Early glasses

The addition of a flux results not only in a mixture of silica and flux having a lower melting temperature than that of the silica, but also in the melt being less viscous, flowing more easily than silica (viscosity is a measure of the resistance of fluids, liquids, and also gases, to flow fluids with high viscosity flow more slowly than do those with low viscosity). As a consequence of its relatively low viscosity, the hot molten mixture of silica and flux, a type of early glass, can be shaped with relative ease. 

Freestone, I.C., Leslie, K.A., Thirlwall, M. and Gorin-Rosen, Y. (2003). Strontium isotopes in the investigation of early glass production Byzantine and early Islamic glass from the Near East. Archaeometry 45 19-32. 

Thus, 60% of the total radioactive particle mass is found in the aerial cloud population. This is partitioned between the early, glass fraction and the late, crystalline fraction in accordance with the areas under the two lognormal curves which combine to produce the observed size distribution with mass. 

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. 

These early cements had relatively poor properties compared with modem glass-ionomers. They set relatively slowly, but quickly became unworkable, and were weak when fully hardened [28]. Also, as the glass G200 was relatively high in fluoride, it was opaque, which meant that the set cements lacked translucency [18]. This meant that the early glass-ionomer cements had relatively poor aesthetics. 

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