They changed significantly about three different times, with lesser evolutions along the way. The earliest ones had a 'pebble-like' texture, and were usually octagon or oval up to about 1845.
About this time the finish was a little nicer, having a 'sandy' texture. These included the nonpareil, acorn, elliptical, double elliptical, and some other variations.
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.
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Use the scaling, straightening and cropping tools to make sure your fabric looks as accurate as possible. Download fabric images from a website, then import into EQ8. EQ8 exports high-resolution images perfect for print publications.
A few simple observations can help narrow the timeframe of an image.
The preserver was prone to breaking at the corners.
The daguerreotype is very easily detected, even by someone who has never seen one before.