40. Mightier Than the Sword: The prohibition against building the altar using metal tools
When you build Me an altar, do not make it from cut stones. Raising your sword on (the altar) will desecrate it (Exodus 20:22)
When building an altar of stones for sacrificing to God, one may not use metal tools. If such tools were used in its construction, the altar is disqualified from use. The reason is that the altar is an instrument of peace and iron tools are implements of war. Things used for bloodshed are unfit for building the altar, which people use to get closer to God. Yes, war is sometimes necessary; both swords and plowshares have their place. Building God’s altar just isn’t one of them.
This principle was carried through in other ways, as well. For example, King David was not permitted to build the Temple because he had spilled blood in warfare (see I Chronicles chapter 22). And when his son Solomon built the Temple, the stones had to be shaped at the quarry because no iron implements were permitted to be used at the Temple site (see I Kings chapter 5). According to Middos 3:4, the altar was whitewashed twice a year and even the paintbrushes used could not contain any metal so as not to violate the prohibition of raising iron tools on the altar.
This mitzvah applied to both men and women while the Temple stood. It is discussed in the third chapter of the tractate Middos (which is Mishna only, no gemara). It is codified in the Mishneh Torah in Hilchos Beis HaBechira. It is #79 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos.