The Batch Redox Number and its Impact on Coloured Glass Manufacturing
Introduction to the Batch Redox Number
The batch redox number is a tool used by glassmakers as an indication of the properties of the final glass formed, as well as in the melt itself. Glass is comprised mainly of silica, usually derived from sand, alongside sodium carbonate and calcium oxide, as well as a whole host of other additives depending on the desired glass. Redox in this case refers to the balance of oxidative and reductive effects afforded to the glass by the added components, and a number can be attained based on these components. The batch redox number is calculated by glassmakers for every batch produced, and the outcome of the calculation is useful in predicting outcomes such as colour. The batch redox number, therefore, is an empirical measure of the oxidation-reduction state of each batch.
Calculation of the Batch Redox Number
Batch redox numbers are calculated on the basis of a glassmaking process using 2mt of sand. The contributive effects of common glass additives are well known(1), and so it is simply a case of taking these numbers, multiplying by the mass and adding the values together. Sand does not contribute to the redox calculation. An example of simple flint glass recipe is as follows:
A batch of flint glass will require the following components:
Sand 2,000 kg
Slag* 100 kg
Salt (NaSO4) 20 kg
Therefore, the redox number can be calculated:
Slag 100 kg x -0.092 = -9.20
Salt 20 kg x 0.670 = 13.40
-9.20 + 13.40 = 4.20
* in this example, slag provides both calcium oxide and sodium carbonate
A glass melt in this case will have a batch redox number of 4.20. For a more complex melt, the calculation will be longer, taking into account all of the additives.
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.
What Are the Effects of Our Products?
As mentioned, additives to glass are broadly categorised into those which cause an oxidising effect or a reducing effect, thereby moving the position of the redox number. Iron is present in virtually all glasses, so the redox contributions below should be considered in addition to iron when engaging in a glass making process. Individual redox numbers for additives have been determined experimentally.
Red Iron Oxide
Iron is one of the most common additives in glass, being present in the vast majority of commercially produced glasses. Red iron oxide (Fe2O3, iron(iii) oxide, ferric oxide) is one of the most widely used and available oxides of iron, providing Fe3+. As an oxide, addition of it will push the position of equilibrium towards the oxidative side. As a colourant in its own right, iron oxide providing Fe3+ gives rise to a blue-green colouration. Iron oxide has been added to glass melts for many years due to its ability to better conduct heat, therefore reducing the amount of external heating requires(7). When reduced to Fe2+, a blue colouration is observed.
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(8), useful for container glasses containing perishable foods.
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 or deadleaf green.
As an organic compound, anthracite has no effect on glass colouration in its own right, but will shift the position of redox equilibrium towards a reducing environment. Pure carbon and anthracite (85% carbon) have redox numbers of -6.70 and -5.70 respectively. As a reducing environment is preferred in terms of the manufacturing process, anthracite is largely added for this reason.
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 glass is achieved with a heat strike and more can be read here.
Impacts of the Batch Redox Number on the Manufacturing Process
According to research, the furnace temperature can run lower at lower redox value, in addition to being easier to refine(9). At a lower redox, there will be fewer bubbles in the melt due to less oxygen in the melt. As with any chemical reaction, redox equilibria are temperature dependent. In modern glassmaking, where environmental concerns are paramount, recycled glass cullet can be used. Often, this glass is used in powdered form and added to the melt, though it should be noted that as cullet often contains moderate amounts of organic material, addition can cause a reduction in the batch redox number - which may or may not be desired.
- The batch redox number is a tool used by glassmakers to predict and monitor the properties of a glass
- Modulation of the redox number provides for different colours of glass, derived from the balance of the many transition metal-based chromophores
- The redox batch number can be calculated in advance to predict outcomes of a glass process, and is calculated throughout the process to monitor progress
- In addition to composition and redox balance, temperature, use of fining agents have an impact on the overall final glass
1 W. Simpson and D. D. Myers, Glass Tech., 1978, 19, 82
2 W. Thiemsorn et al., Bull. Mater. Sci., 2007, 30, 487
3 N. F. Zhernovaya et al., Glass and Ceramics, 2000, 57, 84
4 C. R. Bamford, Colour Generation and Control in Glass, Glass Science and Technology, Elsevier, Amsterdam, 1977
5 P. Laimbock, In-line Oxygen Sensors for the Glass Melt and the Tin Bath, in GlassTrend, Eindhoven, 2013
6 R. Falcone et al., Rev. Mineralol. Geochem., 2011, 73, 113
7 R. L. Shute and A. E. Badger J. Am. Ceram. Soc., 1942, 25, 355
8 W. A. Weyl, Coloured Glasses, Society of Glass Technology, Sheffield, 1951
9 A. Hubert et al., Impact of Redox in Industrial Glass Melting and Importance of Redox Control in 77th Conference on Glass Problems, Columbus, 2017