Then the Lord said to Moses and Aaron, "Take a few handfuls of ashes from a furnace; Moses is to throw them into the air in front of the king.
They will spread out like fine dust over all the land of Egypt, and everywhere they will produce boils that become open sores on the people and the animals."
So they got some ashes and stood before the king; Moses threw them into the air, and they produced boils that became open sores on the people and the animals. 1
References for Exodus 9:10
The magicians were not able to appear before Moses, because they were covered with boils, like all the other Egyptians.
But the Lord made the king stubborn and, just as the Lord had said, the king would not listen to Moses and Aaron.