Pigment History Europe Recipes for blue pigments were mentioned extensively in medieval artists' manuals The recipes varied from some rather straightforward methods of making copper acetate to more mysterious "silver blue" recipes for which, as Cyril Stanley Smith has said, "the chemistry escapes us." The proliferation of these recipes is understandable in light of the fact that the only two blue pigments available to the medieval artist (between the eighth and the sixteenth centuries) were the very expensive azurite and ultramarine (3). Recipes for making artificial blue colors are very old. They are embedded in the literature of a technical tradition dating from the 3rd century CE that managed to survive five centuries of "dark ages" to reemerge in the late 8th or early 9th century in two Latin manuscripts, which contain recipes for making blue pigments from both copper and silver. Other blues with intriguing crystalline forms are still unidentified, including a beautiful blue with a morphology resembling rosettes (Fig. 4). Could it be that the medieval artist was such a good synthetic chemist that, to this day, we have failed to synthesize and characterize compounds produced many centuries ago? The mystery surrounding the blue pigments remains unsolved. an iron, or Prussian, blue (Fig. 5). The iron blues are the first of the artificial pigments with a known history and an established date of first preparation. The color was made by the Berlin colormaker Diesbach in or around 1704. Moreover, the material is so complex in composition and method of manufacture that there is practically no possibility that it was synthesized independently in other times or places. ** all paints have three types of components: Pigments, consisting of small particles of colored compounds Media, serving to suspend the pigments and bind them to the surface of the object painted Diluents, allowing the painter to thin the paint to the best consistency for the work. Pigment, binder or medium, thinners ** http://cator.hsc.edu/%7Emollusk/ChemArt/paint/pigment.html Until the mid-Ninteenth Century, most artist's pigments were derived from finely ground naturally occurring minerals: rocks and ores. Most of the dyes came from organic sources: mostly plants like indigo for blue or madder root for red but also a few animals like cochineal beetles for carmine. (Hampden-Sydney's "garnet and grey" colors date back to the Civil War when the students dyed their civil war uniforms with pokeberries and butternut hickory husks.) With the rise of modern chemistry in the Nineteenth Century, some new "inorganic" or mineral-like compounds like "chrome yellow" and many, many new organic dyes were prepared. Shown below are some examples of a few of the inorganic or mineral pigments used in paints. Many of these have been in use for centuries. COMMON NAME CHEMICAL NAME FORMULA COLOR Cinnabar Mercury(II) Sulfide HgS Vermillion Cobalt Blue Cobalt(II) Oxide Aluminum Oxide CoO-Al2O3 Bright Blue Prussian Blue Potassium Iron(III) Ferrocyanide KFe(Fe(CN)6) Deep Blue Verdigris Copper(II) Acetate Cu(CH3CO2)2 Green Chrome Yellow Lead(II) Chromate PbCrO4 Yellow Burnt Sienna Iron(III) Oxide in clay Fe2O3 Reddish Brown Malachite Copper(II) Carbonate Copper(II) Hydroxide CuCO3Cu(OH)2 Green Cadmium Yellow Cadmium(II) Sulfide Zinc(II) Sulfide CdS-ZnS Lemon Yellow Rutile Titanium(IV) Oxide TiO2 White Chinese White Zinc Oxide ZnO White Lead White Lead(IV) oxide PbO2 White Vine Black Carbon C Black ORGANIC COLORANTS Among the new organic colorants were the "Azo Dyes" which were discovered by accident by Perkin in England in 18xx. Perkin had hoped to make a synthetic version of quinine, the only known anti-malarial treatment which was an extract of a South American tree Because those countries that were the sources of quinine were in the hands of the Spanish and Portuguese, English colonists across the topics were vulnerable to malaria. Perkin used coal tar, an ample and cheap byproduct of the coal and steel industry, to provide as the starting materials for his experiments. The resulting compounds that he made gave colors more brilliant and varied than those available from most vegetable dyes. They were also inexpensive so that the average man could afford colored clothes that had previously been beyond the reach of all but the rich. Though Perkin himself became rich from his patents, the British did not capitalize on their early technological lead in dye-making. The color chemistry industry moved to Germany were several companies, notably I.G. Farber, made huge advances in chemistry and wealth. Shown below in a skeleton of the common diazo "kernel" that all azo dyes have. It has two benzene rings (six carbons in a ring with three double bonds) joined by a double-bonded pair of nitrogen atoms. Shown along with this central piece are several organic dyes that are also used as pigments in paints. You might try to identify those that are azo dyes. PIGMENTS. Most traditional artist pigments are derived from finely ground minerals or inorganic compounds. (Ultramarine, from "across the sea", is the pigment from ground lapis lazuli, a semiprecious stone.) Most traditional dyes came from organic sources such as berries and insect bodies. (The garnet of Hampden-Sydney's colors came from using the juice of the pokeberry to dye their civil war uniforms; the gray came from butternut husks.) With the advent of modern chemistry, many colored synthetic organic and inorganic compounds have become available. The chief class among these, is the group of diazo compounds that became available in the nineteenth century. Some of these compounds have been used as both dyes and pigments. Shown below is one of the diazo dyes, All diazo dyes have benzene rings separated by a double bonded pair of nitrogen atoms. You should be able to recognize a diazo dye by this substructure. TYPE OF PAINT BINDER DILUENT DESCRIPTION OF MATERIALS AND PROCESS Encaustic Beeswax None 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 redominantly of hydrocarbons. Fresco Plaster Ca(OH)2 CaCO3 Water 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. 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. Egg Tempera Egg Yolk Water Until the 15th century, egg yolk was used as the most common binder and medium for paints. Egg tempera is prepared from the separated yolks of eggs mixed 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 Linseed or Walnut Oil Turpentine Mineral Spirits 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 carboncarbon 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 Gum Arabic Water In water paints the pigments are usually very finely ground mineral based transition metal compounds and 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 a 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 Acrylate Esters Water 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. 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. methyl acrylate vinyl acetate