What glass and clay have in common?

Glass is silica (sand) mixed with soda. The soda helps the silica to melt - it acts as a flux.

Clay often contains silica, and sometimes quite a high proportion of a clay is silica minerals. It also ususally contains alumina and a variety of other minerals. The particular combination of minerals is what gives different types of clay their indivdual properties.

In firing the clay doesn't melt like glass, but some of the compoentes of the clay will melt partially.