Jps pt/Pd paper

3 INVESTIGATION OF THE METHOD

There are several parameters involved in platinum printing and choices to be made at each stage. These have been separately investigated and the findings for each are summarised below.

3.1 Choice of Paper Substrate

Traditionally the platinotype calls for a 100% cotton-rag paper of the kind intended for fine-art printmaking or watercolour. Such papers have a high alpha-cellulose content, little non-cellulosic polysaccharides and no lignins. Many varieties of 'rag' paper are listed by the suppliers¹⁶, but we have found that they differ greatly in their suitability for the process. Clearly the absorbency will be a prime factor; papers intended for watercolourists, having Cobb test values in the range 20 to 25, seem satisfactory: more absorbent papers will consume larger amounts of sensitizer with no improvement in image quality, and less absorbent papers are prone to lose metallic platinum from their surfaces during wet processing. The absorbency is controlled by the inclusion of a sizing agent - either internally during manufacture, or subsequently, by application to the surface. Details of the Manufacturers' sizing agents and other additives are not usually published, but it appears that papers internally sized with the alkylketene dimer Aquapel™ are particularly suitable. Some gelatine tub-sized papers were also found to work well with palladium (but not platinum), and even an alum-rosin sized paper, although the archival permanence of the latter is open to question¹⁷. Additional surface sizing with gelatine or dextrin was not found to give any improvement in the performance of modern, well-sized papers. Clearly there is no benefit in allowing the sensitizer solution to penetrate deeper into the paper than light can (see §3.3). Papers containing alkaline buffering agents in large amount may cause hydrolysis of the iron(III) sensitizer or precipitation of calcium oxalate; an unbuffered paper of neutral or slightly acid pH is to be preferred.

As to surface texture, a great variety is available: the heavily calendered 'hot-pressed' papers yield the best image resolution, but cold-pressed (called 'not') or even 'rough' surfaces are available for pictures in which a more obtrusive surface texture is desired. All the experiments in this work were performed on hot-pressed papers.

The weight of the paper may also influence image quality; the lighter weights tend to provide higher maximum optical density and a smoother surface, but at the cost of greater fragility, which becomes apparent when the paper is subjected to the wet processing procedure. Most experiments were carried out with the heavier weights, in the range 200-300 g/m².

Table 2 lists some papers that are readily available in the UK and have been found satisfactory. Every paper has its own idiosyncrasies, and small differences in speed, colour, contrast and ease of clearing must be expected between them. While all these papers should work well with the palladium process, the image quality with the platinum process may be more variable. This greater selectivity on the part of platinum is due to the slower kinetics of the reduction reaction, and the fact that it can be inhibited by ligands such as gelatin binding strongly to Pt(II).

Table 2. Some Papers suitable for Platinum and Palladium Printing

Manufacturers name	Surface	Sizing	Weight g/m^2	Comments
Arches Aquarelle	HP	gelatine	300	Off white
BFK Rives	HP	internal	300	Velin Cuve
Cranes AS 8111	HP	alum-rosin		Writing
Fabriano Artistico	HP	gelatine	300	Off white
Fabriano 5	HP	gelatine	300	50% cotton
Hollingsworth Kent	HP	internal	160	Drawing
RKB Arches	HP	internal	300	White
Van Gelder Simili Japon	HP	internal	225	Cream
Whatman Watercolour	HP	internal	290	White

All papers are 100% cotton unless otherwise indicated. Those sized with gelatin are unsuitable for 100% Pt printing.

3.2 Composition of the Sensitizer Solution

As mentioned in §2.1, the photosensitive iron(III) compound used in most of the traditional recipes was the oxalate, Fe₂(C₂O₄)₃.5H₂O. This is, unfortunately, an ill-characterised, amorphous salt of variable composition¹⁸ and unknown structure¹⁹. It is hard to purify, especially to free it from traces of iron(II) which will tend to fog the image, and it is not generally available from chemical suppliers. Much to be preferred is the salt ammonium trisoxalatoferrate(III) trihydrate, (NH₄)₃[Fe(C₂O₄)₃].3H₂O, (otherwise called ammonium iron(III) oxalate or ammonium ferrioxalate), which is readily available from several suppliers²⁰ in a crystalline form of high purity. It may also be easily prepared from commonly available chemicals according to the new method described in the Appendix. A stock solution, of concentration 1.40 mol/dm³, was prepared by dissolving 30 g of the salt in a minimum volume of warm distilled water and making up to a final volume of 50 cm³. This solution, which is nearly saturated at room temperature, was used in all the tests. It keeps well in the dark, but may deposit some crystals below 20 °C.

Potassium trisoxalatoferrate(III) is also readily available, but is not sufficiently soluble in water to provide an adequate metal coating weight as a sensitizer. Indeed, the presence of potassium ions was avoided altogether in the sensitizer in order to prevent premature crystallization of potassium trisoxalatoferrate(III), which will otherwise be formed by metathesis. Sodium trisoxalatoferrate(III), on the other hand, is very soluble and may be used as a sensitizer, but the hygroscopic character of the Na⁺ cation affects the water content of the sensitized layer after equilibration (see §3.4). It did not yield images of such good quality as the ammonium salt.

For platinum printing, the best compound was found to be ammonium tetrachloroplatinate(II), (NH₄)₂[PtCl₄], which is readily available²¹ and very soluble, although care must be taken to avoid oxidation by the air and precipitation of the insoluble (NH₄)₂[PtCl₆]; moreover, any alkali can cause the precipitation of Pt(NH₃)₂Cl₂. A stock solution of concentration 0.67 mol/dm³ is conveniently obtained by dissolving 5 g of the salt in a minimum volume of distilled water and making up to a final volume of 20 cm³; the solution must be made up at least 24 hours before use, for reasons explained in §5.3. Equal volumes of this and the 1.4 molar ammonium trisoxalatoferrate(III) solution are then mixed to make a sensitizer having the correct chemical equivalence between iron and platinum, with a small excess of iron. If a large excess of the iron compound is used in the sensitizer, "bleeding" of the metal image may occur during the wet processing.

The platinum salt specified by the traditional methods is potassium tetrachloroplatinate(II), K₂[PtCl₄]. While it may also be used in the present sensitizer, it has the disadvantage of a lower solubility than the ammonium salt, giving a saturated solution of concentration only ca. 0.43 molar at room temperature, so appropriate adjustment must be made to the relative volumes taken for the sensitizer. The coating weight and maximum optical density will consequently be lower; moreover, use of the potassium salt introduces the risk of unwanted crystallization of potassium trisoxalatoferrate(III) as the sensitizer dries, which may degrade the image. The double coating procedure, which was often recommended in the past to improve the density of the blacks obtained with potassium tetrachloroplatinate(II), becomes unnecessary if the ammonium salt is used.

For printing in palladium, a 0.70 molar solution of ammonium tetrachloropalladate(II) was used. This may be obtained by dissolving 5 g of the salt in distilled water and making up to 25 cm³, but it is more economic to prepare a solution of it from the less expensive palladium(II) chloride and ammonium chloride, as described in the Appendix.

Sodium tetrachloropalladate(II) may also be used but, as mentioned above, the presence of Na⁺ will affect the humidity and consequent colour of the palladium image. A solution of ammonium tetrabromopalladate was also found to yield excellent image quality, but provides a sensitizer that is 'slower' by a factor of about two in the exposure, due to the more intense absorption spectrum of the anion creating a larger internal filter effect and thereby lowering the value of f (§2.3).

To summarise: the aqueous sensitizer solution consists of a mixture of equal volumes of 1.40 molar (NH₄)₃[Fe(C₂O₄)₃].3H₂O and 0.68 to 0.70 molar (NH₄)₂[PtCl₄] or (NH₄)₂[PdCl₄]. Once mixed, a palladium sensitizer may be used immediately or within a few hours. With platinum, however, tests have shown that a definite advantage is to be gained by allowing the mixed sensitizer to stand in the dark at room temperature for one to two hours before coating the paper with it; the maximum density is improved thereby. Presumably some slow ligand exchange takes place between the two complex anions, but the reasons for this effect are otherwise unclear. No advantage has been found in adding excess oxalic acid or oxalates to the sensitizer, and excess chloride ion tends to inhibit image formation.

3.3 Coating the Paper with Sensitizer Solution

Traditionally the sensitizer solution was brushed over the paper, but this method tends to be wasteful of precious metal, and requires skill to avoid unevenness. A more economical method, giving very homogeneous coatings, was devised for the present experiments: the sheet of paper is clipped or taped to a flat glass plate and a suitable volume of sensitizer (see below) is expelled from a small (1 or 2 cm³) hypodermic syringe (without needle) in an even line across the top of the paper; the solution is then spread by drawing it down the length of the paper using a glass rod of length equal to the desired coating width. The rod is not rotated, but the 'line' of sensitizer is drawn up and down the paper, with very little pressure, slowly and evenly several times (at least three, but more than six offers no additional benefit). Any excess sensitizer is absorbed with the edge of a blotting sheet, otherwise the standing liquid will crystallise. A suitable glass spreader with handle can be simply made by bending Pyrex glass rod of diameter 6 to 12 mm, or preferably thick-walled capillary tubing,( which is fabricated to a higher standard of straightness). Sensitizing may be carried out under bright tungsten lighting, but sunlight and fluorescent light are excluded.

The volume of sensitizer that is imbibed by the paper in one coating operation does not increase significantly after four or five 'passes' have been made across it, each 'pass' lasting five to ten seconds. The use of excess sensitizer only leads to crystallization and a degraded image. A second coat can be applied after a period of drying, and is said to enhance the maximum density, but this was thought unnecessary with the present method, and would have introduced even more variables into it.

For the papers listed in Table 2, specific coating volumes were found to lie in the range 22 to 29 cm³/m², depending on the paper and ambient relative humidity. Pretreatment of the paper before coating, either by drying or humidifying it, will influence the quantity of sensitizer that it can imbibe and, ultimately, the maximum optical density that can be obtained. It was not advantageous either to desiccate or humidify the papers to values outside the normal (UK) ambient range of R.H. 40 to 80 %, prior to coating: very dry paper imbibes more sensitizer without marked improvement in the maximum optical density; very humid paper imbibes less sensitizer and tends to give a weaker image, with diminished maximum density.

A specific coating volume of, say, 28 cm³/m² of the sensitizer described in §3.2 corresponds to a platinum metal coating weight of ca. 0.01 mol/m²; i.e. ca. 2 g/m² for platinum and 1 g/m² for palladium. These figures are comparable with the coating weights of conventional silver-gelatine papers. A typical 10" x 8" print, for example, requires a coated area of dimensions ca. 0.26 x 0.21 m to allow adequate margins, and will take up between 1.2 and 1.6 cm³ of sensitizer solution. Assuming that about half the volume of the paper is a void that can be occupied by solution (a proportion that is consistent with the measured densities of paper and cellulose), these volumes correspond to a depth of ca. 0.05 mm penetrated by the sensitizer. This figure for the thickness of the sensitized layer was confirmed by microscopic examination of cross sections, which showed that it extended about 0.05 mm from the surface in most cases.

The volume of sensitizer used in this method of coating is about half that specified by the traditional methods. The cost of raw materials at 1986 prices is ca. £2.50 for platinum and £0.60 for palladium, per 10"x 8" print.

3.4 Drying and the Control of Paper Humidity

The historical methods stress the importance of complete drying of the sensitized paper, either by a stream of hot air or even an uncontrolled degree of radiant heat. Such complete dehydration of the paper cannot yield a 'print-out' effect, and may even degrade the image quality. A certain humidity in the sensitizer is essential for 'print-out', as was recognised in the past, but the earlier workers lacked any means of controlling the humidity. In the present work, the water content of the sensitised paper was controlled before exposure by allowing it to equilibrate at room temperature for one to two hours within an enclosure of constant, known relative humidity.

Table 3. Saturated Solutions and Desiccants providing Atmospheres of Constant Relative Humidity at 20 °C

Name of Salt	Formula	RH%	p(H2O)	Solubility
SOLID DESICCANTS
Silica gel	SiO2.xH2O	0.1	0.017
Calcium chloride anhydrous fused	CaCl2	2	0.34	-
Calcium chloride anhydrous granular	CaCl2	9	1.52	-
SATURATED SOLUTIONS
Calcium chloride hexahydrate	CaCl2.6H2O	32	5.7	279
Potassium carbonate dihydrate	K2CO3.2H2O	44	7.7	147
Calcium nitrate tetrahydrate	Ca(NO3)2.4H2O	55	9.6	75
Ammonium chloride + Potassium nitrate	NH4Cl + KNO3	73	12.7	30 + 13
Ammonium chloride	NH4Cl	80	13.8	30
Sodium carbonate decahydrate	Na2CO3.10H2O	91	16.0	22
Water	H2O	100	17.54	-

p(H₂O) = pressure of water vapour in mm mercury.at 20 °C

solubility is in g of solid per 100 g water at 20 ºC.

Simple humidifiers were constructed, in which the sensitized papers could be supported face down a few cm above the surface of saturated aqueous solutions of various inorganic salts, containing excess solid. Table 3 lists these salts and the relative humidities that they give rise to in the atmosphere above them²². As will be seen in §4, control of the sensitizer humidity not only governs the degree of 'print-out' but also provides a means of controlling certain characteristics of the image, namely the contrast (for platinum) and the colour (for palladium).

If the sensitised paper is not going to be used within a few hours of coating, then it must be stored in a totally dry state to avoid chemical fogging. Drying was accomplished by placing the coated paper in a warm air stream at ca. 40 °C for ten minutes and then transferring it to a desiccator containing silica gel or anhydrous calcium chloride for storage in the dark at room temperature. In this environment it can be kept without deterioration for some weeks, and possibly months. About two hours before it is required for use, it must be rehydrated in an appropriate constant humidity enclosure.

Atmospheric relative humidities were checked in the present work by a hair hygrometer. Figure 1 shows the water absorption isotherm for cellulose; it should be noted that the 'hysteresis' in the curve causes the ultimate water content of the paper, for a given atmospheric relative humidity, to be slightly dependent on whether the equilibrium is approached from a 'wet' or 'dry' state. In practice this does not seem to cause perceptible differences in the images, provided that sufficient time is allowed for equilibration. At very high values of the relative humidity, 95-100%, the water content of the paper becomes somewhat indeterminate, and may continue to increase with time due to filling of the intermicellar spaces with 'bulk' water. This has the undesirable effect of diluting the sensitizer and allowing it to diffuse deeper within the paper, so the use of such high humidities is not recommended.

3.5 Choice of Light Source

In considering the photochemistry in §2.3, the variation of quantum yield and absorptivity with wavelength made it clear that a light source with predominantly long-wave ultra-violet content (e.g. the mercury emission line at 365 nm) will be most efficient. There seems to be no benefit in using short-wave ultra-violet radiation, with its attendant hazards. Convenient sources are provided by the fluorescent coated mercury discharge tubes of the type widely used for reprographic work. In the present experiments, an array of four Phillips tubes, Type TLADK 30/05, was used; these emit a band of near ultra-violet light from 300 to 460 nm with a peak output at ca. 370 nm. This irradiation unit, with its control gear contained in a convenient housing, is marketed by Gordon Audio-Visual Ltd., as a diazo printer.

Uncoated mercury discharge lamps intended for reprographic purposes, such as the Phillips Model HPR 125W which places about half its radiant energy into the mercury lines at 365, 405 and 436 nm, are also quite suitable. Light sources with a significant infrared content (e.g. the sun and quartz halogen lamps) are less satisfactory because they may cause an unacceptable degree of heating during exposure, which will dry the sensitizer and adversely affect image quality.

3.6 Wet Processing Procedure

The purpose of wet processing is:

(i) to complete the formation of the image via reaction <2> (see §2), in case the 'print-out' has not been total -(remembering that its extent depends on the humidity of the sensitizer)- and

(ii) to remove from the paper fibres the excess unreacted sensitizer and the iron(III) reaction products.

As indicated in §2, disodium ethylenediaminetetraacetate was found to be a better agent than the traditional oxalate baths for both development and clearing. A 0.2 molar solution (ca. 7% w/v - which is nearly saturated at 20 °C) has the advantages of being non-toxic and very effective in cleanly removing excess iron(III), which it binds strongly. A better gradation of tones is obtained in the print, with cleaner highlights. The solution has a pH ca. 4, which is preferable to the alkaline solution provided by tetrasodium ethylenediaminetetraacetate. It also renders unnecessary the use of washing baths of dilute hydrochloric acid, which have a deleterious effect on the paper strength and tend to dissolve palladium. (Although a solution of citric acid can be safely used as a substitute.)

Treatment of the exposed print for five minutes at room temperature in two baths of 0.2 molar disodium ethylenediaminetetraacetate solution usually suffices to remove most of the excess sensitizer agents before washing with water. But in some papers, especially those sensitized with palladium, a slight yellow stain may persist due to occluded sensitizer that has penetrated the cellulose fibres and then become inaccessible to the washing agent - possibly due to irreversible closure of pores in the cell wall. This problem may usually be solved by treatment in a third bath consisting of Kodak Hypoclearing Agent, (predominantly inorganic sulphite) used at the Manufacturers' recommended strength. This penetrates and swells the paper fibres, rendering them more accessible to the subsequent washing water, and the inorganic sulphite in the bath probably has the effect of reducing the residual iron(III) to iron(II), thereby diminishing its mordant-like affinity for the paper. Finally a one hour wash in running water should complete the clearing, even of the heavier weight papers, and leave an archivally permanent print.

3.7 Drying and Finishing

After allowing the washed print to drain, it may be air-dried, face up, at room temperature. It is preferable not to touch, blot or squeegee the delicate wet paper surface. Prints will dry substantially flat with none of the curling typical of a silver-gelatine print on fibre-based paper, although some papers of a lighter weight may show slight dimensional instability by cockling around the edges, so generous margins are desirable. Once dry, platinum and palladium prints are quite robust, easy to retouch (with watercolours, not Spottone), and are less susceptible to surface marking than gelatine emulsion prints.