In the definition of glass as a solid material that is completely permeable to visible light, glass-clear plastics and various other amorphous materials, even metals with amorphous structures, are also considered glass materials. The amorphous structure is what makes a material glass. Light absorption is a characteristic of the crystal lattice. If no crystal lattice is present, no absorption in the visible light will occur. Amorphous structures can be created by subjecting a melt to shock solidification so that the molecules and atoms in the melt have no time left to arrange into a crystal lattice during cooling. A “solidified liquid” is formed. The molecules and atoms remain the same arrangement as in the melt but loose the ability to move. The main component of glass is SiO2. In nature, SiO2 forms a highly crystalline quartz. Thermal shock can make SiO2 solidify completely amorphously. Glass can be present as almost pure SiO2 (>99%). This is called silica glass. Like borosilicate glass (approx. 15% B2O3), silica glass is practically resistant to etching attacks. For this reason, silica glass and borosilicate glass is used as material for vacuum chambers and windows of plasma chambers. verwendet. Some plastics can also solidify completely amorphously, in particular PMMA and polycarbonate. Accordingly, amorphous plastics are also referred to as ‘organic glass materials'.