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
Ball CLay 3.67
Copper Carb 3% = Turq. in ox or purple/red in redux.