BURNISH: A polish given to the surface of an artifact, either to improve its appearance and make it more valuable or to compact it (as with clay) to make it less porous. A pot is polished, often using a spatula of wood or bone, while it is still in a leathery “green” state, i.e., before firing. After firing the surface is extremely shiny. Often the whole outer surface of the pot is thus decorated, but in certain ceramic traditions there is “pattern burnishing” where the outside and, in the case of open bowls, the inside are decorated with burnished patterns in which some areas are left matte. In stroke burnish, the surface is completely polished, but the marks of the burnisher, a pebble or bone slip, remain distinct.
On bronze it was done to improve the appearance; even mirrors could be produced in this way. A burnisher is a metal instrument used by engravers to soften lines or efface them. [burnishing]