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Preparations for Iron-based printing

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Category: Darkroom Printing

Preparations for Iron-based printing - Over the next three articles Dr Mike Ware describes three iron-based processes - new cyanotype, argyrotype and platino-palladiotype. They have the same working methods so, to avoid repetition, the equipment, materials and handling procedures you will need are described here first.

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Words Dr Mike Ware, photos Peter Bargh/Dr Mike Ware

 

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 incandescent (tungsten) light - a 60W bulb at a distant of two metres or more, is quite safe. Avoid fluorescent light if you can because 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 trays 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 (but choose them carefully).

Preparations for Iron-based printing

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

Preparations for Iron-based printing

The following items of equipment are needed

  • Pencil
  • Ruler
  • Masking tape
  • Negative blower brush
  • White blotting paper
  • Sheet of plate glass (4-6 mm thick and larger than the paper)
  • Jug to measure one litre
  • Supply of hypodermic syringes: 1cc, 2cc and 5cc (the plastic disposable type without needles)
  • Measuring cylinder or graduate 50cc or 100cc
  • Small mixing glass (a liqueur or shot glass is ideal)
  • Scales accurate to 0.1g or a chemical balance
  • Small spatula or plastic spoon
  • Hygrometer (as sold for use in greenhouses)
  • Supply of brown bottles for sensitizer solutions (ca. 50cc)
  • Four photographic dishes (larger than the paper)
  • Print tongs or rubber gloves
  • Means of print washing
  • Timer or clock
  • Sheets of artists' paper suitable for coating
  • Glass spreader rods
  • Contact-printing frame
  • Ultra-violet light source
  • Negatives developed to a suitably long density range

Other items needed are described in more detail below.

Choice of paper
The characteristics desired in a paper for alternative printing are:
High(>98%) cellulosecontent (i.e. a paper made solely from cotton linters - once called rag paper).
Internal sizing with Aquapel - the modern neutral sizing agent for fine papers. Gelatine-sized papers are unsuitable for printing in pure platinum, but can be used for palladium and the other processes. With an appropriate choice of paper, additional sizing is unnecessary.
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 can react with the sensitizer chemicals in a disadvantageous way.
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 200gsm (grams per square meter) is generally sufficient to ensure adequate wet strength in sheets of moderate size; large sheets will require heavier paper, but will be slower to wash. Thin tissues can yield exquisite prints - if you have the skill to coat and manipulate them.
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. The watermark must be avoided.
Several mould-made fine art papers intended for watercolorists or printmakers have been found to work well with most of the iron-based processes, including:
Fabriano 5*, Saunder's Waterford*, Whatman Watercolour, Hollingsworth Kent,
Arches Aquarelle*, RKB Arches, Arches Platine, Strathmore 500, Cranes AS 8111, Cranes Crest Parchment, Cranes Platinotype,Wyndstone Vellum, Buxton,
(*gelatine sized, and unsuitable for 100% platinum printing.)
Buxton is a handmade paper from Ruscombe Mill, which I commissioned specifically for iron-based printing. 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 be excluded from the coated area. Always handle paper by the edges, using both hands to avoid creasing; 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

Preparations for Iron-based printingIn 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 12mm, having a straight central section equal in length to one dimension 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 coating; the limbs serve as handles. This implement must be kept scrupulously clean.

Rod-coating paper with sensitizer solution

Preparations for Iron-based printing

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.

2 Mark the sheet of paper very lightly in soft 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 2cm larger than the negative to allow for irregularities and make positioning easy. Avoid including any watermark within the picture area. Preparations for Iron-based printing
   
3 Tape or clip the sheet lightly to a very flat, level surface - a heavy glass plate is ideal. It's best to tape when using light paper and clip when heavier stock is used. 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. Preparations for Iron-based printing
   
4 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.) Preparations for Iron-based printing
   
5 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. Preparations for Iron-based printing

Preparations for Iron-based printing

6 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 (three or four 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).

Preparations for Iron-based printing7 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.
I recommend that you practice this procedure using water coloured with a few drops of vegetable dye, before you embark on sensitising with precious metal salts.

Brush coating
This is the traditional - but rather wasteful- method, for those who prefer the freestyle event and are really painters manqu. It can be fun with an inexpensive sensitizer like cyanotype, but the wastage does not commend this method for platinum. If you must use a brush, avoid those with metal ferrules. A Japanese Hake brush is very suitable.

Drying sensitized paper
Immediately after coating, the paper should be kept horizontal for a few minutes until the liquid is sufficiently absorbed for the surface to lose its reflective sheen. The sheet may then be left to dry in the dark for at least an hour before use. Alternatively some heat may be used - a stream of warm air at 40 C for 10 minutes is adequate - but it should be noted that this treatment can influence the print colour, especially in the palladiotype process, and may cause unevenness.

Contact-printing frame
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 6mm thick - not 2mm 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, all the versions of iron-based processes that I shall 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.

Preparations for Iron-based printing   Preparations for Iron-based printing
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 at car boot sales or photography auctions and are often of high craftsmanship (and cost).

It is easy to construct one from strong commercial picture frame material 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; this acts as a vapour barrier, otherwise diffusion of air or moisture through the hinged joint may cause unevenness in printing. Very large contact prints demand more sophisticated technology like a vacuum easel.
If you prefer clean edges and a white border around your image, rather than dark worked edges, then mask the glass 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 m (micrometers); if a small source at greater distance is used, e.g. the sun, then more robust film of thickness 50 to 100 m can be tolerated without perceptible diffusion of the image.

Ultra-violet light sources

Preparations for Iron-based printing

Any light source with a substantial ultra-violet content will work. However, sources such as 'sun-guns' and quartz-halogen lamps also emit infrared 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 so-called long-wave ultra-violet (UVA), peaking around a wavelength of 365nm (nanometers).
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 320nm, at which point ordinary glass begins to absorb strongly, and UVC below 280nm). 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:

  • The sun. Free, but uncertain and variable in many locations, the sun is effectively a point source giving high acutance. There is considerable heating effect however. The summer sky is a diffuse source with about one eighth to one tenth of the speed of the direct sun.
  • Small domestic sunlamps, such as the Pifco 300 watt UV lamp No. 1012, can still be found in car boot sales. These should be used at a distance of about 30 to 50cm from the printing frame.
  • Mercury discharge reprographic lamps, such as the Phillips HPR 125W, used at a distance of about 30cm from the frame.
  • 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/05UV, about 8cm from the print, and is adequate for an image size up to 25x30cm.
  • A similar array can be made quite cheaply by purchasing a commercial luminaire fitting from a lighting-supplier, which is fitted with all the sockets and control gear to take four 600mm fluorescent tubes.
  • A domestic suntan bed. The smaller facial solarium is a convenient source for prints up to 25x30cm.
  • 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.

Whatever source you use, be sure to protect your eyes.

Requirements for suitable negatives
The light sensitivity of iron-based papers 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 three options:

  • to make very small prints
  • to work with a large format camera
  • to make enlarged internegatives (see making an internegative article)

Preparations for Iron-based printing

Each of these strategies has something to recommend it, but whichever you adopt, it will be essential to develop your negatives to the high contrast suitable for all three printing processes to be described in future articles. The density ranges required in the negatives should be the same as the logarithmic exposure ranges of the iron-based sensitizers, because there is no Callier Effect in contact printing. These lie typically in the region from at least 1.5, equivalent roughly to the range of a Grade 0 enlarging paper, to a range of 2.5, which is far softer than any silver-gelatine emulsion. To yield a full tonal range in the print, negatives must be made with a correspondingly long density range: aim for a value around 1.8 to 2.1 (6 or 7 stops). This will be obtained by giving a good continuous-tone camera film normal exposure, but developing it for 75% to 100% more than the normal time period recommended for silver-gelatine enlargement printing.
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-bar) with development time or with developer concentration. As a guide, negatives for iron-based printing should be developed to a C.I. between 0.8 and 1.0, depending on the luminance range of the subject and your personal interpretation. These C.I. values are incompatible with normal silver printing, so you cannot expect to use the same negative for both. A convenient compromise is to make your original camera negatives in the usual way for silver printing, ensuring adequate exposure to give plenty of shadow detail, and then prepare internegatives of higher contrast from them.
Next month
Mike shows you how to make an argyrotype print
About the author
Mike Ware graduated in chemistry at the University of Oxford (1962) and obtained a doctorate by research in molecular spectroscopy (1965). He became a Chartered Chemist and a Fellow of the Royal Society of Chemistry (1982) and now studies the science, history and art of alternative photographic processes.
Mike acts as a consultant to the National Museum of Photography, Film & Television, Bradford, England, and has supervised postgraduate research in photograph conservation at the Victoria & Albert Museum and the Royal College of Art, and in alternative photographic processes at the University of Derby.
Mike conducts specialist one-to-one workshops and masterclasses in alternative printing techniques throughout the country, and has appeared on BBC Televison in the Open University series The Chemistry of Creativity (1995). Mike also has two books published by the Science Museum, London (or NMSI as it is now known) Mechanisms of Image Deterioration in Early Photographs (1994) and the more recent Cyanotype: the history, science and art of photographic printing in Prussian blue (1999) He is currently working on his third book on Chrysotype.
Contact Mike by e-mail at mike@mikeware.demon.co.uk

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