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The body is Plainsman M390. These are commonly-used base glazes. The top one is an MgO matte, next down is a calcium matte. They react very differently to these additions. Notice also the difference when titanium dioxide is applied thickly. Tin oxide fires whiter than zircon (e.g. Zircopax). Each opacifier has issues. Tin is expensive. Titanium is difficult to mix into the slurry (screening required), not as white or opaque, variations in thickness produce more difference in results and it can turn blue. Zircon is more likely to cutlery mark, twice as much is required and it amplifies the color of any iron present.
Glaze opacity refers to the degree to which it is opaque. There is more than meets to eye to the subject of opacity control.
G1214Z - Cone 6 Silky Matte
This glaze was born as a demonstration of how to use chemistry to convert a glossy cone 6 glaze into a matte.