The limestone strata in which the Theban catacombs were excavated were almost always interspersed with flint nodules, fossils, and petrified shells.

These faults were variously remedied according as the decoration was to be sculptured or painted.

If painted, the wall was first roughly levelled, and then overlaid with a coat of black clay and chopped straw, similar to the mixture used for brick- making.

If sculptured, then the artist had to arrange his subject so as to avoid the inequalities of the stone as much as possible.

When these occurred in the midst of the figure subjects, and if they did not offer too stubborn a resistance to the chisel, they were simply worked over; otherwise the piece was cut out and a new piece fitted in, or the hole was filled up with white cement.