Exodus 9:8

(8) Ashes of the furnace.—Furnaces in Egypt were either for the melting of metal, the preparing of lime, or the baking of 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 sore infliction, their own wrongdoing became to the Egyptians a whip wherewith God scourged them.