Solarization is an artistic effect in photography. A film or photographic paper is briefly exposed to light during chemical development, reversing the brighter tones of the image and creating a white rim around previously dark areas.
Back in the 1950s, I worked with solarization in my darkroom. Here is an example and a step-by-step description of how I did it.

• A photo was taken of a ceramic vase with roses against a black background, illuminated from the side. The camera used was a Rolleiflex (a twin lens reflex).
• A blow-up was made from the 6 x 6 cm negative, but the development process was interrupted and the photographic paper was briefly exposed to light, then the development was continued.
• The finished print was placed face-to-face on a sheet of hard (i.e. high contrast) photographic paper with a pane on top of it, exposed to light and normally processed. See left image.
• This new finished print was again contact-copied. See right image.