http://history1900s.about.com/od/holocaust/a/babiyar_2.htm

Crushing the Bones: The ashes from the pyre were scooped up and brought to another group of prisoners. Since there were usually large pieces of bone that had not burned in the fire, they needed to be crushed to fully destroy the evidence of Nazi atrocities. Jewish tombstones were taken from the nearby cemetery to crush the bones. Prisoners then passed the ashes through a sieve, looking for large bone pieces that needed to be further crushed as well as searching for gold and other valuables.