Can I use white as a glaze?

Yes. This will be an opaque glaze. An opaque glaze tends to be less transparent than a transparent glaze. Both are thinned down with water or glazing medium, but the opaque glaze is made with a paint or color that is opaque. The opaque glazes tend to be lighter and have some white in the composition of the paint, which is what makes them slightly opaque. If an opaque glaze is used, value contrast tends to lessen underneath the glaze and move towards the value of the opaque glaze. For example, a white, opaque glaze over black will slightly lighten the black color.