The history of bricks, one of the oldest building-material in the world, begins at the dawn of civilisation. Sun-dried bricks (basic brick mixed with straw, and dried in the sun) were invented between 10 000 and 8 000 BC; cast bricks were developed between 5 000 and 4 500 BC, but the most important discovery was the invention of burnt bricks, around 3 500 BC. This brick has allowed the building of permanent structures in some regions where this was beforehand not possible. By burning it, the brick becomes as resistant as stone, but the further advantage is that it is easier to put in place. Indeed, the arrival of brick has speeded up the rhythm. Because of its age and the duration of its construction, the Great Wall of China has known all the different types of bricks, apart from the hollow bricks, invented in the XIX century.

   The sun-dried brick building, also called adobe, is one of the oldest and cheapest building technique. The adobe brick is a mixture of clay (30% of the weight whereas burnt brick contains 75% of clay) water and vegetal remains, such as straw, wood shavings, saw-dust hemps, or even animal hairs. Wooden moulds are filled up with this paste, which is removed a few days later and then dried in the sun, for about fifteen days. It is not very demanding in terms of technology and the required raw materials are usually available on the premises. The main problem with sun-dried brick is that it is easily in disrepair; even the mud coat on this kind of building doesn’t protect it from the heavy rainfall very long. The earth parts and sun-dried parts of the Great Wall have been eroded and are now in disrepair.

   The burning of bricks solves the initial problem, but the process isn’t easy; burning a brick in an ordinary fire wouldn’t work. For the material to glaze, it takes between 900 and 1 150°C, for eight to fifteen hours, depending on the type of clay used. The bricks are cooled down slowly for them not to crack. If they are not sufficiently cooked, they will not be strong enough and there will be a risk for them to crumb, and vice versa: if the temperature is too high, the clay gets out of shape and becomes a kind of glazed material. During the drying process, the brick shrinks. The material must contain sand or other material. The percentage of pure clay will determine the mechanical properties. The Great Wall’s walls are half-brick stretcher walls (the longer side of the brick is visible, contrarily to bond-stone constructions) and the corners are closed at both ends. The thermal expansion of the bricks if of 0,005mm/m.°C, SOIT half of the concrete expansion. Thus, if the temperature of a brick wall of one meter rises of 50°C, it will expand by 50x0,005 = 0,25mm. The creep due to humidity (distortion engendered by the water retention) is inferior to 0,1mm/m. The brick parts built by the Ming are very stable. The rampart floor was careered with tree or four layers of bricks, the top ones were square bricks and the others were long bricks.

   A recent archaeological discovery in Qinhuangdao updated fifty one brick ovens which were used for the making of grey bricks of the Great Wall. There are three types of ovens of three to five meters in diameter. The first one is in the shape of a dragon, with a piping system that links four to six or eight oven chambers, all in a row. The second one is in a “U” shape, and the last one looks like a “horn”. The oven walls are built with the same grey bricks of 10,5 kilos each. These were located upstream at the top of the Great Wall.