Glass-forming substances are oxides (SiO2, B2O3, P2O5, TeO2, GeO2), fluorides (AlF3), sulfides and
other substances. Depending on the main glass-forming agent used, the glasses
are oxidic (silicate, quartz, germanate, phosphate, borate), fluoride, sulfide,
and so on.
The most common is silicate glass - one of the oldest materials that
mankind has learned to create from time immemorial. And in composition, the
current glass is not much different from the oldest. Only the manufacturing
technology has changed, and the useless impurities in the composition have
become much less.
The basic method for producing silicate glass is to melt a mixture of
silica sand (SiO2), soda (Na2CO3)
and calcium carbonate (CaCO3). The result is a compound of the
composition Na2O·CaO·6SiO2.
70-75% of silicon dioxide (SiO2) as the main
component contains in the glass. It is obtained from quartz sand, subject to
its corresponding granulation and purity. Venetians used for this pure sand
from the river Po, or even deliberately imported it from Istria. Bohemian
glassmakers received sand from pure quartz.
The second component, calcium oxide (CaO), makes the glass chemically
resistant and enhances its luster. For the glass, it is taken in the form of
lime. Ancient Egyptians received calcium oxide from the crushed stone of seashells,
and in the Middle Ages, it was prepared from the ashes of trees or algae, since
limestone was not yet known as a raw material for making glass. The first to
mix chalk with the glass mass were Bohemian glassmakers in the 17th century.
The next part of the glass are oxides of alkali metals - sodium (Na2O)
or potassium (K2O), necessary for melting and manufacturing glass.
Their share is about 16-17%. They go to glass in the form of soda (Na2CO3)
or potash (K2CO3), which at high temperatures decompose
into oxides. Soda was first obtained by leaching seaweed ash and in an area
remote from the sea, potash was used, derivable by leaching the ashes of beech
or coniferous trees.
There are three main types of glass: soda-lime glass (1Na2O:
1CaO: 6SiO2), potash-lime glass (1K2O:
1CaO: 6SiO2), potash-lead glass (1K2O:
1PbO: 6SiO2).
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