Bill Hall, Age 15, of Boise, Ida. For his question:

What makes glass turn bluish in the sun?

In Boston, Mass., there are lovely old houses with windowpanes of purple blue. Originally this glass was clear. The amethyst tints were added by the action of the ultraviolet rays in sunlight. Artisans of the ancient world did this trick in their glassworks. But they never knew why it worked, for they knew nothing of atoms and their chemical behavior.

A natural mineral called desert glass is tinted deep amethyst by the rays of the sun. Certain types of window glass turn blue or purple or rosy pink. This happens when there are traces of manganese in the glass making ingredients. Iron Impurities add a greenish tint, and a trace of manganese dioxide is added to counteract this and Clear the glass. The iron and manganese dioxide react together and change each other.

In the molten brew, the manganese dioxide loses some of its oxygen. It is reduce the iron grabs some of the free oxygen. It is oxidized iron oxide molecules have no green tint to add to the glass. Traces of manganese can add pinks or blues to the mixture. But if conditions are just right, the reduced manganese adds nothing and the glass is clear    clear enough to make windowpanes.

But the sun shines through the glassy windows. And year-by-year its invisible ultraviolet rays cause changes in the reduced manganese. Molecule by molecule it gain back atoms of free oxygen and become re oxidized. Conditions axe now right for the manganese to add its gem tints to the glass. The window panes turn pink or blue or purple amethyst.

There are countless recipes for making different glass, but the basic ingredients are always soda. Iirae and silica. They are melted and fused at 1200 degrees or more and cooled fast. Traces of other ingredients may be added to give thousands of different qualities to the glass.

Artisans of the ancient world discovered many of these tricks by accident. Many of their recipes were written on clay tables in 88 B.C. and found in the library of old Nineveh in Mesopotamia. They knew haw to use manganese to tone down green tints or stain their glass with amethyst. But they did not know that the tricks were done by the reduction and oxidation of the ingredients.

A trace of copper can stain glass blue or green or rich red. Oxidized copper adds a blue tint, and the ancients made blue glass in Open, smokeless fires. The copper in the molten brew combined with oxygen in the smoke free air. They made the rich red glass in smoky fires and cooled the brew in stuffy ovens. The smoky air was short of oxygen. The finished glass was stained red by suspended atoms of pure, non oxidized copper.