This is done in some sort of pug mill.
"Scientific American Supplement, No. 601, July 9, 1887" by Various
The machines used in the preparation of such clays are grinding-mills and pug-mills.
"Encyclopaedia Britannica, 11th Edition, Volume 4, Part 3" by Various
There are two kilns, six wheels, one slab roller, one extruder and one pug mill.
A grant helped the group purchase a pug mill, used to recycle any unused clay, making the operation cost-efficient.