Ambrotypes were a photographic innovation from mid-1850s. A piece of glass was dipped into a wet collodion concoction and exposed, to produce a positive image. When the back of the glass was covered with black varnish, the lovely image “popped” (esp. with a little hand-tinting). The photo was then protected with a piece of glass, a gold mat and “savers,” and a fitted case. Within a decade this format was supplanted by the hardier tintype.
My inventory project now tells me we have 64 pretty little ambrotypes snugged in their tiny cases. The above slideshow shows 50; the rest are in the Civil War slideshow. If you like your small photos super-sized, visit the collection at Flickr.