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Alternate Names: Fe3O4, Black Iron Oxide, BIO, Magnetite Powder, Iron(II,III) Oxide
Description: Ferrous ferric oxide, Synthetic Magnetite
Oxide Analysis Formula Fe2O3 63.40% 1.00 FeO 28.50% 0.88 LOI 8.00%n/a Oxide Weight 231.92 Formula Weight 252.09The black iron used in ceramics is generally this synthetic form (the natural equivalent mineral magnetite contains 5-15% impurities). Synthetic black iron is much more expensive than the natural finely ground material (-200 mesh) but if there are good reasons for its use and percentages in the product recipe are low enough the cost may be justified. In ceramics, black iron is used as a source of Fe (in preference to red iron) where its black raw color and its better distribution properties are needed. For example, Alberta Slip is a recipe of raw clays and minerals intended to duplicate Albany Slip. The recipe calls for a small amount of iron oxide because the clay blend does not fire to quite as dark of a color. Since the original Albany Slip powder was a dark grey, black iron (rather than red) is employed in the Alberta Slip recipe to match this color better and provide the needed iron to the fired product.
The chemistry shown here is not the actual, synthetic black iron is almost pure Fe3O4. This chemistry is intended to work with INSIGHT where it is normal to define only FeO and Fe2O3.
Synthetic black iron is fluffier and lighter than synthetic red iron oxide (a bag of black iron is much larger than a bag of red). It is a very fine powder, 100% will easily wash through a 325 mesh screen. Synthetic black iron does not agglomerate as badly as red iron, thus it disperses in glaze slurries better (thus avoiding fired speckle). You can determine which form you have by washing a sample through a 325 mesh screen, if there is residue it is natural magnetite.
The exceedingly fine particle size of iron oxides makes them very messy to work with, they stain the skin in a manner that only soap can remove even though they do not dissolve in water.
High purity, low heavy metal content grades of black iron are available. All forms should have 90% or more Fe3O4. Black iron is also used as a colorant for a wide range of non-ceramic products.
Most synthetic magnetites are made by some type of chemical precipitation (0.2-1 micron particle size). However, a high-temperature drying process can be used to convert synthetic hematite into synthetic magnetite (thus the greater cost). The resultant product of this process has a slightly larger particle size (2-10 microns). 100% pure material would contain 72.3% Fe.
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How can there be so many colors? Because iron and oxygen can combine in many ways. In ceramics we know Fe2O3 as red iron and Fe3O4 as black iron (the latter being the more concentrated form). But would you believe there are 6 others (one is Fe13O19!). And four phases of Fe2O3. Plus more iron hydroxides (yellow iron is Fe(OH)3).
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Iron oxide has been added to a buff burning stoneware clay and samples fired at cone 6. They contain black iron oxide (10%, 5% and 2.5%). Even at 2.5% the raw pugged body is very black and messy to work with. Did they fire black? Or even dark grey? No. We have also tried 20% (mix of black and yellow iron) and the fired color is still dark red. Some form of manganese is needed to get an affordable black burning clay.
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Plainsman M340 buff cone 6 stoneware. 3% iron was added has been added to each of these. The yellow iron (left) is clearly not as concentrated (and not mixed in as well). The black (center) gives a maroon color.
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Example of 5% black iron oxide (left), red iron oxide (center) and yellow iron oxide (right) added to GW glaze, sieved to 100 mesh and fired to cone 8. The black is slightly darker, the yellow has no color? Do you know why?
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The freshly thrown piece on the left front is a medium-temperature plastic stoneware body. Its color comes from a natural iron-bearing clay in the recipe. However, that red clay is becoming much more expensive and difficult to obtain because of trucking availability and cross-border issues. We are investigating the addition of iron oxide to a blend of buff burning materials (which can be tuned to match the working and firing properties of the original body). A 3% iron oxide addition is producing the same fired color. But raw color also needs to be matched. The answer is a blend of red:yellow:black iron oxides. The 3% iron addition in the rear centre piece is a 50:50 mix of red and yellow iron oxides, clearly it is too red. The right front piece is a 40:50:10 mix of red:yellow:black iron oxides. This is getting closer, for the next trial we will try more black and less red.
Iron oxide pigments are widely used in many fields such as construction, ceramics, coatings, plastics, etc. due to their excellent weather resistance, stability and color diversity. Especially in the building materials and textile industries, iron oxide pigments have become the ideal choice for many companies due to their environmental protection characteristics and relatively low production costs.
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However, there are many types of iron oxide pigments on the market, and users often face more confusion when choosing suitable pigments. This article will explore the key factors in selecting iron oxide pigments from the aspects of color, stability, environmental protection, process adaptability, etc., to help companies and individual users make more informed decisions.
Color selection
Iron oxide pigments are rich in colors, covering multiple color systems from yellow to black, which makes them applicable in many occasions. Common iron oxide pigments include yellow iron oxide, red iron oxide, black iron oxide, etc. When choosing the color of the pigment, users should give priority to whether the color saturation and brightness of the pigment meet the use requirements.
For example, in the building materials industry, red iron oxide and yellow iron oxide are common choices for making red bricks and floor tiles because of their bright colors and high durability. For products that require high chroma, such as coatings and inks, red iron oxide and yellow iron oxide pigments with higher saturation and better dispersion are more suitable.
In addition, color stability is also crucial. Different iron oxide pigments behave differently under ultraviolet light, acid and alkali environments. During the selection process, it should be ensured that the color of the pigment will not fade or turn white under long-term exposure to meet the needs of outdoor applications and long-term use.
Chemical stability
The chemical stability of iron oxide pigments is of great significance in many industrial applications. Iron oxide pigments have good acid and alkali resistance and weather resistance, and can adapt to different climates and humidity environments, so they are widely used in construction and road engineering.
However, higher weather resistance or chemical resistance may be required in specific environments. For example, in coastal or highly polluted areas, building materials exposed to strong ultraviolet rays, acid rain, and salt spray may require iron oxide pigments with higher weather resistance and chemical resistance. At this time, iron oxide black may be a better choice because it has better stability in acidic and alkaline environments.
Pigment particle size
Particle size of pigment also affects product quality. Pigments with too large particle size may cause uneven coating surface or rough surface of plastic products. For fine process applications, it is recommended to choose iron oxide pigments with high fineness and uniform particles to ensure the visual effect and quality of the final product.
Cost-effectiveness
Although the cost of iron oxide pigments is relatively low, different brands and production process differences may lead to large price differences. In the selection process, in addition to paying attention to cost, you should also consider the cost-effectieness and choose high-quality pigments that meet the product performance requirements, rather than simply pursuing low cost.
For example, in the food, cosmetics and other industries, although high-quality iron oxide pigments may be more expensive, their safety and environmental protection are more prominent, and they can meet more stringent quality requirements, so they are more cost-effective.
Conclusion
The selection of iron oxide pigments involves multiple dimensions, whether it is color effect, stability, environmental protection or process adaptability, users need to consider comprehensively. Especially for application fields such as construction and textiles, the correct selection of suitable iron oxide pigments can not only improve the appearance and performance of the product, but also enhance the market competitiveness of the enterprise.