Fired at cone 6. It is impossibly vitreous, the surface is smooth like a glaze. And it has not warped. In fact, other pieces made from it having walls as thin as 2mm did not warp either!. This comes from a two-foot-thick section of the 3B layer from a Plainsman Clays quarry near Ravenscrag Saskatchewan, Canada. A cretaceous dust storm! It is plastic and feels impossibly smooth. Smoother than any commercial porcelain. It does not fire white because mother nature did include a little iron oxide. It accepts glaze like a porcelain.
Standard porcelains used by potters and for the production of sanitary and table ware have surprisingly similar recipes. But their plasticities vary widely.
How to Find and Test Your Own Native Clays
Some of the key tests needed to really understand what a clay is and what it can be used for can be done with inexpensive equipment and simple procedures. These practical tests can give you a better picture than a data sheet full of numbers.