How Can the Batch Redox Number be Modulated?
Colour in glasses is based on the interactions of additives and the redox balances. Typical glass additives are transition metal compounds which, in different oxidation states, have different valences. The specific ion responsible for a colour is referred to as a chromophore. The valences of the compounds depend on the amount of oxygen in the melt, the overall balance of additives, the redox state and temperature(2).
The most important chromophore in glassmaking is the amber chromophore, which is produced by the interaction of iron and sulphur ions. Sulphates are added to most glasses, removing bubbles and seeds in the melt making for a superior final product, usually added in the form of sodium sulphate. In glass, sulphate can be reduced to sulphide, this happens at low batch redox numbers. The amber chromophore is formed in the presence of both Fe3+ and S2- (sulphide) ions and has an intensely brown colouration(3).
Quite simply, the batch redox number can be changed by the addition of oxidising or reducing components. The addition of reducing species will cause the batch redox number to decrease, whereas the addition of an oxidising species will cause the batch redox number to increase. Measurement of the observed batch redox number is indirect - the glassmaker will establish the ratio of reduced iron (Fe2+) to overall iron in the case of a finished glass product, via optical means(4). A higher Fe2+ to overall ratio indicate more reduced glasses. The iron ratio is used as iron is present in virtually all glasses. In the case of a melt, the partial pressure of oxygen gas is measured, with higher values indicative of a more oxidised glass(5).
For typical container or plate glass, the following batch redox values and iron ratios are associated with colours(6):
| Batch redox number | Fe2+/Fetotal ratio | Glass colour |
| 0 to 5 | 0.10 to 0.40 | Flint (colourless) |
| -15 to 0 | 0.40 to 0.65 | Emerald green |
| -25 to -15 | 0.60 to 0.75 | Feuille morte |
| -30 to -20 | 0.75 to 0.90 | Amber |
It can therefore be said that a lower batch redox number corresponds to a reduced glass. Batch redox numbers in excess of 5 are associated with oxidised flint glass and UVA green colourations.
Iron Pyrite (FeS2, iron(ii) disulphide)
Pyrite is known as a reducing agent in the glassmaking space, with a redox number of -1.20. Therefore, the addition of pyrite will push the position of the redox equilibrium in favour of a reduced glass, and a lower batch redox number. Pyrite is therefore added to produce an amber colouration(9), useful for container glasses containing perishable foods. Whilst the advantages of pyrite are many, care should be taken to not use excess amounts, as elevated levels of sulfides can lead to unstable glasses.
Pyrite is responsible, in the main, for the amber chromophore. The amber chromophore is responsible for the amber colouration and is a ferric iron (Fe3+) centre coordinated by three oxygen species and a sulfide species in a tetrahedral fashion. Production of the amber chromophore can only happen under certain redox conditions, as implied in the earlier table. Typical amber glasses are achieved with no more than 2.5 kg of pyrite per tonne of sand, with a reducing environment ensued by the addition of carbon-type additives such as anthracite.
Iron Oxides In General
It has been experimentally known as early as 1942, that the addition of small amounts of iron oxide to glass melts causes a marked increase in efficiency of the process in the furnace. The leading theory for this phenomenon is that the presence of iron throughout the melt (initially metallic) causes improved thermal conductivity through the melt(10).
Chrome Flour
The redox pathway that is crucial with chromite is between the reduced chromium(ii), the standard chromium(iii) and the highly oxidised chromium(vi). It is worth noting that independent of the overall batch redox number, the green character associated with chromium(iii) will prevail - it is only modulated by the overall redox environment. In chromite, chromium(iii) is the oxidation state of the metal.
In a reducing environment, the redox balance for chrome flour will be in favour of Cr2+ and Cr3+, whereas in an oxidising environment, it’ll be in favour of Cr3+ and Cr6+. The effect of the lowest oxidation number chromium is minimal, and it is regarded as a rarer oxidation state. On the other hand, Cr6+ provides a yellow colouration which in concert with Cr3+ will ‘dilute’ the green colour to a somewhat more muted tone. Notable is the fact that Cr6+ is harmful to humans, so care should be taken to minimise exposure to any gases arising from the melt.
The interaction of chromium and the amber chromophore is particularly interesting, as colours such as olive and antique greens require both the amber and chromium chromophores - the modulation of amber by the chromium causes a shift in the position of the chromium-iron equilibrium, and the commensurate colour changes. It is this effect that gives rise to brown-green coloured glasses such as feuille morte.
Georgia and emerald greens are produced using oxidising methods using both pyrite and chromite, whereas all other greens are produced using reducing methods. Emerald green requires around 6 kg of chrome flour per tonne of sand(11). Georgia green uses less than one tenth this amount of chrome flour. Feuille morte glass uses chrome flour and pyrite in a 1:2 ratio - 1 kg of chrome flour and 2 kg of pyrite per tonne of sand.
Chrome flour is preferred as the agent to modulate oxidation levels as the material it replaced, potassium dichromate, is toxic and dangerous to handle.
Copper Oxide
Under oxidising conditions, copper oxide will remain in the cupric oxide form (CuO). The Cu2+ ion has a green-blue colouration. Under reducing conditions, however, copper oxide will be present in the cuprous oxide form (Cu2O), with the Cu+ ion responsible for an intensely red colour. Therefore, to achieve red or black glass, it is imperative to maintain reducing conditions via a low batch redox number. Black colours are typically achieved after the glass has undergone a process called striking, which is where the glass is heated to ca. 600 - 650 °C post-annealing, more can be read here.
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