I live in central Missouri, USA. And have dug and used a local clay that has been used commercially for over a hundred years for firebrick at the A. P. Green Co. in Mexico, Missouri and is currently used to manufacture kitty litter at a facility east of Columbia, Missouri. The natural clay is loaded with a lot of organic material, like plant roots. When fired it produces an almost snow white color too a very light pinkish off white. The surface is open and course when fired. I am planning a pit firing this coming Fall, 2019, with a blend of quartz sand in the clay. The vessels will have a bisque firing before their sawdust and charcoal pit firing. when brushed with a large wet house painting brush the quartz sand produces a linear patterned texture. This batch will be colored blue with lye water from ashes and the browns from pit firing. Like always we’ll see how it goes.