for an article on "Solargraphy of ESO" by Fosbury and Trygg.

Made using the latent image on photographic paper exposed for months in a pinhole camera. See: and

The following comment from David Malin explains the basic Solargraphy process:

"The colours in B&W photographic paper exposed to light come from finely divided metallic silver growing on the silver halide grains. The latent image, which is typically ~10 silver atoms per billion-atom grain is invisible, but on continued exposure the latent image clumps grow so that the first visible signs of a print-out image are yellowish, darkening to sepia than a maroonish-brown as the particle size increases. Eventually the maximum exposure produces a slate grey shade. Reversing an image with this natural range of variations will produce interesting colours, which are of course unrelated to the real colour of the scene. However, lightly exposed parts will be bluish and shades of green/cyan will likely appear in the mid-tones, both of which will lend the positive images a natural look.

It is possible to fix the printed out image but this reduces the range of tones to more or less sepia-dark brown. Print-out paper used to be made for this purpose and was generally exposed by contact with a negative using sunlight as the source. This is how E. E. Barnard acquired his interest in photography. Other types of photographic materials such as enlarging paper produce different tones with exposure to intense light, mostly unsuited the the solar print process. Extreme exposure results in solarization (hence the name) which is a partial loss of density in the highlight (darkest) regions."
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