Gold, Iron-Blue and the Other Metal Toners

Put a fixed, washed silver print into a solution of a gold salt and something happens that the word “toning” hides. Gold arrives on the image. Silver leaves it — dissolved, carried away, gone from the sheet. The two are not separate events but two halves of one electron transfer, and the ratio between them is set before you open the bottle, by which oxidation state the gold is in. Get that ratio wrong and the bath removes more picture than it decorates.

That arithmetic is the spine of this page. Gold, platinum and palladium work by replacement: the image silver reduces a noble-metal ion and pays for it in silver. Iron-blue and copper toners work another way entirely — they build a new coloured compound at the image, and the silver, oxidised out of the way, contributes almost nothing to the colour you end up looking at. Two mechanisms, two sets of consequences, and one of them ends with a print that must never touch a buffered mount board.

!No safety level, and where each level is argued

This page prescribes no bath, no time and no temperature of its own, and the classification rubric grades an operation, not a subject. Where the operation lives, the level is argued: GP-1 is Level B; T-21 and T-26 are Level C, on the persulfate and the thiourea respectively, and each of those pages carries the argument rather than a summary of it. Platinum is not handled anywhere in this course, on a ruling of 4 September 2026; the reasoning is in the platinum toner entry and in Part XXV. Mercury and uranium appear below only inside historical formulas that contain them, named so that you can recognise such a formula and put it down; no procedure using either follows here or anywhere, and the family belongs on the toners we study and do not use.

The gold arithmetic: one atom of gold, one or three atoms of silver

Gold toning is a redox reaction with a fixed exchange rate. Mike Ware, writing the chemical history of gold in photography, states the mechanism and the accounting in the same paragraph: the image silver is partially replaced by elemental gold, which tends to coat the silver nanoparticles, and in the course of reducing the gold salt to metal a chemically equivalent amount of metallic silver is dissolved out of the image. Both halves matter. It is not a plating in the electroplater’s sense, where metal is added and nothing is taken away; it is a substitution that happens to leave the new metal sitting on what is left of the old.

Silver is only ever oxidised to silver(I). So the exchange rate is decided by the gold’s oxidation state, and there are two possibilities.

3 Ag + Au³⁺ → 3 Ag⁺ + Au

Gold(III): three silver atoms spent for one gold atom deposited

Ag + Au⁺ → Ag⁺ + Au

Gold(I): one for one

Ware’s conclusion follows immediately, and it is the single most useful fact about gold toners: all useful gold toners employ gold(I), because the gold(III) route costs three times the silver for the same amount of gold. James Reilly, writing from the conservation side about albumen and salted paper, says what that costs the print: in an acid bath such as a simple gold chloride solution, one atom of gold replaces three atoms of silver, “a case in which toning action would lag far behind bleaching of the silver”, and the result “would be a flat, lifeless image with a reddish color”. The bath eats the print faster than it colours it.

⚗Why gold plates onto silver at all

Ware’s table of noble-metal redox potentials gives the commonest gold salt as E([AuCl₄]⁻/Au, 4Cl⁻) = +1.00 V, and silver, on the same table, as +0.80 V — the figure Part III tabulates more precisely as +0.7996 V. The gold couple is the higher of the two, so metallic silver will hand its electrons to gold(III) and gold metal will appear where the silver was. That is the whole thermodynamic story of gold toning, and it is why the same trick does not work as freely with the other noble metals — a point this page returns to when it gets to platinum and palladium, whose couples sit below silver’s.

The by-product has to go somewhere. Reilly records that in gold or platinum toning the substitution creates silver chloride as a by-product, and that toning must always be followed by a fixing step to remove it. That instruction belongs to the printing-out processes, where the print is toned before fixing — Bostick and Sullivan’s gold kit for printing-out paper says so in as many words, and their fixing step is where the excess silver finally leaves. On a developed-out silver gelatin print, which has already been fixed and washed, the dissolved silver leaves in the bath and in the wash instead, which is one reason a spent gold toner belongs in the silver-bearing stream and not in the sink.

What the substitution does to the colour

The colour change is not the gold’s own colour arriving. It is the particle changing. Reilly lists the three things that set the colour of a print: the size and shape of the image particles, the distance between them, and the refractive index of the medium they sit in. Replacing silver atoms with gold atoms enlarges the metal aggregates, and enlargement makes the print appear colder — more neutral. That is the same physics the part’s opening lesson sets out for nanoparticle silver, and it explains an observation that otherwise looks like folklore.

Kodak’s 1928 chemistry manual states the folklore version and gets the mechanism right:

Finely divided gold is red, which is not as pleasing as the blue gold obtained by more rapid deposition. To insure rapid deposition it is necessary that the bath should be kept alkaline.

Deposit gold slowly and it lands as very small particles, which absorb where small gold particles absorb, and the print goes red. Deposit it faster and the particles coalesce, and the print goes blue. The rate of deposition is a colour control, and it is set by the bath’s chemistry rather than by the printer’s timing. Getty’s conservation atlas confirms both endpoints on real objects: it reproduces a bright-red developing-out print and a bright-blue one, and X-ray fluorescence finds gold in both.

What a gold bath takes and what it leaves

1. 1Gold deposited on the particle — Ware: the gold tends to coat the silver nanoparticles rather than replace them wholesale, so the picture is a gold surface over a smaller silver core
2. 2Image silver still in the sheet — what is left to carry the density, and the reason Reilly calls the acid gold(III) result flat and lifeless
3. 3Image silver dissolved out — three atoms per gold atom from gold(III), one per gold atom from gold(I); it leaves in the bath and then in the wash
4. 4Density surviving the treatment — drawn in proportion to the silver remaining, five-eighths against seven-eighths

Drawn to teach the ratio, not to depict a particle. The stoicheiometry is Ware's; the consequence for density and colour is Reilly's. Numbers are marked on the left panel only, because the right panel is the same drawing with different counts.

Deeper: the complexing agent is the toner

Gold(I) is where a working toner has to be, and gold(I) is not a place you can simply buy. Ware sets out the difficulty: most simple compounds of gold(I) in water are unstable with respect to disproportionation —

3 Au⁺ → 2 Au + Au³⁺

Disproportionation: gold(I) falls apart into metal and gold(III)

— so the answer has to be found among the complexes of gold(I). Ware names the water-soluble stable ones: the sulphito, cyano, thiocyanato and thiosulphato species, [Au(SO₃)₂]³⁻, [Au(CN)₂]⁻, [Au(SCN)₂]⁻ and [Au(S₂O₃)₂]³⁻. Read the list against the ingredient panel of any published gold toner and the pattern is obvious. The thiocyanate in GP-1 and in the gold-thiocyanate toner is not a bystander; it is the reagent that takes gold(III) down to gold(I) and then holds it there.

This is the reason a bare gold chloride solution is a poor toner and a complexed one is a good one, and the reason is quantitative. A ligand that binds gold(I) strongly does not merely keep it in solution; it lowers the couple’s potential. Ware gives the extreme case: E°([Au(CN)₂]⁻/Au, 2CN⁻) = −0.6 V, against +1.00 V for the uncomplexed tetrachloroaurate. That is a swing of 1.6 volts produced by nothing but a pair of ligands, and it is the same effect Part III teaches on the silver side, where the silver couple falls from +0.80 V free in solution to +0.02 V once two thiosulfates are attached. Complexation is how a photographic chemist turns a violent reagent into a controllable one.

⚗The two ways to reach gold(I), and the twenty-four-hour one

Ware distinguishes two broad categories of gold(I) toner.

Sulfur-ligand baths. A sulfur-containing molecule — thiosulfate, thiocyanate or thiourea — both assists the reduction of gold(III) and stabilises the gold(I) that results. These are the baths that work as soon as they are mixed: the gold-thiocyanate toner is ready cold, and GP-1 is mixed immediately before use.

Alkaline-buffer baths. Sodium tetrachloroaurate(III) is simply dissolved in a mild alkaline buffer — acetate, borate, carbonate, phosphate, or even chalk — and the gold(III) reduces itself to gold(I) over as much as twenty-four hours by oxidising water. No sulfur is involved at all. The gold-acetate toner is the classic case and will tone nothing on the first day; the gold-borax toner is the faster member of the family. Ware adds the limit that saves a wasted afternoon: made too alkaline, the solution becomes ineffective.

Reilly supplies the practical test for which state a bath is in. The acid gold stock is yellow; when it has passed into the active state through contact with alkali it becomes colourless. That decolorization “is the best guide to the state of the toning bath”. He also gives the reason not to push it: a toner made too alkaline tones more quickly but loses its activity much faster, and too-active toners “do not produce pleasant tones and are more difficult to control”.

Two threads meet here. Kodak’s 1928 rule that alkalinity gives rapid deposition and therefore blue gold, and Ware’s rule that alkalinity is one of the two routes to gold(I), are one chemistry seen from the tray and from the flask. Reilly’s warning about over-active baths is why this course treats neither “more gold” nor “more alkali” as an improvement: past a point both trade a controllable reaction for an uncontrolled deposit.

The three published gold routes, and what distinguishes them

Kodak’s G-23 sheet publishes three gold formulas, and they are three different jobs rather than three strengths of one. Each is a formulary entry here, with its provenance, its quantities and its mixing order; the quantities are not repeated on this page.

Bath	What it is for	Where in the scale it acts	Working temperature	Course level
GP-1, gold protective solution	Protection, with the tone changed “only slightly”	Uniformly; the shift is to a slightly bluish black	20 °C	B
T-21, gold toner	Warm to cold brown on warm-tone papers; little effect on cold-tone papers	Highlights and shadows at a uniform rate, so the printer can stop at a chosen hue	43 °C	C
T-26, blue toner	Blue on some papers; soft grey-blue is commoner than a saturated blue	Highlights first, shadows last	20 °C, or 38–40 °C for a faster bath	C

The middle column is the one to read twice, because it decides what each bath can be used for as a picture-making tool. A toner that acts uniformly cannot be split-toned: stop it early and you get less of the same thing everywhere. A toner that starts in the highlights can be stopped anywhere along the scale, and Kodak says what that looks like — “partial toning may produce blue highlights and untoned shadows”. This is the same distinction the selenium page makes from the other end, where the bath reaches the shadows first.

⁇Is T-21 a plating bath or a sulfiding bath?

Kodak’s own sheet describes T-21 as producing brown tones “by ‘plating’ the silver image with gold”. Ware classifies the same formula quite differently. In his table of gold toning methods, T-21 sits under sulphiding gold toners, and his account of it is that Walter C. Nelson patented the bath in 1932 as a modification of the hypo-alum sulphiding toner, in which the persulfate oxidises the thiosulfate to generate colloidal sulfur and so assist the sulphiding of the silver, with gold added at a low concentration.

Both readings fit the ingredients. The bath contains potassium persulfate and a large mass of thiosulfate — the components of the hypo-alum sepia toner — as well as gold. What decides between them is an analysis nobody appears to have done: Ware states plainly that there is no record of prints toned by this means having been analysed to determine the relative proportions of gold and silver sulphide in the final image.

The course’s position: T-21 is a mixed toner producing both gold and silver sulfide, in unmeasured proportions. Kodak’s “plating” describes the intended effect, not the composition, and this page does not read it as evidence of what the image is made of.

⁇What is the reducing agent in T-26?

Kodak’s 2006 printing of Blue Toner T-26 lists four solutes: a one per cent gold chloride stock, thiourea, tartaric acid and anhydrous sodium sulfate. Ware, describing the same formula by name, lists 0.04 per cent gold chloride, 0.2 per cent ferric EDTA, 0.1 per cent tartaric acid and 1.5 per cent sodium sulfate.

Three of the four match Kodak’s table exactly when Kodak’s masses are read as percentages. The disagreement is confined to the one component that decides the mechanism: a sulfur ligand reducing gold(III) to gold(I), or an iron chelate, which would put the bath in a different family altogether.

The course follows the manufacturer’s own sheet and treats T-26 as a gold-thiourea toner, on the principle that a maker publishing its own formula outranks a secondary description of it. The discrepancy is recorded rather than resolved, because it is possible that Ware is describing a different revision; the corpus holds only the 2006 printing, and no earlier one to compare.

Two further gold baths in the formulary belong to other papers: the gold-borax and gold-acetate toners, both alkaline-buffer routes for printing-out and salted papers, used in Parts XXII to XXIV.

🕰Why gold toning exists at all

Gold toning is older than the word “toning”, which Ware dates to about 1855; before that, photographers said “colouring” or “gilding”. Its adoption as a permanence measure has a specific origin, which Ware places at the Photographic Society’s Fading Committee — he gives the year as 1850, and this course has found no second source for that date, so the attribution is his. The committee’s recommendations included toning with gold salts to coat the silver particles in a layer “which is quite impervious to attack”, and Ware’s observation on surviving objects is that albumen prints so treated usually show a rich purplish-brown colour with little evidence of fading.

A recommendation and an observation about survivors are not a measurement. The evidence is the business of the archival-evidence lesson, and this page hands it over.

Gold on a sulfide-toned print, and why the order matters

Take a print that has already been sepia toned — its image now silver sulfide rather than silver — and put it into a gold bath. What comes out is red.

Wall’s 1912 dictionary describes it from practice: a gold and thiocyanate bath of the printing-out kind, applied to a bromide print already toned brown in the sulphide toner, carries the colour “through a series of warm browns to red chalk”. Kodak’s G-23 gives the modern version with numbers on it — sepia or brown toner, a thorough wash, then Blue Toner T-26, the red or orange appearing after approximately 15 to 30 minutes at 32 °C — and adds that cold-tone papers yield a truer red while warm-tone papers give an orange.

Two of Kodak’s cautions are worth more than the recipe. The technique usually costs density in the shadows, so the sheet says to start from a print of higher-than-normal contrast. And it is the sequence that is specified, not merely the pair of baths.

⚗Why the sequence is not reversible

Each bath acts on whatever the last one left, and the two orders present the second bath with different substances.

Sepia first leaves an image of silver sulfide. A gold bath then acts on that, and Ware records what is believed to happen: the crimson is “supposed to be due to the formation of a double sulphide of gold and silver”. Note the word supposed — this is a nineteenth-century interpretation carried forward, not a determination, and the course reports it as such.

Blue first leaves something else entirely: Prussian blue and silver ferrocyanide, with little or no metallic silver left to sulphide. No source read for this course describes the result of running a sulfide bath on a blue-toned print, which is not evidence that it fails — it is evidence that nobody publishing has thought it worth describing. If you try it, you are running an experiment, and the honest way to record it is as one.

The technique has a distinguished user. Ware notes that William Mortensen’s proprietary “Metalchrome” process, published only after his death, uses gold toning of a sulphide-toned print as an essential early step to reach red, before modifying those reds to flesh tones with water-soluble aniline dyes. That is toning and tinting in one workflow, and it is a useful reminder that the two are separable operations even when a single printer runs both.

Where gold toning is actually used, and what it costs

Here is the disappointment, stated by the chemist most enthusiastic about gold in photography. Ware:

In modern bromide papers, the colour change brought about by gold toning is very slight, or even imperceptible, because the silver formed in developed bromide papers has a filamentary structure much larger than nanoparticle silver, and appears neutral black, offering little scope for shifting its colour.

The particle-size argument that made gold a colour treatment for printing-out papers is the same argument that makes it a *non-*treatment for a modern enlarging paper. A print-out image is nanoparticle silver whose colour is exquisitely sensitive to particle size; a developed bromide image is filamentary silver, already large, already neutral. Gold lands on it and changes very little. Ware’s conclusion is the one the market has reached anyway: “gold protection, rather than toning, of silver-gelatin images is recommended when archival stability is paramount”, and GP-1 is the example he names.

This does not make gold toning of developed papers pointless — Kodak sells T-21 for warm-tone papers and Ware’s own account of the 1930s blue-toning vogue is a counter-example. But the counter-examples have a condition attached. Ware records that the blue result “depended critically on the particle size of the original silver image”, so slow chlorobromide papers gave much finer results than faster bromide papers, and that such papers should not be hardened in the fixing bath. Kodak’s sheet reaches the same place by observation: T-21 has little effect on cold-tone papers, T-26 will not change a cold-tone paper at all. If your paper is a neutral or cold-tone bromide, gold is a protective treatment and not a colour one, and you should choose it on that basis.

✕Chrysotype is not gold toning

Ware corrects this misapprehension twice in two different publications, which suggests he meets it often. Chrysotype is a printing process in which the image is made of gold from the start and no silver is involved at all. Gold toning is a retrospective manipulation of an existing silver image. The two share a metal and nothing else, and the confusion matters because it leads people to expect a gold-toned silver print to have a chrysotype’s permanence, which it cannot: the silver underneath is still silver.

The cost, and an honest gap in the shopping data

Gold is why this page carries a £££ cost level, and the cost sits almost entirely in one bottle. The comparison the formulary makes is instructive: both GP-1 and T-26 are built from the same one per cent gold stock, and T-26 takes four times as much of it for a bath of about the same volume, so a litre of blue toner costs roughly four litres’ worth of protective solution in gold. That ratio is the course’s arithmetic from the two published tables rather than a figure either sheet prints, and it comes with the caveat the T-26 entry records: Kodak never states what the blue toner is made up to. Kodak’s own comment on T-26 is blunter — “Blue Toner is fairly expensive toner to produce” — and it is followed by the advice to mix it immediately before use, which means the gold in an unused litre is money poured away.

Two further numbers bear on what a gram of “gold chloride” actually buys. Kodak’s 1928 manual gives the gold content of gold chloride as 65 per cent metallic gold, and of gold sodium chloride as 49 per cent, with the note that the double salt “is neither acid nor deliquescent”. Reilly and Kodak both warn that the substance photographers buy under the name gold chloride is not gold(III) chloride at all but chlorauric acid, and that the dry salt is very deliquescent — sold in hermetically sealed tubes for that reason. The gold(III) chloride and chloroauric acid pages carry the identity problem, the hazard classification and the storage rules; both are handled as Level B, with splash goggles for the solid and plastic rather than metal for every spatula, because these compounds attack most metals. The classification behind that comes from the aggregated GHS notifications rather than from a current supplier’s safety data sheet, which this course does not hold for either salt: the sheet that arrives with the bottle governs, and it should be read before the bottle is opened.

!What the course could not price

src/data/prices.json has no entry for gold chloride in any form, and a search of the UK photographic suppliers the course uses for its price data returned no listing for the raw salt at all. What it holds instead is finished product: the Moersch MT10 gold toner at £32.99 for 250 mL of working solution is now a dated record there, at the top of a band that runs down to about £12 for a bottle of sepia. A Fotospeed blue toner at £23.99 for 150 mL, from a UK dealer’s search results on 6 September 2026, is the same kind of evidence and is not in the file.

So the £ per gram of gold salt that this page’s cost level rests on is unverified, and the £££ band is argued from the finished-product prices and from the fact that the active metal is gold. A made-up toner of undisclosed strength cannot be divided into grams of the salt, which is the reason the file gives for keeping gold chloride on its gap list even now that a gold toner is priced. Treat the band as a floor rather than a figure, and see the planner for what the course can actually price.

Iron-blue toning: a pigment built where the silver was

Iron-blue toning belongs to Kodak’s second category of toning, and the difference from gold is not a matter of degree. Kodak’s 1928 manual sets out the pattern for the whole family in one paragraph: potassium ferricyanide oxidises the silver image and forms silver ferrocyanide from it, and if the bath also contains the salt of a metal whose ferrocyanide is insoluble and coloured, that metal’s ferrocyanide is what appears at the image. Iron citrate gives “a blue iron ferrocyanide”. Uranium nitrate gives the reddish-brown uranium ferrocyanide. Copper citrate gives the red copper ferrocyanide. The operation is sometimes run in one bath and sometimes in two.

Three reactions carry it. The first is the silver’s oxidation, which is the same first step as a rehalogenating bleach — ferricyanide takes an electron from metallic silver and becomes ferrocyanide:

Ag + [Fe(CN)₆]³⁻ → Ag⁺ + [Fe(CN)₆]⁴⁻

The image silver reduces ferricyanide to ferrocyanide

The ferrocyanide it has just made now has two possible partners. One is the silver it came from:

4 Ag⁺ + [Fe(CN)₆]⁴⁻ → Ag₄[Fe(CN)₆]

Silver ferrocyanide, the white by-product Wall describes

The other is the iron(III) supplied by the bath, and this is the reaction that makes the picture:

4 Fe³⁺ + 3 [Fe(CN)₆]⁴⁻ → Fe₄[Fe(CN)₆]₃

Prussian blue, iron(III) hexacyanoferrate(II)

This is why the blue lands where the silver was. Prussian blue needs the ferrocyanide half, and the fastest, most concentrated source of it in the sheet is the image itself, reduced silver being an efficient electron donor. So the pigment appears in proportion to how much silver was there — the general property of toning that the part’s opening lesson sets out, reached here by a route quite unlike sulfide or selenium.

✕Why the whites stain blue as well, and why every procedure ends 'wash until the highlights are clear'

It would be tidy to say that iron(III) and ferricyanide cannot make Prussian blue without the silver. They can, and Ware’s account of the Prussian blue family says exactly how. A mixture of iron(III) and hexacyanoferrate(III) — Prussian brown, or Prussian yellow — is soluble and a powerful oxidant, much stronger than either ion on its own, because the insolubility of the product drives the reaction and puts the effective potential at about +1.5 V. Ware records that the brown mixture “rapidly turns blue when spotted onto cellulose filter paper or any other oxidisable substrate”, and that “it forms in the presence of almost any oxidisable substance”. In cyanotype practice the same reaction is a known defect: ferric ions in the developer cause blue fogging of the background.

A print is paper and gelatin. So a blue toning bath will slowly stain the parts of it that carry no image, and that is the fault every published procedure quietly guards against. Wall’s 1912 bath is washed “until the high-lights are clear”. Kodak Limited’s T-11 gives 10 to 15 minutes for the same reason, and its diagnosis of blue-stained highlights is fog, a stale bath, or a bath mixed out of order — three ways of saying that the bath’s own oxidising power got to the background before the wash did.

The silver still decides the picture, because it is faster and it is where the density is. But “nothing happens where there is no silver” is the ideal case, not the observed one, and this is one of the few toners where the whites are genuinely at risk.

Getty’s conservation atlas confirms the product on real objects. Blue toning formulas work “by converting silver particles to silver salts and then forming a blue image by reaction with a solution of an iron salt”, and the different mechanisms “often yield blue images composed of Prussian blue pigments” — identified by X-ray fluorescence showing a higher concentration of iron in the maximum density areas than in the highlights. The atlas also disposes of a name that still circulates: blue toning was sometimes called cobalt toning, after the colour of the image, and there is no cobalt in the bath.

🔬The same pigment as a cyanotype, made backwards

Prussian blue is ferric ferrocyanide, and it is the image substance of the cyanotype as well as of a blue-toned silver print. The chemistry runs in opposite directions in the two cases. In a cyanotype, light makes iron(II) and the hexacyanoferrate in the coating is the precipitant waiting for it. In blue toning, the iron(III) is supplied ready-made and the silver provides the reducing power that converts ferricyanide to ferrocyanide.

Same pigment, opposite mechanisms, and — as the rest of this page will show — identical vulnerability. The iron-blue toned print entry in the process atlas carries the comparison in full.

The formulas, and what the record actually contains

Published iron-blue formulas exist in quantity and they do not agree about the iron salt. This course holds three primary printings, and they must be told apart because they were written for different materials.

• Wall’s 1912 dictionary gives two ferric baths “for bromide prints”. The second is the one this page’s chemistry describes: ten per cent solutions of ferric ammonium citrate and potassium ferricyanide with dilute acetic acid, in which well-washed prints are immersed until the tone is reached and then washed “until the high-lights are clear”. Wall adds four words that matter later: “This bath intensifies the image.”
• Wall’s 1924 formulary gives Sedlaczek’s single-bath variants, built from ten per cent stocks of ammonia iron alum — ferric ammonium sulfate, not citrate — with a citrate or an oxalate, ammonia alum, hydrochloric acid and ferricyanide, giving a deep blue, a cold blue, or with tartaric acid a grey blue. It also gives a two-bath route: ferricyanide with a little potassium oxalate first, a wash, then iron ammonium oxalate.
• Kodak Limited’s 1949 formula T-11 is titled “Iron toner for blue tones in slides or films”. Its composition is ammonium persulphate, ferric alum, oxalic acid, potassium ferricyanide, ammonium alum and dilute hydrochloric acid; the sheet says the method of compounding is “very important” and that each solid must be dissolved separately and the solutions mixed strictly in the order given, or the bath will not be pale yellow and clear.

!Answering the question the manifest asked about T-11

This lesson’s specification asked whether Kodak Limited’s T-11 is suitable for paper. Read in full, it is not offered for paper at all: its own title says slides or films, and its composition is ferric alum with persulfate and oxalic acid rather than the citrate the covers list names. The citrate-and-ferricyanide description traces instead to Wall’s 1912 bath for bromide prints and to Kodak’s 1928 account of iron citrate, and that is the pairing this page teaches.

T-11 is still worth having in front of you, for two things it says that no paper formula in the corpus says as clearly. Its mixing instruction is that each solid must be dissolved separately and the solutions combined strictly in the order given — a bath that is not pale yellow and perfectly clear has been mixed wrongly. And its troubleshooting is diagnostic: blue-stained highlights mean the material was fogged in development, or the bath was stale, or it was mixed out of order. Neither of those depends on the support.

No iron-blue toner appears in this course’s formulary, and so no quantities appear on this page. That is a deliberate gap rather than an oversight: a formula belongs in a formulary entry with its mixing order, its function-of-every-ingredient table and its waste section, and until one is written the honest form of this lesson is the mechanism, the provenance and the warning.

What blue toning does to the print — and which “blue toner” you mean

Two claims travel with blue toning: that it increases contrast and density, and that it tones the highlights first. Both are true of something. They are not true of the same bath, and the confusion is entirely the fault of a product name.

Kodak’s “Blue Toner” T-26 is a gold toner. It contains no iron at all. Everything Kodak’s sheet says under that heading — that “the contrast and density of prints treated in Blue Toner T-26 appear to increase”, that exposure may be reduced slightly to compensate, that “toning will occur in the highlights first and the shadows last” — is a statement about a gold-thiourea bath. Read as a claim about iron-blue toning it is unsourced.

What is documented for the iron baths is the density effect, from a different direction. Wall’s 1912 bromide-print bath “intensifies the image”, and Kodak’s 1928 account of the whole deposition-of-metal-salts family treats intensification as characteristic of it — the reddish-brown uranium ferrocyanide is described as having “very great printing strength”. That makes sense of the mechanism: the silver has been oxidised out of the way and replaced, particle for particle, by a pigment that may be a better absorber than the silver was. Whether Prussian blue outperforms silver at a given mass is not something this course’s sources quantify, and no number should be attached to the word “intensifies” on this page.

The paper and the developer matter as much here as anywhere in toning. Kodak’s own note on T-26 is that “the exact tone you obtain will vary with the paper and developer you use” and that “a soft gray-blue is more common than a saturated blue” — warm-tone papers reacting well, neutral-tone papers shifting slightly, cold-tone papers not changing. Ware’s account of the 1930s gold blue-toning vogue says why in mechanism: the blue depended critically on the particle size of the original silver image. Both statements are about gold baths, but the underlying dependence — image colour follows particle size, and particle size follows emulsion and developer — is the same one Part XVIII establishes for untoned papers.

=The first prints tone most: a property of the bath, not of the blue

Kodak states of T-26 that “if you tone prints one at a time in Blue Toner, the first few prints will tone the most, and the last few may not tone at all”, and recommends immersing up to five prints together for consistency. Is that a general property of these baths or specific to T-26?

The evidence the course holds says it is a property of an unreplenished noble-metal bath, and it is not established for the iron baths at all. Bostick and Sullivan describe exactly the same behaviour in their gold-thiocyanate kit for printing-out paper — “with every print that is toned, a portion of the gold chloride is used up” — and publish two replenishment rules to deal with it, one adding gold after every print and one waiting until the bath visibly slows. Kodak’s own T-21 sheet does the same thing with a replenishment figure. Three gold baths, one behaviour, and a mechanism that explains it: the gold is consumed, one atom at a time, by the prints.

An iron-blue bath is not consumed in the same way — its iron is a precipitant rather than a metal being plated out — but Wall notes that Sedlaczek’s baths “will not keep well” and should be mixed just before use, which is a different failure with the same practical consequence. The course records Kodak’s statement as being about T-26 and about gold baths generally, and marks the iron case as not established.

The alkali problem, which is a permanence consequence and not a hazard

Every source that describes iron-blue toning describes, in the same breath, the thing that will destroy it. Wall, 1924: “the colour is dependent on the deposition of Prussian blue on the image, and this is soluble in alkalis; therefore, long washing in ordinary water is inadvisable”. Kodak Limited, 1949, on T-11: “since the toned image is soluble in alkali, washing should not be carried out for too long a period, especially if the water is slightly alkaline”. These are not safety warnings. Nobody is going to be hurt. What is at risk is the picture, over a timescale from minutes to decades.

Ware’s Cyanomicon gives the chemistry and, unusually for this subject, a number. Prussian blue has three distinct destruction pathways — photochemical reduction, alkaline hydrolysis and aqueous peptization, which he calls fading, bleaching and dispersing. The middle one is ours. Its cause is any substance of alkaline pH; its products are hydrated ferric oxide, which is colloidal and becomes insoluble, and ferrocyanide ions, which are soluble and wash away; and its reversibility decreases to nil as the ferric oxide ages.

The number is Holtzman’s, quoted by Ware, and it is the sentence to memorise:

A buffer at pH 9.4 completely decolourises (i.e. destroys by irreversible hydrolysis) Prussian blue in 1 to 10 minutes… This is not a highly alkaline pH, and is the same, for example, as that of a saturated solution of calcium carbonate, the buffer commonly incorporated in archival papers and boards.

A 0.25 molar sodium carbonate solution, at about pH 10.7, destroys a cyanotype’s blue in less than half a minute.

What alkali does to a blue-toned image, and the window in which it can be undone

1. 1Prussian blue in the emulsioniron(III) hexacyanoferrate(II), deposited where the silver was
2. 2Contact with anything above pH 7wash water, a carbonate wash aid, a buffered mount, an alkaline paste; at pH 9.4 the reaction is complete in 1 to 10 minutes
3. 3Alkaline hydrolysisthe pigment splits into hydrated ferric oxide, which stays in the sheet, and ferrocyanide ions, which are soluble
4. 4While fresh: partly reversibleWare reports partial restoration by dilute acid with added ferrocyanide, before the ferric oxide has aged
5. 5Once aged: irreversiblethe ferric oxide-hydroxide becomes insoluble in dilute acid, cannot regenerate Prussian blue, and remains as a yellow-brown stain bound to the cellulose

The five stages are Ware's account of alkaline hydrolysis in Cyanomicon, chapter 9; the pH 9.4 timing is Holtzman's, quoted there. The same sequence is what a controlled bleach-back exploits, stopped at stage four.

⚗Why 'archival' board is the enemy here

Calcium carbonate is in the board on purpose. Ware sets out the standard: ISO 9706:1994 for permanent paper requires a pH between 7.5 and 10.0 and an alkali reserve of at least 0.4 moles of acid per kilogram — at least 2 per cent w/w calcium carbonate. The reserve exists to neutralise acid arriving from the atmosphere or from the object itself, and for a silver gelatin print it is a good thing. Part XII’s archival sequence records the same 2 per cent figure from the conservation side and reaches the ordinary conclusion: buffered board for a silver print, unbuffered for a cyanotype.

A blue-toned silver gelatin print is in both columns at once, and the pigment decides. Its support and its binder are a silver print’s; its image substance is a cyanotype’s. Nothing in the sources this course holds addresses that specific object, so the reading here is the course’s own: treat a blue-toned print as a Prussian blue object for every material it will touch, because the alkaline reserve that protects the paper attacks the image, and it is the image you cannot replace.

The practical list follows from that one number.

• Wash water. Kodak Limited’s warning is about the tap. Hardness is not pH, and this course has verified the pH of no supply, but hard water is water that has been through chalk. Wash, and stop.
• Wash aids. Here the course can be specific, because the maker publishes the figure: ILFORD WASHAID at 1+4 is pH 7.00 to 7.20, so it is not the problem. A wash aid built on carbonate rather than sulfite would be; find out the way the sulfite washing aid page does, by looking for a published pH and assuming nothing when there is none.
• Mount board, window mats, interleaving, enclosures. Anything sold as buffered, acid-free or conservation grade probably carries the chalk reserve. Part XIX’s mounting section says what to read on the specification; here you want the unbuffered version of all of it.
• Adhesives and pastes. A starch paste is not automatically neutral.

⚠One genuine hazard, alongside the permanence one

Nothing above is a health matter, but iron-blue toning does carry one that is. Potassium ferricyanide is not itself acutely toxic in the way its name suggests — the cyanide is locked in a very stable complex — but in contact with concentrated acid it releases hydrogen cyanide, and these baths are acidified. That is on its chemical page with its sources, and it is the reason a ferricyanide bath never meets a concentrated acid and never goes down a drain with acid waste. The waste rules for the whole part are on the disposal page and in the general chemical waste SOP. Check your local regulations; they govern, and they differ.

Reversibility: a control and a warning at once

Because the pigment is destroyed by alkali, a blue-toned print can be partly bleached back. Wall records the equivalent operation in the copper family: a one per cent ammonia bath clears the whites. Kodak Limited’s instruction to wash “until the high-lights are clear” is the same effect used deliberately, at the mild end.

As a control that is valuable: very few toning operations can be walked backwards, and a print gone too blue in the highlights can be brought back. Ware states the limit, and it is severe. A freshly bleached cyanotype can be partly restored by dilute acid with added ferrocyanide, but as the ferric oxide–hydroxide ages it becomes insoluble in dilute acid and cannot regenerate Prussian blue; what remains is a yellow-brown stain bound to the cellulose. Kodak Limited notes that even a correctly toned and washed T-11 slide acquires “a very slight permanent yellow colouration of the clear gelatin”.

One property of one pigment is therefore both halves of the story: the print can be corrected today, and ruined slowly by its mount for the next thirty years.

ILFORD state the conclusion from the manufacturer’s side, in a note printed under their optimum permanence sequence: sulphide, polysulphide and “some metal replacement toners (gold and platinum)” give a protective effect, while “other metal replacement toners such as blue (iron) and red (copper) toner may not give extra protection and the image might fade”. Blue toning is a colour decision. It is not an archival one, and no page in this course will claim otherwise.

Copper and the red toners

Copper toning is the same second-category chemistry with a different metal in it, and Kodak’s 1928 one-line summary is that copper citrate with ferricyanide gives “the red copper ferrocyanide”. Wall’s 1924 formulary sets out the working baths — Ferguson’s single solution of cupric sulfate, potassium citrate and ferricyanide, which keeps and does not stain, and Sedlaczek’s variants for particular tones, which do not keep and must be mixed just before use — and his account of the mechanism adds the part that matters:

…the deposition of red cuprous ferrocyanide on the image with the simultaneous formation of white silver ferrocyanide. It is actually an intensification process; therefore, the primary image must not be developed too far.

Both halves of that are practical instructions. The colour arrives as a new compound; the silver stays in the sheet as a white one; and because density is added rather than exchanged, the print must be made lighter than you want it. Wall gives the levers as well: a weak one per cent ammonia bath will clear the whites of the strongest bath, and about five minutes in five per cent hypo makes the toned image markedly more transparent. Like blue toning, copper toning can be walked backwards.

⁇Which copper compound is the red one?

Three sources, three namings.

• Kodak, 1928: “the red copper ferrocyanide”, from copper citrate plus ferricyanide.
• Wall, 1924: “red cuprous ferrocyanide” — that is, a copper(I) compound, which would require the copper to be reduced as well as the silver oxidised.
• The Getty atlas, 2013, analysing a real copper-toned print by X-ray fluorescence, finds copper and iron and attributes the red to “the precipitate of copper ferricyanide complex” — a ferricyanide rather than a ferrocyanide.

XRF establishes the elements present, not the oxidation states or the stoicheiometry, so the atlas’s naming is a description rather than a determination. The course records the agreed part — the red is a copper hexacyanoferrate deposited at the image — and states that the oxidation state of the copper and the exact composition are not settled by the sources it holds. Anyone writing a formula page for a copper toner will have to settle it from the chemical literature rather than from these three.

Why this course is cautious about copper

Not because of the level. A tray of dilute copper toner is a Level B operation on the rubric’s own criteria, in the same band as a good deal of what this course does teach. Three other things make it a poor bargain.

The waste is permanent. Copper sulfate pentahydrate — the hydrate that formulas actually specify — carries a harmonised classification of Danger with H318, causes serious eye damage, alongside harmful if swallowed, skin irritation and H410, very toxic to aquatic life with long lasting effects. That last statement decides the disposal on its own: a spent copper bath is bottled, labelled and taken to hazardous waste collection, never poured away. It also fails Level A’s waste criterion outright, which is the same test that put selenium into Level B.

There is nothing to set against it. Sulfide and selenium buy conversion of the image to a more stable compound; gold, platinum and palladium buy a noble metal on the surface of the particle. ILFORD name red (copper) toner alongside blue as a treatment that “may not give extra protection” and after which “the image might fade”. A permanent environmental liability in exchange for a colour, and no permanence claim to weigh against it, is a trade this course does not recommend — while noting, since Rule 7 asks for the distinction, that this is a judgement about a trade-off and not a statement of chemical fact. The reds copper reaches are not available from any other bath this course will run; the only other route to them is uranium, and uranium is Level D study only.

And there is no formulary entry. No copper toner is published in this course’s formulary, so no quantities appear here, for the same reason no iron-blue quantities do.

Platinum and palladium, named honestly

Silver will reduce a gold(III) salt spontaneously because gold’s couple, at +1.00 V, sits above silver’s +0.80 V. The same table puts platinum at +0.73 V and palladium at +0.62 V — both below silver — and Ware notes alongside it that the iron(II) reductant used in the printing processes “does not reduce platinum(II) or palladium(II), although it will reduce gold(III) and silver(I)”.

Read at face value, that says the image silver has no thermodynamic reason to hand its electrons to either metal, which would make platinum toning a different sort of reaction from gold toning rather than a more expensive version of it. That reading is the course’s own, from Ware’s published figures, and it is directional rather than decisive: these are potentials for particular chloro-complexes, and complexation and acidity move them. What the sources say next is at least consistent with it. Reilly records that the rate of substitution is greater for gold than for platinum, “although the rate may be modified by the presence of other substances in the toning solution”, and that platinum toning “goes on much more effectively in an acid environment”. Kodak’s 1928 manual states the same practice from the other side: “Platinum toning baths are used in an acid condition.”

What platinum toning does when it works is what gold does: Reilly’s account covers both, a partial replacement that encloses the image silver in a metal much harder to oxidise or to sulphide, with the image aggregates enlarged so the print reads colder. The platinum toner for printing-out papers is in the formulary as chemistry, published at Level C and marked historical study.

Should this page describe platinum or palladium toning of a silver gelatin print? The manifest asked, and the answer from the evidence is no, on two independent grounds.

☠The one published silver-gelatin platinum toner in this corpus contains mercuric chloride

Wall’s 1912 dictionary gives a platinum toning bath for bromide prints — potassium chloroplatinite, citric acid, distilled water and mercuric chloride, with the note that increasing the mercuric chloride warms the tone and reducing it cools it. The mercury is not an impurity or an optional extra; it is the tone control.

That places the formula squarely in Level D. It is recorded here so that you can recognise it in an old manual and put the manual down, and it is described on the toners we study and do not use along with the rest of that family. No procedure follows from it anywhere in this course.

The second ground is the course’s own ruling of 4 September 2026: platinum is never handled, on the respiratory sensitisation hazard of the hexachloroplatinates, and Part XXV performs palladium instead. And the third is simply that the modern kits do not do this job. Bostick and Sullivan’s palladium toner is titled “for POP, Vandyke and Kallitype”; its working bath is dilute citric acid with a few drops of a chloropalladite solution, the print is toned before fixing, and the fixing step “will bleach out the excess silver and reveal your final toned image”. That is a printing-out workflow from beginning to end. It is not a treatment for a fixed and washed enlarging paper, and nothing in the corpus adapts it to one.

So platinum and palladium toning belongs beside Part XXV, where the sensitisers, the clearing baths and the papers are, and where a palladium bath is a step in a process rather than an afterthought bolted onto a silver print. This page names them, says what they do and why, and stops.

Selective and multiple toning, and the words for what is happening

Selective toning is masking. Kodak’s method is the one to know because the manufacturer publishes the failure modes with it, which is more than most accounts do.

A liquid frisket is brushed on, or a clear self-adhesive sheet is laid down and cut round the area to be toned, the unwanted film lifted off with tweezers. The print is presoaked — ten minutes for fibre-base, two minutes for resin-coated — and put into the toner, which is allowed to flow freely over the uncovered parts; the sheet says not to worry if the print buckles. Liquid frisket rubs off under the fingers while the print is still in the wash, and maskoid lifts on a piece of sticky tape. With two or more toners in sequence, the whole procedure is repeated for each.

The two cautions are the useful part. The toner will try to bleed under the frisket, and Kodak’s countermeasures are, in order: keep the part you do not want toned physically out of the solution if the geometry allows it, and if it does not and bleeding starts, skip the presoak. A dry print resists creep under the mask better than a swollen one. And the sheet leads with a compositional instruction rather than a chemical one — choose a scene with a distinct line between the areas you want treated differently — which is an honest statement of the technique’s limit. A frisket edge is a boundary, not a gradient, so the method is worst exactly where a print most needs subtlety.

📷Judge the object, not the palette

Multiple toning is not colour mixing and cannot be planned like it. After a sepia bath and a gold bath, a print carries metallic silver, silver sulfide, gold, and — on Ware’s account, tentatively — a double sulphide of gold and silver, in proportions nobody has measured. Add a blue bath and there is Prussian blue and silver ferrocyanide as well. Kodak’s promise for the sepia-then-blue sequence is that a print may show “its original warm image tone, as well as sepia, blue, and red or orange tones” at once, which is a fair description of what a partially converted image looks like and a poor basis for predicting one.

Ware’s verdict on the elaborate mixed baths of the past is worth carrying into the tray: when mixtures become this complicated and the composition of the resulting image is correspondingly uncertain, “the value of the whole enterprise is called into question”. The course’s practical version: work by test strips and by inspection, record what you did in the lab notebook at the level of bath, dilution, temperature and time, and judge the finished object. A prediction you cannot make is not a plan.

Toning, tinting, mordanting: three words that are not synonyms

The course keeps these apart deliberately, and the distinction is mechanical rather than verbal.

Toning changes the image substance, so it acts in proportion to density: where there was no silver, nothing happens, and the whites stay the colour of the paper.

Tinting colours the binder or the base, so it lands everywhere — including the highlights. The test on a finished print is exactly that: a tinted print has coloured whites, a toned one does not.

Mordanting is the hybrid, and Kodak’s 1928 manual describes it as its third category of toning. The silver image is converted into a substance that will hold a dye — treatment with a mixture of potassium ferricyanide and potassium iodide converts it to silver iodide, which “will mordant basic dyes and attach them to the image”; Kodak’s own laboratories published a variant using a uranium mordanting bath that converts the image to a mixture of uranium and silver ferrocyanides before dyeing. The colour lands on the image, in proportion to density, which makes it behave like toning. But the substance carrying the colour is an organic dye.

That last point decides its permanence, and ILFORD state it in one sentence: “Dye toners do not give extra protection.” A mordanted image is only as durable as its dye, which is usually the least durable component in the object. It is the exception to the generalisation that toning improves stability, and the exception exists because the colour has stopped being a metal.

✦One consequence for finishing

Kodak’s sheet closes with a restriction that catches people out: you cannot use reducers on a toned print. Farmer’s reducer works by dissolving silver, and on a toned print the thing you would be attacking is no longer silver — or is silver hidden under gold. Local lightening after toning is a mechanical operation, with an etching knife, not a chemical one. Which is an argument for doing the fine printing work before the toning and not after it.

Carrying this forward

Six things from this page are worth holding.

1. Gold(I), not gold(III). The oxidation state sets the exchange rate between gold gained and silver lost, one-for-one against three-for-one, and every useful gold toner is built to reach gold(I) — by a sulfur ligand or by an alkaline buffer and a day’s patience.
2. The complexing agent is the toner. It reduces the gold, holds it in solution and lowers the couple’s potential, which is what converts a violent oxidant into something you can watch.
3. Gold on a modern bromide paper is protection, not colour, because developed filamentary silver is already large and neutral. Colour from gold belongs to printing-out papers and to Parts XXI to XXV.
4. Iron-blue and copper toners build a new compound rather than converting the old one. The silver is oxidised out of the way and ends up as a white ferrocyanide; the colour is a pigment made at the image by the ferrocyanide the silver itself produced.
5. Prussian blue is destroyed by alkali at pH 9.4 in one to ten minutes — the pH of the calcium carbonate reserve in archival board. Everything a blue-toned print touches must be chosen on that fact.
6. Toning, tinting and mordanting are three mechanisms, and only the first two are told apart by looking at the whites.

The permanence claims made in passing here — that gold protects, that blue and copper do not — are evidence claims, weighed against what conservation science has measured on the archival-evidence page. The substances named here and refused a procedure are on the toners we study and do not use.

Check your understanding

Question 1. A gold toning bath is made by dissolving gold chloride in water with nothing else added, and a print is toned in it. What is the most likely result?

• A rich blue-black, because gold is a cold-toned metal
• A flat, reddish image with noticeably less density than it started with
• No change at all, because gold will not deposit on silver
• An increase in density, because gold is added to the image

Show the answer and why

Answer: A flat, reddish image with noticeably less density than it started with

The gold in a simple acid gold chloride solution is gold(III), so three atoms of silver dissolve for every atom of gold deposited. Reilly describes the outcome exactly: toning action lags far behind the bleaching of the silver, and the result is a flat, lifeless image with a reddish colour. The red comes from the same cause as the flatness — slow deposition gives very small gold particles, and Kodak's 1928 manual notes that finely divided gold is red while faster deposition gives blue.

Question 2. Why does a thiocyanate or a thiourea appear in almost every practical gold toner?

• It buffers the bath so the pH does not drift
• It is a wetting agent that helps the toner penetrate the gelatin
• It reduces gold(III) to gold(I) and forms a complex that holds it in solution, which also lowers the couple's potential
• It dissolves residual silver halide left by incomplete fixing

Show the answer and why

Answer: It reduces gold(III) to gold(I) and forms a complex that holds it in solution, which also lowers the couple's potential

Ware names the stable water-soluble gold(I) complexes — the sulphito, cyano, thiocyanato and thiosulphato species — and explains that a sulfur ligand both assists the reduction from gold(III) and stabilises the gold(I) that results. The complexation is not incidental: the formation constants are large enough to depress the potential dramatically, from +1.00 V for uncomplexed tetrachloroaurate to −0.6 V for the dicyanoaurate. That is the same effect that turns silver's +0.80 V into +0.02 V when thiosulfate binds it in the fixer.

Question 3. In an iron-blue toning bath containing an iron(III) salt and potassium ferricyanide, what supplies the ferrocyanide that Prussian blue needs?

• It is present in the bath from the start as an impurity in the ferricyanide
• The acid in the bath reduces some of the ferricyanide
• The image silver, which is oxidised and reduces ferricyanide to ferrocyanide
• The iron(III) salt disproportionates in solution

Show the answer and why

Answer: The image silver, which is oxidised and reduces ferricyanide to ferrocyanide

This is why the blue lands on the image and not in the whites. Iron(III) and ferricyanide together do not give Prussian blue; the ferrocyanide half has to be made, and the only reducing agent in the sheet is the metallic silver of the image. So the pigment appears in proportion to the silver that was there. Getty's atlas confirms it on real prints by X-ray fluorescence: more iron in the maximum-density areas than in the highlights.

Question 4. You have made a blue-toned silver gelatin print and want to mount it for exhibition. The shop offers buffered conservation board at pH 8.5 with a 2 per cent calcium carbonate reserve, and an unbuffered rag board. Which do you take, and why?

• The buffered board, because a silver gelatin print should always be mounted on buffered stock
• The unbuffered board, because the alkaline reserve will hydrolyse the Prussian blue
• Either; the reserve only matters for prints in direct contact with the atmosphere
• The buffered board, because Prussian blue is stabilised by calcium ions

Show the answer and why

Answer: The unbuffered board, because the alkaline reserve will hydrolyse the Prussian blue

The image substance decides, not the support. A saturated calcium carbonate solution sits at pH 9.4, and Holtzman found that a pH 9.4 buffer completely and irreversibly decolourises Prussian blue in one to ten minutes. The conservation advice that a silver print takes buffered board and a cyanotype takes unbuffered assumes the two are different objects; a blue-toned silver print is both at once, and the pigment is the part you cannot replace. Expect the alkaline hydrolysis to leave a yellow-brown iron oxide stain behind it.

Question 5. Kodak states that if prints are toned one at a time in Blue Toner T-26, the first few tone the most and the last few may not tone at all. What does the course conclude from this?

• That all blue toners behave this way, iron and gold alike
• That the effect is unique to T-26 and follows from the thiourea
• That it is characteristic of an unreplenished gold bath, corroborated by two other gold toners, and is not established for iron baths
• That the bath has been contaminated by carry-over from the fixer

Show the answer and why

Answer: That it is characteristic of an unreplenished gold bath, corroborated by two other gold toners, and is not established for iron baths

T-26 is a gold toner, and gold is consumed one atom at a time by the prints. Bostick and Sullivan report the same slowing in their gold-thiocyanate kit and publish two replenishment rules for it; Kodak give a replenishment figure for T-21. Three gold baths, one behaviour, one mechanism. An iron-blue bath works differently — its iron precipitates rather than plating out — and although Wall notes that those baths keep badly, no source read here establishes the first-prints-tone-most effect for them.

Question 6. Which statement about platinum toning of a silver gelatin print does the evidence in this course support?

• It is a straightforward substitute for gold toning and gives colder blacks
• It is well documented for enlarging papers and simply expensive
• The one published silver-gelatin formula in the corpus is controlled with mercuric chloride, the modern palladium kits are for printing-out processes, and the course rules platinum never handled
• It cannot work at all, because platinum sits below silver in the electrochemical series

Show the answer and why

Answer: The one published silver-gelatin formula in the corpus is controlled with mercuric chloride, the modern palladium kits are for printing-out processes, and the course rules platinum never handled

Three separate reasons converge. Wall's 1912 bromide-print platinum bath uses mercuric chloride as its tone control, which is Level D. The course ruled on 4 September 2026 that platinum is taught but never handled, on the respiratory sensitisation hazard of the hexachloroplatinates. And the palladium toners actually sold are titled for POP, Van Dyke and kallitype, toning before the fix. The potentials — platinum at +0.73 V and palladium at +0.62 V against silver at +0.80 V — explain why the reaction is not spontaneous in the way gold's is, but they are standard potentials and both complexation and acidity move them, so they are a direction rather than a proof.

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