The Red Heifer (Hebrew, parah adumah), was the cow whose ashes were used in the purification rites for one who had been contaminated through having come into contact with a corpse.
As described in the book of Numbers (19:1-22), the cow had to be slaughtered outside the Israelite camp and its blood sprinkled in the direction of the holy of holies in the Tabernacle (in Temple times, the holy of holies in the Temple).
The cow was then burned whole together with cedar wood, a crimson thread, and hyssop. The ashes were mixed in a vessel containing spring water.
The person contaminated was sprinkled on the third and seventh day of his defilement and he was then allowed to enter the sanctuary. This rite was followed in the Temple.
The Talmud states that the red heifer was a rarity since it had to be completely red. But Milgrom has suggested that the word adumah, translated as 'red,' really means 'brown' and the rarity consisted in it having to be completely brown without any white or black streaks or spots.
There is some evidence that ashes of a parah adumah were preserved for centuries after the destruction of the Temple. These ashes are no longer available and since, according to Maimonides, the site of the Temple still enjoys its sanctity and since everyone has come into contact with a corpse or with one who has, Orthodox Jews, nowadays, do not enter the Temple site and a notice appears at the entrance to warn them off.
Defiling the pure
The great paradox of the whole rite is that the priests who performed the purification became themselves defiled. This mystery, that the parah adumah purified the defiled and yet defiled the pure was, for the Rabbis, the supreme example of the unfathomable in connection with some of the divine commands, which the devout were obliged to accept unquestioningly.
Even the wise King Solomon, say the Rabbis, was unable to explain the rite. Rabban Johanan ben Zakkai is reported as saying, when a heathen accused the Jews of practicing sorcery in observing this rite:
'The corpse does not have the power by itself to defile, nor does the mixture of ash and water have the power by itself to cleanse. It is a decree of the Holy One, blessed be He, who declared: I have set it down as a statute, I have issued it as a decree which you are not to question.
Nevertheless, preachers throughout the ages have read a number of ideas into this rite of purification. Rashi quotes Rabbi Moses the Preacher who connected the parah adumah with the golden calf. If Israel had not worshipped the golden calf the people would have been immune from death. When a person suffers corpse contamination, a cow with a gold-like color is the means of purification.
Modern preachers have used the paradox that those engaged in purification become themselves defiled to point out that often those who work for a good cause justify even ignoble means to achieve their purpose and become spiritually defiled in the process.
The section of the Torah dealing with the parah adumah is read on the Sabbath before the month of Nisan as a reminder of Temple times when this portion was read to warn those contaminated to purify themselves in readiness for the offering of the Paschal lamb on the festival of Passover. This Sabbath is called: “the Sabbath of Parah.”