History of glass

Naturally occurring glass, such as obsidian, has been used since the stone age. The first recognized instructions for glass making are in Egypt around 1500 BC, when glass was used as a varnish for ceramic and other items. In the first century BC the method of blowing glass was urbanized and what had once been an enormously rare and valuable item became much more common. During the Roman Empire many forms of glass were created, mostly for use in vases and bottles. Glass was made from sand, plant ash and lime. The initial use of glass was as a colored, opaque, or transparent glaze applied to ceramics before they were fired. Small pieces of colored glass were considered valuable and often rivaled valuable gems as jewelry items. As time passed, it was discovered that if glass is heated until it becomes semi-liquid, it can be shaped and left to cool in a new, solid, independently standing shape. In the first century BC, somewhere at the eastern end of the Mediterranean, a new invention caused a true revolution in the glass industry. This was the detection of glassblowing, both free-blowing and mold-blowing. The color of "natural glass" is green to bluish green. This color is caused by the unstable amounts of naturally occurring iron impurities in the sand. Common glass today usually has a slight green or blue tint, arising from these same impurities. Glassmakers educated to make colored glass by adding metallic compounds and mineral oxides to produce brilliant hues of red, green, and blue - the colors of gemstones. When gem cutters educated to cut glass, they found that clear glass was an excellent refractor of light, the fame of cut clear glass soared, that of colored glass diminished.