Matthew Henry Commentary Exodus 9:8-12

"And Jehovah said unto Moses and unto Aaron, Take to you handfuls of ashes of the furnace, and let Moses sprinkle it toward heaven in the sight of Pharaoh. And it shall become small dust over all the land of Egypt, and shall be a boil breaking forth with blains upon man and upon beast, throughout all the land of Egypt. 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 the magicians could not stand before Moses because of the boils; for the boils were upon the magicians, and upon all the Egyptians. And Jehovah hardened the heart of Pharaoh, and he hearkened not unto them, as Jehovah had spoken unto Moses." — Exodus 9:8-12 (ASV)

When the Egyptians were not moved by the death of their cattle, God sent a plague that seized their own bodies. If lesser judgments do not work, God will send greater.

Sometimes God shows people their sin in their punishment. They had oppressed Israel in the furnaces, and now the ashes of the furnace are made a terror to them. The plague itself was very grievous.

The magicians themselves were struck with these boils. Their power had been restrained before; but they continued to withstand Moses and to confirm Pharaoh in his unbelief, until they were forced to give way.

Pharaoh remained obstinate. He had hardened his own heart, and now God justly gave him up to his own heart's lusts, permitting Satan to blind and harden him.

If people shut their eyes against the light, it is just for God to close their eyes. This is the gravest judgment a person can be under, short of hell.