Lab: Exposing, Developing and Clearing a Palladium Print

Purpose

To take one coated sheet through a measured ultraviolet exposure, a developer that reduces nothing, and a three-bath clearing sequence — and to come away with a print, a record, and a defensible answer to the question of whether the iron has actually left the paper.

The exposure and the development are the easy half. The image will be there within seconds of the developer touching the sheet, and there is very little you can do to it after that. The clearing is the half that decides whether the object survives, and it is the step that the sources most often say is passed over too casually — Photographers’ Formulary say exactly that on their own instruction sheet, and add that a print not properly etched “will darken with age and in this way can be destroyed”.

The metal in the sheet is inert. The iron beside it is not.

Learning objectives

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

• Calibrate an ultraviolet exposure for this sensitiser, this paper and today’s humidity, by a test strip rather than by a number copied from a book.
• Read the faint print-out image in the frame for what it is — a guide to how far the exposure has gone, not the picture.
• Choose between the three published developers and say what each does to speed, colour and the high values.
• Explain why development time is not a contrast control, in terms of what runs out.
• Run a three-bath clearing sequence and say what each bath removes, and what stopping at one costs.
• Judge the clearing by the only published test, and state honestly what that test cannot tell you.
• Account for every solution that leaves the bench, including the one that is worth money.
• Measure the dry-down of a print with no binder, which no source read for this course publishes.

Prerequisites

• Preparing the sensitiser and coating for palladium, and specifically the coated sheet it produced, with its record. A sheet without a record is a print you cannot learn anything from.
• Clearing, toning and the permanence of iron-silver prints, where the same three-bath sequence is met on a cheaper print. Everything about the chemistry of clearing transfers; what changes is what the bath is protecting.
• Build a UV exposure unit and ultraviolet sources and dose, for the enclosed, interlocked source and for what a dose is.
• The wash-testing lab, for the habit of testing a wash rather than assuming one — noting that its residual-thiosulfate test is not the test needed here, for reasons the Analysis section sets out.

Safety classification

Level B, on four criteria of the rubric, and one raised step declared below.

• A concentrated solution of a systemic toxin. The potassium oxalate developer is worked at about 28 to 32 per cent w/v — roughly a third of a kilogram of soluble oxalate in every litre in the tray. Its encyclopaedia page records the notified classification as Warning with H302, H312, H315 and H319, and records CAMEO going considerably further than that: inhalation of the dust “can cause systemic poisoning”, and ingestion causes burning pain, vomiting, severe purging, a weak pulse and possible cardiovascular collapse. The course handles the substance against CAMEO’s description rather than against the notified label.
• Acidic baths on the same bench as hexacyanoferrate chemistry. Whether you use the citric acid route or not, this cluster’s benches carry ferricyanide, and the acid rule from Part XXI is absolute.
• Sensitiser residues that are skin sensitisers. The coated sheet carries sodium tetrachloropalladate(II) until the clearing baths have taken it out, so gloves stay on until the print is in the wash.
• Ultraviolet. An enclosed, interlocked source, used closed, on Part XVI’s terms.

⚠Declared raised step: a warmed developer, and where the course stops

The step. Warming the potassium oxalate developer above room temperature, to no more than 38 °C.

Its level. Still Level B, and it is declared here rather than buried in the Procedure because the controls belong before the step and not inside it: the bath is warmed by standing its tray in a larger tray of hot water, never on a hotplate and never over a flame; it is carried at not more than half depth; it is handled with tongs and gloves and behind splash goggles and an apron; and the hot-water tray is filled before the developer tray goes into it, not after.

Where the course stops, and why the ceiling is 38 °C rather than something higher. The modern kit sheets work this bath at 32 to 38 °C (90 to 100 °F). Bostick and Sullivan state that it can be used “from room temperature up to the boiling point”; Jean-Claude Mougin allows palladium from 7 to 100 °C; and the historic trade worked it at 60 to 77 °C before 1892. This course runs none of that, and the reason is not squeamishness: no source read for this page establishes a safe handling temperature or a scald control for a domestic tray of concentrated oxalate solution, and Willis’s own 1892 announcement names the problem it solved — a method of development at room temperature “rather than at the elevated temperatures which had caused so many workers previously to scald their fingers.” A ceiling the course can defend is better than a range it cannot.

Ware’s own tests ran at room temperature with a 28 per cent bath, and so does the default of this page. The temperature series belongs to the experiment, where it is a measured variable rather than an improvisation.

What is not a hazard here, and why. Three things a reader might reasonably expect are absent.

There is no cyanide question in the developer or the clearing baths. Nothing in this session’s chemistry is a hexacyanoferrate. The only place a ferricyanide appears in this part is the sensitiser spot test of the previous lab, and it is kept on its own station with its own waste bottle precisely so that the acid on today’s bench never meets it. That separation is a control against a real incompatibility, not a ritual.

There is no strong acid in the course’s own route. The historic clearing bath was hydrochloric acid at 1 in 60 for platinum and 1 in 200 for palladium — a corrosive at pH below 1 — and this page does not run it. The sequence specified below is a chelating bath at pH 3 to 4, a sulfite bath and an alkaline chelating bath at pH 9 to 10, and its two chelating agents are Level A substances whose notified records carry no hazard statement at all. Ware’s own reasoning for the substitution is on record and it is not primarily a safety argument: he recommends “instead of hydrochloric acid” a reducing agent together with a modern chelating agent because it clears better, and reports that accelerated ageing of prints cleared that way produced no stain.

And the ultraviolet source is not a hazard while it is closed. The control is the enclosure and the interlock, in that order, and eyewear is the last line rather than the first. What makes ultraviolet dangerous in this cluster is that nothing tells you it is happening: a lamp that does not feel warm is still delivering the dose.

Hazards

Hazard	Where it arises	Control
Systemic oxalate poisoning by ingestion; skin and eye irritation	The developer, at about 28 to 32 per cent w/v; its splashes; the tongs	Gloves and sealed goggles; tongs rather than fingers; nothing eaten or drunk at the bench; hands washed before leaving; the tray covered when not in use
Oxalate dust	Only if you mix the developer from the solid	Mix in a still room, over the tray, with the powder wetted before stirring; EH40 gives oxalic acid 1 mg/m³ long-term and 2 mg/m³ short-term
Vapour from a used developer	A bath carrying dissolved ferric oxalate	Sarah Van Keuren reports that the fumes of her own reused bath gave her headaches and that the tray belongs under a fume hood or outdoors; the course’s control is a ventilated room and a covered tray, and it takes her report as a working printer’s evidence rather than as a classification
Scald	A warmed developer tray, and the hot water warming it	The declared raised step above: water bath, never a flame, half depth, apron and gloves, 38 °C ceiling
Skin sensitisation, H317	The wet coated sheet, until the clearing baths have removed the unreduced palladium	Gloves on from the frame to the wash
Severe skin burns and eye damage, H314	Sodium sulfite as a solid and in the 2.5 per cent bath — carried by 51.8 per cent of the reports that classify it	Gloves and goggles when weighing and mixing; the made-up bath is dilute, and it is still a bath you do not put a hand into
Alkali burn	Only if you make the third bath from disodium EDTA and sodium hydroxide rather than buying the tetrasodium salt	Alkali into water, never the reverse; goggles; and prefer the bought tetrasodium salt, which removes the operation
Hydrogen cyanide, in the one case that matters	Any acid meeting the hexacyanoferrate waste of the previous lab	The absolute rule: no acid near that bottle, ever. Keep it off this bench entirely
Ultraviolet, skin and eye	The exposure unit	Enclosed and interlocked, used closed
Mains	The exposure unit and any warming appliance	Part XVI’s provisions; nothing mains-powered stands in a wet area

Required PPE

Nitrile gloves for the whole session, changed if solution gets inside one. Two hazards are being controlled at once and they need the glove at different times: the palladium sensitiser residue up to the first clearing bath, and the oxalate developer throughout.

Sealed chemical splash goggles. The developer is poured rapidly across a tray at bench height, which is the operation most likely to throw a drop upward, and the palladium salt’s H318 is eye damage rather than irritation.

An apron, and closed shoes, for the warmed-developer step.

Tongs, dedicated to this bench. Not shared with the silver benches of Parts XIX to XXIV: oxalate and silver must not meet, because oxalic acid’s page carries the incompatibility with silver compounds and the reason is silver oxalate, which is explosive when dry.

No respiratory protection is specified for the wet steps. Van Keuren’s headache report is taken seriously enough to put the tray in a ventilated space rather than to specify a mask, because no source read for this page identifies the species involved or gives an airborne concentration, and a control cannot be specified against a hazard that has not been characterised. If you are mixing the developer from the solid rather than buying it made up, that is a dust operation and it needs the mask the price file names as one of its gaps.

Ventilation

A ventilated room, and the tray covered when it is not being used. Nothing here is classified as producing a hazardous vapour, and no source read gives an airborne exposure figure for any of these baths. What the sources do give is one working printer’s account: Van Keuren, running a reused ammonium citrate developer over years, reports that its fumes gave her headaches and that the tray belongs under a fume hood or outdoors.

The course’s position is stated rather than smoothed. That is an experienced practitioner’s observation of her own bath, not a classification and not a measurement, and it concerns a developer carrying dissolved ferric oxalate from many prints rather than a fresh one. It is enough to justify a room that exchanges its air and a lid on the tray between prints; it is not enough to specify extraction, because nobody has said what would be extracted. If you develop a headache at this bench, believe yourself before you believe this paragraph.

Materials

Material	Quantity for this session	Note
Coated, dried sheet from the previous lab	2 or 3	One for the test strip, one for the print, one spare
The standard negative, or a step tablet	1 of each	A calibrated step tablet is what makes the session a measurement rather than a print
Masking material — Rubylith or black polythene	enough for a border	Ware’s three reasons for masking are given below, and the technical one is the reason this is not optional
Distilled or deionised water	about 500 mL	For the developer and for the final rinses; tap water is what Ware specifies for the clearing baths themselves
Blotting or drying screen	1	A horizontal plastic or fibreglass screen, per Ware
Felt blanket or other gas-permeable backing for the frame	1	Not a plastic sheet, for the reason under Expected observations

Chemicals

Five solutions, one of which you may already have from Part XXIV.

Chemical	Quantity this session	Form
Potassium oxalate monohydrate	280 g to make 1 L, or a bought made-up developer	Solid, as a 28 per cent w/v bath; the kits ship it saturated, 227 g stirred into 500 mL with undissolved solid left in
EDTA disodium dihydrate	about 50 g per litre, 1 L needed	Solid, for clearing bath 1 at about 5 per cent w/v
Sodium sulfite	about 25 g per litre, 1 L needed	Anhydrous, for clearing bath 2 at about 2.5 per cent w/v; sodium metabisulfite at the same strength is Ware’s own alternative
EDTA disodium dihydrate with sodium hydroxide	about 45 g with 9.6 g per litre, 1 L needed	For clearing bath 3, the alkaline one. If you hold the tetrasodium salt, weigh 50 g of that and use no alkali at all
Ammonium citrate or sodium citrate with citric acid	optional, for the alternative developers	See the developer comparison below; the quantities are in the formulary

The clearing sequence above is the EDTA and sodium sulfite sequence, which Ware devised with Pradip Malde and published under his own name with its chemistry set out in full. The course uses it in preference to the historic acid baths on his own argument rather than on a safety judgement of its own, and the reasons are under “The clearing agents compared” below.

Equipment

• Five processing trays, one size larger than the print: developer, three clearing baths, and a water rinse. Plastic, not stainless steel and not metal of any kind — both noble metals plate out on a more reactive metal surface, and the same reasoning that closes off metal storage bottles closes off metal trays.
• A larger tray or basin to stand the developer tray in, if the warmed step is used.
• The contact printing frame, split-back, glazed in glass rather than acrylic.
• The ultraviolet exposure unit, enclosed and interlocked, with a timer.
• A thermometer for the developer.
• A hygrometer, still reading the room, because the humidity of the sheet at exposure is a variable you inherited from the coating session.
• Tongs, dedicated, two pairs, one for the developer and one for the clearing line.
• A bluish light source — a daylight-balanced LED or a north window — for the clearing inspection.
• The densitometer of Part XV, if you are printing a step tablet.
• Labelled waste containers, set out before the first sheet is wet.

Estimated cost

Band ££££, and the band belongs to the sheet rather than to the session. Every bath on this page is cheap: potassium oxalate, EDTA and sodium sulfite are ordinary laboratory or photographic solids, and the three clearing baths between them cost a small fraction of the metal on one coated sheet. The developer is effectively a capital item rather than a consumable, because it is reused for years rather than discarded.

What makes the band is that the object at risk in this session is the coated sheet from the previous one. A clearing sequence run badly does not waste a bath, it wastes the metal. That is why this page spends more of its length on clearing than on exposure, and it is the honest form of the cost argument: on this material, the cheap steps are where the expensive mistakes are made.

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.

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
Sodium sulfite, anhydrous	25 g into a 1 L bath, one session’s worth	£13.68 to £19.98 per kg	£0.34 to £0.50
Coated sheet carried in from the previous lab	1 printed, 1 test strip	Not priced, for the two reasons its own page gives: the palladium solution and the ferric oxalate are both gaps the price file names	—
Potassium oxalate monohydrate	280 g into a bath that is then reused indefinitely	Not priced	—
EDTA disodium dihydrate	about 95 g across baths 1 and 3, each good for 50 to 100 prints	Not priced	—
Sodium hydroxide, if bath 3 is made from the disodium salt	9.6 g	Not priced. Cleaning alkali is among the gaps the price file already names	—
Citric acid or a citrate, if an alternative developer is used	20 to 500 g depending on the bath	£10.00 per 250 g of the monohydrate; the citrates are unpriced	£0.80 to £20.00 on the citric-acid route
Distilled water; wash water	about 500 mL, plus the wash	Not priced	—

The priced subtotal is about £0.61 to £1.10 on the standard developer, and it is a floor rather than a total. Five of the eight rows have no sourced price, and the one that matters most — the coated sheet — is unpriced for the reason its own page gives. What the table does establish, and it is worth stating because it inverts the usual expectation: the whole wet chemistry of a palladium print costs about a pound a session and is reused, and the metal on one sheet costs several times that. The discipline of this part is not about the baths. The alternative developer is the exception the numbers now show: the citric-acid route alone runs £0.80 to £20.00 according to which bath is made up, which is more than the standard developer and the gloves together.

Equipment is deliberately absent: a tray is not consumed, and neither is a frame, a thermometer or a pair of tongs.

Waste streams

Four containers, and the fourth is not on this bench.

• Spent developer, when it is finally discarded. It carries oxalate, iron and dissolved noble metal. It is the container commercial printers keep for metal recovery, and Ware’s instruction for the equivalent bath in his own practice is explicit: when spent, save it for recovery of precious metal.
• Spent clearing baths 1 and 3, carrying iron-EDTA complexes and whatever palladium washed off the sheet. Same container as the developer in a student’s practice, for the same reason: the noble metal decides the route.
• Spent clearing bath 2, the sulfite. Made fresh for a session and discarded after it, because sulfite oxidises in air to sulfate and a bath that has become sulfate reduces nothing.
• And the hexacyanoferrate bottle from the previous lab, which stays off this bench entirely. This is the one absolute rule in the cluster: no acid, no acidic bath, no acidic waste and no acidified rinse goes anywhere near it.

The wash water is the one stream that is genuinely dilute, and it is also the largest by volume. It carries the tail of everything above. The course’s position is the one it takes everywhere: label the containers through the labelling SOP, follow the general chemical waste procedure, and check your local regulations, because they govern. See the disposal ruling.

Alternative route

No ultraviolet source. Print in the sun, exactly as Part XXI’s alternative route sets out, and with the same trade: you lose repeatability and you gain the sharpest contact print available, because the sun is nearly a point source. Bostick and Sullivan list sunlight beside a metal halide bulb and ultraviolet tubes without apology and give a sunlamp exposure of about three to eight minutes for a 4 × 5 negative, with dense negatives running half an hour or longer. What you must supply instead of a dose is a written record — date, time, sky, orientation, elapsed time and the appearance at each inspection — through the daylight exposure SOP and the outdoor exposure session SOP. Expose the whole test strip in one unchanging session so the comparison between strips stays valid even though the absolute dose is unknown, and say so in the analysis.

No mains for warming. Skip it. The default of this page is room temperature, which is what Ware’s own tests used, and the temperature variable belongs to the experiment.

No running water for the wash. Ware allows at least three fresh static baths where water is short, in place of the thirty minutes running. Photographers’ Formulary’s own alternative for the kit route is an hour with a complete exchange every five minutes, which is twelve changes and is the same idea counted differently.

No densitometer. Everything except the measurement can be done, and the print is still a print. What you cannot then do is fill the tonal-scale field of the comparison atlas from your own work, and the honest answer there is to mark that row as the course’s published data rather than yours.

No palladium at all. The whole of this page’s wet sequence — the developer, the three clearing baths, the masked-margin inspection, the wash — is the same on a kallitype, which uses the same iron chemistry with silver. A student on the accessibility route runs this session on a kallitype and then tones it in palladium, and the only thing they do not learn is how this particular sensitiser behaves in this particular developer.

One thing has no alternative. There is no version of this session that skips the clearing. Not a shortened version, not two baths instead of three, not “it looked clean”. The evidence for that is under “What the clearing is actually for”, and it is the strongest evidence on the page.

Preparation

1. Mix the developer, at least the day before if you can. 280 g of potassium oxalate monohydrate to make one litre is a 28 per cent w/v bath, which is the strength Ware used throughout his own comparative work. The historical range is wider and the differences are recorded rather than averaged: Willis’s own patent gives 120 grains or more per fluid ounce, which is a nominal 27 to 30 per cent; later practice went to saturation at about 32 per cent; and the modern kits stir 227 g into 500 mL and instruct that not all of it will dissolve, which is deliberately past saturation. The formulary entry carries all four and averages none.

The bath should be neutral or only just acid. Ware records the rule qualitatively and gives both reasons: accumulated alkali from the water or the paper is what the acidity counters, and more than a trace of acid is said to inhibit the formation of the metal. Some recipes add 0.1 per cent oxalic acid or less; some modern practitioners add about 2 per cent, presumably to hold the pH below 6 and inhibit the hydrolysis of iron(III) that leaves a stain. No source read for this course publishes a measured pH for the working bath, and this page does not invent one.

2. Mix the three clearing baths. All three are made with tap water at room temperature, which Ware specifies rather than merely permits.

The three clearing baths, in the order the print meets them

the print's route

1. Bath 1 — disodium EDTA, about 5 per cent w/v, pH 3 to 4About 50 g of disodium EDTA dihydrate dissolved in about a litre of tap water. Ware: "It is important that this first bath should be acidic, pH ~4. Do not use tetrasodium EDTA." Capacity about 50 to 60 prints of 10 × 8 in per litre. This is the bath that takes out most of the iron(III), and it is the one to save for metal recoverymost of the iron
2. Rinse — water, half a minuteNot a wash. Its job is to stop bath 1 travelling into bath 2
3. Bath 2 — sodium sulfite, about 2.5 per cent w/vAbout 25 g of anhydrous sodium sulfite in about a litre. Made fresh for the session and discarded after it, because sulfite oxidises in air to sulfate and a sulfate bath reduces nothing. Ware names sodium disulfite, sodium bisulfite and Kodak Hypo Clearing Agent as alternatives at the same strengthreduces what chelation cannot detach
4. Rinse — water, half a minuteAgain, to keep the baths apart
5. Bath 3 — tetrasodium EDTA, about 5 per cent w/v, pH 9 to 10Either 50 g of the tetrasodium salt in a litre, or Ware's own route from what you have: 9.6 g of sodium hydroxide dissolved in a litre first, then about 45 g of disodium EDTA added to it. Capacity about 100 prints per litre — twice bath 1's, because most of the iron has already gone. This is the bath to prolong if a stain persiststhe iron(II), at the pH that suits it
6. Wash — running water, 30 to 60 minutesOr at least three fresh static baths where water is short. The print leaves alkaline, which is the condition that suits the paper's own keeping

Quantities, times, pH windows and capacities are Ware's own, published for exactly this material. The alkali table for making bath 3 from other bases — sodium carbonate anhydrous 12.7 g, the decahydrate 34.3 g, sodium bicarbonate 20.2 g — is his too.

3. Mask the borders of the coating. Rubylith or black polythene, cut to the image size. Ware gives three reasons and the technical one is why this page treats it as a step rather than a preference: the masked margin, coated with sensitiser but never exposed, is the only visual test of complete clearing you will have. Compared against the uncoated paper beside it, a yellow stain there tells you the print is not cleared. If the margins are exposed and darkened, “one can never tell if the print has been properly cleared”. He calls it a cruelly demanding test and very desirable, and both halves are true. His other two reasons are that a ragged black border distracts from the picture, and that a large heavily exposed non-image area can bleed dense photoproduct into light areas like sky during wet processing.

4. Back the sheet with something gas-permeable. A felt blanket of the papermaker’s kind, not a plastic sheet. This is not fussiness either: carbon dioxide is a by-product of the photolysis, and if it cannot diffuse out through the verso it forms a bubble between negative and paper thick enough to degrade resolution in regions of high local contrast, especially under a light bed rather than a point source.

5. Set out the waste containers, and check that the ferricyanide bottle from the previous lab is not in the room.

Procedure

Stage 1 — Expose a test strip (30 to 45 minutes)

1. Run the ultraviolet unit up for about five minutes before the first exposure. Ware’s own unit takes about three minutes to reach steady output and he allows five.
2. Load the frame: negative emulsion-side down against the coated side, felt blanket behind, glass closed.
3. Expose a strip in third-of-a-stop steps. That is Ware’s own method for exposure determination, and the end point he looks for is a suitable distribution of tones across the scale with maximum density reached in the first two steps of a step tablet.
4. Develop and clear the strip through the whole sequence below before you judge it. A palladium print changes at every bath, and judging it wet out of the developer will mislead you twice over.

🧮What to expect, and why this page gives you a method rather than a time

No number transfers between two ultraviolet units, so the figures below are orientation for the calibration rather than substitutes for it.

• Ware’s own laboratory unit is a 120 W UVA facial tanning unit — six 20 W tubes peaking near 365 nm — and he settled on standard exposures of 5 or 10 minutes for all his tests, for both metals, for ease of comparison.
• His own print-out palladium workshop gives “a few minutes under an average UVA light source”, and about 40 seconds under an 800 W high-intensity-discharge lamp with digital negatives.
• Bostick and Sullivan give about three to eight minutes for a 4 × 5 negative under a sunlamp placed three to four inches above the paper, with dense negatives running half an hour or longer.

And the dose depends on things that changed since yesterday. The negative’s ultraviolet density range, the paper, the coating volume and — the one people forget — the relative humidity of the sheet. Ware’s own figures for his print-out sensitiser have palladium’s relative speed rising from 0.5 at 32 per cent relative humidity to 2.5 at 80 per cent: a factor of five, from a variable you cannot see. Your test strip is a measurement of today.

Stage 2 — Expose the print (10 to 30 minutes)

5. Set the dose from the strip and expose the print, frame closed.
6. Look at the sheet when you open the frame, and know what you are looking at. There will be a faint image, and it is brownish rather than the colour of the print. The Getty atlas identifies it: a faint brownish image made from the photochemically generated ferrous oxalate. It is a picture of the iron, not of the palladium, and its density tells you how far the exposure has gone rather than what the print will look like.

Stage 3 — Develop (5 minutes, of which the reaction takes seconds)

7. Have the developer in its tray, at a measured temperature, before the frame is opened. Develop immediately after exposure; the sheet is not stock at any point in this session.
8. Pour, do not lower. This is the one manual skill of the stage and every source gives its own trick for it. Bostick and Sullivan: pour the developer quickly over the print, fast enough to break any air bubbles, tilting and sloshing the tray. Van Keuren: slip the print face up into the developer and make sure the entire surface is covered quickly. Auerbach: dam the whole volume at one end of the tray, slip the print in at the base of the dam, and drop the tray level so it arrives as a single wave. Wall’s 1924 instruction for hand-coated paper is a fourth answer to the same problem — take both ends of the sheet, immerse one end face down, draw the paper right through the solution and turn it face up.
9. Leave it in for one to two minutes with agitation, and understand that this is a handling convention rather than a chemical requirement. Photographers’ Formulary say at least two minutes and that you cannot overdevelop a platinum print; Auerbach gives 30 to 40 seconds; Mougin gives one minute and says outright that “timing is not critical and doesn’t cause any increase of contrast”.
10. Drain the print.

Stage 4 — Clear (35 minutes)

11. Bath 1, disodium EDTA, 10 minutes with intermittent agitation at room temperature.
12. Rinse, half a minute.
13. Bath 2, sodium sulfite, 10 minutes.
14. Rinse, half a minute.
15. Bath 3, tetrasodium EDTA, 10 minutes.
16. Inspect before you wash. Take the print to a bluish light and look at the masked margin — the area that was coated with sensitiser and never exposed — against the uncoated paper beside it. Any yellow there is residual iron. If it is present, prolong bath 3 and look again.

Stage 5 — Wash, dry and record (60 minutes, mostly unattended)

17. Wash in running water for 30 to 60 minutes, or at least three fresh static baths.
18. Drain face out on a near-vertical sheet of Perspex for about ten minutes, then dry flat at room temperature on a plastic or fibreglass screen.
19. Record everything in the Data to record section, including the wet appearance and the dry appearance, because the difference between them is a measurement this course does not yet have.

Expected observations

In the frame: very little, and brown. The image that appears during exposure is ferrous oxalate, not palladium. On a ferric oxalate sensitiser Ware notes that a dry coating prints out hardly at all, because the photoproduct is insoluble; the small amount you do see comes from the free oxalic acid the kits include, which converts about a tenth of the ferric oxalate to the soluble trisoxalatoferrate(III) anion whose photoproduct can migrate a little.

In the developer: everything at once. The AIC’s process summary is three words long — “the image comes up instantly” — and both suppliers say development is complete within seconds. What you should watch for is not the image arriving but whether it arrives evenly: an area the liquid reached a second late develops differently, and that is what a lap mark is.

A slow or partial appearance is diagnostic. If the image comes up sluggishly or in patches, the causes worth checking in order are: a sheet that was not completely dry before exposure, which both suppliers name as the cause of splotchiness; a developer that is cold; an exhausted or contaminated sensitiser; and a sheet coated at a humidity outside the working range.

In the clearing baths: the print lightens. Some of that is residual sensitiser leaving, which is the process working. Some of it, on a palladium print in an over-strong acid bath, would be image metal leaving, which is why this page’s sequence is chelating rather than acidic.

Out of the wash: a matte surface with no layer on it. The paper’s own texture, the image apparently inside the sheet rather than on it, and a long smooth scale with a maximum density that will disappoint anyone expecting a glossy silver print’s black. Ware’s own densitometry of this family puts maximum density around 1.45.

And on drying: a change nobody has published a figure for. Every print in this course dries down darker or lighter than it looked wet, and Part XVIII’s dry-down lesson treats it for silver gelatin, where a gelatin layer shrinks and its refractive index changes. A palladium print has no layer to shrink. No source read for this course states whether its dry-down therefore differs in size or in direction from a gelatin print’s, and the course does not assume. The Data to record section asks you to measure it, and Further experiments says how to make that measurement worth something.

What is happening chemically

Three baths do three different jobs and the middle one is the one people leave out.

The developer dissolves; it does not reduce

The coating holds ferrous oxalate, which is nearly insoluble — Ware gives 0.022 g per 100 cc — sitting beside a palladium salt that cannot move either. Saturated oxalate ions take the iron(II) into solution:

FeC₂O₄ + C₂O₄²⁻ → [Fe(C₂O₄)₂]²⁻

What a platinum or palladium developer actually does

and the mobile iron(II) then finishes the reduction light had already prepared:

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

The reduction, completed in solution at the image site

So the reaction stops when the iron(II) runs out, not when the developer tires. That is the whole content of “you cannot overdevelop a platinum print”, and it is why Auerbach can write that density of image is determined only by exposure and not by development, and why the AIC records that contrast in this family is achieved mostly by exposure.

What the clearing is actually for

Every print leaves the developer carrying iron. Some of it is unexposed iron(III) that never reacted; some is iron(III) made by the reduction itself, one for each electron the palladium took. Ware’s chemistry of clearing gives the reason it will not simply wash out: iron(III) is chemisorbed onto the cellulose, and above about pH 4 it hydrolyses to a polymeric colloidal hydroxide that lodges in the fibres. That hydroxide redissolves in dilute acid at first — and if it is not removed at the wet-processing stage it transforms irreversibly into iron(III) oxyhydroxide, the mineral goethite, which dilute acid will not touch. The yellow highlight stain of an old platinum print is that mineral.

Why three baths and not one

1. 1Bath 1 chelates the iron(III) it can reachEDTA wraps six donor atoms round iron(III) and holds it in solution, at a pH low enough — 3 to 4 — that the iron does not hydrolyse first. This takes out most of the iron, and it is why Ware insists the first bath be acidic and that tetrasodium EDTA must not be used for it
2. 2But chelation alone cannot detach all of itSome iron(III) is bound to the cellulose more tightly than the chelate can compete for. This is the fact that makes a one-bath clearing sequence insufficient, and it is the reason the second bath exists
3. 3Bath 2 reduces iron(III) to iron(II)Sulfite is a reductant. Iron(II) binds the paper more weakly than iron(III) and is chelated better in the bath that follows. Ware's conservation chapter records the same reductive-dissolution principle as a standard treatment used by professional paper conservators for iron stains
4. 4Bath 3 chelates the iron(II), at the pH that suits itAlkaline, pH 9 to 10, where the iron(II)-EDTA complex is at its strongest. It has twice bath 1's capacity because most of the iron has already gone, and it leaves the paper alkaline, which suits the paper's own keeping
5. 5The wash removes the bathsThirty to sixty minutes running, or three fresh static baths. What leaves here is chelate and salt, not iron

The sequence, the pH windows and the capacities are Ware's, published for platinum-palladium and reprinted unchanged for chrysotype. The reasoning in the second step is the one that makes it three baths rather than one.

The reduction chemistry of the middle bath is worth writing out, because it is the step whose purpose is least obvious:

HSO₃⁻ + 2 Fe³⁺ + H₂O → HSO₄⁻ + 2 Fe²⁺ + 2 H⁺

Ware's reduction of iron(III) by hydrogen sulfite, the reason the middle bath is not optional

Why the developer changes rather than exhausts

Nothing in the bath is consumed by developing. What accumulates is iron and dissolved noble metal, and the consequences are all observable. The colour darkens. A sludge settles and is filtered off. And — the one that surprises people — Ware’s warning that a re-used platinotype developer always contains sensitiser reagents in solution and is therefore still photosensitive: unprotected from actinic light it will steadily precipitate particulate platinum metal, which if left in suspension could find its way onto the surface of subsequent prints.

Two working printers reach opposite conclusions from the same bath and the disagreement is the useful part. Van Keuren reused an ammonium citrate developer over years, replenishing for volume, and reports that “only once in my experience has this developer expired from overuse”. Mougin uses 50 cc as a one-shot for a print up to 8 × 10 and discards it, on the argument that the bath gets loaded with particles of palladium and of ferrous oxalate which in strong concentration eventually veil the print in an indelible way — and he concedes the cost, “which certainly is more expensive”. The variable that decides between them is not in either account.

Data to record

Field	Why it is here
Everything carried forward from the coating record	Bottle strengths, drop volume, coating volume, paper, humidity at coating and drying
Relative humidity of the room at exposure, and dark time since drying	Palladium’s speed varies by a factor of five across the humidity range Ware measured
Ultraviolet source, distance, run-up time, and the dose from the test strip	The only number that transfers to your next print, and only within your own darkroom
Appearance of the print-out image when the frame was opened	The exposure guide, and the first evidence in any later diagnosis
Developer: which one, strength, temperature, age, approximate number of prints through it	Colour, speed and fog all depend on all four
Development: time, agitation, how the liquid arrived	Lap marks are a record of arrival order
Clearing: which sequence, bath times, bath ages, water hardness if known	Water hardness affects clearing time on Bostick and Sullivan’s own note
Masked-margin inspection after bath 3, under a bluish light, in words	The only published test of clearing you have
Whether bath 3 was prolonged, and by how long	A print that needed it is a print to watch
Wash: running or static, duration, changes
Reflection density of two or three patches, wet and again after 24 hours dry	The dry-down measurement this course does not yet hold for a binderless print
Image colour, by the course’s stated method, against a dated physical reference	Colour is a result here, not a description
Faults, in words, before you form a theory about them

Analysis

1. Was the exposure right? From the step tablet, find the step at which maximum density is first reached and the step at which density first lifts off paper white. The difference in log exposure between them is your exposure scale, and it is the number the experiment will make you defend. Ware’s own densitometry of print-out palladium gives about 2.4; his figure for the unmodified traditional platinotype sensitiser is about 2.0.

2. Did the negative suit the process? Compare that scale with your negative’s ultraviolet density range. Ware asks for about 2.4 for palladium; Bostick and Sullivan ask for 1.35 to 1.50 for their kit, which is a shorter negative for a system that includes a contrast agent this course does not use. If your negative is shorter than your measured scale, the print will be flat and no bath will fix it — the fix is in the negative, and the negatives lesson owns it.

3. Is the print cleared? This is where the honest answer is uncomfortable, and it is worth spelling out because it is not the answer the manifest for this page originally expected.

!There is no published spot test for residual iron in a finished print

The course looked for one. What the sources actually publish is three different things, and none of them is a spot test on a corner of the print.

The kits publish a ferricyanide test — for the sensitiser bottle, not for the print. Photographers’ Formulary’s test puts one drop of ferric oxalate into two crystals of potassium ferricyanide in water and reads the blue that forms. It is an assay of a solution’s quality before coating, and the previous lab runs it there.

Ware publishes a visual inspection, and it is the test this page uses: examine the print under a bluish light for any yellow stain of residual iron in the masked, unexposed margin, and prolong the final bath if it is there. Its power comes entirely from the masking, which is why masking is a step and not a preference.

The conservation literature publishes an instrument. The Getty atlas records that even after the best fixing and clearing procedures, enough iron remains in a platinotype to be detected by X-ray fluorescence at the iron K-alpha line at 6.40 keV, with different concentrations in the maximum and minimum density regions. That is a laboratory’s answer, not a darkroom’s.

Why nobody should improvise the obvious test. A ferricyanide solution on a corner of a print would indeed find iron(II) — and it would do it by making Prussian blue in the sheet, permanently, in a print you have just spent an evening on, on a bench that carries acidic baths. The acid rule alone rules it out. If you want the assurance, mask a margin, look at it under a bluish light, and prolong bath 3.

And a clean margin is not a guarantee. Chapman Jones demonstrated in 1895 that unexposed platinotype paper retained iron after four hours in dilute hydrochloric acid, detectable by hydrogen sulphide, and Ware records that his warning was largely forgotten until recently. The visual test finds the stain you can see. It does not find the iron you cannot.

4. What the accelerated ageing evidence says, and why it is the reason for the third bath. Matthew Clarke and Dana Hemmenway analysed residual iron in platinum and palladium printing tests processed four ways — long clear and long wash, long clear and short wash, short clear and long wash, short clear and short wash — and aged the results at 70 °C and 75 per cent relative humidity in the dark for four weeks. Their findings are the case for this page’s whole clearing section:

• In the worst category of processing short cuts, residual iron concentrations were significantly raised, and more so for palladium than for platinum.
• All the specimens showed little or no visible iron stain immediately after preparation and would have been judged acceptable as artists’ prints.
• After ageing, yellow stains appeared that correlated with the measured residual iron, progressively worse in the poorly processed specimens, and closely resembling the stain seen in historical platinotypes.
• The stain did not depend on the presence of platinum salts: a ferric oxalate sensitiser alone gave similar results.
• Papers sized with gelatin retained more iron than those sized with alum and rosin.

Ware adds the comparison that decides the course’s choice of sequence: artificially accelerated ageing of prints cleared by the Malde–Ware method “has been shown to produce no stain”.

5. Measure the dry-down and report it as your own. Density wet, density at 24 hours, on the same patches, with your densitometer’s own precision stated. The course holds no published figure for a binderless print and this measurement is worth more than a description.

Troubleshooting

“The highlights are yellow.” Residual iron: the print is not cleared, and the troubleshooting entry carries the diagnosis in full. Bostick and Sullivan state it directly — yellowing in the highlights means the clearing is incomplete — and their remedy is to increase the concentration of the clearing agent or the time in the bath. On this page’s sequence, prolong bath 3. Note their other observation: water hardness sometimes affects clearing time.

“A faint overall stain appeared and deepened over a week.” The same fault, caught later, and the Clarke and Hemmenway study is exactly this in the laboratory. What confirms it is the masked margin, if you masked; what confirms it definitively is X-ray fluorescence, which you do not have. Can it be saved? Re-clearing a dried print is a conservation treatment rather than a darkroom step, and Ware’s account of the professional method — reductive dissolution with sodium disulfite, then dilute hydrofluoric acid to remove the iron(II) as a fluoro-anion — is a description of what conservators do and emphatically not a procedure for a home bench: hydrofluoric acid is outside anything this course will put in a reader’s hands. The realistic answer is that a print stained by under-clearing is evidence, and the correct response is to change the process and reprint.

“The high values are a brownish grey rather than paper white.” Chemical fog, not iron, and the colour is the diagnosis. Ware’s comparative tests show an oxalate developer reducing some palladium(II) on its own, fogging the highlights brownish-grey — “quite distinct from the yellow stain of iron” — and worse in humid conditions. Willis specified a citrate developer for the palladiotype and his distributor said why. Switch developers and reprint.

“The print lost density in the clearing bath.” The bath was too strong or too acid for palladium. This is the fault the historic literature is full of: the platinum strength of hydrochloric acid, 1 in 60, dissolves palladium and partially bleaches the image, which is why Anderson’s figure for palladiotypes is 1 in 200. The modern kits carry the same asymmetry in citric acid — 2 per cent for platinum, 1.5 per cent for palladium, with the palladium sheet stating the reason in its own words. This page’s chelating sequence avoids the problem rather than managing it.

“The image came up unevenly, in bands or with a hard edge.” A lap mark: the developer reached one part of the sheet later than another. Pour faster, or use Auerbach’s dam.

“The image is weak and looks fibrous or grainy.” On platinum this is Ware’s classic fault — the salts washed out before they finished reacting. On palladium it is more likely under-exposure or a coating that was too thin; check the test strip and the coverage figure from the coating session before you change anything chemical.

“The shadows came out lighter than the tones next to them.” Tonal reversal, and it was decided at the coating stage rather than here. Ware associates it with a coating dried below about 30 per cent relative humidity, an oxalate developer, a lower-absorptivity paper, a contrasty negative and partial print-out. It happens far more with palladium than with platinum, and not at all in the print-out process.

“Fine black dots appear through the image and through the unexposed margins too.” Ware calls it the “plague of black spots” and reports the print-out route as apparently immune to it; the proximate cause here is a re-used developer that has been allowed to see light and has precipitated particulate metal into suspension. The troubleshooting entry separates the faults that look alike.

“The developer has gone very dark.” Expected. It is accumulating iron and metal. Filter the sludge through a coffee filter, replenish for evaporation, and keep it out of the light — a used bath is still photosensitive.

Clean-up

1. Return the developer to its bottle, filtering if a sludge has formed. It is not waste; it is stock.
2. Keep clearing baths 1 and 3; they have capacity left — about 50 to 60 and about 100 prints of 10 × 8 in per litre respectively. Discard bath 2, which does not keep.
3. Rinse the trays into the noble-metal waste container, not the sink, with the smallest volume of water that does the job.
4. Wipe the bench wet and wash the tongs separately from any silver bench’s tongs.
5. Wash your hands before you take your goggles off.
6. Check that nothing acidic has come near the hexacyanoferrate container, which should not be in the room in the first place.

Storage

The developer keeps indefinitely and improves in some accounts. Bostick and Sullivan: an indefinite life, replenished for evaporation, sludge filtered periodically, colour darkening with dissolved metal; “many printers swear the older the developer, the better.” Store it in the dark, because it is still photosensitive, and in glass or plastic rather than metal.

Clearing baths 1 and 3 keep between sessions; bath 3 is changed when discoloration begins to show. Bath 2 is made fresh for a day’s printing and discarded.

The finished print is stored on the general conservation terms: neutral pH paper-board folders, materials passing the Photographic Activity Test, and — the AIC’s own figures — about 20 °C with a drift of not more than 2 degrees, and 50 per cent relative humidity varying by not more than 5 per cent over 24 hours. The cost and permanence lesson takes that argument further.

And the print is easy to retouch, on Ware’s own note, using permanent watercolour pigments — which is a property of having no binder over the image.

Disposal considerations

The noble metal decides the route for almost everything on this bench. Developer, clearing baths 1 and 3, tray rinses and the first wash water all carry palladium in milligram quantities, and the palladium salt’s notified record carries H400 and H410 in about three-quarters of its reports. Ware’s own instruction for his first clearing bath is to save it for recovery of precious metal, and commercial printers do the same with the developer. At a student’s scale, recovery is a question rather than an answer: the cost lesson examines whether it is practical, and what a licensed route would be.

The oxalate is the other real chemistry in the container. Soluble oxalates are systemic toxins, and a 28 per cent bath is a concentrated one. It is not neutralised by dilution and it is not made harmless by being old.

The sulfite bath is the least troublesome stream, and it is worth saying why rather than implying it: sulfite oxidises in air to sulfate, which is why the bath must be made fresh, and a spent sulfite bath is substantially a sulfate solution carrying whatever iron it reduced. It still goes into the collected waste, because it has been through a print.

The general practice is collection and labelling, and the specific rule is jurisdictional. This course publishes no jurisdiction-specific disposal instruction. Check your local regulations; they govern. See the disposal ruling.

Questions

1. Your print develops fully in four seconds and you leave it in the bath for two minutes. What, if anything, has the extra 116 seconds done? Answer in terms of what is being consumed.
2. Two printers run the same developer for years and one throws it away after every print. Give the argument each of them would make, and say what measurement would settle it.
3. A print shows brownish-grey high values. A second print from the same negative, developed in a different bath, shows yellow high values. Which is which, and what does each tell you to change?
4. Why does the middle clearing bath contain a reducing agent rather than a second chelating agent?
5. You are offered a clearing sequence of “one bath of EDTA, five minutes, it’s not critical”. Using the Clarke and Hemmenway result, say what is wrong with judging that sequence by looking at the print on the day.
6. The historic clearing bath for palladium is hydrochloric acid at 1 in 200, and for platinum at 1 in 60. What is the acid removing, and what decides the difference in strength between the two?

Further experiments

Measure the dry-down properly, because the course cannot. Print a step tablet, read the density of five patches wet — blotted, not dripping — then at one hour, six hours and 24 hours. Plot the change against wet density. Then do the same on a fibre-base silver gelatin print from Part XIX. If the binderless print’s dry-down differs in size or in direction, you have measured something no source read for this course publishes, and it belongs in the comparison atlas.

Run the three developers side by side on one sheet. Cut a coated sheet into three, expose the three strips identically, and develop one in potassium oxalate, one in ammonium citrate and one in sodium citrate. The claims to test are specific: that the citrate baths leave clear high values where oxalate fogs them, that oxalate is faster and warmer, and that ammonium citrate is the most neutral of the three. Three independent sources say the last of those and none of them measured it.

Do the clearing sequence one bath at a time. Four identical prints: one cleared in bath 1 only, one in baths 1 and 2, one in the full sequence, one in the full sequence with bath 3 doubled. Inspect all four masked margins under a bluish light at the end of the session, and again at three months, six months and a year. That is the Clarke and Hemmenway experiment run at room temperature and on your own time, and it is the most useful thing anyone in this course can do with four sheets.

Test the carbon dioxide claim. Print one sheet backed by the felt blanket and one backed by a sheet of plastic, from the same negative, and compare the resolution in an area of high local contrast under a loupe. Ware states the mechanism; nobody in this course has looked.

Recap

The exposure makes iron(II) and nothing else you can see; the brown image in the frame is ferrous oxalate and not the picture. The developer contains no reducing agent — it dissolves the immobile photoproduct so that the iron(II) can reach the palladium — which is why the image arrives in seconds, why development time is not a contrast control, and why the bath accumulates iron and metal instead of exhausting. Three clearing baths follow, and they are three rather than one because chelation alone cannot detach all the iron(III) from the cellulose: bath 1 chelates what it can reach at pH 3 to 4, bath 2 reduces the rest to iron(II), bath 3 chelates that at pH 9 to 10 and leaves the paper alkaline.

The clearing is the step that decides whether the print survives, and the accelerated-ageing evidence is that a badly cleared print looks perfectly acceptable on the day and stains later in proportion to the iron left in it. The only test you have at the bench is a masked margin under a bluish light. It finds the stain you can see, and not the iron you cannot.

Check your understanding

Question 1. Your print reaches full density four seconds after the developer covers it. You leave it in for two minutes. What have the extra 116 seconds accomplished?

• They have increased density, since development is a rate process and more time means more metal
• Essentially nothing to the image: the reaction stops when the iron(II) photoproduct runs out rather than when the bath tires, so time past completion adds nothing — which is why the sources say you cannot overdevelop a platinum print and why Mougin says timing is not critical
• They have reduced contrast, since prolonged development compresses the scale
• They have started to dissolve the palladium image back off the sheet

Show the answer and why

Answer: Essentially nothing to the image: the reaction stops when the iron(II) photoproduct runs out rather than when the bath tires, so time past completion adds nothing — which is why the sources say you cannot overdevelop a platinum print and why Mougin says timing is not critical

This is the practical face of "the developer reduces nothing". What is consumed is the iron(II) that light made, and there is a fixed amount of it in each area of the sheet; once it has all reached the palladium, the reaction has finished. Auerbach's statement is the blunt version — density of image is determined only by exposure, not by development. The one thing the extra time does accomplish is real but not photographic: more of the sensitiser dissolves into the bath, which is part of why a reused developer accumulates iron and metal.

Question 2. Why is the middle clearing bath a reducing agent rather than a second dose of chelating agent?

• It neutralises the acidity of the first bath before the alkaline third one
• Some iron(III) is bound to the cellulose too tightly for chelation alone to detach; reducing it to iron(II) gives a species that binds the paper more weakly and is chelated better by the alkaline bath that follows
• It removes residual palladium salts, which EDTA cannot chelate
• It bleaches the yellow stain so that the print looks cleared

Show the answer and why

Answer: Some iron(III) is bound to the cellulose too tightly for chelation alone to detach; reducing it to iron(II) gives a species that binds the paper more weakly and is chelated better by the alkaline bath that follows

The problem the sequence solves is chemisorbed iron(III), and Ware's account is that chelation alone gets only part of it off the paper. Sulfite reduces iron(III) to iron(II); the alkaline bath then chelates that at pH 9 to 10, where the iron(II)-EDTA complex is strongest. Option four is the one to reject hardest: nothing here bleaches a stain, and a treatment that made a print merely look cleared would be the exact failure the Clarke and Hemmenway ageing study exposed.

Question 3. You raise the developer temperature by fifteen degrees, from 22 to 37 °C, and print the same step tablet. Predict what changes.

• Contrast rises sharply and maximum density increases, because the reaction goes further
• Image colour warms, and the practical risk is streaking because development becomes near-instantaneous; contrast is not a variable the developer controls, and the effect on it is at most secondary
• Nothing measurable changes; developer temperature affects only the time to completion
• The print lightens, because a hot bath begins to etch the palladium

Show the answer and why

Answer: Image colour warms, and the practical risk is streaking because development becomes near-instantaneous; contrast is not a variable the developer controls, and the effect on it is at most secondary

Two sources point the same way. The Getty atlas gives the mechanism — hotter development gives smaller metal particles and browner prints — and Photographers' Formulary warn that with a heated bath development occurs almost immediately, so the sheet must be submerged quickly and evenly or it will streak. On contrast, the AIC's summary governs: contrast is achieved mostly by exposure rather than by the developer. Wall's 1924 rule that dilution gives more brilliant prints is a real observation about strength rather than temperature, and it is a weak lever beside the negative.

Question 4. A print looks perfectly clean on the evening it is made and develops a faint overall yellow over the following week. What is the most likely cause, what test would have caught it, and can the print be saved?

• Fog from an old developer; a fresh developer would have caught it; the print can be re-developed
• Residual iron from incomplete clearing; the masked-margin inspection under a bluish light would have caught it on the night; and the honest answer is that re-clearing a dried print is a conservation treatment rather than a darkroom step, so the print is evidence and the response is to change the process and reprint
• Light fading of the palladium; storage in the dark would have caught it; the print recovers if kept dark
• Sulfiding of the image metal; a residual-thiosulfate test would have caught it; the print can be selenium-toned

Show the answer and why

Answer: Residual iron from incomplete clearing; the masked-margin inspection under a bluish light would have caught it on the night; and the honest answer is that re-clearing a dried print is a conservation treatment rather than a darkroom step, so the print is evidence and the response is to change the process and reprint

This is exactly the failure Clarke and Hemmenway reproduced in the laboratory: specimens with raised residual iron showed little or no visible stain immediately and would have been judged acceptable, then stained on ageing in proportion to the iron. Options three and four both misattribute the fault to the image substance, which is the one part of this object that is chemically inert — palladium does not fade and does not sulfide, and there is no thiosulfate anywhere in this process to test for.

Question 5. The historic clearing bath for platinum is hydrochloric acid at 1 in 60 and for palladium at 1 in 200. What is the acid doing, and what sets the difference?

• It dissolves unreduced metal salt, and palladium salts are more soluble so less acid is needed
• It holds the pH far below the point at which iron(III) hydrolyses, dissolves hydroxide that has already formed and supplies chloride to carry iron out as chloro-complexes; the strength is set not by what the iron needs but by what the image will survive, because palladium metal is etched by an acid that platinum tolerates
• It neutralises the alkalinity of the paper, and palladium papers are less alkaline
• It removes the developer, and palladium developers are weaker

Show the answer and why

Answer: It holds the pH far below the point at which iron(III) hydrolyses, dissolves hydroxide that has already formed and supplies chloride to carry iron out as chloro-complexes; the strength is set not by what the iron needs but by what the image will survive, because palladium metal is etched by an acid that platinum tolerates

The bath is aimed at the iron in both cases and the difference is entirely about the metal in the image. Anderson's figure, confirmed by McCabe and by Gottlieb in 1993, is that palladiotypes must be cleared at 1:200 rather than the 1:60 used for platinum, "which otherwise dissolves palladium and bleaches the image partially". The modern kits preserve the asymmetry in citric acid at 2 per cent against 1.5 per cent, and say so on the sheet. It is also why this course uses a chelating sequence instead: a bath chosen for what the image will survive is a bath that has been weakened away from what the iron needs.

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