Print-out platino-palladiotype

Every other noble-metal print in this atlas is developed out. This one is not, and the reason is a single change in what the light leaves behind: a soluble iron(II) photoproduct instead of an insoluble one, which lets the reduction happen in the sheet while the exposure is still running. Read the platinotype first for the developed route, and ammonium iron(III) oxalate for the salt that makes this one possible.

The chemistry

In dry ferric oxalate the photoproduct is ferrous oxalate, insoluble: it stays where it was made and cannot go and find the metal salt, which is why the traditional process needs a developer at all and why a dry ferric oxalate sensitiser prints out hardly at all.

Ammonium iron(III) oxalate provides the trisoxalatoferrate(III) anion instead, and its photolysis leaves [Fe(C₂O₄)₂]²⁻, a soluble complex ion. Ware’s statement of the practical consequence is the pivot of modern siderotype: if the sensitised paper holds enough water — which any cellulose paper does at 70 to 80 per cent relative humidity, where it carries about 8 per cent of water by weight — the ions can migrate and reduce the palladium(II) or platinum(II) to metal during the exposure. The image is formed by light rather than developed afterwards, and only clearing baths are needed.

⚗Three consequences Ware lists, and why the second one changes the workflow

Print-out is self-masking. The darkening shadows inhibit their own further darkening, so a longer density range in the negative can be accommodated simply by exposing longer. Contrast control moves from the sensitiser to the exposure.

Prints can be judged by inspection, without test strips. That is a different way of working from the developed processes, where the sheet looks like almost nothing until it goes in the tray.

The route appears to be immune to the “plague of black spots” that sometimes afflicts ferric oxalate development. Ware reports it as an observation rather than as a mechanism, and this page does the same.

Humidity is a reagent here, not a condition. That is the hardest thing to internalise about the process. A sensitised sheet that is too dry cannot print out at all, because the ions cannot move, and the practice therefore carries a table of saturated salt solutions for constant-humidity enclosures at 20 °C — with lithium chloride at the dry end at 15 per cent RH.

The cation is chosen rather than incidental. Ware’s print-out palladiotype deliberately uses the ammonium salt, or palladium(II) chloride dissolved in ammonium chloride, because the process turns on ammonium ions and controlled humidity. Bostick & Sullivan’s Ziatype kit is built around a lithium palladium solution instead. Both choices are on the same thermodynamic ground: dissolve lithium chloride and the solution warms, with an enthalpy of −37.2 kJ/mol, because the small highly charged lithium ion binds four water molecules tightly; dissolve ammonium chloride and the solution gets cold, because the large diffuse ammonium ion binds water weakly. A cation that holds water tightly changes how much water a coating must absorb before its ions become mobile, and how readily it gives that water up again. Lithium and ammonium are not interchangeable even though both are monovalent cations that leave no residue.

Historical workflow

Ware credits Giuseppe Pizzighelli in 1887 with observing print-out using sodium iron(III) oxalate — the sensitiser whose behaviour the Platinomicon reconstructs. It is the same period in which the trade was using ammonium and sodium ferric oxalates to make its fastest blueprint papers, with Wall’s caveat attached that those papers “do not keep so well”.

The modern practice is a coated sheet held at a known humidity, a contact exposure under ultraviolet watched to completion, and wet processing that begins with clearing rather than development. Bostick & Sullivan’s Ziatype sheet gives an EDTA and sodium bisulfite clearing bath with a stated capacity; the disodium EDTA page records that the sheet lists “250 g EDTA clearing agent” without naming which of four substances sold as EDTA it means, and that where the course cannot establish which salt a published formula meant, it says so rather than choosing quietly.

The image

Colour is set at the bench, before the exposure rather than after it. In a developed process the printer chooses a bath; here the levers are the cation, the humidity and any addition to the sensitiser, and they are all committed before the sheet goes under the lamp. That is a genuinely different way of working and it is the reason the process has a following.

Surface and scale are the noble-metal family’s: metal among the fibres, no binder, matte, with a long scale made longer by the self-masking of print-out.

And the numbers this entry once owed are now in the course. Ware’s Table 7.1 gives relative speed, exposure range, development in log exposure units and image colour against relative humidity, and Part XXV reads it. For palladium in his ammonium system:

Relative humidity	Relative speed	Development still needed, log H	Image colour
32 per cent	0.5	0.4	Sepia
55 per cent	1.3	0.2	Van Dyke brown
80 per cent	2.5	0	Warm black

Read the third column first, because it is what makes this a separate process rather than a technique. The amount of work left for the developer falls to nothing as the sheet gets wetter; at 80 per cent relative humidity the image is complete when it comes out of the frame, and what follows is clearing rather than development. That is the same statement as “humidity replaces the developer”, in numbers.

The first column is a factor of five across the span — more than two stops — and the fourth is why colour is committed before the exposure here. Platinum behaves in the opposite direction: its relative speed falls from 1.8 to 1.0 over the same span while its colour barely moves, which is why Ware recommends a platinum-palladium mixture of about 3:1 to a printer who does not want to control humidity at all — the two speed effects very nearly cancel. And the scale is the family’s longest: about 2.4 in exposure range for palladium at the wet end, with a mid-tone slope near 0.78, against about 1.9 and 0.96 for platinum.

One thing still not established here is a cost per print for the print-out route specifically. Part XXV does the cost arithmetic for the metal, which is the dominant term, but this entry does not claim a figure of its own.

Permanence

The same as the developed noble-metal prints: an inert metal held in the paper fibres, with the survival of the object governed by the paper, the clearing and the storage rather than by the image substance.

One difference is worth recording because it is a permanence claim in disguise. Ware reports the print-out route as apparently immune to the black spots that sometimes afflict the ferric oxalate development route — a defect that appears over time in some prints and is a real conservation problem. Immunity to it, if it holds, is a durability argument rather than a convenience one. The course records the observation and does not have a mechanism for it.

Hazards

Palladium salts, Level B and classified as skin sensitisers. Ammonium iron(III) oxalate, Level B: soluble oxalates are systemic toxins, and the sensitiser is a concentrated solution of one. Lithium chloride, Level B, where a lithium palladium solution is used. EDTA and sodium bisulfite in the clearing baths.

Where platinum is used in the mixture rather than palladium alone, everything on the platinotype entry applies and the course’s ruling with it: students perform palladium and never handle platinum. A print-out process does not change the classification of a platinum salt.

The humidity enclosure is an unusual piece of apparatus for a darkroom and deserves a mention here: a saturated salt solution in a sealed box is a wet chemical hazard sitting in a room with photographic paper in it, and lithium chloride at about 800 g per litre is not a mild solution.

Where the course teaches it

Part XXV, as chemistry taught in full and as a route the labs do not take.

Noble Metal Chemistry and the Platinotype owns this process’s mechanism, and its section Print-out, where humidity replaces the developer is this entry’s subject stated as the part’s single most important variation. It supplies the arithmetic for “enough water” that this entry could only gesture at — paper conditioned near 70 per cent relative humidity carries about 8 per cent water by weight, which works out at roughly ten water molecules per trisoxalatoferrate(III) ion in the amorphous regions of the cellulose, enough for short-range diffusion while the sheet still rattles and looks dry. It carries Pizzighelli’s 1887 discovery with the detail that he used the sodium salt and tested the others, the Ziatype’s lithium palladium and why lithium and ammonium are not interchangeable, and Ware’s Table 7.1 figures above.

The labs of the part take the developed route, and that remains true. Preparing the Sensitiser and Coating for Palladium and Exposing, Developing and Clearing a Palladium Print are both develop-out. So this entry’s original statement stands, and can now be made precise instead of provisional: the course teaches this process’s chemistry and does not set it as a lab.

But humidity is a measured variable in the part rather than a footnote, and that is where a reader gets closest to it in practice. Experiment: Tonal Scale, Contrast and Image Colour in Palladium dries strips of one coating at three different humidities and measures what changes, using saturated salt solutions to hold the boxes at known values. Its design contains the sharpest available demonstration of this entry’s argument: the experiment deliberately does not compare developers at 80 per cent relative humidity, because at that humidity there is nothing left for a developer to do and three developers would be three clearing baths. A variable that vanishes is the strongest evidence that the print-out route is a different process and not a setting.

The Ziatype kit is the living commercial descendant and records what its maker publishes and what it does not. And The Process Comparison Atlas is the assignment that would place a print from this route beside the others on one measured scale, for a reader who pursues it.