ered. The plates prepared to-day do not
resist as well as those formerly made; and as the popularity of works of
art multiplied by the press has considerably increased, it became
necessary to look about for means by which the surface of a copper plate
may be hardened, and be made to yield a large edition. This has been
accomplished by

99. =Steel-facing.=--_Steel-facing_, which was invented by Messrs.
Salmon and Garnier, and which M. Jacquin undertook to render
practicable, consists in depositing a coating of veritable steel, by
galvanic action, on the face of the copper plate, or, in other words, by
the superposition of a hard metal on a soft metal.

This mode of protection, which perfectly preserves the most delicate
passages, even down to the almost invisible scratches of the dry point,
not only guarantees the copper against the contact of the hand and the
rag, which would tell on it more than the pressure of the rollers, but
at the same time makes it possible to print a thousand proofs of equal
purity. Certain plates, owing to the manner of wiping used on them, do
not reach this figure; others, more simply printed, may yield three to
four thousand proofs, and sometimes even a still larger number.

As soon as the plate shows the slightest change, or the copper begins to
reappear, the coating of steel is removed by chemical agents, which,
acting differently on the two metals, corrode the one, while they leave
the other untouched. The plate is thus brought back to its original
state, and is therefore in the same condition as before to receive a
second steel-facing. In this way plates may be _de-steeled_ and
_re-steeled_ a great many times, and the proofs printed from them may be
carried up to considerable quantities.

As a rule, the plates are not steel-faced until after the proofs before
lettering have been printed.

Soft-ground etchings, the biting of which is quite shallow, must be
steel-faced after two to three hundred impressions.

The delicacy of the bur thrown up by the dry point hardly permits the
printing of more than twenty or thirty proofs on an average;
steel-facing carries this number up to a point which cannot be fixed
absolutely, but it is certain that the bur takes the steel quite as well
and as solidly as an etched line. Dry points may, therefore, yield long
editions; the steel-facing must in that case be renewed whenever
necessary.[26]

100. =Copper-facing Zink Plates.=--Zink plates cannot be