On the day when ye shall pass over Jordan; i.e. at the time; "day" is here used in a wide sense (cf. Genesis 2:4; Numbers 3:1; 2 Samuel 22:1; Ecclesiastes 12:3; Isaiah 11:10, etc.). Thou shalt set thee up great stones, and plaster them with plaster, The stones, the number of which is not specified, were to be large, because much was to be inscribed upon them, and they were to be covered with a coating of lime or gypsum ( שִׂיַד), in order to secure a smooth white surface on which the inscription might be clearly depicted.
That the words were not, as Michaelis, Rosenmüller, and others suppose, cut in the stone, and afterwards covered with plaster in order to preserve them, is plain from its being enjoined that they were to be written upon ( עַל) the stones so prepared; and besides, as this was intended to be a proclamation of the Law, the main purpose of the erection would have been frustrated had the inscription been concealed by such a covering as that supposed.
Among the ancient Egyptians the practice of depicting records on walls or monuments covered with a coating of plaster was common (see Hengstenberg, 'Authentic des Pent.,' 1.464, English translation, 1:433); from them, doubtless, it was borrowed by the Hebrews.
It has been suggested by Kennicott that the writing was to be in relieve, and that the spaces between the letters were filled up by the mortar or cement. This is possible, but it is not such a process as this that the words of the text suggest.
"A careful examination of Deuteronomy 27:4, Deuteronomy 27:8, and Joshua 8:30-32, will lead to the opinion that the Law was written upon or in the plaster with which these pillars were coated. This could easily be done, and such writing was common in ancient times.
I have seen specimens of it certainly more than two thousand years old, and still as distinct as when they were first inscribed on the plaster".