Ware's print-out platino-palladiotype

The traditional platinotype has one defect that no amount of skill removes, and it is not the price of the metal. It is that the light-struck iron cannot move. Ferric oxalate’s photoproduct is iron(II) oxalate, which is almost insoluble — 0.022 g in 100 cc of water — so it sits where it was made and cannot go and find the platinum salt. A developer has to dissolve it first, and the moment the sheet enters that bath the soluble platinum salt begins washing out of the paper too. The image is then a race between a slow reduction and a fast extraction, and the reason a traditional platinum print can come out weak, fibrous or grainy is that the extraction won.

Pradip Malde and Mike Ware began, in 1982, from the other end. Change the iron salt to ammonium iron(III) oxalate, whose anion is the well-behaved monomeric trisoxalatoferrate(III), and the photoproduct is a soluble iron(II) oxalato-complex. Give the paper enough water — which any cellulose sheet holds at 70 to 80 per cent relative humidity — and those ions can migrate a few micrometres and reduce the noble metal to the metal itself during the exposure. There is then nothing for a developer to do, and the baths that follow are there only to take the iron out.

Iron solution — 60 per cent w/v ammonium iron(III) oxalate — the only light-sensitive substance in the system

Ingredient	Quantity	Form the source specifies
Ammonium iron(III) oxalate	30 g	the trihydrate, (NH4)3[Fe(C2O4)3]·3H2O, FW 428.07, general purpose reagent grade about 98 per cent; Ware's more exact name for the anion is trisoxalatoferrate(III), and the salt is bought rather than made
Water	33 mL, added	Purified water — distilled, de-ionised or pharmaceutical. Ware adds exactly 33 cc and says the volume then "should be correct (50 cc) so it does not need to be made up"; the Platinomicon adds "check the volume is correct, 50 cc, and if not make it up". So this is a stated volume of water added, and the 50 cc is a check rather than a make-up. The solution goes cold as it dissolves, which is the ammonium ion's endothermic hydration and is the same effect the process depends on in the paper, so the beaker is warmed in a bath at about 50 °C. The result is an emerald-green solution, 60 per cent w/v, 1.40 molar, pH about 5.

Palladium solution — 19 per cent w/v ammonium tetrachloropalladate(II), Ware's Method 2 — the metal that becomes the image, made in the beaker rather than bought

Ingredient	Quantity	Form the source specifies
Ammonium chloride	1.8 g	NH4Cl, FW 53.49, weighed accurately and dissolved first in 20 cc of water, which Ware says takes no effort
Palladium(II) chloride	3 g	PdCl2, FW 177.31, well powdered and added a little at a time to the hot ammonium chloride solution; Ware attaches a dust-mask warning to this step and to no other in the handout
Water	to make 25 mL	20 cc of purified water dissolves the ammonium chloride; the solution is then heated to about 70 °C and the palladium chloride stirred in a little at a time until the brown solid has gone and the liquid is very dark red, which can take an hour. Cooled, transferred to a measuring cylinder and made up to exactly 25 cc — a make-up volume, and the reason every strength on this page is computed against it. The result is 19 per cent w/v, 0.67 molar, pH about 2, and Ware says it is stable indefinitely.

Mixed in the ratio — the working sensitiser for a palladium print

1 part Iron solution — 60 per cent w/v ammonium iron(III) oxalate + 1 part Palladium solution — 19 per cent w/v ammonium tetrachloropalladate(II), Ware's Method 2

"For a palladium print mix equal volumes of the iron and palladium solutions, which may be coated immediately." Mixing is done at room temperature under tungsten lighting, by drawing the liquid gently in and out of the delivery syringe three times, with a separate syringe dedicated to each stock.

Ware's general rule covers the whole system and not only this one case: the volume of iron solution must always equal the combined volume of platinum plus palladium solution, and the two metal solutions may be combined in any proportion within that. Equal volumes are therefore not a convenience. At 1.40 molar against 0.67 the mixture carries about 2.1 iron(III) ions for every metal(II) ion, and the reduction consumes two, which is the stoicheiometry Ware states outright in his discussion of why potassium cannot be the cation. The platinum-containing mixtures are described on this page and are not in this object, for the reasons given in provenance.note; a mixture containing platinum is also matured for an hour in the dark before coating, and a pure palladium mixture is not.

☠What this page will not tell you how to do

The system as published is three bottles: iron, platinum and palladium. The platinum bottle is not in the table above and this page gives no procedure for it. The course ruled on 4 September 2026 that students perform palladium and never handle platinum, and the Level D policy carries the argument — the governing hazard statement is 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 limit anywhere in this course. Ware’s own handout says the same thing in the practitioner’s register: never touch the surface of platinum-sensitised paper, never put ungloved fingers in the processing solutions, and if you develop the allergy then platinum printing is not for you.

What the page does give is the quantity, because Rule 6 forbids altering a published formulation by omission: the platinum solution is 25 per cent w/v ammonium tetrachloroplatinate(II), 5 g dissolved in 18 cc of purified water to a final volume of exactly 20 cc, 0.67 molar, pH about 3, left to stand 24 hours before first use. Everything this page says about platinum — why it is slower, why gelatin destroys it, why its speed falls as humidity rises — is chemistry and it is taught. None of it is an instruction.

Purpose

To reduce palladium(II) — or, in Ware’s full system, platinum(II) — to metal inside the paper while the exposure is still running, so that the print can be judged by looking at it and no developer is needed. The sensitiser is the whole of the light-sensitive chemistry and it is two bottles measured into a shot glass with syringes. The three baths that follow remove the iron, and they are clearing baths rather than developers: by the time the sheet is wet, the picture is finished.

Ware’s own account of what the redesign was for is a list of the traditional method’s faults, and it is worth having because every item on it is a chemical argument rather than a preference. Ferric oxalate is ill-characterised, polymeric, of variable composition and hard to buy. Brush coating wastes platinum. Test strips waste more. The developer is a large volume of a very poisonous salt. The customary clearing baths of dilute hydrochloric acid attack the paper. The customary contrast additive is an oxidising agent that damages the image. And the maximum density is low unless the sheet is coated twice.

The ammonium system answers each of those, and it answers them with one substitution rather than with six fixes. Printing out is not a technique bolted onto the process: it is simply what this chemistry does when the photoproduct is soluble and the paper is damp. Every other property on this page — the self-masking, the exposure by inspection, the colour control, the absence of a developer — follows from that one fact.

Recommended uses

Contact printing in palladium from a negative with a long ultraviolet density range. Ware asks for at least 1.8 and as much as 2.4, the same requirement as his other siderotypes. The sensitiser is far too slow for an enlarger, so the print is the size of the negative and contact printing is the only route; the course’s process entry for the developed relative is the palladiotype.

Where you want the image colour decided at the bench rather than in a toning tray. A palladium print made at 32 per cent relative humidity is sepia and one made at 80 per cent is a cold near-neutral, from the same bottle and the same negative. Nothing has been added to the print to do it: what changed is the size of the metal particles, and the only variable was the air.

Where consistency has defeated you with ferric oxalate. A great deal of the frustration in traditional platinum printing is the sensitiser’s own variability, and the ammonium salt is a crystalline compound of known structure that dissolves readily and keeps for years. If your prints vary and your technique does not, the iron bottle is the first suspect.

Where the metal has to be used economically. Printing by inspection removes test strips, and rod coating uses less than half the sensitiser a brush does. On a metal priced by the gram that is not a detail.

Where the paper is not what it should be. Palladium is forgiving of sizing that platinum is not, which is why Ware tells the newcomer to begin with palladium alone: it is more tolerant of shortcomings in technique and capable of fine quality on a wider range of papers.

When another formula is preferable

If you want the historical process rather than a modern one, the developed platinotype is a different object with a different chemistry, and the course teaches its developer as Willis’s potassium oxalate developer and its low-temperature relative as the sodium citrate developer. A print made by development is not a print made by print-out that happens to have been developed.

If you cannot control humidity at all and cannot wait for the weather, this is the wrong system: below 50 per cent relative humidity the print-out is only partial and the process reverts to a development process with a bath that was never designed to be one. The three-solution drop system is the developed alternative the trade actually sells, and the course records it at Level D for the platinum and the chlorate in it.

If you want a brown print on plain paper and the cost matters more than the permanence, the iron-silver family does that for a fraction of the price: Ware’s argyrotype from one bottle, or the kallitype sensitiser with a developer chosen for the colour. Neither is a noble metal and neither will last the way this does.

If you want blue, the New Cyanotype uses the same ammonium iron(III) oxalate and is the cheapest way to learn coating, humidity and negative scale before spending palladium.

If your only paper is gelatin-sized and you want platinum, no formula fixes that. Gelatin binds platinum(II) into a complex the iron(II) photoproduct cannot reduce, and the print-out route, which needs the sheet damp for hours, is the worst case for it. Palladium is unaffected.

If you would rather buy the chemistry made up, the kit route exists and the course records it separately: the Ziatype kit is a print-out process built on a lithium palladium solution instead of an ammonium one, which is a different piece of physical chemistry and not a repackaging of this.

Mixing

Three stocks, made once, and then a shot glass. Nothing here is mixed in bulk: the sensitiser is made a millilitre or two at a time, immediately before coating, by drawing measured volumes from the stock bottles with a syringe dedicated to each. Ware’s reason for the dedicated syringes is cross-contamination — a trace of iron solution in the palladium bottle will start reducing it — and his reason for the shot glass is that it is the right size and the right shape, “provided you give up drinking out of it”.

The iron bottle

30 g of ammonium iron(III) oxalate trihydrate goes into a 100 cc beaker and takes exactly 33 cc of purified water. Stirring dissolves it; the solution goes noticeably cold as it does so, and the beaker is warmed in a bath at about 50 °C to help it along. Within five minutes it is an emerald-green solution and its volume is 50 cc, which Ware asks you to check rather than to make up.

🧮Where the other 17 cc came from, and why the solution goes cold

33 cc of water plus 30 g of a solid gives 50 cc of solution, so the dissolved salt occupies about 17 cc — an apparent density of about 1.8 g/cc for the ions in solution, which is unremarkable. The arithmetic is the course’s; the two volumes are Ware’s.

The cold is more interesting, because it is the same effect the whole process runs on. Dissolving an ammonium salt is endothermic: Ware gives the enthalpy of solution of ammonium chloride as +15.2 kJ/mol and notes that it proceeds anyway because the entropy change is large and positive. The ammonium ion does not bind water tightly. Dissolving lithium chloride, by contrast, gives out heat — −37.2 kJ/mol — because the small lithium ion clamps four water molecules to itself. Hold that difference: it comes back under The mechanism as the reason this system uses ammonium salts throughout.

Filtered through a Whatman #1 paper into a brown bottle, labelled and dated, the solution is 60 per cent w/v, 1.40 molar, pH about 5, and keeps for several years in the dark.

The palladium bottle, made rather than bought

Ware prints two ways to reach the same solution and marks them EITHER and OR. Method 2 is the one in the object above because it is the cheaper of the two and because the course has encyclopaedia entries for both of its ingredients.

1.8 g of ammonium chloride is weighed accurately into a 100 cc beaker and dissolved in 20 cc of purified water, which happens easily. The solution is heated to about 70 °C and 3 g of well-powdered palladium(II) chloride is added a little at a time with stirring — Ware attaches a dust-mask warning here and nowhere else in the handout, and it belongs to the powder rather than to the solution. The brown solid takes up to an hour to disappear into a very dark red liquid, and Ware’s tip for telling whether it has gone is to look at the beaker from below. Cooled, transferred to a measuring cylinder, made up to exactly 25 cc, filtered and bottled.

🧮Why 1.8 g of ammonium chloride, and why 25 cc rather than 26

Palladium(II) chloride is only sparingly soluble in water on its own. It dissolves as the tetrachloropalladate(II) anion when there are two more chloride ions available for each palladium, and the ammonium chloride is there to supply exactly that:

PdCl₂ + 2 NH₄Cl → (NH₄)₂[PdCl₄]

Making the salt in the beaker

3 g ÷ 177.31 = 16.92 mmol

Palladium(II) chloride, FW 177.31

2 × 16.92 mmol × 53.49 = 1.81 g

Ammonium chloride needed, at two per palladium, FW 53.49

Ware’s 1.8 g is the stoicheiometric amount to within one per cent. Nothing is in excess: unlike the argyrotype, where a deliberate surplus of acid sets the pH and with it the contrast, this solution is a clean conversion with no spare reagent left to do a second job.

16.92 mmol × 284.29 = 4.81 g in 25 cc = 19.2 per cent w/v = 0.677 M

What that makes, as (NH4)2[PdCl4], FW 284.29

And that is the answer to a puzzle in the sources. Method 1 dissolves 5 g of the bought ammonium salt and makes it up not to 25 cc but to 26: 5 g ÷ 284.29 = 17.59 mmol, and 17.59 mmol in 26 cc is 0.677 M. The two methods are matched on molarity, not on mass or on volume, and both come out at Ware’s stated 0.67 M. His earlier web article makes Method 1 up to 25 cc and calls the result 20 per cent w/v, which would be 0.70 M — a four per cent difference and a real one, corrected in the 2009 handout and again in 2017. All the arithmetic in this box is the course’s, from Ware’s own weights, volumes and formula weights.

Mixing the working sensitiser

The volume of iron solution always equals the combined volume of metal solution. For a palladium print that is equal volumes of iron and palladium, mixed at room temperature under tungsten lighting by drawing the liquid in and out of the delivery syringe three times, and coated immediately. Any mixture containing platinum is left to mature for an hour in the dark first.

⚗The equal-volume rule is stoicheiometry, not convenience

The reduction consumes two iron(II) ions for every metal atom it deposits:

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

Two iron(II) per atom of palladium

Mix 1.40 molar iron with 0.67 molar palladium in equal volumes and the coated sensitiser is 0.70 molar in iron and 0.335 molar in palladium — 2.09 iron to one palladium, a hair over the two the equation needs. Ware states the same figure from the other direction when he works out why potassium cannot be the cation of a print-out sensitiser: “if one assumes that the stoicheiometric molar ratio Fe : Pt = 2 : 1 applies in the mixed solution”, and “the Malde-Ware method uses [Fe(C₂O₄)₃]³⁻ = 0.7 molar”. The ratio is designed in, and the syringes are how you keep it.

Tween 20 is added to the mixed sensitiser and never to the stock. Ware’s dose is about 0.25 per cent in the finished sensitiser, delivered as one drop — about 0.05 cc — of a 5 per cent stock per cubic centimetre of sensitiser, or one drop of a 10 per cent stock per 2 cc. Absorbent papers may need none at all; hard-sized ones such as Buxton coat more evenly with it. It does not keep in dilute solution, the right amount depends on the paper, and it interacts badly with gelatin sizing — which are three separate reasons for keeping it out of the stock bottle.

Coating is by glass rod, five to seven passes, about 1.5 cc for the area of a 10 by 8 inch print, with a wide uncoated border for handling and any excess blotted off before it can crystallise and scratch a negative. The room wants to be at 18 to 22 °C: too cold and the sensitiser crystallises, too warm and it soaks in too deep.

The processing solutions

Three baths, made up in bulk and used in a fixed order. Disodium EDTA at 5 per cent w/v, 100 g in 2 litres. Sodium metabisulphite at 2.5 per cent w/v, 25 g — a level tablespoonful — in 1 litre, made fresh for the session. Tetrasodium EDTA at 5 per cent w/v, 100 g in 2 litres. Ware defines per cent w/v in the same handout as X grams of solute in 100 cc of solution, so these are make-up volumes. The course keeps its clearing baths on their own page, at the EDTA, citric acid and hydrochloric acid clearing sequence, where the two EDTA salts can be compared properly; what matters here is that the sequence is part of this formula and not an accessory to it, and that its order is the argument.

Behaviour

It prints out, which changes how you work more than what you get. Under full print-out conditions the image appears during the exposure and you go on exposing until the highlight detail resolves. The shadows do not block up the way a development paper’s do, because print-out is self-masking: metal already deposited absorbs the ultraviolet that would deposit more. A hinged-back frame lets you look.

Humidity is the main control and it works in opposite directions for the two metals. This is Ware’s own table, and it is the most useful page in his handout.

Sensitiser	RH %	Relative speed	Exposure range ΔlogH	Development, logH	Colour
Platinum	32	1.8	1.5	0.9	warm black
Platinum	55	1.7	1.5	0.3	warm black
Platinum	80	1.0	1.8	0	neutral
Palladium	32	0.5	2.0	0.4	sepia
Palladium	55	1.3	2.2	0.2	Van Dyke brown
Palladium	80	2.5	2.4	0	warm black
Platinum-palladium 3:1	32	1.2	1.6	0.6	warm black
Platinum-palladium 3:1	55	1.0	2.0	0	neutral
Platinum-palladium 3:1	80	1.0	2.2	0	neutral

Speed is relative and arithmetic, referring to the middle tones. The exposure range runs from fog + 0.04 to 0.9 of the maximum density. The development column is in logH units, where 0.3 is one stop and zero means total print-out. Ware warns that every one of these numbers moves with the paper.

Three things fall out of it. Palladium gets five times faster between 32 and 80 per cent relative humidity, and platinum gets slower, which is the counter-intuitive result Ware devotes a section to. The development column goes to zero for both metals at 80 per cent, which is the definition of the process working as intended. And the exposure range widens with humidity, so a damp sheet accommodates a longer negative — the opposite of the usual relationship between a soft printing material and a soft negative, because the mechanism is self-masking rather than a change of gradient.

⁇The tables do not all agree, and one of them disagrees with its own book

Three printings of this table exist. The 2009 handout and the 2017 Platinomicon agree in every cell the course can read, which is what allows this page to trust its text mirror of the handout; the Platinomicon adds a fourth block for a 1:1 mixture. Ware’s undated web article differs: it gives platinum exposure ranges of 1.2, 1.2 and 1.5 rather than 1.5, 1.5 and 1.8, and its palladium colour column reads Van Dyke brown, sepia, neutral where the other two read sepia, Van Dyke brown, warm black. The web article is the earliest of the three and the course takes the two later, agreeing printings as the figures.

The remaining disagreement is inside the 2017 book. Table 7.1 calls palladium at 80 per cent relative humidity “warm black”; §11.14, sixty pages later, says the palladium sensitiser passes “through a pleasing purplish-brown at 55% RH to a neutral grey-black at 80% and above”; and §7.11 says that “when well-humidified, near-neutral tones can be achieved”. Two of the three say near-neutral. The course records the table as printed and notes that the direction — warmer when dry, colder when damp — is unambiguous in every source, and that the exact word for the wet end is not.

Carbon dioxide comes off the sheet while it prints, and it can blur the image. Every siderotype throws away an oxalate ligand as gas, and Ware works out how much: at a specific coating volume of 25 cm³/m² and 0.7 molar ferrioxalate, complete conversion releases 0.0175 mol/m², which is 392 cm³ of gas per square metre at standard temperature and pressure — a layer about 0.4 mm thick if it cannot escape. A separation of 0.1 mm between negative and paper is enough to blur an image perceptibly on a light bed. That is why the sheet must be backed with something gas-permeable, a papermakers’ felt rather than a plastic sheet, and it is also why Ware masks the borders: an unnecessary expanse of heavily exposed sensitiser makes gas the picture cannot use.

The masked border is a test as well as a preference. A margin that was coated and never exposed shows residual iron as a yellow stain against the uncoated paper beside it, under a bluish light. Ware calls it a cruelly demanding test and recommends it for exactly that reason: if the border is clean, the print is clear.

Image characteristics

Colour is a nanoparticle-size effect and humidity is how you set it. The image is metal in particles of roughly 15 to 25 nm, small enough that their colour is a surface plasmon resonance rather than the colour of bulk metal. A large reservoir of water in the fibres lets more material react locally, so the particles grow larger and the image reads more neutral; a restricted pool constrains the reaction to small particles, which read brown or sepia. That single sentence accounts for most of the colour column in the table above.

Ware lists eighteen factors that move the colour and the useful ones divide into two groups. Warmer: more palladium in the mixture, more Tween, gelatin sizing, residual iron from imperfect clearing, mercury(II) or lead(II) salts in the sensitiser, and vacuum contact rather than a pressure frame. Cooler: higher relative humidity before exposure, higher temperature, longer exposure at lower intensity, humectants in the sensitiser, post-exposure steaming, and linen paper rather than cotton. To make a genuinely neutral palladium print you have to get most of them right at once, which is why it is difficult and why Ware says it can be done.

A mixed sensitiser does not give a proportionally mixed image. This is the finding that should change how anyone reads the phrase “platinum-palladium print”. Ware’s X-ray fluorescence measurements on a sensitiser mixed 1:1 by moles found the coating matched the solution before exposure and did not after it: the image ran about 1:3 platinum to palladium at low exposures, levelling off near 3:4 at maximum density. A later measurement on a paper without gelatin sizing gave about 1:2 in the middle tones and reached 1:1 only in the deepest shadows. Palladium prints out about 2.5 times faster in the middle tones, and the platinum that has not reacted by the time the sheet goes into the first bath is washed away. Ware’s conclusion is arithmetic: if you want equal molar amounts of the two metals in the image, coat with a sensitiser at Pt:Pd of 2:1 or more.

Surface and scale are the noble-metal family’s. Matte, no binder over the image, the metal lodged among the surface fibres, a maximum density well below a glossy silver-gelatin print’s, and — the compensation Ware claims and the conservation literature echoes — exceptional separation in the middle values and delicacy in the high ones. The print dries down and is judged wet at your peril.

The mechanism

Four steps, and the second is the one that makes this process different from every other noble-metal print in the formulary.

1. Light breaks up the trisoxalatoferrate(III) ion

The photosensitive species is the anion [Fe(C₂O₄)₃]³⁻, a chiral octahedral complex Ware likens to a ship’s propeller. It absorbs in the ultraviolet at a ligand-to-metal charge transfer band with a maximum at 260 nm, and absorption there transfers an electron from one oxalate ligand to the iron:

hν + [Fe^III(C₂O₄)₃]³⁻ → [Fe^II(C₂O₄)₂(·C₂O₄)]³⁻

Hatchard and Parker's first step: an intramolecular electron transfer

The radical anion is in dissociative equilibrium with the complex, and when it meets another unexcited iron(III) complex it hands over the second electron and leaves as two molecules of carbon dioxide:

[Fe^III(C₂O₄)₃]³⁻ + ·C₂O₄⁻ → [Fe^II(C₂O₄)₂]²⁻ + C₂O₄²⁻ + 2 CO₂

The second electron, and the gas

hv + 2 [Fe(C₂O₄)₃]³⁻ → 2 [Fe(C₂O₄)₂]²⁻ + C₂O₄²⁻ + 2 CO₂

Overall stoicheiometry of the photolysis: iron(III) on the left, iron(II) on the right

The ideal quantum yield would be 2, one iron(II) for each electron; the measured value is about 1.2 between 250 and 420 nm, falling to 0.9 at 500 nm and very sharply thereafter, because a dark back-reaction competes. It is largely independent of pH.

2. The photoproduct is soluble, and that is the whole idea

Compare the two iron salts on this one point and everything else follows.

hv + Fe₂(C₂O₄)₃ → 2 FeC₂O₄ + 2 CO₂

Ferric oxalate: the photoproduct is insoluble iron(II) oxalate

Iron(II) oxalate dissolves to the extent of 0.022 g in 100 cc of water and has a polymeric linear-chain structure. It cannot reduce platinum(II) or palladium(II) until something dissolves it, and the something is a concentrated bath of oxalate ions — which is precisely what Willis’s potassium oxalate developer is:

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

What the developer is actually for

The ammonium salt skips that step entirely. Its photoproduct, [Fe(C₂O₄)₂]²⁻, is already a soluble complex ion, so if there is water in the paper the reduction can proceed on the spot. Ware’s summary is the plainest statement of the design: “if the sensitized paper contains a sufficiency of water molecules, as will be the case for any cellulose paper exposed to an ambient relative humidity of 70–80%, the ions can migrate to reduce the platinum(II) to Pt metal in situ.”

3. Water is a reagent, and the cation decides how much you need

Not all water in a paper is available. A cation that binds water tightly into a primary hydration sphere takes it out of circulation, and the sheet has to become much wetter before any of it is free to mobilise the iron and metal anions. Lithium is the worst offender — hygroscopic, deliquescent, and holding what it absorbs — and Ware records the practical consequence: a lithium sensitiser has to be brought to a state of wetness that is uncontrolled and liable to damage a negative in contact.

The ammonium ion does the opposite. It is close to water in size and shape, it disrupts water’s structure rather than organising it, and the enthalpies of solution show it directly: ammonium chloride dissolves endothermically at +15.2 kJ/mol, driven by the entropy of that disruption, where lithium chloride dissolves exothermically at −37.2 kJ/mol. So with ammonium as the sole cation, “only a relatively small amount of water need be absorbed to provide sufficient ion mobility to enable a print-out process”.

⚗Two cations that are excluded, for two different reasons

Potassium is excluded by solubility, not by hydration. Potassium ferrioxalate, K₃[Fe(C₂O₄)₃]·3H₂O, dissolves to only about 10 g per 100 cc at 20 °C, and Ware computes a solubility product from that and tabulates the potassium concentration at which a given ferrioxalate concentration will crystallise. At the 2:1 iron-to-metal ratio this system uses, the critical limit is 0.456 molar in ferrioxalate — and the Malde-Ware sensitiser runs at 0.7 molar. Put potassium tetrachloroplatinate(II), the salt Willis and every successor used, into an ammonium ferrioxalate sensitiser and the mixed solution can throw crystals during coating. That is why the platinum salt of this system is the ammonium one, and it is a constraint the traditional process never met because ferric oxalate solution contains little or no free trisoxalatoferrate(III) to precipitate.

Calcium is excluded by its oxalate. Calcium oxalate is about as insoluble as calcium carbonate, and its formation strips oxalate off the iron complex in three successive steps until the iron is a bare aquo-ion that hydrolyses to a stain:

Ca²⁺ + [Fe(C₂O₄)₃]³⁻ + 2 H₂O → CaC₂O₄ + [Fe(C₂O₄)₂(OH₂)₂]⁻

Chalk taking the first oxalate

Ca²⁺ + [Fe(C₂O₄)(OH₂)₄]⁺ + 2 H₂O → CaC₂O₄ + [Fe(OH₂)₆]³⁺

…and the last

This is the chemistry behind the flat instruction to avoid alkaline-buffered paper, and it is why the paper article matters as much as the sensitiser.

4. Iron(II) reduces the noble metal

The photochemically generated iron(II) oxalato-complex is a moderate reducing agent — E([Fe(C₂O₄)₃]³⁻/[Fe(C₂O₄)₂]²⁻) = +0.02 V — and everything it has to reduce sits well above it: +0.62 V for [PdCl₄]²⁻/Pd and +0.73 V for [PtCl₄]²⁻/Pt. Thermodynamically both are downhill by a wide margin. What separates them is kinetics.

⚗Aquation, and why a fresh platinum solution is not ready to use

There is no easy path for putting electrons onto a square-planar MCl₄²⁻ ion. The reaction goes more readily through the aqua-complex, in which one chloride has been replaced by water:

H₂O + [MCl₄]²⁻ ⇌ [MCl₃(H₂O)]⁻ + Cl⁻

Aquation of the tetrachlorometallate(II) anion

The equilibrium constant at 20 °C is 0.17 for palladium and 0.015 for platinum, so at sensitiser concentrations more than half the palladium is aquated and less than a fifth of the platinum is. Worse, the platinum equilibrium is slow to establish — a measured half-time of 2.4 hours, so ten half-lives is about a day. That is the whole reason Ware’s platinum solution must stand 24 hours before first use, and it is a good example of an instruction that looks like superstition until you know the rate constant.

Ware’s proposed pathway for the electron transfer itself is outer-sphere, through a symmetrical hydrogen-bond bridge between the aquated iron and the aquated metal, with hydrogen atom transfer one way balanced by proton transfer the other; a second iron(II) completes the job:

2 [Fe(C₂O₄)₂(OH₂)₂]²⁻ + [PtCl₃(H₂O)]⁻ → 2 [Fe(C₂O₄)₂(OH₂)₂]⁻ + Pt + 3 Cl⁻ + H₂O

The overall reduction, as Ware writes it

He marks the mechanism as unelucidated and the bridge as probable rather than established, and this page marks it the same way.

Deeper: why platinum is the difficult metal

Palladium and platinum are almost the same size — the lanthanide contraction sees to that, giving metallic radii of 137.3 and 138.5 pm and square-planar ionic radii of 78 and 74 pm — but platinum is a third-row transition metal and palladium a second-row one. The greater spatial extent of the 5d orbitals gives platinum complexes larger ligand field activation energies and slower substitution kinetics. Palladium(II) is labile and is reduced easily; platinum(II) is not.

Three consequences show up in the darkroom. Platinum is slower, and can lose its salt to the first bath before it has reacted, which is exactly the failure mode the print-out route was designed to avoid. Platinum is poisoned by gelatin: proteins bind platinum(II) into a complex the iron(II) cannot reduce, the reaction takes hours and needs water, and a humid print-out sheet gives it both. Ware is emphatic that this is on scientific record rather than a matter of opinion, and that the nineteenth century’s insistence on bone-dry platinum paper was an unwitting way of outrunning it. Platinum shows an “inhibited edge” — a deficiency of image along the border between a very dense area and a moderate one, absent in palladium and absent when the platinum sheet is dried and developed instead. Ware’s best candidate is the chloride ion liberated by the aquation and the reduction, diffusing out of the dense region and pushing the aquation equilibrium backwards where it lands. He offers it as the most likely explanation and not as a demonstrated one.

Function of every ingredient

Ammonium iron(III) oxalate, 30 g — the only thing light acts on. Everything else in the system is waiting. The salt is ammonium trisoxalatoferrate(III) trihydrate, a crystalline compound of known structure containing the discrete monomeric anion, and Ware chose it over ferric oxalate on four grounds: it is universally available at low cost in consistent purity, it dissolves readily to a solution stable for years, its photoproduct is soluble, and it appears to be immune to the “plague of black spots” that sometimes afflicts ferric oxalate prints. More of it than 60 per cent w/v is not available: the solution is close to saturation already, which is why it throws green crystals if it is left below 20 °C. Less of it lowers the metal coating weight the sensitiser can carry and with it the maximum density, and it also moves the mixture away from the 2:1 iron-to-metal ratio the reduction needs, so the metal cannot all be used. The excess over stoicheiometry is only about five per cent, which is a smaller margin than it looks: the photolysis converts only part of the iron, so the working excess is the whole reason the shadows can reach full density.

Palladium(II) chloride, 3 g — the image. In the finished print this is metallic palladium in nanoparticles among the paper fibres, and it is what you are looking at. As bought it is a brown solid that dissolves poorly in water; it goes into solution as the tetrachloropalladate(II) anion once the extra chloride is there, which is what the hot ammonium chloride solution provides. More of it, at fixed volume, is not an option Ware offers: 19 per cent w/v is already what the equal-volume rule pairs with a saturated iron bottle, and raising it would leave the iron unable to reduce it all. Less of it gives a thinner coating weight of metal and a lower maximum density — historically the reason the traditional process resorted to double coating, and a problem the ammonium salts avoid by being more soluble than the potassium ones. The choice of palladium over platinum is the choice of a faster, more forgiving metal that gives warmer images, tolerates gelatin, and — as Ware notes and this page repeats because it is a permanence claim — is somewhat less resistant to later chemical attack than platinum.

Ammonium chloride, 1.8 g — two chlorides and a cation, and both jobs matter. The obvious job is the stoicheiometric one worked out under Mixing: two chloride ions per palladium, converting a sparingly soluble salt into a soluble complex anion. The unobvious job is the cation it leaves behind. Ammonium is the structure-breaking cation this whole process depends on, and sodium or lithium chloride would have donated the same two chlorides while bringing in a cation Ware spends a section arguing against. He does not say that at this step, so read it as the course’s inference from his cation argument rather than as his statement. More of it than the stoicheiometric amount would add free chloride, which pushes the aquation equilibrium of step 4 backwards and makes the metal harder to reduce — the same effect Ware proposes as the cause of the inhibited edge. Less of it leaves undissolved palladium chloride in the beaker, which you will see by looking at it from below. Note too what it is not doing: it is not a humectant, and the ammonium chloride used as a saturated solution in the humidifying box at 80 per cent relative humidity is a different use of the same substance and never touches the paper.

Water, 33 cc to the iron and 25 cc of make-up to the palladium. Purified — distilled, de-ionised or pharmaceutical — and the reason is calcium. Tap water hardness will precipitate calcium oxalate in the sensitiser and start the hydrolysis cascade under The mechanism. The processing baths are allowed tap water because by then the image exists and the EDTA has capacity to spare, although Ware notes that a hard-water district weakens a clearing bath by consuming its chelator on calcium.

Tween 20, about 0.25 per cent in the mixed sensitiser and not in the object above. A non-ionic surfactant, added drop by drop from a 5 or 10 per cent stock so that the dose can be tuned to the paper. It helps a hard-sized sheet accept an even coating and helps the sensitiser reach into the fibres. More of it warms the image, because adsorbed surfactant stabilises small nanoparticles and stops them growing — Ware’s figure for a neutral palladium print is no more than 0.1 per cent and preferably none. It also interacts unfavourably with gelatin sizing. It is not in the formula object because Ware prints it as a separate stock at a strength the reader chooses, added to the working sensitiser rather than to any bottle.

Ammonium tetrachloroplatinate(II), 5 g — described, not tabulated. The third stock is 25 per cent w/v, 0.67 molar, and its function in the system is to be the other image metal: neutral rather than warm, higher in contrast and maximum density, and slower. Ware chose the ammonium salt over Willis’s potassium tetrachloroplatinate(II) for the solubility reason set out above — potassium would crystallise the ferrioxalate — and gained a second advantage, since the more soluble salt supports a higher metal coating weight and makes double coating unnecessary. More platinum in the mixture raises contrast and neutrality and lowers speed; less does the reverse. That is the whole of what this page will say about handling it, and the reasons are in the callout at the top.

Interactions

Iron against metal: fixed by design, and the one ratio you should not improvise. Two iron(II) per metal atom, delivered by equal volumes of 1.40 and 0.67 molar solutions. Departing from it in either direction wastes the expensive half of the formula.

Platinum against palladium: not what you mixed. Covered under Image characteristics, and it is the interaction most likely to mislead a printer, because the sensitiser’s ratio and the image’s ratio are different numbers and only the second one is the print.

Humidity against metal: opposite signs. Palladium speeds up and platinum slows down as the sheet gets damper. A 3:1 platinum-palladium mixture therefore has speed and contrast that barely move between 40 and 70 per cent relative humidity, which is a genuine engineering result: two components whose humidity coefficients have opposite signs, blended to cancel. It is Ware’s recommendation for anyone who does not want to own a humidity box.

Chloride against reduction. Every aquation and every reduction liberates chloride, and chloride pushes the aquation equilibrium back. This links three otherwise separate observations: the inhibited edge, the historical success of mercury(II) and lead(II) additives — which work by scavenging chloride, with Hg²⁺ + 4Cl⁻ having a formation constant of 1.2 × 10¹⁵ — and the platinum solution’s need to mature. The course records the mercury and lead routes at the lead contrast additions and the mercuric sepia platinotype and does not teach them.

Gelatin against platinum, and not against palladium. A paper decision that is really a chemistry decision, and the only one in the system where the two metals want different materials.

The three baths against each other, in order. This is where the sequence earns its name. Bath 1, disodium EDTA at pH 3 to 4, is acid enough to suppress hydrolysis and is at the optimum pH for chelating iron(III):

Na₂H₂EDTA + Fe³⁺ → [Fe(EDTA)]⁻ + 2 Na⁺ + 2 H⁺

Bath 1: iron(III) chelated in acid, as Ware writes it

Bath 2, sodium metabisulphite, reduces whatever iron(III) is still chemisorbed to the cellulose down to iron(II), which binds cellulose less strongly and hydrolyses less:

2 Fe³⁺ + SO₃²⁻ + H₂O → 2 Fe²⁺ + SO₄²⁻ + 2 H⁺

Bath 2: the residue reduced

Bath 3, tetrasodium EDTA at pH 9 to 10, is at the optimum pH for chelating iron(II) and takes out the last of it, leaving the sheet alkaline, which suits the paper’s own keeping. Swap baths 1 and 3 and you get the failure Ware warns about by name: an alkaline first bath does not complex iron(III), it hydrolyses it, and the print is stained yellow or brown. Matthew Clarke and Dana Hemmenway’s X-ray fluorescence comparison found this sequence left less residual iron than any other combination they tested — comparable with, or lower than, the iron already present in the uncoated paper.

Time against irreversibility, in the clearing. Freshly formed iron(III) hydroxide redissolves in dilute acid; left alone it transforms into iron(III) oxyhydroxide, FeO(OH), the mineral goethite, which does not. All the iron has to come out before the print dries, and that is why the sequence is long and why a shortened wash is a permanence decision rather than a convenience.

Variants

Method 1 for the palladium bottle. Buy ammonium tetrachloropalladate(II) and make 5 g up to exactly 26 cc. It is Ware’s own alternative, printed beside Method 2 as EITHER and OR, and it lands on the same 0.67 molar solution. The course cannot enter it in the object because the encyclopaedia has no page for that salt; the arithmetic showing the two are equivalent is under Mixing.

The web article’s printing. Ware’s undated web account differs from the 2009 handout in five places and this page records all of them rather than blending: the bought palladium salt is made up to 25 cc and called 20 per cent w/v; the wet-processing sequence is three baths, with Kodak Hypo Clearing Agent between the two EDTA baths rather than a metabisulphite bath of its own; a 20 per cent Tween stock is specified for pure platinum, giving about 1 per cent in the sensitiser, against the handout’s 10 per cent stock and 0.5 per cent; the palladium salt’s cheaper preparation heats to 70–80 °C rather than about 70; and the table of sensitiser characteristics differs as described above. Where the two later printings agree against it, this page follows them.

The 2014 print-out palladiotype. Ware published a palladium-only version of the same system as its own set of Siderotype Workshop Notes, with the metabisulphite bath explicit in the outline procedure as “Reduce and Clear”. A reader who wants this chemistry without any platinum in it at all should read that document; the course has it in the bibliography and has not yet given it a formulary entry of its own.

Potassium oxalate in place of bath 1, for pure platinum. Where a 100 per cent platinum print still lacks highlight detail or shows the paper’s fibre structure, Ware offers the traditional platinotype developer — about 30 per cent w/v potassium oxalate, which he marks poisonous — as a more energetic first bath, and says it may even be used hot. It is the traditional process’s developer doing the traditional process’s job in the middle of a print-out workflow, and it is an admission that pure platinum does not always print out completely.

One drop of palladium into a platinum sensitiser. Ware calls it “a cheat” and reports that it improves print-out, apparently by acting as a catalyst, and that re-used first bath containing traces of palladium probably works the same way. He offers it as an observation without a mechanism and this page does the same.

Making the EDTA salts you cannot buy. Ware gives two conversions. Tetrasodium EDTA becomes the disodium salt with 31 g of citric acid per 100 g, and he notes the citrate can only help the clearing. From the free acid H₄EDTA, about 35 g in a litre with 9.6 g of sodium hydroxide gives the disodium salt and 19.2 g gives the tetrasodium; the equivalent doses of anhydrous sodium carbonate are 12.7 and 25.4 g and of sodium bicarbonate 20.2 and 40.3 g. These belong to the clearing sequence page and are recorded here because they are printed in the same chapter as the sensitiser.

Contrast additives, which this system does not use. Ware explicitly declines potassium chlorate, potassium dichromate and hydrogen peroxide as contrast agents, on the ground that they do not contract the tonal scale uniformly but simply truncate the high values, and that chlorate causes graining. The course’s drop system page sets out the alternative that does use them and why it is classified where it is.

Safety

Level B for the two solutions in the object, and Level D for the platinum bottle. The split is not a hedge: it is what the substances are.

The platinum salt. Everything in the callout at the head of this page. H334 in 99.5 per cent of 212 notified reports for the closely related potassium salt; EH40’s halogeno-platinum entry at 0.002 mg/m³ as platinum with the Sen notation, against 5 mg/m³ for platinum metal — a limit 2,500 times tighter for the compound than for the element; sensitisation that may take months to appear and for which the only remedy is to stop being exposed. Ware records that platinum allergy was first described in 1911 as an occupational disease of photographic factory workers handling platinotype paper, and adds the reassurance that goes with it: platinum metal is not implicated, so a finished platinotype is harmless to handle.

The palladium salt. Palladium(II) chloride carries signal word Danger, with H301 toxic if swallowed and H302 harmful if swallowed both in the notified record, H317 may cause an allergic skin reaction, H318 causes serious eye damage, H335 may cause respiratory irritation, and H400 and H410 for aquatic life. Ware’s own reading is that palladium salts are moderately toxic and not known to be dangerously irritant or allergenic, and that they are the safer alternative for someone who has developed platinum allergy. The deciding operation is weighing and transferring the dry powder, and it is the one step in the whole preparation to which Ware attaches an explicit dust-mask instruction. Note that EH40 lists no palladium compound at all, and that its own introduction says absence from the list does not mean a substance is without risk.

The iron salt. Soluble oxalates are systemically toxic — kidney damage is the classic injury — and the sensitiser is a 60 per cent solution of one. Ware’s assessment is that several grams would have to be swallowed to do that, that the salt is large non-volatile crystals with little inhalation risk, and that it is an irritant to skin, eyes and mucous membranes. EH40 gives oxalic acid 1 mg/m³ long-term and 2 mg/m³ short-term. Do not put ungloved fingers in the first processing bath: it contains everything that came off the print.

The ultraviolet source. Installed so that your eyes cannot see it directly, or goggles. Skin exposure minimised. Ware adds a specific instruction that is easy to overlook: avoid lamps with significant UVB output, because UVB is much more destructive to tissue and can generate ozone.

What is not a hazard here, and why. There is no volatile solvent, nothing is heated above about 70 °C, and the only heating step is a beaker of water. Nothing evolves a toxic gas — the gas the process does produce is carbon dioxide, and the reason it matters is optical rather than toxicological. The clearing baths are near-neutral and alkaline chelators at 2.5 to 5 per cent, which is a milder tray chemistry than an acid stop bath. And a finished print is inert: noble metal in paper, with no residual sensitiser if the clearing has been done properly, which is the point of the yellow-margin test.

Storage

The iron bottle is the one with a shelf life you can watch. Brown glass, dark, room temperature, several years. Two kinds of crystal appear in it and they mean different things. White needles after a few days are ammonium oxalate, and they are the visible end of an equilibrium: a 1 molar trisoxalatoferrate(III) solution carries about 0.01 molar free oxalate in equilibrium with the complex, and that is enough to exceed the ammonium salt’s solubility. Re-filter and carry on. Green crystals after a spell below 20 °C are the ferrioxalate itself, because the solution is close to saturation; warm gently and swirl.

The palladium bottle made by Method 2 is stable indefinitely. Filtered, stoppered, labelled, dated.

The platinum bottle keeps a year or so and is not stable indefinitely, and separately it is not ready for a day after it is made, for the aquation reason above. Those are two different clocks running in opposite directions on the same bottle.

Mixed sensitiser containing platinum: the sources disagree and the page says so. The 2009 handout and the Platinomicon both say the mixed sensitiser “appears to be stable for years”, so that a batch can be prepared ahead of a session; the web article says to leave it an hour or two “but no longer”. The course does not choose between them and notes that they may be answering different questions — one is about maturation, the other about keeping.

Sensitised paper is not stock. A few hours at room humidity, or six months light-tight and air-tight with silica gel or anhydrous calcium chloride below 10 per cent relative humidity. The failure mode of storing it damp is chemical fogging, and the failure mode of coating it and printing it days later is a slow reaction between the platinum salt and whatever is in the paper.

The metabisulphite bath does not keep at all, because sulphite is oxidised to sulphate by air. Make it for the session and pour it away. The Tween stock does not keep well either.

Incompatibilities

Chalk-buffered paper, for the calcium oxalate cascade set out under The mechanism. Where a buffered paper is unavoidable Ware pre-treats it in 5 per cent v/v hydrochloric acid and washes it, and records a more recent recommendation of a 10 per cent sulphamic acid bath for 20 minutes. He is explicit that oxalic acid must not be used for this, because calcium oxalate is as insoluble as the carbonate it would replace.

Gelatin sizing, for platinum only. Palladium is indifferent to it.

Potassium ions anywhere in the sensitiser, for the ferrioxalate solubility reason. This is the one incompatibility that would not occur to a printer coming from the traditional process, because potassium tetrachloroplatinate(II) is what that process uses.

Lithium and sodium as cations of a print-out platinum sensitiser. Lithium totally inhibits print-out in pure platinum, sodium works poorly, and both can cause solarisation — a reversal of the tonal scale in heavily exposed regions — when used at low humidity. They are usable with the more energetic metals, palladium and gold, at the cost of higher hydration and more risk to the negative.

Tetrasodium EDTA as the first bath. It is alkaline, iron(III) is not effectively complexed at pH 10 but hydrolysed, and the result is a yellow or brown stain. Ware names a supplier who has recommended it and says plainly that it is wrong.

Gold(III) with a ferrioxalate sensitiser, if anyone is tempted to tone in the bottle: gold(III) oxidises the free oxalate that is always present in equilibrium, precipitating gold and evolving carbon dioxide. Mixed gold-palladium sensitisers are possible but need cool, quick working.

Metal utensils and cross-contaminated syringes. The noble metals plate out on base metal, and a trace of iron solution in a metal bottle starts reducing it. Glass, plastic, and one syringe per bottle.

Waste

The first clearing bath is the valuable stream and should not go down the drain. Ware’s instruction is to save the spent disodium EDTA bath for recovery of precious metals — it carries whatever platinum and palladium did not become image, which on a platinum-containing print can be a large fraction of what was coated. Bottle it, label it, and keep it.

The iron is the environmental question rather than the economic one. All three baths leave carrying iron held as a soluble EDTA chelate, which is what makes the stream different from a plain rinse: the chelate keeps the iron in solution rather than letting it settle, and EDTA itself is poorly degraded in conventional treatment.

Sensitiser you do not use is the expensive mistake. The reason Ware tells you to tune the coating volume by experience, and to mask the borders rather than coat generously, is that everything beyond about 1.5 cc for a 10 by 8 sheet is metal you have bought and thrown away.

Check your local regulations. They govern, they differ, and nothing on this page is a substitute for them. The course’s disposal page sets out the general practice.

Troubleshooting

A yellow stain in the masked margin. Residual iron. Prolong bath 3. Look under a bluish light, which is where the stain is most conspicuous. If it has been left until the print dried, the iron may already have converted to goethite and become insoluble in the ordinary baths.

Weak maximum density in a platinum print, or an image that looks fibrous or grainy. In order of likelihood: gelatin in the paper; a sensitiser that was not matured for an hour; insufficient print-out because the sheet was too dry; and a first bath that washed the platinum out before it reacted. The remedy for the third is a generous post-hydration; for the fourth, the more energetic oxalate bath.

Highlights that will not resolve. Post-hydrate — “steam” the print over water at 40 °C — before the first bath. Ware gives 1 to 2 minutes generally and 2 to 4 minutes, or 20 to 30 minutes at room temperature, for pure platinum.

Maximum density falls off when the paper is very damp. Above 80 per cent relative humidity the sensitiser diffuses too deep into the sheet, and beyond an hour at 100 per cent you get weakened density plus clearing problems. Over-humidified paper is also the likeliest way to damage a negative in the frame.

A soft, blurred image, worse in areas of high local contrast, and worse under a light bed than a point source. Trapped carbon dioxide. Back the sheet with a permeable felt, never with plastic, and mask the unused margins so that less gas is made in the first place.

Points of light reflecting off the dried sheet. Crystals of sensitiser on the surface, from a room that was too cold or a coating that was too heavy. They will scratch a negative. Blot excess sensitiser before drying, and warm a crystallising sensitiser with a drop or two of warm distilled water before coating.

Fog on paper that was coated some time ago. It was not dry enough or not desiccated. Sensitised paper keeps for six months only below 10 per cent relative humidity.

An unexplained deficiency of image at the edge of a dense area, in pure platinum. The inhibited edge. It is not a coating fault; masking or cutting off the dense area before exposure makes it disappear.

Reversed tones in the heaviest shadows. Solarisation, associated by Ware with sodium or lithium cations at low relative humidity, and with the rapid reduction of palladium in development processing.

Experiments

Everything below is written for palladium alone, which is the half of Ware’s system this course teaches.

The humidity ladder. Coat six identical sheets, humidify pairs at 32, 55 and 80 per cent relative humidity over saturated calcium nitrate tetrahydrate, saturated common salt and saturated ammonium chloride, and print a step wedge on each under identical exposure. Read the relative speed at the middle tones, the exposure range from fog + 0.04 to 0.9 of maximum density, and the image colour, and set them beside Ware’s table. This is the single most informative afternoon available in the process, because it measures the one variable that has no analogue in any silver process.

Print-out versus development, on one sheet. Expose two sheets identically at 80 per cent relative humidity, then process one immediately and give the other 20 minutes of post-hydration first. The difference is the “development” column of Ware’s table made visible.

The Tween series. Four sheets at 0, 0.1, 0.25 and 0.5 per cent Tween in the sensitiser, one paper, one humidity, one exposure. Predict from the mechanism which will be warmest before you look, and record whether the coating quality changes in the opposite direction from the colour.

The clearing audit. Coat and mask a sheet, expose it, and process it through bath 1 only. Process a second through baths 1 and 2, and a third through the full sequence. Compare the masked margins under a bluish light, then put all three away and look again in six months. This is a permanence experiment disguised as a clearing experiment, and the fact that it takes six months to answer is the point.

The paper trial. The same negative on an unbuffered paper, a buffered paper straight from the pack, and the same buffered paper after an acid pre-treatment and wash. Record the highlight detail and any yellowing in the margins, not just the overall look.

Coating economy. Weigh the sensitiser you use per print for ten prints and work out your own specific coating volume in cm³/m². Compare it with Ware’s 25 cm³/m² assumption. At the price of palladium this is an experiment with a payback period.