Agate Burnishing Tool No21
Agate burnishers create the bright mirror finish when used to burnish or polish the dry gilded surface.
To prevent scratching the gold leaf the surface must be clean and the burnisher should be warm to the touch.
Experts tap the gesso with the burnisher and listen to the sound to tell whether it is ready. There should be a defined "Click", not a soft tap. It is important to burnish at the right moment when the leaf is dry but the gesso still has a little give in it.
Note that if there is any dampness in the gesso the agate will rip the surface instead of polishing it.
Re-burnishing after a few days will usually heighten the gloss.
Burnishers can also be used to produce a deep gloss surface on paper, or paint.