Exodus 9:10
And they took ashes of the furnace, and stood before Pharaoh; and Moses sprinkled it up toward heaven; and it became a boil breaking forth with blains upon man, and upon beast. And they took ashes of the furnace, and stood before Pharaoh; and Moses sprinkled it up toward heaven; and it became a boil breaking forth with blains upon man, and upon beast.
By God's command Moses and Aaron stood before Pharaoh and scattered furnace ashes toward the sky. The ashes became fine dust spreading over all the land of Egypt and turned into painful erupting sores with blains upon man and upon beast. Its time was announced beforehand, so it could not be dismissed as happening by chance.
The act took up the Egyptian custom of casting ashes into the air to ward off evil, but it brought sores on king, priests and people instead, turning their bloody rites into a curse. The furnace recalled the foretold oppression, so the plague assured Israel that God remembered His covenant and that judgment on Egypt and deliverance were near, while pressing the demand to free the Israelites.
Even the magicians and priests were struck, and by no magic or enchantment could they shield themselves, so they could no longer stand before Moses and Aaron because of the affliction. Their failure exposed to the Egyptians how useless it was to trust in their boasted power, since they could not protect even their own bodies and persons. Moses acted by God's power rather than by human feat, while the magicians acted by Satan's power and could not counter the plague, and their removal from standing before Moses cut off the channel through which Satan had been working.
The plague showed Jehovah's supremacy over Egypt's idols and demonstrated that the whole earth is under the command of the God of the Hebrews. Pharaoh remained obstinate and unyielding, and afterward the Lord hardened his heart so that he would not listen, just as God had foretold.