The plate is next bitten with acid nitrate of
silver, and may then be treated in various ways, so as to form either
a printing-block or an engraved plate. The process never came to any
practical use, but led M. Garnier to the invention of the very
valuable and largely used process of acierage or steel-facing, by
which the surface of engraved copper-plates is so hardened and
protected by a thin coating of iron that instead of only a few hundred
impressions, many thousands can be printed from a plate without the
slightest deterioration.
The next invention noticed is the citrate of iron process of M.M.
Salmon and Garnier, in which a paper, coated with a sirupy solution of
citrate of iron, is exposed to light under a positive print for a
period varying from eight to ten minutes in the sun, to half or
three-quarters of an hour in the shade. In the parts where the light
has acted the paper becomes non-hygroscopic in proportion to the
intensity of the action of the light upon it. The paper being left for
a short time to absorb moisture from the air, is dusted over with
lamp-black, which, attaching itself to the unexposed parts, reproduces
an exact image of the original drawing.
M. Garnier has since greatly modified this method of obtaining an
image by dusting, and applied it to various processes of
photo-engraving.
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