A few simple observations can help narrow the timeframe of an image.Mats give us some wonderful clues as to a daguerreotype's (and ambrotype's) original date.
Beginning in the early 1850s, the mats became smooth, with some engraving artistry, simple at first but more complex by the late 50s.
Then around 1859, the mat material became much thinner and complex designs were stamped into the metal.
The preserver is a brass frame that wraps the glass, mat, and image.
Early daguerreotypes were protected by only a mat and glass, and then placed in a case.
General Rule: The thinner mats and preservers date to late 1850s.
The preserver was first used around 1847, and it became a standard part of cased images.The daguerreotype is very easily detected, even by someone who has never seen one before.It exhibits the characteristics of a mirror at many angles.They are frequently broke or chipped as cases were brittle.Note: These were used with daguerreotypes, ambrotypes, and tintypes General Rule: Thermoplastic or Union case dates after 1856.The best way to understand is to view the wide variety of images in the gallery.