It looks like the first one I tried was pretty damaged compared to the others, as the next couple have scanned OK, even on 'auto'.
They're really interesting things. There are two almost identical exposures on each plate, they only appear to be at slightly different points of focus and possibly different exposures, which makes me think that these were taken by a specialist camera with a view to producing a stereoscopic projection. Fascinating!
Alas, I do not have any other information about them yet - I know they were taken in Brittany around the time of WW1 - judging by the car that is in the first plate and the clothes I would guess after rather than before. Must try to find out more.
Here is one of the plates, straight out of the scanner (resized):