Cyanotype Photogram
Summary
UV light reduces1 iron(III) to iron(II) where it strikes coated paper, and the iron(II) then forms insoluble Prussian blue — so objects laid on the paper cast white shadows on a blue field. You’ll be able to explain what the light does to the iron, why the image appears during the water rinse, and why a photogram is a “negative.”
History
The cyanotype process was invented by Sir John Herschel2 in 1842 — the same scientist who gave photography the words positive, negative, and snapshot, and who introduced sodium thiosulfate as the standard photographic fixer. For Herschel, the cyanotype was a scientific curiosity and a way to copy notes; he did not imagine it would outlast nearly every other photographic process.
The first person to use cyanotype as a serious creative tool was the botanist Anna Atkins3. In 1843 she produced Photographs of British Algae: Cyanotype Impressions, which was the first book illustrated with photographs. She placed dried seaweed and algae specimens directly on the coated paper, pressed them in sunlight, and washed the prints in water. The resulting silhouettes — white shapes on deep Prussian blue — remain some of the most beautiful images in photographic history.
The same chemistry was adapted for architectural blueprints: engineers traced their drawings onto translucent paper, laid the tracing against cyanotype-coated paper, and printed them in sunlight. The word blueprint comes from this process, and it was the standard method of copying technical drawings from the 1870s until the mid-20th century, when faster diazo “whiteprint” processes (blue or black lines on a white ground) took over — later giving way to xerographic copiers and CAD plotters.
The chemistry has not changed since 1842. Cyanotype is also one of the very few historic photographic processes that remains commercially available and in active use today.
Hazards & preparation
PPE: gloves (the sensitizer stains); old clothes.
- Potassium ferricyanide — never mix with strong acids. The cyanide is safely bound to iron, but strong acid can release toxic hydrogen cyanide gas. Keep acids away.
- Both salts are low-toxicity, but avoid ingestion and don’t get the powder in your eyes.
- Don’t stare into the UV lamp or the sun during exposure.
Disposal: the dilute rinse water and spent sensitizer are low-hazard — flush with plenty of water. Keep concentrated leftovers for hazmat if you have a lot. See the Safety page.
Materials
- Ferric ammonium citrate (green) — 20 g in 100 mL distilled water (Solution A)
- Potassium ferricyanide — 8 g in 100 mL distilled water (Solution B)
- (Or: pre-mixed cyanotype sensitizer from a photographic supplier)
- Watercolour paper (90 lb/190 gsm or heavier) or cotton fabric
- Foam brush, sponge brush, or wide watercolour brush
- Flat objects to print: leaves, flowers, ferns, feathers, keys, lace, circuit boards, hands
- A sheet of glass (optional, to press objects flat and give sharp edges)
- A sunny day or a UV lamp (365 nm)
- A tray or sink for the water rinse
- A few drops of hydrogen peroxide (optional, to intensify the blue)
Procedure
- Mix the sensitizer: Combine equal volumes of Solution A and Solution B just before use. The combined solution is yellow-green and light-sensitive — work in normal artificial light, away from direct sun or UV.
- Coat the paper: Brush a thin, even coat onto watercolour paper or fabric — one direction, then cross-coat perpendicular. It should look slightly translucent, not pooling.
- Dry in darkness: Hang to dry in a dark room or shade, 20–40 minutes (or a hair dryer on cool). The dried paper turns pale yellow-green.
- Arrange your objects: In dim light, place flat objects face-down on the dried paper. A sheet of glass pressed on top flattens them for sharper edges.
- Expose to light: Carry to direct sunlight. The paper shifts from yellow-green through grey to blue-grey. Strong summer sun: 5–10 min; hazy/winter sun: 15–25 min; UV lamp (365 nm): 3–10 min.
- Develop in water: Rinse under running cold water 2–3 minutes. The image appears as unexposed salts wash away and the blue deepens. Rinse until the water runs clear. (A few drops of hydrogen peroxide in the rinse intensifies and speeds the blue.)
- Dry flat: The image keeps deepening over the next hour as it fully oxidizes.
What you should see
The dried paper is a dull yellow-green. In sun it visibly tarnishes to grey and blue-grey where light hits it. The magic is the rinse: as the water clears the unexposed coating, the exposed areas bloom into a deep Prussian blue while the shadowed shapes stay crisp white — a photographic negative in blue and white.
| Symptom | Likely cause | Fix |
|---|---|---|
| Pale, washed-out blue | Underexposed | Expose longer / in stronger sun; peroxide in the rinse |
| Whole sheet blue, no image | Objects didn’t block light, or fogged before exposure | Press objects flat under glass; keep coated paper dark until exposure |
| Image washes away | Coating too thin, or over-rinsed | Coat a bit heavier; rinse just until water runs clear |
| Blotchy coat | Uneven brushing | Cross-coat evenly; use heavier paper |
The reactions
UV light reduces iron(III) (Fe³⁺) to iron(II) (Fe²⁺) in the exposed areas:
\[\ce{Fe^{3+} + e^{-} ->[\text{UV}] Fe^{2+}}\]
The Fe²⁺ reacts with the ferricyanide ion to produce insoluble Prussian blue:
\[\ce{K^{+}_{(aq)} + Fe^{2+}_{(aq)} + [Fe(CN)_6]^{3-}_{(aq)} -> KFe[Fe(CN)_6]_{(s)}}\]
sunlight turns Fe³⁺ into Fe²⁺, which combines with ferricyanide to make insoluble blue pigment; the unexposed salts rinse away to leave white.
The identical pigment is synthesised in the Prussian Blue experiment.
The Science
The deep Prussian blue colour comes from intervalence charge transfer between iron(II) and iron(III) centres within the crystal structure of the pigment. Light energy promotes an electron from Fe²⁺ to Fe³⁺, absorbing red and orange light and transmitting only blue. This is the same absorption mechanism seen in the lab-synthesised pigment, but produced photochemically in the paper fibres.
The hydrogen peroxide rinse accelerates the final oxidation of any residual Fe²⁺ ions in the paper, giving a deeper, faster blue. Washing soda (sodium carbonate) or ammonia will partially bleach the blue to yellow-brown — this is reversible, and re-exposing to air restores the blue over hours to days.
Cyanotypes are highly lightfast but sensitive to alkali. They should not be framed against alkaline mat board, and should not be washed with soap or alkaline cleaners.
Questions to Explore
What is UV light doing to the iron? It converts Fe³⁺ to Fe²⁺ in exposed areas. Why does UV cause this, and what is the light’s energy used for?
Hint / answer
A UV photon carries enough energy to knock an electron off the citrate onto the iron, reducing Fe³⁺ to Fe²⁺. Visible light photons are too weak to drive it, which is why cyanotype needs sun or a UV lamp, not room light.
Why does the image appear during rinsing, not exposure? The deep blue only shows on washing. What does water do?
Hint / answer
Water lets the freshly made Fe²⁺ meet ferricyanide and crystallize into insoluble Prussian blue, and it washes the unexposed, still-soluble iron salts away to reveal white. Dry, the reaction is stalled and the contrast is faint — the rinse both develops and fixes it.
Why are shadows white and highlights blue? Objects block light, leaving white; exposed areas go blue — a negative. How would you make a positive-tone print?
Hint / answer
Only lit areas form blue pigment, so anything that shadows the paper stays white — the reverse of a normal photo. For a positive, expose through a negative transparency (dark where the scene is light), so the bright parts of the scene end up blue.
Why does ammonia bleach the blue to brown? Alkali partially destroys Prussian blue, and it re-darkens in air. What’s happening?
Hint / answer
Alkali attacks the iron-cyanide lattice, breaking down the blue pigment into brownish iron hydroxide/oxide. Re-exposure to air and acid re-forms Prussian blue, so the bleaching is reversible — the blue comes back over hours to days.
What limits sharpness? Flat objects give sharper edges than fluffy ones. Why does the object-to-paper gap matter?
Hint / answer
Sunlight comes from a slightly spread source, so it sneaks sideways under any gap, blurring the shadow edge. An object pressed flat (under glass) leaves no gap and casts a crisp edge; a fluffy feather held off the paper gives a soft one.
Going further
- Go tonal. Print through an inkjet negative on acetate to get full greyscale tones instead of a plain silhouette.
- Print on cloth. The same recipe works on cotton or silk (use a longer wash) for a permanent fabric print.
- Next in the Redox track: make a silver photograph the 1834 way in the Salted Paper Print.
Footnotes
Reduction — The gain of electrons by an atom, ion, or molecule; always paired with oxidation.↩︎
John Herschel — English polymath (1792–1871) who invented the cyanotype photographic process in 1842.↩︎
Anna Atkins — English botanist and photographer (1799–1871) who made the first book illustrated with photographs, using cyanotypes of algae.↩︎