Roman Glass

Read this article about the history and evolution of Roman glass production. The manufacture of glassware was known throughout the ancient world, but Roman artisans produced glass on an unprecedented scale. During the first century BCE, the invention of glassblowing allowed artisans to quickly create glass products in a wide range of shapes, bringing cheap glass to mass markets.

Chemistry and Colours

	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.
Colourless	Iron(III) oxide (Fe₂O₃)		Colourless glass was produced in the Roman period by adding either antimony or manganese oxide. 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. 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.
Amber	Iron-sulfur compounds	0.2%-1.4% S 0.3% Fe	Sulfur is likely to have entered the glass as a contaminant of natron, producing a green tinge. Formation of iron-sulfur compounds produces an amber colour.	Reducing
Purple	Manganese (such as pyrolusite)	Around 3%		Oxidising
Blue and green	Copper	2%–13%	The natural 'aqua' shade can be intensified with the addition of copper. During the Roman period this was derived from the recovery of oxide scale from scrap copper when heated, to avoid the contaminants present in copper minerals. Copper produced a translucent blue moving towards a darker and denser green.	Oxidising
Dark green	Lead		By adding lead, the green colour produced by copper could be darkened.
Royal blue to navy	Cobalt	0.1%	Intense colouration
Powder blue	Egyptian blue
Opaque red to brown (Pliny's Haematinum)	Copper lead	>10% Cu 1% – 20% Pb	Under strongly reducing conditions, copper present in the glass will precipitate inside the matrix as cuprous oxide, making the glass appear brown to blood red. Lead encourages precipitation and brilliance. The red is a rare find, but is known to have been in production during the 4th, 5th and later centuries on the continent.	Strongly reducing
White	Antimony (such as stibnite)	1–10%	Antimony reacts with the lime in the glass matrix to precipitate calcium antimonite crystals creating a white with high opacity.	Oxidising
Yellow	Antimony and lead (such as bindheimite).		Precipitation of lead pyroantimonate creates an opaque yellow. Yellow rarely appears alone in Roman glass, but was used for the mosaic and polychrome pieces.

These colours formed the basis of all Roman glass. Although some of them required high technical ability and knowledge, a degree of uniformity was achieved.