Guides
The cyanotype process: chemistry and safety rules
How the cyanotype process works: the iron chemistry that makes the blue, and the safety and waste rules that actually apply in a home studio.
What to take away
- The cyanotype process is a contact printthe image forms during exposure and develops in plain water.
- Never let a ferricyanide solution meet a strong acid, and never heat it. That reaction can release hydrogen cyanide.
- Paper choice matters more than chemistry. Alkaline-buffered paper destroys the blue you are trying to make.
- We do not publish gram quantities here. Use the formula on the manufacturer's data sheet for the product in your hand.
What is actually happening
The sensitizer comes from two stock solutions: ferric ammonium citrate, the iron salt, and potassium ferricyanide, the ferricyanide salt, mixed in equal volumes just before coating. Ultraviolet light reduces the iron compound where it strikes. During washing, the reduced iron reacts with the ferricyanide to form Prussian blue, an insoluble pigment that stays in the paper fibers while unexposed sensitizer rinses away.
Sensitizer to Prussian Blue
- Iron salt + ferricyanide, equal volumes
- Coated paper dried in dim light
- UV reduces iron where it strikes
- Wash: reduced iron meets ferricyanide
- Prussian blue locks into paper fibers
- Unexposed sensitizer rinses away
Two things follow. The process is print-out, so you watch the image appear and judge exposure by eye. And because the pigment is an iron blue, anything alkaline attacks it.
The five stages
| Stage | What you are controlling | Typical failure |
|---|---|---|
| Planning | Negative density and contrast | A digital negative that is far too thin |
| Coating | Even film, dim light, full drying | Streaks, and coating that is still damp |
| Exposure | Ultraviolet dose | Guessing instead of running a test strip |
| Washing | Clearing unexposed sensitizer | Stopping the wash too early |
| Oxidation | Time, and a mild acid rinse if used | Judging the print before it has dried down |
Exposure varies with season, time of day, cloud cover and lamp age, so a test strip at the start of every session beats any published time. Wash until the water runs clear and the yellow-green tint has left the highlights. Dry flat and leave the print a day while the blue deepens.
Five Stages, Five Failures
- Planningnegative density and contrast
- Coatingeven film, dim light, full drying
- Exposureultraviolet dose, test strip first
- Washingclear unexposed sensitizer fully
- Oxidationlet blue deepen, dry flat
Safety that is not optional
- Ferricyanide and acid must never meet. No mineral acids, no acidic drain products in a sink you rinse into, and no heating.
- If you intensify with an acid rinse, use a dilute household acid such as white vinegar or citric acid. A formula calling for a mineral acid belongs in a shop with mechanical extraction, not a house.
- Wear nitrile gloves and sealed eye protection while mixing and coating, and weigh powders slowly to avoid dust.
- Keep it out of the kitchendedicated trays and vessels, no food or drink at the bench.
- Read the safety data sheet for each product, and keep it.
Waste needs the same care: a rinse tray of dilute sensitizer is not household water, and an acid rinse strong enough to matter raises a disposal question.
The EPA test for the characteristic of corrosivity under RCRA, at 40 CFR 261.22, turns on an aqueous pH at or below 2 or at or above 12.5. Keep unused sensitizer and any strong rinse separate and labeled. Ask your collection point before pouring anything.
Paper, and why it decides the print
Use unbuffered, slightly acidic, well-sized paper. Alkaline buffering is added to most conservation board and many art papers because it protects against acid, and it bleaches a Prussian blue image over time. The same rule applies later: no buffered mount board, no alkaline detergent.
Handling matters once the print is finished. The National Archives, whose holdings include cyanotypes among many historic processes, sets out handling guidance for photographic prints worth adopting: clean gloves, hold by the edges, support with two hands, dust with a soft brush rather than canned air.
Where this fits in a practice
Cyanotype is one of two processes we recommend for a domestic room, and our Home printmaking: a clear starting point explains why. The room needs a wet zone and a drying plan.
Common questions
Can I use an enlarger?
How long does the sensitizer keep?
Why did my print turn gray?
Is sunlight better than a UV lamp?
In this guide
- Cyanotype process: negatives, toning and limited editionsA practical cyanotype process guide: making digital negatives, toning with tannins, and signing and numbering a limited edition, with the U.S. copyright rules.
- Sequence your cyanotype purchases so the essentials come firstThe buying order for a first cyanotype setup, why premixed chemistry beats raw powders at the start, and the equipment not to buy at any stage.
- Does a finished cyanotype print really cost what you think?A finished cyanotype print carries paper, film, chemistry, failures and framing. Here is where the money goes and when a reproduction is cheaper.
- How to run the cyanotype process safely inside a one-sink flatA cyanotype process for a one-sink flat: coat, dry, expose and wash with chemical, PPE, ventilation, spill and disposal precautions.
- What to look for in a cyanotype process classUse this cyanotype process class checklist to verify chemistry, safety data, wastewater rules, handouts, reference prints, and course format before booking.
