People make glass by melting a particular kind of sand in a fire. This is sand that has a lot of silica in it. People have been using naturally occurring glass since the Stone Age, when they wanted an especially sharp edge on a tool or an arrowhead. Obsidianis a natural glass which forms when sand shoots out of a volcano during an eruption and is heated by the hot gases until it melts into glass. You find it in the Aegean islands between Greece and Turkey, where there are a lot of volcanoes. Even in the Stone Age, people used to sail there specially to get the obsidian.
It wasn’t until around 2500 BC that people began to make glass for themselves. The first glass was little glass beads, which people used for necklaces in place of jewels; these were not fired very hot, so they often had bubbles in them. Later glass-makers learned to make strings of glass, which you could wrap around a clay pot to decorate it. But glass was still very rare in West Asia and ancient Egypt, and pretty much unknown everywhere else.
By about 1550 BC the Phoenicians (who were very good at glass working) could make core-formed glass perfume bottles. They probably used the newly invented bellows to finally get their furnaces hot enough. You made a solid bottle-shaped core out of clay, and then you wrapped lots of these glass strings around it, until it was all covered in them (kind of like a rubberband ball). (Or, some people think you rolled the clay core in powdered glass, or dipped the core into melted glass.) Then you let it cool and scraped out the clay to make a glass bottle.
This glass was about two-thirds silica sand, and the rest lime and soda ash made from burning salt-water plants that grew on the beach. The lime made the glass more stable, and the soda ash made it melt at a lower temperature. But this way of making glass bottles took a long time, and so people only used glass for jewelry and fancy bottles for a long time.
Fancy glass beads were great for trading: small and light, hard to break, but expensive jewelry. By 1300 BC, traders were selling amber in the Mediterranean and carrying Egyptian glass beads up north to Denmark, probably along the Danube and the Rhine, or up the Volga River. A thousand years later, by 300 BC, traders had brought some of these glass beads as far away as China.
How Glass Is Made, by Alan J. Paterson (1985). For kids, but it’s really about modern glass technology.
Studies in Ancient Technology: Leather in Antiquity – Sugar and Its Substitutes in Antiquity – Glass, by R. J. Forbes (2nd revised edition 1997). Only part of the book is about glass, but it will tell you everything you need to know about glass in ancient Greece and Rome. By a specialist, for adults.
Early Glass of the Ancient World: 1600 B.C.-A. D. 50, by E. M. Stern (1995). Marianne Stern is the leading world expert on ancient glass.
Roman, Byzantine and Early Medieval Glass: Ernesto Wolf Collection, by E. M. Stern and others (2002). Same expert author as above.