ASH - purified white ash made from rice is the the ideal support material to warm or burn incense in.
White ash especially beneficial when you don't want to lose precious incense material. Fire needs oxygen, and unlike sand or a flat dish, oxygen has the ability to circulate in ash. Incense you burn in ash will burn thoroughly leaving no unburned bit's.
Ash is white when new, earning character and a grayish tone the more it's used.