Chrome Flour In Ceramics
Chrome flour is finely powdered chromite, itself also known as chromite powder, iron chromite and chrome flour 325. Its occurs in plentiful supply in southern Africa. Although it has numerous applications that rely on its refractory properties, one of its major uses is in the ceramic sector as a pigment and as a glaze. It is also prized for its colouring ability in glass manufacture, offering a pleasing green hue in routine scenarios. It is the principle ore of chromium metal. As a pigment, chrome flour has been intentionally used since the 1800s.
How is it used in pigments for ceramics?
With recent architectural and interior design styles veering towards the direction of maximum choice for the end consumer, it is vital that all types of wall coverings are available in a suite of colours. Ceramic tiles, bricks and pavers are no exception. Pigments are added during the manufacturing process.
Chromite has long been associated with brown pigments for ceramics, typically when mixed with zinc iron chromite, this is due to the specific crystallographic sites that these chromophores in the chrome flour occupy. These colours are produced from the mixture of zinc, chromium and iron oxides occupying the tetrahedral, octahedral and interstitial sites respectively of the spinel, when calcined in excess of 1,000°C. Crucially, chrome flour is stable in temperatures exceeding this, important as all ceramics are fired to some degree.
Taking chrome flour and mixing it in equal proportions with iron oxide, for example, will afford a pigment that will colour porcelain tiles black after calcination at 1,200°C. Using limonite (hydrated iron oxide) with chrome flour in a ratio of 3:1 (in favour of chrome) will yield a conventional brown colour - both of these at just 3% by weight in the overall tile. Increasing the ratio of limonite to chrome flour to 1:1 will, after calcination, produce a ceramic with the highly specific shade of ‘chocolate brown’. In the black pigment example, further cost savings can be realised by using waste hematite as an iron oxide source.
Shades of pink are attainable when chromite is used in concert with zinc and aluminium oxides; at low firing temperatures alongside antimony and titanium oxides, chromite affords a yellow-orange hue. Classic green pigments - akin to those found when chromite is used to colour glass - can be obtained with using pure chromite, but care needs to be taken so as not to react with other metals present. If tin is present then a pink colour may be formed, zinc can cause a dirty brown pigment if present in large amounts. The presence of manganese alongside chrome flour can lead to colours varying between red and green depending on manganese concentration.
Grey pigmentation, especially prevalent for bricks and pavers, can be easily obtained using chromite. When used in a roof tile setting, it brings the added benefit of heat resistance. Shades between pale grey and almost black are also available by variations in powder size, with base body amounts of chromite of up to 10% in the ceramic. As a general rule of thumb, the particle size for most pigmenting applications has chromite in the region of 45um or 325mesh
How is it used in glazes for ceramics?
In comparison to pigments, glazes are applied after the manufacturing process and bear more similarities in behaviour to chromite as a glass colourant. Chromite materials have been identified in ceramic glazes dating from the late Bronze Age, providing for a muted grey-ish glaze alongside other materials. In some cases, chromite in a glaze can act as a nucleation point for the crystallisation of other components dissolved in the glaze; potentially giving rise to glazes that are metallic and/or lustrous in appearance.
Acting alone, like in glass, chrome flour provides for the customary green colour in a glaze. Adding more iron oxide will cause a change in favour of a greenish brown. Like in pigments for ceramics, the zinc iron chromite manifold presents a brown colouration in the glaze world. Should lead be present in large amounts, the colour is modulated to a more yellowish tint, whereas in the complete absence of lead, near black is observed. The presence of lead oxide compounds can also have an effect on the perceived vividness of a glaze, even with a high chromite content.
Chromite should be considered as distinctive from chromium oxide - the latter does not have iron present and can be combined with other metals to afford glazes in pink and purple.
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