I was out in Fredricksburg the other day and ran across a stone flake in a stream bed. The flake had little divots in it and circular cracks. These flakes are commonplace for anyone that has excavated archaeological sites where fire and stone were used in the same area. These little crater attributes are called "pot lids" due to the basin and lid shape of the stone bits that fly off during thermal shock. Prehistoric groups often made hard stone more workable by heat treating tool stone in the sand underneath a bed of coals. This process changes the characteristics of the stone, making it a little more brittle but much easier to work. I wanted to recreate the little pot lid flakes that are so common at areas where ancient stone tool users built fires on top of old discarded flakes. So, I did. It was a quick little project to check out how color changed and how pot lids form.
It really doesn't take all that much heat to affect the chert, a few coals will do the trick. Most folks that heat treat stone these days take the temperature up to about 400 F. One piece of the Edwards chert (gray stuff from Austin) did well, turned waxy and had a slight color change to pinkish cream color. One of the Edwards chert pieces exploded (yay!) and created the dimpled stuff we find all the time. The other material is Alibates chert from West TX. This material had the most significant color change and turned lustrous and easier to flake, although it was markedly brittle.
You can see the color change in the Alibates to a rosy color instead of the original brownish cream color. The heating managed to create those little circular cracks that we were hoping to see. The Edwards chert seems to be a good candidate for a larger scale attempt in the future. You can see that it dried the piece out and will offer flake production with less effort (the bit that didn't explode)
Well fun fun! Not heat is needed to make flakes easier to work. The
key would be to use a controlled increase in temperature and uniform
distribution of heat. Maybe I'll try the process for real on some
I'll use one of these flakes for the new project- making arrows for an upcoming hunt.