the negative
inkjet filmThe negative is what the light passes through. Where it is dark the paper stays white; where it is clear the paper turns blue.
- A photo is turned into a negative and printed on clear film, with its tones corrected so the print looks like the photo.
- A plant is laid straight on the paper. Thin, pale parts of a leaf let a little light through.
- Flip mirrors the image.
Drop or paste any photo to print your own.
the process
- 1coat
- 2expose
- 3wash
- 4dry
The sensitizer soaks into the paper, so the paper decides how crisp and how deep the blue can be.
- Arches Platine: smooth, hard cotton. The crispest detail and the deepest blue.
- Bergger COT320: smooth cotton, a little softer.
- Cold-press cotton: a textured surface; brush marks and grain show.
- Student watercolor: cheap wood-pulp paper; softer, paler, bleeds more.
- Kozo washi: thin Japanese paper of long fibers; soft, organic edges.
- Recycled sketch: flecked and thirsty; uneven.
Sized ↔ thirsty: sizing keeps the liquid at the surface. Thirsty paper drinks it deep, so brush marks freeze where they land and the blue is softer.
The light-sensitive coating: iron salts that turn into Prussian blue where light hits them.
- Classic (Herschel, 1842): two bottles mixed just before use. A is ferric ammonium citrate, B is potassium ferricyanide. About 10–15 min of summer sun; forgiving, with a long range of tones.
- New (Mike Ware, 1994): one ready-mixed bottle based on ferric oxalate. About four times faster and a deeper blue.
- Two coats, dried in between, hold more iron: a deeper blue, but about twice the exposure.
Only ultraviolet light does the work.
- Sun: clear summer noon. The sharpest edges, because the sun is a point of light.
- Shade: an overcast sky; three to four times slower, very even.
- UV box: a 365 nm LED unit; steady and repeatable. Its light comes from a wide panel, so edges under a thick object soften slightly.
How long the print sits in the light. Turn the timer clockwise to add time.
The pencil mark is the standard time for this paper, chemistry and light: the least exposure that gives the full blue, as a printer would read it off a test strip.
- Shorter: paler; the light tones wash away.
- Longer: richer shadows. Much longer and the shadows go gray-bronze, but most of the blue comes back as the print dries.
Lift the frame during the exposure to peek.
Water carries away the yellow salts that never saw light, leaving only the blue.
- Tap: ordinary water (pH about 7.6).
- + Vinegar: a splash in the first bath clears the yellow faster and keeps the highlights clean.
- Hard: alkaline water slowly dissolves the blue. Paler prints, yellowish highlights.
Five minutes clears a print; very long washes lose a little blue.
A print comes out of the wash pale. The blue deepens as the iron takes up oxygen again.
- Hang overnight: it comes back by itself over a day.
- Peroxide: a splash of 3% hydrogen peroxide does the same in seconds.
- Tea / coffee: washing soda bleaches the blue to faint yellow, then the tannin in the brew stains it back: purple-brown to near black with tea, warm brown with coffee. The paper takes on the brew's color too.