The making of mudbricks is a well documented practice, illustrated in models from the Middle Kingdom and on tomb walls. In Egypt, bricks were made of Nile mud, a mixture of clay and sand, sometimes mixed with bits of straw or animal droppings. This was mixed with water to form a malleable mass and then pressed into moulds. These moulds, some actual examples as well as miniature models of which have survived, were rectangular with an open top and bottom. The lump of clay was then left to dry in the sun. Only occasionally, mainly in the Roman period, were the mudbricks also baked in a kiln. Most of the mudbricks made in this way were of a handy size, though bricks of up to a meter in length have also been found. The oldest mudbricks to have been found come from tombs dating to the Predynastic Period at Naqada and from royal tombs at Abydos. Throughout the centuries, mudbrick was continuously used to build houses, palaces, fortresses and many other structures, stone being reserved almost exclusively as a building material for tombs and temples.