Saw dust or smoke firing adds colour to surface. Areas have been masked out with clay , this stops the carbon from the smoke entering the pores and colouring the surface. The areas under the clay are therefore white which is the colour of the fired clay………which is earthstone original.  The kiln firing is 1131 deg C 

These pictures show the clay mask after the piece has been lifted from the smoke firing. Once the piece is cooler it is possible to ping off the clay mas revealing the white surface underneath.