The normal glass is made from quartz sand, soda and limestone, through smelting together. It is a kind of silicate mixture of fluid formation. At the beginning, the glass product is coloured small pieces with poor transparency. The color is not added with artificial works, the real is that the raw materials is not pure, and been mixed with impurity. At that time, colored glass products is used for decoration, differ greatly than now.
After study, people found that if added 0.4%~0.7% colorant in the raw materials, glass will have color. Mostly the colorant is metallic oxide, as every metallic elements have their own optical characteristic, then different metallic oxide show different colors on the glass. For example, glass with Cr2O3 will show green color, with MnO2 will show purple color, with Co2O3 will show blue color.
In fact, glass color is not based on the colorant. Through adjusting the smelting temperature, to change the valence of element, then could made the glass with different color. For example the Cuprum in the glass, if existed by high valence copper oxide in the glass, it is blue green color, but if existed by low valence Cu2O, it will show red color.
Now, people use the rare-earth element oxidate as the colorant to produce different high quality colored glass. The glass with rare-earth element show brighter colour and lustre, even change color under different sunlight. Using this kind glass to make windows and doors, the indoor could keep lightness, no need to use curtain to avoid sunlight, then people called it as automatic curtain.