"Then the Lord said to Moses and Aaron, 'Take handfuls of soot from a furnace and have Moses toss it into the air in the presence of Pharaoh. It will become fine dust over the whole land of Egypt, and festering boils will break out on men and animals throughout the land." Exodus 9:8,9

Many years ago, a visitor to India wrote: "When the magicians pronounce an imprecation [curse] on an individual, a village, or a country, they take ashes of cow-dung, or those 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 surely come upon you.'"

This pagan ritual that developed over years may actually have been taken from what God did in Egypt as recorded in Exodus.

The ashes were a visual sign that it was the God of the Israelites who was causing the disease that came upon the Egyptians because of Pharaoh's stubbornness. Boils broke out on every person and their animals, and nothing could stop it.

Whether God put power into the dust itself to cause the boils or whether the ashes were strictly a sign, doesn't matter. The Lord was showing Himself superior to a Pharaoh who thought himself to be divine.