Chrome Flour in Glassmaking
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).
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. 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 sulphate, whereas low levels of sulphate are retained in reduced glasses. The overall redox number is a product of sulphide levels and levels of other redox active components, including chromite(6).
Chrome-Pigmented Green Container Glasses
Oxidised glasses have negative redox numbers, higher levels of retained sulphate 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 is used compared to chromite.
Reduced green glasses have positive redox numbers, low levels of retained sulphate 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).
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. In addition to container glass, chromite is a colourant in other glass types:
Plate (Architectural) Glass
Green-coloured plate glass has historically not been the most desired, although it was the first tinted plate glass product to be made. Early attempts at using chromite to colour molten glass, which was then pulled from the melt are reported in patent literature(14). When considering the modern process of manufacturing plate glass, whereby molten glass is poured in a thin stream onto molten tin, chromite can be used in the melt in the same way as any other colourant, for example cobalt. Iron compounds are the leading colourants in contemporary green plate glass. It remains true, however, that the major use for green-coloured glass outside of the container space is in the automotive field.
Tinted glass is often used in cars to reduce the impact of solar transmission into the vehicle, in addition to aesthetic and privacy reasons. Early tinted glass for vehicular applications required a pale shade of green, and thus oxides of iron were used(15). More recent attempts utilise chromite as a pigmenting compound alongside oxides of iron(16). In addition, chromite spinel can be used in concert with copper compounds when applied to automotive glasses as an enamel coating(17), or even as a sol-gel-type film(18). It is postulated that the same qualities afforded to UVA glass by the complementary presence of chromite and iron oxides are contributory to the inhibition of solar radiation reaching the inside of the vehicle.
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 sulphate. 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).
- Green coloured glass is achieved using iron chromite as the primary colourant. Overall colour is determined by the identity of other additives, the overall composition and redox balance in the melt
- Chromite is used to produce various shades of green container glasses, chiefly used for foodstuffs
- In addition, there have been some uses of chromite for the colouration of architectural glass, and it is a common pigment in automotive glasses
- As a tool in glass manufacture, chromite provides for better heat tolerances, and in some instances can behave in a flux-like manner
1 I. C. Freestone and M. Bimson, J. Glass Stud., 2003, 45, 183
2 Ü. Güldal and C. Apak, J. Non-Cryst. Solids, 1980, 38, 251
3 D. A. C. Manning, Raw materials for the glass industry, in Introduction to Industrial Materials, Springer, Dordrecht, 1995
4 H.-B. Xu et al., Environ. Sci. Technol. 2008, 42, 19
5 M. Vilasi et al., J. Am. Ceram. Soc., 2010, 93, 1347
6 W. Simpson and D. D. Myers, Glass Tech., 1978, 19, 82
7 H. N. Mills, J. Non-Cryst. Solids, 1982, 47, 27
8 M. Silva et al., Photodermatol. Photoimmunol. Photomed,, 2009, 25, 181
9 US Patent US2974052, 1960, expired
10 US Patent US3332790, 1964, expired
11 R. Falcone et al., Rev. Mineralol. Geochem., 2011, 73, 113
12 Glass Manufacturing, United States Environmental Protection Agency, Columbus, 1976
13 US Patent US3291621, 1963, expired
14 US Patent US2923636, 1959, expired
15 C. R. Bamford, J. Non-Cryst. Solids, 1982, 47, 1
16 US Patent US20180305245A1, 2019, pending
17 G. E. Sakoske et al. Pressure Forming of Automotive Glass and Challenges for Glass-Ceramic Enamels, Ferro Corporation, Washington, PA, 2019
18 T. Yoneda et al., Sol-Gel Coatings Applied to Automotive Windows in Handbook of Sol-Gel Science and Technology, Springer, Cambridge, 2018
19 A. Hubert et al., Impact of Redox in Industrial Glass Melting and Importance of Redox Control in 77th Conference on Glass Problems, Columbus, 2017
20 P. V. Chartii et al., Glass and Ceramics, 2011, 67, 307
21 W. J. Biermann and M. Heinrichs, Can. J. Chem., 1960, 38, 1449
22 H. Franz, J. Non-Cryst. Solids, 1980, 42, 529
23 M. Pelino et al., J. Eur. Ceram. Soc., 1999, 19, 2641