Actually, good raku clay should be a high fire clay, cone 6 upward. It stays open becasue it does not vitrify. If you fired to cone 8, your clay is probabluy a cone 10 clay and, therefore, close to vitrification at cone 8. You may still be able to get some light carbon markings and reduction that could be very interesting and subtle. It would be interesting to try splashes of slip resist and heavily reduce in sawdust to see what happens. I am imagining lovely cloudy greys. If you do dercide to give it a try, let us know and post pictures. Good luck.