Cracks sometimes occur when you are bending a doll to pose it while it is still raw clay or when you are joining raw clay to some partially set clay. Cracks also occur when raw clay has been left exposed to the air. Sometimes the clay is too soft to fix the crack when it happens. It is best to set the doll aside and let the clay firm up before attempting to fix it.
Don’t attempt to just smooth over a crack. Use a sharp tool to carve
out an open area. Then, with a wood
tool, press new raw clay into the open area to fill in any pockets. Finish the
area by smoothing with your thumb or a filbert brush.
A tiny crack on cured clay is difficult to fix because you
can’t carve out a hole. Try using a liquid polymer clay product on the doll and
re-baking it (follow manufacturer directions). You will have to coat the entire
doll to have a consistent color. You can also make a mixture of regular clay and liquid clay. Use the mixture to fill a crack in a cured clay doll but only do that on an area that is going to be completely covered.
The issue of cracking clay is discussed on this blog... be sure to read it: http://madsculptor.blogspot.com/2011/04/head-or-other-partscracking-issues.html