As it is a positive which has acted as matrix,
the uncovered zinc indicates the design, and the ground remains coated
with insoluble bitumen. The plate is then etched with a weak solution
of nitric acid in water, and the lines of the design are thus slightly
engraved; the surface is then re-coated with another layer of bitumen,
which fills up all the hollows, and is then rubbed down with charcoal.
All the surface is thus cleaned off, and the only bitumen which remains
is that in the lines, which, though not deep, are sufficiently so to
protect the substance from the rubbing of the charcoal. When this
is done we have an engraved plate which can be printed from, like a
lithographic stone; it is gummed and wetted in the usual way, and it
gives prints of much greater delicacy and purity than those taken
directly from the bitumen. The ink is retained by the slight projection
of the surface beyond the line, so that it cannot spread, and a kind of
copper plate engraving is taken by lithographic printing. Besides, in
arriving at this result, there is the advantage of being able to use
directly the original plans and drawings, without being obliged to have
recourse to a plate taken in the camera; the latter is indispensable
for printing in the usual way on bitumen where the impression on the
sensitive film is obtained by means of a negative. It will be seen that
this process is exceedingly ingenious, and not only is its application
very easy, but all its details are essentially practical.
Pages:
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62