Answer:  

Dr. Masahide Takahashi, who is actually studying about glass material, kindly answered the question above.
 
 
Please take a look at the periodic table. The elements at the upper right side from the diagonal line connecting B (Boron )in group 13 and At (Astatine) in group 17 are nonmetals, and other elements are metals and transition metals (except for hydrogen). Nonmetallic elements make up about 1/5 of all elements. Therefore, we can say that most elements are metals.
  
Glass materials are widely distributed in our life, including not only windows and dishes but also fiber optics for the internet. Glass is also a very important material for insulation in electronic circuits. The main component of most these types of glass is silicon dioxide. Fiber optics are made of almost 100% silicon dioxide. Silicon is a nonmetallic element according to the periodic table, so we can say that the main component of glass is a nonmetal.
  
Much glass contains transition metals such as iron, because glass is usually made from natural minerals such as silica sand. If glass is made from a pure reagent, glass companies could not do business because of the high cost.
  
When you look at a cross section of a window, you see green. This is because of light absorption of the iron contained in the glass as an impurity. Metal is also useful to remove the color of glass. The light absorption can be adjusted by newly contained metal so that the glass becomes colorless and transparent. This is the way to make transparent glass using cheap ingredients.
  

Venetian glass   Courtesy of  Noble Traders

Colored glass can be made using the properties of the metals above. Venetian glass (see picture on right) is a bright red glass. The glass contains gold so that the glass shows a bright red color. (That is why it’s so expensive!)   Reflection index is an important parameter for glass used in lenses. Glass material that has a high refractive index bends light well. Refractive index is largely dependent on the density of material.

  
High refractive index glass can be made by adding a heavy metal ion such as lead to glass. Such a glass has played an important role as optical material for a long time.

Since the reflection of light depends on the difference of refractive index between air and glass, high refractive index glass appears sparkling. Crystal glass, such as Baccarat crystal glass, uses the properties of glass containing lead (high refractive index). This type of glass looks very beautiful because the cut causes light to reflect in many directions.

Baccarat Crystal   Courtesy of  Hipomi's hippo collection

  
Metal inside glass controls not only color and refractive index but also the melting point of glass. Glass used for electronic parts should have a melting point of below 600 C. Otherwise the electronic parts will be broken. This type of glass, which is called “low-melting glass”, is becoming a more important material in the electronics field. Almost all of low-melting glass contains lead, because the chemical bond between lead and oxygen is weak and easily dissociates at a low temperature so that the melting temperature becomes low. The lead largely decreases the melting temperature, without decreasing the chemical durability of glass. Glass which has a low melting temperature usually has low chemical durability, but glass containing lead is special. Until recently the reason was unknown. However, studies have now shown that it might be related to the structure of lead inside glass.
  
Considering our health as well as environmental issues, it is harmful that there are so many things around us which contain poisonous elements such as lead. So many researchers are now studying how to remove lead from glass. As I mentioned at the beginning, most elements are metallic. So there should be metal which has the same function as lead and is good for our health. However, such a metal has not yet been reported. 
  
Conclusion
Metal in glass is a very important element controlling the optical and thermal properties of glass. Glass containing metal is applied to many fields in this modern society.
 
 
Acknowledgement
We would like to thank Dr. Masahide Takahashi for his kind answer. Please take a look at his website for the details of his study.
  
 

This article is translated by Chemistryquestion.com from the original article in Chemistryquestion.jp.  Please let us know if you find any errors.