Glass is formed by melting silica and other chemical substances together (the main production raw materials are soda ash, limestone and quartz). It is a silicate non-metallic material that forms a continuous network structure during melting, and the viscosity gradually increases and hardens during cooling, resulting in crystallization. The chemical composition of ordinary glass is Na2SiO3, CaSiO3, SiO2 or Na2O · Cao · 6sio2. The main component is silicate double salt, which is an amorphous solid with irregular structure. It is widely used in buildings to isolate wind and transmit light. It belongs to mixture. There are also colored glass mixed with oxides or salts of some metals to show color, and tempered glass made by physical or chemical methods. Some transparent plastics (such as polymethylmethacrylate) are sometimes called plexiglass.