Mata Ortiz burnishing technique: Add finely ground graphite and baby oil to kerosene and apply that, using your hands, over black manganese slip. Then take a highly polished agate and use it to burnish the surface to a shiny black.
Which one of these common centering problems showed on this video have you had? Getting the clay centered, ramp on the edge, mushroom, volcano, or elephants butt. Great Video ~ How to solve some common centering problems.