|Monthly Tech-Tip |
Alternate Names: Fe3O4, Black Iron Oxide, BIO, Magnetite Powder, Iron(II,III) Oxide
Description: Ferrous ferric oxide, Synthetic Magnetite
The 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.
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).
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.
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.
Example of 5% black iron oxide (left), red iron oxide (center) and yellow iron oxide (right) added to G1214W glaze, sieved to 100 mesh and fired to cone 8. The black is slightly darker, the yellow has no color? Do you know why?
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 at Wikipedia
A page showing the amazing range of colors of iron oxide
Natural Red Iron Oxide
YLO-1888D Yellow Iron Oxide
Iron Oxide Yellow
Iron Oxide Red
Red iron oxide is the most common colorant used in ceramic bodies and glazes. As a powder, it is available in red, yellow, black and other colors.
Spanish Red Iron Oxide
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
Metallic based materials that impart fired color to glazes and bodies.
Iron oxide and Hematite
|Oxides||FeO - Ferrous Oxide|
A black, hard, dense and stable crystalline mineral form of iron oxide.
|Frit Softening Point||1420C M|
|Density (Specific Gravity)||5.20|
|pH for dry powder||4-8|
|By Tony Hansen|
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