The Chemistry and Mechanism of Art Materials

Unsuspected Properties and Outcomes

Michael J. Malin

• Publisher: CRC Press
• Year: 2021
• ISBN:
  9781000507805
• Edition: 1
• Topic: Art
• Subtopic: Art General

This unique book presents an integrated approach to the chemistry of art materials, exploring the many chemical processes involved. The Chemistry and Mechanism of Art Materials: Unsuspected Properties and Outcomes engages readers with historical vignettes detailing examples of unexpected outcomes due to materials used by known artists.

The book discusses artists' materials focusing on relevant chemical mechanisms which underlie the synthesis and deterioration of inorganic pigments in paintings, the ageing of the binder in oil paintings, and sulfation of wall paintings as well as the toxicology of these pigments and solvents used by artists. Mechanisms illustrate the stepwise structural transformation of a variety of art materials.

Based on the author's years of experience teaching college chemistry, the approach is descriptive and non-mathematical throughout. An introductory section includes a review of basic concepts and provides concise descriptions of analytical methods used in contemporary art conservation.

Additional features include:

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• Illustrations of chemical reactivity associated with art materials

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• Includes a review of chemical bonding principles, redox and mechanism writing

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• Covers analytical techniques used by art conservation scientists

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• Accessible for readers with a limited science background

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• Provides numerous references for readers seeking additional information

1 Essential Concepts

DOI: 10.1201/9781003053453-1

1.1 Chemical Bonding

Art materials are generally solids (including pigments, metals, ceramics and enamels, which are covered in this book) and also solvents, polymerizable liquid binders, including wax, egg yolk, (linseed) oil and acrylic polymers (which are organic materials) [1]. Of the organic materials, the toxicity of solvents and the aging of oil binders are covered in the book. The focus will be inorganic materials, particularly pigments whose discolorations have been investigated by contemporary analytical techniques and described in detail. Some space is also allotted to ceramics and glazes to provide a chemical foundation.

There are four types of inorganic solids: metallic, ionic, molecular, and network. These are distinguished by the lattice-point particles (i.e., the type of particle, atom or ion), which occupy the lattice points of the respective crystal, and physical properties such as melting point, electrical conductivity, hardness and solubility, see Table 1.1.

TABLE 1.1 Solids and Associated Physical Properties [1]

Solid/property	Metallic	Ionic	Molecular	Network
Particle	Atoms	Ions	Molecules	Atoms
Bonding	Metallic	Ionic	Covalent	Covalent
Melting point	Low to high	High	Low	High
Elec. Conductivity	High	Solid: nil	Nil	Nil
		Melt: high
		Solution: high
Hardness	Hard	Hard	Soft	Hard
Solubility	Insol	Depends on ionic charge	Like dissolves like	Insoluble
Examples	Ag, Cu, Pb	CaCO3	Arsenic sulfides	Diamond
		Prussian blue	Orpiment, realgar	Silica

Solubility is defined as the number of grams of substance which will dissolve in 100 mL of a liquid solvent at a specified temperature. Inorganic pigments are generally ionic compounds with low solubility in water and even lower solubility in organic liquids, e.g., linseed oil, or egg yolk. In oil painting, linseed oil is used as a “binder”, a spreadable liquid in which pigment particles are suspended.

Metallic solids include pure metals (elements) and also alloys in which the two or more species of metal atoms have similar size and are therefore able to co-crystallize. The “sea of electrons” model for metallic bonding consists of a three-dimensional stacked array of cations which are surrounded by loosely bound valence electrons which are shared by all of the cations, see Figure 1.1. The mobility of the valence electrons is consistent with the high electrical conductivity and thermal conductivity of metals.

FIGURE 1.1 Electron-sea model for metal bonding.

Ionic solids are held together by Coulombic forces between anions and cations according to Coulomb’s Law:

$$\text{Force of ionic attraction}=\left[\text{k}\left|{\text{q}}^{+}{\text{q}}^{-}\right|\right]/{\text{d}}^2,$$

where q⁺ and q⁻ are the respective ionic charges, d is the inter-nuclear distance and k is a proportionality constant. Hence, ionic bond strength is directly proportional to the absolute value of the product of the ionic charges and inversely proportional to the inter-nuclear distance. This accounts for the wide range in water solubility of salts, e.g., Na⁺ Cl⁻ (very soluble) and Ca²⁺ S²⁻ (poorly soluble). Further, ionic compounds conduct electricity in the molten state and also when dissolved in a solvent because the ions are mobile. In the solid state, ionic compounds do not conduct because the ions are locked up in a crystalline lattice. When salts dissolve in water, the aquated ions carry electrical current through the solution. Figure 1.2 shows a representation of the NaCl crystal matrix in which the smaller spheres are the sodium cations and the larger spheres are the chloride anions [2].

FIGURE 1.2 NaCl crystal lattice.

In molecular solids, the atoms are held together by covalent bonds. Melting points of molecular compounds are low, generally < 200°C, because the crystal lattice is stabilized by weak attractive forces between molecules (e.g., dispersion forces, dipole–dipole interactions and hydrogen bonds), all of which are considerably weaker than either covalent or ionic bonds. When a molecular crystal is heated to its melting point, the weak intermolecular forces are broken but not the strong intramolecular covalent bonds unless decomposition occurs. This class of compounds does not conduct electricity because there are no ions present. Covalent bonds are “directional” in the sense that they form because of the overlap of hybridized orbitals and as a consequence, specific molecular geometries result which manifest in characteristic bond angles. In contrast, ionic bonds are not directional, and characteristic crystal unit cells are formed because of favorable energetics due to stacking of ions (in ionic compounds) or atoms (in metals).

In discussions of chemical bonding, it is convenient to present three types of bonds. See Figure 1.3, where “I” represents a covalent bond between two identical atoms (e.g., I₂ or H₂). In this case, the electron pair which comprises a single bond must be equally shared between the bonding partners, and therefore, this type of bond is said to be non-polar. The other extreme situation, “III”, can be represented by the ionic bond between Na⁺ and Cl⁻, in which there is little or no sharing of a pair of electrons. In pictorial terms, the pair of electrons in the bond resides very close to the chlorine and far away from the sodium, so that effectively an ion-pair is formed. In the intermediate situation, II, a pair of electrons is unequally shared between the bonding partners. The more unequal the sharing, the more polarized the bond will be. In this situation, partial positive and negative charges are assigned to the bonding pair, and a bond dipole can be drawn over the bond (with the arrow facing the negative e...