Cyanotype chemistry calculator — based on Anna Atkins' 1843 process
Number of sheets to coat in this session
☀️ Estimated UV Exposure — Cold Press Watercolor:
8–14 min direct sun / 20–40 min overcast
Most popular for beginners — textured surface holds chemistry well.
Total Solution Needed:
13.9 ml
Part A: Ferric Ammonium Citrate:
1.39 grams
Dissolve in 7.0 ml distilled water → store in light-tight amber bottle
Part B: Potassium Ferricyanide:
0.70 grams
Dissolve in 7.0 ml distilled water → store separately
⚗️ Mixing rule: Prepare A and B separately. Mix 1:1 only immediately before coating — once combined, the sensitizer degrades within hours. Work under tungsten or UV-safe yellow safelight.
Work under dim tungsten bulbs or UV-safe yellow safelight. Keep windows covered. Lay out your paper on a clean flat surface.
Combine Sol A + Sol B (1:1) in a small cup. Brush evenly across the paper with a foam brush or glass rod. Apply two thin coats rather than one thick coat.
Hang or lay flat to dry completely (20–40 min, or overnight). Any residual moisture causes uneven exposure. A hairdryer on cool works well.
Place your object or negative directly on the sensitized paper. Expose to direct sunlight or a UV lamp. See the exposure guide above for your paper type. The exposed area turns bronze-green — that's correct!
Rinse the print under cold running water for 5–10 minutes. The Prussian blue develops and the unexposed sensitizer washes away — the colour deepens as you watch!
Hang to dry away from direct light. The blue intensifies further over 24 hours as oxidation completes. Optional: a 3% hydrogen peroxide wash after rinsing speeds up the final colour.
🎯 A Simple Example: Botanical Contact Prints Like Anna Atkins
You've found a beautiful fern in your garden and want to make five cyanotype contact prints on watercolor postcards — exactly as Anna Atkins did in 1843.
1️⃣ Click the Postcard preset. The calculator sets 4×6 inches, 10 sheets.
2️⃣ Change Quantity to 5 (you only have 5 sheets today).
3️⃣ Select Cold Press Watercolor as your surface.
4️⃣ The calculator shows you need roughly 1.4g FAC + 0.7g PF, each dissolved in 7ml water — a tiny, precise batch. No waste.
5️⃣ After coating and drying, lay your fern flat on the paper under a piece of glass and expose for 10–12 minutes in summer sun. Wash in cold water. Watch the blue appear.
Pro tip: Press your botanicals flat between newspaper pages for 24–48 hours before printing — any moisture in the plant will cause tide marks on your print.
📷 Building your own camera to pair with this chemistry? Try the Pinhole Focal Length Architect — design a pinhole camera that exposes directly onto cyanotype-coated paper for a fully historic darkroom process. Or use the Camera Obscura Architect to understand the light geometry before you coat your paper.
Data Source: The Silver Sunbeam (John Towler, 1864) • Public domain • Solo-developed with AI
The forgotten pioneer: In 1843, Anna Atkins published Photographs of British Algae: Cyanotype Impressions — the first book ever illustrated with photographs. Not the first book about photography. The first book illustrated with photographic images. She did this a full year before Fox Talbot's landmark publication, using a process she learned directly from Sir John Herschel, its inventor. Atkins was a trained botanist, the daughter of a scientist, and a meticulous observer. She chose cyanotype not because it was easy, but because it was perfect: no darkroom, no enlarger, no silver chemistry — just sunlight, iron salts, and the contact silhouette of a real specimen pressed flat against sensitized paper. Each print was the actual shadow of a real piece of seaweed or fern. Her entire 14-volume atlas contained over 400 hand-made cyanotypes, each one produced by this exact two-chemical process.
The chemistry of Prussian blue: Cyanotype uses two iron salts — Ferric Ammonium Citrate (Part A) and Potassium Ferricyanide (Part B). Separately, they're stable and unreactive. Mixed together and coated on paper, they form a light-sensitive complex. When UV light strikes this complex, it triggers a photoreduction: Fe³⁺ ions are reduced to Fe²⁺. These reduced iron ions then react with the ferricyanide to form ferrous ferricyanide — the insoluble, intensely blue compound known as Prussian Blue (Fe₄[Fe(CN)₆]₃). Wash the print in water, and the unreacted sensitizer washes away cleanly, leaving the white of the paper beneath. No fixer needed. No darkroom. Just water.
Why this is having a revival: Cyanotype is undergoing a genuine renaissance. It's one of the most accessible alternative photographic processes ever devised — the chemistry is inexpensive, widely available, and can be applied to almost any absorbent surface: paper, fabric, wood, leather. Whole communities have formed around cyanotype on denim, botanical contact printing, large-format sun-printing with children, and contemporary fine art. The same process that powered engineering blueprints in the 1890s — which is why engineers still call their drawings "blueprints" today, even though no one uses the process anymore — is now being used by textile artists, printmakers, and photographers who want a tactile, solar-powered alternative to digital imaging.
From Anna's algae to your garden fern: The formula this calculator uses — 20g Ferric Ammonium Citrate per 100ml water (Solution A) and 10g Potassium Ferricyanide per 100ml water (Solution B), mixed 1:1 — was codified by John Towler in The Silver Sunbeam (1864) and has remained the standard for 160 years. It's one of the most reproducible and forgiving photographic recipes ever written. The New Cyanotype formula (Mike Ware, 1994) substitutes Ammonium Iron(III) Oxalate for a richer, more neutral blue with fewer tide marks — a modern refinement of a Victorian original. Anna Atkins would recognize everything about the process. And she'd probably appreciate that we've finally calculated the exact gram weights so you don't waste her precious chemistry.
🐾 From the Lab Cat's Photochemistry Division: I pressed myself against the sensitized paper this morning to make a contact print. The humans said this was "not how the process works" and also "get off the darkroom table." I maintain that a cat silhouette in Prussian blue would have been far more scientifically interesting than another fern. Anna Atkins would have agreed. She was a biologist. I am biologically significant. 🌿