The machine made process begins with the crushing of the material which is performed by a set of high speed rolls. Then the clay is moved into a mixing unit where water is added to lift it to the correct moisture content. A mixing process follows where the many tonnes of pressure allow the stored clay to distribute its water content equally. To achieve the desired shape the clay is fed into an extruder or press where the air is removed by passing the material through a vacuum chamber. The de-aired plastic material is then forced through a shaped mouthpiece at huge pressure, resulting in a formed ribbon of clay. Further forming takes place by cutter or die, producing the final product which is then stacked into setters or cassettes ready for drying or firing. A few days are taken up by drying the newly formed clay products. The water in the clay body is slowly removed by gradually increasing the temperature in the drying chamber or tunnel. A lot of care has to be taken here in order not to crack or distort the clay ware. The tiles are then moved into the kiln for firing, at a top temperature of 1,100 degrees centigrade, which can take several days to complete. Slowly increasing and decreasing the kiln temperature and pressure allows the clay material to change to a hardened durable state. The roofing process does not finish with the manufacturing of the tiles. As with all roofing products, a quality end product requires skilled craftsmanship when laying a clay tile roof. The days of the apprentice are undergoing a revival due to the work of the Roofing Industry Alliance, a government-sponsored industry body which aims to improve the competitiveness of the UK roofing sector, addressing the fundamentals such as product marketing and training.