Why did Hashem tell Yirmeyah to take big rocks and cover them?
Malbim: Normally, a Nevu'ah for evil can be retracted if people improve their deeds. If a Navi did an action [with the Nevu'ah], it is never retracted. This shows that the decree is absolute, like the Ramban says.
What is the meaning of "ba'Melet"?
Rashi: He covered. The Mem is part of the root; sometimes it is omitted, like [the Mem in] Makom, Ma'amad and Ma'aser. The root [without the Mem] is Lat, like "va'Yalet Panav" (Melachim I, 19:13). Yirmeyah covered [the rocks] in the Malben.
Radak #1: It is the material from which they make bricks.
Radak #2: It is another material from plaster and sand.
What is "Malben"?
Radak #1: It is where they burn bricks.
Radak #2: It is where they make bricks.
Radak citing Targum Yonasan: It is plaster used for building.
