Oxidised and Reduced Glass, Colourings and More
A key aspect in the modern glass maker’s work is the manipulation of glass colouration - which is defined by the balance of pigments added and the overall redox state (how oxidised or reduced, relatively, a glass melt is) as defined by the batch redox number. The choice of pigment not only has a profound impact on colour but also on the manufacturing process. African Pegmatite is a leading miner, miller and supplier of the broadest range of pigments for all types of glass production.
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 work will only deal with silicate glasses. A more detailed discussion on the idea behind the batch redox number is available elsewhere on this website.
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. If strong colouration is sought for plate (window) glass, it is typically added after production in the form of a stain or lacquer - in much the same way as the stained glass windows found in churches.
How Is Glass Coloured?
In the simplest sense, to colour a glass, all one needs to do is add pigment to the melt. Such pigments include powdered metal oxides, sulfides, or other molten compounds. Many of the compounds used for glass production have been unchanged for many years - it is rather the understanding of such pigments, how they behave and interact with each other that has transformed modern glass making.
A combination of the chromophores present (provided by the pigments) and the overall oxidation state of the melt are the modern arbiters of how a glass is coloured and how it will turn out. Glass colouration with pigments often rely on the components of the pigment being in a particular oxidation state, producing or modulating a distinct chromophore. Additionally, concentrations and amounts added play significant roles. Intuitively, the more pigment that is added produces a stronger colouration.
The characteristic hue of amber glass, found everywhere from beer bottles to medicine jars, is due to the amber chromophore. This chromophore is composed of ferric iron, Fe3+, coordinated tetrahedrally by three oxygen ligands and a single sulfide. The redox balance is therefore crucial: too 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 sulfur 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. Excess levels of the amber chromophore can lead to a greyish colouration, which is rare, and is largely avoided via careful cooling practices.
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. Another advantage to using red iron oxide as an iron source, over pyrite for example, is that it does not add any sulfides along with it - making for an easier manufacturing process.
Iron(ii) oxide, on the other hand, will afford a blue - green colouration. When combined with pyrite (see below), deeper green colourations can be realised.
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 an 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, therefore when combined with other pigments, the redox equilibrium will be pushed towards the left.
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 tonne 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 tonne 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). Copper oxide pigments in highly reducing environments (i.e. low batch redox numbers) will produce black coloured glass.
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.
Niche Pigments And Colours
Aside from the conventional selection of pigmenting materials and the resultant - relatively narrow - bouquet of colours, other colourations can be achieved using lesser used pigments. These are lesser used primarily because of reasons of low demand for that colour and in some cases a rarity of the pigment itself, or even if it has a hazardous nature.
Although highly toxic, cadmium can be used in glassmaking to produce vivid red and orange colourations when used with selenium. Cadmium is added as cadmium sulfide, and has a moderate reducing effect.
Can be added in modest amounts to remove any green pigmentation caused by iron, but is primarily used to afford a purple hue when used in high concentrations. Manganese oxides have long been used in glass pigmentation.
When combined with cobalt, nickel produces a decolourised glass. In its own right, nickel pigmentation can give rise to blue or, at high concentrations, violet glass.
Is a mixture of the elements praseodymium and neodymium that can produce green or lilac glasses when used in the right concentrations. The glass for safety spectacles often uses this compound as it effectively blocks light at 589 nm - the wavelength associated with the bright yellow light from sodium in glass. As such, spectacles containing didymium are used extensively by glassmakers to prevent excess light exposure.
Responsible for yellow or yellow-brown pigmentation when used alone, titanium compounds are very rare in the glass pigmenting space - but can be used to ‘boost’ the colouration effect of other pigments.
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 tonnes 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 sulfate, 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 Use Of Cullet
Many glassmakers will seek to reduce costs or increase the environmental performance of their process by using cullet - which is made of finely ground recycled glass. In many cases, this is advantageous as the cullet will behave as a flux, reducing the temperature needed to produce the melt in a satisfactory fashion. This cost effective solution is widespread in the manufacture of container glasses, but less so for plate/window glasses.
One concern with regards to cullet is that it may be sourced from already coloured glass - this phenomenon needs to be considered in the batch redox calculation, with ground gullet glass tending to be on the more reducing end of the scale, due to the presence of organics. It is imperative to include this in calculations and make the necessary remediations - which can be as easy as adding more oxidative pigments.
- 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
The modern glass maker is highly aware of the need for the highest quality pigments to produce the most striking colourations - all whilst ensuring stability of the final product. African Pegmatite is the go-to industrial partner for the supply of superior pigments and other additives to ensure glass products are resilient, of high standard and are easy and economical to produce.
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10 US Patent US10246370B2, 2017
11 US Patent US3830639A, 1972, expired
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