A cyanotype is a camera-less iron-salt process from 1842: paper is coated in ferric ammonium citrate and potassium ferricyanide, exposed to UV, then washed in water. Where light struck, Prussian blue forms and stays. Where it did not, the coating rinses away and the paper shows through. There is no black and no white in a cyanotype — only blue and bare paper.
That is why a blue tint over a greyscale photo never reads as a cyanotype. The process is not a hue shift; it is a three-point tone mapping with a hard floor and ceiling. This filter maps shadows to a deep iron blue, midtones to the classic Prussian blue, and highlights to warm paper rather than to white, then interpolates between those anchors across the luminance range. A fine grain follows on top, because hand-coated paper never absorbs the solution evenly.
| Tone | Maps to | Why |
|---|---|---|
| Shadow | Deep iron blue | Maximum Prussian blue density, never pure black |
| Midtone | Classic cyanotype blue | The signature of the process |
| Highlight | Warm paper | Unexposed coating washes off; the paper is the white |
The related filters on this page continue the same wet-process family: Ink Wash and Watercolour for pigment bleeding into fibre, and Paper for the substrate texture on its own. Raise the contrast slider to push more of the frame to the blue end, which is what a longer UV exposure does. Everything runs in your browser through Canvas 2D and exports a full-resolution PNG.