Frequently Asked Questions - Hoover Color
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: 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. The non-coloring contaminants are most often natural extenders used in industry as industrial fillers, including clays, talc, and calcium carbonates.
As processing technology continues to improve, the tinting strength of the Naturals approaches that of the Synthetics. This results from the ability to remove higher amounts of the contaminates contained in the Naturals.
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, such as Hoover Color, can vary the dispersibility of a pigment by different grinding processes which break down these pigment clusters into smaller clusters and particles. Often the terms Atomized, Micronized or Jet-Milled are used to describe pigments that have undergone this milling step. These terms are used interchangeably to describe pigments that are easily dispersable under high speed dispersion. 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.
At Hoover Color, we measure the dispersibility of each pigment by evaluating both its tinting strength and the amount of grit greater than 44 microns or 325 Mesh. Most of Hoover Color’s standard blended iron oxide products have grit levels of less than 0.1%. 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. In general, if a Hoover Color Iron Oxide is dispersed in a Long-Oil Alkyd on a Cowles type High-Speed Disperser, one would expect to achieve a grind of 5 to 6 Hegman. Our Atomized Pigments, such as our 510BU Bt.Umber, 417RU Raw Umber, and 778BN Brown, can achieve better dispersion because of their better grind. They can normally achieve a 6 to 7 Hegman in our standard high speed dispersion test.
The term lightfastness is defined as the ability of a pigment to resist fading after exposure to sunlight. Almost all of the pigments that Hoover Color sells are lightfast. This includes all of our Iron Oxide Pigments, both the Naturals and Synthetics, Chromium Oxide Greens, Mixed Metal Oxides, BC-35 Lampblack Substitute, and Bone Blacks. Most prehistoric art uses the same mineral pigments as those sold by Hoover Color. Therefore, we have as much as 30,000 years of exposure information confirming the permanance of these colors.
There are two exceptions to this rule. The first is the Tuscan Red and Orange Pigments that we manufacture. These pigments are blends of iron oxides with brighter organic pigments. These organic pigments are less durable then then the iron oxides and will fade over time. However, even though they will fade over long-term exterior exposure, they will never fade completely. For example, an organic red would fade to an almost whitish pink, while Tuscan Red would only fade to the duller, yet permanent, iron oxide red.
The other nonpermanent pigment type that Hoover Color sells is our Van Dyke Brown materials. Both the real Van Dyke and our 925 Van Dyke Natural Dye are not permanent iron oxides, but are based on lignite and humic acid. Long term exterior exposure will fade these materials.
Most of the pigments that Hoover Color 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:
- 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.
All Iron Oxide pigments are very chemical resistant. Only in very highly concentrated acids are these materials soluble. Mixed Metal Oxides and Chromium Oxide Greens are also very chemical resistant. The only materials that Hoover Color offers with limited chemical resistance are the Tuscan Reds and Orange and the Van Dyke and 925 Browns.
All of the pigments that Hoover Color markets are compatible with water-based systems, as well as any water reducible or solvent based systems.
When most people think of Hoover Color, they think of browns. However, Hoover Color offers a wide range of inorganic pigments including yellows, oranges, reds, brown, greens, blues, violets, and blacks.
Yes, any of Hoover Color’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.