Sun Printing
Frontline units occasionally used a "Blueprinting" process to create extra copies of trench maps. They placed a map drawn on tracing paper over a sheet of light-sensitive paper and allowed the ultraviolet rays of the sun to do the rest. On a clear day a "Sun Print" could be made in less than half an hour, but if the weather was overcast, it could take several days to generate as few as a dozen copies.
The image below was taken from a "Sun Print" in our collection (ID #199ww1map).
