The white slip I use on the clay slabs while they are wet is Kurt's Shino Slip.  This slip is fairly high in sodium, giving the surface a buttery texture when fired at ^6.  It also has a body to it, allowing you to create thick textured areas that don't melt out inthe firing.  In oxidation the slip stays beautiful white - though it is a bit transparent.  In the reduction, the slip responds to the surplus of carbon much like a shino glaze with an orange blush.

Kurt's Shino Slip
Kona F4           9
Spodumene   13
EPK                24
Soda Ash        3
Neph Sy.        38
OM4 Ball      13