Clae have produced a porcelain slip for use on raw clay.  It can be used as a white base layer to underglaze or glaze over, so that regardless of the colour of your clay you will get vibrant results.   Because porcelain is clay you will get excellent coverage, creating a matte white colour finish. 
 
The Porcelain Slip comes in 700ml containers.
 
Clae slip are tested for 1220- 1240 deg C and are suitable for all mid-fire clays and firings.