Charles Ellicott 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)

Ashes of the furnace.—Furnaces in Egypt were for either melting metal, preparing lime, or baking bricks. It was probably from a furnace of this last kind that the ashes were now taken. Much of Goshen had been converted into a brick-field (Exodus 1:14; Exodus 5:7–13); and though most of the bricks made would be simply dried in the sun, a portion would be subjected to artificial heat in brick-kilns. When ashes from one of these kilns were made the germs of a disease that was a severe affliction, their own wrongdoing became for the Egyptians a whip with which God scourged them.