> why did Southworth and Hawes reportedly "double plate" all their dag
> plates with an extra layer of silver?
As I know they used (mainly) 20, 30 and 40 quality-signed plates. The known
range: from 10 to 60 (1/10-th part of silver to 1/60-th part of silver in
the whole mass of the plate).
The dag. technology is/was difficult so the result can be a quite lot
wrong photograph/plate. The photographers cleaned off the (wrong) picture
(mercury amalgam) layer from these plates and reused them.
The thicker silver layer (mainly in the case of galvanized plates)
was able for more times reusing (max 3-4 times).
However an overexposed or/and developed, or a hammered plate (uneven)
is not good for multiple using.
Balint Flesch
PS: The double plate can be a name of a size also (a double size of a
whole plate).