וַיֹּ֣אמֶר יְהוָה֮ אֶל־מֹשֶׁ֣ה וְאֶֽל־אַהֲרֹן֒ קְח֤וּ לָכֶם֙ מְלֹ֣א חָפְנֵיכֶ֔ם פִּ֖יחַ כִּבְשָׁ֑ן וּזְרָק֥וֹ מֹשֶׁ֛ה הַשָּׁמַ֖יְמָה לְעֵינֵ֥י פַרְעֹֽה׃
And the LORD said unto Moses and unto Aaron: ‘Take to you handfuls of soot of the furnace, and let Moses throw it heavenward in the sight of Pharaoh.
Rashi on Exodus
מלא חפניכם HANDFULS (more lit., as much as will fill your fists), old French joincheiz; Engl, double-handfuls.
Rashbam on Exodus
פיח, ash, dust particles remaining at the bottom of the furnace light enough to be carried by the wind.
Sforno on Exodus
לעיני פרעה. In order that he would see that the plague was not the result of mixing something moist with something else, i.e. something subject to the influence of the sun or the atmosphere. Both of these phenomena sometimes produce what are known as “natural disasters.”