
Figure 17.—SIDEROGRAPHY SPECIMEN PLATE. 1819. Reduced 
from 8x 6 inches. (Original in Constance Meade Collection, 
Oxford University Press.) 
by the old case-hardening method. Next a small 
cylinder of mild steel was rolled and pressed re- 
peatedly over the engraving, by means of a special 
machine, forcing the softer metal into the engraving 
until the image was transferred in raised lines to the 
cylinder surface. The cylinder was then hardened 
and used in its turn to impress the image into any 
number of mild steel plates, this time once again in 
sunken lines. These plates were then hardened for 
printing. Thus a single engraving, however complex, 
could be copied exactly on to any number of plates, 
PAPER 71: SIR WILLIAM CONGREVE AND HIS COMPOUND-PLATE PRINTING 
or the engraving could be repeated several times on 
the same plate. The blacks and whites of an image 
could be reversed by a change in the order of trans- 
ferring: if an extra cylinder was introduced, or if 
the initial engraving was made on a cylinder rather 
than on the flat plate then the final printing plate 
would hold the image in raised rather than sunken 
lines, and the printed result would be a white line 
design on a black ground. 
The siderography process had enormous advantages 
for bank-note printing. Since the plates were copied 
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