By MAJOR J. WATERHOUSE, B.SC.
In one of the upper rooms of the Electrical Exhibition in Paris, there
is an interesting collection of plates and proofs produced by various
methods of photo-engraving, invented by M. Henri Garnier, whose name
is so well known in connection with these processes, and whose
beautiful plate of the Chateau of Maintenon gained for him a gold
medal at the Paris International Exhibition of 1867.
Some interesting details of these processes are given in an extract
from a report on them by M. Davanne to the Société d'Encouragement
pour l'Industrie Nationale, read at its sitting on the 22d July last,
of which copies are distributed gratis in the exhibition.
The report opens with a brief allusion to M. Garnier's continuous
labors in permanent photographic printing, commencing with the
ingenious mercury process worked out in conjunction with M. Salmon,
and published in 1855, in which a print which has been exposed to the
fumes of iodine is laid down on a plate of polished brass, so that the
iodine, absorbed by the printed lines, slightly attacks the brass;
mercury being then rubbed over the brass, forms an amalgam with the
iodized parts. If a roller charged with printing ink be now passed
over the plate, the ink will only be taken on the pure brass, and not
on the iodized parts. 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.
The report then proceeds to give the following details of a process of
photo-engraving, which was exhibited before the society by M. Garnier
in March last: