Photographic plates were used in still photography and consisted of a glass plate coated with a light-sensitive emulsion of silver salts.
Photographic plates ceased to be used by amateur photographers in the early 20th century, as they switched to photographic film, but glass plates continued to be used until the 1970s by some photographic businesses, and until the 1990s for astronomical surveys before these moved to digital imaging.
Glass plates were far superior to film for research-quality imaging because they were extremely stable and less likely to bend or distort, especially in large-format frames for wide-field imaging.
Glass photographic plates using the wet collodion process, which was invented in 1851, replaced the earlier Daguerreotype process that used a polished silver coated plate of tin or copper. The wet collodion process was inconvenient and required portable darkrooms for field photography. Gelatin dry plates were first invented in 1871 and in 1878, it was discovered that heating the plates made them more resistant to friction, and the emulsion more sensitive to light. During the 1880s dry plates largely replaced wet collodion plates, although wet collodion plates continued to be used for some special purposes.