John Gill Commentary Exodus 9:10

"And they took ashes of the furnace, and stood before Pharaoh; and Moses sprinkled it up toward heaven; and it became a boil breaking forth with blains upon man and upon beast." — Exodus 9:10 (ASV)

And they took ashes of the furnace
which was near at hand, perhaps in Pharaoh's kitchen: and stood before Pharaoh ;
not in his palace, or in any covered room, but in some place open to the heaven, a courtyard or garden adjoining to the palace:

and Moses sprinkled it up towards heaven; cast it up in the air; this being again ascribed to Moses, seems to confirm the notion of those who think he only did it; but, for the reasons before given, both may be thought to be concerned: and it became a boil breaking forth with blains, upon man, and upon
beast ;
these failing down in the manner before described, on whomsoever they lighted, whether man or beast, produced sore boils and inflammations, and raised blisters and blotches;

and hence arose those lying scandalous stories of the Israelites being a scabby people, and of their being driven out of Egypt on that account, affirmed by Manetho, Lysimachus, Diodorus Siculus, Tacitus, Justin, and others; (See Gill on Exodus 4:6)

with this plague the first vial poured forth on mystical Egypt, or antichrist, has some agreement, (Revelation 16:2) .