John, if you want to marble on clay, try Mason stains as underglaze and a clear glaze to protect them vs. other marbling pigments. Some colors will burn out without glaze but cobalt and other strong colors are less likely to. A blue and white suminagashi-like sink would be quite stunning! Use oxgall or photo flo and distilled water to make the pigments float.
We saw a great demonstration of marbling tiles at the International Marbler's Gathering at Arrowmont several years ago--who remembers the marbler's name? I believe he was from Massachusetts, US.
The Plains, VA 20198