These are cone 03 porcelains with G2931K clear glaze base plus Mason 6021 encapsulated stain (which is very expensive). The one on the left has 15% stain added. The one on the right is slightly less dense red but has only 10% stain plus 2% zircon (zircopax). Notice the zircon has smoothed the surface also, this is a known benefit of using it with encapsulated stains. The 2% addition is not enough to opacify, the color remains deep and translucent. But the surface is still not completely smooth and glassy as is the base transparent glaze on its own. This can be improved using a drop-and-hold firing schedule.
This is a type of stain manufacture that enables the use of metal oxides (like cadmium) under temperature conditions in which they would normally fail.
Cone 04 Drop-and-Hold