3,831. A Stone That Was Touched by Iron

Hilchos Beis HaBechirah 1:15

Stones from the Temple or the Azarah that were damaged or cracked were invalidated. Such stones cannot be redeemed for secular use; they must be hidden away. Any stone that was touched by iron may not be used for the altar or its ramp, even if the stone is undamaged. This is derived from Exodus 20:22: “if you raise your sword upon it, you will invalidate it.” If someone builds the altar or ramp using a stone that was touched by iron, he violates a Torah prohibition and is punished with lashes, as that same verse says, “Do not build them with hewn stone.” If someone builds the altar or ramp using a damaged stone, he violates a Torah obligation (rather than a prohibition).

Hilchos Beis HaBechirah 1:16

If a stone became damaged or was touched by iron after it was built into the altar or the ramp, only that stone is rendered unfit; the others remain fit for use. The altar was whitewashed twice a year, before the holidays of Pesach and Succos. When they whitewashed it, they used a cloth rather than a trowel so that iron not touch a stone and render it unfit.