Exodus 9:8, commentary and pictures

God directed Moses and Aaron to take handfuls of ashes of the furnace, and Moses was to sprinkle them toward heaven in the sight of Pharaoh. As he did so the fine particles spread as dust over all the land of Egypt, and wherever it settled it became painful boils breaking forth with blains upon man and upon beast. In the order of judgments this came after the murrain upon Egyptian cattle in which not one of the Hebrews' cattle died, and before the plague of hail mingled with fire, lightning and thunder. Like the other plagues its time was announced beforehand so it could not be dismissed as chance, proving to the Egyptians that the whole earth is under the command of the God of the Hebrews whom thunder, hail and storm obey.

The furnace ashes signified that Israel's long oppression in the furnace of affliction was nearing its end and recalled the covenant promise to Abraham to judge their oppressors and bring the people out of bondage. The ashes likely came from rites of human sacrifice to Typhon, whose priests scattered them in the belief they would ward off evil, a means the Egyptians may also have used to try to turn the plagues aside. By turning a rite meant to avert harm into a judgment of boils, the act exposed the impotence of Egyptian religion, affirmed Jehovah's supremacy, and pressed the demand for Israel's deliverance.

As one of the signs done before Pharaoh, it attested that God had sent Moses to demand Israel's release, showing the work to be God's power rather than human ability and greater than the magicians' counterfeits. The boils struck even the priests and magicians so that by none of their enchantments could they shield themselves, and they could no longer stand before Moses and Aaron because of the affliction. This let the Egyptians see the folly and uselessness of trusting in the boasted power of the magicians when it could not protect even their own persons.