If oven-bake clay feels stiff, the clay should be pre-warmed to aid in conditioning. You can do this by letting the block of clay sit beneath a low-wattage light bulb for 5 minutes or so, and then rolling it briskly between your hands. The clay will become soft and flexible and the initial stiffness will subside. You can also use your hands by simply holding the clay between your hands for several minutes prior to rolling.