With a small brush and ordinary asphaltum or black varnish, paint
the design, the margin and the entire back of the metal. When this
coat has dried put on a second and then a third. The asphaltum is
to keep the acid into which the metal is to be immersed later from
eating any part of the metal but the background. Two coats or more
are needed to withstand the action of the acid.
The acid bath is composed of nitric acid and water, about half and
half, or, possibly, a little less acid than water, the mixture
being made by pouring the acid into the water, not the water into
the acid, which is dangerous. Keep this solution off the hands and
clothes, and do not inhale the fumes.
Put the asphalt-coated metal in the bath and allow it to remain
for four or five hours, depending upon the thickness of the metal
and the strength of the acid. With a stick, or a pair of old
tongs, take the metal out of the acid occasionally and examine it
to see how deep the acid has eaten it--1/32 in. is about right for
the No. 16 gauge.
When etched to the desired depth, remove the piece and with an old
knife' scrape off the asphaltum.
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