There are at least two major elements in choosing glass: density and clarity. A third is the “hot/cool” effect of colours. The appropriate combination of these elements leads to a panel with bright or hot spots where you want them. You can create a dramatic image or a more restrained one with more gradual gradations of light without obvious bright or dark areas.
Density relates to the amount of light the glass allows through. Clearly black is the most dense glass – allowing no light through. In general, glass can be divided into opalescent and transparent.
Opalescent glasses range from the very dense opaque to less dense translucent glass.
Transparent glasses have a variety of densities too, although almost always less dense than opalescent glass. The density of transparent glasses relates to the intensity of the colour and the texture of the glass.
The intensity of the colour is related to the amount of light allowed through. The intensity is directly related to the saturation of the colour. A further effect on colour intensity is the thickness of the glass. If you look at a handmade sheet of glass with different thickness on one end to another end, you can see the gradation of the colour and the amount of light that comes through.
The texture of the glass affects the density of the glass. A smooth glass will have less density as the light passes through without dispersion. As the glass becomes more textured, the light is more dispersed and so appears more dense.