Jamieson, Fausset and Brown on Exodus 9:8

Ex 9:8-17. Plague of Boils.

8. Take to you handfuls of ashes, &c.--The next plague assailed the persons of the Egyptians, and it appeared in the form of ulcerous eruptions upon the skin and flesh (Le 13:20; 2Ki 20:7; Job 2:7). That this epidemic did not arise from natural causes was evident from its taking effect from the particular action of Moses done in the sight of Pharaoh. The attitude he assumed was similar to that of Eastern magicians, who, "when they pronounce an imprecation on an individual, a village, or a country, take the ashes of cows' dung (that is, from a common fire) and throw them in the air, saying to the objects of their displeasure, such a sickness or such a curse shall come upon you" [Roberts].