משנה על סוכה 1:2

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.

[An object] one handbreadth square and one handbreadth high conveys uncleanness and blocks uncleanness. How does it [block uncleanness]? In the case of a covered drain beneath a house, if it has a space a handbreadth wide and its outlet was a handbreadth wide, and there is uncleanness inside it, the house remains clean; And when there is uncleanness in the house, that which is within [the drain] remains clean, for the manner of the uncleanness is to go out and not to go in. If it had a space one handbreadth wide but its outlet was not one handbreadth wide, when there is uncleanness in it, the house becomes unclean; But when there is uncleanness in the house, that which is within it remains clean, for the manner of the uncleanness is to go out and not to go in. If it did not have a space one handbreadth wide and its outlet was not one handbreadth wide, when there is uncleanness within it, the house becomes unclean; And when there is uncleanness in the house, it [also] becomes unclean. It makes no difference if the cavity was carved out by water or by a sheretz or if it had been eaten out by salt. And similarly [if it is in] a row of stones or a pile of beams. Rabbi Judah says: any "tent" not made by a person is not considered a tent’. But he agrees that crevices and crags [can be considered as ‘tents’].