Before the law was written on the large stones set up on Mount Ebal, what were they coated with?

Answer: Plaster

Moses commanded Israel to set up large stones on Mount Ebal, coat them with plaster, and write on them all the words of this law.

“And on the day you cross the Jordan into the land that the LORD your God is giving you, set up large stones and coat them with plaster.”— Deuteronomy 27:2 (BSB)