from The American Heritage® Dictionary of the English Language, 4th Edition
- n. Decoration produced on pottery or ceramic by scratching through a surface of plaster or glazing to reveal a different color underneath.
- n. Ware decorated in this manner.
- adj. Scratched; -- said of decorative painting of a certain style, in which a white overland surface is cut or scratched through, so as to form the design from a dark ground underneath.
- n. A kind of pottery made in England, in which clays of different colors are laid one upon another and the pattern is produced by cutting away the outer layers, as in cameos and cameo-glass.
from the GNU version of the Collaborative International Dictionary of English
from The Century Dictionary and Cyclopedia
The large plate you so greatly admired is called sgraffito or scratched work, sometimes called slip engraving.
Used in a technique called sgraffito, whereby a thin layer of Overlay paste is applied to the glazed porcelain or glass, allowed to dry, and a pointed instrument used to scratch a design into the layer, allowing the porcelain or glass beneath to appear.
Then the master made his sketch in white, or "sgraffito" (i.e. graven on the plaster), as in the architectural lines of the pictures of patron saints in the Uffizi, and the _Marriage of S. Catherine_ in the Pitti Palace; he also put in the shadows in monochrome.
I believe the clay coated board is so that you can incise or abraid through the clay to expose a white surface for fine detail, like scratchboard or sgraffito.
He decorated the exterior of many Roman palaces in sgraffito, a form of painting where, over a dark background, often stucco, a lighter-coloured layer was painted, and designs, scratched through the light layer, only showed dark on light (en camaïeu).
The word 'sgraffito' was borrowed from the Italian, and comes ultimately from the Ancient Greek 'graphein' ("to write or scratch.").