The priest shall be unclean until the even, i.e; the priest who superintended the sacrifice, and dipped his finger in the blood. Every one of these details was devised in order to express the intensely infectious character of death in its moral aspect.
The very ashes, which were so widely potent for cleansing (Numbers 19:10), and the cleansing water itself (Numbers 19:19), made every one that touched them, even for the purifying of another, himself unclean.
At the same time the ashes, while, as it were, so redolent of death that they must be kept outside the camp, were most holy, and were to be laid up by a clean man in a clean place (Numbers 19:9). These contradictions find their true explanation only when we consider them as foreshadowing the mysteries of the atonement.