Agate Burnishers
Burnishing is an integral part of the water gilding process and burnishing, or polishing, the leaf using a burnisher is what gives a water gilt finish it's distinctive mirror shine.
Unfortunately, only water gilded surfaces can be burnished in this way, metal leaf that has been laid on top of a surface prepared with gold size doesn't have the flexibility and grip to take a burnish. Attempting to do so will damage your leaf and your finish.