The clay is a buff stoneware. The upper two samples are G2934, an MgO matte. The one on the right has 10% zircon added to opacify.
The bottom two are G1214Z (transparent version and opacified-with-zircon version).
Glaze opacity refers to the degree to which it is opaque. There is more than meets to eye to the subject of opacity control.
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