Albert Barnes Commentary Jeremiah 43:9

"Take great stones in thy hand, and hide them in mortar in the brickwork, which is at the entry of Pharaoh`s house in Tahpanhes, in the sight of the men of Judah;" — Jeremiah 43:9 (ASV)

Brickkiln - Possibly, a pavement of brick. Jeremiah was to take a few large stones, ones he could nonetheless carry in his hand, and build with them, in the propylaea before the royal palace, something that would serve to represent the dais on which the seat of kings was usually placed. Hiding them in the clay means plastering them over with mortar.