Florida cherts were heated slowly to 350-400°C and maintained at this temperature for long periods, producing a desirable change in the fracture properties -- a structural, not a mineralogical, change. Some testing was also done on obsidian, English flint, Arkansas novaculite and pure quartz. Techniques used were: differential thermal analysis; petrographic thin section analysis; scanning electron microscopy; x-ray diffraction; tests of rock mechanics; atomic absorption spectrophotometric analysis; gas absorption procedure. The application of heat to flint materials may have conferred an advantage to primitive man in the manufacture of chipped. stone implements. Photographs. -- AATA