Why ? 
because as we says in French “qui peut le plus, peut le moins” translate in English “who can do more can do less”
© 2017 David Damour Course given in 2015 on how to ground pigments with oil on marble

Making your own paints allows you to know and to choose better commercial ready-made paints and materials with relevance and discernment because you have the knowledge of what is the best and so you can know and compare with material in use in trade otherwise how do you know what is the best.

© 2017 David Damour Paints Oil make at my studio
© 2017 David Damour Paints Oil make at my studio
Why ? 
because it affect the luminosity and the colour strength of the paint

In order for the light distribution to be as effective as possible, the diameter of the pigment should be slightly less than half the wavelength of the light to be dispersed. Since the human eye is more sensitive to green yellow light (wavelength about 0.55 μm), the theoretical primary particles of the pigments are 0.1 to 0.3 μm, microscopic studies had confirmed this range of primary particle size. The optimal granulometry of the pigments to maximize their covering and colouring power in order to produce a layer of luminous paint could be between 40, 63 and 100μm, size observe in supplier’s shop.

Because the size of the powder of the pigment has a primordial role in the refraction of light. Consequently, the painter must choose pigments of granulometry which are suitable according to the technique he wishes to produce. The more a pigment is fine the more it is colouring, for water painting like “illumination or gouache” it is perfect, unfortunately, too fine pigments are inadequate with some techniques like “fresco” for example and even with “oil painting”, because if the pigment is too fine, the paint will crack, thus it is preferable to choose pigments that are bigger, even if they have to be crushed for a longer time, or choose from the minerals. You can also ask your supplier to grind them to grain sizes acceptable between 63 and 100 μm.

© 2016 David Damour Limonite ( iron oxide at 68%) 4 levigations
Why ? because the forms of pigments give to the paint film a lot of good characteristics

Rods or stick form gives more resistant paint film that can be compared to reinforced concrete with iron bars Tor embedded in its mass.

Thorn-like particles tend to settle in the paint film and create a lattice structure, and are more easily dispersed than a compact layer formed by nodular, round or square particles. These sharp pigments are long and they can create asperities across the film of paint and thus give mat to satin coatings that lower their shine. This rough surface can serve the base of the under layers and intermediate layers creating a whole network of irregularities which allows a better cohesion and adhesion of the following films.

The plate-like pigments create thin slices that will consolidate the film. They create bridges inside the paint film, thus making them airtight to the passage of liquids and vapors.

The knot-shaped, round, and square particles absorb less oil than other types of pigment form, allowing the preparation of rather dull paint, easy to apply while the needle-shaped particles, in the form of a blade and In the form of bars tend to increase the consistency of the paints because of their affinity for the oil.

Different shapes of pigment
Why ? because the paint should not be too thin or it may crack in oil painting.

Extra fine painting are not good for oil painting, only for water painting, oil glazes and oil sfumati. By choosing pigment that have a granulometry between 63 and 100µm you can do quickly, better and good paint that have the right finesse for impasto and velature.

© 2016 David Damour grinding of cobalt blue courses given in November 2015
Why ? because the purity of the paint allow you to make paintings more vivid, more luminous, more opaque, so more beautiful, impossible to do with paint which has too much filler in.

For a long time the suppliers of grinded and paints in tube have used fillers; so the pigments do not dissociate from the binder in tubes. In recent years, they have used modified organic clays called “Tixogel®” and aluminium stearate, high-efficiency coagulating agent, for oil colours for the best paints, for the worst “Meudon white” (chalk), which make oil paintings more transparent and greyish. You have to prefer silicates as filler for oil, like baryum white or permanent white, because they are more stable than carbonates. Some pigments, such as titanium white, which tends to flour, manganese blue or manganese violet, sometimes require a filler to better agglutinate under the glass muller or mill rolls. These extenders contribute to the covering power and hiding power of low-cost paints and are used to control gloss, texture, suspension and viscosity. The main types of extender are carbonates (chalks), silicates (silica, kaolin), sulphates (gypses), oxides (iron and manganese) and aluminates. Their particle sizes range are from 4 to 200 μm.

Except in very special cases, a pigment never needs fillers.

Suppliers are obligated to incorporate them in paint for storage, but painters does not need them at all.

In order to check if a pigment is pure, pour 50 grams of it into 200 ml of white spirit, stir and let rest (if you have a sedimentation cone, it’s better), leave 24 hours at rest, if there is a filler, you should see it very visibly, like another layer in the mass. for water paint replace the white spirit with water.

© 2016 David Damour Fillers for oils and waters painting
Why ? because it affect the stability of the painting and the substract, and despite the fact that the paint uses oil, the goal is to use as little as possible oil to make a good film.

Low particle size pigments have a high specific surface area per unit mass; as a result, a greater amount of oil is absorbed by the surface.

Similarly, needle, blades and irregular pigments particles have an high oil absorption rate compared to the round, square and node particles due to a larger area.

Pigments with spongy structure such as carbon black have also an high oil absorption rate raised due to oil penetration in the interstices.

Oil absorption rate is a value on the amount, and therefore, the density of the pigment is important. Dense pigments often have lower oil absorption rates than the pigments whose density is low, provided that their size and shape of particle are of the same order. Apart from the special characteristics of the pigments mentioned above, other factors such as energy during grinding, duration and the shear force.

© 2016 David Damour Fillers in oil paint ; on left good quality, on right weak oil paint

If you want to know more, 
I have written a book about pigments, paint and the craft of the painter of the 21st in French language, that I am in the process of translating into English, it will be available around March-April 2017 you could find my book on my website

Like what you read? Give David Damour a round of applause.

From a quick cheer to a standing ovation, clap to show how much you enjoyed this story.