Charles Spurgeon Commentary Numbers 19:5-6

"And one shall burn the heifer in his sight; her skin, and her flesh, and her blood, with her dung, shall he burn: and the priest shall take cedar-wood, and hyssop, and scarlet, and cast it into the midst of the burning of the heifer." — Numbers 19:5-6 (ASV)

All was to be burnt, and then the ashes, the essence and product of it, were to be preserved to make the water of purification needed to remove those constant defilements which fell upon the people of the camp.

So, the merits of our Lord Jesus Christ, which are the very essence of him, are perpetually preserved for the removal of our daily pollution. There was also the essence of cedar wood; that is, the emblem of fragrant immortality, for cedar was an unrotting wood. And hyssop, and scarlet. There must be the humble hyssop used, yet there must be some degree of royalty about the sacrifice, as the scarlet colour signified; and all this is mixed with the blood and the flesh and the skin of the creature, to make the ashes of purification.