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.
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.
A highly sought after property in porcelain, they are fired close enough to melting to pass considerable light. It can be very difficult to fire translucent ware without it warping.