Underglaze Potters Pens feature a unique tip-activated valve that doses underglaze ink onto the bisque. Generally, a line with a thickness of 1mm is achieved. In order to attain a very opaque decoration, draw at a deliberate rate. Govern the flow by gripping the pen body more or less firmly. Shake well before using and applying to bisque ware. There is also a wax resist pen which fires away and resists the glaze. Potter's Pens can be fired to Cone 9 and are AP Non-Toxic.