Cyanotype Sensitizer Calculator

How It Works

Cyanotype is a photographic printing process that produces distinctive blue-and-white images using UV-sensitive iron compounds. The sensitizer consists of two solutions mixed in equal parts immediately before use:

• Solution A: Ferric ammonium citrate (25g per 100ml water) — the light-sensitive iron(III) compound
• Solution B: Potassium ferricyanide (10g per 100ml water) — reacts with reduced iron to form Prussian blue

When exposed to UV light, the iron(III) in Solution A reduces to iron(II), which reacts with the potassium ferricyanide to form the insoluble Prussian blue pigment (iron(III) hexacyanoferrate(II)). Unexposed areas wash away in water, leaving white paper.

Coverage rate is approximately 2ml of mixed solution per 100 cm² per coat.

How to Use

1. Select calculation mode — "By Paper Area" if you know your print size, "By Target Volume" if you want to prepare a batch of sensitizer
2. Enter dimensions (area mode) — paper width and height in centimeters, plus number of coats
3. Or enter target volume (volume mode) — how many ml of mixed sensitizer you want to prepare
4. Read chemical weights — the exact amounts of each chemical and water to weigh out
5. Prepare solutions separately — dissolve each chemical in its water amount, then combine equal parts A and B just before coating

Tips

• Prepare solutions A and B separately and store in brown glass bottles. They last months individually but only ~2 hours once mixed.
• Use distilled water — chlorine and minerals in tap water can cause inconsistent results or staining.
• Coat in subdued light (tungsten or dim room). The sensitizer is UV-sensitive, not visible-light-sensitive, but prolonged exposure to bright light can fog it.
• Apply with a foam brush or glass rod for even coverage. Avoid metal tools — they can react with the iron compounds.
• Dry coated paper completely in darkness (1-2 hours). Incomplete drying causes blotchy results.
• Double coating produces deeper blues and better tonal range, especially on absorbent papers. Allow the first coat to dry completely before applying the second.

FAQ

What paper works best for cyanotype?

Heavy watercolor paper (200-300 gsm) produces the best results — it absorbs the sensitizer well, handles water development without warping, and provides a bright white base for contrast. Hot-press (smooth) gives finer detail; cold-press (textured) adds character. Avoid glossy or coated papers.

How long should I expose the print?

Exposure time depends on UV intensity. In direct summer sunlight: 5-15 minutes. Under a UV lamp: 5-20 minutes depending on wattage and distance. The print is ready when the exposed areas turn a steel-gray or olive-bronze color (it turns blue during the water wash, not during exposure).

Can I tone cyanotypes to change the color?

Yes. Common toning methods include: tannic acid (tea/coffee) for warm brown-black, sodium carbonate (washing soda) for yellow-orange split tones, and bleach-and-redevelop for various effects. Toning is done after the print is fully washed and dried.

How long do the stock solutions last?

Individual solutions A and B, stored in dark bottles away from light and heat, last 6-12 months. Once mixed together, the working solution should be used within 2 hours — sensitivity degrades rapidly after mixing. Pre-coated paper stored in a light-tight container can last 1-2 weeks.

Why is my print coming out pale or washed-out?

Common causes: under-exposure (expose longer), insufficiently concentrated sensitizer (weigh chemicals precisely), paper that doesn't absorb well (switch to watercolor paper), or washing too aggressively (a gentle 5-minute rinse in cool water is sufficient). Double-coating also helps deepen blues.

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Last reviewed: June 2026