Dilute alkaline thiosulfate fixer for iron-silver prints

An iron-silver print goes into this bath as a red-brown image and comes out browner and stronger. Leave it there and it goes pale and stays pale. Both of those are the same reaction, and the ten grams of carbonate exist to slow the second one down without stopping the first.

Ingredient	Quantity	Form the source specifies
Sodium thiosulfate	50 g	“crystals”, which is the pentahydrate
Sodium carbonate	10 g	King names no hydrate
Sodium sulfite	2 g	anhydrous
Water	to make 1000 mL	dissolve in 750 mL, then make up

Purpose

To dissolve the silver that the light did not reduce out of a kallitype, a Van Dyke Brown or another iron-silver print, and to take as little of the image with it as possible.

On a gelatin-silver print those two aims do not compete. The image there is comparatively coarse filamentary silver, locked inside a hardened gelatin layer, and a strong fixer removes the undeveloped halide without touching it. On an iron-silver paper they compete directly. The image is colloidal silver — Ware puts the particles of a brown plain-paper silver image at around 20 nm, far smaller than the wavelength of the light they scatter — sitting in the cellulose fibres of the paper with no binder over it at all. The bath that can complex silver(I) out of a residual salt is the same bath that, with the oxygen of the air helping, can complex the image away.

One kit maker says so twice, in two different sheets and in almost the same words, and it is the single most important instruction on this page. Photographers’ Formulary, for the kallitype: do not use a standard photographic fixing bath; the finely divided, unprotected silver metal will be etched from the print. Photographers’ Formulary, for the Van Dyke: do not use a standard photographic fixing bath; the very finely divided silver metal of the Van Dyke print will be etched off the paper.

So this formula is a fixer designed around a restraint. Its three defences against its own fixing agent are dilution (5 per cent, a fifth of F-5), alkalinity (ten grams of carbonate, five times what Reilly puts in a printing-out bath), and time (four minutes, and every source that publishes a limit puts it at five).

Recommended uses

Kallitype, which is what King published it for. It sits at step 10 of his fourteen-step procedure, after development in 20 per cent sodium citrate, a neutral or slightly acid first rinse, clearing in 3 per cent citric acid, a second rinse, toning, and a third rinse. Four minutes, or better, two minutes in each of two baths with a 30-second running-water rinse between them and the second bath always fresh.

Van Dyke Brown, argentotype and argyrotype, as the course’s general iron-silver fixing bath. This is a reading and is marked as one. King publishes the formula for kallitype only. What justifies extending it is that all five kit sheets the course has read specify a bath of the same thiosulfate strength for Van Dyke as for kallitype — 50 g per litre in four of them, and 100 g in 2000 mL, which is the same strength, in the Formulary’s Van Dyke kit — give the same warnings about bleaching and about ordinary fixers, and are working on the same colloidal silver. What King adds to those baths is the alkali, and Ware’s argument for alkalinity is about print-out silver in general and not about kallitype in particular.

After clearing, and after toning. Both orderings matter and both have reasons attached. Clearing comes first because it is done in acid — 3 per cent citric acid in King’s method, tetrasodium EDTA at Bostick & Sullivan, potassium oxalate in the Formulary kit — and because King warns that an alkaline rinse at that stage forms ferrous hydroxide compounds in the paper that make complete clearing “difficult or impossible”. Toning in gold, platinum or palladium comes before fixing because a toned print does not bleach: King gives increased shadow depth, minimised bleaching and a more dramatic tone change as the three reasons, and Bostick & Sullivan put it as a rule — a sufficiently toned print will not bleach back. Selenium is the exception both authors make: it reacts with residual silver nitrate and stains, so selenium toning is done after fixing.

Where the print is going to be kept. King’s position is that all untoned kallitypes will eventually fade because residual iron(II) oxidises the image silver, and that the fixing bath is one of three places where permanence is decided; the other two are the clearing bath before it and the wash after it.

When another formula is preferable

• For film, plates and gelatin-silver papers, F-5, F-6 or TF-2. This bath is a fifth the strength of F-5 and carries no hardener and no acid: on a modern film it would fix slowly, incompletely, and with developer still working in the tray. It is not a weak fixer, it is a fixer for a different material.
• For salted paper and albumen, Reilly’s alkaline 15 per cent bath. Same idea, three times the hypo, a fifth of the carbonate, and the strength is justified by a much heavier silver load: a printing-out paper carries far more silver than an iron-silver sheet, most of it as unexposed chloride that has to be complexed and removed.
• For a kit process, the kit’s own bath. The plain thiosulfate fixer for printed-out papers is the course’s entry for those baths, and four of the five kit sheets specify plain 5 per cent hypo with no alkali at all. They work. What they give up is the acid tolerance discussed under Interactions, which the Formulary’s own New Kallitype sheet identifies precisely: the use of distilled water or water with a neutral or slightly base ph is important. Acidic water accelerates the tendency of thiosulfate to bleach highlights.
• Not, on the evidence, Ware’s pH 9 to 10. That figure is the only published alkalinity target in the iron-silver literature the course has read, and the sentence carrying it does not say clearly which bath it belongs to. It is set out under Variants and it is not usable as a formula.
• Where nothing is available but a bottled fixer, the Formulary’s own advice for its New Kallitype kit is a pH-neutral rapid fixer diluted from 1:9 to 1:19. That is a maker recommending a competitor class of product for its own process, which makes it worth recording; the course cannot verify what is in any particular bottle, and the two words that carry the whole recommendation are pH neutral.
• Never plain hypo at 40 per cent, and never an acid hardening bath. Both are on this site and neither belongs anywhere near an iron-silver print.

Mixing

Water first, three quarters of it. King: dissolve the solids in 750 mL, then add water to make 1000 mL. The order matters for the strength, not for the chemistry: a make-up volume defines the concentration, and a bath mixed by adding a litre of water to the solids is a different and weaker bath. This is the Kodak convention the rest of this formulary runs on, and it is the reason the water line reads “to make”.

Then hypo, carbonate, sulfite, in the order King lists them. Nothing in this formula can be got dangerously wrong by reordering it. There is no acid to meet a thiosulfate, and there is no developing agent to protect, so the developer-mixing rule that puts the sulfite before the alkali has no force here. Watch each solid dissolve before the next goes in, which is the only real discipline: 50 g of thiosulfate crystals dissolve slowly and endothermically, and undissolved carbonate sitting under a layer of hypo is easy to miss.

Temperature. King publishes none. Both Photographers’ Formulary sheets dissolve their thiosulfate in water at 52 °C / 125 °F and both work the bath at 20 °C / 68 °F; Bostick & Sullivan mix in cold tap water and work at about 68 °F, adding the observation that matters most here — warmer water accelerates bleaching. Mix warm if you want the crystals to go quickly; use the bath at room temperature.

The 4X concentrate. King offers a concentrate at four times the strength, diluted 1 + 3 for use. That is 200 g of thiosulfate, 40 g of carbonate and 8 g of sulfite to make a litre, and all three dissolve at that strength. He publishes no keeping time for it and neither does anyone else, so the course cannot tell you it keeps better than the working bath — only that a concentrated thiosulfate solution is the form in which hypo is normally stored, and that the New Kallitype sheet’s warning about the working bath’s short life was written about a bath at working strength.

🧮What five per cent, one per cent and two grams actually are

50 g ÷ 248.18 g/mol = 0.20 mol/L, a 5 % w/v bath

Thiosulfate, as the pentahydrate

10 g ÷ 105.99 g/mol = 0.094 mol/L

Carbonate, read as the anhydrous salt

2 g ÷ 126.04 g/mol = 0.016 mol/L

Sulfite, anhydrous

Three comparisons put those numbers in the formulary. The thiosulfate is a third of Reilly’s 150 g/L printing-out bath and a fifth of F-5 at 240 g/L; it is twice Ware’s argyrotype bath at 25 g/L. The carbonate is five times Reilly’s 2 g/L. And the sulfite is about a seventh of the 15 g/L that TF-2 carries, in a bath a fifth the strength.

The form of the hypo is worth a second look. 50 g of the anhydrous salt is 0.32 mol/L, fifty-seven per cent more thiosulfate than the crystals give. On a fixer whose entire design is restraint, that is not a rounding error. Weigh the pentahydrate, or convert deliberately.

Behaviour

It intensifies before it reduces, and both are the same reaction seen at different times. Every source that describes the bath describes the first half: the print darkens, the colour moves from red towards brown, and the shadow gradation strengthens. Ware, for the argyrotype: the bath “removes any traces of insoluble silver salts and intensifies the image: the shadow gradation strengthens, and the colour rapidly shifts from red to brown”. The Formulary, for the Van Dyke: “the print will darken and become brown during this fixing period”. Then the second half arrives. Ware again: “overlong treatment in this bath and exposure to air can result in loss of image density especially in the highlights”. The Formulary: “a longer soak will cause the print to fade”. Bostick & Sullivan: “over-fixing can lead to bleaching of the image, while under fixing will cause archival permanence issues.”

The highlights go first. That is not an accident of technique but a consequence of geometry: a highlight is a thin, sparse deposit of very small particles with a great deal of surface per unit of silver, so a reaction that eats the surface removes a larger fraction of it.

Four minutes, and the published ceiling is five. The times published for a bath of this kind run from two minutes to five: Ware gives two to three, Bostick & Sullivan’s kallitype sheet about two followed immediately by fresh water, the Formulary’s New Kallitype sheet two to five, and three sheets — both Van Dyke sheets and the Formulary’s kallitype kit — five, with two of them saying in terms that a longer soak makes the print fade. King’s single bath of four minutes sits at the long end of that range and his preferred two-plus-two sits at the short end of it twice over. Bostick & Sullivan add the reason a very short fix is credible here: the kallitype’s silver “is not enmeshed in gelatin and fixes much faster than a silver bromide print”, so even more dilute fixer and shorter times may be equally effective. That is a manufacturer saying, in a kit sheet, that its own instruction is probably conservative.

Two baths beat one. King’s preferred form is two minutes in each of two, with a 30-second rinse between, and the second bath always fresh. The reasoning is Reilly’s and belongs to every plain thiosulfate fixer: thiosulfate must be present in excess or the silver-thiosulfate complexes that form carry fewer ligands, are insoluble, and cannot be washed out of the paper. A fresh second bath is the guarantee that what the print leaves with is the soluble complex.

It exhausts on a print count, and nobody publishes one for this bath. King gives no capacity. The figures the course has for comparable baths are these. Bostick & Sullivan: ten 8 × 10 kallitypes per litre of 5 per cent plain hypo, discarded after each session. The Formulary’s New Kallitype kit: four or five 8 × 10 prints per 50 g packet, which is a litre. Ware: about ten 10 × 8 prints per litre of 2 to 2.5 per cent. The Formulary’s Van Dyke kit states no volume at all — just enough to cover the print, discarded after two or three. Four to ten 8 × 10 prints per litre, from three sheets that state a volume, is a narrow enough spread to plan a session around and too wide to publish as this formula’s number.

🧮Why the bath is thrown away at a few per cent of its capacity

King’s sensitiser is equal parts 10 per cent silver nitrate and 20 per cent ferric oxalate, about 2 mL of the mixture for an 8 × 10 print. So each print starts with about 1 mL of the 10 per cent silver nitrate:

0.1 g AgNO₃ × (107.87 ÷ 169.87) = 0.064 g of silver

Silver on one 8 × 10 sheet

0.20 mol/L ÷ 2 = 0.10 mol/L = 10.9 g of silver per litre

Silver the thiosulfate could complex, at two ligands per silver

A litre of this bath could in principle complex the entire silver content of about a hundred and seventy prints. The published discard rates, where a sheet states one per litre, are four to ten. So a working bath is thrown away having used two to six per cent of its stoichiometric capacity, and that is not waste — it is the direct consequence of Reilly’s rule that thiosulfate has to be in large excess for the soluble complex to be the one that forms. A fixer for this material is never limited by how much silver it can hold. It is limited by how much thiosulfate has to remain unused.

The arithmetic is the course’s, from King’s own sensitiser figures. Not all of that 64 mg reaches the fixer: much of it leaves in the development and the clearing baths, so the true load per print is lower still and the conclusion is stronger, not weaker.

Mixed, it does not keep. The Formulary states it plainly for the plain version — sodium thiosulfate “does not have an extended shelf life once mixed with water” — which is why its kit ships two small packets instead of one large one. Ware instructs that the argyrotype bath should not be stored and re-used. Reilly, on the same chemistry in a different process, says even unused fixing solutions break down rapidly and that the bath should be made up just before use. King publishes nothing on the point. Mix what the session needs.

Image characteristics

Judge nothing wet, and judge nothing in the fixer. Bostick & Sullivan give the practical rule for the Van Dyke: the print leaves the fixer at about three-quarters of its final density and darkens further as it dries. Ware puts the dry-down of an argyrotype at “at least one Zone”. A print that looks right in the tray will dry too dark; a print that looks right coming out of the fixer is usually about right.

The colour change is the image being altered, not stained. Ware’s account is the most specific in the literature the course has read, and it is worth following because it explains why a fixing bath changes an image colour at all. He believes an argyrotype becomes partially sulphide-toned in the thiosulfate bath: the yellow silver nanoparticles acquire a coating of silver sulphide “only to a depth of a few atoms, perhaps even a monolayer”, which is enough to change the surface plasmon resonance absorption and therefore the colour profoundly. Energy-dispersive X-ray analysis of argyrotypes by Ellie Young at RMIT found sulphur as well as silver in the image, which is evidence for the proposal rather than proof of it, and Ware presents it as a belief.

The consequence is a genuine double edge, and Ware states both halves. Silver sulphide is very insoluble and very stable, so a thin sulphide skin should make the image more resistant to hostile chemicals — an unintentional toning step. But “overlong immersion in thiosulphate completely transforms the silver nanoparticles into silver sulphide, causing the image to become badly faded”. The same reaction that improves the print in two minutes destroys it in twenty.

What the bath must never contribute is a brown or black stain. If the print picks one up, the fault is upstream: see Troubleshooting.

The alkali contributes nothing visible of its own. No source reports a colour difference between a plain 5 per cent bath and an alkalised one. What the carbonate changes is the rate at which the second half of the curve arrives.

The mechanism

Three reactions run in the same tray, and the formula is an attempt to favour the first over the second without stopping the third from starting.

Reaction one: what the bath is for

Ag⁺ + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻

The fixing reaction: residual silver(I) into the soluble dithiosulfatoargentate complex

Thiosulfate binds silver(I) strongly enough to pull it out of the salts an iron-silver sensitiser leaves behind, and the anion that results is soluble and can be washed out of the paper. Ware writes the same reaction for the silver chloride of a printing-out paper:

AgCl + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Cl⁻

The same complexation, written for a silver chloride paper

What the residual silver actually is in an iron-silver print, the course cannot tell you precisely, because no source it has read names the compound. The Formulary says the fixer “will remove the excess silver ions”; Ware says it “removes any traces of insoluble silver salts”; King says fixing removes “unused silver”. The sensitiser puts down silver nitrate with an iron(III) salt of an organic acid, so silver salts of that acid are the obvious candidates, and the course leaves it there rather than naming a compound nobody published. What every source agrees on is that most of the unreduced silver leaves in the development and clearing baths, and that what reaches the fixer is a residue.

Reaction two: what the bath does by accident

4 Ag + 8 S₂O₃²⁻ + O₂ + 4 H⁺ → 4 [Ag(S₂O₃)₂]³⁻ + 2 H₂O

The bleaching reaction: image silver, thiosulfate and the oxygen of the air

This is the reaction the whole formula is designed against, and it is the one Ware names when he writes that the colloidal silver of a print-out image “is easily oxidised by air in the presence of thiosulphate ions”. Metallic silver sitting in plain water is barely touched by the oxygen dissolved in it. Metallic silver in a thiosulfate solution is not the same proposition at all, and Ware’s own table of standard redox potentials says why.

⚗Thiosulfate drops the silver couple by 0.79 volts, and that is the whole problem

Ware tabulates these standard potentials in Argyronomicon §23.8:

E°(Ag⁺/Ag) = +0.799 V

Silver on its own

E°([Ag(S₂O₃)₂]³⁻/Ag, 2S₂O₃²⁻) = +0.01 V

Silver in the presence of thiosulfate

E(O₂,4H⁺/2H₂O) = +1.229 − 0.059 pH

Oxygen, which depends on pH

His general statement of the principle is that “any involvement of highly insoluble products or complex ions in the redox equilibrium greatly increases the ease of oxidising metallic silver”. Here the complex ion drags the silver couple down by 0.79 V. Put the two half-cells together and the driving force for air to dissolve the image is the difference between the oxygen couple and the silver couple:

0.816 − 0.799 = +0.017 V — essentially at equilibrium; the image is safe

In water at pH 7, with no thiosulfate present

0.816 − 0.01 = +0.806 V — strongly favourable; the image is being attacked

In a neutral thiosulfate bath, pH 7

0.545 − 0.01 = +0.535 V — still favourable, but 0.27 V less so

In this bath, if it sits near pH 11.6

That is the whole argument for an alkaline fixer in three lines, and it is also the honest limit of it. Alkalinity does not make the reaction stop. It removes about a third of the driving force and, with it, a great deal of the rate. The timer is still the main control, and Ware’s own phrasing is that alkalinity “inhibits” the dissolution of silver rather than preventing it.

The arithmetic combining Ware’s half-cells is the course’s; the potentials, the principle and the conclusion are his. Standard potentials describe thermodynamics and this page is really about a rate, so treat the numbers as an explanation of direction, not a prediction of how long a print survives.

Ware states the pH dependence directly elsewhere in the same book, discussing storage rather than processing: “it is chemically certain that molecular oxygen of the air is a more potent oxidising agent under acidic, than under alkaline conditions”, so image silver “will be less susceptible to aerial oxidation under the alkaline conditions”. The same physics governs a tray and an archival box.

Reaction three: what acid does to the bath

H₂S₂O₃ → H₂SO₃ + S

An acid-decomposed fixer: thiosulfuric acid falls apart into sulfurous acid and sulfur

Kodak’s 1928 primer sets this out plainly. Add a strong acid to a weak hypo solution and it goes milky: the acid converts sodium thiosulphate into free thiosulphuric acid, which is unstable and decomposes into sulphurous acid and sulphur. This is Reilly’s first reason for making a printing-out fixer alkaline — “the slight alkalinity prevents any acids which might be inadvertently introduced into the fixing bath from decomposing the sodium thiosulfate and liberating sulfur” — and his second is reaction two above: “an acid fixing bath would tend to attack the finely divided metallic silver of the image, causing excessive bleaching of the highlights and middletones”.

The primer also gives the reverse reaction, and it is what King’s two grams of sulfite are for. Boil sulphite with sulphur and thiosulphate re-forms, so “while acids liberate sulphur from the hypo, sulphite combines with the sulphur to form hypo again”, and enough sulphite present prevents acid from decomposing the bath at all.

🧮What pH is this bath, and why the page does not simply tell you

No source publishes a pH for it. What can be calculated, from the carbonate’s own second dissociation constant as recorded on the sodium carbonate page:

K_b = 10⁻¹⁴ ÷ 4.7×10⁻¹¹ = 2.1×10⁻⁴

Hydrolysis constant, from pKa₂ = 10.33

[OH⁻] = √(2.1×10⁻⁴ × 0.094) = 4.5×10⁻³ mol/L

Hydroxide from 0.094 mol/L carbonate

pOH 2.35, so pH ≈ 11.6

Which is

The sulfite contributes almost nothing beside that: its own hydrolysis gives about 5×10⁻⁵ mol/L of hydroxide, a hundredth as much.

Three reasons not to print 11.6 as a fact. It assumes an ideal solution containing nothing but carbonate; a real bath absorbs carbon dioxide from the air, which converts carbonate to bicarbonate and pulls the pH down towards the 10.3 that a carbonate–bicarbonate pair holds; and every print carries in a little acid from the clearing bath. The measured pH of a working bath will be lower than 11.6 and the course has not measured it.

It is worth noticing that even a bath that had fallen all the way to the 10.3 a carbonate–bicarbonate pair holds would still sit above the only published alkalinity figure in the iron-silver literature, Ware’s pH 9 to 10 — a figure whose own sentence is ambiguous about which bath it is for, as the callout under Variants sets out. Measuring this bath is the first experiment at the foot of the page.

Function of every ingredient

Sodium thiosulfate, 50 g of the pentahydrate. The fixing agent, and the only ingredient that does the job the bath is named for. It complexes residual silver(I) two ligands at a time into the soluble dithiosulfatoargentate anion, which the wash can remove. Why this little: at 0.20 mol/L the bath already carries a hundred and seventy prints’ worth of stoichiometric capacity, so nothing is bought by making it stronger, and Ware names “too concentrated a solution of thiosulphate” as one of the three things that cause serious loss of image density. More of it: faster bleaching, more thiosulfate driven into the paper fibres, and a longer wash to get it out — which Reilly identifies as the mechanism by which prolonged or over-strong fixation costs permanence. Less of it: the excess that keeps the complex soluble starts to disappear, and the insoluble lower-ligand complexes Reilly warns about form in the image layer where no wash will reach them. Its interactions are with everything else here: with the carbonate, which decides how fast it is destroyed by acid; with the sulfite, which rebuilds it when acid does; with the air, which turns it from a fixing agent into a silver solvent; and with the print’s own image, which is the thing it must be prevented from dissolving. Its form matters: the anhydrous salt at the same weight is 57 per cent more thiosulfate.

Sodium carbonate, 10 g. The alkali, and the ingredient that distinguishes this formula from the four plain baths its neighbours publish. It does three separate jobs, all sourced, none of them visible in the tray. First, it holds the bath on the alkaline side, where — in Ware’s words — molecular oxygen is a weaker oxidising agent, so the reaction that dissolves the image silver runs with less driving force behind it. Second, it is a reserve against acid: a print carries in citric acid from the clearing bath, a tray may last have held a stop bath, and Kodak’s primer and Reilly between them describe exactly what a dose of acid does to an unbuffered hypo solution. Third, following the Formulary’s own diagnosis, it removes the water supply as a variable — that sheet requires distilled water or “water with a neutral or slightly base ph”, because acidic water accelerates bleaching, and ten grams of carbonate makes the question moot. More of it: no source the course has read reports a benefit from a more alkaline fixing bath than this, and the one published alkalinity figure in the iron-silver literature — Ware’s pH 9 to 10 — is below where this bath probably sits and is ambiguous about which bath it describes. Less of it: two grams is Reilly’s figure for a printing-out bath, and none at all is the plain kit bath, which works and is more sensitive to what it is mixed with. Its interaction that matters: with the clearing sequence, which must come before it — see Interactions.

Why ten grams and not Reilly’s two, King does not say. The course will not invent a reason. What can be said is that the two formulas are in different positions: Reilly’s 2 g sits in a bath three times as strong, discarded after ten to fifteen prints, whose job is to hold a printing-out paper just off neutral, while King’s 10 g sits in a bath a third as strong that has to survive citric acid arriving on every sheet. A fivefold alkali reserve in a threefold weaker bath is a fifteen-fold difference in how much acid the bath can absorb before it stops being alkaline. That is a plausible design and it is not King’s stated one.

Sodium sulfite, 2 g. King assigns it no function, and the schema has no way of saying so: an ingredient’s function field is a required string, so it always reads as though somebody explained the ingredient. That shortfall is open item 10 in the course’s own schema notes, and this paragraph is where it gets confessed instead. What follows is established chemistry from other sources, not King’s explanation. Sulfite has two plausible jobs at this concentration and one job it is definitely too dilute to do. As an oxygen scavenger: sulfite has a strong affinity for oxygen and is oxidised to sulfate in preference to whatever else is in the solution, which is why it is the preservative in nearly every developer and most fixers. Since the reaction that destroys the image here is aerial oxidation of silver in the presence of thiosulfate, a reagent that consumes dissolved oxygen is attacking the problem at its source. As the thiosulfate’s insurance policy: Kodak’s primer states the reversible relation directly — sulphite combines with liberated sulfur to re-form hypo, so enough sulphite present prevents acid from decomposing the bath. That is the same job the carbonate does by a different route, and two mechanisms against one failure is a reasonable thing for a formula to carry. What it is not: 2 g per litre is about a fiftieth of the 100 g per litre that D-76 carries, which is where sulfite’s solvent action on silver halide begins to matter, so it is not helping to fix anything. More of it: nothing in the literature the course has read suggests a benefit, and sulfite is itself oxygen-demanding waste. Less of it, or none: the bath becomes the plain one the kit sheets publish. Note the same substance appears twice in King’s method: 1 per cent sodium sulfite is also his hypo-clearing bath two steps later, where it does a completely different job — displacing thiosulfate from the paper by ion exchange, not scavenging oxygen.

Water, to make 1000 mL. A make-up volume rather than a measured addition, which is what makes the percentages on this page mean anything. King gives no temperature for it. Distilled or de-ionised water is worth using here for the reason Ware gives about the clearing stage rather than a reason about the fixer itself: chlorinated water attacks colloidal silver, and the Formulary’s Van Dyke sheet warns that hard water usually carries dissolved iron salts which contaminate the print.

Interactions

With the clearing bath before it, which must come first and must be acid. This is the one ordering in the whole iron-silver sequence that cannot be swapped, and both King and Ware explain it from opposite directions. King: the rinse after development must be neutral or slightly acidic, because an alkaline rinse forms ferrous hydroxide compounds in the paper that make complete clearing “difficult or impossible”. Ware, describing the historical problem that the argyrotype was designed to solve: the difficulty of the iron-silver processes is clearing the print of iron salts without dissolving the image silver, and the old alkaline developers avoided the dissolution at the cost of hydrolysing the excess iron(III) to insoluble ferric hydroxide in the image. Iron comes out in acid; silver survives in alkali; so the acid step goes first and the alkaline step goes last, and the clearing sequence is a separate bath for that reason.

With the rinse between them, which is not optional. Ten grams of carbonate is a large reserve but it is still finite, and every sheet arriving straight from a 3 per cent citric acid bath spends some of it. King specifies a 30-to-60-second rinse after clearing and a 60-second rinse after toning.

With the toner, decisively. A print toned in gold, platinum or palladium does not bleach in this bath, because the silver it would have lost has been replaced by a metal the reaction does not touch. King and Bostick & Sullivan give the same instruction from different premises and it is the single most effective control over everything described under Behaviour. Selenium is toned after fixing instead, because it reacts with residual silver nitrate and stains.

With the wash after it, which finishes the job or undoes it. Ware’s third pitfall of thiosulfate fixation is insufficient washing, which leaves the print to fade by sulphiding — the fixer’s own residue converting the image to silver sulphide over years instead of minutes. King’s sequence is a one-minute rinse, two minutes in 1 per cent sulfite, and 20 to 30 minutes of running water; without the sulfite step he asks for a full hour. Anchell and Troop’s argument for alkaline fixers generally is exactly this: they wash out faster than acid ones.

With potassium salts, and this is a caution rather than a finding. Anchell and Troop record that developers or stop baths high in potassium salts can partly convert sodium thiosulfate into potassium thiosulfate, “which is inactive compared with the ammonium or sodium salts”, and advise against a potassium-based stop bath before a fixer. Potassium reaches an iron-silver sequence readily: potassium oxalate is the Formulary kallitype kit’s clearing bath and the classical platinotype developer, and Rochelle salt, one of the three kallitype developers in this formulary, is a sodium-potassium tartrate. The warning was written about film fixers, the course has found no measurement of the effect at these dilutions, and King’s own developer and clearing bath are both sodium salts. It is a reason to rinse properly, not a reason to change developer.

With the air above the tray. Ware names exposure to air, alongside time and concentration, as a cause of image loss. A print floating face up in a shallow tray, agitated vigorously, is being aerated. Keep it submerged and agitate gently.

Variants

The Photographers’ Formulary kallitype fixing bath: ammonia instead of carbonate. Sodium thiosulfate pentahydrate 50 g and 12 mL of 28 per cent ammonia in 1000 mL of distilled water at 52 °C, the thiosulfate dissolved first and the ammonia added afterwards in a well-ventilated area. Same thiosulfate strength, a different alkali, and a different hazard: the sheet’s own safety section says concentrated ammonia “releases extremely choking ammonia gas when opened” and singles it out as one of four chemicals in the kit needing special attention. Its fixing instruction is not more than five minutes, “a longer soak will cause the print to fade”. The course records it here rather than as an entry of its own because it is the same formula with the alkali substituted, and because a bath that needs ventilation to mix is a worse recommendation than one that does not.

Ware’s ammonia option, and what it is not certain to be. In his account of the argyrotype process, immediately after describing what overlong immersion in the 2 per cent thiosulfate bath does to the highlights, Ware writes: “If, on the other hand, very delicate highlight detail is desired, a little ammonia may be added to the clearing bath to make it distinctly alkaline (pH 9 to 10); this inhibits the dissolution of silver, but may raise the level of residual iron in the image.” It is the only published alkalinity target in the iron-silver literature the course has read, and it needs the callout below rather than a place in the formula. His standard instruction, in both the web account and the 2009 workshop notes, is the plain bath with nothing added.

⁇Which bath did Ware mean?

The sentence says clearing bath and the paragraph it sits in is about the thiosulphate bath, and the two readings lead somewhere different.

Read as the thiosulphate bath: the passage is continuous — “overlong treatment in this bath … on the other hand …” — and inhibiting “the dissolution of silver” is a description of what happens in the fixer, because that is the bath in which silver dissolves. On this reading Ware is prescribing exactly the kind of bath this page publishes, at a stated pH.

Read as the clearing bath: Ware’s own workshop notes make the clearing bath a separate, deliberately acid tray — de-chlorinated water with about 2.5 g of citric acid per litre, pH about 4 — and the caveat in his own sentence fits that tray far better than it fits the fixer. “May raise the level of residual iron in the image” is precisely what alkalinity does at the clearing stage: it hydrolyses iron(III) to insoluble ferric hydroxide, which is the failure King warns about in his first rinse and which Ware himself names as the historical defect of the alkaline iron-silver developers. By the time a print reaches the fixer the iron should already be out, so alkalinity there ought not to raise residual iron at all.

The course does not resolve it and does not use the sentence as evidence for an alkaline fixer. What it takes from Ware instead is the statement in Argyronomicon that carries no ambiguity at all — that molecular oxygen is a more potent oxidising agent under acidic than under alkaline conditions, and that image silver is correspondingly less susceptible to aerial oxidation in alkali — together with the redox potentials that explain it. The formula on this page rests on King’s published quantities and on Reilly’s two stated reasons, neither of which depends on how this sentence is read. If the clearing-bath reading is right, Ware’s pH 9 to 10 is not a target for this bath and the honest position is that no source publishes one.

The plain 5 per cent baths, which are the majority practice. Bostick & Sullivan for the Van Dyke, 50 g/L in cold tap water, five minutes. Bostick & Sullivan for the kallitype, publishing Dick Stevens’s formulas, 50 g/L at about 68 °F, about two minutes, ten prints per litre. Photographers’ Formulary for the Van Dyke, 100 g in 2000 mL of water at 52 °C, five minutes at 20 °C, discarded after two or three prints. Photographers’ Formulary for the New Kallitype, 50 g/L, two to five minutes, with the requirement that the water be distilled or neutral-to-slightly-basic. Four sheets, one strength, no alkali. The course’s entry for that construction is the plain thiosulfate fixer for printed-out papers, and this formula is best understood as those baths with the water-quality requirement built in as a reagent.

No variant of the course’s own. There is no reason to modify a formula whose author published it whole and whose three quantities each have a defensible job, and Rule 6 would require this page to say what was changed and what followed. Nothing is changed.

Safety

Level A. Three solids in water, none of which requires more than the standard darkroom controls.

The three solids are not equally innocuous and the smallest of them is the one with the most serious aggregated classification, which is a useful thing to notice about how hazard data works.

Sodium thiosulfate carries no agreed GHS classification at all: of 281 reports to the ECHA inventory, 98.6 per cent state that it does not meet the criteria — a thin evidence base rather than a clean bill of health, as its own page says. Sodium carbonate is Warning, GHS07, H319, causes serious eye irritation, with two national authorities going further to H318, serious eye damage; Chemical Safety Card 1135 adds that harmful airborne concentrations of the powder are reached quickly on dispersion. Sodium sulfite, all two grams of it, carries the heaviest aggregate — Danger, GHS05 and GHS07, H302 harmful if swallowed, H314 causes severe skin burns and eye damage, H315 and H319 — and that aggregate is a spread of opinion rather than a ruling: a quarter of the 2,482 reports behind it say the substance meets no GHS criteria at all, and of those that do classify it, about half give H314. The classification describes the pure powder in the tub, not two grams dissolved in a litre of water.

So the controls are: nitrile gloves, chemical splash goggles rather than glasses while either powder is weighed, an apron, and weighing done without raising visible dust. Once the bath is mixed, ordinary splash protection is enough.

What is not a hazard here, and why. There is no acid in the formula, so the sulfur dioxide that dominates the safety section of every acid fixer on this site has no route to form — the whole design point of the carbonate is to keep it that way. There is no hardener, so no alum and no aluminium salt. There is no ammonia, which is the substantive safety difference between this formula and the Formulary’s ammonia-alkalised bath above: that one has to be mixed in a well-ventilated area and this one does not. And there is no silver nitrate in the tray — the genuinely hazardous reagent of an iron-silver session is handled at the sensitising bench, under the silver nitrate SOP, several steps earlier.

Ventilation is not among the controls for this bath, because nothing in it produces a vapour at room temperature. That statement stops being true the moment an acid reaches it.

Storage

The dry solids keep well in labelled, airtight containers that have never held food. Thiosulfate crystals deliquesce slightly and cake; anhydrous carbonate is hygroscopic and a caked tub no longer weighs what the formula assumes; sulfite that has caked has been oxidising to the sulfate and has lost preservative strength. All three want to be away from acids, and the carbonate away from the sulfite’s own enemies.

The mixed bath does not keep, and the honest position is that nobody publishes how long it lasts. The Formulary states that thiosulfate has no extended shelf life once dissolved; Ware instructs that the argyrotype bath is not to be stored and re-used; Bostick & Sullivan discard after each session; Reilly says solutions of this kind should be made up just before use. King is silent. The course’s practice, and it is a practice rather than a citation: mix for the session, use it, tip it into the silver-bearing waste container.

The 4X concentrate is the only part of this formula that is meant to be stored, and King gives no keeping time for it either. Store it as you would any hypo stock — full, stoppered, dark, cool, labelled with the date and the dilution — and remember that the label must say concentrate, because a print given four minutes in undiluted 4X stock is a print given four minutes in a 20 per cent fixer.

Incompatibilities

• Acids of any kind. The bath’s alkalinity is a reserve, not immunity. A serious dose of acid gives free thiosulfuric acid, sulfur precipitates and the bath goes milky and useless — and, before it looks useless, it starts eating highlights.
• Acid stop bath, and the tray that held one. Never mix this in a vessel that has held stop bath, and never use fixer tongs in the clearing tray.
• Silver nitrate carried in unwashed. Ware’s first pitfall of thiosulfate fixation: excess silver nitrate must be washed out with water before the print reaches the thiosulfate, “otherwise the image will be seriously stained with brown silver sulphide”. That is an incompatibility between two solutions the same print carries five minutes apart.
• Oxidising agents — potassium ferricyanide, potassium dichromate, persulfates, peroxides. Thiosulfate and sulfite are both reducing agents, and any of these in the tray attacks the bath and, through it, the print.
• The toning bath, in the other direction. Fixer carried backwards into a gold or platinum toner contaminates it. Keep the tongs separate, and rinse between.
• Ordinary rapid fixer, in the same tray or in the same session. Ammonium thiosulfate at working strength will strip an iron-silver image, which is the point Photographers’ Formulary makes in both of its iron-silver kit sheets when it says not to use a standard photographic fixing bath.

Waste

A used bath is silver-bearing and does not go down a drain. Collect it, label it, and follow the silver-bearing waste SOP and the standing jurisdictional caveat on the disposal page: what the local authority permits is the authority, not this page.

Most of the silver is not in the fixer, and that is the part worth planning for. In an iron-silver process the sensitiser’s unreduced silver leaves the sheet across the whole wet sequence, and the development and clearing baths carry a large share of it before the print ever reaches the thiosulfate. The course has no measured concentration for any of those streams and refuses to borrow a figure from a gelatin-silver process, so its practice is to collect the whole wet sequence rather than to guess which tray is the silver-bearing one.

The bath’s own load beyond silver is thiosulfate and sulfite, both oxygen-demanding, and carbonate, which makes a fresh bath alkaline enough to sit outside the pH window a sewer code typically allows — the sodium carbonate page carries the sourced figures and the disposal page carries the rule. Dilution is not treatment; collection is.

Troubleshooting

The print pales in the fixer, or the highlights disappear. The bath is doing reaction two. Look, in order, at: time — four minutes, five at the outside; temperature — Bostick & Sullivan are explicit that warmer water accelerates bleaching; toning — an untoned print bleaches and a toned one does not; aeration — is the print floating face up in a shallow tray with vigorous agitation; and acid carried in — a skipped rinse after the citric acid clearing bath. The two answers the trade gives are King’s and Bostick & Sullivan’s, and they are the same: tone before fixing, or overprint by a stop or two.

A brown or black stain appears in the image. Silver nitrate that was not washed out before the print met the thiosulfate, forming silver sulphide in the image. Ware names it as the first of the three pitfalls. The fix is upstream, in the wash and the clearing bath, and it is not recoverable in the fixer.

The bath turns milky, or smells of sulfur dioxide. Acid has reached it and thiosulfuric acid has decomposed. Discard it. Then find the acid: a contaminated tray, a print that skipped its rinse, or a graduate used for citric acid and not washed.

A yellow stain remains in the unexposed areas. That is residual iron and it is the clearing bath’s problem, not the fixer’s. Fixing longer will not remove it and will cost image density. King’s test is that a paper needing more than about four minutes to clear is unsuitable for the process.

The print looks weaker every time you check it. Two possibilities with the same symptom. If it is weakening in the tray, that is bleaching. If successive prints in a session come out progressively weaker with the same time, the bath is exhausting — the published discard rates for a comparable bath run from four to ten 8 × 10 prints per litre, and the sensible response is a fresh second bath rather than a longer first one.

The print fades over months or years. Residual thiosulfate, residual iron, or both. Ware’s third pitfall is insufficient washing; King’s permanence argument is that residual iron(II) will oxidise the image silver whatever the wash was like, which is why he tones everything. The tests are HT-2 for residual thiosulfate and ST-1 for residual silver, and the residual hypo and silver SOP runs both.

Nothing happens: no darkening, no colour shift. Check that the bath is what you think it is. A thiosulfate bath always changes an iron-silver print visibly, and a tray that does nothing is a tray of water, a tray of hypo-clear, or a bath so exhausted that it should have been discarded several prints ago.

Experiments

Measure the pH of the bath, fresh and after each print. The calculation above gives about 11.6 for an ideal carbonate solution and argues that a real bath will read lower; nobody has published a measurement, so this is a genuine open question a reader with a calibrated meter can close for themselves. Record the fresh reading, then after one print, three prints and six, with the clearing bath’s own pH for comparison. The interesting number is how fast the reserve is spent.

A fixing-time series, which is the experiment this whole page is about. One negative, six identical strips, all cleared and rinsed identically, fixed for 1, 2, 4, 8, 16 and 32 minutes, washed identically and dried. Read the maximum density and a highlight density on each. The curve should rise and then fall; the useful output is where your peak is, with your paper, water and light source. Run it twice, once untoned and once toned in gold, and the second curve should be much flatter — that is King’s claim, tested.

Alkali series. The same strips, all fixed four minutes, in baths carrying 0, 2, 5, 10 and 20 g of carbonate per litre. Ten grams is King’s; two is Reilly’s; zero is what four kit sheets publish. If the carbonate is doing what this page says it does, the difference should be largest in the highlights and should grow with fixing time — so run the series at four minutes and again at twelve.

Sulfite in or out, with the tray aerated or still. Two baths, one with King’s 2 g and one without, and each used both with gentle agitation and with deliberate vigorous aeration. If the sulfite is acting as an oxygen scavenger, the difference between the two baths should be much larger in the aerated pair than in the still pair. This is the only experiment on the page that tests an explanation the course supplied rather than a source.

One bath against two. Six prints through a single bath for four minutes, six through King’s two baths of two minutes each with a fresh second bath, all washed identically. Test every print with ST-1 for residual silver and HT-2 for residual thiosulfate. Reilly’s argument predicts that the single-bath prints will show more residual silver as the bath ages, and that the two-bath prints will not.

Water quality as a variable. The Formulary’s New Kallitype sheet claims that acidic water accelerates bleaching. Mix the plain 5 per cent bath in distilled water, in tap water, and in tap water acidified to about pH 5 with citric acid, fix identical strips in each for four minutes, and then repeat the whole comparison with King’s carbonate added to all three. If the carbonate does what this page says, the second set of three should be indistinguishable and the first should not.