Photographers' Formulary platinum and palladium printing kits

Two boxes from Condon, Montana, and a documentary situation that runs the wrong way round twice over.

The ordinary bought product in this formulary hides its composition behind a marketing description and discloses fragments of it on a safety data sheet, because the law obliges a maker who sells a classified mixture to classify it. Photographers’ Formulary does the reverse. Its instruction sheet for the platinum kit prints a table headed CHEMICALS CONTAINED IN THIS KIT with a weight or a volume against every one of seven lines, gives the strength of all three solutions as a percentage, names both noble-metal salts by formula, spends the better part of a page on the chemical identity of its own ferric oxalate, and explains in one clear sentence what the potassium chlorate is doing. Almost nothing about the composition of this kit is secret.

And there is no safety data sheet for it. Not a thin one, not a generic one — none. The company maintains two safety data sheet libraries on its own site: 105 sheets for its kits, and 124 for the chemicals it sells loose. Neither box in this entry appears in the first. And the second, which does cover the arrowroot starch, the citric acid and the potassium oxalate, covers none of the four chemicals that make a platinum or palladium kit what it is: there is no sheet for ferric oxalate, none for potassium chlorate, none for potassium chloroplatinite and none for sodium tetrachloropalladate. The one document in that library filed under the platinum and palladium sensitiser is a sheet for ammonium iron(III) oxalate bound to one for silver nitrate, and the company’s own product page for the same bottle says it contains neither.

So the concentrations in the table below were not read off a hazard document, because no hazard document exists for the things in the bottles. They were read off the maker’s own instruction sheets, which are the only place the maker states what is in its own box — and which are, on that narrow question, better documents than most safety data sheets.

☠Level D: this page describes a product and gives no procedure, and two separate course rulings put it here

Platinum. The course ruled on 4 September 2026 that students perform palladium and never handle platinum. Potassium tetrachloroplatinate(II) carries H334 — may cause allergy or asthma symptoms or breathing difficulties if inhaled — and HSE’s EH40 sets halogeno-platinum compounds at 0.002 mg/m³ as platinum with the Sen notation, the tightest workplace exposure limit anywhere in this course. Ware’s own health warning is the practical form of it: never touch the surface of sensitised paper, keep ungloved fingers out of the baths, and treat dust as the worst hazard.

Potassium chlorate. Its encyclopaedia entry is at Level D on the rubric’s second criterion rather than on acute hazard — a safer modern route to the same result exists and this course teaches it. That the supplier weighs the sachet, in a place set up to weigh oxidisers, removes the deciding operation from the darkroom; it does not remove the reason.

So this page gives no procedure. It has no coating routine, no drying regime, no exposure time offered as an instruction, no processing sequence and no protective-equipment list, and the dilutions and development fields of its product record are deliberately empty. What it does instead is describe a document and a box: what the maker publishes, where the publishing stops, and what a reader holding one of these bottles is actually holding. The drop tables themselves are published, once, on the open entry that owns them — the three-solution drop system — where they are printed so that a formula met in a manual or on a shelf can be identified, and for no other purpose.

The relatives this course does teach are the palladiotype at Level B, with the contrast put into the negative rather than into the bottle, and the kallitype sensitiser, which is this same iron chemistry with silver in place of the noble metal and no oxidant anywhere in it.

Photographers' Formulary, Inc. — sold as kit — discontinued Not stated by the maker. The instruction sheets for both kits are still served from the supplier's own store, and neither kit appears in the Alt Process category of that store as read on 6 September 2026, where the only surviving component of either is Sensitizer B. This entry records that state of affairs and draws no conclusion from it: a product absent from one category listing is not a product the maker has announced the end of.

Components the maker discloses — from the safety data sheet, which classifies hazards and does not state a formula

Component	Concentration as the sheet gives it	Hazard codes
Arrowrootnamed on the sheet as Arrowroot starch20 g, for sizing the paper. The sheet has it boiled five minutes in a litre of water and brushed on; it is the only item in the box that never touches the image chemistry	—
Ferric oxalatenamed on the sheet as Ferric Oxalate: Sensitizer A and Sensitizer BTwo 30 mL bottles, and the only light-sensitive substance in the kit. The sheet states it is a 20 per cent solution of trihydrogen ferric oxalate made by the iron alum and oxalic acid procedure, photosensitive around 460 nm, to be handled under a red safelight and not heated above 50 °C	20%
Oxalic acidnamed on the sheet as a slight excess of oxalic acid, in the ferric oxalate solutionNamed by the maker as a constituent of both iron bottles and given no quantity. It holds the pH down, keeps the iron salt soluble, and converts any of the inactive potassium double salt present to the active acidic form	—
Potassium chlorate0.18 g for platinum and 0.26 g for palladium, supplied as a pre-weighed sachet to be tipped whole into one bottle of ferric oxalate. The sheet states its purpose in its own words: it reconverts ferrous ions to the ferric state, and so "acts as a restrainer, increases contrast, and maintains the whites"	—
Potassium tetrachloroplatinate(II)named on the sheet as Platinum Salt Solution, 20%5, 15 or 30 mL according to catalogue number, and the image metal of the platinum kit. The sheet names it as potassium chloroplatinite, K₂PtCl₄, states that platinum salts are not light sensitive, and warns that the solution is near saturation	20%
Sodium tetrachloropalladate(II)named on the sheet as Palladium Salt solution, 15%15 or 30 mL, and the image metal of the palladium kit. Not light-sensitive either; it waits in the paper for the iron(II) the developer sets free. The sheet gives the strength and never the hydrate	15%
Citric acid (anhydrous)named on the sheet as Citric Acid30 g, for the clearing baths — 30 g made to 1500 mL for platinum and to 2000 mL for palladium, because palladium metal is etched off the print by an acid strong enough to be routine for platinum	—
Potassium oxalate monohydratenamed on the sheet as Potassium Oxalate227 g, and the developer — 500 mL of water for platinum and 700 mL for palladium, in both cases a saturated solution with undissolved solid left in the bottom. It contains no developing agent: it dissolves the photoproduct rather than reducing anything	—

Not disclosed. Photographers' Formulary publishes the composition of these kits and withholds the specification of what the composition is made of, which is the opposite way round from most bought products and is not a smaller gap. The sheets say "Ferric Oxalate" and give a strength of 20 per cent, and never say which of the several hydrates sold under that name is in the bottle, what its assay is, or how the percentage was calculated — a question with teeth here rather than in the abstract, because the formula weights in circulation for ferric oxalate run from 375.76 to 483.84, so two bottles both honestly labelled 20 per cent can differ by a quarter in iron. They say the solution "contains a slight excess of oxalic acid" and never say how much, although that excess sets the pH, converts any inactive potassium double salt to the active form, and measurably changes how much of the image prints out during the exposure. They give the platinum salt as 20 per cent potassium chloroplatinite and state that it carries "varying amounts" of the much less soluble hexachloroplatinate without saying what those amounts are or how a buyer would know. They give the palladium as 15 per cent sodium tetrachloropalladate and never state the hydrate, and the anhydrous and trihydrated salts differ by about a sixth in palladium. They publish no pH for any solution, no keeping time for the kit or for any bottle in it except the warning that the chlorate decomposes over weeks, no coating volume in millilitres against which the drop tables could be checked, no drop volume other than the bare assumption of twenty to the millilitre, and no sensitometry of any kind — no exposure scale, no maximum density, no speed, no curve, and no exposure time for a named light source beyond "10-20 minutes". The sheets even disagree with themselves about the yield, giving 12 sheets of 8 × 10 from the 15 mL kit and then "about ten" two sentences later. And the boundary that matters most is documentary rather than chemical. The company maintains two safety data sheet libraries: 105 sheets for its kits, none of which is for 07-0001, 07-0003, 07-0005, 07-0007 or 07-0009, and 124 sheets for the chemicals it sells loose, which cover the arrowroot starch, the citric acid and the potassium oxalate in these boxes and cover none of the four chemicals that make them platinum and palladium kits: there is no sheet for ferric oxalate, none for potassium chlorate, none for potassium chloroplatinite and none for sodium tetrachloropalladate. The single document in that library filed under the platinum and palladium sensitiser, the sheet for Sensitizer B 07-0022 and 07-0023, turns out to be a safety data sheet for ammonium iron(III) oxalate trihydrate bound to one for silver nitrate, and the company's own product page says the bottle contains neither. A reader who wants to know what has been classified about the liquids they will actually handle will find that the answer is nothing at all.

Nearest open formula. Three-solution drop system for platinum and palladium — They are the same thing seen from two sides, and the comparison is worth making precisely because the overlap is so nearly total: the open entry's provenance is this kit's own instruction sheet. What the kit adds is the objects — two bottles of iron already made up to a stated strength, a chlorate sachet weighed by somebody with a balance and a place to use it, a bottle of noble metal that would otherwise have to be bought as a solid and dissolved, and a sheet with a porosity test, a sizing recipe, a drying regime, an etch and a wash on it. What the open entry adds is everything the box cannot supply: the identity of the material behind the words "ferric oxalate", the published disagreement between three suppliers who call the same bottle 20, 25 and 27 per cent, an account of what the chlorate does to a tonal scale that is not the maker's phrase "increases contrast", the redox chemistry that explains why palladium needs half again as much of it, and the two rulings that put both entries at Level D. Neither is a route this course opens: the open entry publishes the composition and no procedure, and this page publishes what the maker publishes and no procedure. The formula this course does teach in this family is the palladiotype with the contrast put in the negative instead of in the bottle.

Purpose

To put a platinum or palladium print within reach of somebody who owns no balance, has no way of buying four difficult chemicals in small quantities, and has never seen the process done.

That is a narrow offer and a real one. Of the seven things in the box, three are ordinary — starch, citric acid, potassium oxalate — and four are not. Ferric oxalate is not sold in a hardware shop, does not keep, and, as the sheet itself goes to some trouble to explain, is sold in two chemically distinct forms only one of which works. Potassium chlorate is a strong oxidiser that a domestic balance reads badly at 0.18 g. The noble-metal salts cost more per millilitre than anything else a darkroom contains and arrive as solids that have to be dissolved to a strength near their own solubility limit. A kit removes all four problems at once, and that is the whole of its commercial reason for existing.

The two kits are the same kit with the metal changed, and the maker says so in the first paragraph of the palladium sheet: “The mechanics of printing with palladium and platinum are very similar; therefore, these directions are almost identical to Formulary’s Platinum Printing directions.” The platinum sheet returns the compliment by advising a beginner to learn on palladium first, “the less expensive palladium salt kit”, before spending money on platinum — which is, as it happens, the same order of learning this course arrives at from an entirely different direction.

	Platinum Printing Kit	Palladium Printing Kit
Catalogue numbers	07-0001, 07-0003, 07-0005	07-0007, 07-0009
Metal solution	5, 15 or 30 mL at 20 %	15 or 30 mL at 15 %
Metal salt named	potassium chloroplatinite, K₂PtCl₄	sodium tetrachloropalladate, Na₂PdCl₄
Ferric oxalate	two bottles, 30 mL each, 20 %	two bottles, 30 mL each, 20 %
Potassium chlorate	0.18 g	0.26 g
Citric acid	30 g, made to 1500 mL	30 g, made to 2000 mL
Potassium oxalate	227 g, in 500 mL	227 g, in 700 mL
Arrowroot starch	20 g	20 g
Negative asked for	density range 1 to 1.5	density range 1.3 to 1.5
Stated print colour	neutral-grey	warm-black to sepia
Safety data sheet for the kit	none published	none published

Three of those rows are the whole chemical difference between the two products, and two of them come with the maker’s own reason attached. The chlorate is nearly half again as much “because palladium is less sensitive to contrast control with potassium chlorate than is platinum”. The acid is a third weaker because “palladium metal can be etched from the print if the clearing solution contains too much Citric Acid”. The developer’s extra 200 mL of water comes with no explanation at all.

The third product in this entry is the one still on the shelf. Sensitizer B Platinum/Palladium, catalogue 07-0022 and 07-0023, is the kit’s own second bottle sold on its own: “30 ml of Ferric Oxalate 20% solution plus a packet containing 0.26 g potassium chlorate for palladium printing or 0.18 g potassium chlorate for platinum printing”. It is cited here because it prints the same three numbers a second time, independently, on a page written for a different purpose — and because it is the reason there is a safety data sheet in this entry at all, filed under its catalogue numbers, describing something else.

Recommended uses

What the sources record, and what this course does with them. The course recommends none of the printing uses below: it recommends the reading. The section is here because a reader who meets one of these bottles — on a shelf, in a bequest, in a workshop store, in a second-hand kit bought at auction — has to know what the numbers on it were for.

Identifying a bottle. This is the use the course has for the page. A brown glass 30 mL bottle labelled “Sensitizer A” or “Sensitizer B” and nothing else is, on this supplier’s published composition, a 20 per cent solution of trihydrogen ferric oxalate with a slight excess of oxalic acid, and, if it is the B bottle, carrying the chlorate sachet as well — which works out at about 0.6 per cent for platinum and 0.87 for palladium on this course’s arithmetic from the maker’s weight and volume, and is not a figure the maker prints. A 15 mL bottle marked 20 per cent is platinum; one marked 15 per cent is palladium. Nothing else in an alternative-process cupboard looks quite like the pair, and no other supplier uses the same two strengths.

Reading a kit sheet critically, which is a skill this formulary exists to teach. These two documents are unusually good practice material: they publish a full composition, they explain their own chemistry in places, they carry at least four internal inconsistencies, and they end with a sentence transferring all risk to the buyer. A reader who can find the four inconsistencies without being told where they are has learned most of what Rule 7 asks for.

Understanding what a “contrast grade” meant before there were filters. The five drop tables are one of the oldest surviving pieces of working photographic craft still in commercial print, and the design is genuinely elegant: the metal holds constant at 0.521 or 0.522 of the mixture through all five grades, so the only variable that ever reaches the paper is the chlorate concentration. That is explained in full, with the arithmetic, on the open entry.

Not for a negative made on purpose for the process. This is what the sources themselves converge on and it is the position the course takes. Ware’s conclusion is that with modern negative-making, especially by digital means, contrast agents such as chlorate “become unnecessary when a correctly calibrated negative is made”. A correction curve on a digital negative buys the same contrast for nothing, without the graininess and without a strong oxidiser on a domestic bench.

Not as a first alternative process. The kit’s own sheets say so more gently than this course would: learn on palladium before platinum, run a test strip weekly because the chlorate bottle drifts, and expect to lose prints to paper you have not tested. A reader who cannot yet coat an even sheet should be practising on the classic cyanotype sensitiser or the kallitype, where a spoiled sheet costs pennies rather than the price of a meal.

Not as a way of learning what a platinum print is. Read the sheets and you will know the drop counts and not the chemistry. The sheets never say that the developer contains no developing agent, never explain why development is instantaneous, never mention that the image is metal precipitated among the paper fibres, and never once use the word siderotype.

When another formula is preferable

• The palladiotype, with the contrast in the negative. The course’s route to a noble-metal print, and the one that removes both Level D grounds at a stroke: no platinum and no oxidant. Willis’s potassium oxalate developer and the sodium citrate developer are its two published developers, and the clearing sequence is the step that decides whether the print survives.
• The three-solution drop system, if the question is what is actually in this box. It is the open entry this page compares against, its provenance is this kit’s own instruction sheet, and it carries the composition, the five drop tables, the mechanism and the arithmetic in full. Everything the kit’s chemistry is, that entry says; everything the kit’s documentation is, this one says.
• Ware’s print-out platino-palladiotype, if the objection is to the traditional method itself. It replaces the ill-characterised ferric oxalate with ammonium iron(III) oxalate, takes its contrast from controlled humidity and from the metal ratio rather than from an oxidant, and prints out, so a test strip becomes unnecessary. Every complaint this page makes about the specification of the iron falls out with the change of salt.
• The kallitype sensitiser, to learn the craft on. Identical iron photochemistry, identical coating, drying, exposing and clearing skills, silver instead of the noble metal, and a fraction of the cost per sheet.
• The Pizzighelli and Hübl formulations of 1886, if the question is where the drop system came from. The four-bottle ancestor, in which the chlorate is Solution C.
• Bostick & Sullivan’s platinum and palladium kits, if the comparison wanted is between the two surviving suppliers. They sell the same three-bottle system at a 27 per cent ferric oxalate against this supplier’s 20, organise their drop charts by print size rather than by contrast grade, publish no weight for their chlorate and no strength for either metal solution, and state in words what this supplier states in grams — that palladium requires twice as much chlorate. Their numbers are corroboration of the system and are not transferable to these bottles, because their iron is more than a third stronger. That entry is planned and not yet written.
• The Photographers’ Formulary cyanotype kit, if what you want is this supplier at its most transparent on a process the course teaches, or the Bostick & Sullivan cyanotype kits for the opposite documentary case, where the composition appears in a parenthesis on a shop page and the safety data sheet library contains ninety sheets and no mixture.

What the maker publishes

A great deal, in two documents that were plainly written from one another, and the useful way to read them is by what kind of statement each thing is.

The composition, in a table, with units. Arrowroot starch 20 g; Ferric Oxalate: Sensitizer A 30 ml, Sensitizer B 30 ml; Citric Acid 30 g; Potassium Chlorate 0.18 g; Platinum Salt Solution, 20% 5 ml (15 ml or 30 ml); Potassium Oxalate 227 g. The palladium sheet’s table is the same with 0.26 g of chlorate and a 15 per cent palladium solution. That is a complete inventory of a photographic product, published by its maker, with the quantity of every item — which almost no bought product in this formulary does.

The chemical identity of the iron, at length and with an argument. The sheet devotes most of a page to the point that “the photographic term ‘ferric oxalate’ is a misnomer, which has given rise to a considerable amount of confusion in the photographic literature”: there are two common forms, tripotassium ferric oxalate and trihydrogen ferric oxalate, both photosensitive, and “only the acidic form is sufficiently photosensitive to be useful in photography”. It states that the green solid potassium salt is thermally stable to 110 °C, is destroyed by ultraviolet, and is photoactivated only by being put into acid, and then says plainly: “Photographers Formulary Does Not recommend the use of the green, solid tripotassium ferric oxalate for platinum, palladium, or kallitype printing. Its photoactivity is low and it is difficult to convert to the more active form.” Its own product is “a 20% solution of trihydrogen ferric oxalate… prepared by Photographers Formulary by the iron alum-oxalic acid procedure”, containing a slight excess of oxalic acid, photosensitive around 460 nm, to be used under a red safelight, not to be heated above 50 °C, very water-soluble, yellow to yellow-green in room light. The same paragraphs appear in the supplier’s kallitype kit sheet, which makes them the company’s standing account of the material rather than a remark about one product.

A quality-control test the buyer can actually run. Two crystals of potassium ferricyanide in about 2 mL of water; one drop of ferric oxalate added under a red safelight. A slight darkening means the iron is in good order; a very dark or black — “It actually turns blue” — means excess ferrous ions. In room light the good solution looks yellowish-brown to orange and a contaminated one looks green or blue, “the deeper the blue, the poorer the quality”. Held beside a 100-watt frosted bulb for a minute, a deep blue appears on the side nearest the lamp, and the sheet names the reason: it is Prussian blue, formed from the newly made ferrous ions and the ferricyanide. That is a supplier publishing a way to check its own product before the customer commits an expensive metal salt to it, and it is rare enough to be worth admiring.

A warning about its own platinum salt. The 20 per cent solution “is near saturation”, the commercial salt “contains varying amounts of potassium hexachloroplatinate, K2PtCl6, which is rather insoluble”, a small fine red precipitate is therefore common, and it must not be transferred into the sensitiser. If the precipitate is heavy and will not dissolve on warming, the sheet asks the buyer to contact the company for an exchange. A maker documenting the ordinary imperfection of its own material, and offering to replace it when the imperfection is more than ordinary, is a better piece of technical writing than most datasheets contain.

The three solutions the buyer makes up. Sensitizer B, by tipping the whole 0.18 g sachet into one of the two ferric oxalate bottles — “be sure that all of it is transferred to the ferric oxalate solution” — and shaking to dissolve. The clearing acid, 30 g of citric acid in 1000 mL of warm water made to 1500 mL for platinum and to 2000 mL for palladium. The developer, 227 g of potassium oxalate in 500 mL of water for platinum and 700 mL for palladium, “Not all of the solid will dissolve”, the saturated solution being the developer.

What the chlorate is for, in the maker’s own sentence. “In the photo process, when ferric oxalate is struck by light, ferric ions are reduced to ferrous ions which subsequently convert the platinum salt to the free metal. The purpose of the potassium chlorate is to reconvert the ferrous ions back to the ferric state. Thus the chlorate acts as a restrainer, increases contrast, and maintains the whites.” The mechanism is right; the phrase “increases contrast” is where the sheet and the conservation literature part company, and Image characteristics below sets out how.

That the chlorate bottle ages. “The chlorate in Sensitizer B will slowly decompose over a period of weeks and the solution will lose its potency. It is wise to run a test strip about once a week or just before a printing session.” This is the only keeping statement in either document, and it is a candid one: it says, in effect, that the contrast grade printed on the table is not a fixed property of the bottle.

Five contrast grades, as parts and as drops. For very soft, soft, average, moderately contrasty and very contrasty negatives, each given as a parts column and as drop counts for a 4 × 5 and an 8 × 10, with a worked example converting the parts to a 55-drop mixture and rounding it up to 56 because “you cannot measure a fraction of a drop”. The tables are printed in full on the open entry and are not repeated here.

The negative, specified by density range. Platinum: “between 1-1.5”. Palladium: “between 1.3 and 1.5”. Both add the same rule of thumb — a negative that gives a good print on grade 1 paper will print correctly with the average mixture. This is a maker telling a buyer the truth about the process’s appetite, and it is the number that determines whether any of the rest works.

The paper, with reasons. Pure linen, 100 per cent rag, is best; watercolour and etching papers are satisfactory; cheap wood papers give poor results; Bristol has too hard a surface — “The platinum falls off during development” — and two- and three-ply papers separate in the wash. The quality varies “from manufacturer to manufacturer and, we have heard, even from box to box labeled with the same lot number”, so the sheet’s advice is to keep one standard negative and print it on every new paper. That is a supplier admitting that its product’s performance depends on something it does not sell.

A porosity test with a threshold. Measure 50 drops of Sensitizer A and try to cover an 8 × 10. If you cannot, size the paper or change it. The kit provides an excess of Sensitizer A precisely so this can be done.

The sizing recipe. The 20 g of arrowroot starch, worked to a cream in about 20 mL of hot water, brought to a litre with constant stirring, boiled five minutes, cooled, skimmed or decanted.

The exposure, as a range without a source calibration. Ferric oxalate absorbs in the ultraviolet, so sunlight or a sunlamp is required and “An enlarger will not work”; the platinum sheet names a General Electric 275 or 300 watt bulb as satisfactory and the palladium sheet drops the bulb; “Ferric Oxalate is extremely slow. Exposure will take 10-20 minutes”; and a test strip is advised. There is no distance, no irradiance and no calibration behind the range.

Development, etching and washing, in numbers. Development in saturated potassium oxalate at room temperature or 90 to 100 °F, plastic or wooden tongs, the image arriving almost immediately, at least two minutes in the bath, and the flat statement that “You cannot overdevelop a Platinum Print”. Etching in three successive trays of the dilute citric acid at five minutes each, the first tray discarded when it clouds and yellows and the trays rotated so the third is always clear — with the warning that this step “is usually passed over too casually” and that a print not properly etched “will darken with age and in this way can be destroyed”. Washing at a complete water change every five minutes for an hour, no colder than 68 °F, distilled water recommended.

A safety paragraph, and it is worth reading twice. Potassium oxalate is singled out as the chemical needing special attention: “This compound is an anticoagulant (prevents blood clotting) and a poison”, used as the developer and therefore easily reaching skin, so tongs or rubber gloves are strongly advised. Ferric oxalate “like potassium oxalate, is a poison”, used in very small amounts. Potassium chlorate is “a dangerous and explosive chemical… However, the amount is so minuscule that no special precautions need be taken”, not to be put in a wastepaper basket or a bin because it is an oxidiser, and to be flushed down a drain with lots of water if it must be discarded. Then: “Consult with local sewer and water authorities regarding proper disposal of darkroom chemicals in your area. The user assumes all risks upon accepting these chemicals. IF FOR ANY REASON YOU DO NOT WISH TO ASSUME ALL RISKS, PLEASE RETURN THE CHEMICALS FOR A COMPLETE REFUND.”

And a bibliography, which almost no kit sheet has. Tice’s 1981 Modern Photography article, Crawford’s The Keepers of Light, Wall and Jordan’s Photographic Facts and Formulas as revised by Carroll, and — on the palladium sheet and on the older printing of the platinum one — Rexroth’s The Platinotype and Shillea’s Instruction Manual For The Platinum Printing Process. Two of the five carry the supplier’s own catalogue numbers, so the sheet is partly a sales list; it is still a sheet that tells you where its instructions came from.

🧮Arithmetic on the maker's own numbers, and what each figure is worth

Everything in this box is this course’s arithmetic on the supplier’s published figures, not a number the supplier prints, and each one additionally assumes that the unlabelled percentages are weight in volume — the convention Ware defines as the weight in grams of solute in 100 cm³ of made-up solution, and the one every formula in this family uses.

The chlorate in Sensitizer B. 0.18 g tipped into 30 mL is 0.6 per cent w/v for platinum; 0.26 g in 30 mL is about 0.87 per cent for palladium. The palladium dose is therefore about 1.44 times the platinum one, where Bostick & Sullivan describe the same difference more roundly as twice as much. The sheet publishes neither percentage.

The coating volume. The five drop tables all come to 46 drops for an 8 × 10 — 22 + 24, 8 + 14 + 24, 14 + 8 + 24, 18 + 4 + 24 and 22 + 0 + 24 — which at the sheet’s own 20 drops per millilitre is 2.3 mL, or about 45 cm³/m². Ware’s specific coating volumes for typical papers fall in the region of 24 to 36 cm³/m², and his working guide is 1.4 cm³ for an 8 × 10 on Cranes cotton. The kit therefore coats at something like one and a half times the conservator’s figure — which is consistent with the sheet’s porosity threshold of 50 drops, about 48 cm³/m², being treated as the outer limit of an acceptable paper rather than as an unusual amount.

The yield, and why it cannot be checked. At 24 drops of metal per 8 × 10, the 5 mL platinum bottle holds about 4.2 sheets’ worth and the 15 mL bottle about 12.5 — which is exactly what the older printing claims and within rounding of the 12 the current one claims. So the sheet’s headline coverage figure is arithmetic, not measurement, and the sheet then contradicts it two sentences later with “in practice you will find that 15 ml will cover about ten 8X10’s”. Both statements cannot be checked against anything, because no drop volume is published: 20 drops per millilitre is the sheet’s own stated assumption and a different dropper gives a different drop.

The developer. 227 g of potassium oxalate in 500 mL of water is far past saturation, which is why the sheet says not all of it dissolves; the working strength is therefore whatever saturation gives at the temperature of the room, and neither sheet states it. The palladium kit’s 700 mL is 40 per cent more water for the same solid, and remains saturated, so the difference between the two developers is a volume of bath rather than a strength — which is presumably the point, and the sheet does not say so.

The clearing baths. 30 g of citric acid in 1500 mL is 2 per cent w/v; in 2000 mL it is 1.5 per cent. The palladium bath is three-quarters the strength of the platinum one, and the sheet’s own reason is that the stronger bath etches palladium off the print.

What is not disclosed

No safety data sheet for either kit, and this is the finding that shapes the page. The supplier’s own Kit SDS index carries 105 sheets, keyed by catalogue number, from the 01- developer kits through to 07-0110. In the 07- series it has sheets for 07-0010, 07-0022 and 07-0023 together, 07-0024, 07-0028, 07-0070, 07-0075, 07-0080, 07-0090, 07-0091, 07-0095, 07-0100, 07-0106 and 07-0110. It has none for 07-0001, 07-0003, 07-0005, 07-0007 or 07-0009, and the filenames those catalogue numbers would take on the same server all return 404.

No substance sheet for any of the four chemicals that matter. The company’s other library, for the chemicals it sells loose, runs to 124 sheets — and covers exactly three of the seven things in these boxes: arrowroot starch, citric acid and potassium oxalate. It carries six iron sheets — ferric ammonium citrate, ferric ammonium oxalate, ferric ammonium sulfate, ferric chloride, ferric nitrate, ferric sulfate — and not one for ferric oxalate. It carries fourteen potassium sheets and not one for potassium chlorate. It carries nothing whatever for platinum or palladium.

!The one sheet that does exist, and what is on it

Linked from the supplier’s own kit index under 07-0022 and 07-0023 — the catalogue numbers of Sensitizer B Platinum/Palladium — is a fourteen-page PDF. It is the only document in the company’s literature filed under the platinum and palladium sensitiser. It contains two safety data sheets:

• Sigma-Aldrich 12302, “Ammonium iron(III) oxalate trihydrate”, CAS 13268-42-3, formula C₆H₁₂FeN₃O₁₂·3H₂O, molecular weight 428.06, version 5.2 of 22 December 2014. Classified Acute toxicity Oral 4 and Dermal 4, signal word Warning, H302 + H312. Its section 3.1 gives one component, “Triammonium iron(3+) trioxalate trihydrate”, at <= 100 %. Its section 11 carries no toxicological data at all: acute toxicity, irritation, sensitisation, mutagenicity and reproductive toxicity all read “No data available”, and the sheet adds that the properties “have not been thoroughly investigated”.
• Columbus Chemical Industries 4730, “Silver Nitrate, Crystal, ACS”, revised 31 August 2012. Oxidizing solids 2, Acute toxicity Oral 4, Skin corrosion 1B, Serious eye damage 1, and the two aquatic categories. UN1493, class 5.1, packing group II.

The supplier’s own product page for 07-0022 and 07-0023 says the bottle contains a 20 per cent ferric oxalate solution and a packet of potassium chlorate. Ammonium iron(III) oxalate is a different iron salt from ferric oxalate — different cation, different CAS number, different behaviour in a sensitiser, and the salt Ware’s print-out process substitutes precisely because it is not the same material. Silver nitrate is not described as being in any platinum or palladium product this company sells.

This page records what the document contains and does not speculate about how it came to contain it. One observation is safely within the evidence: these two sheets are the first two sheets of the same company’s kallitype kit bundle, in the same order. Whether that is the explanation is not something this course can establish, and the reader is owed the fact rather than the theory. What follows for a printer either way is exact: nothing anywhere states the hazard classification of the liquid in the Sensitizer bottle, and the sheet that appears to is describing something else.

No identity for the iron beyond its name. “Ferric Oxalate”, 20 per cent, trihydrogen form, made by the iron alum and oxalic acid route. No hydrate, no assay, no formula weight, no lot, no iron content. That is a larger gap than it looks. Ware calls ferric oxalate “a chemists’ nightmare” — polymorphic, apparently uncrystallisable, sold in states of hydration that the literature gives variously as the tetrahydrate, the pentahydrate and the hexahydrate, with formula weights running from 375.76 to 483.84. Two bottles both honestly labelled 20 per cent can therefore differ by about a quarter in the amount of iron they carry, and everything photographic follows from that number.

No figure for the excess oxalic acid. The sheet names it and stops. Ware found that the customary 2 per cent of free oxalic acid converts about a tenth of the ferric oxalate to the soluble trisoxalatoferrate(III) anion, whose photoproduct is soluble — so a little of the image forms during the exposure rather than waiting for the developer, and partial print-out is one of the conditions for the tonal reversal palladium prints sometimes show. “A slight excess” could be a tenth of that or twice it, and the two are different sensitisers.

No number for the hexachloroplatinate. “Varying amounts” is the whole of it. The insoluble platinum(IV) salt is not a contaminant in the ordinary sense — it is the same substance the trade sells as a contrast agent under other names — so its content is a photographic variable, not just a cosmetic one, and the sheet gives the buyer no way to know how much is present beyond looking at the sediment.

No hydrate for the palladium. Na₂PdCl₄ is 294.2 g/mol anhydrous and is usually sold as roughly the trihydrate; the difference between the two is about a sixth of the palladium. A 15 per cent solution of one is not a 15 per cent solution of the other.

No pH, anywhere, for anything. Not for Sensitizer A, not for Sensitizer B, not for the mixed sensitiser, not for the saturated developer and not for either clearing bath. That is a real absence in this family rather than a pedantic one: the clearing chemistry is entirely pH-driven, since chemisorbed iron(III) hydrolyses above about pH 4 and, once it has gone as far as goethite, the ordinary baths will not remove it at all.

No keeping time for anything except the chlorate. Nothing for the unopened kit, nothing for either iron bottle, nothing for the metal solution, nothing for the made-up developer, nothing for the made-up acid, and nothing for the mixed sensitiser in the shot glass.

No sensitometry of any kind. No exposure scale, no maximum density, no speed, no characteristic curve, no reciprocity note, and no measured result for any of the five contrast grades. The nearest either sheet comes to a measurement is the density range it asks of the negative.

No coating volume in millilitres, and no drop volume. The drop tables are the whole of the published coating instruction, and “assuming 20 drops per ml” is the whole of the published conversion. Which means the yield claim, the coverage claim and the contrast grades are all stated in a unit the supplier does not define and does not supply an instrument for — the kits ship no dropper at all.

And the sheets disagree with themselves. Four inconsistencies survive into the current printings: the 15 mL platinum kit covers 12 sheets in one sentence and “about ten” two sentences later; the palladium sheet calls its 15 per cent bottle “the 15-ml 20% palladium solution”; the palladium worked example prints 0.304 × 55 = 1.67 drops and then rounds it to 17; and the two sheets convert 50 °C to 122 °F on one and 120 °F on the other. A fifth was corrected between printings — the older platinum sheet asks for “1000 ml of water at 120 ° C” for the clearing acid, which the current printing gives as 120 °F. None of these is dangerous. All of them are the reason a page like this reads the printings separately instead of merging them.

Disclosed components

Eight, from two boxes, and every concentration on this page comes from an instruction sheet rather than from a hazard document, because no hazard document for these products exists. The consequences of that are worth stating once, plainly, before the list.

The hazard column is empty on purpose. A component’s hazard codes belong to the sheet the component was read from, and the sheets these were read from are technical documents that carry no GHS classification at all. The company does publish classified sheets for three of the eight, in its bulk-chemical library, and those classifications are quoted below in their proper place — as classifications of the loose chemical, which is a different question from the classification of a 20 per cent solution in a bottle.

Four of the eight carry no concentration, and that is the maker’s doing rather than an omission here: the kit ships them as weighed solids, so the sheet gives an amount and not a strength. What that amount becomes in a bath depends on a make-up volume the sheet gives separately, and those volumes are recorded under What the maker publishes.

Arrowroot starch, 20 g, for sizing. The one item in the box with no part in the image chemistry: boiled to a size and brushed onto the paper so that the image sits on the surface rather than sinking into the fibres, and so that a porous sheet does not drink an expensive sensitiser. The sheet is candid that it may not be needed — “Sizing was imperative in the early days of platinum printing because of poor paper quality. Today sizing may or may not be needed depending upon the quality of the paper you choose” — and gives the 50-drop porosity test as the way to decide. The supplier’s own substance sheet for it is for something else. Under the heading Arrowroot Starch in the bulk library sits Sigma-Aldrich S4251, “Starch, from potato”, CAS 9005-25-8, EC 232-679-6, version 5.4 of 27 February 2015 — the same substitution the course records on the cyanotype kit page, where the identical sheet is bound behind an instruction sheet that says arrowroot. Whatever is in the packet, the sheet classifies what it describes only as a combustible dust, signal word Warning, with no precautionary statements, its section 3.1 component named “High-polymeric carbohydrate material” at <= 100 %, an ACGIH time-weighted average of 10 mg/m³ annotated “Dermatitis” and “Not classifiable as a human carcinogen”, OSHA Table Z-1 limits of 15 and 5 mg/m³, and an incompatibility list reading “Strong oxidizing agents” — which, since the same box contains 0.18 or 0.26 g of potassium chlorate, is a fact with a use, and Incompatibilities below returns to it.

Ferric oxalate, 20 per cent, in two 30 mL bottles. The only light-sensitive substance in the kit, and the substance the sheet works hardest to describe: trihydrogen ferric oxalate, H₃Fe(C₂O₄)₃, prepared by the iron alum and oxalic acid procedure, sensitive around 460 nm, to be handled under a red safelight, not to be heated above 50 °C, very water-soluble, yellow to yellow-green when first brought into room light. Under ultraviolet an electron transfers from a coordinated oxalate to the iron, reducing iron(III) to iron(II) and destroying the ligand as carbon dioxide; the iron(II) is the reducing agent that will make the picture. The two bottles are chemically identical until the chlorate sachet goes into one of them, which is the design point of the whole system. And it is the least specified substance in the box: no hydrate, no assay, no formula weight, no iron content, and no safety data sheet anywhere in the supplier’s 229 published sheets. The company sells it, packs it into three different kits, publishes three near-identical essays about its chemical identity, and has never classified it.

Oxalic acid, “a slight excess”, in the same two bottles. A disclosed component with no quantity attached, which is an unusual thing to find and worth taking at face value: the maker is telling you there is a second substance in the iron bottle and declining to say how much. It earns its place — it holds the pH down, keeps the iron complex soluble, and converts any of the inactive green potassium double salt present to the active acidic form — and it is not photographically inert, since Ware found the customary 2 per cent converts about a tenth of the iron to a soluble anion whose photoproduct is also soluble, producing a little print-out during the exposure. This is also the strongest classification attaching to anything the iron bottle is known to contain. The supplier’s own bulk sheet, Sigma-Aldrich 247537 for oxalic acid dihydrate, CAS 6153-56-6, gives Acute toxicity Oral 4, Acute toxicity Dermal 4 and Serious eye damage Category 1, signal word Danger, H302 + H312 and H318, with an oral LD50 in the rat of 1,080 mg/kg, a pH of 1 at 126.1 g/L, and incompatibility with bases, metals, acid chlorides and alkali metals. That sheet is for the pure solid and the bottle holds a slight excess in a 20 per cent iron solution; the point is not that the bottle is a Danger-labelled product, but that the acid is the reason a splash of Sensitizer A deserves the eye protection neither sheet asks for.

Potassium chlorate, 0.18 g for platinum and 0.26 g for palladium. The contrast agent, the whole difference between Sensitizer A and Sensitizer B, and one of the two reasons this page is at Level D. It is a strong oxidising agent that takes no part in the light reaction and none in the reduction of the metal: it re-oxidises part of the iron(II) that exposure has just produced, so that less of it survives to make an image. The maker’s own sentence — “acts as a restrainer, increases contrast, and maintains the whites” — is mechanically correct and aesthetically misleading, and Image characteristics below explains why. It arrives pre-weighed in a sachet, and the sheet insists the whole of it goes in, because at 0.18 g the fraction left in the paper matters. No safety data sheet for it exists in either of the supplier’s libraries, and the only thing either instruction sheet says about its hazards is that it is “a dangerous and explosive chemical” whose quantity here “is so minuscule that no special precautions need be taken” — a risk assessment in one clause, with no classification behind it. The full hazard account, and the reason this course teaches a calibrated negative instead, are on its own page.

Potassium tetrachloroplatinate(II), 20 per cent, 5 to 30 mL. The image, in the metal this course will not let anyone handle. The sheet calls it potassium chloroplatinite and gives the formula, states that platinum salts are not light sensitive, and warns about the near-saturation and the red precipitate of the much less soluble hexachloroplatinate(IV). It waits in the paper doing nothing until the developer sets the iron(II) free, then takes two electrons per atom and precipitates as metallic platinum — as single ellipsoidal nanoparticles 15 to 25 nm across, distributed through the body of the surface fibres, in the electron microscopy Ware reports. Ware’s own standard solution is about 18.5 per cent against this supplier’s 20, and he notes both that the salt’s true solubility is poorly documented and that a fresh solution is left to mature for a day because the dissolved anion aquates with a half-time of 2.4 hours. Its hazard is not acute toxicity but sensitisation, and that is what decides its classification: see Safety.

Sodium tetrachloropalladate(II), 15 per cent, 15 or 30 mL. The same job in the metal the course does teach as a process. Na₂PdCl₄, not light-sensitive either, reducing faster than platinum and giving a warmer, browner, lower-contrast image with a greater tendency to fog in an oxalate developer and a vulnerability to acid that platinum does not share. The sheet gives the strength and never the hydrate. Ware’s own standard is a 0.5 M solution, made either from the solid double salt or from palladium(II) chloride dissolved in hot sodium chloride solution; the potassium salt is the same anion with a different cation and about a tenth less palladium per gram.

Citric acid, 30 g, for the clearing baths. The step the sheet says is “usually passed over too casually” and the one that decides whether the print survives: it removes the iron left in the paper, which would otherwise darken the print over years. Made to 1500 mL for platinum and 2000 mL for palladium — 2 per cent and 1.5 per cent respectively, on this course’s arithmetic — with the maker’s own reason for the difference, that a stronger bath etches palladium metal off the print. The supplier’s bulk sheet for it, Sigma-Aldrich 791725 for citric acid anhydrous, CAS 77-92-9, classifies it Eye irritation Category 2A, signal word Warning, H319 and nothing else, with an oral LD50 in the rat of 5,400 mg/kg, a pH of 1.8 at about 50 g/L, and incompatibility with oxidizing agents, bases, reducing agents and nitrates. It is the mildest thing in either box.

Potassium oxalate, 227 g, and the developer. The most interesting item in the kit and the one both sheets single out for warning. It contains no developing agent: what it does is supply free oxalate ions that take the immobile ferrous oxalate the light made into solution as a mobile complex, at which point the iron(II) can finally reach the metal salt. That is why development is over in seconds and why a platinum print cannot be overdeveloped. The kits make it as a saturated solution with undissolved solid left in the bottom, so the working strength is whatever saturation gives at the temperature of the room. The interesting thing is the mismatch between the two documents the supplier publishes about it. The instruction sheet calls it “an anticoagulant (prevents blood clotting) and a poison” and asks for tongs, because the developer is the one bath a printer’s hands go near. The supplier’s bulk sheet, Sigma-Aldrich 379727 for potassium oxalate monohydrate, CAS 6487-48-5, classifies it Acute toxicity Oral 4 and Dermal 4, signal word Warning, H302 + H312 — and its section 11, the toxicological section, reads “No data available” on every line: no LD50, no irritation, no sensitisation, nothing. The strongest warning the company gives about this chemical is in the document that is not the safety data sheet. Elsewhere that sheet is useful: pH 7.0 to 8.5 at 50 g/L, relative density 2.127, stable but to be kept from moisture, and incompatible with halogens, ammonia, cyanides and heavy metals — the last of which is worth a moment’s thought in a darkroom that also holds a bottle of platinum.

Behaviour

What the sources record. None of it is offered as an instruction.

Almost nothing happens during the exposure, and what does happen is the wrong colour. The Getty atlas describes the exposed sheet as carrying “a faint brownish image made from the photochemically generated ferrous oxalate”. The picture is not there; a distribution of iron(II) is. This is the behaviour that makes the process so unlike a printing-out one and so hard to judge, and it is why the sheets ask for a test strip rather than offering an exposure time.

Then the image arrives all at once. Development is complete within seconds, which is why the sheet warns that a heated bath must be entered quickly and evenly to avoid streaking, and why air bubbles on the surface have to be brushed away immediately: an area the developer reaches a second late is an area that develops differently. The corollary is the sheet’s own: “You cannot overdevelop a Platinum Print.”

The developer is a variable, not a constant. The sheets offer it at room temperature or at 90 to 100 °F and say only that heating makes development faster. The Getty atlas gives the direction of the effect the sheets omit: hotter development produces smaller metal particles and warmer prints. So the same sensitiser, the same negative and the same exposure give two different prints in two different trays, and neither sheet says so.

The chlorate bottle ages faster than anything else in the box. The supplier states it plainly and recommends a weekly test strip. The consequence is worth spelling out: the contrast grade printed against a drop count is not a property of the bottle but of the bottle this month. A mixture counted out in the same numbers in March and in May is not the same mixture.

Humidity is a hidden variable and a large one. Ware attributes the tonal reversal seen in some palladium prints principally to low relative humidity in the dried coating, below about 30 per cent, with the effect disappearing at 67 per cent on otherwise identical prints. Neither kit sheet mentions humidity at all; both prescribe a hairdryer.

Palladium solarises and platinum does not. The palladium sheet is the only document in this entry to say so — “Under certain conditions, palladium prints, unlike platinum prints, will solarize” — and gives no conditions.

Drying is a variable with a stated failure at both ends. Too little and the sensitiser sits on the surface, the print lacks contrast because the paper was damp during the exposure, and metal floats off during processing; too much, or too hot, and the sensitiser sinks into the fibre, the print goes flat because the paper scatters the light, and heat may decompose the sensitiser and give uneven densities. That is an unusually complete account of a failure mode for a kit sheet, and it has no numbers in it beyond the drying times.

The clearing baths are not interchangeable between the two metals. 2 per cent citric acid for platinum and 1.5 per cent for palladium, with the maker’s reason stated: the stronger bath etches palladium off the print. Ware records the same asymmetry from the historical record at 1 : 60 against 1 : 200 hydrochloric acid, and notes that American printers who used the platinum strength on palladiotypes were partly rescued by a second error, because the stronger acid removed the palladium fog their oxalate developer had produced.

The clearing bath tells you when it is spent. “The acid in the first tray will soon become cloudy and yellowed in appearance. This is due to any remaining Ferric Oxalate being etched from the paper.” That is a self-indicating bath, and the sheet’s tray-rotation scheme is built around it.

The paper is a chemical variable and the supplier admits it. Quality varies between manufacturers “and, we have heard, even from box to box labeled with the same lot number”, so the sheet’s remedy is a standard negative printed on every new paper. Ware adds the mechanism the sheet does not: gelatin sizing binds platinum(II) strongly and irreversibly and de-activates it, so a paper choice is a chemical decision before it is an aesthetic one.

Image characteristics

Colour: neutral-grey for platinum, warm-black to sepia for palladium, in the palladium sheet’s own words, with the observation that palladium prints have “almost the same scale, richness, and delicacy as do platinum prints but are warmer and have a smoother tone”. Neither sheet explains the difference or offers any way to change it. The chemistry behind it is straightforward and is on the open entry: palladium reduces faster, faster precipitation gives smaller particles, and smaller particles look warmer.

Contrast: five grades, and the control is entirely in the ratio of the two iron bottles. The metal holds at 0.521 or 0.522 of the mixture in every grade, so neither the amount of metal nor the volume that has to be spread over the sheet ever changes; the only thing that moves is the chlorate concentration. The grades are pinned to the negative rather than to the print — highlight densities of about 1.1, 1.3 and 1.5 — and no measured result is published for any of them.

And “increases contrast” is not what the chlorate does. This is the most important sentence on the page and it is Ware’s: the effect of these oxidising agents “is not to uniformly contract the tonal scale, but simply to truncate the high values”. A harder grade of silver paper redistributes every tone; a chlorate dose removes the weakest exposures altogether and leaves the rest where they were. The oxidant is spread evenly through the coating and consumes iron(II) wherever it finds it, so in a shadow that received a large exposure plenty survives and the tone is barely touched, while in a highlight that received little the whole of it can be consumed and the tone disappears. What you buy is separation in the mid-tones and empty paper in the highlights — what Ware calls a false sparkle of lost gradation, and describes as aesthetically unacceptable for a process whose whole point is subtly nuanced high values.

And it costs texture. Ware records an increase in graininess and a deterioration in image quality. The Getty atlas records the same defect from the analytical side and its observation is the one a conservator uses: the chlorate itself cannot be detected, because it is too soluble to leave a residue, but it is responsible for a visually detectable patchiness in the platinum image.

Tonality: unstated by the maker at every grade. No exposure scale, no maximum density, no curve. Ware’s figure for the unmodified sensitiser — an exposure scale of about 2.0 — is the nearest independent number, and the kits’ own negative specification of 1 to 1.5 is the practical statement of the same fact: the process wants a longer-scale negative than a silver print does, and the drop tables exist because most negatives are not that.

Maximum density is a known weakness of this route and the sheets do not mention it. Ware attributes the historical practice of double-coating to it and traces the underlying cause to the platinum salt’s solubility: a sensitiser near saturation cannot carry as much metal as the image would like. The kit’s 20 per cent solution is, by the sheet’s own admission, at that limit.

Surface and support: set by the paper, and the paper is specified without being named. Pure linen or 100 per cent rag, single ply, not Bristol. Five specific papers are named by neither sheet, which is a gap the competing supplier fills.

The mechanism

Rather more of this is the maker’s than usual, and the parts that are not are marked. The sheet states that platinum salts are not light sensitive, that ferric ions are reduced to ferrous ions by light, that the ferrous ions convert the metal salt to the free metal, and that the chlorate reconverts ferrous to ferric. That is a correct three-step account of a siderotype, printed on a kit sheet, and it is more than most makers offer. What follows fills in what the sheet leaves out, and every step of that is the open literature applied to the composition the maker publishes rather than an account the maker gives.

⚗Three acts, and the chlorate acts between the first and the second

One: light makes an insoluble iron(II) salt. Ferric oxalate absorbs in the ultraviolet — the sheet says 460 nm — and an electron transfers from a coordinated oxalate to the iron, reducing iron(III) to iron(II) and destroying the ligand as carbon dioxide. In the dry coating the product is ferrous oxalate, which is almost insoluble.

UV + Fe₂(C₂O₄)₃ → 2 FeC₂O₄ + 2 CO₂

Ware's equation for the photolysis of solid iron(III) oxalate

The metal salt is sitting in the paper alongside it and nothing happens, because the reducing agent cannot move. This is why the exposed sheet carries only a faint brownish print-out and not an image.

Two: the developer dissolves rather than reduces. The fact that makes this family unlike every other developing process in the course, and the fact neither sheet states. Saturated potassium oxalate supplies free oxalate ions, which take the immobile ferrous oxalate into solution as a mobile complex.

FeC₂O₄ + C₂O₄²⁻ → [Fe(C₂O₄)₂]²⁻

Oxalate ions dissolve the immobile photoproduct into a mobile complex

Now the iron(II) can reach the metal salt and the reduction finishes itself, two iron(II) per metal atom because each metal(II) takes two electrons and each iron supplies one.

2 [Fe(C₂O₄)₂]²⁻ + [PtCl₄]²⁻ → 2 [Fe(C₂O₄)₂]⁻ + Pt + 4 Cl⁻

Reduction of platinum(II) by the iron(II) oxalato complex

2 [Fe(C₂O₄)₂]²⁻ + [PdCl₄]²⁻ → 2 [Fe(C₂O₄)₂]⁻ + Pd + 4 Cl⁻

The same reaction with palladium, which is the metal the course teaches

Ware’s redox potentials say why it goes at all: the iron(III)/iron(II) oxalato couple sits at +0.02 V, tetrachloroplatinate at +0.73 V and tetrachloropalladate at +0.62 V.

Three: the chlorate removes part of act one before act two can use it. Potassium chlorate is dissolved in the coating from the start. It does not touch the light-sensitive iron(III) and it does not touch the metal salt; it re-oxidises some of the iron(II) that exposure has just made, so that less survives. Because it is spread evenly and consumes iron(II) wherever it meets it, it removes the weakest exposures entirely and barely touches the strongest, which is what “truncating the exposure scale” means physically.

This page writes no balanced equation for that third step, and the reason is on the chlorate’s own page: no source read gives the stoichiometry or says what the chlorate is reduced to in a ferric oxalate sensitiser at the pH it is used at. An equation assembled from first principles would be an invention with a citation attached.

Can the class of thing be read off the disclosure, the way a developer disclosing hydroquinone and a phenidone salt can be read as a PQ developer? Yes, and this is a much easier reading than most, because the maker has done half of it already. A kit disclosing a light-sensitive iron(III) oxalate, a tetrachlorometallate(II) of platinum or palladium, an oxidising chlorate held in a second bottle of the same iron, an oxalate developer and an acid clearing bath is a drop-counted, chlorate-controlled siderotype of the Pizzighelli and Hübl line, and nothing else. The identification is secure. What cannot be read off the disclosure is anything about specification — which hydrate the iron is, how much oxalic acid rides with it, how much hexachloroplatinate the platinum carries, which hydrate the palladium is. Those four unknowns are the difference between a formula and a material, and this course marks the boundary rather than guessing across it.

Why palladium needs half again as much chlorate, which the sheet asserts and does not explain, is worth one paragraph because it explains several other things at once. The electron transfer proceeds not through the [MCl₄]²⁻ ion but through its aquated form, in which water has displaced one chloride, and at 20 °C the equilibrium constant for that substitution is 0.17 for palladium against 0.015 for platinum. So at sensitiser strength more than half the palladium is already in the reactive form and less than a fifth of the platinum is; the palladium reduction is fast, and a fast reaction gives the oxidant less time to intercept the iron(II). The same speed accounts for palladium’s lower native contrast, its greater tendency to fog in an oxalate developer, and its warmer colour. That explanation is Ware’s, applied here to the maker’s two numbers; the maker offers none.

The nearest open formula

The three-solution drop system for platinum and palladium, and the comparison is unusual in this formulary because the two entries are not rivals. The open entry’s own provenance is this kit’s instruction sheet. What separates them is not chemistry but what each kind of page is for.

The open entry is the formula, read as a formula. It carries all three solutions with their strengths, both chlorate doses, all five drop tables in parts and in drops, the mixing relationships, the mechanism with its equations and redox potentials, the arithmetic that converts the parts column into a volume, the disagreement between three suppliers who call the same bottle 20, 25 and 27 per cent, and the historical line back to Pizzighelli and Hübl in 1882 and forward to Irving Penn’s four bottles. It exists so that a set of numbers met in a manual or on a shelf can be identified.

This page is the product, read as a product. It carries what the maker publishes and — the part the formula page has no field for — what the maker does not: no safety data sheet for either kit among 105, no substance sheet for any of the four decisive chemicals among 124, and one sheet filed under the sensitiser that describes a different iron salt. It carries the internal contradictions of the two printings, the yield claims that cannot be checked, the coverage figure the sheet contradicts two sentences later, and the arithmetic that shows the kit coating at half again the conservator’s volume. Those are facts about a bought object rather than about a formula, and they are the ones that decide whether the object is worth buying.

Where they differ in substance, it is on the strength of the iron. Photographers’ Formulary supplies 20 per cent; Bostick & Sullivan 27; Ware’s own standard siderotype solution is 25 per cent with 2 g of oxalic acid per 100 cc. A drop count written for one of those bottles is not a drop count for another, and neither this page nor the open entry averages them.

And the formula this course actually teaches in this family is neither. It is the palladiotype with no oxidant in the bottle at all, developed by Willis’s potassium oxalate developer or by the sodium citrate developer, cleared by the sequence that decides whether the print lasts, and given its contrast by a calibrated negative. Remove the platinum and the chlorate from these two boxes and what is left is a Level B process this course does teach; the two things removed are exactly the two things that put this page at Level D.

Safety

⚠Level D, on two independent grounds, and neither is about the palladium

Platinum, for irreversibility. Potassium tetrachloroplatinate(II) carries H334 — may cause allergy or asthma symptoms or breathing difficulties if inhaled — with H301, H315, H317 and H318 alongside it, and HSE’s EH40 sets halogeno-platinum compounds at 0.002 mg/m³ as platinum with the Sen notation, against 5 mg/m³ for platinum metal itself: a limit 2,500 times tighter for the compound than for the element. Ware’s health warning is the practical form of the same fact — never touch the surface of platinum-sensitised paper, never put ungloved fingers in the processing solutions, treat dust inhalation as the worst hazard, and do not store large amounts of dried sensitised paper. Platinum allergy was described in 1911 as an occupational disease of photographic factory workers handling platinotype paper.

Potassium chlorate, for the availability of a better route. Its own page sets out the classification — signal word Danger, H271 as a strong oxidiser under the harmonised CLP entry, with the acute-toxicity statement genuinely disputed between notifiers — and concludes that the contrast the reagent buys is available from a calibrated negative without the graininess and without an oxidiser in the darkroom.

Nothing above is a control and none is offered. The Level D policy is that a classification is not a statement about how difficult the controls would be; it is a statement that a page cannot assume them.

The kit’s own safety paragraph, and what is wrong with it. It is not a bad paragraph. It correctly singles out the developer as the bath a printer’s hands go near; it correctly says that potassium oxalate is “an anticoagulant (prevents blood clotting) and a poison”; it asks for tongs or rubber gloves; it says ferric oxalate is a poison too; it says the chlorate is an oxidiser that must not go in a bin. Three things are missing and each of them matters. It says nothing about the platinum, which is the one substance in the box with an irreversible hazard and the tightest exposure limit in this course — the words allergy, asthma and sensitiser do not appear in either document. It offers no first aid beyond “wash immediately with soap and water”, no eye-contact instruction and no emergency contact. And it disposes of the whole chlorate question in one clause: “the amount is so minuscule that no special precautions need be taken”. That may well be true of 0.18 g in a sealed sachet, but it is a risk assessment asserted rather than shown, made by the seller, about the substance the buyer is least likely to know anything about.

The dilution is doing most of the safety work and should be understood as doing it. The classified sheets in the supplier’s library describe solids at 100 per cent. The liquids in the box are at 20, 20 and 15 per cent, which the maker states. Below that, every figure is this course’s arithmetic on the maker’s own numbers rather than anything the maker prints: the mixture in the coating dish comes to roughly 10 per cent iron and 10 per cent metal, because the drop tables hold the metal at about 0.52 of the volume and the two iron bottles share the remaining 0.48, and the clearing tray is 2 per cent acid for platinum and 1.5 for palladium. The concentrate is the thing to respect; the tray is comparatively benign. The usual mistake runs the other way.

The developer is the bath with a real systemic hazard and the weakest paperwork. The company’s own substance sheet for potassium oxalate gives Acute toxicity Oral 4 and Dermal 4 with H302 + H312 — harmful if swallowed or in contact with skin — and then leaves every line of its toxicological section reading “No data available”. The strongest statement the company makes about this chemical anywhere is the instruction sheet’s word poison. Both documents agree on the practical conclusion, which is that hands do not go in the developer.

The eye hazard nobody labels. Neither instruction sheet asks for eye protection, and neither mentions oxalic acid as a hazard at all — yet the iron bottles contain free oxalic acid by the maker’s own statement, and the maker’s own sheet for that acid carries H318, causes serious eye damage, signal word Danger. Splash protection is not optional around a bottle of Sensitizer A, whatever the instruction sheet omits. See gloves and first aid.

Dust is the exposure route the sheets never name. Three of the seven items ship as solids — the starch, the citric acid and 227 g of potassium oxalate — and the fourth, the chlorate, is a fine solid in a small sachet. The starch’s own sheet carries a combustible-dust classification and an ACGIH limit of 10 mg/m³; the oxalate’s carries an acute dermal and oral classification; and Ware’s warning about platinum is specifically about dust. Opening and dissolving 227 g of anything is a dust operation.

And the sentence at the end of the safety section. “The user assumes all risks upon accepting these chemicals. IF FOR ANY REASON YOU DO NOT WISH TO ASSUME ALL RISKS, PLEASE RETURN THE CHEMICALS FOR A COMPLETE REFUND.” It is recorded here without comment except this one: a transfer of risk is not a hazard communication, and it is the last thing either document says about safety.

Storage

What the sources record.

The iron is the perishable one and the sheets barely acknowledge it. Neither document gives a keeping time for either ferric oxalate bottle. What they do give is the handling: red safelight, no ultraviolet, and nothing above 50 °C. Ware records six to nine months in the dark as the figure “said by some”, and notes that at least one well-respected printer makes the solution fresh the night before every session. The competing supplier gives its own about a year, longer refrigerated, and insists it be brought back to room temperature before use.

The chlorate bottle is perishable in a way that is easy to miss, and here the sheet is candid. It decomposes over weeks, the solution loses potency, and the remedy offered is a weekly test strip. Nothing about the bottle looks different; only the print does.

The metal solutions keep and their enemy is contamination, not time. Nothing in either sheet suggests otherwise, and the one storage-adjacent warning both give is about the platinum bottle’s red precipitate, which must not be carried into the sensitiser. Glass, and a separate dropper for each bottle: the noble metals plate out onto a more reactive metal surface, which rules out metal vessels and metal droppers throughout.

The sachet is the item most likely to be mishandled in a cupboard. A paper packet of 0.18 g of a strong oxidiser, stored among paper and cloth, is the situation every hazard entry for potassium chlorate describes — and its own page identifies the handling of the dry salt as the operation that decides its classification. Neither instruction sheet says where to keep it.

Nothing here is labelled well enough to survive a cupboard. The bottles say “Sensitizer A” and “Sensitizer B”, and after the sachet has gone in, one of the two is a different solution from the other with no outward sign. Anything decanted from either is a container nobody can identify: write the substance name, the percentage and the date on it — see labelling a container — and if you have already inherited one that is unmarked, the unlabelled container procedure exists for exactly that.

And a housekeeping note the sheets do not make. The developer is 227 g of a solid whose own sheet says “Avoid moisture”, stored in a room where trays of water are open all day. A jar that has taken up water is a jar whose 227 g is no longer 227 g.

Incompatibilities

A strong oxidiser and a combustible dust are packed in the same box. Potassium chlorate’s whole hazard profile is its behaviour with combustible material — a very flammable mixture with combustibles, explosive if the combustible is finely divided, ignitable by friction — and the supplier’s own sheet for the starch names strong oxidizing agents as its incompatible materials. Nothing in the kit brings the two together and nothing in either instruction sheet warns that they are in the same package. Keep the sachet away from the starch packet, from paper, and from anything dusty, and dispose of it as an oxidiser rather than as rubbish.

Ferric oxalate against the metal solutions, in the bottle. The two are meant to meet in the coating vessel and nowhere else. A dropper used in both, or a brush rinsed carelessly, contaminates a bottle that costs more per millilitre than anything else in the darkroom, and iron(II) reaching a metal solution reduces metal in the bottle rather than in the paper.

The developer against heavy metals, on the supplier’s own sheet. Sigma-Aldrich’s sheet for potassium oxalate names halogens, ammonia, cyanides and heavy metals as incompatible materials. In a darkroom running this process the developer and the noble metal meet on purpose, in the paper, which is the point; what should not happen is a metal solution finding its way into the developer jar, or a developer tray being used for anything silver.

Palladium against acid. The clearing bath that suits platinum dissolves palladium: 2 per cent citric acid against 1.5 per cent in these kits, and 1 : 60 against 1 : 200 hydrochloric acid in the historical record. This is an incompatibility between a metal and a bath strength rather than between two bottles, and it is the one the palladium sheet warns about in its opening paragraph.

Oxalic acid against metals and bases, on the supplier’s own sheet for it — which matters because the free acid is in both iron bottles: no metal funnels, no metal spatulas, and nothing alkaline poured into the same vessel.

Citric acid against oxidising agents, bases, reducing agents and nitrates, again on the supplier’s own sheet. In this workflow it meets none of them; in a shared sink it can meet all four.

Gelatin against platinum. Ware records that gelatin sizing binds platinum(II) strongly and irreversibly and de-activates it, which is why the sheet’s insistence on rag and linen papers and its rejection of hard-surfaced Bristol are chemical instructions as much as aesthetic ones — and why the kit supplies a starch size rather than a gelatin one.

Buffered paper and hard water. Calcium strips oxalate from the iron complex and starts the hydrolysis that leaves a yellow stain; once the iron has gone as far as goethite the ordinary clearing baths will not remove it. That is why both sheets ask for distilled water for rinsing and for the final wash, and it is the reason an alkaline-buffered paper is a poor choice even though neither sheet says so. See calcium carbonate and cross-contamination between alt processes.

Anything acidic near a ferricyanide test. The spot test the sheet publishes uses potassium ferricyanide, which liberates hydrogen cyanide on contact with strong acid. Two crystals in a shot glass is a small quantity, and the rule is still absolute: see incompatibilities.

Waste

The bulk of it is iron, and it is the clearing baths. Three trays of citric acid carrying the unreacted ferric oxalate etched out of the paper, plus an hour of wash water behind them. The first tray announces itself — cloudy and yellow — and the sheet’s own instruction is to discard it and rotate the others. The standing arrangement is on the disposal page and in general chemical waste, and the first thing to establish is what your local authority accepts.

The developer is the one to keep out of a drain on its own merits. A saturated solution of a substance the maker itself calls a poison and an anticoagulant, in litre quantities, with an aquatic profile its own safety data sheet declines to give — section 12 of the Sigma-Aldrich sheet reads “No data available” for toxicity, persistence, bioaccumulation and mobility alike. Absence of data is not evidence of safety, and the sheet’s own section 8 says “Do not let product enter drains”.

Spent metal is worth recovering rather than discarding, and this is the one waste stream in the formulary with a positive value. Platinum and palladium in a spent bath or on a spoiled print are the most expensive thing in the room; a refiner will take them. Nothing in either instruction sheet mentions recovery.

The chlorate, and the disposal instruction this course does not adopt. The sheets say: if you must dispose of solid potassium chlorate, “flush it down a drain with lots of water”, and do not put it in a wastepaper basket or a bin because it is an oxidiser and can feed a fire. The second half of that is sound and important. The first half is a 1990s-vintage instruction that this course does not repeat: an oxidiser is not neutralised by dilution, and the aggregated notifications for potassium chlorate carry H411, toxic to aquatic life with long lasting effects, in the great majority of reports. Treat an unused sachet as oxidiser waste and ask the local authority, which is what the same sheet says one paragraph later about everything else.

Paper and packaging. Sensitised offcuts and test strips carry iron and noble metal; dry them and keep them out of general recycling, and — for platinum specifically — remember Ware’s advice not to accumulate dried sensitised paper at all.

Troubleshooting

There is no procedure here to troubleshoot. What follows is what the sources record going wrong, kept because each entry explains a piece of the chemistry and because anybody working from these sheets will meet all of it.

“The highlights are empty and the print looks brittle.” Too much chlorate. The tonal scale has been truncated rather than compressed, and the weakest exposures were cancelled outright rather than moved.

“The print is grainy and patchy, and the analysis finds no contrast agent.” The Getty atlas’s diagnostic: chlorate leaves no analysable residue because it is too soluble, so patchiness with nothing detectable is its signature.

“The same drop count gives a flatter print than it did last month.” The chlorate in Sensitizer B has decomposed. The supplier’s own remedy is a weekly test strip, which is an admission that the bottle’s grade drifts.

“The highlights are yellow and will not clear.” Residual iron: the print has not been etched enough, or the acid was exhausted. The sheet’s own warning is that an improperly etched print “will darken with age and in this way can be destroyed”, which makes this the failure that decides whether the print survives rather than how it looks. See iron-silver highlights that will not clear and, for the stain that will not come out at all, platinum stain from incomplete clearing.

“The palladium print lost density in the clearing bath.” The acid was too strong — the platinum kit’s 2 per cent bath used on a palladium print, or a bath made up to the wrong volume.

“The palladium highlights are a brownish grey rather than paper white.” Chemical fog from an oxalate developer reducing palladium(II) on its own, distinct from the yellow of iron and worse in humid conditions. Willis’s citrate developer leaves clear highlights.

“The shadows went backwards.” Tonal reversal, which Ware’s tests associate with a dried coating below about 30 per cent relative humidity, an oxalate developer, a contrasty negative and a little print-out. The palladium sheet’s own phrasing for it is that palladium prints “will solarize”.

“There is a red precipitate in the platinum bottle.” Normal, per the sheet: the commercial salt carries some potassium hexachloroplatinate(IV), which is much less soluble. It should not be carried into the sensitiser. If it is heavy and will not dissolve on warming, the sheet asks you to contact the company for an exchange.

“The sensitiser has gone cloudy.” See sensitiser gone cloudy. On this kit’s composition the first suspect is the platinum bottle’s own sediment and the second is contamination between droppers.

“The metal floated off the paper during development.” The sheet’s own two diagnoses: a Bristol or hard-surfaced paper, or insufficient drying, which leaves the sensitiser on the surface rather than in the fibre. See also thin coat.

“The print is flat and the paper texture shows through.” The sheet’s third drying failure: too long or too hot, so the sensitiser sank too far into the fibre and the surface scatters the light.

“The coating is streaked or blotchy.” Development that began before the print was fully immersed is the sheet’s own explanation, and the remedy it gives is to submerge quickly and evenly and to brush away air bubbles at once. For the coating itself see blotchy and streaked hand coating, mottled coating from humidity and, where the solution beaded instead of wetting, repellency spots.

“The image is soft or doubled at the edges.” Registration rather than chemistry: see uneven contact print. The split-back frame exists so the print can be inspected without losing alignment.

“The print faded or yellowed years later.” Almost always the clearing again. See an iron-silver print that faded; the mechanism, and the sequence that prevents it, are on the clearing sequence and in the iron-silver clearing sequence.

“Two prints from the same box do not match.” Ask three questions in order: how old is the chlorate bottle, what was the humidity when the paper dried, and is this the same paper from the same box. If none of those explains it, the unstated iron content is the variable nobody can check, which is the argument of this whole page.

Experiments

No experiment below involves making or using either kit.

Enumerate a supplier’s safety data sheet library and see what is missing. This is the exercise that produced this page, it needs nothing but a browser, and it generalises to every manufacturer in the formulary. List every sheet a company publishes, list every chemical it sells, and take the difference. The interesting products are not the ones with a sheet; they are the ones without. Then write down, for one product, what a buyer can and cannot learn about the hazard of what they are holding, and read recording a safety data sheet for what a complete record looks like.

Read a sheet against the sheet it is supposed to be. Take the Sensitizer B document, the product description of the same catalogue number, and the kit instruction sheet, and set the three side by side. One says the bottle holds ferric oxalate and potassium chlorate; another classifies ammonium iron(III) oxalate and silver nitrate. Decide what you can conclude and — harder, and the real exercise — what you cannot.

Find the inconsistencies before you are told where they are. Both kit sheets are freely available. There are at least four in the current printings and one more that was corrected between printings. Two concern a number that contradicts itself in the same paragraph, one is a decimal point, one is a unit and one is a temperature conversion. It is a better lesson in reading a technical document than any amount of being told to read carefully.

Rebuild the drop table from the parts column. Check, for each of the five grades, that the metal holds at 0.521 or 0.522 while the two iron bottles trade places, then work out the chlorate concentration in the final mixture at each grade from the sheet’s own 0.18 g per 30 mL and 20 drops per millilitre. The five numbers you get are the actual contrast scale of the system, and their smallness explains both why the control is delicate and why a month-old bottle behaves differently.

Check the yield claim against the coating volume. The sheet says the 15 mL platinum bottle covers 12 sheets of 8 × 10 and then says about ten. Work out which the drop tables support, then convert the same tables to cubic centimetres per square metre and compare against Ware’s 24 to 36. Two of those three numbers will not agree, and deciding which to believe is the whole of the skill.

Calibrate a dropper, since neither kit supplies one. Count drops from whatever dropper you own into a tared weighing boat until you have a gram, and repeat ten times — see balance and thermometer check for the instrument. The single number you get converts every drop table in this family into millilitres, and it will not be 20 drops per millilitre.

Use the ferricyanide spot test as a redox lesson on a cheaper sensitiser. The kit’s own quality test is a Prussian blue reaction being used as an iron(II) detector. Run it instead on a classic cyanotype sensitiser: before exposure, after exposure, and after a day in the dark. It teaches act one of the mechanism directly, and it is the same measurement a platinum printer makes on a bottle worth several hundred pounds.

Test the substitution the course actually makes. The claim replacing the chlorate is that a correction curve on a digital negative delivers the same contrast without the cost. That is testable on any ultraviolet process — cyanotype is the cheapest — by printing one step tablet, measuring the exposure scale, building a curve to match it, and comparing against a print from an uncorrected negative.

Separate “truncating” from “compressing” on paper you already have. Print one negative on grade 2 and grade 4 silver gelatin paper and plot both curves. Neither is what a chlorate dose does. Draw what a truncation would look like on the same axes — the top of the scale cut off, the rest unmoved — and you have the shape Ware objects to, without mixing anything.

Assess the classification yourself, which is what Part XXVI’s assignment asks for. This entry has two independent Level D grounds with completely different logic: one about an irreversible health outcome that no control recovers from, one about a superseded technique for which a better route exists. Write the assessment for each, decide whether either alone would be enough, and then read Safety above to see whether you arrived by the same route.

Record what you find. A bought product earns a line in the formula version record as much as a mixed one does: the catalogue number, the printing of the sheet you read, the date you read it, and — for this supplier above all — which of its two safety data sheet libraries you looked in, and what was not there.