upon the metal, the negative must be made accordingly. After
having dissolved the unaffected coating the plate may he subjected
to the action of acids, etching the plate and leaving the picture in
high relief. The metals commonly used for photo-engraving are
copper and zinc; zinc being used for line work and copper for half
tones.
Etching.—ln making engravings the metal is first cleaned and
then carefully polished. When perfectly smooth it is coated with
the sensitive solution. This is merely a solution of albumen—
the white of eggs—water and bichromate of ammonium. After
coating the plate it is dried and exposed to light under the pre
pared negative. When the plate has been sufficiently printed it is
removed, given a thin uniform coating of transfer ink and then
washed in water with a tuft of cotton. This removes all the
soluble matter leaving the picture, on the clean zinc surface, in
black lines. To further increase the resistivity of the plate to
acids it is dusted with dragon's blood, a reddish-brown resin, and
then carefully brushed, with a soft brush, free from all surplus
powder. This adheres to the ink alone. The plate is next heated
until the powder is incorporated with the ink, which is indicated
by turning a glossy black. The plate is now ready for etching,
but, before placing in the acid bath, the back is coated with as
phaltum varnish to protect it. To etch or " bite " the plates
dilute nitric acid is used, it being allowed to act from three to five
minutes, leaving the picture in slight relief. It is now taken out,
washed, dried, repowdered, reburned, and again returned to the
acid for its second bite. In all about four bites are given, each
succeeding one deeper than the previous bite, which finishes the
operation. The plate is taken out, scrubbed with lye to remove
the ink and dragon's blood and then turned over to be mounted
type-high for the printer's use.
ror the production of . half tone engravings the modus ofierandi
is much the same. A copper plate is coated with the sensitized
Albumen and exposed under the negative. The etching is done
by a strong solution of perchloride of iron, which is allowed to
Act continuously until the etching is deep enough. In this pro
cess inking and powdering are not resorted to, but the compound
on the plate is heated until it is dark brown or black; this being
sufficient to resist action. If zinc be used in place of copper the
etching may be made in a manner exactly like that described for
line re oroductions.