Palladiotype

Palladium entered photography as a substitute and stayed as a preference. It is the noble-metal print this course actually teaches, and the honest reason is a safety ruling rather than an aesthetic one — but the process turns out to be better as well as permissible, which is a happier position than most substitutions reach. The platinotype is the entry to read beside it.

The chemistry

The same two-stage siderotype chemistry, with one metal changed. Paper carries light-sensitive ferric oxalate and a tetrachloropalladate that is not light-sensitive; ultraviolet reduces iron(III) to iron(II) and takes the oxalate off as carbon dioxide; an oxalate or citrate developer dissolves the insoluble ferrous oxalate so the iron(II) can move; the mobile iron(II) reduces palladium(II) to metal. Two iron(II) per palladium atom, because palladium(II) takes two electrons.

⚗Why palladium is the easier metal

The reason lies one step earlier than the reduction. Ware’s account has the electron transfer proceed not through the [MCl₄]²⁻ ion but through its aquated form, in which water has displaced one chloride. At 20 °C the equilibrium constant for that substitution is 0.17 for palladium against 0.015 for platinum, so at sensitiser strength more than half the palladium is aquated and less than a fifth of the platinum. Palladium’s solution chemistry is more labile as well, so the reduction is faster.

Ware’s summary of what that buys: palladium is faster to print and achieves a finer, smoother quality than platinum under a wider range of conditions and paper choices. It is not the cheaper compromise it is usually described as.

Contrast has to be put back. Palladium gives a lower-contrast, browner, very smooth image, and the standard method for raising contrast is an oxidant in the sensitiser. Photographers’ Formulary supplies 0.26 g of potassium chlorate in the 30 mL ferric oxalate bottle for palladium against 0.18 g for platinum, and explains the mechanism plainly: light makes iron(II), iron(II) converts the palladium salt to free metal, and the chlorate reconverts iron(II) to iron(III), so the chlorate acts as a restrainer, increases contrast and holds the whites. Bostick & Sullivan put the same comparison in words instead of grams — palladium requires twice as much chlorate as platinum to achieve contrast.

The cation is not always a spectator. At 326.4 against 294.2 for the anhydrous sodium salt, a gram of the potassium salt is about a tenth less palladium; and Ware’s print-out palladiotype deliberately uses the ammonium salt, or palladium(II) chloride dissolved in ammonium chloride, because that process turns on ammonium ions and controlled humidity. The palladium salt’s page works through both.

Historical workflow

Coat, dry, contact-print under ultraviolet, develop, clear in three baths, wash. What distinguishes the palladiotype from the platinotype in the historical record is that Willis treated it as a different process rather than as a cheaper platinum, and Ware’s reading is that a shrewd and painstaking chemist would not have changed the chemistry if the old developer had served.

Willis specified trisodium citrate dihydrate at 20 per cent w/v with citric acid, used for four to five minutes, and clearing baths from a stock of 20 per cent citrate with 9 per cent citric acid, diluted eightfold for use, in three sequential baths of at least ten minutes each followed by a thirty-minute wash — rather than the potassium oxalate and hydrochloric acid of the platinotype. Sodium citrate’s page records that Ware gives the developer’s acid strength twice and not quite identically, 2 per cent in one chapter and 2.2 per cent in the one where he converts the original imperial units, and that the course quotes both rather than splitting the difference.

His reason was not about the image. The palladiotype developer and clearing baths, his distributor’s notice says, “have no tendency to injure the beautiful surface of the paper employed for coating”, where prolonged immersion of a matt platinotype in the oxalate and acid baths does destroy its bloom. Photographers’ Formulary’s modern kit sheet preserves the same distinction, warning that palladium metal is etched from a print by acid strong enough to be routine for platinum.

🕰What happened when printers ignored the instructions

On Paul L. Anderson’s evidence, which Ware quotes, many American workers developed their palladiotypes in their pre-existing, used platinotype developer — nearly saturated potassium oxalate, complete with its accumulated metal residues — rather than mixing the trisodium citrate bath the manufacturer specified, and cleared them in hydrochloric acid diluted 1 in 60 rather than 1 in 200.

Ware’s reading is that the two departures tended to be self-cancelling: the stronger acid etched away the palladium fog that the oxalate developer had produced. The prints came out well enough that the discrepancy went unexamined. That is why the two traditions of palladiotype processing give different prints from the same sensitiser, and it is a good argument for recording what you actually did rather than what the sheet said.

⁇1916 or 1917?

The Getty atlas dates the introduction of the palladiotype process to 1916. Ware dates it to 1917 and gives the evidence: an unannounced demonstration of a “brand new printing paper” by W. H. Smith, technical manager of the Platinotype Company, to the Croydon Camera Club in January 1917; the public demonstration of the revealed “Palladiotype” to the Camera Club of London on 8 March 1917; and Willis & Clements advertising “Palladiotype Sepia” in the United States that year.

This course follows Ware, because his date is tied to named meetings and dated advertisements while the atlas gives the year alone. A reader who needs the date for a catalogue entry should cite both.

The image

Warmer, browner and very smooth, and lower in contrast from the same negative than platinum. Ware records that adding mercury(II) salts to a platinum sensitiser produces an image resembling palladium’s, which is how the sepia platinotype was made before palladium was used — so the look this process gives for nothing was previously bought with a mercury compound.

Surface is the noble-metal family’s: metal among the fibres, no binder, matte, the paper’s own character part of the picture.

Scale is usually a little longer than platinum’s, which is another way of saying that contrast has to be put in deliberately.

Permanence

The same argument as platinum, and the same everyday enemy. A noble metal held in the paper fibres with no binder is about as chemically inert as a photograph gets; what governs a print’s survival is relative humidity, the paper, and how well the iron was cleared out of it.

The clearing is where the practical permanence lives, and it is where the kits are least clear. Ware’s palladiotype practice uses disodium EDTA at about 5 per cent w/v and gives its capacity; the disodium EDTA page records the trap — Bostick & Sullivan’s platinum/palladium and Ziatype kits list “250 g EDTA clearing agent” without naming the salt, their kallitype sheet names the tetrasodium salt, and the difference between the disodium and tetrasodium salts is not a conversion factor but six units of pH. Ware’s instruction elsewhere is blunt: do not use tetrasodium EDTA, which is alkaline and will cause iron stains.

!The permanence claim that may have moved metals without anyone checking

The Getty atlas records that only after 1917 did photographers begin to use palladium and then platinum–palladium mixtures on paper they coated themselves, and that most prints made before the First World War are simple platinotypes. Today the “Pt/Pd print” of a portfolio is a mixture and the ratio is a control.

That puts a hard question to every permanence claim made about platinum printing: if most such prints contain palladium, the claim has been transferred from one metal to another without anybody checking. The course states the question and does not answer it, and the honest thing to put on a label is which metal made the image, and in what proportion.

Hazards

Palladium salts are Level B and classified as skin sensitisers, despite palladium’s reputation beside platinum. That is the substance-level classification, and it is why gloves are not optional.

Soluble oxalates are systemic toxins; the sensitiser and one of the two developer traditions are built on them. Potassium chlorate, the contrast agent, is an oxidiser with its own entry. Hydrochloric or citric acid clearing baths are the routine wet chemistry.

The developer may itself reduce palladium. Ware’s tests show that an oxalate bath can reduce palladium(II) to some extent by itself, causing a brownish-grey fogging of the highlights by palladium metal, quite distinct from the yellow stain of iron — a fault worth knowing because it is diagnosed by its colour.

Waste carries palladium, iron and oxalate and is collected rather than discharged. The metal is worth recovering.

Where the course teaches it

Part XXV, and it is the practical heart of that part.

Noble Metal Chemistry and the Platinotype is the lesson, and its organising claim is the one this entry assumes without arguing: the developer reduces nothing — it dissolves, and understanding that explains the image, the colour, the permanence and the price in one move. It also settles what sets the image colour, and the answer is four things of which one is not a bath: the metal, the humidity, the developer and its temperature, and any additive.

Lab: Preparing the Sensitiser and Coating for Palladium and Lab: Exposing, Developing and Clearing a Palladium Print are the two Level B halves — about two millilitres of sensitiser mixed in a shot glass, then a measured ultraviolet dose and a bath in which the image arrives at once. Experiment: Tonal Scale, Contrast and Image Colour in Palladium runs eight conditions on three papers and is where this entry’s claims about scale and colour become your own measurements. Cost, Permanence and When a Platinum Print Earns Its Price does the arithmetic on what the metal in one print actually weighs and examines the permanence claim rather than repeating it. The Additives We Study and Do Not Reproduce is the Level D lesson — mercury in a platinum sensitiser, lead in Willis’s coating solutions, and the dichromate in the developer — and Assignment: The Process Comparison Atlas closes the cluster on one negative printed in every process a reader can reach.

⚗What Ware's Table 7.1 puts numbers on, and why humidity is the surprise

The part reads Ware’s table for relative speed, exposure range and image colour against relative humidity, and the result is the strongest argument in the cluster that humidity is a reagent here rather than a condition.

Relative humidity	Palladium speed	Palladium colour	Platinum speed
32 per cent	0.5	Sepia	1.8
55 per cent	1.3	Van Dyke brown	—
80 per cent	2.5	Warm black	1.0

The two metals move in opposite directions, which is why Ware recommends a platinum-palladium mixture near 3:1 to a printer who does not want to control humidity: the effects very nearly cancel. Palladium’s own span is a factor of five in speed — more than two stops — and moves the colour further than a change of metal ratio would.

Two more figures from the same source belong beside them. The exposure scale is long: about 2.4 for Ware’s print-out palladium and about 2.0 for the unmodified traditional platinotype sensitiser, against the classic cyanotype’s 0.9. The maximum density is modest, about 1.45 — well under a glossy silver gelatin print, which is worth knowing before an expensive process disappoints somebody expecting a deeper black. And palladium’s mid-tone slope is about 0.78 against platinum’s 0.96, which is the numerical form of “lower contrast and greater delicacy in the high values”.

And the part’s ruling on which metal a student handles is not a matter of difficulty. Palladium is performed and platinum is never handled, because the notified classification for the platinum salt carries H334 — respiratory sensitisation — in almost every report, and HSE sets halogeno-platinum compounds 2,500 times tighter than platinum metal, the tightest figure anywhere in this course. The palladium salt’s record carries no respiratory sensitisation, and that absence is the whole of the difference: a skin sensitiser is controllable by a glove and an asthmagen is not controllable by anything a page can assume.

The accessibility route this entry records is argued in full in the cost lesson rather than mentioned: palladium alone rather than platinum, small prints, and the palladium-toned kallitype as a legitimate substitute. It is part of the design and not an afterthought.