Naturally occurring glass, like obsidian, has been used ever since the Stone Age. The first accepted instructions for glass making are in Egypt about 1500 BC, when glass was used as a glaze for pottery and additional items. In the first century BC the method of blowing glass was developed and what had once been a tremendously rare and expensive item became much more common. During the Roman Empire numerous forms of glass were created, generally for use in vases and bottles.
The Glass was made from sand, plant ash and lime. The most primitive use of glass was as a colored, opaque, or clear glaze applied to ceramics before they were fired. Small pieces of colored glass were considered costly and often rivaled valuable gems as jewelry items. As time passed, it was revealed (most likely by a potter) that if glass is heated in anticipation of it becomes semi-liquid, it can be shaped and left to cool in a new , solid, separately standing shape. In the first century BC, someplace at the eastern end of the Mediterranean, a new discovery caused a true revolution in the glass industry. This was the invention of glassblowing, both free-blowing and mold-blowing. The color of "natural glass" is green to bluish green glass. This color is caused by the changeable amounts of naturally occurring iron impurities in the sand. Common glass at present usually has a slight green or blue tint, arising from these same impurities.
The Glassmakers learned to make colored glass by adding metallic compounds and mineral oxides to make brilliant hues of red, green, and blue - the colors of gemstones. When gem cutters learned to cut glass, they create that clear glass was an outstanding refractor of light, the attractiveness of cut clear glass soared, that of colored glass diminished.
The Glass objects from the 7th and 8th centuries have been found on the island of Torcello close to Venice. These form an essential link between Roman times and the later on importance of that city in the production of the material. About 1000 CE, an essential technological breakthrough was made in Northern Europe when soda glass was replaced by glass made from a much more readily obtainable material: potash obtained from wood ashes. From this point on, northern glass differed considerably from that made in the Mediterranean area, where soda remained in general use.