Pigments, Dyes, Inks and Glazes

Two Chemical Classes of Pigments

Many molecules and solid state compounds have generated interest for thousands of years simply because they possess color. These colors have been valued and complex technologies have been developed for their harvesting, mining or synthesis. But these colors have been used in diverse applications such as painting, textile dyeing and inks, as well as coloring glass and glazes. Later on this page we will discuss the origin of color in these compounds and molecules, as well as providing additional information on the individual pages for Inorganic Pigments and Organic Pigments and Dyes.

One common way to classify pigments is by whether they contain metals or not. Most inorganic pigments contain metals while organic pigments, which are composed primarily of carbon, hydrogen, oxygen and nitrogen, generally do not with the exception of those that may be found as salts of the alkali metals. Originally, the distinction between inorganic and organic would have been determined by the source of the compounds. Inorganic pigments were originally obtained from non-living materials, and these were mostly minerals and charcoal, while organic pigments were extracted from plants and sometimes more rarely from animals or their byproducts. But as with most things in more recent times, the classifications become fuzzy, especially with advances in the understanding of atoms and molecules and in the synthesis and the introduction of new man-made pigments that can blur the lines. For example, verdigris, Prussian blue, cobalt yellow (aureolin), phthalo blue and phthalo green are metal-based pigments that are coordination compounds and not minerals (see Inorganic Pigments). But in verdigris and the phthalo compounds, the metals are bound to organic molecules, and the organic molecules may have color of their own, even in the absence of the metal. Chemists call any molecule or ion that is bound to a metal a ligand and give it an abbreviation of L. Additionally, many organic pigments and dyes contain acidic functional groups such as a carboxylic acid (-CO2H) or sulfonic acid (-SO3H) unit. These groups can be deprotonated with bases such as sodium hydroxide, potassium hydroxide, etc. and the result is a salt that contains a metal cation and the anion of the parent organic molecule. Since the alkali metal ions do not contribute to the color, the molecules are still largely considered to be organic regardless of the presence of the metal cations.

We also need to distinguish between the way different colored materials are used. While all of these the compounds here possess color and that is the primary reason they are being used, the chemistry of their usage in specific to the application. Some pigments may also be used as dyes, but this is not true of all of these substances. Most minerals are not good dyes. Pigments get their value from being largely insoluble in the medium that they are being used in. Pigments in oil paints and acrylics for example, are not dissolved in the medium, rather they are dispersed in and retain their nature as small particles. This property keeps the particles locked in the paint once it dries or hardens. But to use a colorant as a dye, it often needs to be soluble in water or some other solvent in order to apply it to the textile. There are some engineering tricks to get around the limitations - such as synthesizing the insoluble dye directly inside the fibers of the textile, but most commercial dyes are soluble in water. Similarly, most inks are soluble in water or sometimes ethanol.

The Origin of Color

Light dispersion conceptual waves Light being split into its components by a triangular prism. Lucas Vieira/Public domain.

In order to fully understand the origin of color in pigments, we need an understanding of some biology, physics and chemistry. First, we need to understand that color derives from our ability to detect certain wavelengths of light. This is both a function of the light itself and our eyes, which detect that light. When you were young, you may have done the simple experiment of passing sunlight through a triangular prism. The prism produces a rainbow because the different wavelengths of light that comprise sunlight are bent to different degrees as they pass through the prism. If the opposite sides of a piece of glass are parallel, then no separation of color is observed because the different wavelengths of light get recombined into white light. Regardless of the wavelength of light, each wavelength is traveling at the same speed. Another term used to describe light is frequency, and the wavelength (represented by the Greek letter λ) multiplied by the frequency (denoted by the Greek letter ν) is equal to the speed of light, which is conventionally given the symbol c. Thus we can write c = ν × λ. But even though the speed of of light (E = each wavelength is the same, the energy is not. Energy is directly proportional to the frequency of the radiation, E = ℎ × ν. We can also write that the energy is inversely proportional to the wavelength, E = (ℎ × c)/λ). As physicists discovered the properties of light, they found that the visible spectrum that we see is only a small part of a much larger electromagnetic spectrum.

The absorbance of light by the rods and cones in the eye.

Our eyes, however, do not act as a prism and they are limited to seeing only the visible part of the electromagnetic radiation spectrum. They are complex organs that have evolved over time to permit us to distinguish some different wavelengths of light. Interestingly, other animals' eyes have evolved to view a different portion of the spectrum. Bees, for example, do not see into the red wavelengths but they are able to observe ultraviolet wavelengths that we cannot.

The retina of the eye contains two different types of specialized structures - rods and cones. Rods are very sensitive to visible light but they cannot distinguish color. Cones, on the other hand, are less sensitive but have been developed into three types that allow us to distinguish different wavelengths, as seen in the figure for the red, green, blue cones and the rods. It is the underlying biology of these structures in the eye that makes the color wheel and the complementary color system used in Art work the way it does. Vision, however, does produce a simple 'photographic' image, and the brain employs short cuts with already stored images. How the brain processes these signals is not completely understood but is actively studied by neuroscientists. Optical illusions take advantage of the way the brain processes its signals to achieve their sometimes astonishing affects. But each individual sees color somewhat differently. An interesting video that illustrates how the human eye responds to color is found at What is color? by It's Okay to be Smart.

Colorblindness is one condition where an individual has cones of one or more particular types that do not respond to the intended wavelengths of light. The most common type is red-green color blindness, in which the individual cannot easily distinguish red and green objects, and the second most common is blue-yellow.

When visible light strikes an object, there are several things that can happen to the light. If the object is colorless, most of the light will simply pass through the object. If that object has flat faces, then some of the light can be reflected off the surface. But if an object has color, some of the light striking the object will be absorbed. The color an object appears will arise from the light that is not absorbed.

White is unique because it arises from the reflection of all light from colorless crystals. While a macroscopic crystal appears colorless abd transaparent, that same substance ground to a fine powder will appear white because of light scattered from many small crystallites. Similarly, a colored pigment will appear lighter in color the more finely it is ground. The coarseness has another effect. A smooth surface will appear shiny, while a rough surface will appear to have a mate finish also because of the diffuse scattering of light. Black pigments are black because they absorb all wavelengths of visible light.

Because the result of color mixing is different for transmitted and reflected light, it is divided in two classes: additive and subtractive. Subtractive color is the process of mixing different paints to produce a new color. The individual pigments subtract, or absorb, certain colors from the the light that is reflected to the eye. This is the basis for the CMYK color system used in ink jet and laser printers. In printers cyan, magenta, yellow and black inks are mixed to produce the full range of colors. Black is necessary here because mixing pigments rarely produces a true black color. On the other hand, when light of different colors is mixed, additive color results. This means that the wave pattern for each of these wavelengths of light are added to each other to achieve the color we see. This is the basis for RGB projectors, where RGB stands for the red, green and blue lights that are combined to produce the full color spectrum. For an RGB projected image, black is the absence of light, while white is the presence of all wavelengths of the visible spectrum.

But visible light is not the only electromagnetic radiation that interacts with matter. The entire spectrum of radiation displays such interactions, and each particular wavelength can be used to probe the structure and properties of the materials, even though these interactions are invisible to the naked eye. Scientific instruments and techniques have been developed that allow us to learn a tremendous amount about what materials were used and how they were manipulated. You can learn more about these techniques here.

reflected light On the left, an object that appears blue reflects blue light, while on the right, the blue color of a transparent object, such as a pane of glass in a stained glass window, is due to the blue light that is transmitted through it. Some glass objects are dichroic, which means that they appear different colors depending on whether they are viewed with reflected or transmitted light, as has been observed for the 4th century Roman glass Lycurgus cup.
Additive and Subtractive Color The two systems for mixing color: subtractive and additive. Subtractive color is the process of mixing pigments that remove various wavelengths of light from the reflected light, while additive color is the result of mixing various wavelengths of light as occurs in an RGB projector.

The electromagnetic sprectrum. Modified from the origianl at Inductiveload, NASA/CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/). The figure compares the wavelengths and their frequencies to objects that are on the same length scale as the wavelength of the electromagnetic radiation.
Effect of grain size on pigment colorThe effect of grain size on the perceived color of a pigment. Both of these samples are azurite, but the one on the left is ground much finer than the one on the right. Note in the coarser material that there are some green flecks due to the presence of a malachite. See Inorganic Pigments for an explanation of the relationship between azurite and malachite.

Light, matter and the origin of color

How light interacts with matter. Different ways that light interacts with matter.

Before talking about how color arises in inorganic and organic compounds, we should review the different ways that light can interact with matter. Electromagnetic radiation can simply pass through an object without being affected. This is what happens when visible light passes through a colorless pane of glass, and it is called transmission. Note that transparency and color describe two distinct properties. A substance can be transparent and have color, such as stained glass windows, or they can be opaque and have color.

During the transmission process, light can be bent as it passes through the object. This is known as refraction, and the amount that light is bent in different substances is known as the refractive index. You may have observed this in looking at the stems of flowers where they enter water when emersed in a transparent vase. It appears as if the stems are disconnected between the stem that is in the water and that part that is above the surface. If the opposite sides of a transparent object are parallel, light appearing on the opposite side appears to be undistorted from the incident light. It is not following the exact path of the incident light but is following a parallel direction that is slightly displaced from that of the incident light. But if the opposite sides are not parallel, as in a triangular prism, then splitting of the light into different wavelengths is observed. This arises because the different energies of light travel at different speeds owing to the refractive index of the material.

Electromagnetic radiation of all wavelengths can be absorbed by the material, and a variety of things can take place in the material as a result depending on the magnitude of the energy absorbed. Infrared radiation will cause bonds to vibrate, radio frequency radiation will make nuclei that have a spin moment flip their spins, visible light and near UV light will cause electrons to move from one molecular orbital to another, while more energetic UV and X-ray radiation may cause electrons to be ejected from the material. X-rays that are not absorbed by a substance may be diffracted. In this case, the distribution of electrons in a solid acts the same way that a series of closely spaced slits acts for visible light. In both cases, interference between the different waves of the radiation creates a pattern of reinforced intensity and cancelled intensity known as a diffraction pattern. Diffraction patterns may be analyzed to give structural information about the material. A related phenomenon to absorption is emission. After a molecule or compound absorbs electromagnetic radiation, it can release that energy in a process called emission, which can also be measured.

Light can also be reflected, in which case the color of the reflected light is the observed color of the object. But objects can also absorb some of that light. If all colors are reflected, the object is perceived as white, and if all colors are absorbed, the color seen is black. If only some of the wavelengths are absorbed, then the object will have the color of the reflected light, or the complementary color to the light that was absorbed.

The way that light is reflected from a surface is further complicated by whether the surface is smooth or rough. If it is rough, then light will be scattered in many random directions. This is the reason that colorless solids look white when they have rough surfaces or when ground into powders. This is also the reason that some objects appear shiny or glossy, a result of smooth surfaces, as compared to dull or matte, which arises from irregular surfaces.

All of these processes have particular utility in studying art objects. While the human eye is good at observing a large number of these effects, it is neither perfect nor quantitative. Over the past century, scientists have developed instruments capable of measuring all of these processes and they become tools that can be used to study art objects in fine detail. Unlike the human eye, these instruments employ special detectors for measuring the wavelengths and intensities of electromagetic radiation absorbed, transmitted, reflected or emitted. Details about the different techniques and the type of information they can provide can be found at Analytical Methods.

Absorption by rods and cones in the human eye.

Color is something we perceive because of the biology of our eyes and how that biology interfaces with our brains. We see things indirectly. The images produced arise because light is reflected off of the surface of objects and this reflected light is captured by our eyes. Specifically, the eye is a complex set of lenses comprising the iris that focuses the incoming light onto the retina at the back of the eye. Light is fundamentally packets of energy that are related to the frequency or wavelength of the light by the equation E = ℎν, where ℎ is Planck's constant (6.62607004 × 10-34 J·s-1) and ν is the frequency of radiation given having units of Hz or s-1.

The structure of retinal

These packets of light activate retinal, one of the forms of Vitamin A, in the back of the eye. This causes 11-cis-retinal to isomerize to all-trans-retinal resulting in an electrical signal to travel via the optic nerve to the brain, where the signal is processed. Interestingly, retinal is derived from β-carotene, one of the naturally-occurring pigments we have seen on the Organic Pigments page. It is responsible for the orange color of carrots. The middle C=C bond in β-carotene reacts with O2 to produce two molecules of the all-trans-retinal.

There are two types of structures in the eye that detect light - the rods and the cones. The rods only differentiate light and dark, while the cones are responsible for detecting color. The cones require more intense light for activation than do the rods, which explains why objects appear to us only in shades of gray in low light. The cones come in three types: red-, green- and blue-absorbing.

11-cis-Retinal, C20H28O.
All-trans-retinal, C20H28O.

Color wheel and color mixing.

If you have studied art, you are likely to be familiar with the color wheel. The wheel is famous for showing the relationship between complementary colors, which lie directly across from one another on the wheel. Thus red and green, blue and orange, purple and yellow form complementary pairs. Color theory has it that these colors look good to our eyes, and the reason for this lies in the structure of the eye that allows us to perceive color - the red, green and blue cones. The way that color is detected by these structures and processed by the brain is also responsible for a large number of optical illusions. Once such optical illusion is an afterimage that you see after you stare intensely at something for a brief period and look away. The rods or cones involved in perceiving that object become desensitized and the image will show up using the structures in the eye that are not already saturated.

There are two different ways of mixing colors. The first type of color mixing is subtractive. If we combine pigments, the result is subtractive color mixing, because the absorptions of the two pigments combine to remove the wavelengths of light associated with pigments both individually. If we mix pigments that absorb all colors of visible light, then the object appears black. In reality it is difficult to produce a 'true' black by mixing pigments, and so pigments that are naturally black, such as charcoal, are often used instead. The other type of color mixing is additive, which is the process that occurs when we look at a project image from an RGB projector. In this case, red, green and blue light are mixed and the result is white light rather than black.

calcite A comparison of a single crystal of calcite and powdered calcite. Public domain images from Parent Géry and Walkerma.

But then how do we get white pigments? White pigments are actually colorless solids if you looked at them as single crystals. What makes them white is reflection of all wavelengths of visible light. If the crystals are large, they appear transparent, but if they are very small, the reflected light from the surfaces is enhanced and the particles will be scattering all wavelengths of light in a multitude of directions. The result is that the compound appears white even though it is really colorless. This is illustrated with a large single crystal of calcite when compared to powdered calcite as used in gesso for the initial treatment of a surface before it is painted.

The effect of grain size on color of a pigment The effect of grain size on the color of azurite. The sample on the left has a much smaller grain size than the one on the right. These samples were obtained from NaturalPigments.com

Crystallite size is not only responsible for making colorless compounds appear white, but it will also affect the apparent color of colored minerals. An example of the apparent color of azurite shows that the more finely crystalline material appears to be much lighter in color owing to the diffuse scattering of light. This is an important consideration in the manufacture of pigments where the crystallite growth conditions must be rigorously controlled in order to achieve uniform color matching in the products.

Color in Inorganic Compounds

d-orbital energies The energy levels for the five d-orbitals with an octahedral and tetrahedral set of ligands.

There are several ways in which minerals and other inorganic compounds can produce color. These include:

The presence of transition metals often results in colored compounds. This is because these metals may have electrons d-orbitals. For isolated transition metal atoms or ions, the d-orbitals all have the same energy, but in the presence of atoms or ions bound to them, which are called ligands, the d-orbitals will have different energy levels according to the geometry of the ligands around the ion. It just so happens that the energy difference between these levels falls in the visible region of the electromagnetic spectrum, meaning that transition metals with some d electrons will generally absorb visible light. For this reason copper-containing minerals are often green or blue, while manganese will usually produce a pink or red color or Fe3+ compounds are often yellow, orange or brown. Similarly, transition metals that have no electrons in d-orbitals such as Ti4+ or Sc3+, or whose d-orbitals are completely filled, such as Ag+ and Zn2+ will have colorless compounds (unless some other method of color production is present).

A comparison on colorless and irradiated fluorite. Images are from C. Millan/CC BY-SA and E. Strauhmanis, Public Domain.

Irradiation by high energy sources by may cause naturally-occurring minerals to have electrons excited to molecular orbitals in the crystal where they become trapped in a metastable situation. These are called color centers and they occur frequently in common minerals such as halite (sodium chloride, NaCl), fluorite (calcium fluoride, CaF2) and quartz (SiO2) that are otherwise colorless. Halite can be blue, and fluorite is usually green or purple, and smokey quartz becomes gray or black. Additional examples of minerals that possess color due to irradiation are found ate Gems & Minerals. These colors, however, are not stable and the color and may fade over time or if the sample is heated, which allows the electrons to leave the trapped state and return to their lowest energy configuration. Color can also be introduced into metal compounds by intentionally irradiating them to produce or enhance color for the commercial market.

For some compounds, there can be charge transfer, or a movement of an electron from the ligands to the transition metal or vice versa, and these are known simply as metal to ligand charge transfer (MLCT) or ligand to metal charge transfer (LMCT). They can result in very intense absorptions.

Other minerals may get their color from the presence of trapped radicals. This is the case for lapis lazuli where the color is attributed to the presence of S3- radical ions. Lapis lazuli had particular usage in midievil and Renaissance paintings as the color associated with divine religious figures such as the Virgin Mary. Lapis lazuli was mined in Afghanistan and also went by the name ultramarine, meaning across the sea.That pigment was so expensive that some artists would scrape it off older canvases for reuse, and a synthetic version was created in the 1800s to provide a less expensive option (see Inorganic Pigments).

Color in Organic Compounds

But without transition metals, how do organic moleclues produce color? As for all pigments, the color is related to the orbital energies of the molecule or solid compound in question. Most inorganic pigments have their colors because of the electron in d-orbitals, and the absorption of energy caused a transition of on electron from one d-orbital to another, higher energy d-orbital. A similar phenomenon occurs with organic molecules, but organic molecules lack d-orbitals of the appropriate energy for absorptions in the visible spectrum.

But organic molecules posses molecular orbitals that can be tuned to absorb in the desired part of the electromagnetic spectrum. Whenever two or more atoms combine to form molecules, they create a set of molecular orbitals. These orbitals are classified as bonding - the ones that hold the molecule together, antibonding - the ones that, if occupied, tend to reduce the bonding between atoms, and nonbonding, which do not contribute to the bonding between atoms. In the most simple molecules, the bonding and antibonding orbitals occur in pairs, and the antibonding orbital is always higher in energy than the bonding orbital that is corresponds to. Once the set of molecular orbitals is created, the electrons occupy the lowest energy molecular orbitals first, regardless of what type they are. As these orbitals increase in energy, a point is reached where the orbitals are not unoccupied. This often occurs between an orbital that is primarily bonding and one that is primarily antibonding.

Energy level diagram A simplified schematic of an energy level diagram for a molecule or chemical compound.

The highest energy molecular orbital that has electrons in it is called the Highest Occupied Molecular Orbital (HOMO) and, similarly, the lowest energy orbital that has no electrons is alled the Lowest Unoccupied Molecular Orbital (LUMO). If one looks at moving an electron from the bonding orbitals to the antibonding orbitals, the one with the smallest energy gap will usually be the transition from the HOMO to the LUMO, and the energy difference between them is known as the HOMO-LUMO gap. According to quantum mechanics, if one adds an energy, such as shining photons of light on the molecule, that matches the diffence in energy between two orbitals - an electron will move fro the HOMO to the LUMO.

In the figure, the different horizontal lines represent the energies of the individual molecular orbitals (MOs) that make up the molecule. The small arrows represent the electrons that occupy the orbital. According to quantum mechanics, only two electrons can occupy a given orbital and their spins must be paired – electrons have spin similar to that of a spinning top – some will spin clockwise and the others counterclockwise, and in a single orbital two electrons must have paired spins. When energy equal to the difference between two orbitals is added by irradiating the molecule with the appropriate energy photons (indicated by the wavy line), an electron will move from the lower energy orbital to the higher energy one. This results in absorption of energy and this can be measured by a spectrophotomer. If the molecule is already excited with the electron in the higher energy state, the electron can spontaneously move back to the lower energy orbital. When it does, it will emit a photon of the same energy. This is known as emission, and it can also be measured by spectrophotometers.

Normally, if a molecule has only single bonds, the HOMO-LUMO gap is quite large and falls in the UV region of the spectrum. Double bonds will have a smaller HOMO-LUMO gap, but one that is still in the UV region. But, as one begins to string these double bonds together in a chain of alternating double and single bonds, the energy gap becomes smaller and smaller, to the point where it ultimately lies in the visible region of the spectrum, which can be seen approximately for annatto, β-carotene, lycopene and saffron. The length of the double-bonded carbon backbone of the chain will correlate with the wavelength of light absorbed. At the extreme end is the organic molecule known as polyacetylene, which is comprised of very long one-dimensional carbon chains with alternating double and single bonds, and polyacetylene is black. One way of looking at this is the size of the molecular orbital. For each of these molecules, there is a molecular orbital that extends the entire length of the carbon chain. This means that the chain with the most number of carbons is longer, the wavelength of the electrons in that orbital will be the longest and will therefore have the lowest energy (remember that wavelength and energy are inversely proportional to each other). Remember also that the color absorbed by the molecule is not the color that is seen - the color observed is the light that is reflected from the object, so polyacetylene will absorb all frequencies of visible light radiation.

The structure of benzene.Various ways to draw the structure of benzene, C6H6. These structures are all equivalent.

But the double bonds do not have to be arranged in chains, and other atoms besides carbon can also be present. Common functional groups that are often present in pigment and dye molecules are aromatic rings, where the alternating double and single bonds are found in a ring. The simplest example of this is benzene, C6H6, shown in the figure that shows multiple ways to draw the benzene molecule. Because one could draw the bonds in either of the ways shown in the top two drawings, these structures would have the same energy, and in the early days of understanding these molecules the two forms were said to "average" to one in which the C-C bonds were neither double nor single, but rather having a bond order of 1.5. In reality, the carbon-carbon bonds in benzene are all the same (1.40Å) and are intermediate in length between the longer single bonds (ca. 1.54Å) and the shorter double bonds (ca. 1.34Å). This is now referred to as delocalization, and the structure on the bottom left with a circle in the center of the ring is drawn to emphasize that the electrons in the double bonds are spread out over the entire ring. Benzene itself is a colorless molecule, but stringing lots of benzene groups together will also lower the energy of absorption. In the limit, that is what happens in a graphene layer (see ) as found in graphite, which is black for the same reasons that polyacetylene is black.

Other organic groups on the molecule can cause the HOMO-LUMO gap to move into the visible region of the spectrum. Similar to the effect observed by making polyacetylene, The group responsible for absorbing the light is known as the chromophore, while the groups added to the molecule to tune the light absorption are known as auxochromes. Auxochromes can either increase or decrease the energy difference. Those that increase the HOMO-LUMO gap are known as Hypsochromes, while those that decrease the energy gap are known as bathochromes. Some common hypsochromes are nitro (-NO2), cyano (-CN), sulfonic acid (-SO3H) or sulfonate (-SO3-), and nitro (-NO). Typical bathochromes include amine (-NH2), hydroxo (-OH), and alkoxo (-OR, where R can be a common organic group such as methyl, ethyl, phenyl, etc.). The difference between these two groups is that hypsochromes tend to pull electron density away from the chromophore, while the bathochromes tend to push electrons towards the chromophore. A hypsochromic shifts the absorption towards shorter wavelengths and higher energies, so these groups are said to cause a blue shift in the absorption, while bathochromes result in a red shift. The addition of ionizable groups like the sulfonic acid or carboxylic acid groups to the molecule will help to make these materials water soluble. These acids will react with bases to produce salts that are more water soluble than the parent neutral molecules.

Pigments, Dyes, Inks and Glazes

While pigments, dyes, inks and glazes all are used because of their color, the requirements for the use can be very different. While some materials can be used for more than one of these applications, there are general guidelines for what makes a compound suitable for each of these applications. Here are some rough guidelines:

Pigments: To be useful in paints, pigments must generally be insoluble, whether they are inorganic- or organic-based formulations. They are not dissolved in their medium such as oil, acrylics, tempera, etc. so much as dispersed within the medium. The lack of solubility helps to keep the pigment fixed in its place once the paint is applied. Many inorganic pigments that contain heavy metals (Pb, Cd, Hg) are toxic and while they pose little threat once they are fixed in a dried medium on a painted object, preparation of such pigments as fine powders exposes the artist or the manufacturer to toxic dust, and modern, commercially prepared paints generally avoid highly toxic compounds. Lead paints used in houses are notorious too for contaminating soils when the paints are scraped off during a re-painting process and children have been known to have been poisoned by ingesting paint chips containing lead-based pigments.

Naturally dyed yarnDifferent colors obtained using madder root as a dye, Colonial Williamsburg. Madison60 / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Dyes: Dye molecules must often be soluble or at least dispersible in a suitable solvent in order to facilitate the dyeing process. They are first dissolved in a suitable solvent, most often water, to which the textile is added. Most dyes are organic molecules, but some do contain inorganic materials. A notorious example is Paris green, an arsenic-containing compound that was used for dyeing fabrics for clothing the 1800s (see Inorganic Pigments). An important consideration, especialy with organic dyes, is the molecules response to pH. Many change color depending on the acidity of the solution and also find application as acid-base indicators in the laboratory. The triaryl methane-based dyes (see ) are a good example of this. Thus pH must be carefully controlled. The color of the dye will also depend on the particular fiber that it is attached to, so that a variety of shades are produced depending on whether the fabric is wool, cotton, linen, silk, etc. See the example of dyeing fabrics using madder root from Colonial Williamsburg.

A model for one possible way that the mordant Al3+ could bind to a fragment of cellulose and cochineal.

There are two fundamental types of dyes, substantive and adjective. Substantive dyes do not require special treatment in order for the dye to adhere to the fabric, but adjective dyes do no bind strongly and require the use of a mordant. Indigo is a substantive dye, while cochineal is an adjective dye. The role of the mordant is very simple - it is usually a metal ion that is capable of binding to functional groups in the organic molecules of the textile as well as to functional groups in the dye molecule. Common mordants included alum (KAl[SO4]2·12H2O), and other salts of Al, Cr, Cu, Fe, K, Na and Sn. The mechanism by which binding to the metal occurs is through lone pairs of electrons on functional groups in the organic molecules. This is illustrated for a cellulose fragment, an aluminum cation and cochineal dye. Because of the number of oxygen atoms in both cellulose and cochineal, there are many possible ways in which the Al3+ can bind to these molecules to hold them together. The cochineal molecule (see Organic Pigments & Dyes for more information on cochineal and its structure) is recognized by the three fused aromatic benzene rings, while cellulose is made of a string of glucose rings. The transition metal ions such as Cr, Cu and Fe would possess their own color arising from the electonic transitions in the d-orbitals (see Inorganic Pigments for an explanation of how this happens) and these would impact the end color of the dyed material.

Tannic acid, C76H52O46.

Inks: Inks are pigments dispersed in a liquid, often water or alcohol, or a gel medium for writing on paper or parchment. Black inks can be made used soot, or carbon black, or graphite. A common ink in the middle ages was iron gall ink. Iron gall ink was prepared by mixing ferrous sulfate (FeSO4) with an extract from galls on oak or other trees. This extract contains tannins, and the result is an iron tannate complex. Upon exposure to air, the Fe2+ ions would oxidize to Fe3+ and the resulting ferric tannate ink would become insoluble in water. Iron gall ink often caused deterioration of the parchment or paper on which it was used owing to its acidity.

Glazes: Glazes are colorants that painted onto ceramic objects and then fired to achieve various types of designs. These are most often inorganic pigments mixed with a glass-forming material such as silica which melts to form a glassy coating on the ceramic.

The Greeks developed these techniques to a high level using by firing the pottery at several temperatures and reducing (oxygen-poor) or oxidizing (oxygen-rich) conditions. The main body of the pottery was red, and it was painted with a slip — a slurry of clay and pigmnent — that contained iron oxide, which would turn black upon reduction of the iron oxide to metallic iron. The vessel was first fired at ca. 800°C under oxidizing conditions that would produce an object that was completely red. Upon increasing the temperature to 950°C and shutting off the supply of air to the kiln, the the object would become solid black. The object would then be re-exposed to air and cooled. During this stage the red color would return to bulk object, but the areas that had been painted with the black slip would remain black.

An alternative method for producing black pottery was developed by the Puebloan Native Americans in northern New Mexico beginning in the early 1900s and envolved firing the ceramic in the presence of organic matter, such as cow dung, under reducing conditions. Those pieces are now iconic collector's items.

Problems with Pigments

While many paintings have survived centuries in very good condition, that is not true of all works. A few examples will be noted here.

Organic pigments, which owe their colors to their abilities to absorb light, may also undergo photoreactions when exposed to ultraviolet radiation. A notable example is the fading of some red and yellow organic pigments in works by van Gogh (Van Gogh’s Fading Colors Inspire Scientific Inquiry). Both his "Roses (1890)" and "Field with Irises near Arles (1888)" suffer from his use of cochineal and eosin that faded with time. This left the roses, which were described as pink, turning white and the irises, which were said to be purple, being blue.

But inorganic pigments are also subject to degradation. van Gogh also used red lead, Pb3O4 (see Inorganic Pigments), that is known to convert to colorless lead sulfate, plumbonacrite, Pb10(CO3)6O(OH)6 and (hydro)cerussite, Pb3(CO3)2(OH)2, when exposed to too much light.

Hydrocerussite, Pb3(CO3)2(OH)2.
Plumbonacrite, Pb10(CO3)6O(OH)6.

Mercuric sulfide, whose mineral name is Cinnabar and also known as vermilion when used as a pigment, darkens to gray or black. This has been attributed to reaction with chloride ions, most probably arising from exposure to humans. The initial reaction of HgS with chloride is thought to produce corderoite, Hg3S2Cl2, which the decomposes into calomel, Hg2Cl2, S(s) and a cubic form of HgS, called metacinnabar, that is black.

Hg3S2Cl2(s) + light → Hg2Cl2(s) + S(s) + HgS(s)

Cinnabar, HgS. Cinnabar is a brilliant red compound found in the trigonal crystal system.
Metacinnabar, HgS. Metacinnabar is black and has a cubic crystal structure that is similar to that of zinc blende, ZnS.

Red lead, Pb3O4 and lead white, PbCO3 are subject to reaction with sulfur-containing compounds such as H2S in the atmosphere or other pigments such as cinnabar or orpiment (As2S3). The product is galena, PbS, which is black.

Another problem with zinc white, ZnCO3, and lead white, PbCO3, is the reaction with the drying oils in oil paintings to produce soaps. We generally think of soaps as the reaction of lye (sodium or potassium hydroxide) with fatty acids, but soaps can be made with any metal cations. The drying oils are long chain fatty acids that have C=C double bonds in the chain. The carboxylic acid function, -CO2H, reacts with these metal carbonates to produce M(O2CR)2 (M = Zn, Pb). The problem with these soaps can flake off the painting creating serious issues in restoration and conservation.

MCO3(s) + 2 HO2CR(l) → M(O2CR)2(s) + H2O(l) + CO2(g)

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