All the iron-based processes have working methods in common; so to avoid repetition, the equipment, materials, and handling procedures are summarized here.

The Working Environment

A benefit of the low sensitivity of alternative printing processes is that you do not need a darkroom or safelighting to carry them out. Ordinary curtains or blinds should subdue daylight sufficiently, but preferably you should work under a normal domestic (tungsten) light: a 60W bulb, distant 2 metres or more, is quite safe. Avoid fluorescent light if you can; some types of tube have a significant UV output.

You will need a clean, flat, dry surface for preparing the sensitized paper and enough wet-processing space for four photographic dishes of an appropriate size, together with running water for print washing and a drying facility that need be no more complicated than a 'clothesline' and pegs. The sensitizers and processing chemistry used are generally odourless and no fumes are evolved, so there is no need for special ventilation in the workplace. However, clean working methods are vital and any spilt substances must be cleaned up immediately.

Materials and Equipment

The following items of equipment will be needed:

1. Pencil
2. Ruler
3. Adhesive drafting tape (low tack)
4. Negative blower brush
5. Spirit level (small)
6. White blotting paper
7. Sheet of plate glass (4-6 mm thick and larger than the paper)
8. Large clips, preferably plastic 'bag clips'
9. Jug to measure 1 litre
10. Small graduated syringes 1 cc, 2 cc and 5 cc (Disposable plastic type without hypodermic needles)
11. Measuring cylinder (in USA a 'graduate') 50 or 100 cc capacity
12. Two Pyrex glass beakers 150-200 cc capacity
13. Thermometer (0-110 °C)
14. Small conical filter funnel (5-6 cm diameter)
15. Filter papers to fit funnel: Whatman no.1 8-10 cm diameter
16. Small mixing glass (a liqueur or 'shot' glass is ideal)
17. Scales accurate to 0.1 g, e.g. Salter's Model 1250, or a chemical balance
18. Small spatula or plastic measuring spoon
19. Hygrometer (as sold for use in green-houses)
20. Several brown bottles for sensitizer solutions (ca. 50 cc is suitable)
21. Self-adhesive labels and indelible pen
22. Four photographic dishes (larger than the paper)
23. Print tongs or rubber gloves
24. Means of print washing
25. Timer (a wristwatch will do)
26. Glass spreader rods*
27. Sheets of Artists' paper suitable for coating*
28. Contact-printing frame*
29. Ultra-violet light source*
30. Suitable negatives*
31. Boxes for controlled hydration of paper*

Choice of Paper

• High (>98%) alpha-cellulose content (i.e. a paper made solely from cotton linters). Papers with a mixed furnish can absorb sensitizer unevenly, causing granularity or blotchiness.
• Internal sizing with 'Aquapel' -or other equivalent alkylketene dimer- the modern neutral sizing agent for fine papers. Gelatine-sized papers should be avoided for printing in pure platinum. Gelatine combines chemically with the platinum complex and can seriously inhibit the formation of the platinum metal image, which may then appear weak and fibrous. Alum-rosin sized papers (such as Cranes AS 8111) seem to work well, but are archivally questionable (although Willis's commercial paper was of this type). Gelatin is not inimical to palladium and the other processes, however, and is especially recommended for chrysotype.
• The paper should be free of alkaline buffering agents such as calcium carbonate (chalk) which is often added to protect archival quality papers from the effects of acid. Calcium carbonate reacts with the sensitizer chemicals in a disadvantageous way, precipitating calcium oxalate and hydrolysing the iron salts.
• A fairly smooth, hot-pressed (HP) surface is desirable - unless you wish to make a highly textured print. 'Not' and Rough surfaces may prove difficult to coat and cause loss of resolution. However, some very smooth, heavily calendered commercial papers do swell and roughen on wet processing.
• A weight of paper around 160 to 240 gsm is generally sufficient to ensure adequate wet strength; very heavy papers will be slow to wash completely. Thin tissue can yield exquisite prints -if you have the skill to coat and manipulate it.
• The paper should not contain any other additives, such as clay fillers, wet-strengthening agents, optical brightening agents, bleaches, alkaline buffers or dyes.
• Dimensional stability -i.e. little tendency to shrink or distort on wet processing- is a desirable attribute, especially if multiple printing is to be used. In this respect, hand-made papers are better than machine-made, which have an intrinsic 'direction'.
• A 'wove' mould paper texture is preferable to 'laid' for pictorial purposes.

(* these are gelatine sized, and should be avoided for pure platinum printing)

A handmade paper which has beeen specified by the author for platinum-palladium printing is the 'Buxton' paper from Ruscombe Mill available as 160 gsm and 240 gsm; it works very well with all the iron-based processes. It is also available gelatin-sized as 'Talbot' paper.

The greatest imponderable in 'plain paper' printing is the effect of paper-manufacturers' additives on the reactive chemicals of the sensitizer. Each commercial paper has its own idiosyncrasies, which may vary from batch to batch or be changed by the manufacturer without notice. There is no substitute for personal trial and experiment.

The two sides of a sheet of fine paper often differ distinctly, reflecting the method of manufacture: the so-called 'wire' side shows, on close examination under bright light, a very fine geometrical mesh pattern; the 'felt' side has a random texture. Paper may be coated on either side, according to your taste. The watermark should always be excluded from the coated area. Always handle paper by the edges, using both hands; never touch the picture area, either before or after coating, and do not touch the reverse either, because moisture from the fingers can diffuse through.

Glass Spreader Rods

In place of the traditional brush, the more economic coating implement is a glass rod (or, preferably, thick-walled capillary tube, which is usually manufactured to higher standards of straightness) of external diameter between 6 and 12 mm, having a straight central section equal in length to the width of the coating area. The end portions of the rod should be bent at an angle, using a powerful gas burner; a 'bicycle handlebar' shape: \_____/ is simple and ergonomically effective. The centre portion acts as the spreader, and determines the width of the coated area; the limbs serve as handles. This implement must be kept scrupulously clean.

Rod-Coating Paper with Sensitizer Solution

Follow this procedure:

1. Cut or tear your sheet of paper to a size that allows generous borders around the picture area - this not only looks well, but the margins also facilitate handling and protect the image. Mark the sheet of paper very lightly in pencil to indicate the corners of the area to be coated. Time and trouble may be saved by devising a card template to guide this marking up. The coating dimensions should be 1 to 2 cm larger than the negative to allow for irregularities and make positioning easy. Avoid including any watermark within the picture area.
2. Tape or clip the sheet lightly to a very flat level surface: a heavy glass plate is ideal. Check with a spirit level that the paper is horizontal; this is critical, so adjust it if necessary. Dust off the surface with a blower brush.
3. Draw up the mixed sensitizer into a syringe of appropriate size, and adjust it to the required volume. (Ignore the air bubble in the syringe and take the volume reading from the bottom of the plunger.)
4. Expel the liquid slowly and gently from the syringe (use two hands) as you move it steadily from left to right across the width of the coating area between the pencil marks at the top of the paper. For steadiness, touch the paper lightly with the syringe nozzle and try to expel an unbroken strip of sensitizer, but do not be too slow or fussy - irregularities will even out in the coating.
5. Holding the spreading rod with one end in each hand, place the straight central portion onto the paper parallel to, and a little above, the strip of sensitizer. Then, with slight pressure draw the spreading rod down into the strip of solution. Pause briefly (a couple of seconds) while the liquid distributes itself uniformly along the length of the rod, then steadily draw the strip of solution down the paper, like a tiny tidal wave running in front of the rod. The rod is not rotated, and very little pressure need be applied. When you reach the pencil marks at the bottom of the coated area, hop the rod over the strip of solution and push it back to the top of the coating; then hop over the sensitizer strip once more and draw it down to repeat the spreading. Just five passes over the paper will suffice for its surface layer to become saturated with a uniform coating of sensitizer. The first two passes should be made quite rapidly (3 or 4 seconds each) to ensure complete wetting of the surface, and the last three as slowly as possible (10 to 15 seconds each) to allow maximum absorption. At the end of the fifth pass, drag the spreader well below the picture area, with its excess sensitizer (when you have fine-tuned the exact volume to use with your chosen paper, there should be very little waste).
6. Gently lift off the spreader, and soak up any residual liquid left at the bottom of the coating with the edge of a clean strip of blotting paper, otherwise crystals may be formed which can damage the negative. Clean the spreading rod.

Marking	Drawing solution
Coating 1	Coating 2
Coating 3	Coating 4
Blotting

Brush Coating

Drying Sensitized paper

Contact-Printing Frames

Some form of contact-printing frame is essential for applying enough pressure to maintain the close contact between negative and paper. The simplest option is a sheet of plate glass (4 to 6 mm thick - not 2 mm picture glass) and a flat baseboard with a thin sheet of foam rubber or plastic to take up unevenness. This sandwich is held together by strong clips. Alternatively, one of the commercial contact frames for proofing negatives may be used.

However, the versions of iron-based processes I describe give a print-out - that is, a fully detailed image is formed during the exposure and subsequent processing involves little or no development, only clearing and fixing. It is therefore a great advantage to be able to inspect the progress of the printing (away from the UV source!) without destroying the registration between negative and print. This can be achieved with a hinged-back printing frame of the traditional design common in the nineteenth-century, when printing-out was the standard practice. These can still be bought and are often of high craftsmanship (and cost), but it is easy to construct one from a strong picture frame made of extruded aluminium with a deep rebate. It is important to include an impervious sheet of rubber or plastic larger than the sensitized area sandwiched between the paper and the hinged pressure back; otherwise diffusion of air or moisture through the hinged joint may cause unevenness in printing. Very large contact prints demand more sophisticated technology - the vacuum easel.

If you prefer clean edges and a white border around your image, rather than the currently fashionable overexposed worked edges, then mask the glass front of the printing frame with a window of rubylith, or other opaque adhesive film. Such masking brings two extra benefits: it avoids the generation, in the wet processing, of large quantities of redundant (non-image) metal, which may bleed into and stain the picture area. Moreover, the unexposed but coated borders will provide a check on the effectiveness of your clearing chemistry, as evidenced by the absence of any perceptible yellow stain.

Precious negatives may be protected during contact printing by interposing a very thin polyester film -such as Mylar or Melinex- between paper and negative. The negative will be particularly vulnerable if the paper is fully humidified. Such a 'sandwich' will, of course, diminish the edge sharpness (acutance) of the print, depending on the geometry of the light source: if a diffuse 'light bed' is used at a close distance, the protective film must be no thicker than 20 microns; if a small source at greater distance is used, e.g. the sun, then more robust film of thickness 50 to 100 microns can be tolerated without perceptible diffusion of the image.

Ultra-violet Light Sources

Any light source with a substantial ultra-violet content will work. However, sources like 'sun-guns' and quartz-halogen lamps also emit infra-red radiation, which has the undesirable effect of heating the paper and drying it out as the exposure proceeds. The best sources are fluorescent coated mercury-discharge tubes, emitting mostly the so-called 'long-wave ultra-violet' (UVA) around a wavelength of 365 nm. Tubes with a peak output around 410 nm are also effective, but 'cool white' fluorescent tubes are not.

There is no advantage, and much additional risk, in employing the more dangerous 'short-wave' ultraviolet mercury lamps which rapidly damage eyes and living tissue (UVB below 320 nm, at which point ordinary glass begins to absorb strongly, and UVC below 280 nm). Long-wave UVA lamps are manufactured in several forms for reprographic purposes as well as for domestic sun-tanning. The following UV sources are listed in ascending order of cost:

1. The sun. Free, but uncertain and variable in many locations, the sun is effectively a point source giving high acutance, and is about 4 times faster than source no. 4. There is considerable heating effect however. The north summer sky is a diffuse source with about half the speed of no. 4.
2. Small domestic sunlamps such as the Pifco 300 watt UV lamp No. 1012. These should be used at a distance of about 30 to 50 cm from the printing frame, providing about half the speed of no. 4.
3. Mercury discharge reprographic lamps, such as the Phillips HPR 125W, used at a distance of about 30 cm from the frame.
4. A convenient unit such as was marketed by Gordon Audio-Visual as a Diazo printer. This provides a 'light bed' of four Phillips fluorescent tubes, type TLADK 30W/05 UV, about 8 cm from the print, and is adequate for an image size up to 25 x30 cm.
5. A similar array can be made quite cheaply by purchasing a commercial luminaire fitting from a lighting supply house, which is fitted with all the sockets and control gear to take four 600 mm fluorescent tubes. UVA tubes marketed for 'insect attractors' are suitable.
6. A domestic 'suntan bed' - if you can afford to make platinum prints this big, then the cost of lighting must be immaterial! The smaller 'facial solarium' is another convenient source; Phillips have marketed the alluringly-named "Decolleté Half Body Solarium" which uses 6 Phillips "Cleo" UVA tubes (Actinic/09). This is a very efficient and moderately-priced unit for prints up to 30 x 35 cm.
7. If cost is no object, a commercial mercury exposure system such as those manufactured by NuArc for the graphic arts and screen printing industry, may be obtained for £1000-£2000.

Requirements for Suitable Negatives

The light sensitivity of iron-based papers like the platinotype is about a million times less than that of silver-gelatine enlarging papers; so with the technology commonly available, they can only be contact-printed. The reproduction scale of 1:1 leaves us with four options:

• to make very small prints
• to work with a large format camera
• to make enlarged internegatives
• to employ digital imaging technology

Making Negatives Directly

Trial and error can be lessened by consulting the film manufacturers' published data for the variation of Contrast Index (Kodak's C.I. or Ilford's G) with development time or with developer concentration. As a guide, negatives for platinum printing should be developed to a C.I. between 0.6 and 1.0, depending on the luminance range of the subject; the softer palladium process will require negatives developed to C.I. values in the range 0.7 to 1.3, again depending on subject and interpretation. Palladium, in particular, has a long 'toe' in its characteristic curve which can confer great delicacy on the highlight tones.

These C.I. values are more or less incompatible with normal silver printing in an enlarger, so you cannot expect to use the same negative for both. A convenient compromise is to make your original negatives in the usual way for silver printing, ensuring adequate exposure giving plenty of shadow detail, and then prepare internegatives of higher contrast from them.

Making Enlarged Internegatives

• Direct reversal duplicating film makes a negative directly from a negative. A convenient (but expensive) method is to make internegatives by enlarging onto Kodak Professional Direct Duplicating Film Type SO-339, which is available in 5"x4" and 10"x8" formats only. Dodging and burning may be carried out at this stage if desired. Very vigorous development is necessary to achieve an adequate maximum density in this material. As a starting point for experiment, develop it in the Kodak high contrast developer D19 (undiluted) for 6 to 8 minutes at 22-24 °C with continuous agitation. This film has orthochromatic sensitivity, so a red safelight is necessary. See the Kodak literature for further information. N.B. SO-339 is a very slow material and needs a heavy exposure under the enlarger. Even modest enlargements may require a minute's exposure at full lens aperture. Remember that it is a reversal material - more exposure gives less density. Remember always to make your internegatives as mirror images: i.e. place the original negative emulsion up in the enlarger so the duplicate will make a "right-reading" image when exposed emulsion-to-emulsion on the print.
• The longer route is via an interpositive, using normal negative-working continuous tone films such as Kodak Gravure Positive Film 4135 or Agfa Gevatone N31p or N33p, which are available in large sheet sizes. The choice of size for the interpositive will depend on your available enlarger formats and budget, but to minimise the degradation in quality introduced by the extra copying step, the larger you make the interpositive, the better. (Contact-printing an interpositive from a 35mm negative can introduce serious dust problems.) The interpositive should be printed quite heavily to ensure no loss of information; development to a Contrast Index value of one is recommended. The negative is then made by enlargement, or contact, from this interpositive, on the same film, using more vigorous development to raise the contrast. Kodak HC-110 is a convenient developer, offering control of contrast by varying concentration as well as time. Agfa market a continuous-tone orthochromatic sheet film, GO210p, which has the advantage that it has a variable contrast that can be adjusted by yellow or blue filters.
• By starting with a positive transparency from the camera (such as Agfa Dia-Direct), an enlarged internegative can be obtained in one step on the films specified in (2).If the original small-format image is a colour transparency, projection onto these orthochromatic materials (which are insensitive to red light) will not faithfully reproduce the tonal balance. To maintain this, a panchromatic sheet film, e.g. Ilford's FP4 or HP5, will be necessary, possibly with a colour head or a filter to bring the enlarger light source to a 'daylight' colour balance.
• The sheet film on which a negative is projection-printed may be reversal-processed to give another negative directly. Kodak market a reversal-processing kit for their T-Max films. It is not certain that sufficient contrast for palladium printing can be achieved this way. (N.B. T-Max 100 is now manufactured with a strongly UV absorbing layer, which greatly prolongs alternative printing exposures.)
• Paper internegatives are a very economical but low-quality option. Obviously a thin-based paper stock will be needed to avoid long printing exposure times. The 'Resin-coated' (RC) variety is said to work well. Variations in paper texture, including any lettering on the back, can be cancelled out by giving white light exposure to the back of the paper, in addition to the normal image exposure to the emulsion side. A test strip is necessary to determine this additional 'compensating fog' exposure.
• A final remark on internegatives: Lith films, if they are developed with little or no intermediate tone, are quite inappropriate to the subtle and costly medium of noble metal printing.

Digital Negatives

The essential point is that ink-jet negatives are completely different optically from the familiar negatives made with silver-gelatin emulsions. The latter have a neutral density which does not vary much with wavelength: what is seen and measured in the visible region of the spectrum is a fairly accurate reflection of what is occurring in the near UV, where the negative is actually doing its work when used for alternative printing. This is not so for digital negatives, which are made with dyes or pigments, having absorbances in the near UV which are quite different from their values in the visible region. Ordinary densitometers, which measure the visible wavelengths, are useless, and tell you nothing about what is going on in the UV. If you are not fortunate enough to own a UV densitometer, it is necessary to proceed empirically. One easy way of matching the negative to the printing process is based on the following reasoning:

1. The full range of UV densities of the ink laid down on the transparency depends on three things: your particular printer, the manufacturers' ink set, and the best choice of printer driver settings. The last of these can only be found and optimised by experimentally printing test negatives: we are, to an extent, at the mercy of these pre-set parameters, and can only judge by the way an ink-jet negative works in producing the final print. A driver setting is needed which will provide a UV ink density range equal to, or greater than the printing range of the process.
2. The first priority is to match the image ink density range to the exposure range of the printing process, so that an image (on-screen) opacity value (in the negative) of 100% prints "just" white, and 0% prints "nearly" the maximum density of the process.
3. As a means of matching negative and process, many workers recommend using 'the Curve', or transfer function, that can be found in Adobe Photoshop. But, with its infinitely variable nature and large number of defining parameters, this seems an inappropriate tool to use initially.
4. Only one parameter is needed for the overall match: the range of output levels, which must be reduced from 0-255 to Lw-255, by raising the lower (black) output level, Lw, above zero.
5. All the image tones are then re-mapped linearly and automatically onto this contracted scale of output levels; no information is lost. (This can be checked with the eye-dropper sampling an on-screen step tablet.)
6. This procedure is entirely equivalent to simply shifting the origin point of the curve from (0,0) to (0,Lw); the curve itself is still a straight line running to (255,255). (I refer to a curve applied to the negative - not the positive).
7. Curving the transfer function, can, of course, also be applied to modify the internal relative contrasts of the image, but this curve will now be much less drastic than the ones commonly recommended, if one has correctly defined the end points.

1. Find the exposure time with your standard alternative printing setup to produce a 'near' maximum density through the clear film base: ideally the 2% step of the tablet should be just visually distinguishable from this.
2. Inspect a print of the 50 step made with this standard exposure, to find the step that prints 'just' white - i.e. the one after the step with last perceptible tone. Let this be Qw% opacity.
3. Convert this opacity value to its corresponding luminosity level , Lw, by subtracting it from 100 and multiplying by 2.55. i.e.:
   Lw = 2.55 (100 - Qw).
4. When making a negative for this process, in the histogram adjustment window, reset the Lower Output Level from its default value of 0 to Lw. Keep the Upper Output Level at 255. This negative will therefore be output onto the transparency material with an ink density range that just matches the process, so that a negative image opacity value of 100% will print 'just' white, and a value of 0% will print with nearly the maximum density, under the standard exposure time.

© Photographs copyright Peter Bargh, ephotozine.com 2001-2