Instructables
Picture of Sgraffito on Greenware
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Once project is leather hard, apply 3 coats of underglaze.  Using a small ribbon tool incise (scratch) through the underglaze to reveal clay underneath.  For straight lines use a ruler. :-)  Whatever is scratched through will fire as the clay color (white in this example).  After bisque fire add a clear glaze to make it glossy.