So the Lord said to Moses and Aaron: Each of you take handfuls of soot from a kiln, and in the presence of Pharaoh let Moses scatter it toward the sky.
Then the Lord spoke to Moses: Early tomorrow morning present yourself to Pharaoh when he sets out toward the water, and say to him: Thus says the Lord: Let my people go to serve me.
So they took the soot from a kiln and appeared before Pharaoh. When Moses scattered it toward the sky, it caused festering boils on human being and beast alike.
But although Pharaoh found upon inquiry that not even so much as one of the livestock of the Israelites had died, he remained obstinate and would not let the people go. Sixth Plague: The Boils.