Art Pigments

Paints All paints have three types of components: Pigments Media Diluents

Pigments Pigments consist of small particles of colored compounds. Are derived from finely ground naturally occurring minerals:  rocks and ores.

Media Media serves to suspend the pigments and bind them to the surface of the object painted. Examples are: beeswax, linseed oil, walnut oil, plaster, gum arabic and egg yolk.

Diluents Diluents such as water, turpentine, or mineral spirits allow the painter to thin the paint to the best consistency for the work.

Gemstone Paints The only two blue pigments available to the medieval artist were the very expensive azurite and ultramarine. Picture-azurite Azurite ("Blue verditer," "mountain blue," "lapis armenius," "azurium citramarinum," "blue bice"):  Origin and History: Latin borrowed a Persian word for blue, lajoard, which in the form of lazurium became azurium, and gave us our word azure. It is composed of a basic carbonate of copper, found in many parts of the world in the upper oxidized portions of copper ore deposits. Azurite mineral is usually associated in nature with malachite, the green basic carbonate of copper that is far more abundant. Azurite was the most important blue pigment in European painting throughout the middle ages and Renaissance by contrast, despite the more exotic and costly ultramarine having received greater acclaim.  Making the Pigment: To prepare a color from it, lump azurite is ground into a powder, and sieved. Coarsely ground azurite produces dark blue, and fine grinding produces a lighter tone; however if not ground fine enough, it is too sandy and gritty to be used as a pigment. The medieval system included washing it to remove any mud and then separating the different grains by some process of levigation. If plain water is used it is a slow, laborious process, so they used solutions of soap, gum and lye. When azurite is washed, the very fine particles are rather pale, greenish sky-blue, and not much admired for painting. The best grades of azurite for painting were coarse: not sandy, but so course that it could be quite laborious to lay them on. Chemical Properties: Cu3[CO3]2[0H]2, H3.5, SG-3.7, monoclinic. Azurite sometimes looks a little like lapis lazuli, and the two were often confused in the Middle Ages. To tell them apart with certainty the stones were heated red-hot. Azurite turns black when this is done, and true lapis is not injured.  It does not blacken from the effects of sulphur gases as some chemists have supposed, but from the action of the strong alkalis improperly used in picture cleaning, and from the purely optical effect of darkened varnish surrounding its particles. The color can, however, be ruined by the presence of acids. Artistic Notes:  Glue size was often used as a binder to hold the pigment grains firmly in place. (Size is more easily affected by protracted dampness or by washing than egg tempera, and blues in wall paintings have therefore sometimes perished through the destruction of their binder where colors in tempera have stood.) It was necessary to apply several coats of azurite to produce a solid blue, but the result was quite beautiful. The actual thickness of the crust of blue added to the richness of the effect, and each tiny grain of the powdered crystalline mineral sparkled like a minute sapphire, especially before it was varnished. The open texture of a coat of azurite blue has often been its undoing on panels; the varnish sinks into it and surrounds the particles of blue. As the varnish yellows and darkens, the power of the azurite to reflect blue light is destroyed, strangled by the varnish---a large number of blacks in medieval paintings were originally blues, only obscured by the discoloration of the varnish. It is incredibly permanent.

Types of Paints

Encaustic The Egyptians, Greeks, and Romans often used beeswax as the medium for pigments. The encaustic method was in very common use until the 8th century A.D. and is still used by a few painters today. In this technique finely ground pigment is mixed in melted wax and applied to the surface. Waxes are polymers composed predominantly of hydrocarbons. Fayum Funeral Portrait, Mummy Portrait of a Woman, Antinoopolis, End of the Reign of Trajan, 98-117 A.D., Wax portrait on wood.

Fresco In fresco painting, the medium and the surface are the same. Several processes involved in making a fresco are of chemical interest. Limestone or marble which is made up from calcium carbonate is heated in a lime kiln to produce quicklime, CaO. CaCO3   -->   CaO + CO2 The quicklime is then "slaked" with water to produce lime, Ca(OH)2, which is used to prepare the plaster. CaO + H2O   -->   Ca(OH)2 As the plaster dries and ages, the calcium hydroxide in it undergoes a chemical reaction with CO2 in the air changing it from Ca(OH)2 to CaCO3, which is the mineral that comprises limestone and marble. Ca(OH)2 + CO2   -->   CaCO3 + H2O This calcium carbonate matrix, which now contains the pigments of the painting, is insoluble so that the painting is almost impervious to water. However, calcium carbonate in all its forms reacts with sulfuric acid to give CaSO4 which has a much greater solubility in water than does aCO3. Thus acid rain, which contains sulfuric acid, slowly converts the CaCO3 in frescoes and in marble sculpture to CaSO4. The CaSO4 then flakes of or washes away eventually destroying the artwork. In fresco painting, the medium and the surface are the same. An aqueous suspension of the pigment is applied directly to a wet plaster of calcium hydroxide and fine sand. The pigment is absorbed and is bound into the surface as the plaster dries.

Egg Tempera Until the 15th century, egg yolk was used as the most common binder and medium for paints. Egg tempera is prepared by mixing egg yolks with a slurry of artist's pigment in water. Enough water is added to provide the proper consistency for painting. This paint dries extremely rapidly, and when applied thinly, it gives a translucent glaze that allows either a white surface ground or an undercoat to show through. The drying and hardening process of the medium involves the both the denaturation of the proteins from the egg and polymerization of the fats in the yolk. The proteins form many hydrogen bonds with each other and with the surface, locking the pigments into a solid matrix. As they age, these proteins form covalent bonds with each other, making the matrix very stable and permanent.

Oil By the 15th century, oil paints, using vegetable oils as the medium, replaced egg tempera as the most common paint. The oil most commonly used is linseed oil which is obtained from the seed of the flax plant. The oil does not dry but rather is cross-linked where there are carbon-carbon double bonds in the oil. This process is initiated by oxidation by oxygen in the air or by metal oxides. Early oil paints were very slow "drying" because the initiation step of air oxidation is quite slow. However, it was soon discover that adding some metal oxides like ZnO or MnO2 could also start the cross-linking process and speed up this hardening process. Ironically, the relative slow pace of drying compared to that of egg tempera was considered an advantage since paintings could be reworked and the composition modified before the paint hardens.

Watercolor In water paints, the pigments are usually very finely ground mineral-based transition metal compounds. The vehicle is an aqueous solution of gum Arabic, a resin prepared from the sap of the African acacia tree. This resin is a translucent water-soluble polymer. The resulting paintings usually retain a translucent quality; they appear bright in part because the whiteness of the paper is reflected through layers of the paints. A much more opaque water based paint, Gouache, has more coarsely ground pigment and sometimes incorporates CaSO4 as an opaque whitener. The medium is still gum arabic. 

Acrylic These paints use an aqueous suspension of both the pigment and monomers of compounds such as methyl acrylate and vinyl acetate. The paint does not become plastic until the monomers combine. In a process similar to the "drying" of oil paints, these monomers are linked together by a chain reaction to form a polymer molecule that is insoluble in both water and most organic solvents. Since 1945, plastic media such as acrylics have become popular. These man-made media have not replaced oil paint as the vehicle for pigments but rather have provided an alternative method.

Art Pigments