Saqqara was the main necropolis of Memphis that was the capital of ancient Egypt during the time of the Old Kingdom, and it has a lot of tombs dating back along the ancient Egyptian history from the time of the first and second dynasties to the Greco-Roman period. Some of the ancient Egyptian tombs from the fifth and sixth dynasties have decorated burial chambers. One of them is the tomb of Memi that might date back to the late fifth dynasty and the beginning of the sixth dynasty. Although his burial chamber has three beautiful decorated walls, the most of the decorated plaster fell down off the bed rock walls. There are some deterioration factors that have affected the mural paintings at Memi's burial chamber, some internal and the other external, and they led to the deterioration of the mural paintings. High humidity is one of the most dangerous factors that carries salts out of the inside structure of the bed rock to the surface and under the plaster, when humidity decreases. It leaves salt under the plaster, that pushes the plaster and causes it to fall down. The internal structure of the bed rock has clay minerals that are affected by the presence of high humidity that leads those minerals to swell and push the plaster off the bedrock walls. Gypsum plaster was used at Memi's burial chamber, whose main components of the plaster are gypsum, sand, calcite and clay minerals. Those components are inhomogeneous and they led to different effect by weathering factors. The thickness of the plaster at Memi's burial chamber varies according to the regularity of the bedrock. However, in general, the thickness of the plaster is thin and the thinness makes it easy to be cracked and broken when it falls off the walls. As a result of high humidity, there is a clear growth of microorganisms on the bedrock walls and the plaster. Salts such as gypsum and halite are the worst problem in the burial chamber because they crystalized to form pins that pushed the plaster to separate and fill off the bed rock walls.
Ewais et al. (Thu,) studied this question.