creation

making process

glass types

A flux is needed to make glass from sand because it lowers the melting point. In the past, this made glassmaking technically possible. Today, it allows glass to be produced at commercially viable temperatures. Fluxes contain high concentrations of alkali-element compounds such as sodium or potassium. Lead salts can also lower the melting point. Glass containing only silica from sand and alkali from a flux is unstable, so it also requires a stabilising component, typically lime. Lead, which acts as a flux in glassmaking, also has a stabilising effect within the glass. Glass is classified into different types depending on the flux and stabiliser used. Because glass has been recycled since historical times and the type of glass was not always taken into account in this process, in practice there are gradual transitions between the types of historical glass.

Soda-lime glass

Originally, soda-lime glass was made from sand and natural soda, sometimes mixed with a source of lime (calcium carbonate), such as ground limestone or shells. The soda consisted of ash from halophytic plants, which contained high levels of sodium and calcium, as well as potassium and various other components. Modern soda ash is synthetic and is produced from salt or sea salt using the Solvay process. This soda ash is virtually pure sodium carbonate and is currently used in the industrial production of glass, such as container glass and window glass.

Natron-lime glass

In ancient Egypt, the mineral natron from salt lakes around the Nile Delta was used as a flux from around 1000 BCE. Natron is a mixture of sodium carbonate, sodium bicarbonate and small amounts of sodium sulphate and sodium chloride and contains virtually no potassium salts. This type of glass therefore contains very little potassium and relatively high levels of sodium. It was the predominant type of glass in the Western Roman Empire and surrounding regions until the early Middle Ages.

Wood-ash glass and potash glass

In medieval Europe, wood ash was used as a flux in glass production. Beech was among the woods used to produce this ash. Potash can also be made from wood ash by dissolving the ash in water, removing insoluble components such as calcium, magnesium and phosphate salts, then evaporating the remaining liquid. Wood ash and potash contain a high proportion of potassium and virtually no sodium, as do the resulting glasses. Potash glass contains much less phosphorus and magnesium than wood-ash glass. Because potash contains virtually no lime, lime from another source, such as limestone, was added when making the glass.

Lead crystal or crystal

Historically, lead crystal, usually referred to simply as crystal, was made using a lead salt as a flux. The resulting glass is clearer and produces a clearer ringing sound than the glasses described above. Under guidelines issued by the European Coal and Steel Community (ECSC), a predecessor of the EU, crystal must contain at least 30% lead oxide. Previously, glass with a lower lead oxide content could also be classified as crystal. Because lead can leach out from the glass, posing a toxic risk, lead-free crystal instead uses zinc, barium or both.