In order to generate the plague of the rash, Moses threw ashes from a furnace heavenward. The ashes scattered throughout the land of Egypt and caused a rash on the Egyptians’ skin. According to the Sages, in addition to the wonder of the rash itself, the throwing of the ashes and their dispersal were also miraculous.

Rabbi Yehoshua ben Levi said: A great miracle was performed in the plague of rash, as when a person shoots an arrow upward it does not fly even one hundred cubits, while Moses threw two handfuls of the ashes of a furnace, an insubstantial substance, and Moses threw it heavenward until it flew so high that it reached the Throne of Glory. There was yet another miracle that was performed in the plague of rash: Moses’ one hand held two handfuls of ashes. There was yet another miracle that was performed in the plague of rash: When a person scatters a kav of dust, it scatters only four cubits. Moses took two handfuls of ashes and he scattered the ashes over the entire land of Egypt, which was four hundred parasangs by four hundred parasangs, as it is stated: “It will become dust over the entire land of Egypt” (Exodus 9:9). (Shemot Rabba [Shinan] 11)