What kind of wrapped plate was fogged by Becquerel's uranium salts in 1896?

The wrapped plate fogged by Becquerel’s uranium salts in 1896 was a photographic plate.

Henri Becquerel wrapped a Lumière photographic plate in thick black paper so ordinary light could not expose it. He placed uranium salts against the wrapping and expected sunlight-excited phosphorescence to produce an image. The plate nevertheless darkened even after the materials had been stored in a dark drawer.

That result showed that uranium compounds emitted penetrating radiation without needing prior exposure to sunlight. Becquerel first suspected phosphorescence, but further experiments with non-phosphorescent uranium salts led him to conclude that the radiation came from uranium itself. This became the discovery of spontaneous radioactivity.

A common mix-up is to call the object a photographic film. Film is a reasonable modern description, but the historical apparatus used a glass photographic plate coated with a light-sensitive emulsion. Marie Curie later studied the phenomenon systematically and coined the term radioactivity.