Colourant Content Comments Furnace Conditions 'Aqua' Iron(II) oxide (FeO) 'Aqua', a pale blue-green colour, is the common natural colour of untreated glass. Many early Roman vessels are this colour.[1] Colourless Iron(III) oxide (Fe2O3) Colourless glass was produced in the Roman period by adding either antimony or manganese oxide.[2] This oxidised the iron (II) oxide to iron (III) oxide, which although yellow, is a much weaker colourant, allowing the glass to appear colourless. The use of manganese as a decolourant was a Roman invention first noted in the Imperial period; prior to this, antimony-rich minerals were used.[1] However, antimony acts as a stronger decolourant than manganese, producing a more truly colourless glass; in Italy and northern Europe antimony or a mixture of antimony and manganese continued to be used well into the 3rd century.[25] Amber Iron-sulfur compounds 0.2%-1.4% S[1] 0.3% Fe Sulfur is likely to have entered the glass as a contaminant of natron, producing a green tinge.