Patina

Patina (probably from the Latin word for a flat dish, from patere, to lie open; cf. "paten"), a thin coating or incrustation which forms on the surface of bronze after exposure to the air or burial in the ground. It is looked on as a great addition to the beauty of the bronze, especially when it is of the green colour found on antique bronzes (see Bronze). By extension, the word is applied to the discoloured or incrusted surface of marble, flint, &c., acquired after long burial in the ground or exposure to the air, and also to the special colour given to wood surfaces by time.