18.1 Glass Structure
The descriptions of glass types in this book are largely based on the descriptions by Clement. The Sellmeier coefficients are derived from vendor data as cited in ZEMAX™. The wavelength increment of the Sellmeier index is 1%. The transmission data are also derived from vendor data over 25 mm of internal transmission, as cited in ZEMAX.
A glass is a random network of fused inorganic material. It may be considered the amorphous form of a ceramic. Crystallization is defeated by a sufficient rate of cooling for the viscosity of the glass. A glass with thick viscosity may cool slowly due to the poor mobility of the components. A glass with low viscosity must cool rapidly before crystals grow.
A krone, or crown (K), is basically soda lime glass:
where silica (SiO2) is derived from sand, sodium oxide (Na2O) is derived from soda, and calcium oxide (CaO) is derived from lime. Soda lime glass might also contain potassium oxide (K2O), which is derived from potash (K2CO3). The silicate is a glass network former. Sodium oxide and calcium oxide are network modifiers that lower the melting point and the viscosity, respectively. Diversity in glass defeats crystallization and promotes homogeneity in the glass melt. Silica is the base network of most glass.
A borosilicate krone (BK) is frequently described as
where the alkali metal oxide (M2O) is a network modifier, such as sodium oxide (Na2O) and potassium oxide (K2O).