Platinotype

☠Level D, and for a reason that is not acute toxicity

The course ruled on 4 September 2026 that students perform palladium and never handle platinum, and the Level D policy carries the argument. Platinum is not acutely toxic in the way mercury and cyanide are. The statement that governs the decision is H334 — may cause allergy or asthma symptoms or breathing difficulties if inhaled — which appears in 99.5 per cent of 212 notified reports for the platinum salt, and HSE’s EH40 sets halogeno-platinum compounds at 0.002 mg/m³ as platinum, with the Sen notation. That is the tightest limit anywhere in this course.

This entry gives no procedure. The chemistry, the workflow in outline, the image characteristics and the hazards are here; no quantity, no sequence and no conditions are.

!Two levels for one substance, and they are not in conflict

The platinum salt’s own encyclopaedia entry classifies the substance Level C, and the Level D policy page lists platinum among the families Level D covers. A reader will notice, so the atlas states it.

They are answering different questions, and the rubric says so: a level is assigned to a procedure, and it is not a permanent verdict on a substance. Level C is a real level with real pages — a fume cupboard, controlled waste and a competent second person make a procedure acceptable, and the course will give the procedure to a reader who has those. Level D is the level at which the course gives no procedure to anyone, because the controls that would be needed are not ones a page can assume of a domestic reader. The salt is a Level C substance; the platinotype, as this course would have to teach it, is a Level D procedure.

The chemistry

Paper carries a mixture of light-sensitive ferric oxalate and a platinum salt that is not light-sensitive at all. Ultraviolet reduces iron(III) to iron(II) and takes the oxalate away as carbon dioxide; a developer then lets the iron(II) reduce platinum(II) to platinum metal, and that metal is the image. Two iron(II) are needed for each platinum atom, because platinum(II) takes two electrons and each iron supplies one.

The developer contains no developing agent. This is the most surprising fact in the noble-metal family and it is worth stating slowly. In dry ferric oxalate the photoproduct is ferrous oxalate, which is insoluble: it sits where it was made, the platinum salt sits where it was coated, and neither can reach the other. Potassium oxalate supplies free oxalate ions which take the iron(II) into solution as a mobile complex. Now it can move, and now it can hand two electrons to one platinum(II). Every other developer in photography reduces something; this one dissolves something and lets the reduction light had already prepared finish itself. Potassium oxalate’s page carries the equations.

⚗One equilibrium constant explains most of the difference from palladium

There is no easy route for putting electrons onto the [PtCl₄]²⁻ ion itself. Ware’s account has the reaction proceed through the aquated anion instead, in which one chloride has been replaced by water — and the equilibrium constant for that substitution is 0.015 for platinum against 0.17 for palladium at 20 °C. So at sensitiser strength fewer than a fifth of the platinum ions are aquated while more than half the palladium ions are.

That single number explains most of the practical difference between the two metals: why palladium prints faster and more smoothly, and why platinum needs help. It also explains a piece of darkroom folklore. Establishing the aquation equilibrium is slow for platinum — Ware cites a measured half-time of 2.4 hours, so ten half-lives is about a day — which is why a freshly made solution is left to mature for twenty-four hours before first use, and may be why the published solubility figures for the salt disagree with one another.

Contrast used to be bought with an oxidant. Pizzighelli and Hübl’s 1882 formulae adjusted contrast by adding potassium chlorate to the sensitiser, which re-oxidises some of the iron(II) photoproduct so that it is unavailable to reduce platinum, truncating the exposure scale. Ware records that this costs image quality and grain, and argues that with a correctly calibrated negative none of the period oxidants is necessary. Part XXV owns a lesson called “The Additives We Study and Do Not Reproduce”, and mercury and lead salts were among them.

A dichromate was another, and it leaves this course by two independent routes rather than one. The Getty atlas records the trick — a trace of dichromate added to raise contrast, still detectable under X-ray fluorescence a century later — and the chromium policy rules chromium(VI) out at any level anywhere. But the platinum sensitiser the dichromate went into is also no longer handled, on the separate ruling above. Two rulings remove the same bottle, and a page that mentions only one of them tells half the story.

Historical workflow

Willis began in 1872 where everyone before him had begun, with platinic chloride — platinum(IV) — and by his own later account “all my early experiments were naturally made with platinic chloride”. They failed, and Ware explains why: platinum(IV) is not reduced to the metal readily enough, and the surviving specimen in the National Media Museum has deteriorated because platinic chloride is strongly acidic and oxidising and the paper was never washed.

In 1873 he turned to the little-known platinous salts and recalled that “after a troublesome operation, I made some potassic chloro-platinite”. Magnus had first reported a preparation of platinous chloride in 1828, but his method was difficult and uncertain and the compound had remained a rarity; Ware suggests that Willis’s finding a use for it is what prompted preparative chemists to look for easier syntheses.

Ware names three things the process needed and could not easily get, and the third is the one people forget: the platinum salt, the ferric oxalate, and the use of potassium oxalate as a developer. Willis admitted that his attention was directed to the last by “…a note by a French chemist…”, who has never been identified, and — the detail that dates the whole enterprise — he had to prepare the potassium oxalate himself, because at the time he could not obtain it in London.

The commercial arc closes quickly. The Getty atlas records that the first platinotype papers went on sale in 1880, that the Platinotype Company listed fifteen kinds in 1911, that the British government declared platinum a strategic metal during the First World War and forbade its use in photography, and that commercial manufacture ended in the United States in the 1930s and in Great Britain in 1941. What revived it was not a paper but a kit: chemicals sold for hand-coating, which is how almost every platinum print since has been made — and which is why the hazard that used to belong to a factory now belongs to whoever opens the bottle.

The image

It is the reference this atlas compares other prints to. A long, smooth tonal scale; a matte surface with no binder over it; and a noble metal for an image substance.

Colour is neutral to cool black. The period sepia platinotype was made by adding mercury(II) salts to the sensitiser, which produces an image resembling palladium’s — lower in contrast, brown and very smooth — and that is how it was done before palladium was used.

Surface is the family’s, and the paper is a real variable: the Getty atlas lists starch, agar-agar, gum arabic and gelatin among the sizing materials described in the photographic literature.

And most platinum prints are not platinum. The Getty atlas records that low concentrations of palladium turn up in some early platinotypes without any deliberate addition, that most platinotypes made before the First World War are simple platinotypes, and that only after 1917 did photographers begin to use palladium and then platinum-palladium mixtures on paper they coated themselves. The “Pt/Pd print” of a modern portfolio is a mixture, and the ratio is a control. The palladiotype entry puts the awkward question that follows.

Permanence

A noble metal in the paper fibres is about as chemically inert as a photograph gets, and the process has the strongest permanence claim in this atlas. What is worth stating is what the claim is not about.

The everyday enemy is not chemistry but relative humidity. The coating’s behaviour, the print-out that happens during exposure and the developer’s work all depend on how much water the paper holds, and a print made at one humidity is not the same print made at another.

Residual iron is the other one. Clearing is a three-bath sequence in this family rather than an afterthought, and iron left in the paper stains it and goes on reacting.

And the claim may have been transferred without anyone checking. If most prints sold as platinum are in fact platinum-palladium or palladium alone, then a permanence claim made for platinum has quietly become a claim about a different metal. The course states that as a question rather than answering it.

Hazards

The one that decides the classification. Potassium tetrachloroplatinate is a respiratory sensitiser. HSE’s paragraphs 53 to 56 are explicit that exposure to an asthmagen should be prevented wherever reasonably practicable, that once the airways are hyper-responsive further exposure to tiny quantities may provoke symptoms, that it is impossible to identify in advance who will become hyper-responsive, and that health surveillance is appropriate for anyone exposed or liable to be exposed. A glove and a good intention are not that.

The EH40 limit of 0.002 mg/m³ as platinum is five times tighter than silver’s soluble compounds and five times tighter than chromium(VI), both at 0.01 mg/m³, and 2,500 times tighter than platinum metal’s own 5 mg/m³ — which is the clearest possible statement that the hazard belongs to the compound and not to the element.

Soluble oxalates are systemic toxins, and both the sensitiser and the developer are built on them. The developer was worked hot before 1892. Waste carries platinum, iron and oxalate and cannot lawfully enter a domestic drain.

The first recorded victims are worth naming: the workers who coated the paper.

Why we study it and do not reproduce it

Because it sets the standard the alternative processes are judged against, and a reader needs to know what it actually offers and what it costs. Every other entry in this atlas that says “long smooth scale” or “matte, no binder, inert metal” is being measured against this one.

Because the chemistry is the chemistry of the process the course does teach. Every equation on this page is the palladiotype’s equation with one metal changed, and a reader who understands the platinotype understands the palladium print they will actually make.

Because the reason for the classification is itself the lesson. The Level D policy puts it well: a control failure whose consequence is permanent cannot be recovered from by tightening the control afterwards, and sensitisation may take weeks, months or years to appear, after which the only remedy is to prevent further exposure. Two very different hazards — mercury and platinum — arrive at one classification, because the classification is about controls rather than about how nasty a substance is.

Part XXV teaches it as chemistry in “Noble Metal Chemistry and the Platinotype”, and gives its Level B labs in palladium. The course’s answer to a process it will not let anyone perform is a neighbouring process it will, and knowing why is more useful than a substitution made quietly.