FAQs on Iron Oxide

What is the difference between Natural and Synthetic Iron Oxides ?

Three different iron oxide minerals normally are the basis for all Iron Oxide Pigments.These minerals are:Goethite (Yellow Iron Oxide) Hematite (Red Iron Oxide) Magnetite (Black Iron Oxide) These minerals can be produced naturally by geologic activities or can be synthetically produced in chemical reactions.

The color shade of the iron oxide is determined by the size of the individual mineral crystal.
Reds - Small Particle/Yellow Cast - Large Particle/Blue Cast Yellows - Small Particle/Green Cast - Large Particle/Red Cast Blacks - Small Particle/Brown Cast - Large Particle/Blue Cast
While the iron oxide mineral in both Naturals and Synthetics is similar, Natural Iron Oxides contain contaminates which often reduce their tinting strength in comparison to their Synthetic counterparts.

These contaminants are of two types:

(1) Those with coloring properties and those that act as an extender. The coloring contaminants are most often Manganite, MnO, which gives the Umbers their dark colored masstones.

(2) The non-coloring contaminants are most often natural extenders used in industry as industrial fillers, including clays, talc, and calcium carbonates.

How does particle size affect a pigment’s dispersibility ?
The color of a given pigment is determined by its particle size and shape. However, pigments are usually found as clusters of particles rather than individuals. These pigment clusters influence the tinting strength and grind that a given pigment can achieve.

The particles in the clusters are held together by many different mechanisms. The most common of these is the soluble material that cemented pigment particles together in the drying process. Another mechanism is the presence of electrical charges between individual particles. As pigment particles become smaller in size, decreasing in surface area, the strength of the electrical charge increases, making the pigment more difficult to disperse.

Pigment manufacturers can vary the dispersibility of a pigment by different grinding processes

which break down these pigment clusters into smaller clusters and particles.
The final customer will also break down these various pigment clusters in their processing. The degree of de-agglomeration depends on the energy of their dispersion process.
The actual grind of a product depends not only on the pigment but on the vehicle system in which it is being dispersed and the grinding technique used to disperse the pigment in the vehicle.

What are the heat stability properties of Pigments?
Most of the pigments that manufacturer markets are heat stable in most applications, with the exception of two important types of material. These include any pigments based on Yellow or Black Iron Oxides. Yellow Iron Oxides, FeO-OH, will begin to change into Red Iron Oxides at temperatures above 350 degrees Fahrenheit. Black Iron Oxides, Fe304, will begin to change into Red Iron Oxides at temperatures above 300 degrees Fahrenheit.

Groups of pigments containing these materials include:
Ochre Raw Sienna Synthetic Yellow Iron Oxide Synthetic Black Iron Oxide Raw Umber Many Brown Blends of Iron Oxides

The Iron Oxide Industry has addressed the problems of heat stability by developing heat stable yellows and blacks. These are not pure iron oxides, but rather combinations of yellow or black iron oxides processed together with other materials. These heat stable yellows are Zinc Ferrites; the blacks are Iron Manganese Oxides.

Can One manufacturer’s pigments be combined with other pigments?
Yes, any of manufacturer’s pigments can be combined with other pigments; this is how they are normally used. Most often the pigment is combined with the white pigment Titanium Dioxide to produce a tint shade.

This process is often used when a gray color is desired. Often color formulators will make a gray by combining a carbon black with a TiO2 white and shading to the tone of gray desired. A more effective gray can be achieved by matching the undertone of the dark pigments available with the desired end shade. This process requires a lot less expensive shading. Many of the greenish grays that we associate with office equipment, like computers, are achieved this way by mixing raw umber with TiO2 white.

Raw Umbers are also utilized as toning pigments because of their neutral shade. This enables a formulator to darken a color without affecting the chromaticity of the brighter color. This is often useful when formulating pastel shades using bright organic pigments.