Inorganic Pigments

Egyptian blue —The Egyptians were quite adept at using natural mineral pigments as these are found in ubiquitously in the artefacts that arose from their civilization, but they were also good at creating new technology. Egyptian blue was a pigment that they prepared by melting silica (SiO₂), lime (CaO), alkali (NaOH) and copper. The copper is responsible for its characteristic blue color.

Naples yellow and Chrome Yellow — The chemical formula of Naples yellow is Pb₂Sb₂O₇. While it occurs in nature as the mineral bindheimite, it was rarely used in mineral form. The ancient Egyptians were known to have prepared the pigment, which was widely used from about 1700 to 1850. Its usage fell off with the availabilty of the other synthetic pigments such as lead chromate (PbCrO₄, chrome yellow), cadmium sulfide and cobalt yellow. Lead chromate was in use by the early 1800s but was also replaced by the cadmium yellow pigments in use.

Cobalt yellow — Cobalt yellow, or aureolin, is potassium cobaltinitrite, K₃[Co(NO₂)₆], first prepared by Nicholas Wolfgang Fisher in 1848. It is prepared by oxidation a solution of cobalt(II) salts mixed with potassium nitrite.

Cadmium Yellow Red & Orange — Cd was independently discovered by Friedrich Strohmeyer and Karl Samuel Leberecht Hermann in 1817, but the cadmium compounds did not become widely available until 1840. Cadmium reacts with sulfur to form a simple binary compound CdS, which is bright yellow. This is analogous to mercuric sulfide, which is the pigment vermilion, as cadmium and mercury are in the same group in the periodic table. The most stable for of CdS adopts the same cubic structure as zinc sulfide, ZnS, whose mineral form is called zinc blende. ZnS also forms a hexagonal crystal form that is the mineral wurtzite. It is slightly less stable than the zinc blende form. CdS can also be crystallized in the wurtzite form. Mercuric sulfide on the other hand exists in a trigonal crystal lattice.

By replacing some of the sulfur in CdS with selenium, the color changes from yellow to orange to red with increasing Se content. This is not simply a mixture of CdS and CdSe. The Se atoms replace some of the sulfur atoms in the structure and are statistically distributed throughout the crystal. This is known in solid state chemistry as a solid solution, and the formula is written as CdS_1-xSe_x. Usually these numbers are written such that x denotes a small amount of the dopant element. This substitution process is used to tune the electronic properties of the material, which are also related to the colors observed. Cadmium sulfide is a semiconductor with a band gap of 2.38eV, corresponding to a λ of 520nm. In comparison, cadmium selenide's band gap is 1.74ev and λ = 710nm.

Verdigris — Verdigris includes a number of basic copper acetate compounds that have been manufactured as pigments since the middle ages. The reaction to produce them is quite simple - copper metal is simply placed in vinegar, whose active ingredient is acetic acid, and allowed to stand. Oxidation of Cu⁰ yields Cu²⁺ while the H⁺ ions available from the acetic acid are reduced to hydrogen gas, H₂.

The conditions employed will lead to different compounds which, at first glance, appear to be a simple combination of copper(II) acetate and copper(II) hydroxide as seen in the formulas below. While this is a common way to write these formulas, they are misleading in that the structures are not made up of isolated copper acetate and copper hydroxide units. They are much more complicated as seen in the interactive drawing for one of the compounds shown later on this page. That compound shows complex two-dimensional sheets of copper ions linked by both hydroxide and acetate ions. The colors for each of these compounds is different, ranging from blue to green.

Massicot (or Masticot), Litharge and Minium — Lead oxides have also been used as pigments. Massicot is lead(II) oxide,PbO, while minium is Pb₃O₄, which has lead in both the +2 and +4 oxidation states. It coud be written as (Pb²⁺)₂Pb⁴⁺O₄ to emphasize the oxidation states of the lead. Minium is also known as red lead. Massicot was prepared by the Romans by heating lead carbonate. Most metal carbonates undergo this process that is based on the elimination of carbon dioxide gas. It is the same process that is used to prepare lime (CaO) from calcite (CaCO₃). Once the PbO is obtained, it can be heated in air to oxidize it to minium. Lead(II) oxide itself exists in two crystalline forms - orthorhombic and tetragonal. The orthorhombic forms produces a yellow powder that is the pigment massicot, while the tetragonal form is known as litharge and is red.

Prussian Blue — Ferric ferrocyanide is a dark blue pigment discovered in the early 1700s. It was produced originally via the formation of iron(II) ferrocyanide, Fe₂[Fe(CN)₆], an insoluble white compound used as a pigment known as Berlin white. The subsequent oxidation of this compound produces ferric ferrocyanide, Fe³⁺₄[Fe²⁺(CN)₆]₃. Today it can be made more simply in the lab by reacting aqueous iron(III) chloride, FeCl₃, with K₄[Fe(CN)₆]. It gets its color from charge transfer that takes place upon absorbtion of red-orange light. In this process, the absorption of light causes an electron to move from the Fe²⁺ in the [Fe(CN)₆]^4- to the Fe³⁺ counterions. The compound is highly insoluble in water and forms a precipitate. As as pigment, it was originally known as Berlin blue, but became known as Paris or Parisian blue when used in oil paintings. Turnbull's blue is the same compound but it is prepared by adding Fe²⁺ salts to salts of the ferricyanide ion, [Fe(CN)₆]³⁺.

Cerulean ble was another blue pigment that was discovered in 1789 by Swiss chemist Albrecht Höpfner. It is cobalt stannate, CoSnO₃ and became popular for painting skies.

French Ultramarine Blue — In 1826, the French chemist Jean-Baptiste Guimet created a substitute for lapis lazuli, known up until that time as ultramarine. He heated kaolinate, with sodium carbonate and sulfur. Kaolinite, Al₂(OH)₄Si₂O₅, provides the aluminosilicate framework while the sodium carbonate provides the alkali metal cations and additional oxygen. The sulfur provides the source for the S₃^- radicals that are the source of the blue coloration. The result was a vibrant blue pigment that was in many regards superior to the natural mineral, even though the synthetic version is based on the same chemistry. To distinguish the synthetic material from the mineral-based pigment, the new compound was marketed as French ultramarine blue in homage to the place of its origin.

Phthalo Blue and Phthalo Green — In the 20th century chemists discovered the active light-harvesting center in green plants. It is shown in the figure to the right. Four nitrogen atoms point towards the center of the molecule, and the size of the space between the nitrogen atoms is ideally suited for bonding to a metal cation, which is Mg²⁺ in the case of the chlorophylls. This unit is common to all of the chlorophylls, although there are side chains attached to the ring that have different organic groups that result in slightly different green colors. Leaves turn colors in autumn because the chlorophyll molecules decompose leaving other natural pigments to be observed.

Another term used to describe the organic ligand in chlorophyll is chlorin, and it belongs to a general classification known as macrocyles, ring-like organic molecules that can bond to metal ions. Over the past several decades, there has been a considerable amount of research done in studying macrocyclic ligands with a large range of different transition metals and p-block metals bound to them.

Other macrocycles related to chlorin that are used for pigments and dyes include porphyrin and phthalocyanine, and their derivatives. While transition metals bound in the center of the ring affect the color, the network of multiple bonds in the molecule create color as well. The metal replaces the two hydrogen atoms attached to the nitrogens pointing towards the center of the ring, so one can think of the porphyrin as the negatively charged ion [C₂₀N₄H₁₂]^2- when it binds a metal ion. Porphyrins related to the macrocycles present in chlorophyll are present in many living organisms. For example an iron-porphyrin complex is the active center in the hemoglobin molecule that transports oxygen in the blood. These molecules often have other organic groups replacing the hydrogen atoms on the periphery of the molecule that allow them to bind to other biological molecules.

A closely-related molecule is phthalocyanine. It has additional nitrogen atoms in the ring system. Its metal compounds have become common inorganic pigments, including phthalo blue and phthalo green. The syntheses of these two compounds are given below. Phthalo blue is readily converted into phthalo green by reaction with chlorine, although complete chlorination of the ring is hard to achieve and they are not readily separated because of their insolubility. The actual phthalo green pigment will be a mixture of varying degrees of substitution – Cu[C₃₂N₈Cl_16-xH_x] – and consequently display different shades of green.