I used something called plasticine in college. It was clay and plaster and old motor oil all mixed together. It varied in hardness but was pretty hard. Sort of like that bright modeling clay you can find at arts/crafts stores. Maybe harder, even. Sticky and stank. But I would think that it needed to be fairly hard to hold it's shape and fine detail well.
You have to warm it some so it gets pliable. Heat your oven on the lowest setting for 10 minutes and shut it off. Put the clay in there on a tray for 10-15 minutes. Keep an eye on it so it doesn't over heat.
When I was doing more sculpting, I made a "warming box" for my clay. It was a large metal box that fancy cookies came in. I took an old fashioned metal dome photo reflector with a lower wattage bulb and placed it over the tin box with the clay inside. It kept the clay at the perfect "sculptabilty" consistency. The clay would firm-up on the armature which gave it better density for detail work. If I needed to radically change a large area, rewarming with a heat gun or even a hair dryer would get the job done. You can buy very soft clays but they don't hold details well.