Composition: Silica (SiO2) (main component), Soda lime glass (SiO2, Na2CO3 and CaO), Additives: Potasium oxide (lowers the melting point), boron oxide (improves heat durability), metallic oxides (coloured glass- chromium (green), cobalt (blue)), Na, K or Ba nitrates/sulfates – decolourise glass so it is clear (salts oxidize Fe from Fe2+ → Fe3+ (green → yellow)