The Platino-Palladiotype Process
IntroductionSubtle quality, total permanence and ease of working place the platinum-palladium print at the summit of alternative photographic printing. This nineteenth-century process, long thought to be obsolete, has in recent years enjoyed a renaissance among photographers who find it meets an aesthetic and expressive need in their personal work. This account comprises a set of instructions for making platinum-palladium prints by a modern method that employs readily-available materials and the greatest possible economy of use. This method was developed in collaboration with Professor Pradip Malde, a distinguished platinum printer, and we hope that the new controls introduced in the following pages will permit a consistency that has been difficult to achieve with traditional platinum-palladium printing.
Using the methods described in these notes, platinum-palladium images can be obtained in colours ranging from blue-black, through neutral greys, to rich sepia browns. Although the maximum reflection density obtainable on a totally matte platinum-palladium print is considerably less than that of a glossy silver-gelatin print, the tonal separation in the middle values is excellent, and great delicacy can be achieved in the highlights. What platinum prints may lack in impact, they make up for in their beguiling luminosity.
The process of making a platinum print differs considerably from using silver-gelatin papers. First, there is the 'hand-crafted' aspect. What may seem to some photographers a time-consuming labour, can convey to others a sense of satisfaction in being "true to one's materials" in the making of every print.
The very low sensitivity of platinum-palladium printing paper requires an intense ultra-violet light source for exposure. An adequate through-put of light can only be achieved (with commonly available technology) by means of contact-printing. This in turn imposes its own discipline and modus operandi, characterized by working with large formats, a careful control of negative making, and unmanipulated, 'straight' printing. The low sensitivity also has an advantage: there is no need for a darkroom. Fine prints can be made with simple resources in ordinary room light.
Many potential platinum printers are put off by the thought of the expense of the process; it is true that the cost of raw materials is high, but only wastage through failure is truly expensive. With this firmly in mind we have done our best to provide a method which, if followed carefully, will minimize the likelihood of failure.
This manual does not propose platinum-palladium as a substitute for, or competitor with, silver-gelatin printing, as it was at the turn of the century. Not all images are improved by printing in platinum. And even a well-executed platinum print may become almost indistinguishable from one in silver-gelatin if its subtle qualities are obscured by crude glass or bad lighting. In this situation its only remaining virtue is the invisible one of permanence. Our purpose in trying to revitalize interest in the process of platinum-palladium printing is simply to enhance the richness and variety of available photographic print-making techniques, as our visual sensibility tends to become dulled by the homogeneity of commercial reprographic media. In the end, the aim is to place a greater control of the print into the hands of the printmaker.
Disadvantages of the Traditional PlatinotypeThe traditional development platinotype process used for the last hundred years, has consisted in coating paper with an aqueous solution of ferric oxalate and potassium tetrachloroplatinate(II) and drying it thoroughly before exposure. Development follows in potassium oxalate solution; then unreacted materials are dissolved out in a succession of clearing baths of dilute hydrochloric acid in order to 'fix' the image before finally washing in water. There is no doubt that this traditional method is capable of beautiful results -in skilled hands- but in attempting to follow it, some of the following difficulties may be encountered:
- Ferric oxalate is virtually unobtainable in the UK (it is not listed in the Fine Chemicals Data Base) and is an exorbitant price. Chemists describe it as an 'ill-characterised' substance, that is to say - not easy to prepare as a solid, of dubious purity and a highly variable composition depending on its method of preparation.
- The customary technique of coating the paper with a brush wastes unnecessarily large amounts of platinum.
- The necessity for test strips as a guide to printing exposure consumes even more.
- The recommended developing bath involves a large amount of a very poisonous substance (potassium oxalate) and the use of hydrochloric acid for clearing the excess chemicals is likely to damage the structure and permanence of the paper base.
- An additive (potassium chlorate) is often recommended for contrast control which tends adversely to affect the image quality.
- The maximum print density obtainable is rather low unless special measures are taken, such as double-coating or using super-saturated sensitizer solutions at elevated temperatures.
Improved Method for Platinum-Palladium PrintingAll these disadvantages have been overcome in the new method described here, in which:
- Ferric oxalate is replaced by ammonium iron(III) oxalate, which is easily obtainable at low cost in a highly pure form.
- A glass rod provides an economical method of spreading the sensitizer.
- The new method is a printing-out process which requires little or no development, unlike traditional platinotype, in which the full image does not appear until development. The sensitized paper is allowed to acquire a controlled degree of humidity, which permits the formation of the platinum/palladium image during the exposure, a process that can easily be monitored, making test strips unnecessary.
- Unreacted sensitizer and reaction products are removed from the paper by the non-toxic, modern chelating agents - disodium and tetrasodium Edta - leaving the paper at a safe, high pH, with no residual iron.
- Contrast control is achieved either by mixing platinum and palladium or by controlling the relative humidity.
- The maximum density is higher because a more soluble platinum salt is used, yielding a higher metal coating weight which makes double-coating unnecessary.
- The printing-out process has a self-masking effect, which accommodates a wide range of negative density.
There is one recommendation that we would particularly stress for the newcomer: begin with palladium alone; it was one third of the price of platinum (before the days of the 'cold fusion' red herring, and its use as a catalyst in automobile exhausts), it is more forgiving of shortcomings in technique, and it is capable of finer quality than platinum on a wider range of papers, with a greater variety of colour. The permanence of palladium is, for all practical purposes, hardly less than that of platinum.
With the soaring market price of palladium sometimes exceeding even that of platinum, the making of pure platinum prints becomes a more desirable objective than it was recently, when palladium cost one third of the price of platinum. Unfortunately, it is more difficult to achieve a print in 100% platinum, than it is in palladium or a palladium-platinum mixture, because the platinum chemistry is intrinsically slower, and many substances, which do not affect palladium, will inhibit the platinum chemistry. This fact has not been acknowledged by many printers, who have claimed that their preference for palladium, or palladium-platinum, was simply economic. Because of the ease of palladium-platinum printing, and the high quality achievable, there are very few workers using 100% platinum at the present time. At least one famous US printer confesses that he can't make satisfactory pure platinum prints any more by the traditional method, using ferric oxalate, as he could 15 years ago. He attributes the failure to new industrial methods introduced for manufacturing fine papers.
The present print-out 'ammonium system' of platino-palladiotype, however, is designed to provide good 100% platinotypes, provided some care is taken.
Chemicals required for the Sensitizers
- Ammonium Iron(III) Oxalate 30 g
- Ammonium Tetrachloroplatinate(II) 5 g
- Ammonium Chloride 1.8 g
- Palladium(II) Chloride 3 g
Making up the Sensitizer SolutionsThe sensitizer is prepared when required by mixing equal volumes of an iron solution with either a platinum solution or a palladium solution or, for a mixed metal print, a combination of both. It is not advisable to store large quantities of these solutions, especially the ammonium tetrachloroplatinate(II) solution, which tends to decompose with time. The instructions given below for making them up are scaled to realistic quantities which should suffice to make about sixty 10"x8" platinum-palladium prints.
- The iron solution (Fe) contains the complex salt Ammonium Iron(III)
Oxalate at a concentration of 60% w/v. This substance is also known as Ammonium
Ferrioxalate, or as Ammonium Trisoxalatoferrate(III) Trihydrate. Its formula
To make up:
- Weigh out 30 g Ammonium Iron(III) Oxalate into a small beaker (ca100 cc).
- Add exactly 33 cc of distilled water (from a measuring cylinder) and stir well to dissolve the solid.
- The solution becomes cold, so gently warm the beaker in a bath of hot water (about 50 °C) to assist dissolution.
- Within 5 minutes nearly all the solid will have dissolved to form an emerald-green solution. The volume will be correct (50 cc) so it does not need to be made up. A little remaining solid may be ignored.
- Filter the solution (Whatman # 1 paper, is adequate) directly into the clean, dry, brown storage bottle, and store at room temperature in the dark.
- The platinum solution (Pt) contains Ammonium Tetrachloroplatinate(II),
at a concentration of 25% w/v. This substance used to be called Ammonium
Chloroplatinite. Its formula is (NH4)2[PtCl4].
To make up:
- Weigh out 5 g Ammonium Tetrachloroplatinate(II) into a small beaker.
- Add exactly 18 cc of distilled water (from a measuring cylinder) and dissolve the solid with stirring at room temperature. This gives a final volume of 20 cc ­p; there is no need to 'make it up'.
- Filter the solution through a small filter paper (#1) directly into the brown storage bottle.
- N.B. Allow the solution to stand for at least 24 hours before first use.
- The palladium solution (Pd) contains Ammonium Tetrachloropalladate(II)at
a concentration of 20% w/v. Its formula is (NH4)2[PdCl4].
The solution can simply be made by dissolving 5 g of this substance in distilled water and making up to 25 cc. However, the following method of making it is usually less expensive.
To make up:
- Dissolve 1.8 g Ammonium Chloride in about 20 cc of hot (70-80 °C) distilled water.
- Add 3 g Palladium(II) Chloride with stirring (which should be well-powdered. HAZARD: wear a dust mask)
- Keep hot, with stirring, until all has dissolved (about an hour).
- Make up to 25 cc with distilled water.
- Filter and bottle when cool.
Chemicals required for the Processing Solutions
- Ethylenediaminetetraacetic acid disodium salt, Na2Edta
- Ethylenediaminetetraacetic acid tetrasodium salt, Na4Edta
- Kodak 'Hypoclear' powder or sodium metabisulphite or sulphite
Making up the Processing SolutionsThe wet-processing procedure requires reagents which perform both the developing and clearing (chiefly the latter). These are solutions of the sodium salts of ethylenediaminetetraacetic acid - otherwise known as Edta, for short. Two separate clearing baths, each of strength about 5% w/v, are recommended: dissolve 50 g of the solid in each litre of water at room temperature. The first bath is of disodium Edta, with a 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 10"x8" prints.
Clearing of the residual iron compounds from the paper is improved by immersion next in a bath of Kodak Hypoclearing Agent interposed between the two Edta baths; alternatively a solution of sodium sulphite can be used. 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; the advantage is that these last two baths have a high pH (ca. 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 PrecautionsSince platinum printing involves handling materials that are normally unfamiliar to most photographers, it would be irresponsible not to indicate the hazards involved. Bluntly stated, these 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 to those particularly susceptible to allergic reactions, and of course to children.
All chemicals should be clearly labelled and stored in a safe, child-proof place. Any spillages should be promptly mopped up; if you get any of these chemicals on your skin, then wash them off immediately with plenty of cold running water and it is very unlikely that you will come to any harm. Dry powdery solids present a greater risk than solutions, because the dust can be inadvertently inhaled, so it should be well-contained and a mask worn.
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 large, 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; 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). Appropriately enough, 'platinum allergy' was first observed in photographic workers handling platinotype paper in 1911. It is a wise precaution 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. It is better not to store large amounts of dried sensitized paper; the workplace should be well-ventilated. 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 airborne dust. They offer a safer alternative for the sufferer from platinum allergy.
The Ultra-violet Light Source. This should be installed in such a way that your eyes cannot be directly exposed to the radiation; otherwise suitable protective goggles must be used. Skin exposure should be kept to a minimum. Do not use lamps with a significant 'short wave ultra-violet' (UVB) content, because this is much more destructive to living tissue, and may also produce the carcinogenic gas, ozone.
Choosing the SensitizerPlatinum 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 exposure range) with great 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 in order 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. The characteristics of some sensitizers are summarised below showing their dependence on the Relstive Humidity.
Table. Characteristics of Platinum-Palladium Sensitizers
|Platinum-palladium (3:1)||32||1.2||1.6||0.6||warm black|
Speed is relative, arithmetic, referring to middle tones.
logH is the Printing Exposure Range, extending from fog+0.04 to 0.9 Dmax.
Extent of Development is in logH units (0.3 = 1 stop). 0 indicates total print-out.
N.B. These parameters will vary with the choice of paper.
Mixing the SensitizerMixing should be done 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"x4" print should take 0.3 to 0.4 cc of sensitizer; a 10"x8" should require between 1.2 and 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 platinum print mix equal volumes of Fe and Pt; let the mixture stand for one hour in the dark at room temperature before coating.
- For a palladium print mix equal volumes of Fe and Pd, which may be coated immediately.
- 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 hypodermic 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 glass makes an ideal mixing vessel - provided you give up drinking out of it! The volume of iron solution (Fe) must always be equal to the total volume of platinum (Pt) plus palladium solutions (Pd). Mix the solutions well by drawing the liquid gently in and out of the syringe three times. If the sensitizer contains platinum, it is definitely advantageous to allow it to 'mature' before coating: draw it up into a large syringe to minimise evaporation, and leave in a dark place for an hour or two, but no longer. If the sensitizer is pure palladium, it may be coated immediately.
A possible problem with coating sensitizer at rather low room temperatures is crystallization. If this is observed, add a drop or two of warm distilled water to dissolve the crystals before coating.
Coating the PaperAll manipulation of the sensitized paper can be carried out under quite bright 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 'drink' 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). Coat the paper by the glass rod technique described in Preparations.
DryingAfter coating, allow the sensitized paper to rest horizontally at room temperature until its surface has dried sufficiently to appear non-reflective to the subdued (tungsten) light. Then dry the sensitized paper for about ten minutes in a warm (40° C) air stream, or at room temperature for an hour, and preferably in the dark. Examine the dried sheet of paper carefully for signs of crystals on the surface, which show up as points of reflected light: these can damage negatives.
StorageThe sensitized paper should be used within a few hours; otherwise, it must 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.
HumidifyingThe key to the printing-out process lies in controlling the humidity of the sensitized paper just before exposure. The effect of relative humidity on the extent of print-out, colour and contrast is summarised in the Table from which you will see that optimum results are obtained between 50% and 80% RH. 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. If you have a hygrometer, you can simply make use of the prevailing relative humidity (if suitable) to achieve a predictable result by hanging the paper in a dark place at room temperature (close to 20 °C) for an hour or two before exposure.
Greater control, however, is provided by a humidifying tank i.e. a tray with close-fitting lid, in which the paper may be placed 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 are: ammonium chloride, which provides an RH of 80% at 20 °C, and calcium nitrate tetrahydrate which provides an RH of 55%. 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 the tank, which therefore contains an atmosphere of 100% RH; but in this case the timing of the humidification is critical: from 5 to 20 minutes for a warm-toned result; a longer humidification of 30 to 40 minutes in the water vapour will yield fuller print-out and a colder image tone. Humidification 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 ExposurePlace the sensitized paper in the printing frame in contact with the emulsion side of the negative, interposing a protective polyester film if you think it advisable. Check that the coated area adequately covers the image, and ensure that the rubber backing sheet is in place between the paper and the back of the frame. Tighten the pressure back or clips of the frame.
Position the frame centrally under your UV lamp and switch on, starting the timer. (Some UV sources require a warm-up period before they reach their optimum output - these you may prefer to have switched on in advance.)
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, so no generalizations can be made beyond saying that 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 result from the humidity control.
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 be frightened of extending the exposure in order to resolve detail in the highlights; the shadows will not 'block up' totally, like 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 protective goggles and gloves.
Wet Processing Procedure for Platinum-Palladium PrintsTray-process the exposed paper with intermittent agitation at room temperature in:
|1||Disodium Edta (5% w/v)||10 mins|
|2||Rinse in water||half min|
|3||Kodak Hypo Clearing Agent (working)||10 mins|
|4||Rinse in water||half min|
|5||Tetrasodium Edta(5% w/v)||10 mins|
|6||Wash in running water||minimum 30 mins|
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 bath (1) is about 50 10"x8" 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.
You should judge the success 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), which may cause subsequent embrittlement of the paper base. If no yellow stain is perceptible, clearing has been adequate.
The procedure is appropriate to palladium or mixed platinum-palladium prints. In a pure platinum print the print-out is less vigorous, and special care is needed as follows.
Printing in Pure 100% PlatinumTo achieve the best quality, the following are the cardinal points to observe:
- The choice of paper is critical. It must contain no trace of gelatin size (see below) or alkaline buffer (calcium carbonate). Several of the Crane's papers (Crest parchment, AS8111, Cover Natural White) work well, as does Buxton. Wyndstone Vellum and other 'parchmentized' papers may also give fine results.
- Newly-mixed sensitizer should be allowed to 'mature' in the dark for an hour before coating, to obtain the best Dmax. This needn't interrupt the flow of work, because this mixed sensitizer appears to be stable for years, so a batch can be prepared in advance of printing sessions.
- A generous post-hydration (after exposure, but before immersion in the wet processing baths) will give the image the best chance to complete its print-out. 20-30 minutes over water at room temperature, or 2-4 minutes over water at 40 °C are recommended.
- If highlight detail is still deficient, or 'grain' evident due to the fibrous structure of the paper, then the first processing bath (disodium Edta) may be replaced with the more energetic traditional platinotype developer bath of 30% potassium oxalate (poisonous!). It may even be used hot.
- Although it is a 'cheat', adding just one drop of palladium solution to the sensitizer can improve print-out. It appears to work as a catalyst. Re-used 'developer', that may contain some palladium, probably also helps in this way.
- The judicious use of Tween 20 can be beneficial to assist the sensitizer penetrate the paper fibres and smooth out the tones. The optimum concentration depends on the chosen paper. Tween does not keep very well in dilute solution. It is best to make up a 20% solution in distilled water as stock: one drop (ca. 0.05 cc) of this per 1 cc of sensitizer gives a final Tween concentration of ca. 1%. Less than this may suffice.
Drying and FinishingAfter about one hour's wash, drain the print without touching its delicate wet surface and air-dry it, face up, at room temperature. There should be no tendency to curl; any slight cockling around the edges may be counteracted by drying the sheet while still humid under pressure, 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 -but use best quality permanent watercolour pigments, not Spottone, which pentrates the paper too deeply.
Control of Print ContrastThere is no substitute for making the negatives correctly in the first place. Having prepared a negative of approximately the right density range, the contrast may be fine-tuned in the printing process by two main controls:
- mixing of platinum and palladium in various ratios
- controlling the humidity of the sensitized paper before exposure
If humidity control seems too bothersome, then the most consistent results can be obtained at normal UK ambient RH (40-70%) by mixing the platinum and palladium solutions in the ratio of about 3:1, respectively; this sensitizer has a contrast and speed that are fairly constant over wide variations in humidity and it yields a long range of well-graduated neutral tones and a good Dmax.
Control of Print ColourThere are several factors, including the nature of the paper, 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:
- A lower RH tends to produce browner tones.
- Palladium yields warmer prints than platinum.
- Gelatine-sized papers tend to yield warmer tones than those sized with Aquapel or starches.
Platinum Printout and GelatinIn choosing a paper substrate for the humid, print-out method of 100% platinum printing, it is very important to avoid those that are sized with gelatin. This is not a 'view' - it is on scientific record: proteins can bind strongly to platinum(II), and the resulting complex is not readily reducible to Pt metal by the iron(II) photoproduct.
In the last century, popular wisdom had it that "Water is the Great Enemy of the Platinotype". This is incorrect. But water does assist the slow reaction of the platinum salt with gelatin, which is its true enemy:
chloroplatinate(II) + gelatin + water -> irreducible Pt(II) gelatin complex
This reaction takes several hours to complete. The rapid and complete dehydration recommended for platinum papers in the past simply prevented this reaction by minimising the time of contact with hostile ingredients in the paper, such as gelatin, thereby retaining the reactivity of the platinum(II) salt towards reduction by the iron(II) photoproduct - but only upon development, in hot, concentrated potassium oxalate solution.
The platinotype can therefore tolerate gelatin, only if the paper is rapidly dried immediately after coating, and kept so. Hence all the C19th strictures about desiccation that I referred to earlier. If the paper is *pure*, humidity can be tolerated, so the printing-out platinotype (R.H. ca. 80%) needs a gelatin-free paper for best results. Papers sized with polysaccharides (starches or Klucel G) or alkyl ketene dimer (Aquapel) are compatible with the process. It is a matter of historical record that Willis only succeeded with pure platinum printing by avoiding gelatin-sized papers.
It should be emphasised that this problem does *not* arise with palladium printing, because it is much more reactive and is not inhibited by gelatin. This may account in part for the swing towards palladium by many contemporary, so-called 'platinum' printers. Certainly, the 100% platinum print is rarely made successfully today.
It has always been my aim to make Pt/Pd printing easier and more reliable for the artist. It is often the case in science that, to reach a 'simple' goal, one has to travel through a jungle of complexities. The Pt/Pd medium demands a careful acknowledgement of the chemical facts, and great attention to detail.
I spent several years puzzling over 'unaccountable' failures in my platinum prints before I grasped the problem. The following summary may assist others to understand their difficulties or failures, and hopefully diminish them in the future:
- Gelatin inhibits the printing of platinum, but not palladium. (There are many types of gelatin - some are likely to be more inhibiting than others). It greatly 'desensitizes' the chemical response by preventing the precipitation of Pt metal: loss of highlight detail will be the first consequence. Almost complete loss of image is an extreme possibility.
- The damaging reaction between gelatin and the usual platinum compound (tetrachloroplatinate(II)) is slow at room temperature - taking hours to complete. Heat makes it go faster. This reaction also requires the presence of water to bring the reagents into contact.
- Temperature, Humidity, Concentration and Time are therefore all crucial factors in determining the extent of the ill-effects caused by the gelatin. Obviously, high humidity (print-out) systems are most at risk; the bone-dry (development) systems least.
- In some circumstances (e.g. rapid drying, low humidity, low gelatin concentration, short delays between coating and printing, cold workrooms) an acceptable platinum print may be obtained in the presence of gelatin. But it will never be as good as one properly made, without gelatin.
- None of these problems arises with palladium, which prints easily and freely. So anyone using a Pt/Pd mixture should satisfy themselves that there is actually platinum in their prints, and they are not just washing it down the drain and making palladium prints.
- The acid test is your ability to make a premium quality 100% platinum print with your materials. You may be surprised how difficult it can be! (It took Willis 20 years to get it right.) Choice of a 'pure' paper is vital.
- I now use 'Buxton' paper, hand-made to my specification (see Paper article). Using the printing-out procedure described above, one can make a platinum print with the same delicate highlight gradation as a palladium print - but it still requires care. There are also some commercial papers that work well with 100% Pt, but almost any paper will work with Pd.