I am an artist who is always exploring different materials, tools, and processes.
What Are Fillers and Pigment Extenders?
So what are fillers and pigment extenders? Why are they used in paint? There are two main reasons why these kind of materials are used in paint. One is for economical reasons because pigments are very expensive. The second reason is more important: Many fillers and extenders improve the performance and handling of the paint while being applied.
Most fillers and pigment extenders are white or colorless powders. Many are naturally occurring minerals with widespread occurrence. Usually, though, the mined minerals need to go through processing to remove impurities. Others are synthetic to produce a product ready for paint formulation.
Calcium Carbonate (whiting, chalk, drop chalk, or limestone) is a white powdery substance. It is a simple compound of the element calcium bonded to carbonate. It occurs naturally as marble and natural chalk. It is mostly made synthetic since it’s very inexpensive to precipitate a calcium chloride solution with a sodium carbonate solution. Mixing both clear solutions forms calcium carbonate in a finely divided form.
How Calcium Carbonate Used for Paints?
It is mainly used as a filler in paints to extend pigments. I find calcium carbonate quite satisfactory when used to extend pigments when preparing dispersions. It lightens the color a bit but is translucent, especially in solvent based paints, which means adding it to another color will not instantly turn them into pastels. It is also used to prepare “French Mineral“ pigment. These are basically an organic pigment mechanically bonded to a calcium carbonate base. This combination is much easier to disperse in water-based paint mediums.
Another use for calcium carbonate is to make painting grounds for canvases and hard board panels used for artists' paintings. Gesso is essentially chalk in an acrylic or vinyl binder that prepares porous surfaces to be painted on by acrylic and oil paints.
Calcium carbonate is used to make chalk paint for application on porous substrates like wood. Adding calcium carbonate to latex or acrylic paint makes the paint gritty and textured. It lends itself well to decorative distressing and chalk paint does not need a primer to be applied.
Talc is a naturally occurring mineral composed of magnesium silicate. It is often refined to a bright white powder and is used as a thickener, lubricant, and as a filler in paint and 2 part epoxy resin. In paint, talc adds texture and body to the paint vehicle. Also, talc is often used to make universal tints and pigment dispersions. Its unique structure can aid in the dispersion of organic pigments.
Like calcium carbonate mentioned above, talc is also useful for making grounds.
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Kaolin clay is a white clay that is used to make porcelain, chinaware, and even toilets. Like all fillers mentioned thus far, kaolin can be used as a painting ground. It can also be ground with organic pigments to make inexpensive paint colorants that are easier to disperse in water-based media.
Kaolin not only extends pigments, but for latex paints, it also contributes to a harder paint film with improved adhesion.
Barium sulfate, also referred by its natural mineral form barite, is a dense white powder. It is mainly used as a pigment in coatings. In oil-based paint it is nearly transparent and used as a filler and extender and to modify consistency. Titanium white, by itself, is an unsatisfactory white pigment for oil paints due to the spongy film it forms. A mix of titanium white and barium sulfate fixes this and is referred to as permanent white. When barium sulfate is combined with zinc sulfide, it’s referred to as lithopone.
Aluminum stearate is a white powdery compound that is a metal salt of a fatty acid. It's often referred to as an aluminum soap. Aluminum stearate is added to oil paints to stabilize the dispersion of pigments and to give the paint body as well. When a high enough concentration is added to a drying oil, aluminum stearate can cause it to thicken and gel. This is the same process of making napalm from petrol.
Not only can aluminum stearate thicken drying oils, but it can also assist in dispersion of pigments into the oil. The stearate is allowed to coat the pigment particles, therefore preventing agglomeration of the pigment particle. Some oil, like linseed oil, can be heated and a 2% solution of aluminum stearate can be created to add to the pigment prior to grinding. Alternatively, the correct amount of aluminum stearate can be added to the pigment and ground together in a mortar and pestle prior to mulling on the slab with drying oil.
Diatomaceous earth—DE, or Kieselgur—is a silica containing sedimentary rock. It is mainly used as a filtration medium in water purification. It also finds applications in the concrete, pesticide, and explosive industries. It is formed from fossilized exoskeletons of diatoms.
Diatomaceous earth is lightweight and porous and has several properties beneficial to paints. One is that diatomaceous earth is amorphous silica and not crystalline silica. This mitigates the health risk associated with crystalline silica. Diatomaceous earth lends durability and hardness to a paint film and therefore less resin is require. Also, due to its unique porous structure, diatomaceous earth speeds up drying of paint film. Diatomaceous earth useful as a matting agent when a less glossy finish is desired.
Wollastonite is a mineral consisting mainly of calcium metasilicate. It is a white to off white powder often used in ceramics because it acts like a flux and also an additive to reduce shrinking of clays during firing. This last use is attributed to wollastonite’s lack of moisture and other volatile component. It also has applications in concrete and coating industries.
Its main use for paints and coatings are attributed to its high whiteness, high alkaline pH, and it’s ability to enhance durability of coatings. The alkaline pH of 10 is especially favorable to the formulation of water borne emulsion paints. Alkalinity stabilizes emulsions and helps reduce rusting of iron and steel substrates. This alkalinity also means less ammonia is needed reducing the odor of the emulsion paints.
Nephelene Syenite is silica deficient sodium aluminosilicate mineral filler. It is essentially free of crystalline silica and is being sought after by coating industry because of this property. The particles are an angular platelet type that easily interlock and have low overall surface area. With the hardness of nephelene and the interlocking particles, this filler adds durability and scuff resistance to coatings. The low surface area make for easy dispersions in any paint medium. The alkaline nature of this mineral is beneficial for stability in water based coatings as already noted for wollastonite.