A technical dictionary of printmaking, André Béguin.

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Burnishing
An operation which consists in polishing the surface of a metal plate so as to make it smooth and shiny. Such work is done with a burnisher, a tool made of polished steel. The tool end is quite strong and has a swelling as well as rounded shape so that it can be used to level the irregularities of the metal plate, close up the incorrect lines, rub out the scratches, and remove an undesired grain. In the past artisans who planed copper plates charged extra for the burnishing of a plate [* planing].
In the past the burnisher was much used in the last phase of polishing a plate after pumicing* and using charcoal*. The plate was then rubbed for a long time transversally and always in the same direction with very line sand mixed first with chalk and later mixed with red haematite. Often the work was done using two hands so as to have more leverage.
At present the burnisher is above all used for repairing accidents, for smoothing the plate after scraping with a scraper*, to obtain the whites in mezzotint* work (see also black), after scraping, or just to lighten certain parts of the plate. Of course the reason for which the burnisher is no longer used in polishing* (see also abrasives) is that there are more efficient ways of doing it today.
Burnishers are also used to rub the back of sheets of paper when relief engravings are manually printed [* wood, frotton, relief].
Various types of burnishers exist, some of which are flatter while others are move curved. They are cleaned and polished with leather and tin powder. A burnisher should never be used on a dry plate. One may use a drop of oil (olive oil according to Bosse) or if nothing better is available a bit of saliva will do. The burnishing of metal that is to be engraved should not be confused with the burnishing of a decorative metal. The latter used to be done with an agate or red haematite burnisher and lustering agents. This second kind of burnishing leaves the metal slightly darker than when it has a mat finish. [* scouring].
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