Scratch Boards Great Way to Reverse Your Thinking
Working from White to Black
What are they?
Scratch boards have been in use since the 19th century and were often used to make pieces of art that were easy to re-produce and could resemble woodcuts. The base is made of specially prepared clay which is white in color. The clay is near the thickness of cardboard and is covered with black ink.
How do I use it?
Instead of adding more material on top of the board, which is what we do on canvas or paper when painting and drawing, the black ink is scraped off to show the white clay beneath. The broader the strokes when being scraped off, the lighter that area of your piece will seem. Likewise, thinner strokes relay that there is less light, and are useful when trying to add values other than black or white. Just about anything may be used to scrape off ink that isn’t wanted: needles, exacto blades, wire brushes, nails, etc. Specific tools are available for purchase as well, one of which that is useful has a rounded arrowhead shape. The tip is very narrow and can make quite thin lines. Modify the angle of the tool to use the edge until the curved ends of the arrowhead, and thick, bold lines may be used for the lightest areas of highlights. Any amount of tipping or angling the tool may lead to varying thicknesses of line, which you won’t have to achieve by continually switching tools otherwise. If the need to “erase” ever arises, simply paint over the unwanted mistakes with India ink and let dry before trying to work in that area again.
Why is it helpful?
Using a scratch board helps us to better recognize light: the different shapes that it contains, and the varying intensity of light etc. When using paper or canvas, focus tends to be on the shadows. The scratch board helps us to think differently and ideally we will be able to recognize both the shapes and values of light in addition to those of shadow, which help to give our art a more convincing quality.
http://www.britannica.com/EBchecked/topic/529824/scratchboard for some historical insight