The Batch Redox Number and its Impact on Coloured Glass Manufacturing
Modern coloured glass making revolves around two key aspects: the batch redox number and quality pigmentation materials. African Pegmatite is a leading supplier of the purest additives for coloured glass production. With purity comes a more accurate determination of batch redox number, leading to a more efficient production process.
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
|0 to 5
|0.10 to 0.40
|-15 to 0
|0.40 to 0.65
|-25 to -15
|0.60 to 0.75
|-30 to -20
|0.75 to 0.90
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. It should be noted that adding compounds to the melt needs to be executed with great care. Any addition can cause a drop in temperature, which could lead to devitrification - i.e. crystallisation of or within the molten glass(7).
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(8). When reduced to Fe2+, a blue colouration is observed. One of the major advantages to using red iron oxide to achieve pigmentation is that as a source of iron, it does not provide excess sulfides when these are not required.
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).
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.
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. It has been reported that anthracite can eliminate imperfections caused by errant gases in the melt, and can lower the temperature required of the melt especially when used with chrome and manganese pigments(12). Anthracite is responsible for enhancements to the glass melt chemistry. It is considered a good source of carbon that does not release harmful gases when heated in the melt. In glasses containing sodium sulfate, anthracite is particularly adept at reducing gaseous imperfections.
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.
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.
A Note On Manufacturing And Additives
When considering which pigments to add to achieve a certain colour, it is imperative to also consider other additives that may be added to the melt at the same time as the pigment:, such as the popular additive feldspar. Feldspar is a material that is added as a ‘flux’, that is, it lowers the temperature required to produce the melt. When combined with a mixture tailored to produce a reducing environment (lower batch redox number), the feldspar further lowers the temperature required in the furnace. Hardness and durability of the glasses are also increased.
Glassmakers also need to consider pre-existing additives and pigments if using cullet or recycled glass in their manufacturing process. The use of cullet and re-used glass is a cost effective and environmentally responsible measure that is often employed in the manufacture of container glasses. However, cullet may be sourced from already coloured glass. This needs to be considered when incorporating - and typical values for batch redox numbers used to make the original glass should be sought. Pre-existing glass and cullet tends to be on the reducing end of the scale with respect to its batch redox contribution, owing to pigments and other reducing materials present such as organics.
- 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
- Such chromophores form only under certain redox chemistries, so it is useful to be able to use additives that do not add pigment themselves but instead can push the redox position in a certain direction
- 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
Ensuring the highest quality coloured glass means buying the best pigments and additives. African Pegmatite is the trusted industry partner that provides the purest oxides, minerals and materials for the most consistent pigmentation every time - oftentimes offering enhanced mechanical and chemical resistance properties to the glass itself.
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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
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10 Glass Manufacturing, United States Environmental Protection Agency, Columbus, 1976
11 W. Vogel, Glass Chemistry, 2nd ed., Springer-Verlag, Heidelberg and Berlin, 1994
12 A. Koroviakovskii, Masters thesis, Lappeenranta University of Technology, 2016
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