If the rest of the surface were sufficiently
resisting, the plate might be bitten at once; but light alone is not
enough to produce complete impermeability: the action of heat must be
combined with it. The plate is, therefore, placed on a grating, with
wide openings, a large flame is applied underneath, and it is heated
till the borders where the copper is bare show iridescent colors. The
sugary coating thus becomes very hard in the exposed parts, but under
the powder it is broken, porous, and permeable to acids. The surface
is then covered with the biting fluid, which is a solution of
perchloride of iron at 45 deg. Baume, and after few minutes' contact the
plate is engraved. It only remains to clear off the bichromated sugary
coating which forms the reserve, and which, being hardened by the
heat, resists ordinary washing. It is removed perfectly by rubbing the
surface with a hard brush and warm potash lye; the plate is then ready
for printing. Sometimes it may be necessary to give several successive
bitings, or to use a resinous grain; in such cases the various methods
of the engraver's art are employed.
B.--_Photo Engraving for Half-Tones._--To reproduce by engraving the
image of any object, a portrait, or a landscape, the gradation of tint
is obtained by repeating three times in the following manner the
operation A, just described:
The copper plate being prepared as before, it is exposed to the light
under a positive, and given a long exposure, say four minutes, in the
electric light.
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