To prepare your paper for cyanotype, mix equal volumes of the two solutions just before use, spread them in a thin layer in subdued light with a brush or a glass rod, then leave the sheet to dry flat in the dark. Between the chemistry in its bottles and the image in the sun, there is a step that many people neglect and yet which accounts for a large part of success: coating the paper.
This is the moment when you lay the sensitive solution on the sheet and decide the thickness, the evenness and the boundaries of the area that will turn blue, and a poorly coated paper gives a blotchy or uneven print before the sun has even done its work.
In short, in 5 steps:
- Mix the two solutions at the right moment
- Work in subdued light
- Apply with a brush or a glass rod
- Dry the sheet away from light
- Store the sensitised paper
This page sticks to that specific technique: preparing and coating the paper, drying the sheet and storing it once sensitised. The choice of paper itself, its weight and its fibres are covered in the page on which paper to choose for cyanotype, and the process as a whole is put in context in the practical guide to cyanotype if you are a beginner. Here, we assume the paper has already been chosen and the solutions are ready.
Mix the two solutions at the right moment
Cyanotype relies on two iron salts that are kept separate and only combined at the last moment, because it is mixing them that makes the paper sensitive to ultraviolet light. So take an equal volume of each solution, one pipette of ferric ammonium citrate and one pipette of potassium ferricyanide, and combine them in a small container just before coating.
The proportions and the logic behind this mixture are detailed in the cyanotype recipe. The Sténoflex cyanotype kit brings together the two bottles of solution, the watercolour paper and the pipettes, so you can coat your first sheets without hunting down each reagent or measuring by guesswork. Potassium ferricyanide has low toxicity at the amounts used in the process, but it can release cyanide on contact with an acid, or under UV light when in solution.
Only prepare the quantity you need for the session, because once mixed, the solution only keeps for a week, away from light and air. For a medium-sized sheet, a few millilitres are enough, and in workshops it can be seen that about ten drops of each salt easily cover a postcard-sized area. It is better to mix a fresh batch than to waste any.
Work in subdued light
From the moment the two solutions are combined, the paper becomes light-sensitive, so the preparation must be done away from the sun and from any source of ultraviolet. This does not mean a darkroom, simply subdued light: a room lit by an ordinary bulb, shutters half-open, away from a sunny window. Soft artificial light does not have enough UV to expose the sheet during the time it takes to coat it.
The most common mistake is to coat near a window so as to see clearly, which starts the exposure too early and gives an overall bluish fog. Get into the habit of preparing in a dark corner of the room, then only taking the sheet out into the sun once the negative or the plants are in place and the surface is ready.
Apply with a brush or a glass rod
Two techniques coexist in workshops, and each has its supporters. A broad, soft brush, preferably without a metal ferrule that would react with the salts, lays the solution down in criss-cross layers, one horizontal pass then one vertical pass, until you get an even yellow-green tint. This is the most accessible method, and it sometimes leaves visible brushstrokes that many people actually seek out as a signature.
A glass rod, or a coating rod, spreads the solution, first poured in a thin line along the edge of the sheet, in two or three passes back and forth. It gives a thinner, more even layer, saves solution and works well on hot-pressed surfaces, such as those of a smooth watercolour paper whose sizing holds the solution on the surface. In both cases, work quickly, without going over and over an area that is starting to dry, as you would tear the layer away instead of evening it out.
Coat an area slightly larger than the planned image, so as to leave a margin, and avoid excess solution, which forms puddles, dries into crusts and marks the print with darker patches. A thin, even layer is always better than a thick, irregular one.
Choose between a broad brush and a glass rod before coating. The sheet here is blank; coating is then done away from UV.
Dry the sheet away from light
Once coated, the sheet must dry completely before exposure, because paper that is still damp gives blurred blues and sticks to the negative. Lay it flat or hang it up in the dark, always away from daylight, and let it dry in the open air.
A hairdryer on a cold or warm setting speeds up the process if you are in a hurry, as long as it never heats the layer enough to damage it. Allow anything from thirty minutes to a few hours of drying at room temperature, depending on the paper weight and the humidity of the air.
Drying flat prevents the solution from running down the sheet under gravity, which would create an unintended gradient. When the surface is no longer tacky to the touch and the yellow-green tint is matt, the paper is ready to receive the negative and the sun.
Store the sensitised paper
You can coat in advance and keep sensitised paper for several days, provided you protect it from light and humidity. Slip it flat into an opaque sleeve or a card folder, in a cool, dry place, and it keeps its sensitivity for one to two weeks for first uses, sometimes longer depending on the paper and how it is stored.
Over time, sensitised paper can turn yellow or lose some of its reactivity, and small oxidation spots appear if the air is damp. For the most consistent prints, coat on the day itself, but preparing a few sheets in advance remains practical for an outing in the sun or a workshop.
Éric Marais — Photographer, author of two books on pinhole photography (published by Pearson) and creator of the Sténoflex. He has run introductory workshops in alternative photography, from pinhole to cyanotype, from the very beginning.