The ambrotype photograph is a particularly lovely format of antique photography. Developed after the Daguerreotype, they used what is called the Wet Plate Collodion Process.
As opposed to their predecessor the Daguerreotype, which were generally produced on copper, these photos were negative images produced on glass.
To transform the negative into a positive image, some form of integration is required. Typically you will either see the back of the plate covered in varnish, or a piece of black velvet behind the plate.
Photographers quickly adapted to this additional requirement by using dark-colored glass for the base. The most common is red glass (which bears the moniker of Ruby ambrotype); but I have seen examples on violet and green glass as well. Using dark-colored glass largely negates the need for backing.
Ambrotypes photos were popular in the 1850s into the early 1860s, but were largely superseded by other more practical formats like tintypes and CDVs.
Like Daguerreotypes, Ambrotypes are fragile and typically found housed in protective mats and cases. The most obvious reason for this is the fact it is a piece of glass, as opposed to copper. However, the emulsion is much more durable than the surface of a dag.
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