For a cyanotype on a pebble:
- choose a porous, matt stone, or rough up a pebble that is too smooth;
- coat the side to be printed in thin layers;
- expose it to the sun with a leaf or a piece of lace laid on top;
- rinse under a very gentle trickle of water.
A pebble picked up on a beach already catches the eye with its shape and its polish, and cyanotype adds a blue imprint that seems to come from the sea rather than from a studio. Stone is an unexpected support but one that stays true to the process, two iron salts, ultraviolet light and a little water, provided you accept what the mineral imposes: an often domed surface and a porosity that changes completely from one rock to another.
This page deals only with pebbles and stone, and with what sets them apart from a nice flat sheet.
Understanding the porosity of stone
Success depends entirely on the ability of the mineral to hold the iron solution for a moment. A porous, matt stone, limestone, sandstone, tuffeau or an open-grained slate, soaks up the solution and holds it long enough for the reaction to take place, whereas a very smooth sea pebble, flint or quartz polished by the water, has a closed surface on which the solution beads and slides off without taking hold.
This is the first thing to check, with a drop of water placed on the stone: if it soaks in slowly, the support is suitable; if it stays as a shiny bead, you will need to open up the surface or choose another pebble.
First clean the stone in clean water and let it dry completely, because sea salt and dust prevent an even coating. On a pebble that is too polished, a light pass with fine abrasive paper roughens the area to be coated and gives it just enough tooth to hold the solution.
Examine the surface of a pebble before coating it, telling matt stone apart from a highly polished support.
Coating a surface that soaks up and curves
The solution does not change: the two iron salts mixed in equal parts, ferric ammonium citrate and potassium ferricyanide. Under the UV from the sun, 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 directly in the mineral. The cyanotype kit supplies these two salts in bottles, 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.
Several thin coats rather than one thick one
Apply the solution with a soft brush, in thin coats, only on the side you want to print. Stone often absorbs a lot, and the common mistake is to load on a single thick coat that dries into an uneven crust, whereas a first thin coat followed by a second once it is lightly dry gives a denser ground. On a domed pebble, turn the stone as you go to follow the curve and avoid runs.
Drying takes place away from light, in a cupboard or a dark room, because the solution is already sensitive to UV. Wait until the stone is dry to the touch and an even yellow-green, which can take longer than on paper because the mineral holds on to moisture.
Exposing a surface that is not flat
Here stone clearly parts company with paper, because the contact technique that gives cyanotype its fine detail assumes a flat surface against which you press a negative or an object. On a domed pebble, this firm contact cannot be maintained, and the image loses sharpness as soon as you move away from the top of the curve.
Stone is therefore better suited to the photogram, by placing directly on the coated surface flexible elements that follow the shape a little, a thin leaf, a blade of grass, a piece of lace. To transfer a photograph, you need a transparent negative at the size of the print laid in contact, which calls for an almost flat face, a slab or a piece of slate, rather than a round pebble.
Exposure is read from the colour change, the exposed area turning from yellow-green to a bronzed grey-blue, and in full sun a few minutes to around ten minutes are often enough.
Rinsing the stone without drowning the blue
Rinsing is the most delicate point on stone, and it runs into the limit already familiar from non-absorbent supports. Do not plunge a pebble into a long bath, because the still fragile pigment sits on the surface and generous rinsing washes it out before it has even darkened. Instead, pass the stone under a very gentle trickle of cold water, or wipe it with a damp, well wrung-out sponge, just long enough to clear away the unexposed salts.
The aim is to stop the reaction and clean the ground, not to soak the stone. Then leave it to dry flat, out of direct sunlight, and the blue darkens over the following hours as the pigment oxidises.
Stone never gives the deep darks of a fine paper, as the pigment sits on the surface rather than anchoring itself in a fibre, and you have to accept a more discreet blue that plays off the colour of the rock.
Living with the limits of the support
A cyanotype on a dry pebble benefits from some protection, because the pigment stays on the surface and the stone will be handled. A matt acrylic varnish in spray form, applied from a distance as a light mist, protects the blue without dulling it, and always test it on a trial pebble before the piece you care about.
Stone thus joins the other closed surfaces where the preparation layer and a measured rinse matter more than a chemistry that never changes, whether it is rigid glass, glazed ceramics or wood that soaks up water.
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.