Hey TN. Good info.
I went with crushed limestone on his because that's what the archaeological reports from his area show to be most common during that time period.
Of course that changes by region and local available material.
Rock.. the oldest pottery was fiber tempered here in the U.S. then it progressed into a variety of different materials.. I've seen.. fiber..like Spanish moss and palm fibers to.. crushed oysters,limestone and even crushed pottery.
The sand seems like the best constructed and durable.
Here is a piece of some of the oldest pottery in my state.(fiber tempered)
Pottery in the southeast U.S. seems to have originated around the Savannah river area separate and older than the rest of the continental U.S. that followed a pattern of progression from South America.
The pottery in the Savannah river area dates back to 4,500 BP.
It reached a Site called MT. Elizibeth close to my home around 4,000 BP.