To understand how sand is turned into glass, we must first consider the structure of pure silica in crystal form. A crystal is nature's most perfectly arranged form of matter. It is a rigid symmetrical pattern of atoms, which is characteristic of each molecule of a substance and is repeated without variation throughout the substance. This pattern is known as the molecular structure, a molecule being the smallest unit into which a substance can be divided without any chemical change taking place. Atoms are the component parts of a molecule. In a molecule of silica, the atoms are arranged so that each silicon atom (abbreviated Si) is surrounded by four oxygen atoms (abbreviated O). However, each oxygen atom is attached to two silicon atoms. In effect, each silicon atom divides four oxygen atoms in half, giving the formula SiO² or silicon dioxide, which is the chemical name for silica. In crystalline silica sand, this pattern is repeated over and over, varied only by occasional impurities.

When silica is subjected to intense heat, the atoms vibrate to and fro and eventually the regular crystalline pattern flows and disappears as the material becomes a liquid. If the heat is removed and the silica is allowed to cool, the atoms slow down and try to return to their original positions of the crystalline form. If you take a lump of ice and place it in a heated saucepan, it will melt and then boil and become steam -- the gaseous form of water. Then put what water is left back into a freezer and within a short time the water will return to its crystalline state of ice. Silica, on the other hand, crystallizes with such difficulty that at normal cooling rates it becomes the "solid" we call glass. Crystals of silica need to form slowly, and since the rate of cooling is too fast, the various atoms are trapped in their liquid positions -- irregular, random groups, very different from the strict pattern of the crystalline form. The chemical term for a substance that is solid without being crystalline is amorphous solid.

In other words, much to the surprise of most people, glass is actually more of a fluid than it is a solid. In fact, if you could view a long-term ultra-high-speed motion picture of a pane of window glass you would see the glass actually slowly flowing to the bottom of the pane. Take a good close-up look at an old window pane and you will discover that the glass is slightly thicker at the bottom edge than the top edge. This occurs because the liquified silica never fully solidifies and gravity gradually pulls the mass downwards, albeit quite slowly.

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Science Service. Sand and Glass. Garden City, NY:Nelson Doubleday, 1974.

This page was last updated on 04/06/2010.