Water of separation

The water of separation (Hebrew mey niddah, "water of impurity" or "water for the removal of sin") was a specially prepared mixture of spring water and the ashes of a red heifer, used to cleanse persons and objects defiled by contact with a human corpse — the most severe form of ritual impurity under the Mosaic law (Numbers 19). A perfect red heifer without defect, never yoked, was slaughtered and burned entirely, along with cedar wood, hyssop, and scarlet thread; the ashes were preserved and mixed with running water as needed. A clean person sprinkled the mixture on the defiled person with a hyssop branch on the third and seventh days of impurity, after which the defiled person washed and was clean by evening. The paradox of the rite — that the person who prepared it became unclean while the unclean person became clean — was noted by ancient rabbis and Christian commentators alike. The author of Hebrews contrasts the limited efficacy of this purification with the blood of Christ, which "purge[s] your conscience from dead works to serve the living God" (Hebrews 9:13–14).