Lab: Preparing the Sensitiser and Coating for Palladium

Purpose

To make about two millilitres of palladium sensitiser for one sheet, spread it evenly over a measured area of unbuffered cotton paper, dry it at a recorded humidity, and come away with a coated sheet, a written record and a measured figure for what your own dropper actually delivers.

Two millilitres is the whole session. That is the difference between this lab and every other coating lab in the course: the volume is trivial and the discipline is not, because the solution in the shot glass costs more per millilitre than anything else you will handle. Nothing is mixed in advance, nothing is mixed for two sheets when one is being coated, and the rehearsal happens before the metal comes out of the bottle.

Platinum is not in this session and will not be. The course teaches platinum fully — its chemistry is the previous lesson, its history is there too, and the economic decision is argued out two pages on — and never asks you to open a bottle of it. The Safety classification section gives the reasoning rather than the assertion.

Learning objectives

By the end of this session you will be able to:

• State what is in each bottle of a platinum or palladium kit and what it does, including which one is light-sensitive and which is not.
• Measure the volume of one drop from your own dropper and convert any published drop count into millilitres for your own sheet size, instead of trusting a number written for somebody else’s bottle.
• Judge a bottle of ferric oxalate by the supplier’s own ferricyanide spot test, and say what a deep blue in the dark means as against a deep blue formed towards a lamp.
• Coat a sheet by glass rod at a volume you chose, and state the coverage you achieved in cubic centimetres per square metre.
• Measure and record the relative humidity at which the sheet was coated and dried, and say what a coating dried below about 30 per cent is likely to do at the tray.
• Choose a paper on chemical grounds — sizing, buffering, filler — rather than on how it feels.
• Explain what a student who cannot afford this session should do instead, and what that substitution costs.

Prerequisites

• Noble metal chemistry and the platinotype, which is where the two-bottle logic, the equal-volume rule and the aquation argument come from. Do not coat before you have read it: the coating volume is a stoicheiometry.
• Printing a kallitype, for the same iron chemistry with silver in place of the metal, at a fraction of the cost. If you have not coated an even sheet there, do not practise here.
• Negatives and papers for the alternative processes, for the unbuffered-paper argument and the standard negative. This part will not re-teach it.
• Measurement and uncertainty, because the drop calibration below is a measurement with an uncertainty and is worth nothing reported without one.

Safety classification

Level B, on three criteria of the rubric, and one operation is excluded from the session altogether rather than being classified.

• Concentrated solutions of a substance classified as a skin sensitiser. Sodium tetrachloropalladate(II) carries signal word Danger with H317, may cause an allergic skin reaction, and H318, causes serious eye damage, in about 76 per cent of 176 reports across 10 notifications. That combination puts splash goggles and gloves beyond negotiation and takes the session out of Level A.
• A soluble oxalate as a working solution. Ferric oxalate is classified harmful if swallowed and harmful in contact with skin, on the thinnest aggregation in the encyclopaedia — one company, one notification — so the course classifies it upward and handles it as the oxalate it is. HSE’s EH40 gives oxalic acid 1 mg/m³ long-term and 2 mg/m³ short-term.
• A dried residue that must never become dust. Both a spilled drop allowed to dry and a coated sheet cut or torn dry can raise particles of a sensitiser. The control is to wipe spills wet and to keep cutting to uncoated paper.

Level C, and therefore not in this session: weighing a platinum or palladium salt powder. Potassium tetrachloroplatinate(II) carries H334 — may cause allergy or asthma symptoms or breathing difficulties if inhaled — and HSE sets halogeno-platinum compounds at 0.002 mg/m³ with the Sen notation, the tightest exposure limit in this course. HSE’s paragraphs 53 to 56 are explicit that exposure to an asthmagen should be prevented wherever reasonably practicable, that once the airways are hyper-responsive further exposure to tiny quantities may provoke symptoms, that it is impossible to say in advance who will become hyper-responsive, and that health surveillance is appropriate for anyone liable to be exposed. That is a fume cupboard, controlled waste and occupational health surveillance — Level C by the rubric’s own definition — and it is why this page specifies bought solutions and no weighing of any noble-metal salt, palladium included. Ware’s own preparation of the palladium solution from palladium(II) chloride carries a dust-mask warning that he attaches to no other step in his handout, and this course does not open that route either.

What is not a hazard here, and why. There is no ultraviolet in this session: the sheet is coated and dried and goes into the dark, and the lamp stays off until the next lab. There is no heat beyond a hair dryer, no acid, no alkali, no mains apparatus beyond that dryer, and nothing here can generate a gas: the carbon dioxide of the photolysis is made under the lamp, not in the shot glass. There is also no cyanide question, unlike every hexacyanoferrate bench in Parts XXI and XXIV — the only ferricyanide in this session is two crystals in a test tube for the spot test, and it is handled at a separate station under the acid rule the cyanotype part established. And the palladium salt, unlike its platinum neighbour, carries no respiratory sensitisation in any block of its notified record: a glove is a real control against a skin sensitiser in a way that no glove is against an asthmagen. That asymmetry is the entire reason this page exists as a Level B lab rather than as a second Level D lesson.

Hazards

Hazard	Where it arises	Control
Skin sensitisation, H317	The palladium solution, the mixed sensitiser, the wet coating, the rod and the blotting strip	Nitrile gloves throughout; change them if a drop gets inside one; never touch a coated sheet with bare fingers even when dry
Serious eye damage, H318	Drawing the palladium solution into a syringe, and any splash from the shot glass	Sealed chemical splash goggles, not spectacles, from the moment the first bottle is opened
Harmful if swallowed and by skin contact, oxalate	Ferric oxalate solution and everything it touches	Gloves; nothing eaten, drunk or smoked at the bench; hands washed before leaving
Dust from dried residue	A spill left to dry; a dry coated sheet trimmed or torn; a hair dryer blowing over a coating that is not yet dry	Wipe every spill wet, at once; trim paper before coating and never after; if a hair dryer is used, the supplier’s own instruction is a dust mask, and the course’s is to dry in still air where possible
Aquatic toxicity, H400 and H410	Every rinse of the shot glass, the syringe and the rod	Everything that has touched palladium goes into the noble-metal waste container, not the sink
Cross-contamination of a costly stock	One syringe used in two bottles; a rod washed carelessly	One dedicated syringe per bottle plus a third for delivery; glass only, never metal, because both metals plate out on a more reactive metal surface

Required PPE

Nitrile gloves, worn from the first bottle to the last rinse. This is a control against a classified skin sensitiser rather than a general tidiness measure, and it is the one Ware states in his own words for the platinum case: never put ungloved fingers in these solutions. Follow the glove ruling for thickness; the course’s control is HSE’s 0.2 mm single-use nitrile, and a thin examination glove sold from the same shelf is not that.

Sealed chemical splash goggles. H318 is eye damage rather than irritation, and the operation that generates the risk — expelling a syringe of solution towards a sheet at bench height — points the nozzle in roughly the wrong direction. Safety spectacles with side shields are not the same control.

An apron or an old shirt, because a palladium stain on cloth is not a laundry problem, it is a noble-metal residue you have now lost track of.

No respiratory protection is specified for this session, and the reason is a design decision rather than an absence of hazard: the session contains no powder and no heating, because the operations that would need a mask have been removed from it. If you deviate — if you buy a solid rather than a solution, or dry with a hot air stream — you have reintroduced the operation and you need the control, which for fine particulates means a mask rated for them and which the price file names as one of its gaps.

Ventilation

General room ventilation, and it is here for the solvent nobody thinks of: water. No step in this session produces a vapour, a fume or a gas — the only reaction that could is the photolysis, which happens under a lamp in the next lab and releases carbon dioxide into an open frame. What ventilation actually does here is carry away the water evaporating from a drying sheet, which is why a still, draught-free corner dries a coating evenly and an open window dries one edge first.

So: a room that exchanges its air, a bench away from the draught, and no extraction. If you are working in a space with no ventilation at all, the limiting factor is your own comfort over a three-hour session rather than any airborne hazard from these solutions.

Materials

Material	Quantity for this session	Note
Unbuffered 100 per cent cotton paper	4 sheets, cut to 12 × 10 in for an 8 × 10 image	Ware’s named list: Buxton or Herschel from Ruscombe Mill, Arches Platine, Weston Diploma Parchment, Crane’s Platinotype, Awagami Masa, Wyndstone Vellum, Atlantis Silversafe Photostore, Hahnemühle Platinum Rag. 160 g/m² is adequate to this size
A negative or a mask of the image size	1	Used only to mark the coating area today
Blotting paper, cut in strips	a few	For lifting residual sensitiser at the foot of the coating
Low-tack tape	a few pieces	Scotch Removable 3M811 is Ware’s own specification
Graphite pencil, hard	1	For the corner marks; nothing else touches the coating area
Distilled or deionised water	about 250 mL	Rinsing glassware and the rod between coatings; calcium is a chemical problem in this family, not a cosmetic one

Chemicals

Two bottles do the work, and one test tube checks one of them.

Chemical	Quantity this session	Form
Ferric oxalate	about 1.0 mL from the bottle	Bought solution, 20 per cent w/v (Photographers’ Formulary) or 27 per cent w/v (Bostick and Sullivan). Trihydrogen ferric oxalate, H₃Fe(C₂O₄)₃, not the green tripotassium salt
Sodium tetrachloropalladate(II)	about 1.0 mL from the bottle	Bought solution, 15 per cent w/v (Photographers’ Formulary) or unstated strength (Bostick and Sullivan “Palladium #3”). Na₂PdCl₄
Potassium ferricyanide	2 crystals	Solid, dissolved in about 2 mL of water for the spot test only, at a separate station

No potassium chlorate. The third bottle of the classical drop system is ferric oxalate carrying an oxidant, and potassium chlorate is Level D in this course on the rubric’s second criterion — a better route to the same result exists and the course teaches it. If your kit came with a Sensitizer B bottle, it stays shut; the section below explains what it would have done and what you do instead.

No platinum, on the ruling in the Safety classification section above. If your kit contains a platinum solution or an Na2 solution, both stay shut, and both are noble-metal waste rather than rubbish when you decide what to do with them.

Equipment

• A balance reading to 0.01 g, for the drop calibration only. The price file names this as a gap and the planner shows its band alone.
• Three disposable calibrated plastic syringes, 1 mL or 2 mL, without needles: one dedicated to the iron bottle, one to the palladium bottle, one to delivering the mixed sensitiser. Ware’s rule, and the cheapest insurance in the part.
• A small glass shot glass or liqueur glass as the mixing vessel. Glass only. The AIC’s own process summary calls it a shot glass, which tells you how small the volumes are.
• A glass coating rod, and a heavy flat glass plate to tape the paper to.
• A small spirit level. Ware calls levelling the plate critical, and it is: a rod-coated sheet on a sloping plate pools at one end.
• A hygrometer, ideally with a remote probe, reading the air at the bench and inside the drying enclosure.
• A drying enclosure — a lidded box, a drawer, anything light-tight and still.
• A dedicated waste container, glass or HDPE, labelled for noble-metal-bearing solutions, standing open on the bench before the first bottle is opened.

Estimated cost

Band ££££, and this is the most expensive session in the course by a wide margin. The capital is modest — three syringes, a shot glass, a rod, a plate, a level, a hygrometer — and almost all of it is on the planner’s list already or is a kitchen object. What makes the band is the two millilitres of solution.

That inversion is worth stating plainly, because it is the opposite of Part XXI. In cyanotype the paper costs more than the chemistry and the arithmetic there puts the sensitiser at around sixpence a sheet. Here the chemistry costs several times the paper, and the discipline of the whole session follows from that one ratio.

Estimated consumables cost

Every figure with a number comes from the planner’s dated UK price file, and every quantity from the Materials and Chemicals sections above. Five of the seven rows have no number in them, and that is this page’s most useful finding rather than a blemish.

Consumed	This session	Sourced price	Cost this session
Nitrile gloves	2 pairs	£6.64 to £14.99 per box of 50 to 100	£0.27 to £0.60
Unbuffered cotton paper, 12 × 10 in	4 sheets	Not priced. Hot-pressed cotton paper is among the gaps src/data/prices.json already names, and the sizes this part prints at are not among the listings it holds	—
Ferric oxalate solution	about 1.0 mL	Not priced. It is a gap src/data/prices.json names, and the reason it gives is this page’s: three suppliers publish three strengths under one name	—
Sodium tetrachloropalladate(II) solution	about 1.0 mL	Not priced. The palladium and platinum salts are a gap the price file names, and this is the most expensive consumable anywhere in this course	—
Potassium ferricyanide, for the spot test	2 crystals	£17.99 per 230 g, checked 7 September 2026, which is about £0.08 a gram	Under a penny: two crystals is a small fraction of a gram
Distilled water	about 250 mL	Not priced	—
Blotting strips, tape, pencil	a few of each	Not priced	—

The priced subtotal is about £0.27 to £0.60, and it is a floor rather than a total — by a very long way, because the one row that dominates the real cost is the one with no number in it. What the table does establish is the shape of the problem: the consumable that decides whether a student can do this part is the only consumable the course cannot yet price, and until a dated UK listing for a palladium salt or a made-up palladium solution goes into the price file, the cost lesson can price this process in metal per print and not in money.

Equipment is deliberately absent from the table. A syringe is arguably consumed and a shot glass is not, a hygrometer certainly is not, and a page that quietly counted them would stop measuring what the consumables calculator needs.

Waste streams

One stream dominates and it is worth money.

• Noble-metal-bearing liquid: the rinse from the shot glass, the delivery syringe and the rod, the blotting strip, the wiped-up spills, and any sensitiser you mixed and did not use. Small in volume, high in value, and carrying H400 and H410 — very toxic to aquatic life, with long-lasting effects — in about three-quarters of the notified reports for the palladium salt. It is collected in a labelled container that stands open on the bench from the start of the session, and it goes nowhere near a drain. Ware’s instruction for the equivalent bath in his own practice is the same one: save it for recovery of precious metal.
• Iron and oxalate: the rinse from the iron syringe, and any sensitiser residue that never met palladium. In practice this ends up in the same container, and there is no reason to separate it: the disposal route is decided by the noble metal.
• The spot-test tube: two crystals of ferricyanide, a millilitre of water and a drop of iron solution, now containing a trace of Prussian blue. It is a hexacyanoferrate waste and it goes in the iron-bearing container of Parts XXI and XXIV, not in the noble-metal one, because the acid rule for hexacyanoferrates governs that bottle and this part’s waste has no acid in it today but will have in the next lab.

Label every container through the labelling SOP and follow the general chemical waste procedure. Check your local regulations; they govern. See the disposal ruling.

Alternative route

Three readers cannot follow this page as written, and the honest answers differ in how much they cost.

You cannot afford palladium, and this is the commonest case. The route the course specifies is the palladium-toned kallitype of Part XXIV: a silver image made by the same iron chemistry on the same paper with the same rod, then plated with palladium from a bath that Bostick and Sullivan build from 7 to 15 drops of palladium solution in a litre of 1 per cent citric acid. The whole toning bath uses less palladium than one coated sheet. What you get is a print whose surface, scale and craft are this part’s; what you do not get is an image made of palladium, and the permanence argument is different in a way the cost lesson sets out rather than glossing.

You can afford some palladium but not much. Print small. The published drop counts scale with area, so a 4 × 5 in sheet is roughly a quarter of an 8 × 10’s metal — 12 drops against 40 on the Formulary’s own tables — and Bostick and Sullivan’s own note is that small negatives make beautiful jewel-like prints. Every measurement in this part works at 4 × 5, including the step tablet, provided you say what size you worked at.

You cannot buy the chemistry at all, or cannot keep it at home. Everything on this page except the coating itself can be done, and should be: read the two kit entries in the formulary (Photographers’ Formulary, Bostick and Sullivan) for what each bottle contains, do the drop calibration with plain water and your own dropper, and coat two sheets with the classic cyanotype sensitiser at the identical volume by the identical rod method. The coverage arithmetic, the humidity record, the levelling and the technique are all genuinely rehearsed; what is not is the behaviour of this particular sensitiser on this particular paper.

One thing has no substitute. There is no version of this session that produces a palladium print without palladium, and no cheaper metal behaves like it. The substitutions above give you the craft and the record; the metal is the metal.

Preparation

Do this before the session, not during it.

1. Read the two bottles you actually have. Suppliers do not agree, and the disagreement is not cosmetic. Photographers’ Formulary supply ferric oxalate at 20 per cent; Bostick and Sullivan at 27 per cent; Ware’s own standard siderotype solution is 25 per cent with 2 g of oxalic acid per 100 cc, which he says is the strength Pizzighelli and Hübl’s analysis originally specified. A drop count written for one of those bottles is not a drop count for another, and this is the commonest way a formula from a book fails in a darkroom stocked from a different supplier. Write the actual strengths of your two bottles at the top of today’s record. If the supplier does not publish one — Bostick and Sullivan do not publish a strength for their palladium solution — write “not published” rather than a guess.

2. Choose the paper on chemical grounds. Three properties decide it and none of them is texture.

What a paper does to this process, in order of how much damage it can do

from decisive to merely annoying

1. An alkaline buffer — the one that ruins the printNearly every conservation-grade paper carries a calcium carbonate reserve. Calcium strips oxalate from the iron complex and starts the hydrolysis that leaves the yellow stain, and Part XXI's arithmetic on ISO 9706 puts the reserve at an eightfold molar excess over the iron you are about to coat. Ware's remedy where a buffered paper is unavoidable is a 5 per cent v/v hydrochloric acid bath or a 20-minute soak in 10 per cent sulphamic acid, then an hour's wash — and he calls it tedious and undesirabledecisive
2. Gelatin sizing — decisive for platinum, not for palladiumWare: gelatin binds platinum(II) strongly and irreversibly and de-activates it, so Willis avoided it in his own papers. His instruction for this course's metal is the useful half: gelatin-sized papers "may be used for pure palladium prints, upon which gelatin has no adverse effect", though the tones run warmer than on an alkylketene-dimer sizemetal-dependent
3. Absorptivity, which sets your coating volumeThe specific coating volume is a property of the paper, not of the process: Ware gives a working range of 24 to 36 cm3 per square metre and a guide of 1.4 cm3 for an 8 by 10 area on Crane's. A thirstier sheet costs more metal per print, which on this material is a real number rather than a nuisance
4. Weight, which decides whether the sheet lies flat160 g/m2 is adequate to 10 by 8 in or A4; A3 wants 240 g/m2 or more, or the wetted area cockles and bellies as the fibres take up water and the negative loses contact
5. Fillers, which change the surface but not the chemistryWare discusses silica and aluminosilicates in papers; they alter how the sensitiser sits and how the print looks, and they are the variable to change last rather than firstleast urgent

Every property and every figure here is Ware's, from his chapters on paper choice and coating. The order is the course's judgement of what will cost you a sheet soonest.

3. Test the ferric oxalate, because it is the component most likely to have gone off. Ware calls ferric oxalate a chemists’ nightmare and records six to nine months in the dark as the keeping figure “said by some”, with at least one well-respected printer making it fresh the night before every session. Bostick and Sullivan give about a year, longer refrigerated, and insist it be brought back to room temperature before use because a cold concentrated solution is not the solution it was.

⚗The kit's own spot test, which is a cyanotype used as a quality control

Photographers’ Formulary publish a test instead of a keeping date, and it is a lovely piece of bench chemistry. Two crystals of potassium ferricyanide in about 2 mL of water; under a red safelight, add one drop of the ferric oxalate and look through the tube.

• Only a slight darkening means the solution has little free iron(II) in it. Good.
• Very dark, or black — actually a deep blue means excess iron(II) is already there: the solution has partly reduced itself and will give a weak, foggy print.
• In room light the tube should read yellowish-brown to orange; green means a trace of iron(II), and the deeper the blue cast, the poorer the ferric oxalate.
• Now hold the tube beside a 100 W lamp. Within a minute a deep blue should form on the side nearest the light. That is the photoactivity test, and it is positive when the blue appears.

The reaction is the one Part XXI is built on: iron(II) meeting hexacyanoferrate(III) makes Prussian blue.

Fe²⁺ + [Fe(CN)₆]³⁻ → Fe³⁺ + [Fe(CN)₆]⁴⁻

The electron transfer whose product precipitates as Prussian blue — here used as an iron(II) detector on a platinum bench

So a cyanotype is being run, deliberately and at microscale, as a quality-control assay on a solution that is about to be mixed with something far more expensive. Run it at a separate station with its own waste tube, and keep it away from anything acidic, under the rule Part XXI established.

4. Set out the waste container before you open anything, and put the noble-metal label on it now.

Procedure

Stage 1 — Calibrate your own dropper (20 minutes)

The entire published literature of this process counts drops, and a drop is not a unit. Do this before you mix anything, and do it every time you change dropper.

1. Tare a small beaker on the balance.
2. Deliver 50 drops of distilled water into it from the dropper or syringe you will actually use, held vertically, at the rate you will actually work at.
3. Read the mass in grams. At room temperature 1 g of water is close enough to 1 mL for this purpose.
4. Divide by 50.

V₁ = m ÷ 50

Volume of one drop

n = 1 ÷ V₁

Drops per millilitre

Repeat the whole measurement three times and report the mean with its spread, following Part II’s practice. Then convert every published drop count you intend to use:

V = (drops published) × V₁

Any published drop count, in millilitres for your dropper

🧮Two suppliers, two drops, and a 25 per cent disagreement

This is not a hypothetical problem. Photographers’ Formulary’s worked example assumes 20 drops per millilitre, so a drop is 0.05 mL, and their sheet rounds a 55-drop mixture up to 56 with the remark that you cannot measure a fraction of a drop. Bostick and Sullivan state, on their palladium toning sheet, that 10 mL of their palladium solution #3 “is equal to 255 drops” — which is 25.5 drops per millilitre, or 0.039 mL a drop.

1 mL ÷ 20 = 0.050 mL

The Formulary's assumed drop

10 mL ÷ 255 = 0.039 mL

Bostick and Sullivan's stated drop

The two differ by about 25 per cent, from two suppliers of the same process, for the same liquid. Run one supplier’s drop count with the other’s dropper and you have coated a sheet with a quarter more or a quarter less sensitiser than the table intended — and on this material that is money as well as density. Neither figure is wrong; a drop depends on the orifice, the surface tension of the liquid and how fast you push. Which is why the number that governs your session is the one you just measured.

Stage 2 — Rehearse the coating with something cheap (30 minutes)

Do not learn to coat on palladium.

5. Cut four sheets to 12 × 10 in, giving generous margins round an 8 × 10 image, and mark the corners of the coating area in light pencil using the negative or a mask as a template. Ware’s rule is that the coated area should be about 1 cm larger than the negative to allow for irregularities.
6. Tape one sheet at top and bottom to the glass plate with low-tack tape, and check with the spirit level that it is horizontal left to right. Ware calls this critical; shim under the plate if it is not.
7. Coat that sheet with classic cyanotype sensitiser or with plain water, at the volume you intend to use for palladium, by the rod method in step 12 below. Watch what the rod does, where the liquid runs out, and how many passes it takes.
8. Repeat until two consecutive rehearsal sheets look even. This costs pence.

✕What the rehearsal does and does not establish

It rehearses the volume, the pour, the levelling, the rod pressure and the pass timing, which is most of what goes wrong on a first attempt, and it does so for a fraction of a penny a sheet.

It does not establish that your palladium sensitiser will cover the same area at the same volume. No source read for this course publishes a comparison of the specific coating volumes of a cyanotype sensitiser and a palladium one on the same paper, and the two liquids differ in solids content and surface tension. The honest position is that the rehearsal is technique practice and the coverage figure has to come from your own first palladium sheet — which is why the Data to record section asks for it as a measurement rather than assuming it. If you want to close that gap yourself, the Further experiments section says how.

Stage 3 — Read the humidity, and decide whether to coat today (10 minutes)

9. Read the hygrometer at the bench and inside the drying enclosure, and write both figures down with the temperature.

Ware’s own working condition for this process is ambient relative humidity of about 55 per cent plus or minus 5 in his studio, with at least one hour allowed for the paper to come to equilibrium with the atmosphere — which he says is typical of hand-coated practice where the sensitised paper is printed the same day.

Below about 30 per cent, do not coat for a print you care about. Ware’s tests on tonal reversal in palladium name a coating dried below roughly 30 per cent relative humidity as the first of five factors promoting it, and reversal is a defect that appears in the tray after all the metal has been spent. If your room is that dry, put a tray of water in the drying box, wait an hour, and read it again.

Above about 80 per cent, Ware’s caution for the print-out process is that the sensitiser diffuses too deeply into the paper and the maximum density weakens, and that over-humidified paper is more likely to damage a negative in the frame. He states that for his own ammonium sensitiser rather than for this one, and the course records it as the neighbouring evidence it is rather than as a figure for today’s coating.

Stage 4 — Mix the sensitiser for one sheet (10 minutes)

Nothing is mixed until the paper is taped down, levelled and marked. The mixture is made for the sheet in front of you and for no other, and the reason is not shelf life — it is that a mixture you do not coat is metal you have thrown away.

10. Work out today’s volume. Start from the published drop count for your kit and your sheet size, convert it to millilitres with your own drop volume from Stage 1, and check it against the coverage arithmetic:

V = A × v

Coating volume from the paper's own absorptivity

where A is the coated area in square metres and v is the specific coating volume in cubic centimetres per square metre. Ware’s working range is 24 to 36 cm³/m², and his guide for Crane’s 100 per cent cotton is 1.4 cm³ for a 21 × 26 cm area, which is an 8 × 10 in print. An 8 × 10 in coating area of about 0.052 m² at 25 cm³/m² gives about 1.3 cm³; the same area at 36 gives about 1.9. Both suppliers’ drop tables land in that band, which is the useful confirmation:

Source	Iron	Palladium	Total for an 8 × 10	In mL at 20 drops/mL
Photographers’ Formulary, “very contrasty negatives” row (no chlorate)	22 drops of Solution A	24 drops	46 drops	2.3 mL
Bostick and Sullivan, Na2 digital-negatives chart, chlorate-free portion	20 drops of Ferric Oxalate #1	20 drops of Palladium #3	40 drops	2.0 mL
Ware, glass rod, 10 × 8 in	equal volume	equal volume	—	1.5 mL

The gap between 1.5 and 2.3 mL is not a contradiction: Ware coats by rod, the kit sheets assume a brush, and Ware’s own comment is that brush coating “consumes more and is expensively wasteful”. On this material, choosing the rod is choosing to spend a third less metal per sheet.

11. With the sheet ready, draw equal volumes of the iron solution and the palladium solution into their own dedicated syringes, expel them into the shot glass, then mix by drawing the liquid gently in and out of the delivery syringe three times.

⚗Why equal volumes, and why the chlorate bottle is not in this session

The equal-volume rule is stoicheiometry, not convenience. Ware’s general statement covers the whole system: the volume of iron solution must always equal the combined volume of metal solution. At his standard strengths — 1.40 M in iron against 0.67 M in palladium — that supplies about 2.1 iron(III) ions per palladium(II) ion, and the reduction consumes two. Coat with less iron and the metal is under-reduced; coat with much more and you are adding residual iron for the clearing baths to remove.

The third bottle is a contrast control the course does not use. In the classical drop system, Solution 2 is Solution 1 with potassium chlorate dissolved in it — 0.26 g in a 30 mL bottle for palladium against 0.18 g for platinum, because, as the palladium sheet says in its own words, “palladium is less sensitive to contrast control with potassium chlorate than is platinum”. Drawing more drops from bottle 2 and fewer from bottle 1 raises the chlorate concentration without changing the iron, the metal or the volume, which is an elegant design and is worth admiring even while declining to use it.

What the chlorate does is re-oxidise part of the iron(II) that light has just made, so less survives to reduce the palladium. Because it is spread evenly and consumes iron(II) wherever it meets it, it removes the weakest exposures entirely while barely touching the strongest: Ware’s phrase is that the effect “is not to uniformly contract the tonal scale, but simply to truncate the high values”, and he calls the result a false sparkle of lost gradation with increased graininess. The Getty atlas confirms the defect from the analytical side — the chlorate itself leaves no detectable residue, but it is responsible for a visually detectable patchiness in the image.

And it is Level D in this course, on the rubric’s second criterion rather than on acute hazard: a better route to the same result exists. Ware’s own conclusion is the course’s: with modern negative-making, “contrast-enhancing agents, such as chlorate, hexachloroplatinate(IV) or dichromate, become unnecessary when a correctly calibrated negative is made”. Your contrast goes in the negative, and the negatives lesson is where that is done.

Stage 5 — Coat (10 minutes for the coating, an hour for the drying)

Work under dim tungsten, not fluorescent light and not near a window. Room temperature 18 to 22 °C: below that, Ware warns, the sensitiser may crystallise.

12. Dust the surface with a blower brush. Draw the mixed sensitiser into the delivery syringe.
13. Point the nozzle at the bottom left mark and expel the liquid slowly and evenly in one unbroken strip as you move it left to right across the coating width, touching the paper lightly with the nozzle for steadiness. Do not be fussy; irregularities even out.
14. Hold the rod with one end in each hand, place it on the paper parallel to and just below the strip, and with moderate pressure push it up into the liquid. Pause about two seconds while the liquid distributes itself along the rod.
15. Push the strip steadily up the sheet, like a small tidal wave running ahead of the rod. At the top marks, hop the rod over the strip, pull it back to the bottom, hop over again and push up. Four passes should saturate the surface layer. The first pass is quick — three or four seconds, moderate pressure — and the last three are as slow as you can make them, ten to fifteen seconds each, with very little pressure, to allow maximum absorption. Two more passes are allowed if much liquid remains.
16. Lift the rod off and soak up any residue at the foot of the coating with the edge of a clean blotting strip. That strip is noble-metal waste.
17. Let the sheet rest flat for about ten minutes before you move it, so the sensitiser is absorbed by the fibres rather than sitting on them. Bostick and Sullivan give the same ten minutes; Ware describes the end point as the surface ceasing to look reflective.
18. Dry in the dark for about an hour at room temperature, in the still enclosure, with the hygrometer inside it.

✦If you must dry faster

Ware’s alternative is a warm air stream at about 40 °C for ten minutes after the rest. Photographers’ Formulary put a ceiling on it: do not heat the solution or the sensitised paper above 50 °C, because trihydrogen ferric oxalate is probably heat-sensitive and the extent is not known.

Bostick and Sullivan attach the hazard note that goes with a hair dryer: tiny particles may be blown into the air and inhaled, and they advise a dust mask. That is the operation this session was designed to avoid, so if you use it, use the mask.

Stage 6 — Decide what happens next (5 minutes)

19. Print today if you can. A coated sheet is not stock. Ware’s figure for sensitised paper is that it should be used within a few hours; Bostick and Sullivan’s sheets simply have the paper coated and printed the same day.
20. If it must wait, the storage is specific rather than casual: light-tight, air-tight, with a desiccant — silica gel or anhydrous calcium chloride — below 10 per cent relative humidity, in which condition Ware says the paper keeps six months without loss of quality. That is not a drawer. It is the same principle Willis’s factory used, selling platinotype paper in sealed tins containing anhydrous calcium chloride.
21. A sheet left overnight in the open at room humidity is the failure case, and it is not a small one: Ware records that sensitised platinotype paper does not keep unless thoroughly dried, because drying inhibits the decomposition reactions. What you will see is chemical fog — a greyish or brownish veil in the highlights that no clearing bath removes, because it is metal.

Expected observations

The mixed sensitiser is dark and yellow-green to olive, the colour of the ferric oxalate dominating a darker orange-brown palladium solution. It should be clear. A cloudiness or a precipitate is a reason to stop: on the platinum side the sheets warn that a fine red precipitate of the much less soluble hexachloroplatinate(IV) is normal in the bottle and must not be carried into the sensitiser, and the same discipline applies to anything you can see in the glass.

The coating darkens the paper visibly and looks wet and reflective for the first minutes, then dulls. Ware’s own end point for the rest period is exactly that loss of reflectivity.

The dried sheet is a pale to mid yellow-ochre, even across the coated area, with a soft edge at the margins where the rod ran out. A mottled or patchy sheen when dry is the single most common first-attempt fault and the Troubleshooting section deals with it.

Very little should be left in the shot glass. If more than a drop or two remains, your volume was too generous, and the number to change next time is the volume rather than the technique.

What is happening chemically

Almost nothing, and that is the point of this session. No reaction runs today. The ferric oxalate and the tetrachloropalladate are simply mixed and put into paper together, and they sit there — iron(III) that light has not yet reduced, beside palladium(II) that light cannot reduce — waiting for the ultraviolet of the next lab.

Three things are nonetheless happening while you work, and each is a control you are exercising.

The sensitiser is being placed in the fibres rather than on them. The rest period exists so that the liquid penetrates the surface layer of the cellulose, because that is where the image will form: the electron microscopy Ware reports finds the metal particles distributed through the body of the surface fibres. A coating that dried before it soaked in makes an image lying on the sheet, which looks different and abrades.

The paper is being loaded with water, or emptied of it. This is the variable that will decide the result, and it is being set now. At about 70 per cent relative humidity a cellulose paper holds roughly 8 per cent of its weight as water, which Ware calculates as about ten water molecules to each sensitiser ion — enough for the short-range ion diffusion that lets a little of the image form during the exposure. Below 30 per cent that mobility disappears and the reversal risk arrives. You cannot see any of this, which is why the hygrometer reading goes in the record beside the drop count.

And a slow decomposition is starting. Ferric oxalate in a dried coating decomposes on its own, which is why coated paper does not keep and why the commercial papers were sold desiccated in tins. Every hour between drying and exposure is a small loss of speed and a small gain in fog.

Data to record

Use the batch SOP for the bottles and record the sheet itself in your notebook. Everything below is needed to make a second print comparable with the first.

Field	Why it is here
Date, time, room temperature	The frame for everything else
Bottle strengths as the supplier states them, or “not published”	Three suppliers give three iron strengths; a drop count means nothing without one
Bottle batch and date opened; result of the ferricyanide spot test	The iron is the perishable component and the test is the only published check
Drop volume measured this session, with its spread	The unit the whole published literature is written in, and the only one you can trust
Drop counts used, and the volume in mL they came to	Both, because the second is what another printer can reproduce
Sheet size, coated area in m², and the specific coating volume in cm³/m²	The number that turns a drop count into a cost and a coverage
Paper: maker, name, weight, sizing if known, buffered or not, any decalcifying pre-treatment	A chemical variable, not a surface one
Coating method: rod or brush, number of passes, timing of first and later passes	Rod and brush differ by about a third in metal per sheet
Relative humidity at the bench and in the drying box, at coating and at the end of drying	The variable this process is most sensitive to and the one hardest to control
Drying: method, temperature, elapsed time; dark time before exposure	Both keeping and fog depend on it
Appearance of the dried sheet, in words	The evidence you will diagnose from if the print fails
Volume left unused in the shot glass	The waste figure, which is the discipline of the session made numerical

Analysis

Work out your own coverage, and then decide what to do about it.

v = V ÷ A

Specific coating volume achieved

with V the volume you actually used in cm³ and A the coated area in m². Compare it with Ware’s range of 24 to 36 cm³/m². If you are above it you are spending metal you did not need; if you are below it and the coating was even, you have found a genuinely economical combination of paper and technique and should record it.

Then convert coverage into metal. This is the arithmetic that makes the cost argument concrete, and it uses only figures from the previous lesson:

n = V × c

Palladium applied per sheet

where c is the palladium concentration of the mixed sensitiser — half the stock’s, because equal volumes were mixed. For Ware’s 0.5 M stock that is 0.25 mmol/mL, so 2.0 mL of mixed sensitiser carries 0.5 mmol of palladium, or about 53 mg of palladium metal, for one 8 × 10 sheet. Ware’s independent statement of coating weights — 6 to 9 millimoles of metal per square metre — agrees to within the range: 0.052 m² at 6 to 9 mmol/m² is 0.31 to 0.47 mmol, or 33 to 50 mg.

Note what that arithmetic assumes and does not claim. It is the metal applied, not the metal deposited. What is not reduced washes out in the clearing baths, and Ware’s X-ray fluorescence work shows that the fraction reduced varies with exposure. The applied figure is nonetheless the one that decides what you spend, and it is the number the cost lesson carries forward.

Finally, ask what your record could not settle. If two sheets coated the same way look different, the record above should let you name the variable. If it does not, add the field that was missing.

Troubleshooting

“The dried sheet has a mottled sheen, and the print later shows the same pattern.” Two causes, and one measurement separates them; coating mottle and drying marks and blotchy and streaked hand coating carry the full diagnoses. Uneven absorption, from a plate that was not level or a first pass that was too slow, gives a pattern that follows the direction of the rod; incomplete drying before exposure gives a pattern that follows nothing in particular, and both suppliers attribute splotchiness to paper that was not completely dry. The measurement that distinguishes them is the spirit level, checked with the sheet still taped down, plus the hygrometer reading inside the drying box: a coating that is even on a level plate and mottled anyway was not dry.

“The coating ran out before I reached the top.” Your volume was too low for this paper’s absorptivity. Do not chase it with more solution mid-pass; finish the sheet, measure the area you did cover, and recalculate v upward for the next one.

“There is a lot of sensitiser left at the bottom edge.” Too much volume, or too much pressure on the later passes. Blot it, record the leftover, and reduce next time. This is money.

“The ferric oxalate turned the spot test deep blue in the dark.” The solution has already reduced itself and should not be mixed with palladium. Buy fresh; on Bostick and Sullivan’s figures the bottle has about a year, less if it has been warm.

“The bottle of palladium has a deposit.” Stop. The metal solutions do not oxidise or age on the supplier’s own account, so a deposit means contamination — most likely ferric oxalate carried in on a shared pipette, or contact with a metal surface, on which both noble metals plate out. A contaminated palladium bottle is the most expensive mistake available in this part.

“The paper is buffered and I did not notice until now.” If the sheet is coated, it is probably lost — expect a yellow stain that the clearing baths will not fully remove. If it is not, Ware’s remedies are a 5 per cent v/v hydrochloric acid bath or 20 minutes in 10 per cent sulfamic acid, then an hour’s wash. Both are extra Level B wet work on a sheet you have not yet spent anything on, which is the right order.

“My drop count came out very different from the published table.” That is the expected result, not a fault, and it is why Stage 1 exists. Report both.

Clean-up

1. Rinse the shot glass, the delivery syringe and the rod into the noble-metal waste container with a small volume of distilled water — the first rinse is the one that matters, and it is worth doing with the smallest volume that works so the container fills slowly.
2. Wipe the bench wet, before anything dries. A dried droplet of sensitiser is a dust source and this is the only step of the session that addresses it.
3. Dispose of the blotting strips and any wipes into the same container’s solid stream, or into a sealed bag labelled with it: they carry palladium.
4. Cap and label the bottles, note the volume remaining, and put the iron back in the dark.
5. Wash your hands before removing your goggles, and after removing your gloves.
6. Empty the spot-test tube into the iron-bearing hexacyanoferrate container, not the noble-metal one.

Storage

The iron is the perishable one. In the dark, at room temperature, tightly capped. Bostick and Sullivan give their ferric oxalate about a year, longer refrigerated, and require it to be brought back to room temperature before use. Ware records six to nine months in the dark as the figure “said by some”. It is the only light-sensitive substance in the kit, so it stays away from sunlight and any ultraviolet source including the one in the next lab.

The metal solutions keep almost indefinitely and their enemy is contamination. Bostick and Sullivan: they do not oxidise or age, refrigeration is unnecessary, and the two things to avoid are cross-contamination with ferric oxalate and any contact with metal. Glass only, and a separate pipette per bottle, for the whole life of the bottle and not just for today.

Coated paper is not stock. Print it today. If it must keep, the specification is light-tight, air-tight and desiccated below 10 per cent relative humidity, for up to six months on Ware’s figure.

Ware’s own note on a freshly made solution belongs here even though this course does not make one. A platinum solution must stand at least 24 hours before first use — not a keeping property but its opposite, because the aquation equilibrium with its 2.4-hour half-time has not finished running. Palladium’s equilibrium constant is more than ten times larger and its solution, on Ware’s account, is stable indefinitely once made.

Disposal considerations

Three chemistries leave this bench and only one of them decides the route.

The noble metal decides it. Palladium salts carry H400 and H410 — very toxic to aquatic life with long-lasting effects — in about three-quarters of the notified reports, and they are also worth money. Commercial platinum printers keep spent solutions rather than discarding them, and the metal accumulates as a sludge that can be filtered off. At a student’s scale one session’s rinse is a few milligrams in a few hundred millilitres, which is neither worth refining nor acceptable in a drain, so it accumulates in one labelled container across the whole part and is disposed of once, as one hazardous waste stream. The practicality of recovery at this scale is argued in the cost lesson rather than asserted here.

The oxalate is a systemic toxin and the general chemistry is not in dispute: soluble oxalates precipitate calcium, which is the basis of their toxicity, and CAMEO records systemic poisoning from inhalation of the dust of the related potassium salt. In the quantities of this session it is a milligram-scale trace in the same container.

The hexacyanoferrate of the spot test is a separate stream for one reason: the acid rule. A hexacyanoferrate must not meet acid, and the next lab in this part puts acidic clearing baths on the same bench. Keeping the two bottles apart is not fastidiousness; it is the one absolute rule in this cluster.

Local regulation decides what may lawfully be done with any of it, and this course cannot tell you what it says where you are. See the disposal ruling and check your local authority’s hazardous waste route.

Questions

1. Your dropper delivers 23 drops per millilitre. A published table for your kit gives 20 drops of iron and 20 of palladium for an 8 × 10. What volume of sensitiser will you actually mix, and is it inside Ware’s coverage range for that area?
2. Why does the palladium version of the chlorate bottle carry more chlorate than the platinum version? Answer in terms of the reaction rates of the two metals rather than by quoting the sheet.
3. A friend offers you a jar of green solid labelled “ferric oxalate” that they say works for cyanotype. Why is it the wrong substance for this session, and what would convert it?
4. You coat two sheets identically on two days, at 28 per cent and 55 per cent relative humidity, and print them identically. What single defect are you specifically looking for in the drier one, and why does it appear in the shadows?
5. Your kit contains a bottle labelled “Na2” and the instructions say to use it in place of both Ferric Oxalate #2 and the platinum solution. What is it, on the course’s reading, and why does this page not use it?
6. Explain, to somebody who has only made cyanotypes, why the equal-volume rule for iron and metal is not an arbitrary convention.

Further experiments

Measure the coverage question this page could not answer. Coat three sheets of the same paper at 25, 30 and 36 cm³/m² with the iron solution alone, dry them, and expose and process them alongside a palladium sheet at your chosen volume in the next lab. The iron-only sheets make no image, but they show you where the coating covered and where it did not, at a cost of a few pence per sheet instead of a few pounds. If the coverage boundary falls in the same place, you have established that iron alone is a fair rehearsal for this paper; if it does not, you have established the opposite, and either result is worth recording.

Calibrate three droppers against each other. A pipette, a plastic dropper bottle and a syringe, on the same liquid, three trials each. Then repeat on water and on the iron solution and see whether the drop volume changes with the liquid. The published literature assumes it does not.

Test the buffering claim on a scrap. Take a sheet you believe is unbuffered and one you know is conservation-grade, put a drop of dilute acid on the back of each, and watch for effervescence — carbon dioxide from a calcium carbonate reserve. It is a two-minute test that will save a sheet of expensive paper one day, and the chemistry is Part XXI’s.

Weigh the sheet before and after coating and drying. The difference is the solids you actually put in the paper, and comparing it with the calculated solids from your volume and the stated bottle strengths is a check on the strengths themselves — which, given that three suppliers publish three different numbers for the same bottle, is a check somebody should be making.

Recap

You mixed about two millilitres of sensitiser from two bought solutions in equal volumes, because the equal-volume rule is the stoicheiometry of a reduction that needs two iron(II) per palladium(II). You measured what a drop from your own dropper actually is, because two suppliers of the same process differ by a quarter on that question. You chose the paper for its buffering and its sizing rather than its feel, you levelled the plate, you coated by rod because a rod spends about a third less metal than a brush, and you recorded the humidity at coating and at drying because that is the variable this process is most sensitive to and the one you cannot see.

Platinum was never in the room. The reason is one hazard statement — H334, and an exposure limit 2,500 times tighter for the compound than for the metal — and the palladium salt’s notified record carries no equivalent, which is what makes this a Level B lab rather than a second lesson.

Check your understanding

Question 1. Your dropper delivers 23 drops per millilitre. The published table for your kit gives 20 drops of ferric oxalate and 20 drops of palladium for an 8 × 10 print, whose coated area is about 0.052 m². What volume will you mix, and where does it sit against Ware's coverage range?

• 2.0 mL, at the top of the range, because the table's drop counts are the volumes
• About 1.7 mL, which is about 33 cm³/m² — inside Ware's 24 to 36 band and towards its thirsty end
• About 2.3 mL, which is above the band and means the table is wrong
• It cannot be calculated without knowing the strength of the bottles

Show the answer and why

Answer: About 1.7 mL, which is about 33 cm³/m² — inside Ware's 24 to 36 band and towards its thirsty end

40 drops ÷ 23 drops per mL = 1.74 mL, and 1.74 cm³ ÷ 0.052 m² = about 33 cm³/m². That is inside Ware's working range of 24 to 36 and rather above his own guide of 1.4 cm³ for the same area, which is the expected result: his figure is for rod coating on Crane's and the kit tables assume a brush. Option four is a reasonable instinct and wrong here — the bottle strengths decide how much metal that volume carries, but the coverage question is about volume against area.

Question 2. Why does the palladium kit's contrast bottle carry 0.26 g of potassium chlorate where the platinum kit's carries 0.18 g?

• Palladium solutions are more dilute, so more of everything is needed
• Palladium(II) is more labile and more of it is aquated at sensitiser strength, so its reduction is faster and gives the oxidant less time to intercept the iron(II); more oxidant is needed to remove the same fraction
• Chlorate decomposes faster in the presence of palladium
• Palladium prints are larger on average, so the bottle is scaled up

Show the answer and why

Answer: Palladium(II) is more labile and more of it is aquated at sensitiser strength, so its reduction is faster and gives the oxidant less time to intercept the iron(II); more oxidant is needed to remove the same fraction

The sheet states the fact — "palladium is less sensitive to contrast control with potassium chlorate than is platinum" — and Ware supplies the chemistry. The aquation equilibrium constant is 0.17 for palladium against 0.015 for platinum, so more than half the palladium is in the reactive aquated form at sensitiser strength against fewer than a fifth of the platinum, and palladium(II) complexes are more labile in any case. The chlorate and the metal compete for the same iron(II); the faster competitor wins more of it. The same single fact explains palladium's lower native contrast, its tendency to fog in an oxalate developer and its warmer colour.

Question 3. A friend gives you a green solid labelled "ferric oxalate". Why is it not the substance this session needs?

• It has decomposed; fresh ferric oxalate is green and old ferric oxalate is brown
• Two substances are sold under that name — trihydrogen ferric oxalate, which is the active one, and the green tripotassium salt, which Photographers' Formulary state they do not recommend for platinum, palladium or kallitype printing; the acidic form is the sensitive one, and acid converts the green salt to it
• The green solid is ferrous rather than ferric, so it has already been reduced
• Nothing is wrong with it; the colour is a grade marking

Show the answer and why

Answer: Two substances are sold under that name — trihydrogen ferric oxalate, which is the active one, and the green tripotassium salt, which Photographers' Formulary state they do not recommend for platinum, palladium or kallitype printing; the acidic form is the sensitive one, and acid converts the green salt to it

This is a matter of chemical identity rather than of condition. H₃Fe(C₂O₄)₃ and K₃Fe(C₂O₄)₃ are both light-sensitive; only the acidic form is sensitive enough to be useful, and the supplier says so in terms. It is also why the kits ship the iron with a slight excess of oxalic acid — which keeps it soluble, converts any of the potassium double salt present, and, at around 2 per cent, converts about a tenth of the ferric oxalate to the soluble trisoxalatoferrate(III) anion and so produces a little extra print-out during exposure.

Question 4. You coat and dry one sheet at 28 per cent relative humidity and an identical one at 55 per cent, then print both identically. What are you specifically looking for in the drier print?

• A general loss of maximum density across the whole print
• Partial tonal reversal, in which the heaviest shadow exposures come back lighter than the tones next to them
• A yellow stain in the highlights that the clearing baths will not remove
• A cooler image colour, because dry coatings give neutral tones

Show the answer and why

Answer: Partial tonal reversal, in which the heaviest shadow exposures come back lighter than the tones next to them

Ware's tests name a coating dried below about 30 per cent relative humidity as the first of five factors promoting tonal reversal in palladium, alongside an oxalate developer, a lower-absorptivity paper, a contrasty negative allowing heavy shadow exposure and a degree of partial print-out. Option four inverts the colour relationship: for palladium, drier gives warmer — sepia at 32 per cent, Van Dyke brown at 55, warm black at 80 on Ware's own table. Option three is residual iron, which is a clearing failure in the next lab and not a coating-humidity effect.

Question 5. Why does this page specify bought solutions and refuse to have anyone weigh a palladium salt powder, when the palladium salt carries no respiratory sensitisation?

• Because bought solutions are cheaper than the equivalent solids
• Because the operation, not the substance, sets the level: weighing any fine noble-metal salt is a dust-generating step, Ware attaches a dust-mask warning to exactly that step and to no other, and removing the operation removes the control it would need — which is also the rule the safety rubric states for raised steps
• Because sodium tetrachloropalladate cannot be weighed accurately on a domestic balance
• Because the solutions keep better, which is the only reason given

Show the answer and why

Answer: Because the operation, not the substance, sets the level: weighing any fine noble-metal salt is a dust-generating step, Ware attaches a dust-mask warning to exactly that step and to no other, and removing the operation removes the control it would need — which is also the rule the safety rubric states for raised steps

The rubric says a level is assigned to a procedure and not to a substance, and its raised-step rule prefers avoiding an operation entirely — by buying a solution ready made — over declaring it and writing out its controls. Weighing is the step that generates dust; dust is the route that matters for the platinum salts and is the one operation Ware singles out for a mask on the palladium side too. Option four is true and secondary: the solutions do keep better, and Ware calls his own made-up palladium solution stable indefinitely, but keeping is not what decides the classification.

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