Solid panels surfaced with polished and textured  lime plaster. Holes are drilled through panels and darkened with charcoal. 

Quicklime is the result of a chemical reaction made by burning limestone with wood or coal CaCO3 + heat → CaO + CO2.  It is then "slaked" with water to make mortar.

Lime kilns (lime works) are likely the earliest technology for transforming the sedimentation of the detritus of life into a useful product with an atmospheric byproduct of CO2.