For a cyanotype on wood, sand a pale, unvarnished board, prime it if needed, coat it with the two iron salts in thin layers, expose it to the sun for a little longer than paper, then rinse it sparingly, without soaking it. Wood is an appealing cyanotype support, because the board keeps its figure and its grain under the Prussian blue, and the image seems to rise out of the material rather than sit on top of it. The process remains that of cyanotype, two iron salts, ultraviolet light and water, but wood sets its own conditions: a surface that soaks up the solution and cannot take heavy rinsing.
In brief, in 5 steps:
- Choose, sand and prime the wood
- Coat the wood and dry it away from light
- Expose to the sun by contact
- Rinse without drowning the wood
- Protect the print once dry
This page deals only with wood and with what changes compared with paper. For the principle of the process and an overview of the other surfaces, the page on cyanotype supports serves as a general reference, and the complete cyanotype guide puts it all in context if you are a beginner.
Choosing, sanding and priming the wood
Not all woods are equal for cyanotype. Pale, close-grained species, lime, birch, poplar or a good-quality plywood, give stronger contrast, because the blue reads better on a pale background. Raw wood is suitable, provided you rule out any panel that is already varnished, waxed or painted, because the solution cannot penetrate a sealed surface, and untreated medium board or MDF also works.
Sanding determines what follows. Use a medium-grit paper, then a fine grit, along the grain, to open up the wood and give it an even surface that will soak up the solution uniformly, then wipe off all the dust with a cloth, because the slightest trace would prevent clean contact between the object and the wood at the moment of exposure.
Controlling how much the wood absorbs
Then there is absorption, because bare wood soaks up the solution unevenly and the blue then turns blurred, denser in porous areas and paler elsewhere. If wood soaks up more solution than paper, it is because of this porosity, which varies from one area of the board to another. It is a very common mistake on this support: coating a raw board directly and getting a mottled background instead of a clean, flat colour.
The answer is a primer, a coat of matt acrylic or ground medium applied thinly, which partly closes up the porosity before sensitising, and which you sand very lightly once dry to keep a matt surface that grips the solution.
This primer is optional for a soft result where the blue blends into the grain, but for a sharp motif it makes all the difference.
Sand along the grain of the wood, then remove the dust before preparing the support.
Coating the wood and drying it away from light
The solution is the same as everywhere else, the two iron salts mixed in equal parts, ferric ammonium citrate and potassium ferricyanide. Under the UV in sunlight, iron in oxidation state III is reduced to iron II, which reacts with the ferricyanide to form Prussian blue, the insoluble pigment that anchors itself in the wood fibre. These two salts are supplied as ready-diluted solutions, in 15 ml bottles, in the cyanotype kit, with two pipettes. 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.
Applying the solution in thin layers
Apply the solution with a soft flat brush or a sponge, in thin, criss-crossed layers, along the grain. As wood absorbs more than paper, a second thin coat once the first has dried slightly gives enough density without waterlogging the surface.
Drying takes place away from light, in a dark room or a cupboard, because the solution is already sensitive to UV. A coated board dries more slowly than paper, as the wood holds moisture deep inside, so wait until it is dry to the touch and an even yellow-green before exposing it.
Exposing to the sun by contact
Lay the black-and-white negative or the object, a leaf, a piece of lace, a key, directly on the wood, making sure of firm contact, under a sheet of glass if needed.
Exposure lasts from a few minutes in full summer sun to several tens of minutes in overcast weather, and wood often needs a little longer than paper because its surface is less reactive. The reliable guide is still the colour change, with the exposed area going from yellow-green to a bronzed blue-grey, and when this tone clearly appears, the exposure is sufficient.
Rinsing a cyanotype on wood without drowning it
Rinsing is where wood differs most from paper. You cannot hold a board under a heavy stream of water for minutes on end, because the wood swells, warps and can split, and too much water washes out the pigment, which is still fragile on the surface.
So rinse sparingly, in cold water, passing the surface gently under a low flow or wiping it with a well wrung-out damp sponge, just long enough to remove the unexposed salts that would leave the wood sticky.
The aim is to stop the reaction and clean the background, not to soak the board. Then leave it to dry flat, away from direct sun, and the blue darkens over the following hours as the pigment oxidises.
Protecting the print once dry
A dry cyanotype on wood benefits from protection, because the pigment stays on the surface and the wood will be handled. A coat of matt acrylic spray varnish, applied from a distance as a light mist, protects the blue without dulling it; avoid warm-toned varnishes and waxes, which shift it towards green, and always test on an offcut before treating the final piece.
Like raw wood, unprimed artist's canvas soaks the solution into its fibres and lends itself to the same soft result in which the blue rises out of the material. In this, wood joins the other surfaces where priming and contact matter more than the chemistry, which never changes, whether rigid glass or cyanotype on soft fabric.
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.