Oxidised and Reduced Glass, Colourings and More
Introduction to Coloured Glass
Glass is an amorphous solid comprising of mostly silicon dioxide (SiO2, silica) alongside additives to modulate the colour, strength and heat performance of the product. To attain a certain colour of glass, various transition metal compounds can be added. Coloured glass has been around since at least the Roman times, when glass dating from the fourth century in Galilee was found to have colours ranging from pale blue to green to amber - with the colours arising from an iron-sulfide chromophore(1).
Oxidised and reduced glasses are terms that are often used but little understood. In brief, various sulfur-based additives can be used in the manufacturing of glass, and if the sulphur is in an oxidised state such as a sulfide, then the glass is referred to as ‘oxidised glass’. Conversely, ‘reduced glass’ is when the sulfur is present in the reduced state, such as sulphate. The reduced/oxidised denotation does not relate at all to the silica content of the glass.
Glass colour is determined by the concentration, identity and redox balance of sulfur compounds and counterions (the aforementioned transition metals, including iron, for example), manipulation of this balance gives rise to different colours in the melt and thus the final product(2).
This article will only deal with silicate glasses.
Why is Glass Coloured?
Glass may be coloured for a variety of reasons, from requiring a distinct hue to better present or advertise a product, to providing the contents of the glass with protection from ultraviolet radiation, in addition to purely decorative purposes. Coloured glass, as thought of in the traditional sense, is mainly used only for container glass, but certain additives to plate glass have been used over the years which modulate the colour from clear ever so slightly.
The characteristic hue of amber glass, found everywhere from beer bottles to medicine jars, is due to the amber chromophore. This chromophore is comprised of ferric iron, Fe3+, coordinated tetrahedrally by three oxygen ligands and a single sulphide. The redox balance is therefore crucial: to reducing an environment and there can be insufficient ferric iron (as it has been converted to ferrous iron) and excess sulfide; if the environment is too oxidising, there will be an excess of ferric iron and insufficient sulfide present. It is imperative to know the balance of ferrous to ferric iron in the melt, the atmosphere both within and above the melt, and the ratio of sulphur compounds. In addition to physical components, the temperature of the melt itself can impact colour and transmittance of amber glass(3). In many cases, a darker amber colouration can be achieved with a small addition of copper oxide to the melt.
Red Iron Oxide
The advantage of using iron oxide as a pigment is that it adds an amber colouration, from the aforementioned amber iron chromophore, yet it doesn’t add other components to the melt. It is an oxidising additive, and therefore will move the redox number to the oxidative side, associated with amber and brown coloured glasses. Iron oxide is routinely used to increase iron content in iron-deficient glasses, such as those which sometimes can occur with pyrite.
Pyrite is one of the most common ores of iron, and thus it is a relatively inexpensive source of iron for glassmaking. As a pigment, pyrite is added to glass melts to produce and amber colouration(4), and its use provides the glass with a resistance to ultraviolet radiation, effectively absorbing light in the sub-450 nm regime. This makes pyrite amber glass particularly well suited to foodstuffs, drugs and laboratory chemicals. Unlike iron oxide, pyrite is described as a reducing additive.
Green glass is the other most popular colour choice for glass, mostly as a container glass for beers, wines and sparkling water. Iron and chrome are chiefly responsible for the green colouration, and by using chrome flour (chromite powder, iron chromite) and iron pyrite in concert, a wide variety of hues of green can be produced, from emerald green all the way through to feuille morte or dead leaf colours.
Georgia green (the famous hue associated with bottles of Coca-Cola) and emerald green are produced using oxidising methods, using both pyrite and chromite, whereas all other shades of green are typically manufactured using reducing methods. Up to six kilograms of chromite per metric ton of sand are used for emerald green glass(5). The use of chrome flour is preferable to the older method of achieving such high oxidation levels, potassium dichromate. Potassium dichromate is toxic and thus dangerous to handle.
For Georgia green, an amount of chromite no more than 10% of what would be used for emerald green glass is used. Feuille morte coloured glass is another pyrite-chromite hybrid, where chrome flour and pyrite are used in a 1:2 ratio (i.e. 1 kg chrome flour and 2 kg pyrite per metric ton of sand).
Noteworthy is chromite’s history as a glass pigment, appearing in 1849, some fifty years after it was used as a glaze pigment. The nature of chromite as a refractory material precluded it somewhat from use in glasses, however the optimal balance of grind size and temperature ensured its wider adoption(6).
Blue, Red and Black Glasses
Red opaque glass has been used since Egyptian times, and since then developed in Iron Age Britain, it was mostly formed by glass formed when doped with colloidal copper, and later with lead oxide(7,8). In contemporary glassmaking, colloidal copper and lead are not used, rather their oxides are.
When added to glass melts, copper compounds form an equilibrium that differs depending on whether it is under oxidative or reductive conditions. An oxidative environment will establish a Cu2+-Cu+ system, which gives rise to a blue colouration akin to copper sulfate; whereas under reducing conditions, a Cu+-Cu0 system will establish, giving a ruby red colouration. The Cu0 ion has no colouration(9). Copper oxide itself is an oxidising dopant, and adding it in sufficient quantity will produce black glass. Perhaps the most visually appealing of all glasses is the deep blue colour afforded by the addition of cobalt oxide, CoO, to the melt(10).
Deep red coloured glass can also be achieved with the addition of cadmium selenide whereas, manganese oxide can be included in the melt(11), which will result in purple/violet hues, though manganese-doped glasses tend to not be very ultraviolet resistant.
Quantifying Additives: The Batch Redox Number
Glassmakers at scale tend to use a term called the ‘batch redox number’ to determine the ingredients needed for a certain colour of glass, which is broadly a proxy for how reducing or oxidising a melt is. Redox numbers are calculated for the sum of all redox active components in the melt by adding together the redox factor multiplied by the mass fraction component per two metric ton of sand, for each component. Particular attention needs to be paid to the use of cullet - a filler - which often contains large amounts of organic (reducing) material, which could easily throw off a batch redox calculation. In addition, the redox of the glass is also influenced by the conditions inside the furnace, such as temperature and how oxidising the atmosphere is. It is therefore critical to ensure that only high purity and high-quality additives are used.
As a general rule, and especially in the case of glass containing iron (which is the most used glass colourant): a redox number between 20 and 0 will produce colourless glass, between 0 and -15 will produce green glass, between -15 and -25 will produce ‘feuille morte’ glass and between -20 and -30 will produce amber. For iron-doped glass these values, as they decrease, correspond to a higher ratio of Fe2+ ratio to Fe3+ (i.e. more reducing).
Impacts on Manufacturing Processes
In brief, container glass is made by melting together raw materials in a furnace to produce a melt, the melt is then refined, formed and annealed. Cyclical finishing processes can occur between and after forming and annealing steps. Annealing is the technique employed to remove points of stress in the glass(12). This process is broadly the same for plate glass, however shaping is replaced by a drawing and rolling scheme.
As a general rule of thumb, a lower redox (i.e. a more reducing environment) allows for better refining and for the furnace to run at a lower temperature providing economic and environmental benefits(13). In addition, a lower redox means there will be less sulphate, which in turn will mean a better refined glass.
As early as 1942, it was known that the addition of small amounts of iron oxide to melts caused an increase in efficiency of the process in the furnace. It has been theorised that the acceleration of melting rate is due to the presence of the iron causing better thermal conductivity in the melt overall(14). Adding compounds to the melt needs to be done with great care, as a drop in temperature could cause devitrification - i.e. crystallisation of the molten glass(15).
Anthracite and feldspar are common additives to the melt, each modulating the properties of the melt:
Anthracite isn’t a glass pigment in its own right, but it is often used alongside iron and iron sulfide compounds to enhance the properties of the glass, providing for yellow/amber colours. The addition of anthracite modulates the overall redox in favour of reduction, which may have an impact on ensuring the level of colouration. It is known that anthracite can eliminate imperfections caused by gases in the melt, and can lower the temperature of the melt when used with chrome and manganese pigments(16).
Feldspar in terms of glassmaking is referred to as a ‘flux’, that is, it reduces the melting point of a solid. Its inclusion can mean a lower temperature requirement in the melt. Fluxes are widely used throughout the glass and ceramic industries for their ability to promote complete liquefaction (17). In addition, feldspar can afford the glass desirable properties such as increased hardness, durability and chemical resistance(18).
● The colour of coloured glass is determined by the identity of additives, their compositions and redox balance in the melt
● Pyrite and chromite are largely responsible for amber and green colours respectively, with blues and reds provided by copper compounds, amongst others
● The redox balance and other additives such as feldspar and anthracite have an impact on the overall manufacturing process and final glass performance
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10 US Patent US10246370B2, 2017
11 US Patent US3830639A, 1972, expired
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13 A. Hubert et al., Impact of Redox in Industrial Glass Melting and Importance of Redox Control in 77th Conference on Glass Problems, Columbus, 2017
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18 A. O. Tanner, Feldspar and Nepheline Syenite, 2015 Minerals Yearbook, United States Geological Survey, Reston, United States, 2015