The Plague of Boils, Exodus 9

John Currid writes,
"The type of furnace spoken of here was probably a kiln for burning bricks. The
furnace, then, was a symbol of the oppression of the Hebrews, the sweat
and tears they were shedding to make bricks for the Egyptians. Thus the very
soot made by the enslaved people was now to inflict punishment on their
oppressors."

 God was making Israel's curse a blessing and was turning Egypt's blessing into a curse.