Underglaze Pens come in eighteen colours. You use the Potter’s Pens by drawing on bisque ware and covering with a clear glaze. All colours are stable to Cone 9, except Rose (cone 4). Pens are refillable using 59ml. refills. Pens can only be refilled with specially formulated and manufactured refills. Refills make an excellent brush-on underglaze as well.