Use the Potter's Pens by drawing underglaze precisely on green or bisque ware and covering with a clear glaze. Black colour is rated to cone 9 but has been used up to cone 11 with acceptable results.
Make sure the tip is depressed and squeeze the pen while you're drawing, so the underglaze comes out smoothly.
To refill underglaze pens first you need to remove the tip. The big trick being the thread is reversed from normal, so you need to remove in a clockwise direction and maybe will need to apply a little pressure to break the seal. Once open you can refill the pen.