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Glas is typically composed of about 75% sand or silica (SiO2) plus Na2O (sodium oxide) and CaO (calcium oxide). The kind of glass you get is dependent on which kind of silica is used. In e.g. France and the Netherlands you can find particularly fine sands that you can produce very clear glass with. Glass is essentially a very thick liquid that moves, but VERY slowly. This is why ancient windows made out of glass are thicker at the bottom than at the top.