Ancient Roman glass was made by mixing two ingredients: silica and soda. Silica is actually sand which is made of quartz. To make the silica melt at a lower temperature, the Romans used soda (sodium carbonate). The source of soda during this period was natron, a type of salt found in dry lake beds. Natron was imported from Egypt from a place called today Wadi El Natrun or "natron valley" in Arabic. Glassmakers would also use a stabiliser such as lime or magnesia. Lime was the primary stabiliser and it was naturally present in beach sand. They would also use colorants if they wanted the glass to have a specific color.
The production process started at large facilities which contained large furnaces and giant tanks where the molten glass would be poured. These large facilities would then distribute the primary glass broken into chunks to smaller production facilities across the Empire.
Wikimedia Commons CC BY 3.0
Initially, ancient Roman glass was mainly used to make vases, cups, pitchers and other containers that held liquids. Glass was thick, heavily colored and not very translucent. It required a lot of polishing. It was also very expensive. Only wealthy people could afford to have cups made of glass.
The 1st century A.D. saw a revolution in glass production technology with the introduction of glass blowing. Glassblowing was invented by Syrian craftsmen from Sidon and Babylon between 27 BC and 14 AD. The ancient Romans copied the technique consisting of blowing air into molten glass with a blowpipe making it into a bubble. The resulting inflated glass was thinner, more viscous and easier to work on than the initial thicker glass. The Roman glassworkers would also use the mold-blowing technique. Molten glass would be placed at the end of the blowpipe and then inflated to take the shape of a carved mold which could be wooden or metal. This allowed for the production of glass objects with a variety of shapes and on an industrial scale.
With glass blowing, glass became thinner, colorless and translucent. Romans produced glass industrially at various locations and glass also became cheaper. The average Roman could now afford to purchase it. We estimate that a glass cup was worth less than a sestertius by the end of the 1st century AD. There were different types of glass and the higher quality ones remained very expensive. It is worth noting that the best Roman glass was produced by Syrians and Phoenicians.
Glass was also used to make mosaic tiles. Glass blowing and improvements in furnace technology allowed for the production of molten glass which led to the production of the first window glass. It is worth noting that Roman houses did not have glass windows up until the first century AD, rather they had holes with shutters with very few facing the street for safety reasons. These windows were often not very transparent, their primary objective being to only let light through.