Platinum toner for printing-out papers

The toner behind the olive-black studio portrait of 1900, and a formula this course publishes and does not use.

⚠Read this before the formula: the course never handles platinum

Two things settle this page’s status and they point the same way.

The course’s own scope ruling. Students perform palladium. Platinum is taught in full as chemistry, history and comparison — its curve shape, its image colour, why it was prized, why it cost what it cost — and is never handled. The hazard that decided it is that the platinum salts used in these processes are recognised respiratory sensitisers, that sensitisation is not reversible, and that a sensitised person then reacts at doses which affect nobody else.

The reagent’s own classification. Potassium tetrachloroplatinate(II) is Level C on two of the rubric’s criteria: a sensitiser at the concentrations used, where engineered control genuinely is the recognised measure, and waste that cannot lawfully enter a domestic drain. Its page carries the exposure limit — two thousandths of a milligram per cubic metre — and the reasoning, and neither is restated or softened here.

So this is a Level D-style entry at Level C: the chemistry, the history, the image characteristics and the hazards, with no procedure. It is here because a formulary that could not say what a platinum-toned print was made of would leave a reader unable to identify one.

Ingredient	Quantity	Form the source specifies
Potassium tetrachloroplatinate(II)	50 mL of a 1% solution	printed as "potassium chloroplatinite, 1% sol."; the platinum(II) salt, not the platinum(IV) chloroplatinate of the earlier and largely ineffective toners
Citric acid (anhydrous)	4 g
Water	750 mL, added	Reilly gives 750 mL of water rather than a make-up volume; with the 50 mL of platinum stock the liquid is about 800 mL and the strengths quoted on this page are computed against that.

Purpose

To replace image silver with platinum, for a brown or brownish-black tone, in an acid bath. Reilly’s contrast is exact: J. Reynolds discovered in 1886 that potassium chloroplatinite was a very energetic toner of silver prints, yielding brown and brownish-black tones instead of the purplish and bluish-black tones obtained with gold toners.

Its second purpose is combination. Matte collodion papers were generally toned with both gold and platinum, and that pairing produced the familiar olive-black colour so often seen in the studio portraits of 1895 to 1920.

Recommended uses

Recorded as the historical practice this formula belongs to, not as a recommendation.

Matte surfaces. Reilly: the effects obtainable with platinum toning harmonise especially well with a matte surface, and platinum toning was preferred for the many kinds of arrowroot and matte albumen paper as well as for the matte collodion and matte gelatin printing-out papers popular between 1895 and 1925.

Matte gelatin printing-out paper, toned in platinum alone, which is why so much of it is brown.

Matte collodion, toned in gold and then platinum, for a neutral or olive black.

Not glossy albumen with a platinic salt. Reilly is emphatic that all platinic chloride formulae — the platinum(IV) salts of De Caranza’s 1856 suggestion — have so little toning energy that at best they are restricted to matte papers and are totally ineffective with albumen paper. This formula uses the platinous salt for exactly that reason.

When another formula is preferable

• In every practical case in this course, a gold toner. Gold-borax, gold-thiocyanate and gold-acetate all do the archival job on a printing-out paper at Level B, and the course’s noble-metal printing is palladium rather than platinum for the same reason this page is not a procedure.
• Where a neutral black is the aim, Reilly’s own combined route is the historical answer, and his own assessment of it is worth carrying: it is not possible to consistently attain the absolutely neutral black characteristic of the platinum print itself. A platinum-toned silver print is not a platinotype.
• Where the paper is glossy, this toner was not what the period used, and the platinic formulae that preceded it did not work at all.

Mixing

Recorded from the source. This course does not ask anyone to mix it.

1. Water, 750 mL.
2. Citric acid, 4 g.
3. 50 mL of a 1 per cent potassium chloroplatinite solution.

Reilly gives no make-up volume, no temperature and no mixing order beyond the order of his own table.

🧮What the bath carries

The liquid is 750 mL of water plus 50 mL of stock, about 800 mL, and Reilly states no final volume:

50 mL × 1% = 0.5 g, in 800 mL = 0.625 g per litre

Potassium chloroplatinite

4 g in 800 mL = 5.0 g per litre

Citric acid

0.625 g of the platinum salt per litre is more, by weight, than any gold bath in this formulary: Reilly’s own strongest gold formula is his thiocyanate toner at 0.6 to 0.8 g/L and his albumen gold baths run at 0.4 to 0.5 g/L. That is consistent with what he says about the metals — a gold(I) toner exchanges one silver for one gold, while platinum toning is described only qualitatively as “very active” with no exchange ratio published anywhere the course has read.

The acid is in large excess: citric acid at 192.12 g/mol gives 26 mmol/L against the platinum’s roughly 1.2 mmol/L, about twenty to one. Reilly’s warnings turn on that ratio — a bath that is too acidic yellows the highlights — but he publishes no window, only the direction.

Behaviour

Acid, where a gold toner is alkaline. Reilly states the difference as the main technical one between the two metals, and Kodak’s 1928 primer says the same thing in four words: platinum toning baths are used in an acid condition.

The bath is poisoned by what the print brings in. This is the property that most distinguishes platinum toning in practice. Reilly: any impurities such as silver nitrate or sodium thiosulfate rob the toning bath of its activity by altering the platinum to an irreducible condition. Hence a preparation that no gold toner requires — the print is well washed, treated for two minutes in a 5 per cent sodium chloride solution, and washed again for five minutes in running water, before it goes into the toner.

Over-toning shows in the highlights, not the shadows. Yellowish highlights follow if the toning is carried on too long, or if the solution is too strong or too acidic.

It must not be carried into the fixer. Reilly’s closing instruction is to wash the prints well after toning so that none of the acidic platinum toner reaches the fixing bath.

No time, no temperature, no capacity and no shelf life are published. All four gaps are the source’s, and none is filled in here.

Image characteristics

Brown to brownish-black, which is the whole reason the period used it alongside gold.

Olive-black in combination. Gold first, to a lilac colour, then platinum to a neutral — Reilly’s own sequence, and the origin of the colour of a great many studio portraits.

Not a platinotype’s black. Reilly says so in the same breath as giving the recipe, and it is the most useful sentence on the page for anyone trying to identify a print: the absolutely neutral black characteristic of the platinum print itself cannot consistently be attained by toning a silver print.

Yellow highlights are the characteristic fault, and they are a symptom of too much rather than too little.

The mechanism

Platinum(II) against platinum(IV), which is the whole of the formula’s history.

⚗Why the platinous salt and not the platinic one

Reilly’s account is a comparison rather than an equation, and this page keeps to it because the corpus holds no equation for platinum toning.

Platinic chloride — platinum(IV) — was tried first, by De Caranza in Istanbul in 1856, as an acidified solution. It has only a very slight toning action and a strong tendency to bleach the silver image, and all platinic chloride formulae have so little toning energy that at best they are restricted to matte papers.

Platinous chloride salts — platinum(II), chiefly potassium chloroplatinite — are very active toners when combined with acids, and have a smaller tendency to attack the silver image.

The parallel with gold is close and instructive. There, the problem is that gold(III) costs three silver atoms per gold atom and bleaches faster than it tones, so every useful gold toner is a device for reaching gold(I) — by a sulfur ligand or by an alkaline buffer. Here the lower oxidation state can simply be bought, as the chloroplatinite, and the formula is correspondingly simple: the metal and an acid.

What the course does not have is the exchange ratio. For gold it can say one silver for one gold, or three for one, with the equations on the chloroauric acid page. For platinum no source read for this course gives a balanced equation, an exchange ratio or a formation constant, so no reaction is printed on this page and the mechanism is stated as its sources state it.

Why the acid. Reilly and Kodak both state that the bath must be acid and neither gives a mechanism. What can be inferred without going beyond them is the negative: an alkaline platinum bath is not what the period used, and the direction of the failure Reilly does describe — yellowish highlights from a bath that is too acidic — means the acid has an optimum rather than a floor.

Why the salt bath before toning. Silver nitrate and thiosulfate both alter the platinum to an irreducible condition, in Reilly’s words. A 5 per cent sodium chloride bath converts free silver nitrate in the paper to silver chloride, which the subsequent wash and the eventual fixer deal with, and which does not reach the toner as a soluble silver salt.

Function of every ingredient

Potassium tetrachloroplatinate(II), 50 mL of a 1 per cent solution, so about 0.625 g per litre. The platinum, and the only substance that ends up in the print. Reilly prints it by its period name, potassium chloroplatinite, and the distinction that name carries is the whole point of the formula: it is the platinum(II) salt, not the platinum(IV) chloroplatinate of the earlier platinic formulae, which bleach more than they tone. More than this concentration yellows the highlights, per Reilly’s own warning. Less is untested by any source read here. Note the form: 50 mL is not a quantity and 50 mL of a 1 per cent solution is. Its own page carries the classification, the exposure limit and the reason this course never handles it — including the one reassurance that is well founded, that platinum metal itself is not implicated in the allergy, so a fully processed print is not a source of exposure.

Citric acid, 4 g, so about 5 g per litre. The acid, and the condition the whole toner depends on. Reilly gives platinum toning’s requirement for a neutral or acidic condition as the main technical difference between it and gold toning, and Kodak’s primer says the same. More acid is one of the three causes Reilly names for yellowish highlights. Less moves the bath towards the neutral end of the window, where he gives no guidance at all. The choice of citric acid over Stieglitz’s nitric acid is Reilly’s; neither he nor any other source read here says why, and the difference between a weak organic acid and a strong mineral one in a platinum bath is not something this page will assert.

Water, 750 mL of it. Reilly gives an amount of water rather than a volume the bath is made to, so the bath’s final volume is not published and the two strengths above are computed against the liquid he names.

Sodium chloride at 5 per cent, which is not in the bath but is part of the formula. It is the pre-treatment, and it belongs in this section because leaving it out makes the toner fail rather than makes the print different: two minutes in it, then five minutes’ wash, before the print goes near the platinum.

Interactions

With silver nitrate and with thiosulfate, and both are fatal to the bath. Reilly’s words: they rob the toning bath of its activity by altering the platinum to an irreducible condition. This is a stronger statement than the gold toners’ “ruined by a trace of fixer”, because it names two contaminants and a mechanism.

With the fixing bath, in the other direction. The acidic toner must not be carried into it; Reilly’s instruction is to wash the prints well after toning.

With a gold toner, deliberately. The combined route: the borax gold bath until the print is lilac, then this bath until the colour is neutral.

With metal. The platinum salt’s own page records that platinum plates out of solution onto a metal container: glass only, never metal.

With dust, which is the hazard rather than an interaction. Its page carries the controls; the relevant one for a toner is that there is nothing to inhale from a made-up solution and a great deal from a dried film of it.

Variants

Stieglitz’s 1889 formula, one of the pioneering platinum toners for silver papers, consisting simply of nitric acid, potassium chloroplatinite and water. Reilly names it and gives no quantities, so it is recorded here as history and is not printed as a formula.

De Caranza’s 1856 acidified platinic chloride, the first published platinum toner and effectively a failure: a very slight toning action and a strong tendency to bleach the silver image. It is the reason the platinous salt matters.

The combined gold-and-platinum route, which is not a variant of this formula but a sequence containing it, and which is where the olive-black of the period comes from.

No course variant is offered, and no dilution is proposed. The course does not handle platinum, so it has nothing to offer here that would not be a procedure by another name.

Safety

Level C, from potassium tetrachloroplatinate(II), on two of the rubric’s criteria at once, and read that page before anything else.

The course never handles platinum at any level, on a scope ruling taken on its own grounds: these salts are recognised respiratory sensitisers, sensitisation is not reversible, and a sensitised person then reacts at doses that affect nobody else. Palladium is what the course prints with, and this page does not qualify that ruling in any direction.

The controls that would be required, from the salt’s own page and stated here so that the classification is legible rather than as an equipment list that makes the procedure acceptable: the hazard is inhalation, so the control is that there is nothing to inhale — no drying of coated or spilled material with warm air, no dried film of the salt anywhere, gloves changed when contaminated rather than rinsed, glass rather than metal throughout, and health surveillance for anyone liable to be exposed. HSE’s guidance on asthmagens is that exposure should be prevented wherever reasonably practicable and that it is impossible to identify in advance who will become hyper-responsive.

The one reassurance that is well founded is on the same page and is worth repeating because it concerns the finished object rather than the bath: platinum metal itself is not implicated in the allergy, so a fully processed platinum-toned print is not a source of exposure. Reading and identifying such prints is safe work; making them is what this course declines.

Storage

No shelf life is published for the bath or the stock and none is invented here.

Glass, never metal, because the platinum plates out onto metal.

Away from silver nitrate and from thiosulfate, which are what destroy the bath’s activity rather than merely contaminate it.

Dry material is the hazard, so a spilled or evaporated solution is not a stain to wipe up casually. The salt’s own page carries the reasoning.

Label it with the metal and the strength, because a 1 per cent chloroplatinite stock and a 1 per cent gold stock are both faint yellow liquids in small bottles and cost about the same to be wrong about.

Incompatibilities

Silver nitrate and sodium thiosulfate, which alter the platinum to an irreducible condition.

The fixing bath, into which the acid toner must not be carried.

Metals, onto which platinum plates out of solution.

Alkalis, which take the bath out of the acid condition it requires.

Developer, in either direction, where one is in use at all.

Waste

A platinum-bearing acid stream, and the salt’s own page records that its aquatic classification is one of the two reasons it is at Level C: this waste cannot lawfully enter a domestic drain.

Not neutralised, not mixed with a fixer stream, not mixed with a gold toner’s.

Collect it, label it as platinum-bearing, and follow the general chemical waste SOP and the disposal ruling; where the bath has toned prints it is also a silver-bearing stream. Local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

Recorded from the source for the sake of understanding the formula, not as instructions for running it.

Yellowish highlights. Reilly’s three causes, in his order: toning carried on too long, a solution that is too strong, or a solution that is too acidic.

The bath will not tone at all. Silver nitrate or thiosulfate got into it and altered the platinum to an irreducible condition. It is not recoverable, and the preparation that prevents it is the salt bath and the wash before toning.

The print bleached rather than toned. The platinic salt rather than the platinous one — the failure of every formula between 1856 and 1886.

Nothing happened on a glossy paper. Expected. Platinum toning belongs to matte surfaces, and the platinic formulae are described as totally ineffective on albumen.

The black is not neutral. Reilly’s own assessment: the absolutely neutral black characteristic of the platinum print itself cannot be consistently attained by toning a silver print, however long it is toned.

Stains after fixing. Acidic toner carried into the fixing bath, which Reilly’s closing wash exists to prevent.

Experiments

The course does not perform these. They are recorded as what a laboratory with the controls would do, and as the questions this formula leaves open.

Identify rather than make. The useful exercise available to this course is on the other side of the process: learn to tell a platinum-toned silver print from a platinotype and from a gold-toned print by colour, surface and, where an instrument is available, by X-ray fluorescence. Reilly’s own sentence — that the neutral black of a platinum print cannot be reached by toning — is the first thing such an exercise tests.

Establish the exchange ratio. No source read for this course gives one for platinum, where gold’s is published in two forms. A step wedge toned to exhaustion, with the silver loss and the platinum uptake both measured, would give the number the literature does not.

Find the acid window. Reilly warns that too acidic a bath yellows the highlights and gives no lower bound. The formula at half, one and twice its citric acid would bracket it.

Reproduce the olive-black. Gold to lilac, then platinum to neutral, on matte collodion. It is the characteristic colour of a quarter-century of portraiture and no modern reference print of it exists in this corpus.