These are porcelains are made using Grolleg kaolin. P700 (Plainsman) has 19% G200 Feldspar and Coleman (popular among potters in the US) has 31% (P700 switches the 12% to kaolin). Although the Coleman porcelain is more vitreous (top right) it is not more translucent. But it is a lot more problematic in fired warping and plucking (lower left). Grolleg kaolin has a high natural flux content so less feldspar is needed than in recipes calling for American kaolins. And that extra kaolin in P700 gives us something: More plasticity. Another thing: These are not as translucent as what can be achieved at middle temperature oxidation.
Polar Ice is made by Plainsman Clays, it is by far the most expensive body they make (because of the use of New Zealand kaolin and VeeGum). I call these my "sunshine mugs". They are fired at cone 6 (2200F) with a transparent glaze on the inside (G2926B) and G2934Y yellow silky matte on the outside. This yellow glaze showcases the translucency in sunlight better than any other I have seen. The high plasticity and this "pie crust" method of making them enables thinner walls than any other method I know of, even casting. Although the walls of this piece are about 3.5 mm thick, I have achieved 2 mm using stiffer clay! Even with very thin walls the weight of the handle does not pull the lips of these into an oval-shape.