I have extracted natural clay from the ground near my house in NC. The clay is pretty sandy. I refined and dried it and bisque fired a pinch pot. The result was a bright orange pot which was solid but felt a bit chalky. I figured, what the heck, protected my kiln shelf and fired it to cone 6 with clear glaze. The result was a brownish pot which felt rough textured and very light. I assumed success, but discovered quickly that the pot was fragile and broke easily. Any ideas? Is there something I can add to the clay to make it more plastic and sturdy?