Lead additions to platinum sensitisers

Point an X-ray fluorescence spectrometer at a sheet of William Willis’s Platinotype paper from about 1906 and a lead line comes up. Point it at a finished print of the same period and the lead is there too — but spread more or less evenly through the sheet, as strong in the high values as in the deepest black. Whatever the lead was doing, it was not making the picture. It was making the picture possible, and then staying behind.

☠Level D: this page is not a procedure

Lead(II) nitrate is classified for historical study only, and the Level D policy governs what this page may contain. It may give the composition, the chemistry, the historical workflow in outline, the image characteristics, the analytical evidence and the hazards. It may not give a procedure, and it does not: no coating routine, no exposure, no development time, no clearing sequence and no washing schedule appears below as an instruction.

The composition is published because a formula that cannot be read cannot be identified, and identification is the main use a reader has for this entry. The course asks nobody to weigh any of it. Lead is regulated as lead rather than as a particular salt, its harm is cumulative rather than acute, and there is no protective equipment list that makes a lead salt on an open bench acceptable in a domestic darkroom. None is offered.

Ware’s own position, from the chemist who reconstructed the historic formulations in order to make reference materials for conservation scientists, is that additives such as mercury and lead salts “would not be generally recommended today for reasons of health and safety, and image permanence”. The platinum and palladium printing this course does teach is on the three-solution drop system and the potassium oxalate developer pages, and neither contains a lead salt.

Ingredient	Quantity	Form the source specifies
Lead(II) nitrate	2.59 g	40 grains, at Ware's 0.0648 g per grain. Ware converts the same line to 9.12 per cent w/v and 0.275 molar at a formula weight of 331.2, which treats the fluid ounce as the volume of the finished solution; Willis's own wording is "40 grains of the salt to 1 oz. of water", a volume of water added. See Mixing.
Water	28.4 mL, added	One imperial fluid ounce, at Ware's 28.413 cm3, and a volume of water the salt goes into rather than a volume the solution is made up to: Willis writes "40 grains of the salt to 1 oz. of water". Ware's percentage conversion of the same line reads the ounce as the finished volume. The difference is small and it is real, and it is discussed under Mixing.

!Read the identifier of this page sceptically

This entry is registered in the course’s own plan as “lead contrast additions to platinum sensitisers”. The sources do not support the word contrast. Ware states flatly that “Willis makes no mention of contrast control in his patents or sensitizer formulae”, and the course has read all five of those patents in transcription and found none either. The agents the platinum trade actually used to raise contrast were potassium chlorate — the drop-counted system of Pizzighelli and Hübl that survives in every platinum kit sold today — together with sodium hexachloroplatinate(IV) and, in the developer, potassium dichromate.

What a lead salt did was different and is set out below: it made the platinum reduce faster and more completely, so that more of it survived the wash. Ware’s phrase is “smoothing out and intensifying the tones”; the Getty Conservation Institute’s is “a more uniform reduction of platinum salts during development”. A stronger maximum black does lengthen a print’s scale, and that is why the two effects get confused. They are not the same thing, and a page that let them merge would teach the reader to misread an XRF spectrum.

Purpose

To record the one lead dose Willis published as a solution in its own right, to explain what a lead(II) salt does to platinum chemistry, and to make a whole class of nineteenth-century print identifiable from its analysis. That is the whole of the entry. It publishes no procedure and asks nobody to carry one out.

There is a second reason it earns its place. Lead is the clearest case in the whole of the platinum literature of an additive that is demonstrably present in an object and demonstrably absent from its image. That distinction is invisible to the eye, invisible to a catalogue, and obvious in ten minutes of X-ray fluorescence — and getting it wrong is how a conservator ends up reporting a “lead print”.

Recommended uses

Reading Willis’s patents without being misled by their summaries. The five specifications are the only formulations Willis ever published, and they contradict each other on lead in a way that maps exactly onto the commercial history:

• 1873, No 2011. Lead nitrate is a coating of its own, between the platinum and the iron, at 40 grains per fluid ounce. Silver nitrate at 8 grains per fluid ounce is offered as an alternative in the second method.
• 1878, No 2800. The three coatings have collapsed into one solution, and the lead is inside it as “preferably two grains of plumbic chloride” per fluid ounce. Mercuric chloride at four grains is offered in its place — the sentence that founds the mercuric sepia platinotype.
• 1880, No 1117. Lead is gone. Willis writes that “it was found necessary to use a salt of lead or of silver” and that “I can dispense with the lead and silver salts, and by avoiding their use can obtain greatly superior results”, his stated reason being that “all danger of discolouration of the white portion of the prints arising from the use of metals other than iron and platinum is completely avoided”. What replaces the lead is platinum: not less than 1.7 grains of the platinous salt per square foot of coated paper, and usually about four, against not more than two-thirds of a grain before.
• 1887, No 1681. Lead is back, named in the coating alongside mercury.
• 1887, No 16,003. Lead is back and now compulsory: “the lead or mercury, one or the other of which is in all cases essential in the practice of my invention”. Willis’s reason is chemical and he says so — “by ensuring the presence or contact of a salt of mercury or of lead with the image at the time of its development, I obtain a better reducing action”.

Fourteen years of one man’s laboratory notebook, compressed into five legal documents, and the argument is always the same: the platinum will not reduce fast enough on its own.

Identifying a print or a paper, and knowing what the lead line means. The AIC’s conservation catalogue lists lead among the elements detectable by XRF in a platinum or palladium print. The Getty’s atlas prints an XRF detail of an early platinotype showing lead and attributes it to the lead oxalate of early formulas. Ware reports Jacqueline Rees’s 1993 measurement of Willis’s “KK” Platinotype paper of about 1906 at the Victoria and Albert Museum, in which zinc and lead were found at very low levels — the paper being unexposed and unprocessed, so the lead is the manufacturer’s and nobody else’s. And Alisha Chipman sorted Paul Strand’s platinum prints into five categories by which of lead, mercury, palladium and gold each contained. A lead line is a dating and attribution signal, not a defect.

Knowing that the patents and the objects disagree, and that the objects win. Willis’s 1880 patent says the lead has gone. Ware states that analysis shows lead was still present in some of his papers after 1892. Both can be true — a patent describes an invention, not a production line, and the Platinotype Company never published the formulation of its most successful papers at all. What follows for a reader is a rule of method: a patent is evidence of what was claimed, and a spectrum is evidence of what is there.

Understanding why platinum needs help at all. This is the point that carries over to modern practice. Plain ferric oxalate with a platinum(II) salt gives, in Ware’s words, “a weak, fibrous or ‘grainy’ platinum image”, because the reduction of platinum(II) by iron(II) is slow and the wet developer washes the unreacted sensitiser out of the paper before the image has finished forming. Four remedies were tried, and lead is one of a family: a hot developer, a paper sized so the sensitiser stays put, glycerine to slow diffusion, or an additive that speeds the platinum chemistry up. Every platinum printer still meets that problem; the modern answers are different.

Conservation, and one specific trap. The Getty’s atlas carries an explicit caution: some mounting boards and paper substrates contain small amounts of lead themselves, so a print permanently attached to a board has to be measured both on and off the image and the board’s signal subtracted before the lead is attributed to the photograph. A lead line is not automatically Willis’s lead.

Not for making a print. There is no version of this that this course teaches.

When another formula is preferable

• For contrast, the chlorate drop system — which is what the question was actually asking. The three-solution drop system makes contrast a count of drops from two ferric oxalate bottles, one of which carries potassium chlorate. Its costs are documented and real: Ware notes that chlorate can coarsen the image and produce a “false sparkle” of empty highlights, and the Getty’s atlas records a visually detectable patchiness. It is nonetheless the agent the trade used, the one still sold, and the one this course explains.
• For contrast with less image damage, sodium hexachloroplatinate(IV). Ware records it as a nineteenth-century recommendation still sold today at a high price and said to cause less deterioration than chlorate, with the caveat that it cannot be used with any ammonium salt in sensitiser or developer, because ammonium hexachloroplatinate(IV) is barely soluble and crystallises out.
• For contrast in the developer, potassium dichromate. Willis and Clements recommended a very small amount in the potassium oxalate bath for “brilliant prints”; Strand and Ned Scott used it on flat negatives. Ware’s warning is chemical: dichromate reacts with oxalic acid and is reduced to oxalato-chromium(III) complexes, so the additive is not stable in an oxalate developer indefinitely and loses effectiveness, especially at low pH. It is a chromium(VI) salt, with everything that implies.
• For contrast without any additive at all, a better negative. Ware’s own recommendation, and the AIC’s summary of contemporary practice, is that contrast in this process is set mostly by exposure and by the negative. The unmodified platinotype has an exposure scale of about 2.0; a step tablet spans about 3.0. Nineteenth-century camera negatives were long-scale, which is precisely why Willis never needed a contrast agent.
• For a vigorous, smooth image without a heavy metal additive, palladium. Ware is explicit that mercury and lead “are unnecessary in the Palladiotype process, which readily gives a smooth result on a wider range of papers with a simple sensitizer formulation”. Palladium(II) complexes are more labile than platinum(II) ones and reduce easily; the additive exists to fix a problem palladium does not have.
• For an image that never leaches at all, print-out. Ware’s print-out platino-palladiotype forms the metal during exposure, so nothing is washed away before it has reacted. That removes the reason lead was there.
• Never mercury instead. The other historical additive is Level D for its own reasons; see the mercuric sepia platinotype.
• Never this, for any practical purpose. The classification is not a difficulty rating.

Mixing

This section gives no mixing procedure. What can be described without instructing is the shape of the thing, the arithmetic that turns Willis’s units into modern ones, and the two quantities the patent leaves in a state a table cannot hold.

Three coatings, dried between each. Willis’s first method is not a sensitiser in the modern sense. The paper receives the platinum salt and is dried; then the lead nitrate and is dried; then the ferric oxalate and is dried. Only after that is it exposed. The lead is therefore laid down underneath the light-sensitive layer and on top of the platinum, which is a fair physical picture of what it does: it sits in the fibres where the reaction will happen, waiting. By 1878 Willis had combined all three into a single solution, and from that point on “the sensitiser” means one bottle.

The units. Willis writes in grains and imperial fluid ounces. Converting with the table in Appendix VIII of the Platinomicon — 1 grain = 0.0648 g, 1 imperial fluid ounce = 28.413 cm³, and therefore 1 grain per fluid ounce = 0.228 per cent w/v — gives the figure in the table above. Ware converts this same line himself, to 9.12 per cent w/v and 0.275 molar at a formula weight of 331.2, and both agree exactly with that factor.

🧮The 1873 first method in modern units, which is the course's arithmetic and not Willis's

Three coatings, each stated as grains of salt to one fluid ounce of water:

Coating	As Willis prints it	Grams in 28.4 mL	Per cent w/v	Approximate molarity
1. Chloro-platinate of potassium	10 gr to 1 oz	0.648 g	2.28	0.047 M as K₂PtCl₆, 0.055 M as K₂PtCl₄
2. Nitrate of lead	40 gr to 1 oz	2.59 g	9.12	0.275 M
3. Ferric oxalate	60 gr to 1 oz	3.89 g	13.68	—

Per litre those are 22.8 g, 91.2 g and 136.8 g. The lead is in molar excess over the platinum by a factor of between five and six, whichever of the two candidate platinum salts Willis meant; and the 2 grains per fluid ounce of plumbic chloride he used from 1878, at 0.0164 molar, is about seventeen times weaker than the 1873 lead coating. What cannot be compared is the amount of each metal that actually reached a square foot of paper, because the 1873 patent publishes no coating rate.

The percentages and molarities are this course’s arithmetic, done so that the doses under Variants can be set against each other. Willis’s statement is the grains and the ounce; the lead figure is also Ware’s, and it agrees.

Added water, not a made-up volume. Willis writes “40 grains of the salt to 1 oz. of water”. That is a volume of water the salt goes into, not a final volume the solution is brought to, and this course records the two differently because sources state them differently. Ware’s 9.12 per cent w/v reads the ounce as the finished volume. The gap between the two readings is small — a few per cent — and it is worth naming rather than smoothing over, because a nineteenth-century patent that says “to 1 oz. of water” and a modern formulary that says “water to make 30 mL” are describing two different solutions with the same words.

The platinum salt that cannot be tabled. Willis’s first coating is “the chloro-platinate of potassium”. The Patent Office abridgment of the same specification calls it “potassium platinochloride”. Nineteenth-century usage was not consistent, and the two readings differ in the platinum oxidation state: the -ate salt is platinum(IV), the potassium hexachloroplatinate that a modern printer knows as a contrast additive, while by 1878 Willis is unambiguously writing “chloro-platinite”, the platinum(II) salt this course has a page for as potassium tetrachloroplatinate(II). Nothing read for this page settles which he used in 1873. Neither candidate for the 1873 salt has an encyclopaedia entry, so an ingredient line for it would point nowhere, and the table records the coating this page is about rather than pretending to the whole method.

The ingredient with no quantity. The third coating is ferric oxalate “with as little oxalic acid as is sufficient to render the ferric oxalate soluble”. There is no figure, in the patent or in its abridgment. Ferric oxalate is awkward to keep in solution and free oxalic acid is what keeps it there, so the omission is a real gap. The Pizzighelli and Hübl entry carries a published figure for the same job from a different formula, and it has been kept out of this table because it is not Willis’s.

Behaviour

Fourteen years of one man changing his mind, told through the documents that record it. Everything below is history; the dates are the argument and the quantities are evidence, not settings.

1873 to 1878: lead is load-bearing. Willis’s own retrospective, in the 1880 patent, is that in practising the earlier process “it was found necessary to use a salt of lead or of silver”. The Getty’s atlas says the same from the other end: the chemistry of his first platinotype process “included the addition of silver or lead nitrates into the sensitizing solution”. Neither metal was decoration. Pizzighelli and Hübl, reviewing the patents in 1886, describe the 1878 modification as enabling prints “showing much greater uniformity, and possessing a higher degree of permanence” — the same word, uniformity, that the Getty uses of lead oxalate more than a century later.

February 1880: the process is attacked on permanence, and lead is the weakness. Van Monckhoven treated platinotypes with hydrogen sulphide gas and complained of yellowing in the high values. In the same month John Spiller subjected them to what Ware calls “a whole barrage of destructive tests, including strong acids that would obliterate any silver print”, and found exactly one substance that caused any deterioration at all: ammonium sulphide, which dulled the highlights. The attribution at the time, and Ware’s now, is the lead and silver salts of the sensitiser.

March 1880: Willis removes it, and pays for it in platinum. The third patent dispenses with lead and silver, and its stated compensation is a large increase in the platinum: not less than 1.7 grains of platinous salt per square foot and preferably about four, where the earlier process used no more than two-thirds of a grain. That is a six-fold increase in the most expensive ingredient in the formula, accepted in order to drop a cheap one. George Dawson tested the new papers and found them unaffected by ammonium sulphide. It is the clearest single measurement of what the lead had been worth.

1887: the criticism returns, and so does the lead. Andrew Pringle published his observation of platinotypes yellowing in hydrogen sulphide solution, and an acrimonious correspondence ran through the British Journal of Photography, with Herbert Berkeley of the Platinotype Company defending the process and H. H. O’Farrell confirming Pringle. Ware’s judgement is that by then Willis “had recently been obliged, in the interests of image quality, to re-introduce either lead or mercury salts into his sensitizers” — which the two 1887 patents confirm in his own words.

November 1887: Willis states the rule. The last of the five specifications says that lead or mercury is “in all cases essential”, and offers three places to put it: in the coating, in the developer, or in both. The developer route is the one that produces the page’s oddest published instruction. Willis prepares a bath of 90 grains of potassic oxalate and 9 grains of potassic chloro-platinite per fluid ounce and then adds a lead salt, “preferably the acetate”, until a permanent precipitate begins to form. That is a titration to a visible end point rather than a weighed dose, and it is a perfectly sensible way to saturate a solution with a sparingly soluble salt when nobody has a solubility table to hand. It is also, for the same reason, unrepeatable from the printed page.

The one incompatibility Willis names himself. In the same specification: “when a salt of lead is employed, there must be no salt used in the developer which would entirely prevent the solution of the lead salt”. His previous patent’s developer had been built on phosphates, and lead phosphate is insoluble; adding lead to that bath would simply precipitate it. His fix was to abandon the phosphates altogether, and he says so, adding that their presence “is the cause of, or is accompanied by the defect in the tone or colour of the picture”.

After 1892: the record goes quiet and the objects speak. Willis never patented or published the cold-development papers of 1892, which were his most successful product, and no Platinotype Company records in the United Kingdom are thought to have survived the Second World War. What survives is the paper itself. Rees’s 1993 XRF of an unexposed “KK” sheet of about 1906 found lead at very low levels, and Ware states that analysis shows lead was still present in some of Willis’s papers post-1892.

Image characteristics

Smoother, stronger, and the same colour. Ware’s summary of what lead(II) and mercury(II) salts do is that they “hasten the response of the platinum chemistry, thus improving the image quality by smoothing out and intensifying the tones”. The Getty’s atlas puts the lead half of that as uniformity twice over — lead oxalate was added “to facilitate good uniform development of the platinotype image” and, in later formulas, “to facilitate a more uniform reduction of platinum salts during development”.

No colour shift, and this is the sharp difference from mercury. Mercury(II) turns a platinum print brown, because it is itself reduced to the metal and co-precipitates. Lead is not reduced at all, for reasons given under The mechanism, so it contributes nothing to the image substance and shifts nothing. Willis’s lead papers were the black ones. When he wanted sepia he reached for mercury, and said so.

The one apparent exception is a different practice. The Getty’s atlas records that a warm-red tone can be got “by development in a solution modified by the addition of copper chloride, mercury chloride, or lead acetate”, and notes that no red-toned platinotype had been analysed at the time of writing. That is lead in the developer, at an unpublished concentration, in a bath whose oxalate chemistry differs from the sensitiser’s; it is not evidence that a lead-bearing sensitiser gives a warm image, and the two should not be run together.

The lead is not in the picture. Clarke’s extensive XRF measurements are the decisive result: the lead signal does not correlate with image density, but appears as a more or less uniform background across the paper. Ware’s reading is that the lead is present as an insoluble colourless salt — the oxalate or the chloride — distributed through the fibres rather than deposited where the light fell. A platinum print with lead in it is a platinum image sitting in a leaded sheet of paper.

What is not published. No source read for this page gives a densitometric comparison of the same negative printed with and without a lead salt, an exposure-time comparison, or a measured maximum density for either. Everything above is qualitative, and the people who wrote it were describing prints they could see rather than curves they had measured.

The mechanism

Take it in three steps: what light does, what the developer does, and what the lead changes.

What light does

Iron(III) oxalate is the only light-sensitive substance in the coating; the platinum salt and the lead salt are both inert to light at printing intensities. Absorbing ultraviolet, the iron(III) oxalate loses one oxalate to carbon dioxide and drops to iron(II):

light + Fe₂(C₂O₄)₃ → 2 FeC₂O₄ + 2 CO₂

Photolysis: iron(III) oxalate to insoluble iron(II) oxalate

The product, iron(II) oxalate, is almost insoluble, so at the end of the exposure the image is a faint, immobile brownish deposit that can reduce nothing because it cannot move. This is the “faint brown image” Willis’s patent tells the printer to expose until.

What the developer does

Free oxalate from the hot potassium oxalate bath dissolves that deposit as a mobile complex, and the mobile iron(II) then reduces platinum(II) to metal:

FeC₂O₄ + C₂O₄²⁻ + 2 H₂O → Fe(C₂O₄)₂(OH₂)₂²⁻

Dissolution: the immobile photoproduct becomes a mobile bisoxalatoferrate(II)

2 Fe(C₂O₄)₂(OH₂)₂²⁻ + PtCl₄²⁻ → 2 Fe(C₂O₄)₂(OH₂)₂⁻ + Pt + 4 Cl⁻

Development: iron(II) reduces platinum(II) to platinum metal, releasing chloride

Two features of that last equation decide this page. The first is that oxalate complexation collapses the iron redox potential: E°(Fe³⁺/Fe²⁺) is +0.771 V, but the oxalato couple is only about +0.02 V, which is why oxalate-complexed iron(II) is a strong enough reductant to bring down a noble metal at all. The second is the four chloride ions on the right.

What the lead changes

Platinum(II) is reduced through its aquated form, and there is not much of it. The tetrachloroplatinate anion is hard to hand electrons to; the aqua-complex is much easier, and the two sit in equilibrium:

H₂O + PtCl₄²⁻ ⇌ PtCl₃(OH₂)⁻ + Cl⁻

Aquation, the equilibrium that decides how reducible the platinum is

Ware gives the equilibrium constant at 20 °C as 0.015 for platinum against 0.17 for palladium, so at sensitiser concentrations less than a fifth of the platinum is aquated where more than half the palladium is — and the equilibrium is slow, with a measured half-time of 2.4 hours.

The reaction poisons itself. Every platinum atom reduced releases four chloride ions into the wet paper, and those chlorides push the aquation equilibrium back to the left, making the remaining platinum harder to reduce. Ware’s evidence that this is real rather than tidy is the reverse experiment: deliberately adding chloride to a platinotype sensitiser, as an alkali metal chloride or as hydrochloric acid, inhibits image formation. He has also observed the effect locally in print-out platinotype, where values next to a heavy shadow print short because chloride diffuses out of it during the exposure. Self-inhibition is his explanation for why platinum exposures run longer than palladium ones from the same iron sensitiser.

Lead(II) takes the chloride away. That is the whole hypothesis. Lead(II) forms a series of chloro complexes — PbCl⁺, PbCl₂, PbCl₃⁻ and PbCl₄²⁻ — and lead(II) chloride is barely soluble, with a solubility product of 1.6 × 10⁻⁵ at 25 °C:

Pb²⁺ + 2 Cl⁻ → PbCl₂

Chloride scavenging: free chloride removed as sparingly soluble lead(II) chloride

H₂O + PtCl₄²⁻ + Pb²⁺ → PtCl₃(OH₂)⁻ + PbCl⁺

The consequence: more of the platinum sits in its reducible form

The first of those two equations is ordinary solubility chemistry. The second is this course’s, written by analogy with the mercury(II) equation Ware prints on the same page: the lead chloro-complexes in it are his and so is the analogy, and only the arrow is ours.

Pull chloride out of the paper and the aquation equilibrium moves right, more of the platinum sits in the easily reduced aqua form, the reduction runs faster and further before the wash can carry the sensitiser away, and the result is the smoother, stronger image the period sources describe. The same argument explains Willis’s other early additive: silver(I) precipitates silver chloride, and the reaction Pt(II) + 4 Ag⁺ → the tetraaquaplatinate(II) ion + 4 AgCl has been used in the laboratory to prepare aquated platinum species. On Ware’s hypothesis, and it is offered as a hypothesis, Willis was scavenging chloride with whatever cation came to hand, fifty years before anybody could have said so.

Lead is not reduced, and that is why it stays colourless. Lead is far too electropositive for the iron(II) photoproduct to reduce it: E°(Pb²⁺/Pb) is −0.126 V, and for the chloride couple E°(PbCl₂/Pb, 2Cl⁻) is −0.266 V, against about +0.02 V for the oxalato-iron couple that is doing the reducing. Mercury(II) sits at +0.854 V and is reduced without difficulty; lead sits below the reductant and cannot be. Ware’s conclusion is that it “should remain in the Pb(II) state in the sensitizer and not contribute to the image substance”, and Clarke’s XRF confirms it: uniform background, no correlation with density.

⚗Lead oxalate: an insoluble salt that is not quite insoluble

Several later formulas, and Pizzighelli and Hübl’s print-out variant, used lead oxalate rather than a soluble lead salt, and at first sight that makes no sense. Lead oxalate has a solubility product of 2.74 × 10⁻¹¹ at 18 °C against 1.6 × 10⁻⁵ for the chloride; the molar solubilities those constants imply — 5 × 10⁻⁶ M against 1.6 × 10⁻² M, which is this course’s arithmetic from Ware’s two figures — put the oxalate about three thousand times further out of solution. How does a salt that insoluble scavenge anything?

Ware’s answer is complexation by the free oxalate that is everywhere in this process:

PbC₂O₄ + C₂O₄²⁻ ⇌ Pb(C₂O₄)₂²⁻

The bisoxalatoplumbate(II) complex, which redissolves lead oxalate in an oxalate-rich medium

The formation constant is 3.45 × 10⁶, from which Ware derives the concentration of the soluble complex as 1.656 × 10⁻³ times the free oxalate concentration. In a developer that is 25 to 33 per cent potassium oxalate, that is a small but genuinely mobile reservoir of lead(II), replenished from a solid that is sitting in the paper fibres and cannot wash out. Lead oxalate is therefore not an inert filler: it is a slow-release lead(II) source that survives the coating, the drying and the first seconds of the developer. It also explains Clarke’s result, since what stays behind after processing is exactly this kind of insoluble colourless salt.

The course has found no published quantity for lead oxalate in any platinum formula. The Getty states the practice three times and gives no numbers. Ware’s own cross-reference for “some recipes” points to two sections of his book, and neither prints one; the nearest thing in either is Imogen Cunningham’s interest in adding lead oxalate to the sensitiser, cited to an undated typescript of about 1910 in her own archive, which is not a published document and which this course has not read. That is why nothing about lead oxalate appears in the table above, and it is a hole in the record rather than a gap in this page’s searching.

⁇What actually caused the yellow stain: lead, or iron?

The sulphide staining of platinotypes in the 1880s is the reason Willis dropped lead, and the cause was disputed at the time and has not been fully settled since.

The period attribution, which Ware follows, is the heavy metals. The yellow discolorations were “attributed to the presence of the lead(II) and/or silver(I) salts that Willis employed in his early sensitizers”, forming lead sulphide and silver sulphide in the paper — the sulphide arriving from an alkali sulphide bath in a test, or from the air in a polluted city. Ware adds the physical explanation the period lacked: lead sulphide and silver sulphide are black in bulk, but as the particle size falls the absorption edge moves towards the blue and the colour shifts to yellow. The strongest evidence for this account is experimental — Dawson tested Willis’s lead-free papers of 1880 and found them unaffected by ammonium sulphide.

Pb²⁺ + S²⁻ → PbS

The reaction the 1880 controversy was about: residual lead(II) in the paper meets sulphide

Henry Chapman Jones, a chemist at the Royal College of Science, attributed it mainly to iron. In 1887 he discussed the yellowing explicitly and put it down to residual iron in the paper; in 1895 he showed that even after four hours in dilute hydrochloric acid an unexposed platinotype paper still held iron enough to discolour under hydrogen sulphide, and that processed platinotypes hold more iron in the dark parts than in the light. Ware’s own comment is that Chapman Jones’s demonstration “has been largely forgotten until the present time”, and Ware devotes a separate section of his book to a slow yellow iron stain endemic to platinotype throughout its history and quite distinct from deliberate sulphiding.

Both can be true, and the distinction matters practically. A lead sulphide stain needs residual lead and a sulphide source; an iron stain needs only bad clearing, which every platinotype of the period had, because commercial sheets were coated to the edge and left no unexposed margin to judge clearing against — the problem the modern platinum clearing sequence exists to solve. The course records the disagreement and takes no side, except on the historical question, where Dawson’s controlled test on lead-free paper is good evidence that the 1880 stain specifically was the lead.

Function of every ingredient

Lead(II) nitrate, 40 grains per fluid ounce, 2.59 g in 28.4 mL of water, 9.12 per cent w/v, 0.275 molar.

What it is. A freely soluble lead salt — about 56 g per 100 cm³ in water at 20 °C — and also a nitrate, which makes it an oxidiser in its own right. In this formula neither property matters as much as the third: it is a source of the lead(II) cation, and the cation is the active thing. Willis himself treated the anion as interchangeable, naming the nitrate and the chloride as “the salts of lead which I find best for my purpose”, and using the acetate in a developer.

Why it is there. Because platinum(II) is reduced too slowly by iron(II) for the image to finish forming before the developer washes the sensitiser out. Lead(II) binds chloride, chloride is what holds the platinum in its least reducible form, and every atom of platinum reduced makes more of it. The lead is a scavenger placed in the paper in advance of a reaction that will otherwise choke on its own product.

What it does chemically. Nothing photochemical, and nothing to the image. It removes chloride as PbCl⁺ and, past the solubility product, as solid PbCl₂; it shifts the platinum aquation equilibrium towards the reducible aqua-complex; and it then stays where it is, because at −0.126 V it is far below the reach of the reductant that is precipitating the platinum. The finished print contains lead that is chemically inert to everything in the process and to almost everything after it, with one large exception: sulphide.

What photographic consequence follows. A smoother, more uniform, stronger image from the same negative and the same platinum — or, put the other way round, the same image from very much less platinum. Willis’s 1880 patent measures that trade for us: dropping the lead cost him a rise from two-thirds of a grain of platinous salt per square foot to between 1.7 and 4 grains.

What happens with more or less. Neither Willis nor any later source read for this page publishes a series, so what follows is reasoning from the mechanism and is labelled as such. More lead scavenges more chloride and, past a point, precipitates lead(II) chloride inside the sensitiser layer — which Ware calls “rather undesirable”, since a crystalline solid in a coating is a coating defect — and leaves more lead in the finished sheet for a sulphide to find. Less lead returns the print towards the weak, fibrous image of the plain platinum sensitiser. There is no published optimum, and the fact that Willis’s own published doses fall by a factor of seventeen between 1873 and 1878, and by another factor of two by 1887, suggests he never found one either.

What it interacts with. Chloride, platinum(II), oxalate, phosphate and sulphide, in that order of importance. Those are the subject of the next section.

What is not in the table, and why. The lead coating is one of three, and the other two are described under Mixing rather than tabulated: the platinum salt because the patent’s name for it is ambiguous between two oxidation states and neither candidate has an encyclopaedia page, and the ferric oxalate because the oxalic acid that goes with it is given as “as little as is sufficient” and has no number at all. Both are genuine gaps in the record, not editorial decisions, and inventing a figure for either is precisely what this course’s first rule forbids.

Interactions

Lead(II) and chloride. The interaction the formula exists for. It runs both ways: the lead removes chloride, and the chloride removes the lead, as solid PbCl₂. Which of the two matters depends on concentration, and the concentration changes during development as the platinum reduction pumps chloride into the paper.

Lead(II) and platinum(II). Indirect and entirely through chloride. Unlike mercury(II), which is known to form platinum–mercury bonded complexes, no lead–platinum species is proposed by any source read for this page. Lead does not alloy with the image, does not colour it, and does not appear in it.

Lead(II) and oxalate. The interaction that makes lead oxalate usable, described above: the insoluble oxalate redissolves to a limited extent as the bisoxalatoplumbate(II) complex in the strongly oxalate developer, which is how a salt with a solubility product of 10⁻¹¹ can act as a reservoir at all. The same complexation is why the lead that remains in a processed print is most likely the oxalate.

Lead(II) and phosphate — Willis’s own warning. His February 1887 developer was built on sodium, potassium or ammonium orthophosphates, chosen because the ferrous oxalate image is insoluble in them. His November 1887 patent abandons them, and says why in two places: with a lead salt “there must be no salt used in the developer which would entirely prevent the solution of the lead salt”, and the phosphates were anyway “the cause of, or accompanied by, the defect in the tone or colour of the picture”. Willis does not name the compound that would form, and neither does this course; what the patent states is the practical consequence, which is that a lead salt tipped into a phosphate developer does not stay in solution to scavenge anything.

Lead(II) and sulphide. The interaction that ended the practice. Any sulphide — ammonium sulphide in a test, hydrogen sulphide in polluted city air — converts residual lead(II) in the paper to lead sulphide, and at the particle sizes involved that reads as a yellow stain in the highlights rather than a black one. This is the only reaction in the whole page that changes what the print looks like, and it changes it for the worse.

Lead(II) and mercury(II) together. Willis’s November 1887 coating carries one grain of each per fluid ounce, which is the only formulation read for this page that uses both. Ware’s account of the two is that they act by the same chloride-scavenging mechanism but that mercury is additionally reduced to the metal and shifts the colour, so a coating carrying both is a lead formula with a mercury colour adjustment on top. Willis’s own note is on the mercury side: “I find the mercuric salt very useful where a warm tone or effect somewhat resembling sepia is desired”.

Lead(II) and silver(I). Alternatives, not partners. The 1873 patent’s second method substitutes 8 grains of silver nitrate per fluid ounce for the 40 grains of lead nitrate, and the two metals were dropped together in 1880 for the same reason. Silver has the additional property of actually joining the image, which lead does not — a re-examination of Willis’s first American platinotype found both silver and gold in it.

Lead(II) and the paper. Ware’s criteria for a platinotype rawstock are cotton or linen, no lignin, a non-gelatin size and — critically — no chalk buffer, because calcium carbonate is destructive to the iron chemistry. Gelatin sizing binds platinum(II) strongly and de-activates it, which is one of the things the lead was compensating for. A modern fine-art paper with an alkaline buffer is the wrong substrate for every formula on this page.

Variants

Every lead dose Willis or his contemporaries published, with its source. None of them is published here as a separate variant entry, because the course reconstructs none of them as a working formula; they are recorded so that a reader meeting a lead line in a spectrum can see the range of doses that could have produced it, and so that the fall from 1873 to 1887 can be read as one sequence.

The 1878 sensitiser: 2 grains of plumbic chloride per fluid ounce. British Patent No 2800, 12 July 1878. The single coating solution carries “about 15 grains of potassic chloro-platinite, 70 grains of ferric oxalate (with enough oxalic acid to render this ferric oxalate freely soluble), and preferably two grains of plumbic chloride”. Ware converts the lead to 0.456 per cent w/v and 0.0164 molar in PbCl₂ at a formula weight of 278.1 — about seventeen times weaker than the 1873 coating on the two molarities, which is the course’s arithmetic on Ware’s figures, and now inside the sensitiser rather than under it. The same sentence four paragraphs later offers four grains of mercuric chloride in its place, which is the origin of the mercuric sepia platinotype. Lead(II) chloride has no page in this course’s chemical encyclopaedia, which is why this formula could not be the entry’s own.

The first silver-free platinum print: 10 grains of lead acetate. The verso of the print of 17 March 1878 in the Metropolitan Museum of Art carries Willis’s own pencilled working — “1° Starch & 12 gr O.P / 2° 10 gr Acetate of Lead / 3° Sensitd with 1 part Pt, 1 part neutral Fer Ox, 1 part gold” — under his ink note that this was the “first print made by the above process in platinum alone without aid by silver salts”, witnessed the same day. It is a laboratory note rather than a formula: it gives no volume, so no concentration can be computed from it, and it uses a third lead salt, the acetate. It is worth having because it is the only lead dose on this page that can be tied to a surviving object.

February 1887: lead named, mercury numbered. British Patent No 1681 specifies a coating of about 60 grains of ferric oxalate per fluid ounce and says “I sometimes mix or dissolve in the ferric oxalate a salt of mercury or of lead or of a mixture of these salts” — and then gives a quantity only for the mercury: “from one grain to three grains of mercuric chloride”. Ware’s summary of this patent states the range for lead and mercury together, as “amounts of lead and/or mercury similar to the earlier Patent no. 2800, namely, 1 to 3 grains per fluid ounce”. The transcript of the specification, printed in the same book, numbers only the mercuric chloride. The course records what the specification says and notes the summary, because a reader who meets the summary alone will believe a lead figure was published in February 1887, and it was not.

November 1887, in the coating: 1 grain of plumbic chloride and 1 grain of mercuric chloride per fluid ounce. British Patent No 16,003, in a ferric oxalate solution of about 60 grains per fluid ounce. At 0.228 per cent w/v the lead is about 0.008 molar, half the 1878 dose. The patent adds that the lead may be left out if the mercuric chloride is raised to two grains or more.

November 1887, in the developer: lead acetate to a visible end point. The same patent’s second method sensitises with ferric oxalate alone and puts everything else in the bath: 90 grains of potassic oxalate and 9 grains of potassic chloro-platinite per fluid ounce, to which “I add a solution of a salt of lead, preferably the acetate, until a permanent precipitate begins to form”. Five grains of mercuric chloride per fluid ounce is offered as the alternative, with the honest caveat that “the salts of mercury do not in all cases ensure a monochromatic tone in the picture”. A third practice in the same specification applies plumbic acetate to the exposed paper first and the rest of the developer immediately after, without washing or drying between.

Lead oxalate: recorded, never quantified. The Getty Conservation Institute states that early platinotype formulas used lead oxalate “to facilitate good uniform development”, that Pizzighelli and Hübl recommended it for their print-out variant, and that several later formulas used it “to facilitate a more uniform reduction of platinum salts during development”. Ware records Imogen Cunningham — a chemistry graduate who had learned platinum printing in Edward Curtis’s Seattle studio and went to Dresden in 1909 to study photographic chemistry — investigating the addition of lead oxalate to the sensitiser about 1910. No quantity for any of these is published in any source read for this page, for the reasons set out under The mechanism.

!A published disagreement about the 1873 quantity

Pizzighelli and Hübl’s 1886 review of Willis’s patents gives the first method as ratios: 1 part chloro-platinate of potassium in 48 parts of water, 1 part nitrate of lead in 48 parts of water, 1 part ferric oxalate in 8 parts of water.

Three of those four figures — including the silver nitrate of the second method — back-convert exactly against the patent at 480 grains to the ounce: 10 grains gives 1 in 48, 8 grains gives 1 in 60, 60 grains gives 1 in 8. The lead line does not. At the same factor, “1 in 48” is 10 grains per ounce, a quarter of the 40 grains the specification prints.

That arithmetic is the course’s, and it does not resolve the disagreement; it only shows that the lead figure is the one line in the table that does not follow the rule the other three obey, which is what a copying or typesetting error looks like. The 1886 book is a translation, from a German original of 1882, of an English patent. The specification is the primary document and its figure is the one in the table above. A reader computing a historical lead concentration should know that a widely-read period source gives a figure four times lower.

Safety

Level D: historical study only, and the classification is about lead rather than about this formula. The classification rubric puts a reagent here when there is no combination of domestic controls that makes it acceptable and a safer route is taught instead. Both conditions hold, and the second holds unusually strongly: modern platinum and palladium printing does not use lead at all, so nothing is lost.

Why protective equipment is not the answer. Lead is regulated as lead, not as a salt. NIOSH defines “lead” to cover metallic lead, lead oxides and lead salts, sets a recommended exposure limit of 0.050 mg/m³ over eight hours, and — the part that matters — adds that air concentrations should be held so that a worker’s blood lead stays below 0.060 mg per 100 g of whole blood. The controlled quantity is the one that has accumulated in the person, not the one in the air on a given afternoon. Great Britain does not give lead a workplace exposure limit in EH40 at all: the document notes that asbestos and lead are regulated separately and points to the occupational exposure limit specified in regulation 2(1) of the Control of Lead at Work Regulations. A regime built on biological monitoring and a separate set of regulations is not one a home darkroom can satisfy with gloves.

The label, and how much it varies. The aggregated ECHA classification for lead(II) nitrate carries the signal word Danger with H272 (may intensify fire; oxidiser), H302 and H332 (harmful if swallowed, harmful if inhaled), H317, H318, H351 (suspected of causing cancer), H360Df (may damage the unborn child; suspected of damaging fertility), H372 (damage to organs through prolonged or repeated exposure) and H410 (very toxic to aquatic life with long-lasting effects). The agreement between notifiers is poor — H360Df at 62 per cent, H351 at 48 per cent, and 11 per cent of reports stating that the substance meets no GHS criteria at all — which is a reason to read the occupational and waste regimes rather than the label. The Environment Agency’s waste guidance notes separately that inorganic lead compounds are classified as carcinogenic by IARC. Full hazard data, with its sourcing, is on the lead(II) nitrate page.

Handling a historical print is a different question, and a much smaller one. The lead in a platinotype is a trace of an insoluble salt locked in cellulose fibres, at levels Rees described as “very low”. Nothing in this page argues against handling nineteenth-century photographs; the ordinary conservation practices — clean hands or gloves for the object’s sake, no eating at the bench — are what apply. The hazard on this page belongs to the making, and nobody is making these any more.

Storage

Not stocked. The course keeps no lead salt, so there is no storage practice to recommend.

What the historical record says about the solutions. Nothing directly. No patent read for this page gives a keeping time for the lead coating solution, and the only related statement found is Wall’s, for a different formula, that a made-up lead bath keeps in the dark. Lead(II) nitrate solutions are not light-sensitive, unlike the mercury(II)–oxalate developer of the sepia papers, so the historic insistence on darkness for platinum stock solutions is about the iron rather than the lead.

What the coated paper was. Willis says only that the paper is dried between coatings and that “the paper thus coated, after being thoroughly dried, is ready for use”. The keeping problem of a platinotype paper is the ferric oxalate and the humidity, not the lead.

Where the lead ended up. In the paper, and it is still there. That is the storage fact that matters now: a collection holding early platinotypes holds lead, at trace levels, bound in an insoluble form, and the relevant precaution is against sulphide-bearing atmospheres and sulphide-bearing enclosure materials rather than against the metal itself.

Incompatibilities

• Phosphates, which precipitate lead and stop it working, stated by Willis himself in the November 1887 specification.
• Chloride in excess, which both precipitates the lead as PbCl₂ and directly inhibits the platinum image, the second effect being Ware’s experimental evidence for the whole chloride-scavenging hypothesis. Excess sodium chloride or hydrochloric acid in a sensitiser works against the process.
• Sulphides, in a bath or in the air, which convert residual lead to lead sulphide and yellow the highlights. This is the incompatibility that ended the practice.
• Chalk-buffered and gelatin-sized papers. Not a lead incompatibility but a platinotype one, and worth stating on the same page: Ware’s criteria exclude an alkaline carbonate buffer, which decomposes the iron sensitiser, and gelatin sizing, which coordinates platinum(II) and de-activates it. See calcium carbonate and gelatin.
• Combustible and reducing material, because lead(II) nitrate is an oxidising nitrate as well as a lead salt. Not a photographic incompatibility, but a storage one, and the reason its GHS entry carries H272 at all.

Waste

None is generated by this course, because the substance is not used. What follows is the chemistry of the problem, not an instruction.

There is no neutralisation to describe. An acid can be neutralised and a dichromate can be reduced, but lead(II) is an element in a stable oxidation state: every treatment moves it out of solution and into a solid, and the solid is lead waste in a filter instead of lead waste in a bottle. In this formula the chemistry does that by itself — much of the lead ends up as insoluble oxalate or chloride in the paper — which is a disposal problem that has been converted into a permanence problem rather than solved.

A spent lead-bearing photographic solution falls under the Environment Agency’s WM3 entry 09 01 05*, bleach solutions and bleach fixer solutions, an absolute hazardous entry needing no assessment. WM3 also notes that inorganic lead compounds are classified as carcinogenic by IARC and that the carcinogenic classification has to be determined when such a waste is assessed. GOV.UK’s household guidance sends hazardous waste from a home to a household hazardous waste collection point, never to the bin or the drain. Check your local regulations; they govern.

Troubleshooting

These are the faults the historical record attaches to lead in platinum printing, with the diagnosis the sources give. They are here to be recognised in an existing object, not to be worked around in a darkroom.

Yellow or dulled highlights after exposure to a sulphide. The 1880 fault. Diagnosis: residual lead(II) — or silver(I) — in the paper, converted to a finely divided sulphide that reads yellow rather than black. Evidence for it: Dawson’s test of the lead-free 1880 papers, which were unaffected. Competing diagnosis: residual iron, per Chapman Jones, which needs no heavy metal at all and is consistent with the endemic slow yellowing of platinotypes generally. Distinguishing them today is an XRF question, and the lead answer is only available if a board signal has been subtracted first.

A lead line in the spectrum that is not the photograph’s. The Getty’s explicit caution. Some mounting boards and paper substrates contain lead; a print permanently attached to a board must be measured on and off the image and the board’s contribution subtracted.

A lead line that is uniform across the sheet. Not a fault, and the expected result. Clarke’s measurements show the lead signal does not track image density. A lead signal that did track density would be the anomaly and would need explaining.

A weak, fibrous, grainy platinum image. The fault the lead was added to cure, and the one a modern printer still meets. The modern remedies are a properly matched negative, a paper without a chalk buffer or gelatin size, adequate humidity in the paper, and palladium in the mix — not an additive.

Crystals or patchiness in a coated sheet. Reasoning from the chemistry rather than from a source: past its solubility product lead(II) chloride will come out as a solid in the coating, which Ware calls “rather undesirable”. The visually similar patchiness that is documented belongs to potassium chlorate, per the Getty’s atlas, and is a reason to prefer other contrast controls.

Experiments

None of these uses a lead salt. They test the chemistry this page explains with reagents the course does handle.

1. The chloride inhibition test — the falsification experiment for the whole mechanism. Ware’s evidence that chloride scavenging is real is the reverse effect: added chloride inhibits the platinum image. That is directly testable with the course’s ordinary platinum or palladium sensitiser by adding a measured amount of sodium chloride or dilute hydrochloric acid to one half of a split batch, coating two strips from the same negative and the same exposure, and comparing maximum density. The hypothesis: the chloride strip prints weaker. The control: the untreated half of the same batch. The one variable: chloride concentration. If chloride inhibits, then removing chloride should promote — which is the lead argument, tested without lead.

2. Platinum against palladium, as a test of self-inhibition. Ware’s explanation of why platinotype exposures run longer than palladiotype ones is that platinum’s aquation equilibrium is fifteen times less favourable (0.015 against 0.17) and that its own reduction releases the chloride that shifts it further. Print the same negative at the same time on a platinum-only and a palladium-only sensitiser from the drop system, and record the exposure each needs for the same maximum density. Record humidity: it is the largest uncontrolled variable in this process.

3. Contrast, properly attributed. Run a chlorate drop series on a step tablet and measure where the scale actually moves. This is the experiment that separates the two effects the title of this page confuses: a chlorate series shortens the exposure scale by destroying iron(II), while an additive that scavenges chloride raises maximum density by letting more platinum reduce. Plotting both against a step tablet shows two different changes to the curve.

4. An archival question, from the sulphide controversy. Spiller’s 1880 experiment is the model: subject test strips to a barrage and see what, if anything, marks them. A modern version on lead-free modern platinum prints — humid ageing, and a controlled sulphide atmosphere — asks whether Ware’s claim that a pure platinotype cannot fade survives contact with a real enclosure. Record the paper, the size, the clearing regime and the residual iron, because on the evidence of Chapman Jones the iron is the variable that matters.

5. A reading exercise rather than a bench one. Take the two 1887 specifications and Ware’s summary of them side by side, and find the place where the summary attaches a quantity to lead that the specification attaches only to mercury. It takes about ten minutes, it is recorded under Variants above, and it is the most useful thing on this page: the habit of checking a summary against the document it summarises is what keeps a formulary honest.