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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 Tolerance 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.
You may have encountered black oxide-coated steel frequently and not thought much about it because it looks unremarkable. At Xometry we instantly quote metal black-oxide coated parts all the time; it remains one of our most popular finishes. But the truth is, it’s an old, simple, and very powerful method for rust-protecting steel that extends the cosmetic and functional life of dry-environment steel parts for many years.
Black oxide, also known as blackening, is a type of chemical-conversion coating by deliberate and controlled oxidation, applied to ferrous metals for corrosion resistance and aesthetic appeal. It is often coupled with an oil film for improved oxidation resilience. In addition to these characteristics, black oxide coatings offer good abrasion resistance due to the hardness of the oxide film, and they also improve the thermal stability of parts. This is typically achieved in a more cost-effective manner than other coating techniques, such as electroplating or painting.
If you want to learn more, please visit our website Black Iron Oxide Pigment.
Black oxide coating is commonly preferred for steel components that will not be subjected to moisture or chemical exposure. It quickly turns to brown oxide under wet conditions and will suffer badly from acid marking when handled.
This article will offer you a solid background in black oxide coating, its importance, purpose, applications, and advantages.
Black oxide coatings are a type of chemical-conversion coating and are created through a chemical reaction between iron atoms on the substrate surface and oxidizing salts in a chemical bath. This forms a moderately integrated and well-attached layer of Fe3O4 magnetite or black iron oxide. Black oxide coating is typically applied to ferrous materials like steel or iron to improve the substrate’s corrosion and wear resistance. Additionally, the coating adds an aesthetically appealing matte black finish to parts, which is why the process is also sometimes referred to as “blackening.” This class of coating has been applied by metalworks and blacksmiths for centuries. The process of blueing steel and iron, which means heating it in the air to induce the resulting cobalt-blue color, is chemically similar to black oxide coating and has been used since the Iron Age for cosmetic and corrosion resistance purposes and to impose a less robust but easier to achieve Fe2O3 coating on the metal.
In this day and age, black oxide is used in various industries and applications due to its unique combination of properties. Xometry’s customers in aerospace, automotive, electronics, and firearms frequently make use of the finish for their custom parts.
Black oxide coating is distinct from most other metal coatings in its application to parts. Unlike other finishes that Xometry offers, such as painting and powder coating — processes that blanket the substrate with a protective layer that is at best only mildly chemically bonded to it — black oxide coatings are created through a direct chemical reaction of the surface atoms to form a closely coupled film. Black oxide is present as a distinct layer on the surface, but it transitions from oxide to metal in a progression through the thickness. As a result, it is well integrated with the metal substrate, offering a more robust and uniform finish.
While black oxide coatings are generally the more eco-friendly option compared to other coating types like painting or electroplating, the black oxide coating process utilizes some chemicals that are harmful to the environment. Chemicals such as nitrates, sodium hydroxide, and others are hazardous but generally less harmful than chemicals used in other coatings. The environmental impact of the process pivots on the disposal of waste and wash-contaminated fluids. If this is managed in compliance with best practices, no toxic chemicals are released.
As mentioned previously, black oxide functions by a chemical conversion process applied to the surface of ferrous metals. The steps for the process of applying black oxide coatings are:
The resulting black oxide coating provides mild corrosion resistance, surface albedo (reflection, luster, etc.), and improved aesthetics. The process is also exploited for its ability to maintain dimensional accuracy and conductivity while enhancing durability and scratch resistance.
We see customers from a wide range of industries who use black oxide coatings for a variety of component applications. These applications most commonly include:
There are two basic classifications of black oxide coatings — hot and cold.
We recommend using a black oxide coating for a number of reasons:
Despite its many advantages, black oxide coatings do have some drawbacks:
If these shortcomings have you considering other coatings, feel free to explore more of our metal finishing options via our metal finishes gallery.
There are some common misconceptions about black oxide that we’d like to clear up. Firstly, it is a widespread belief that all black oxide coatings provide adequate corrosion resistance in humid or acidic environments. If the sealing step is skipped, then the blackening process alone will not improve resistance to corrosion. We recommend ensuring the surface is sealed and is maintained by re-oiling over time, as well as keeping parts dry to assist with keeping corrosion at bay. Ultimately you may need to consider other types of coatings and finishes for especially wet or corrosive environments where greater protection is needed.
Another misconception is that black oxide coatings suffer from weak adhesion and are prone to chipping or peeling. When applied correctly by following proper surface preparation and process parameters, black oxide forms an integral layer with the metal, which ensures strong adhesion.
Finally, some believe that black oxide can significantly affect part dimensions. While the coating does have a measurable thickness, typically between 0.5 and 2.5 microns (0.” - 0.”), it is negligible for most parts and applications.