Moses and the elders of Israel charged the people, saying, “Observe all the Instruction that I enjoin upon you this day. As soon as you have crossed the Jordan into the land the L-RD your God is giving you, you shall set up large stones. Coat them with plaster and inscribe upon them all the words of this teaching. When you cross over to enter the land that the L-RD your God is giving you, a land flowing with milk and honey, as the L-RD, the God of your fathers promised you- upon crossing the Jordan, you shall set up these stones, about which I charge you this day, on Mount Ebal, and coat them with plaster. There too, you shall build an altar to the L-RD your God, an altar of stones. Do not wield an iron tool over them; you must build the altar for the L-RD your God of unhewn stones. You shall offer on it burnt offerings to the L-RD your God, and you shall sacrifice there offerings of well-being and eat them, rejoicing before the L-RD your God. And on these stones you shall inscribe every word of this teaching most distinctly (Deuteronomy 27:1-8, JPS translation). (Several English commentaries on this portion use the term steles to refer to the surfaces upon which the Teaching (=Torah)was written. A stele is an upright stone or slab with an inscribed or sculptured surface, used as a monument or as a commemorative tablet in the face of a building. We will not discuss here the various views as to whether the entire Torah was written on the steles or only parts of it.)

Chapter 8 of the book of Joshua records the fulfillment of this commandment, as well as the fulfillment of the commands described in the latter part of the chapter; namely, the readings of the blessings and the curses.

At that time Joshua built an altar to the L-RD, the God of Israel, on Mount Ebal, as Moses, the servant of the L-RD, had commanded the Israelites- as is written in the Book of the Teaching of Moses- an altar of unhewn stone upon which no iron had been wielded. They offered on it burnt offerings to the L-RD, and brought sacrifices of well-being. And there, on the stones, he inscribed a copy of the Teaching which Moses had written for the Israelites. All Israel-stranger and citizen alike- with their elders, officials, and magistrates, stood on either side of the Ark, facing the Levitical priests who carried the Ark of the L-RD’s covenant. Half of them faced Mount Gerizim and half of them faced Mount Ebal, as Moses the servant of the L-rd had commanded them of old, in order to bless the people of Israel. After that, he read all the words of the Teaching, the blessing and the curse, just as is written in the Book of the Teaching. There was not a word of all that Moses had commanded which Joshua failed to read in the presence of the entire assembly of Israel, including the women and children and the strangers who accompanied them (Joshua 8:30-35, JPS translation).

Regarding the prohibition of hewn stones, we find elsewhere in the Torah as well the statement: And if you make for me an altar of stones, do not build it of hewn stones; for by wielding your tool upon them you have profaned them (Exodus 20:22).

Why did God prohibit the use of an iron tool to cut the stones for the altar? The Mishnah (Massekhet Middot 3:4) gives an answer. First the Mishnah in great detail which describes the procedures involved in preparing proper stones from the altar in the Bet Ha-Miqdash. It writes as follows:

The stones both of the ascent and of the altar were taken from the valley of Beth Kerem. They dug into virgin soil and brought from there whose stones on which no iron had been lifted, since iron disqualifies by mere touch, though a scratch made by anything could disqualify. If one of them received a scratch it was disqualified, but the rest were not. They were whitewashed twice a year, once at Passover and once at Tabernacles, and the Hekhal was whitewashed once a year, at Passover. Rabbi says: They were whitewashed every Friday with cloth on account of the blood stains. The plaster was not laid on with a trowel of iron, for fear that it might touch and disqualify.

(A trowel is a flat-bladed hand tool for leveling, spreading, or shaping substances such as cement or mortar.)

Finally, the Mishnah concludes:

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 would be lifted against that which prolongs.

The Mekhilta of R. Yishmael to Exodus 20:23 (ed. Lauterbach, [Phila., 1949, p. 290] quotes the statement found in the Mishnah of Middot (in the name of R. Shimon ben Elazar) and adds the following comment in the name of R. Johanan ben Zakkai:

Behold it says, You shall build of (whole) stones (Deut. 27:6). They are to be stones that establish peace. Now, by using the method of kal va-homer, you reason: the stones for the altar do not see nor hear nor speak. Yet because they serve to establish peace between Israel and their Father in heaven the Holy One, blessed be He, said (Deut. 27:5) Do not wield an iron tool over them. How much the more then should he who establishes peace between man and his fellow-man, between husband and wife, between city and city, between nation and nation, between family and family, between government and government, be protected so that no harm should come to him.

(The commentary of Rashi, for his part, to Exodus 20:22 is based upon these sources as well.)

Remarkably, Rashi’s grandson Rashbam, on the other hand, in his commentary to Parashat Yitro, advances a different explanation for the prohibition to hew the stones for the altar with iron. In his view, since sculptors who would use iron to mold stones would draw figures and idols on the stones with the iron implements, the Torah, in attempting to steer the Israelites from any idolatry, forbade the use of iron.

Rashbam quotes Isaiah 44:12-13: The craftsman in iron, with his tools, Works it over charcoal, And fashions it by hammering…He forms it with scraping tools, marking it out with a compass. He gives it a human form….(See Martin Lokhshin (ed.), Perush Ha-Rashbam ‘al Ha-Torah (Jerusalem, 2009), p. 250 and n. 46 ad loc., and his English translation, Rashbam’s Commentary on Exodus (Atlanta, 1997), pp. 222, and note 42 ad loc., for a discussion of the Rashbam’s view.)