The hardest glass may be etched and frosted with a peculiar liquid
acid, and also with this acid in the condition of vapor. When powdered
fluor spar is heated with concentrated sulphuric acid in a platinum or
a lead retort, and connected with a refrigerator by a tube of lead, a
very volatile, colorless liquid is obtained, which emits copious white
and suffocating fumes. This is hydrofluoric acid, a dilute solution of
which attacks glass with avidity, while neither sulphuric, nitric, nor
muriatic acid has the least effect upon it. In a diluted state it is
employed for glass etching, for which purpose it is kept in a lead
vessel, because it has very little affinity for this metal. The vapor
of this acid is also used for the same purpose. The glass to be
operated upon is first coated with a ground of wax, and the design to
be etched is then traced through the wax with a sharp instrument. In a
shallow lead basin some powdered fluor spar is then placed, and a
sufficient quantity of sulphuric acid poured upon it to convert it into
a thin paste. The glass to be etched is now placed in the basin, to
which a gentle heat is applied, when the vapor of the acid is
disengaged and attacks the traced lines from which the wax has been
removed. The operation is completed in a few minutes, the glass is
removed, and the wax cleaned off with warm oil of turpentine. All those
parts which have remained covered with the wax are now clear as before,
while the other parts drawn by lines to represent figures have a
frosted appearance. Any person can produce figures on glass with this
acid, but, for reasons before stated, it is dangerous to use.
In October, 1859, a patent was granted to James Napier, of Glasgow,
Scotland, for a very simple method of ornamenting glass with fluoric
acid. Instead of drawing patterns and figures on the glass with the use
of varnish and a graver to prepare the glass for etching, the glass is
prepared by simply transferring pictures from prints, which can be
performed by almost any person. The method is, to take a print,
lithograph, or picture made with printer's ink, and fix the printed
surface to the glass by any ordinary paste made from starch. All the
air must be carefully excluded from between the print and glass. When
perfectly dry, liquid hydrofluoric acid about the specific gravity of
1.14 is applied for about three minutes, when it is washed in water to
remove the paper and the acid, and the figure of the print is then
found upon the glass. The printed portion of the paper may also be cut
in outline and pasted on the glass, then transferred. Glass that is
"flashed" on the surface with another color may be treated in this
manner, when a portion of the flashing or surface will be removed, and
the picture will remain in color.