 They did not apply the whitewash with an iron trowel, out of the concern that the iron trowel would touch the stones and render them unfit, for iron was created to shorten humanity's days, and the altar was created to extend humanity's days, and it is not proper that that which shortens humanity's days be placed on that which extends humanity's days.The Tosefta reported that Rabban Joḥanan ben Zakkai said that singled out iron, of all metals, to be invalid for use in building the altar because one can make a sword from it.The sword is a sign of punishment, and the altar is a sign of atonement.