Has this ever happened to you? You make an armature and add and bake an undersculpt. Then you add some top clay and begin to sculpt your doll but get sidetracked and your sculpt hangs around in your studio for awhile. When you get back to it, the top clay cracks and is crumbly and difficult, if not impossible, to continue sculpting. What happened? The oil in the top clay gets 'absorbed' into the baked undersculpt clay. So here is the guideline recommended by several doll makers; once the undersculpt clay is on the wire armature and is baked, it can wait for a very long time for you to finish it; but once you add the top clay over an undersculpt you must finish sculpting and bake your doll within a week.