decorative spacer that prevents a framed image from touching the cover glass is called a mat. These were also called passé-partout, Th is is a French term generally applied to a window mat. ... specifically the passé-partout; (mount included a deep mat and a reverse painted cover glass often decorated with a gold pin line around the opening. These were very common in Europe for the mounting of daguerreotypes and collodion positive images) ... a French term referring to the window opening that allows the image behind the mat to be seen from the front.
All daguerreotypes, ambrotypes, and ferrotypes were first covered with a mat before placing them into frames or protective cases. The earliest daguerreotype mats were made of thin paper. Eventually stamped metal mats of copper or brass became the standard for cased images. Framed prints were usually matted with paper, heavier card stock, or pasteboard. The cheap ferrotypes called bon tons were placed in a special paper mat with an integral folding paper cover.
...tinsmiths of the 19th century occasionally finished their hand-crafted products with a baked-on coating of asphaltum. The glossy black finish, which resembled Japanese lacquerware, was very popular and was applied to a wide variety of domestic objects. The most typical formula for Japan varnish was solvent, asphaltum, and Canada balsam.
Japanned black-plate iron was the support material for melainotypes and ferrotypes, popularly known as tintypes. The plates were not made by the ferrotypist but commercially manufactured in standard cased sizes or in large sheets to be cut to size as needed.
A double sided case with two ambrotype plates. Left plate has been taken apart, resealed, and replaced in case well without its brass preserver. The image package remains disassembled, with plate, mat, cover glass, and preserver displayed.