Optical glasses are molten from well-defined mixtures of raw materials. Generally, they are substances of variable compositions, which are expressed by convention as oxides of the constituent elements (for example, SiO2, Na2O, CaO, B2O3). However, they are not a mixture of individual compounds such as metals or oxides. In fact glass is a noncrystalline inorganic macro-molecular structure. During the melting process, the raw materials react, creating a new chemical substance that is completely different from the starting materials. The physicochemical, toxicological, and eco-toxicological properties of the substance called glass are totally different from those of the raw materials or oxides from which it is made.
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