Inert pigments  There for reasons other than color
These are the main inert fillers that are used to impart various physical properties other than color such as opacity or texture, or simply to make the paint handle better. Over used these same substances can be regarded as adulterants. All these substances should be used in the minimum concentration possible to enable the maximum color concentration in the circumstances.

Calcium Carbonate
Precipitated Chalk
The most important inert filler for the artist to use. It is synthetic chalk that is more pure and softer than Whiting, and is recommended for most applications that require chalk.

All of the various forms of Calcium Carbonate such as chalk, whiting, marble dust, limestone and so on are all chemically identical, differing only in crystalline structure and degree of impurities. Precipitated Chalk is the most pure form of Calcium Carbonate available.

It is an ingredient in Gouache, Pastels, and Chalks. It can be used as a base for making lake pigments. It makes particularly fine gesso's.

Whiting (Gilders Whiting)
Whiting is ground natural chalk. It varies considerably in purity and is often labeled with such names as Gilders Whiting that differentiate the various qualities. It is less expensive than Precipitated Chalk and tends to be coarser. It is most useful for making hard pastels or for gesso's that require its particular color or texture. Mixed with Linseed Oil it makes putty. The best grades are called Paris White.
This name covers all forms of Calcium Carbonate on the Earth and makes up 4% of the Earth's crust. As a pigment it is different to other forms of the mineral and that is due to its crystalline structure. It is the form of Calcium Carbonate that is best suited to making modeling pastes in Acrylic and is a useful addition to gesso's for its tooth.
Marble Dust
iAn impure form of Calcium Carbonate that can come in a variety of colors. It is a useful additive to very intense organic colors, toning them down and improving workability. Can also increase the brilliance of some dark colors like Viridian.
Marble Chip
Fine chip (like in Terrazzo) can be used as a textural additive to Acrylic paint
Paris White
The finest whitest grade of natural chalk, significantly whiter than the grades sold as Gilders Whiting (see above).

Kaolin (China Clay)

A fine white clay that originally comes from weathered Granite. It can be calcined to produce the whitest shades, It can impart smoothness and other characteristics in Pastels, Chalks, and Gouaches. Do not accept just any offering as it is possible to find particularly white grades but they are not necessarily easy to find.
A natural Magnesium Silicate that has properties between Talc and asbestos that is used by paint manufacturers because it aids in holding pigments in solution and helps in the formation of strong paint films. Not recommended for making paint in the studio.
Talc has many uses for the artist. It is firstly a soft easily carved stone for sculpture. As sticks it is called French Chalk and writes well on steel for marking out metal for fabrication and then as a pigment powder it can be useful when making paints. It is good for increasing the opacity of very weak colors while minimizing the chalky look of using straight Precipitated Chalk. In Pastels its greasy feel can modify some pigments that tend to set to too hard a pastel. It can exhibit a pearly luster in some circumstances too.
Silica / Quartz
Available in various particle sizes from fine powders to sand to small pebbles. Used to provide tooth or visual texture to paint. The larger sizes should only be used with acrylics.
Alumina Hydrate
Aluminum Hydroxide is used as an inert base for precipitating dyes in the making of lakes. Manufacturers like it because it imparts good brushing qualities to oils and so is often found as an extender. The most transparent of the various bases for precipitating dyes for making lakes so is most suitable for delicate colors, An excellent extender for the Pthalocyanines. It is made from Bauxite.
Aluminum and Potassium Sulfate is found naturally as Kalunite. This was the material that lakes were precipitated onto until recent times. Is readily available from pharmacies.
Blanc Fixe (Permanent White)
Synthetic Barium Sulfate is sometimes called Permanent White. It is a common extender for Pthalocyanine pigments where its use is regarded as beneficial to pigment properties. It is commonly co-precipitated with Cadmium colors as the two are chemically similar and hard to separate. Industry standards allow up to 15% Barium Sulfate in Cadmium colors. It is more expensive to produce chemically pure Cadmium and this is why there is a premium on the pigment.
It can be used as a base for precipitating strong dyes in making lake pigments and is an excellent white to use for Watercolor, Gouache, or Fresco.
A white pigment prepared from Blanc Fixe (see above). It is Zinc Sulfide and Barium Sulfate with a little Zinc Oxide. It is a common pigment for exterior paints and is often added to Zinc White to increase opacity.

Natural Barium Sulfate that is a heavy inert pigment with low oil absorption, and easy dispersibility that is a popular (among manufacturers) adulterant in paint. In oil it is nearly transparent but does cloud the color just a little. It should not be confused with Blanc Fixe which is the synthetic version of Barium Sulfate but has quite different properties (see above). The Barium Sulfate in many Cadmium's is in the form of Blanc Fixe (see above) or Lithopone (see above).
A gray lightweight volcanic scoria which is ground into a powder and used in paints and grounds to provide a toothy textural surface.
Infusorial Earth / Diatomaceous Earth
This type of silica is ground from a rock that is the fossilized remains od Diatoms, a single cell algae from millions of years ago. It is found in chalky deposits and is commonly used in abrasives, cleansers, toothpaste's, and filters. It can be used for making paints to coat papers for pastels or other gesso's and grounds to modify tooth.
Used to impart sparkle to paints it can weaken Oil Paint films, especially where the mica particles are larger as the mica continues over time to cleave into smaller flakes. The finest grades of Mica flakes seem to avoid this problem. Mica Titivates are stable Mica pigments giving a wide range of effects including metallic, iridescent and diffraction. For more info on the mica pigments click here.
A type of clay derived from volcanic ash with occasional use  in Pastel making. It has the ability to absorb large amounts of water (and swell accordingly). This quality may have some use in experimental techniques.
A special kind of phyllosilicate mineral resembling Mica which has the unusual property of expanding when heated to form a lightweight porous substance. Most people are familiar with the larger sized materials used in landscape architecture or medium sizes used in some kinds of kitty litter but it can be found in many sizes down to half millimeter. The smaller sizes up to 2 or 3 mm are most useful for mixing into acrylic for textural effects.
Glass Beads
Small glass balls of various diameters that are particularly good in Acrylic but can be used in oil as well.
Ketone Resin
Can be a very effective textural additive to acrylic as the resin pellets and the dried acrylic are similar in refractive index and the Ketone requires alcohol to dissolve so is very permanent as a solid inclusion in the acrylic. Has a different look to glass beads.

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