Chemical glass decolorizing

In the glass (qv) and ceramic industry (see Ceramics), barite can be used both as a flux, to promote melting at a lower temperature or to increase the production rate, and as an additive to increase the refractive index of glass. The viscosity of barite-containing glass often needs to be raised. Alumina in the form of feldspar is sometimes used. To offset any color produced by iron from the barite addition, more decolorizer may be needed. When properly used, barytes help reduce seed, increase toughness and brilliancy, and reduce annealing time. Barite is also a raw material for the manufacture of other barium chemicals. 

Iron oxide is always present as an impurity in glass. It is introduced through the natural raw materials such as sand and limestone. Another source is from trap iron mixed in the cullet and abraded metal from the handling of batch. All of this adds up to several hundred parts per million which causes light absorption at the ends of the spectrum rather than the middle and causes a yellow-green color in the glass. This can be overcome by a process known as decolorization. There are two types chemical and physical decolorizing. )... 

To chemically decolorize a glass, oxidizing materials are added to change the iron from the ferrous to the ferric state. 

Benzaldehyde reacts with many chemicals in a marked manner (1) with aramomo-silver nitrate ( Tollen s solution ) to form metallic silver, either as a black precipitate or as an adherent mirror film on glass (but does not reduce alkaline cupric solution. Fehling s solution ) (2) with rosaniline (fuchsine, magenta) that has been decolorized by sulfurous acid ( Schitf s... 

Decolorizer. A material added to glass to counteract the colour imparted by impurities such as iron the decolorizer may be an oxidizing agent, removing the colour by chemical action, or it may counteract the colour already present by introducing the complementary colour. The materials used include the oxides of As, Ce, Co, Mn, Nd and Se. 

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