The combination can be modified. Lead oxide used as the flux makes the glass less brittle and more stable but harder to work when hot. Various oxides and silicates add colour.
Tin oxide makes the glass opaque.
By 2000BC, Egyptian craftsman were skilled at making glass articles. This technique can produce objects formed from bands of coloured glass. The glass was often coloured to resemble a semi-precious stone. Sometimes a ground paste of glass was poured into a mould and then melted. Around the first century BC, probably in Syria, glass blowing was discovered.