself reacting with the processing solutions. Small blisters sometimes disappear on drying; large or broken ones usually mean the print is lost.

A milky bath, or a smell of burnt matches. Acid has decomposed the thiosulfate to sulfur. Ventilate and discard.

Prints yellowing or fading months later. Under-fixing or under-washing, and the two are hard to tell apart after the fact. Check the print count against the ten-per-litre rule, check that two baths were used in the right order, and check the final wash time against the 30 minutes Reilly specifies.

A print that looks far weaker after fixing than it did before. Expected. Read Image characteristics before you change anything about the exposure.

A bath that was made the night before. Remake it. This is one of the few solutions in the formulary whose source says outright that unused solution breaks down rapidly.

Experiments

Measure the colour change. Photograph or scan a print immediately before fixing, immediately after, after the first wash and after drying, under fixed lighting. Four states of the same print, and the sequence purple to yellow-brown to red to neutral is a physical account of a silver image that no verbal description matches.

Test the ten-prints-per-litre rule against the hundred-and-fifty. Fix matched prints from the same negative in a bath at print number 1, 10, 50 and 140 of a litre, wash them identically, and store them together. Reilly’s argument is that the difference will not be visible on the day; the experiment is only worth running if you are prepared to look again in a year, and it should be entered in a laboratory report so that a year later you know what you did.

Isolate the carbonate. Fix two identical prints, one in the alkaline bath and one in the same bath with the carbonate omitted, then compare the highlights against the unfixed reference. This is the single variable the formula exists for, and it is a one-session experiment.

Measure the pH of both, and of your tap water. Calibrate a meter with the pH SOP and read the alkaline bath, the plain bath and the water they were made from. Reilly publishes no pH for this formula, and the number you get is a measurement the course does not have.

Check the temperature drop on mixing. Reilly’s instruction to use water slightly warmer than the working temperature is a testable claim. Measure it, and you will know how much warmer “slightly” needs to be in your darkroom.