Keys to History

In the mid-1850s, another photographic process became popular, following the daguerreotype. The collodion, or wet-plate, process quickly replaced the daguerreotype as the preferred form of photography. It was faster, cheaper and easier to learn. Three main forms of collodion photographs were sold to the public: the ambrotype (on glass), the ferrotype (on metal coated with black enamel) and paper prints made from glass negatives (the true negative-positive process).

Ambrotypes are actually underexposed wet-plate negatives. When placed against a dark background, they appear to be positive. They were popular from about 1852 to 1863.

Source : In the Eye of the Camera [Web tour], by Nora Hague, McCord Museum (see Links)