The platino-palladiotype process

Writer / Dr. Mike J. Ware
Photography / Dr. Mike J. Ware and Peter Bargh

The following method of printing in platinum, palladium, or a mixture of both metals, has been evolved to overcome the difficulties and disadvantages of the traditional platinotype. Newcomers are strongly advised to begin with palladium alone – even though it is at present as expensive as platinum: palladium more readily gives a finer image on a wider range of papers. Its permanence is, for all practical purposes, hardly less than that of platinum.

Always be careful when handling chemicals. Read the health and safety instructions.

This method is a printing-out process, unlike traditional platinotype which requires development. The sensitized paper acquires a controlled degree of humidity, which allows formation of the platinum-palladium image during the exposure – a process that can easily be monitored, making test-strips unnecessary.

For those who have no experience of Iron printing please check out the article Preparations to Alternative Printing that explains what you need and how to prepare for the processing.

The printing-out process has a self-masking effect, which accommodates a wide range of negative density.

Chemicals required for the Sensitizers:

Ammonium Iron(III) Oxalate 30g
Ammonium Tetrachloroplatinate(II) 5g
Ammonium Chloride 1.8g
Palladium(II) Chloride 3g

GPR reagent grade is adequate in all cases.

Making up the Sensitizer Solutions

The sensitizer is prepared by mixing the iron solution with an equal volume of either the palladium solution or the platinum solution, or, for a mixed-metal print, a combination of both. It is not advisable to store large quantities of the ammonium tetrachloroplatinate(II) solution, which tends to decompose with time. The instructions below are scaled to realistic quantities, which should suffice to make about 60 10x8in platinum-palladium prints.

The iron solution (Fe) contains the salt Ammonium Iron(III) Oxalate at a concentration of 60% w/v. This substance is also known as Ferric Ammonium Oxalate, or Ammonium Ferrioxalate, formula (NH4)3[Fe(C2O4)3].3H2O.

To make up

1Dissolve 30g Ammonium Iron(III) Oxalate in 30cc of warm (about 50°C) distilled water with stirring.

2Make up to a final volume of 50cc with distilled water at room temperature.

3Filter (Whatman #1 paper, is adequate).

4Store in a brown bottle at room temperature in the dark. This solution keeps indefinitely.

The palladium solution (Pd) contains Ammonium Tetrachloropalladate(II), formula (NH4)2[PdCl4], at a concentration of 20% w/v. The solution can simply be made by dissolving 5g of this substance in distilled water and making up to 25cc. However, the following method is less expensive.

To make up:

1Dissolve 1.8g Ammonium Chloride in about 20cc of hot (70-80 °C) distilled water.

2Add 3g Palladium(II) Chloride with stirring (which should be well-powdered. HAZARD: wear a dust mask).

3Keep hot, stirring occasionally, until all has dissolved (about an hour).

4Make up to 25cc with distilled water. Filter and bottle when cool. This solution keeps indefinitely.

The platinum solution (Pt) contains Ammonium Tetrachloroplatinate(II), at a concentration of 25% w/v. This substance used to be called Ammonium Chloroplatinite, formula (NH4)2[PtCl4].

To make up:

1Dissolve 5g Ammonium Tetrachloroplatinate(II) in 15cc of distilled water at room temperature.

2Make up to 20cc with distilled water.

3Filter or decant off any small amount of yellow solid formed, and bottle immediately, avoiding unnecessary exposure to the air. This solution should keep for six months or so.

4NB. Allow this solution to stand for at least 24 hours before first use.

Chemicals required for the Processing Solutions

Ethylenediaminetetraacetic acid disodium salt
Ethylenediaminetetraacetic acid tetrasodium salt
Kodak ‘Hypoclear’ powder, or sodium sulphite

Making up the Processing Solutions

The wet-processing procedure uses solutions of the sodium salts of ethylenediaminetetraacetic acid – otherwise known as EDTA.

Two separate clearing baths, each of strength about 5% w/v, are recommended: dissolve ca. 50g of the solid in each litre of water at room temperature.

The first bath is of disodium EDTA, with an acidic pH around 3 to 4, which is optimum for complexing iron(III) and is acid enough to avoid hydrolysis leading to yellow iron stains; the capacity of a one-litre bath will be about 50 10x8in prints.

Clearing of the residual iron compounds from the paper is improved by immersion next in a bath of Kodak Hypoclearing Agent at working strength; the inorganic sulphite in this tends to reduce any residual iron(III) to iron(II) which is then removed in the final tetrasodium EDTA bath; these last two baths have an alkaline pH (around 9) which is optimum for complexation of iron(II) and leaves the paper in a beneficial alkaline condition.

The wet-processing sequence is summarised below.

Hazards and Safety Precautions

The chemical hazards always sound worse than they really are in practice, at least for the small quantities required by platinum printers. Provided that you observe clean working methods and adopt a modicum of protection, the risk is very low, except for those particularly susceptible to allergic reactions, and of course to children.

Ammonium Iron(III) Oxalate
Like all soluble oxalates, this is classified as highly toxic, i.e. capable of causing death or permanent injury (especially kidney damage) if taken by mouth. However, several grams would have to be ingested to cause this. The chemical takes the form of non-volatile crystals, so there is little risk of inhalation, but it is an irritant to skin, eyes and mucous membranes.

Ammonium Tetrachloroplatinate(II)
Contact with this, and other chloro-complexes of platinum, is known to cause symptoms of asthma and dermatitis or urticaria; some allergic individuals may become particularly sensitized to these chemicals. The symptoms disappear on removing the cause: if you develop this allergy, then platinum printing is not for you (but you could still use palladium). It is wise to wear polythene or rubber gloves when handling the material; never touch the surface of sensitized paper and never immerse fingers in the processing solutions. Do not store large amounts of dried, sensitized paper. Incidentally, platinum metal itself is not implicated in this – so curators may take comfort that there is no risk in handling processed platinotypes!

Ammonium Tetrachloropalladate(II) and Palladium(II) Chloride
These are classified as moderately toxic and are not known to be dangerously irritant or allergenic. The chief risk is from inhalation of airbore dust. They offer a safer alternative for the sufferer from platinum allergy.

Mixing the Sensitizer

Solutions should be mixed at room temperature under tungsten lighting. The volume of sensitizer required is proportional, of course, to the area to be coated, but it also depends on the paper type and ambient conditions. As a guide, a 5’x 4′ print should take 0.3-0.4 cc of sensitizer; a 10’x 8′ should require between 1.2-1.6 cc. This is less than half the volume specified by the traditional methods – an economy made possible by the rod-coating technique. It is usually convenient to batch-coat several sheets at one time.

For a palladium print mix equal volumes of Fe and Pd; the mixed sensitizer may be coated immediately, but will keep for years.

For a platinum print mix equal volumes of Fe and Pt; let the mixture stand for at least one hour in the dark at room temperature before coating (a ‘maturing’ that improves the maximum black).

For a platinum-palladium print you may combine Pt and Pd in any ratio; the volume of Fe used must equal the combined volume of Pt plus Pd. Let the mixture stand for one hour in the dark before coating.

These small volumes are conveniently measured and delivered by means of disposable plastic syringes (without needles!) of capacity 1, 2 or 5 cc. Dedicate a separate syringe for each solution to avoid cross-contamination of the stock solutions, and use another syringe for dispensing the mixed sensitizer. A small liqueur or ‘shot’ glass makes an ideal mixing vessel – provided you give up drinking out of it!

Handling the Paper

For a description of rod-coating see Preparations to Alternative Printing. All manipulation of the sensitized paper can be carried out under moderate tungsten lighting, but you should avoid fluorescent light or daylight. Room temperature should be normal (18-22 °C), if too low the sensitizer may crystallise, if too high it may penetrate the paper too deeply.

Paper that has been stored at low relative humidity (less than 50% RH) may imbibe excessive amounts of sensitizer and coat unevenly. If you live in a dry climate, it may be advantageous to pre-humidify the paper to 70-80% RH before coating (see below).

Drying

After coating, allow the sensitized paper to rest horizontally at room temperature until its surface sheen has disappeared. Then either dry the sensitized paper in a warm (40° C) air stream for about ten minutes, or at room temperature for an hour in the dark. Examine the dried sheet of paper carefully for signs of crystals on the surface, which can damage negatives.

Storage

The sensitized paper is best used within a few hours; otherwise, it should be stored in a light-tight, air-tight container, in the presence of a desiccant such as silica gel or anhydrous calcium chloride, below 10% RH, in order to prevent chemical fogging. Paper may be stored for six months in this way without loss of quality.

Humidifying

The key to the printing-out process lies in controlling the humidity of the sensitized paper just before exposure. Optimum results are obtained at relative humidities (RH) between 50% and 80%. Below 50% RH there is only partial printout and considerable development, above 80% RH the maximum density of the image tends to weaken because the sensitizer can diffuse too deeply into the paper. You can simply make use of the prevailing RH (check with a hygrometer if it is suitable) by allowing the paper to dry in air in a dark place at room temperature for an hour or two before exposing it.

Greater control, however, is provided by a constant-humidity box, i.e. an enclosure (cat litter trays are very suitable!) with close-fitting lid, in which the paper may be held face down, over, but not in contact with, a saturated aqueous solution which provides an atmosphere of constant, known relative humidity. The most useful saturated solutions at 20° C are:

ammonium chloride, RH = 80%
sodium chloride (common salt) RH = 75%
calcium chloride RH = 45%

It is important that there should be excess solid salt in contact with its saturated solution, and that the paper should be evenly exposed to the vapour. The time of exposure in the humidifying tank should not be less than half an hour, for the sake of evenness; the upper limit is not critical and can be a few hours.

A simpler method of humidifying is to use pure water in an enclosed hydrating tray, which therefore contains an atmosphere of 100% RH; but in this case it is essential to time the hydration: from 5 to 20 minutes for a warm-toned result; a longer hydration of 30 to 40 minutes in the water vapour will yield fuller print-out and a colder image tone. Hydration for more than an hour may lead to weakening of the image density, and clearing problems (chemical fogging or an irremovable yellow stain of iron hydroxide) because the paper will absorb an excessive amount of water. Over-humidified paper is also more likely to damage negatives during contact printing.

Making the Exposure

Interpose a protective polyester film between paper and negative if you think it advisable. Ensure that the rubber backing sheet is in place between the paper and the back of the frame. Exposure times will, of course, depend on the power and efficiency of your particular light source, the negative density range, and the proportion of platinum to palladium; times will probably be in the order of a few minutes, and palladium printing will be about twice as fast as platinum. The correct exposure is readily found by inspection of the printed-out image, without the need for test-strips, if you bear in mind the small degree of development that may occur in the wet processing.

Users of this printing-out process will discover that it has advantages over development printing. Under conditions of full print-out (80% RH) you simply continue exposing until the image has the desired appearance; do not hesitate to extend the exposure to ‘print down’ detail in the highlights; the shadows will not ‘block up’ totally, as in development papers, because the printing-out process has a self-masking action in regions of high print density. If you decide to try dodging or burning (which is usually unnecessary) be sure to wear UV-protective goggles, and gloves.

Wet Processing Procedure for Pt-Pd Prints

Tray-process the exposed paper, face down, with intermittent agitation at room temperature in:

Disodium EDTA (5% w/v) 10 minutes
Rinse in water half minute
Kodak Hypo Clear (working) 10 minutes
Rinse in water half minute
Tetrasodium EDTA (5% w/v) 10 minutes
Wash in running water 30 minutes minimum

This is simple and non-critical, and may be carried out in ordinary tungsten lighting; its purpose is chiefly to remove excess chemicals and reaction products. The capacity of one litre of bath (1) is about 50 10x8in prints. The spent solution should be saved for recovery of precious metals. Do not allow the processing solutions to come in contact with your skin; use print tongs or gloves.

Judge the completion of the wet-processing by examining the print, under a bluish light, for any yellow stain of residual iron in regions of unexposed sensitizer (in the area under the mask). If no yellow stain is perceptible, clearing has been adequate.

100% Platinum printing

The procedure above is appropriate to palladium or platinum-palladium prints. In a pure platinum print the chemistry is less vigorous, especially at low RH or if gelatine is present. If you wish to use 100% platinum then observe the following points carefully:

The choice of paper is critical. It must contain no trace of gelatin size or alkaline buffer (calcium carbonate). Several of the Crane’s papers (Crest parchment; AS8111; Cover Natural White) work well with platinum, as do Buxton, and Wyndstone Vellum.

Newly-mixed sensitizer should be allowed to ‘mature’ in the dark for at least an hour (and preferably 12 hours) before coating, to obtain the highest density. This needn’t interrupt the flow of work, because the mixed sensitizer appears to be stable for months, so a batch can be prepared in advance of printing sessions.

A generous post-humidification (after exposure, but before immersion in the wet processing baths) will give the image the best chance to complete its print-out. Leave the print over water for 20-30 minutes at room temperature, or for 2-4 minutes over water at 40°C.

If highlight gradation is still deficient, or ‘grain’ evident due to the fibrous structure of the paper, then the first processing bath (disodium EDTA) should be replaced with the more energetic traditional platinotype developer bath of 30% potassium oxalate. (CAUTION: highly poisonous.) It is more effective used hot.

Addition of Tween 20(tm) surfactant can assist the sensitizer to penetrate the paper fibres and smooth out the tones. The optimum concentration depends on the chosen paper, but should be in the order of 0.1-0.5%.

Although it is a cheat, adding just one drop of palladium solution to the sensitizer can improve print-out. It appears to function as a catalyst. Re-used developer Bath (1), which may contain some palladium, probably also helps in this way.

Drying and Finishing

After a half to one hour’s wash, drain the print without touching its delicate wet surface and air-dry it, face up, at room temperature. Any slight cockling around the edges may be eliminated by pressing the sheet while still humid, between thick sheets of photographic-quality blotting paper or Multisorb or papermakers’ felt – there is no danger of it sticking because the surface is non-tacky. The print is easy to retouch with best quality permanent watercolour pigments.

Control of Print Contrast

There is no substitute for making the negatives correctly in the first place. Having prepared a negative of approximately the right density range (about 2), the contrast may be fine-tuned in the printing process by two main controls:

The choice of platinum: palladium ratio
The RH of the sensitized paper before exposure

Platinum and palladium sensitizers differ in the contrast and colour of print they produce: palladium yields the warmer tones and a softer image (i.e. a longer printing exposure range) with greater delicacy in the high values. This provides a useful control, because the platinum and palladium solutions can be combined in any ratio in the sensitizer to fulfill your wishes for the finished print. However the two metals print out with different speeds, palladium being the faster by a factor of about two. A mixed print will therefore not contain the two metals in the same ratio as in the sensitizer, but will be depleted in platinum.

Control of Print Colour

There are several factors that influence the colour of a platinum-palladium print, which can range from gingery-brown to bluish-black. This is an area for experiment and personal taste:

Lower RH on exposure tends to produce browner tones.
Palladium yields warmer tones than platinum.
With pure palladium, gelatine-sized papers yield warmer tones than Aquapel(tm)- or starch-sized papers.