John Wesley on Exodus 9:10

Ashes of the furnace - Sometimes God shews men their sin in their punishment: they had oppressed Israel in the furnaces, and now the ashes of the furnace are made as much a terror to them as ever their task - masters had been to the Israelites. This is afterwards called the botch of Egypt, Deu 28:27, as if it were some new disease, never heard of before, and known ever after by that name.