Tractate Middot - Chapter 3 - Mishnah 4 מידות פרק ג׳ משנה ד׳

The stones both of the ascent and of the altar were taken from the valley of Bet Kerem. They dug into virgin soil and brought from there whole stones on which no iron had been lifted, since iron disqualifies by mere touch, though a flaw made by anything could disqualify. If one of them received a flaw, it was disqualified, but the rest were not. They were whitewashed twice a year, once at Pesah and once at Hag, and the Sanctuary was whitewashed once a year, at Pesah. Rabbi says: they were whitewashed every Friday with a cloth on account of the blood stains. The plaster was not laid on with an iron trowel, for fear that it might touch and disqualify. Since iron was created to shorten man's days and the altar was created to prolong man's days, and it is not right therefore that that which shortens should be lifted against that which prolongs.