I can't find the thread where Lorna suggested I write to Ampersand and see about a white scratchboard which would scratch down to black or sepia. So I'm starting a new one, here is the answer I got from Ampersand. I may try this!
We donít have a surface that scratches from white to black, and given the nature of our production, Iím not sure how we could do that. We also donít produce a sepia scratchboard, but the good news is that youíre one only one step away from having one. Our Claybord Black is made by taking our Claybord Smooth and spraying a layer of black ink on it. If you wanted, you can take the Claybord Smooth and apply your own ink to the surface in whatever color you choose. An airbrush would probably give you the most even coating, but brushing on will work as well, and can potentially allow you to vary the depth of color to compliment your illustration/painting. The inventor of Claybord, Charles Ewing <http://www.charlesewing.com/default.htm>, has done some very fine pieces in this manner. I recommend a water-based permanent India Ink for your ink application. Hope this helps, let me know if you have any other questions!