Albert Barnes Commentary Exodus 9:8

"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." — Exodus 9:8 (ASV)

This marks a distinct advance and change in the character of the visitations. Until now, the Egyptians had not been attacked directly in their persons. It is the second plague that was not preceded by a demand and warning, probably on account of the special hardness shown by Pharaoh in reference to the murrain.

Ashes of the furnace - The act was evidently symbolic: the ashes were to be sprinkled toward heaven, challenging, so to speak, the Egyptian deities. There may possibly be a reference to an Egyptian custom of scattering to the winds ashes of victims offered to Typhon.