My example of flint glass was just to show a common usage of high density glass. There are 3 general components in glass production. Silica, as you mentioned, a flux ingredient such soda and a stabalizer, such as lead, limestone, or magnesia among others. The addition of the stabalizer is generally where the increase in density and strength comes from. Without the stabalizer silica glass is brittle and would be worthless for crafting something like armor. The percentages you mentioned while correct for flint glass are not a maximum. Glass in fact is generally not manufactered with at such densities, as a density of approx 6.2g/cm^3 it begins to loose its transparency but it is possible to produce non-trasparent glasses with densities around 8.0g/cm^3.