신명기 27:2 해설: Rashi, Ibn Ezra, Rabbeinu Bahya & Chizkuni

והקמת לך THEN THOU SHALT RAISE THEE UP [GREAT STONES] — in the Jordan, and afterwards you shall take out from there others and build an altar of them on Mount Eval. Consequently you must say that there were three sets of stones: twelve in the Jordan, an equal number in Gilgal, and another twelve on Mount Ebal, as is stated in Treatise Sotah 35b.

with plaster to preserve the writing.

ושדת אותם בשיד, “and you shall coat them with plaster.” The Torah did not use the term גיר, chalk, in this instance as the prophet Isaiah 27,9 used when describing shattered altar stones. This in spite of the fact that גיר is a far stronger material than שיד. The reason the Torah ordered the stones to be covered with שיד is that this is a material which stands up to rain better than גיר.