Glass soda-rich

The first glasses were made by mixing silica-rich sand with alkali-rich plant ashes or naturally occurring alkali-rich minerals, along with some lime, either as natural material in the sand or as deliberately added limestone. Thus a lot of early glass has compositions a lot like that of obsidian, i.e., about 70% silica, 15% Na O-i-K O, and 10% CaO-i-MgO. Most glass produced today still has this basic soda-lime composition. 

Nitrum is a naturally occurring soda, an important ingredient in both ancient and modern glasses. The ashes of plants also provided the glassmaker with a rich source of sodiuHL The plants saltwort and glasswort (known as halophytes) were both used to supply sodiuna. 

Figure 6.11 Thermal expansion heating curves for silica, soda-lime-silica and tin-rich soda-lime-silica glasses.

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