this is a biface that I skint out of a piece of gravel. it was extraordinarily tough so I put it in a turkey roaster close to the edge and cranked up the heat to max for twenty four hours. If that helped it any I couldn't tell it. It was still tough, so I elected to use the ol 3/8" copper rod held under the knee indirect.
Heres the biface then the rod shown with the biface with edges beveled and ground.
the last pic shows a stack getting started which I felt like was due to not enough grinding