Glass is not a material, but a state of matter. Nearly any metal or metalloid can be put into the glassy state. As we speak of it here, glass is a synthetic substance made from silica (usually from sand, silicon dioxide). The silica is heated above its melting point and then cooled s that it does not recrystalize.
Ancient furnaces could not reach the melting point of sand (ca. 14300 C), as their maximum was around 11000 C. A flux was added (usually soda or potash) to bring down the melting point. A stabilizer (usually lime) was also added, apparently accidentally in ancient times.
Colorants and opacifiers were also added. With just copper and iron (both common by the time glass was invented) and using the right furnace (air blowing through, just open or closed) nearly any glass color can be made. The science of glass and its colorants is a product of the last two centuries or so.
Glass beadmaking has become very popular, especially in the US, in the last few decades. From a few pioneers, the movement had grown very quickly and very large. Glassmakers guilds and individual glassmakers are listed here. The collecting of their work has become a major force in the bead world.
Glass analysis is complex and its interpretation even more so. Ron Hancock of Toronto U. analyzed specimens of Indo-Pacific beads I collected from India and Southeast Asia. They have shown that: 1. Indo-Pacific beads were not made from glass imported from the West, as many had guessed, 2. Arikamedu used an unusual colorant in the form of wad or bog manganese, and 3. Most of the Indo-Pacific beadmakers made their own glass. The complete tables of Indo-Pacific glass analyses are here.
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