Chrome flour is a highly useful material that finds wide ranging uses in the glassmaking sector where it is chiefly used as a pigment for green glass manufacture, which itself is mainly used to produce container glass. African Pegmatite is a leading miner, processor and supplier of premium chrome flour products - combining global reach and strong experience to provide the right product, to the right specifications, first time.
Chrome flour (also known as chromite, iron chromite and in its chemical notation Cr2O3) is a widely used pigment in the production of container glass, providing many different hues of green depending on the oxidation state and concentration of chromite used. Container glass is simply that; glass which is used for containing, with bottles being the leading example.
Chromite has been used as a glass pigment since at least the 1840’s, some fifty years after its use as a glaze for ceramics. Its wide adoption has been much more recent due to the requirement of specific grind sizes and temperatures in manufacture, in addition to more advanced glassmaking techniques(1). Green colouration, however, is rarely just due to chromite acting alone. The most complete range of colours is achieved when chromite is used in concert with another compound, such as iron pyrite. Manipulation of the ratios of these compounds leads to a full complement of greens, from feuille morte all the way through to deep, emerald green, due to the interaction of the Cr3+ ⇌ Cr6+ and Fe2+ ⇌ Fe3+ redox pathways playing a role. Chromite refers to any mineral that is an iron chromium oxide. In this case, chromite refers exclusively to iron(ii) chromite, FeCr2O4. For the classical emerald green colouration, chromium is present exclusively in the +3 oxidation state, giving rise to light absorption bands at 450 and 650 nm(2).
Chromite refers to any mineral that is an iron chromium oxide. In this case, chromite refers exclusively to iron(ii) chromite, FeCr2O4. For the classical emerald green colouration, chromium is present exclusively in the +3 oxidation state, giving rise to light absorption bands at 450 and 650 nm(2).
Soda lime glass is the most used glass type for container glass, and is predominantly composed of silica, sodium carbonate (‘soda’) and calcium carbonate (‘lime’), alongside much smaller quantities of other compounds added for strength, durability and colour. Its primary component is silica, the other major components are added as ‘fluxes’; present to ensure a lower melting point and more easily controlled viscosity, as pure quartz-type glass (just silica) can be difficult to work with.
As a mineral, chromite is naturally found in below ground deposits, with the highest quality supplies being found in southern Africa(3). It is important that when using chromite, it is stored and handled properly. Under certain conditions, chromium(iii) can oxidise to chromium(vi), which is highly toxic to humans, and lacks the same glass pigmentation characteristics. Though should an excess conversion to chromium(vi) be noticed, conversion back to the safe chromium(iii) oxidation state is possible using chemical means(4). The presence of hexavalent chromium will give rise to a strongly yellow colouration. It is noteworthy that in the soda lime glass melt, chromium is also present in the +2 oxidation state, at least temporarily(5).
Oxidised and reduced glasses refer to the overall redox state in the melt that is used for glass production. Highly oxidised glasses retain high levels of sulfate, whereas low levels of sulfate are retained in reduced glasses. The overall redox number is a product of sulfide levels and levels of other redox active components, including chromite(6).
The nature of chromite as a refractory material precluded it somewhat from use in glasses owing largely to expense relative to other materials, thus it is most likely the case that glass pigmentation is the close second most prevalent use case. Its wider adoption in the glass making sector is due to advancements in milling and processing and an overall greater availability, driving down prices. Chrome is, regardless of cost, one of the best glass pigments around. High quality chromite occurs naturally in ore form and is relatively easily mined, with modern production scales and contemporary processing techniques making chromite one of the most popular choices for coloured glass manufacture.
Chrome-Pigmented Green Container Glasses
Oxidised glasses have negative redox numbers, higher levels of retained sulfate and are associated with traditional green coloured glasses, such as emerald and Georgia greens. Emerald green often possesses a redox number of -5 and is made using around 0.2 wt% chromate and 0.5 wt% iron oxides. Georgia green is a light green, almost blue-ish hue, colour of glass associated with Coca-Cola bottles. It takes this colour due to the ratio of chromite and iron oxide highly in favour of the iron, with quantities of chromite in the region of 0.05 wt%. Georgia green glass typically has a dominant wavelength at circa 555 nm(7). Dead leaf - or feuille morte - is achieved when twice the amount of iron oxides are used compared to chromite.
In the redox environment, the green character associated with chrome(iii) will predominate given little outside influence. The overall balance is between reduced chromium(ii), ‘standard’ chromium(iii) and the highly oxidised (and toxic) chromium(vi). Chromium(iii) is the oxidation state of the metal centre in chromite. It is also the most common oxidation state of the metal.
In a reducing environment, the redox balance for chrome flour will be in favour of chromium(ii) and chromium(iii). On the other hand, in an oxidising environment, in favour of chromium(iii) and potentially also the highly toxic chromium(vi). The lowest oxidation state of chromium does not impact the formation of any chromophores and has no impact on colour - it is regarded as a rare oxidation state. The highest oxidation state, chromium(vi) provides a strongly yellow colouration. When present with chromium(iii), the green colour will take a somewhat more muted tone. Such yellow colour production is indicative of chromium(vi) production.
Reduced green glasses have positive redox numbers, low levels of retained sulfate and are associated with ‘UVA glass’ - that is, yellow-green glass that is resistant to ultraviolet radiation. This is particularly attractive to the glassmaker as items contained within UVA glass are shielded from radiation which may cause them damage(8). UVA glass is of a significantly less green colour than emerald green glass, this is due to the presence of small quantities of stabilised hexavalent chromium, which moderates the green colour from chromium(iii) using its own yellow. As little as 0.1 wt% of chrome oxides by mass is sufficient to produce UVA green(9,10), the contribution of iron oxides is in the region of 0.4 to 1 wt%(11).
Rarely is chromite used as a pigment on its own, however. Attainment of colours such as olive and antique greens requires both the amber and chromium chromophores. The modulation of amber chromophore by the chrome chromophore causes a shift in the position of the chromium-iron equilibrium, and therefore gives rise to the commensurate colour changes. Manipulation of this redox balance is relatively facile considering the ease of adding more chromite or iron sources.
Uses of Green Glass
Container glass is made by cooling a melt in/over a mold, or via a glassblowing-type process. In the molding process, cycling finishing/annealing processes occur. Annealing removes points of stress in the glass(12). Green container glasses are popular for both aesthetic reasons and due to their ability to prevent foodstuffs from spoiling due to the moderate ultra violet protection afforded by the chromite in the glass(13). It should be noted however, that superior UV performance is attained with amber coloured glasses, but such protection comes at the expense of being able to easily visually inspect the contents. Green glass - produced using the amber and chrome chromophores - is therefore used as a highly effective and scalable compromise.
In addition to container glass, chromite is a colourant in other glass types such as in plate and automotive glasses. The major difference between plate and container glasses is that the former is made by molten glass poured onto a flat panel or onto a molten metal bath, which is then often passed through rollers for the annealing process. Automotive glasses are almost a halfway house between plate and container glasses, with their intricate shapes made by rolling warm plate-type glass over specialist rollers and/or via pressure forming. Notably, plate and automotive glasses are not tolerant of recycled glass, such as cullet.
Impacts on Manufacturing and Usage
Lower redox numbers are associated with more efficient manufacturing processes, such as at lower temperature(19). Glasses will be easier to refine if there is less sulfate. Heat tolerance of glass is important; container glasses need to not shatter or crack on cooling and need to be able to withstand a moderate amount of heat when they have fully cooled and in conventional use. It has been shown that higher quality glasses often have a slightly higher iron oxide content - in the case of green glass, this oxide can be provided by the chromite(20). It is imperative that no traces of sulfuric acid are present during the manufacturing process, as it can rapidly cause the formation of poorly soluble chromium sulfate compounds, which will severely inhibit glass production(21).
In general usage, plate/flat glass degradation is a product of its environment (weathering), and container glass its contents. As with during the manufacturing process, highly concentrated acids should not be stored in chromite-coloured container glass(22). In a pseudo-flux different application type, when small amounts of chromite have been added to iron-rich glass melts, it has been found that the rate of spinel formation is increased, leading to an enhanced degree of crystallisation in the finished glass(23). Chromium(vi) is highly toxic even in small quantities, and therefore care should be taken to ensure it isn’t produced, by ensuring conditions are kept to prevent runaway oxidation from Cr(iii) to Cr(vi).
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