Chukat - Rishon
How Now, Red Cow?
G-d gave Moshe the instructions for purifying someone from corpse contamination. They were to take a completely red cow, without any imperfections, that has never had a yoke on it. The cow would be slaughtered outside the camp under the supervision of a kohein.
The kohein takes some of the blood and sprinkles it in the direction of the Mishkan, seven times. The cow is then burned. The kohein throws cedar, hyssop and red wool into the pyre. After this, the kohein must immerse himself and his clothes in a mikvah. He would remain unclean until nightfall, after which he could return to the camp.
The ashes of the cow were gathered by a ritually clean person and placed in a ritually clean location outside the camp. The gatherer likewise had to immerse himself and his clothes, remaining impure until nightfall.
Now, as for what was done with the ashes, they were used for the water that would purify someone who had become unclean through contact with a corpse. Such a person would be unclean for seven days, during which time he would be twice sprinkled with this water, on the third and seventh days. This is the only way to remove corpse impurity. If a person enters the Temple or Tabernacle while impure from corpse contamination, the penalty is kareis.
If a person dies in a tent (or in a building or under a canopy, etc. – under any roof), everything in the tent becomes unclean for seven days. Any open vessel whose lid doesn't have a tight seal is unclean. Similarly, anyone who touches a corpse outside will become unclean.
The ashes of the cow are ground to powder and put in a vessel filled with water from a running source.