For a cyanotype on glass, degrease the plate with alcohol, cover it with a thin layer of gelatine that holds the solution, sensitise it with the two iron salts, then expose it to the sun and rinse it in cold water without ever rubbing. Glass is one of the most demanding supports for cyanotype: a transparent plate placed in front of a window, where Prussian blue takes on a different depth from the one it has on paper. But glass has no fibre for the solution to cling to, and that is the whole difficulty: you first have to create the grip that the material does not provide.
This page sticks to this one support: how to prepare it, sensitise it and rinse it gently.
Why glass resists cyanotype
On paper or fabric, the solution of the two iron salts soaks into the fibre, and the Prussian blue that forms there stays trapped in the material. Glass, on the other hand, is smooth and non-porous: the solution stays on the surface, beads up and shrinks back as it dries, forming uneven islands instead of an even layer.
The chemical reaction itself does not change: under UV, iron in oxidation state III is reduced to iron II, which reacts with the ferricyanide to form Prussian blue. The problem is therefore not the chemistry but the grip, because with nothing to hold on to, the pigment washes away in the rinse, and the whole method that follows stems from this fact: lay down on the glass a thin layer that can hold the solution and then keep the blue. Without gelatine or another bonding layer, cyanotype does not hold on glass: the solution beads up, shrinks back as it dries and the blue washes away in the rinse. 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.
Degrease, then apply the bonding layer
New or handled glass always carries an invisible greasy film, fingerprints, dust, manufacturing residues, and this film is enough to make the bonding layer lift off in patches; in a workshop, it is the step most often neglected.
Clean the plate with methylated spirits or 70% alcohol, rubbing with a lint-free cloth until the surface is perfectly transparent, then touch it only by the edges, as a single fingerprint creates an area where the blue will be missing.
To give the glass the porosity it lacks, coat it with a thin layer of gelatine, the same food product sold in sheets or as a powder, which once dry plays the role of the paper fibre and keeps the Prussian blue attached to the plate.
Soak a few sheets in cold water to soften them, then dissolve them in warm water, never boiling, and pour this liquid gelatine onto the degreased glass held perfectly flat, in a thin, even layer that you leave to set and then dry away from dust.
Some people then harden the gelatine slightly, with a brief dip in a very dilute alum bath, so that it stands up better to the rinsing water. For a first attempt, plain, thoroughly dry gelatine is enough, and practice will show whether the layer holds or needs strengthening.
Sensitising the prepared plate
Once the bonding layer is dry, sensitise the glass as you would paper, by applying the mixture of the two iron salts in equal parts. Do this away from the sun, under the light of an ordinary bulb, which gives off too little UV to expose the plate. The cyanotype kit supplies these two salts in separate bottles, with two pipettes.
Coat with a wide soft-bristled brush or a foam applicator, criss-crossing your strokes to cover the gelatine with an even layer, without going over it endlessly, as you risk scratching the still fragile layer; the proportions of the two salts are given by the cyanotype recipe, which is the same on glass.
The sensitised plate then dries in turn, flat and away from light, until it is dry to the touch. A smooth surface holds less moisture than paper and drying is often quick, but a plate that is still damp would stick to the negative, so patience is still needed.
Exposing and gently rinsing the glass
On the dry plate, lay a negative on a transparency, or plants or objects, in contact, then expose it in the sun. As glass is rigid, pressing everything flat is easier than on fabric: press the negative against the layer under a second clean sheet of glass for sharp contact.
The exposure time follows the usual rules, from a few minutes in full summer sun to several tens of minutes in overcast weather, the reliable cue remaining the colour change on the surface, the layer turning from pale yellow-green to a bronzed grey-blue.
Rinsing is the most delicate moment of the process on glass, because the gelatine layer swells in water and scratches at the slightest touch.
Rinse in cold water, without ever rubbing, letting the water run gently over the plate or immersing it flat in a tray, until the unreacted iron in the protected areas has gone and the water runs clear. Water that is too hot would melt the gelatine and carry away the whole image, so cold water is a safeguard here.
A glass plate is rinsed gently, without rubbing the layer that carries the image.
Failures specific to glass
A layer that lifts off in patches during rinsing is the most common failure, and it almost always comes from insufficient degreasing or from gelatine applied to glass that was still greasy. An image that partly fades away under the water points to a layer that is too thin, which you can correct by slightly hardening the gelatine, whereas a pale plate points to underexposure or washing out, causes common to all supports.
From glass to other smooth supports
The logic seen here, degreasing and then creating a bonding layer, applies to all smooth, non-porous surfaces: glazed ceramics, metal or some plastics are treated on the same principle. Wood, on the other hand, is porous and takes the solution almost like paper, which makes it a more direct support.
Glass needs more care than paper, from degreasing to the bonding layer, and it is this preparation that determines how well the image holds.
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.