I went over Ute Pass Colorado last summer. The east side of the mountain is granite but the west side is sedimentary rock. There was one layer of black heavily organic shale or mudstone. I collected about 5 gallons of the stuff, brought it home, and ran it through the ball mill. The first picture is of the raw material. When water was added to the milled material it formed a clay. It had poor plasticity. I made a 10 cm bar of the black clay and bisque fired it. It's color was bright orange Terra cotta. I then fired it to cone 6 anticipating that it would melt before cone 6. But as seen in the second picture it did not melt. It vitrified and the surface is a bit shiny. The color is a milk chocolate with a hint of orange. The bar before drying was 10 cm long and after cone 6 firing was 9.7 cm. There was very little shrinkage ( about 3%).
Does anyone have an idea what this stuff is? I looked up a Geological survey of the area which said it was sedimentary deposits from the tertiary age. There is a Molybdenum mine just up the ridge from this deposit.
Any ideas how to make this a workable clay. As I said it is not very plastic. What would I need to add?