A good white base glaze is Colemenite White.  Although the glaze is made for an electric kiln, some of the color variations listed require redux.

Colemanite White by N. Nelson

Cone 5-8 Oxidation.  Shiny white.

Frit 14                  25.35

Kingman Feldspar    34.93

Whiting                  5.63

Colemanite             5.07

Ball CLay                3.67

Silica                   25.35 

Color variations:

add:

  Copper Carb 3% = Turq. in ox or purple/red in redux.