Cyanotype Sensitizer Calculator

How It Works

Cyanotype is an alternative photographic process that produces Prussian-blue prints from a light-sensitive iron chemistry. Two aqueous stock solutions are mixed in equal parts just before coating: Solution A is ferric ammonium citrate at 25 g / 100 ml water, and Solution B is potassium ferricyanide at 10 g / 100 ml water. Combined, the sensitizer is applied to paper, exposed to ultraviolet light through a negative or object, and then washed in water — during the wash, the reduced iron reacts to form the deep blue pigment.

The chemistry traces to Sir John Herschel's 1842 discovery, republished with modern working procedures in Christina Z. Anderson's Alternative Photographic Processes, Ch. 8: Cyanotype (2019 edition). Mike Ware's Cyanotype (Science Museum, 1999) documents the "New Cyanotype" variant with different molar ratios and citric acid stabilizer — the tool covers the classic Herschel formula only.

The calculator sizes the batch by paper area. Coating rates from Bostick & Sullivan technical data are roughly 2 ml of mixed sensitizer per 100 cm² with a soft foam brush, or 1 ml per 100 cm² with a puddle-pusher glass rod. For a 20-print series at A5 (148 × 210 mm = 311 cm²), two-brush coats total: 2 × 311 × 2 × 20 / 100 = 249 ml mixed sensitizer, which is 124.5 ml each of Solutions A and B, requiring 31.1 g ferric ammonium citrate and 12.4 g potassium ferricyanide. The tool computes these numbers from paper area and drops the manual arithmetic.

Both chemicals are supplied dry and dissolved in distilled water (tap water can contain calcium and iron that fog the sensitizer). Stock solutions keep 6-12 months in dark bottles; the mixed working solution should be used within 2 hours because iron gradually reduces even in the dark.

How to Use This Calculator

1. Choose calculation mode — By Paper Area for a print series, or By Target Volume if you already know how much working solution you need.
2. Enter paper dimensions — Length × Width in mm, plus the number of prints. Optional single-side or both-sides coating flag.
3. Set coating method — Foam brush (2 ml / 100 cm²) or puddle-pusher (1 ml / 100 cm²).
4. Set the number of coats — 1 coat for light prints, 2 coats for deeper blacks.
5. Read chemical weights — Solution A ferric ammonium citrate in grams, Solution B potassium ferricyanide in grams, and the water volume for each.
6. Read mixed working solution volume — the tool adds a small buffer for waste on the coating tray.

Real-World Use Cases

Botanical print series

A photographer makes 20 A5 (148 × 210 mm = 311 cm²) botanical cyanotype prints, single-sided, with two brush coats each. Total sensitizer volume: 2 × 311 × 2 × 20 / 100 = 249 ml mixed working solution, split into 124.5 ml each of Solutions A and B. That works out to 31.1 g ferric ammonium citrate in 124.5 ml water and 12.4 g potassium ferricyanide in 124.5 ml water. The photographer mixes each stock a day ahead and combines them 15 minutes before coating.

Large sun print for a wall piece

A crafter makes a single 60 × 90 cm sun print on watercolor paper, single-coat, foam brush. Area is 5400 cm², so total sensitizer is 5400 × 2 / 100 = 108 ml mixed solution — 54 ml each of Solutions A and B. The crafter also scales working time: coating a piece this large under safelight conditions takes 6-8 minutes, and the sensitizer starts to skin over at the tray edges after 20 minutes.

Testing a new paper

A student compares three paper stocks (Arches, Fabriano, Stonehenge) with 4 test strips each at 5 different UV exposures — 60 small tests total. Each strip is about 50 × 200 mm = 100 cm², one coat, puddle-pusher. Sensitizer needed: 60 × 100 × 1 / 100 = 60 ml mixed solution — 30 ml each of A and B, or 7.5 g ferric ammonium citrate and 3 g potassium ferricyanide. Because the batch is small, the student can afford to test multiple sensitizer freshness levels (freshly mixed vs 6-hour-old) as another variable.

Tips & Safety Notes

• Work in dim light during coating and drying. Direct sunlight in the coating area starts exposure before you're ready.
• Use distilled water for both stock solutions. Calcium and iron in tap water fog the sensitizer and dull the blues.
• Never combine potassium ferricyanide with strong acids. Contact with concentrated HCl or nitric acid releases hydrogen cyanide gas — the calculator's safety notes flag this in the interface.
• Dry-coated paper in the dark for 30-60 minutes before exposure. Damp sensitizer fogs during exposure and produces washed-out prints.
• Wear nitrile gloves and eye protection when weighing and dissolving the dry chemicals. Both are skin sensitizers with repeated exposure.

Limitations & What This Tool Cannot Do

• Not a Prussian-blue chemistry model. Predicts sensitizer volume from paper area; does not predict development times, tonal range, or archival stability — those depend on paper choice, UV source, exposure duration, and wash chemistry.
• Toxicity awareness required. Ferricyanide is a mild toxin and reacts with strong acids to release hydrogen cyanide gas. The tool does not enforce safe handling; treat the safety notes as required reading and always work in a ventilated space away from acidic solutions.
• Not calibrated for Ware's New Cyanotype variant. Ware's variant uses different molar ratios, adds citric acid, and produces a more stable working solution. This tool covers the classic Herschel formula only; use the Ware paper directly for that variant.

FAQ

What is a cyanotype and how does the sensitizer work?

A cyanotype is a photographic print in Prussian blue, made by coating paper with an iron-based sensitizer and exposing it to UV light through a negative or object. UV reduces the iron; the water wash then reacts the reduced iron with ferricyanide to produce the blue pigment. The process was invented by Sir John Herschel in 1842.

What is the difference between the classic and Ware's "New Cyanotype" formulas?

The classic Herschel formula uses ferric ammonium citrate and potassium ferricyanide in a simple two-solution mix. Ware's "New Cyanotype" (1994) substitutes ammonium ferric oxalate for the citrate, adds citric acid, and requires different molar ratios. Ware's version produces higher contrast and more stable tonal range but is more expensive and less forgiving.

How long does mixed sensitizer last before it goes bad?

Stock Solutions A and B stored separately in dark amber bottles last 6-12 months. Once combined, the mixed working solution should be used within 2 hours — after that, the sensitizer degrades and prints come out light and washed out. Mix only what you plan to coat.

What paper works best for cyanotype prints?

Heavy watercolor paper at 200-300 gsm (Arches, Fabriano Artistico, Stonehenge) is the standard choice — it absorbs sensitizer evenly, survives the water wash without warping, and holds detail. Hot-press papers give finer detail; cold-press adds texture. Avoid papers with optical brighteners, which fog under UV.

Can I make cyanotypes without a UV light box?

Yes — direct summer sunlight for 5-15 minutes exposes cyanotypes well. Overcast days work but need 30-60 minutes. A UV nail-lamp or grow-light also works and gives more repeatable exposures. Any UV source in the 350-450 nm range is effective.

Why do my prints come out light blue and washed out instead of deep Prussian blue?

Three most common causes: sensitizer was too old (mixed solution over 2 hours), paper was too damp during exposure, or exposure was too short. Try mixing fresh sensitizer, drying coated paper for a full hour before exposure, and doubling exposure time to see which fixes it.

Related Tools

• HLB Calculator for Emulsifiers — Formulation-adjacent tool
• Lye & Water Calculator — Chemistry-adjacent tool with precise weight-based mixing
• Varnish Dilution Calculator — Solvent and dilution math

References

• Christina Z. Anderson, Alternative Photographic Processes, Ch. 8: Cyanotype (2019 edition) — classic Herschel formula and modern working procedures
• Mike Ware, Cyanotype: The History, Science and Art of Photographic Printing in Prussian Blue (Science Museum, 1999) — historical context and New Cyanotype variant
• Bostick & Sullivan technical data on cyanotype chemistry — coating rates and paper recommendations

Reviewed by the Craft Calc Lab team on August 1, 2026.