Ambrotype of unidentified soldier in Union uniform
Among the photographic processes available to a widening public and used by Civil War soldiers were ambrotypes. Ambrotypes became popular in the mid-1850s, due in large part to their shorter exposure times (as compared to daguerreotypes), which also made them a good choice for churning out portraits of soldiers. They consist of a collodion image fixed to a glass plate, which, when held up to light, appears negative. To create a positive image, the plate was placed against a black background. Like daguerreotypes, ambrotypes are unique objects—there is no negative, so the plate is the photograph.