Red iron oxide is the most common colorant in ceramics and has the highest amount of iron. It is available commercially as a soft and very fine powder made by grinding ore material or heat processing ferrous/ferric sulphate or ferric hydroxide. During firing all irons normally decompose and produce similar colors in glazes and clay bodies (although they have differing amounts of Fe metal per gram of powder). Red iron oxide is available in many different shades from a bright light red at a deep red maroon, these are normally designated by a scale from about 120-180 (this number designation should be on the bags from the manufacturer, darker colors are higher numbers), however in ceramics these different grades should all fire to a similar temperature since they have the same amount iron. The different raw colors are a product of the degree of grinding.
In oxidation firing iron is very refractory, so much so that it is impossible, even in a highly melted frit, to produce a metallic glaze. It is an important source for tan, red-brown, and brown colors in glazes and bodies. Iron red colors, for example, are dependent on the crystallization of iron in a fluid glaze matrix and require large amounts of iron being present (eg. 25%). The red color of terra cotta bodies comes from iron, typically around 5% or more, and depends of the body being porous. As these bodies are fired to higher temperatures the color shifts to a deeper red and finally brown. The story is similar with medium fire bodies.
In reduction firing iron changes its personality to become a very active flux. Iron glazes that are stable at cone 6-10 in oxidation will run off the ware in reduction. The iron in reduction fired glazes is known for producing very attractive earthy brown tones. Greens, greys and reds can also be achieved depending on the chemistry of the glaze and the amount of iron. Ancient Chinese celadons, for example, contained around 2-3% iron.
Particulate iron impurities in reduction clay bodies can melt and become fluid during firing, creating specks that can bleed up through glazes. This phenomenon is a highly desirable aesthetic in certain types of ceramics, when the particles are quite large the resultant blotch in the glaze surface is called a blossom.
Iron oxide can gel glaze and clay slurries making them difficult to work with (this is especially a problem where the slurry is deflocculated).
Iron oxide particles are very small, normally 100% of the material will pass a 325 mesh screen (this is part of the reason iron is such a nuisance dust). As with other powders of exceedingly small particle size, agglomeration of the the particles into larger ones can be a real problem. These particles can resist break down, even a powerful electric mixer is not enough to disperse them (black iron oxide can be even more difficult). In such cases screening a glaze will break them down. However screening finer than 80 mesh is difficult, this is not fine enough to eliminate the speckles that iron can produce. Thus ball milling may be the only solution if the speckle is undesired.
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Iron Oxide Red 110, Iron Oxide Red 130, Iron Oxide Red 190 are most popular, also Iron Oxide Red 180, Iron Oxide Red 120, Iron Oxide Red 160, Iron Oxide Red 140, Iron Oxie Red 101 are well used in ceramics.
More than 10,000.00kg in Chinese market.
Currently, there are two production process of synthetic iron oxide red, sulphate process and mixed acid process, mixed acid process is also called nitric acid process. The quality of later one for ceramics is better than first one.
Iron oxide red is used as colorant into ceramics, it is the pigment that forms the colors such as brown series, black series of ceramic glaze, cobalt iron series of glaze, black , coffee color, dark brown of ceramic body composition. Brown series play an import role in ceramic industry, the crystal structure of brown series is Fe-Cr-Zn-Al and Fe-Cr-Zn, Fe III exhibits red color, it shows golden yellow to black brown after combined with chrome oxide green
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The main colors for ceramic wall brick and floor tile production are black, yellow and coffee tone. Black is more than 35% among ceramic body colors, iron oxide red is the main raw materials of ceramic body black formular, the content is 50-80%. The low quality of iron oxide red has bad heat resistance, and it is only suitable for lower quality black ceramic body.
Currently, coffee ceramic body is produced by mixing buring of chromite and iron oxide red, yellowish iron oxide red such as 101, 110 and 120 makes red and yellow tone ceramic body, while bluish iron oxide red forms black coffee ceramic body, especially iron oxide red 180, as it is the most dark violet iron oxide red.
The best iron oxide red for brown ceramics is the one with bright red appearance and dark violet tone, especially the formular of zirconium iron oxide red for glaze and coral red for ceramic body, iron oxide red produced by mixed acid process.
It is required more than °C for underglaze pigments, this also make the pigments high stability, only some complex inorganic pigments can meet this heat resistance, such as Cobalt Blue (pigment blue 28, pigment blue 36), Chrome Black (Pigment Black 30), Chrome Titanium (pigment brown 24), Chromium Iron Oxide (pigment brown 29), Zinc Iron Brown (pigment brown 33), Cobalt Green (pigment green 26, pigment green 50), Nickel Titanium Yellow (pigment yellow 53), Manganese Titanium (pigment yellow 164). While on-glaze application, heat resistance is 600-800°C, there are lots of pigments meeting reaching the temperature, besides above pigments, these pigments can also be used, Cadmium Red (pigment red 108), Manganese Ferrite Black (pigment black 26), Copper Chromite Black (pigment black 28), Zinc Ferrite Yellow (pigment yellow 119). When applicable for ceramic body, there are even more pigmetns, it requires lower heat resistance. All of these pigments give very colorful ceramics.
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