Iron (III) oxide is rust, and the pigment behind many ‘earth color’ paints of varying hues. How is it that the same substance can appear yellow, red, purple, brown, green and black? The answer lies in how different material properties of pigment can influence its apparent color, namely: lattice structure, impurities and particle size. I’ll go through a set of familiar pigments in a sensible order to explain the differences in color.
Yellow iron oxide
‘Yellow ochre’ Is the naturally occurring mineral known as ‘limonite’ (yellow→limon) which is itself really a complex mixture of different iron oxide hydrates (mainly ‘goethite’). Rather than describe the complex natural mixtures (which will have natural variations depending on the source), we can describe the the synthetic and relatively pure version we would expect to find in a tube of oil paint (PY 42) that we can call ‘yellow iron oxide’. The natural minerals are generally only used in applications where consistency and purity are not major concerns.
There are in fact a number of processes for making yellow iron oxide in large vats, but the Penniman-Zoph process is the most common. The process begins with iron metal and sulfuric acid, the hydrogen ions are reduced to hydrogen gas and you get the iron sulfate salt:
Fe + H₂SO₄ → FeSO₄+H₂↑
FeSO₄ will then dissociate and oxidize in water(this isn’t such an easy reaction to follow, but I’ve put it here for completeness, the iron oxidation state goes from oxidation 2+ to 3+):
FeSO₄ + 1/2 O₂(g) + H₂O → α-Fe(OOH) + H₂SO₄
The α-Fe(OOH) is the yellow iron oxide, and it precipitates out as small nuclei which continue to grow into larger crystals as the same two reactions keep going with scrap iron metal in the reactor. Those crystals are sent off for grinding for the pigment powder that winds up in your paint.
The α-Fe(OOH) is sometimes written as the ‘hydrate’ α-Fe₂O₃•H₂O (see how it’s really just double the same atoms?) which is a bit misleading. Oxygen and hydrogen are in there, but are really are part of atomic lattice, and it can’t be dried out the way wet laundry can.
Like any chemical process, there is an element of craft, precise conditions and impurities can affect the final color of the product.
Red iron oxide
We can apply super high heat to Fe₂O₃•H₂O to rearrange the atoms into a new lattice and release water to get α-Fe₂O₃, which is red iron oxide. The natural mineral is known as haematite (haem→blood→red). This is the pigment in indian/light/english/venetian-red.
Purple Ochre
Is just red iron oxide ground to a coarser particle size. Particle size affects light scattering in a wavelength selective way.
Umbers and Siennas
If we add manganese oxide to iron oxide, we get umbers for dark brown/green colors. These paints also dry faster since the manganese catalyzes the drying of oil. Siennas contain less manganese oxide compared to the umbers.
Mars black
Can be made the same way yellow iron oxide is made but stopping the oxidation process early to get a mix of iron 2+/3+ oxidation states instead of just 3+.
I am reasonably sure that all the iron oxide pigments in artist paints, unless specially noted, come from the synthetic iron oxides given the volume they are manufactured for use in construction etc.
(no credit for photo)
Additional links:
https://www.usgs.gov/centers/nmic/iron-oxide-pigments-statistics-and-information
