ashes of the furnace: i.e. one of the altars on which human sacrifices were sometimes offered to propitiate their god Typhon (i.e. the Evil Principle). These were doubtless being offered to avert the plagues, and Moses, using the ashes in the same way, produced an- other plague instead of averting it.
Related Commentaries of Exodus 9:10
Exodus 9:10
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.