Sgraffito

Definition: Scratching into top layers to reveal the lower ones using anything from the end of a paint brush to a needle.

Used in the middle ages in the illumination of manuscripts when the ground was often gold leaf. Also used in pottery and wall paintings

The lower layer is left to dry before a second is applied and then scratched into to reveal the lower layer.

This is a wonderful image of a house decorated with sgraffito:

ref:  http://en.wikipedia.org/wiki/Sgraffito#/media/File:Pyrgi_house1.JPG