The walls of houses, palaces, and tombs, whether of mud brick or of stone, were often too rough to be decorated. The same was true of the surface of (wood or stone) statues. For this reason the Egyptians added a layer of plaster to them. There were two types of plaster. One was based on clay, often mixed with straw. The other consisted of a type of gypsum, readily available in Egypt. A layer of gypsum was almost always applied last, even if the wall already had a layer of mud plaster. In El-`Amarna, however, the artists painted directly onto the mud plaster. It was not only possible to paint on walls smoothened by a layer of plaster, it was also possible to carve it, much more easily than stone. Additionally, any mistakes made could be easily corrected by adding a small amount of new plaster. Although with the passage of time much plaster has come loose from the stone surface, Egyptologists are often able to work out which scenes were added in relief because the decorators scored the relief deeper than the plaster and left scratches in the stone beneath. Pieces of (repair or restoration) plaster that have fallen out teach Egyptologists a great deal about the mistakes made when adding the texts and depictions, and about texts whose content have been changed later. In the Old Kingdom the body of the deceased was sometimes covered with plaster and funerary masks were also made with it. The well-known portrait heads from El-`Amarna are also made of plaster.