A quantity of plates may be prepared in this way, and all the adhering oil, etc., may be removed by a clean hand, or lathe buff, after which each plate must be heated to the point necessary to burn off the remaining oil great care being required not to overheat the plate.

A very slight excess of temperature will at once destroy all the polish previously obtained.

The test for ascertaining the right temperature is at hand; the adhering oil will be driven from the plate in the form of smoke when the right temperature is reached.

The moment the smoke ceases to rise from the plate, the heat must be removed, and the plate quickly cooled upon a piece of iron.
A quantity of plates thus prepared may be kept on hand for any required time, and the labor of one minute, with a lathe or hand-buff with dry charcoal, or rather, prepared lampblack, will perfectly polish the surface ready for indexing, etc.

This lampblack also requires some care in preparing.