And the Lord said unto Moses and unto Aaron
This very probably was the day following, on the third day of the month Abib, about the eighteenth of March, that orders were given to bring on the following plague: take to you handfuls of ashes of the furnace;
either in which the bricks were burnt, or rather in which food was boiled, since it can scarcely he thought there should be brickkiln furnaces so near Pharaoh's court; though perhaps some reference may be had to them, and to the labour of the children Israel at them, and as a just retaliation for their oppression of them in that way. These ashes were such as were blown off the coals, and though fresh, yet not so hot but that they could take and hold them in their hands: and let Moses sprinkle it towards the heaven, in the sight of Pharaoh;
this was to be done before Pharaoh, that he might be an eyewitness of the miracle, he himself seeing with his own eyes that nothing else were cast up into the air but a few light ashes; and this was to be done towards heaven, to show that the plague or judgment came down from heaven, from the God of heaven, whose wrath was now revealed from thence; and Moses he was to do this; he alone, as Philo F26 thinks, or rather both he and Aaron, since they were both spoken to, and both filled their hands with ashes; it is most likely that both cast them up into the air, though Moses, being the principal person, is only mentioned.
F26 De Vita Mosis, l. 1. p. 622.