MOSAIC AND PIETRA DURA. 431
authority of the government, for the purpose of protecting the cannon
and balls, exposed to the weather in the Brooklyn Navy Yard, against
rust; for this purpose, when treated with the various coloring
pigments,—such as oxide of manganese, umber, terra di sienna, ochre,
Venetian red, ultramarine, &c,—it is admirably adapted.