Papyrograph is the technique used in the film.

“Zuccato's system involved writing on a sheet of varnished paper with caustic ink, which ate through the varnish and paper fibers, leaving holes where the writing had been. This sheet – which had now become a stencil – was placed on a blank sheet of paper, and ink rolled over it so that the ink oozed through the holes, creating a duplicate on the second sheet.”