Cyanotype

What It Is

Cyanotype uses two iron compounds — ferric ammonium citrate (Solution A) and potassium ferricyanide (Solution B) — mixed together as a sensitizer. When exposed to UV light, a photochemical reduction occurs: the iron(III) is reduced to iron(II), which reacts with the ferricyanide to form Prussian blue (iron(III) ferrocyanide). Unexposed areas wash away with water, leaving the blue image.

Chemistry

Standard stock solutions:

• Solution A (Ferric ammonium citrate): 20–25g per 100ml distilled water
• Solution B (Potassium ferricyanide): 8–10g per 100ml distilled water
• Working sensitizer: Mix A + B in equal parts (1:1) immediately before coating. Once mixed, use within 1 week. Store separately in dark bottles.

Important: Powder must be fully dissolved before mixing A and B. Mix all powder fully in each solution before combining. Combine in subdued light or red-safe light only.

Exposure

• On paper: 4–6 minutes in full direct sunlight (varies significantly with UV index, season, and latitude). Use test strips in 2-minute intervals.
• Exposure indicator: The coated surface shifts from yellow-green to grey-blue when properly exposed.
• Wet vs dry coating: Wet exposures can produce teal tones and expose faster. Dry exposures produce classic deep Prussian blue.

Development

1. Immerse in plain water immediately after exposure. Agitate gently.
2. The unexposed yellow-green sensitizer washes away, leaving the blue image.
3. For richer, deeper blue: add a small amount of hydrogen peroxide to the final water wash — this accelerates oxidation and deepens the blue immediately.
4. Dry flat, away from UV light.

Toning with Botanicals

From the San Casciano workshop (bleaching and toning with garden botanicals):

• Bleaching with wood ash: Alkaline wood ash solution bleaches the blue toward pale yellow-brown. Partial bleaching before toning creates split-toning effects.
• Toning with botanicals: After bleaching, immerse in a strong tea/infusion of rosemary, lavender, sage, onion skins, or coffee. The tannins in the botanicals deposit over the bleached areas, creating warm brown-gold tones over the original blue. Toning works better when wet — tone immediately after bleaching before fully drying.
• Timing: 24–40 hours of oxidization for iron salt processes after exposure before the full depth of blue stabilizes.

Cyanotype on Glass — Agar Coating

The San Casciano workshop introduced agar as a coating medium for glass plates — replacing gelatin, which interferes with the cyanotype chemistry.

Why gelatin fails: Gelatin is sensitive to both humidity and acidity. The cyanotype sensitizer's iron chemistry and moisture cause gelatin to degrade and lose adhesion to glass.

Agar coating recipe: 7.5g agar agar / 375ml water = 2% solution

1. Clean glass plates thoroughly with sodium carbonate solution — twice.
2. Warm the glass plate with a hairdryer (not too hot — just to room temperature or slightly above) to prevent thermal shock when the warm agar hits cold glass.
3. Pour warm agar solution onto the plate. Spread evenly and thinly. Allow to set and dry.
4. Coat with cyanotype sensitizer over the dried agar layer.
5. Allow to dry in the dark. Expose and develop as normal.

Agar sets at approximately 35–40°C. Pour at 45–50°C for the largest working window.

Cyanotype on Fabric

For dyeing fabric with cyanotype (from the San Casciano workshop notes):

• Use soy milk mordant instead of alum for cellulose fabrics — soy protein acts as a protein layer that helps the iron chemistry bond to plant fibres.
• Fabric cyanotype produces softer, less contrasty images than paper due to fibre diffusion.

Tools & Safety

• Dark glass or amber bottles for storing stock solutions
• Red/orange safelight or subdued indoor light for working
• Coating brush or glass rod for application
• UV light source (sun or UV lamp)
• Flat tray for development

Safety: Ferric ammonium citrate and potassium ferricyanide are low-toxicity when used as directed, but ferricyanide releases hydrogen cyanide if combined with strong acid or heated. Never mix with strong acids. Do not heat solutions. Dispose of waste in copious water. Wear gloves during mixing.

Common Failures

• Weak or pale blue: Under-exposure. Increase UV time or intensity.
• Image washes away entirely: Sensitizer not dried before exposure, or over-diluted solutions.
• Brown/olive colour instead of blue: Alkali contamination (from hard water, residue on surfaces). Use distilled water. Clean all surfaces.
• Uneven coating: Coat in one direction, then perpendicular. Work quickly — the sensitizer begins to react with oxygen immediately.
• Agar peeling from glass: Thermal shock during application, or glass not clean. Clean twice with sodium carbonate. Warm glass before pouring agar.

Jay's Studio Note

The San Casciano workshop (Almudena Romero, 2025) treated cyanotype as both a technical and poetic process. Day 2 focused on paper (bleaching with wood ash, toning with rosemary and onion skins); Day 3 returned to the glass plates after their drying time. The glass cyanotype is different in quality from paper — the image sits on the surface rather than in the fibre, making it feel more like a drawing than a photograph. When lit from behind, the Prussian blue is transmitted rather than reflected — a completely different quality of colour.

Related Entries

Agar Bioplastic — agar used as glass plate coating · Chlorophyll Printing · Anthotype · Lumen Printing

References

• Almudena Romero — cameraless photography workshop, San Casciano, Tuscany, 2025
• Sir John Herschel — cyanotype process, 1842
• Penumbra Foundation (New York) — alternative photographic processes

Lumen Cyanotype (Cyanolumen)

Cyanolumen is a hybrid process that combines cyanotype chemistry with lumen exposure. The photographic paper is first coated with cyanotype sensitizer, then exposed with botanical objects placed directly on the surface. The iron chemistry of cyanotype produces the blue shadow areas; the organic chemistry of the plants produces warm organic tones in the contact areas.

Process: Coat cyanotype sensitizer onto silver gelatin photographic paper. Allow to dry. Place botanical objects. Expose to UV. The result combines the cool cyanotype blue (shadow areas) with the warm lumen tones (areas of organic contact). Fix with water wash as normal for cyanotype, followed by sodium thiosulphate fixer to stabilize the lumen areas.

This process was introduced on Day 3 of the San Casciano workshop — the afternoon session exploring hybrid processes between the distinct chemistry of the week's techniques. See also: Lumen Print