Clayboard starts off as a  hardwood board that is coated with a thin layer of Kaolin Porcelain clay.  The artist may then use this as his painting support.  The medium used most on clayboard is colored inks but some artists use oils or acrylics that have been diluted with water.
Scratchboard or Clayboard Black has a thin layer of Black India ink on top of the layer of Kaolin Porcelain Clay. The artist scratches off the black ink revealing the white clay. Shades of gray can be achieved by how much ink is removed or by applying an ink wash. The ink wash can be re-scratched to reveal the white again.

http://www.scratchboardsociety.com
http://www.ampersandart.com
http://www.wetcanvas.com

The Process of creating art on scratchboard.

​As in drawing, it is important to know your value scales when working on scratchboard or clayboard.

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1. Transfer you image onto scratchboard.

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2. Start creating

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5. Finished - "Boa" 8 x 10 Scratchboard