Not a dag, an ambrotype. Dags were the first "type" that was produced, starting in 1839. The process used silver-surfaced copper plates, and the image was IN the silver surface. Copper was used for the plate only for its' stiffness, and it was much cheaper than using a plate made entirely of silver.
In 1851, and Englishman invented the ambrotype, a glass plate that had a gooey collodion spread onto one surface. It was then soaked in a silver solution, and, while still wet, the plate was exposed. Chemicals then fixed the silver image permanently, and a dark background was added to the plate to cause the produced negative to be able to be viewed as a positive. It was cheaper and much less labor to produce than the daguerreotype. The process was patented, and it wasn't until 1854 that ambros were were franchised and produced in the US. "Ambrotype" is from the Greek "ambrotos" = immortal, plus "type" = image, thus immortal image.
In 1856, an American came up with a variant of the ambro process, applying the silvered collodion to a surface of an iron sheet that was darkened with a dark japanned lacquer. The was known as a tintype, also melainotype or ferrotype. By 1860, the dag was obsolete, being replaced by the ambro and tintypes, as well as the process for albumen prints, using a paper backing (such as CDV's).
Dags of soldiers taken during the Civil War are quite rare. Almost all we find are images made before the conflict. Due to the lack of iron that was available, most Confederate hard images were ambrotypes.