into
a shallow dish, lay a sheet of foil upon a flat board, and with a fairly
stiff brush (a flat hog's-hair as wide as possible) proceed to coat the
sheet of foil with a thin but perfectly even coating of glue. The thickness
of the coating can only be found by trial, for if the coating is too thick
a longer time will be required for printing; but it must not be thin enough
to show interference colours. After the coating has been laid on, a soft
brush, such as photographers use for dusting dry {121} plates, should be
passed up and down, and across and across, with light, even strokes to
remove any unevenness. A glue solution used by professional photo-engravers
is as follows:

Fish-glue 12 oz.
Bichromate of Ammonia 3/4 oz.
Water 18 to 24 oz.
Ammonia .880 30 minims.

The bichromate should be dissolved in the water, and, when added to the
glue, stir very thoroughly in order that complete mixing may take place.
The coating may be done in a good light, not bright sunlight, but _it must
be dried in the dark_, because, although insensitive while in a moist
condition, it becomes sensitive immediately on desiccation. If allowed to
dry in the light the whole coating will become insoluble, and for this
reason the brushes used should be washed out as soon as they are finished
with. The sheets will take about 15 minutes to dry in a perfectly dry room,
but it is not advisable to prepare many sheets at once, as they will not
keep for more than two or three days.

The prepared negative must now be placed in an ordinary printing frame, and
a print taken off upon one of the metal sheets in the same way as a print
is taken off upon ordinary sensitised paper. In daylight the exposure
varies from 5 to 20 minutes, but in artificial light various trials will
have to be made in order to get the best results, the exposure varying with
the amount of bichromate in the coating; the proportion of the bichromate
to the glue should remain about 6 per cent. Light from a 25 ampere arc lamp
for 2 to 5 minutes, at a distance of 18 inches, will generally suffice to
"print" the impression on the metal sheets. The printing finished, the
metal print should be laid upon a sheet of glass and held under a running
stream of water. The washing is complete as soon as the unexposed parts of
the glue coating have been entirely washed away leaving the bare metal, and
this will take anything from 3 to 7 {122} mi