Truly fortunate that so many of these have survived. Probably a million wet plate photos were made during the civil war on glass plate. Popular during the war, they lost their appeal afterwards and so many were sold for the glass. Many used in green houses. Over the years the sun caused the images to disappear.
These are pretty amazing considering they were taken up to 150 years ago: A compendium of photos from the Civil War era. Run the cursor over the photograph and the picture caption will pop up. Click photo to enlarge.
"Original Civil War Pictures"
http://www.mikelynaugh.com/VirtualCivilWar/New/Originals2/index.html