This and other characteristics will help you in dating ambrotypes.Mats, the brass inner frame that laid on top of the glass, experienced design evolutions that are datable.
See the Photographers used many of the same pieces and methods for packaging an ambrotype as they did for the daguerreotype (see Daguerreotype page).
Nearly all ambrotypes will have a preserver (if the packaging is original).
Even the softest camera lens brush will scratch the delicate surface.
At best use compressed air to remove loose dust or particles.
Smooth or finely textured surfaces adorned mats up to about 1859. Then, intricate designs began appearing, stamped into very thin mats. Along with mat designs becoming more ornate, the preserver also became more intricate, and delicate.
Preservers before 1859 were usually plain along the edges, with a singular and simple design.
The ambrotype's life span was very short, wide-spread use was less than ten years, but produced a vast collection of beautiful images.
While the name ambrotype was derived from the Greek word ambro, meaning imperishable, it was still a delicate, easily damaged photograph.
Note: Don't mistake the reflection of the cover glass to be the mirror look.
If the image is out of the case, you will be able to confirm the nature of the photograph.
One advantage it did possess over the silver daguerreotype was that it did not tarnish.