TABLE OF CONTENTS Cations with intermediate ionic strength such as Ti, Al, Be, Zr are intermediate elements. Al creates Q 3 at the expense of Q 4 and Q 2 units in silicate glasses. Al "lengthens" the glass, i.e. increases its working range, improves mechanical and chemical resistance and reduces the tendency for de-mixing. Al is a network former at low concentrations and occurs as AlO 4 tetrahedra in the glass structure. This structural unit improves the glass stability and hence the chemical durability. Alkalis (e.g. Na) are located near AlO 4 tetrahedra and balance their negative charge so that alkalis (Na) are no longer modifiers in the silicate network. Being strongly bonded to AlO 4 tetrahedra these alkali cations are not readily leached compared to alkalis more weakly bonded to NBO. This is true if the content of Al is relatively small, when the ratio 
. Al 2O 3 in glasses significantly reduces the diffusion coefficient of water, which also improves glass durability. Increasing the Al 2O 3 content has the same effect as decreasing the total alkali oxide content. However, adding too much Al 2O 3 may be deleterious for the processing efficiency as higher processing temperatures are required. In practice partial replacement of SiO 2 by Al 2O 3 from 3 to 10% is considered to significantly improve the water stability of the glasses. The states of B and...
