Fi Yellows and Pigment Anachronisms

The elements iron (Fe), arsenic (As), lead (Pb), tin (Sn), chromium (Cr), antimony (Sb), cadmium (Cd), and oxygen (O) are contained in various inorganic yellow pigments. Table F.1 lists six of the pigments and their composition.

Prior to the seventeenth century the choice of yellow pigments was limited to (1) lead-tin yellow (Pb2SnO4), and (2) ochres of various hues, prepared from naturally occurring iron oxide ores. Orpiment was an alternative to these but an unpopular one, since its arsenic content made it poisonous and offensive-smelling. The glass colorant, the lead antimonate Naples yellow, was introduced in the seventeenth century into the artist's palette as an alternative to ochre. Vauquelin's 141

Six Inorganic Yellow Pigments and Their Composition

Name

Composition

Six Inorganic Yellow Pigments and Their Composition

Name

Composition

1. Orpiment (king's yellow)

2. Lead-tin yellow

3. Yellow ochre

4. Naples yellow

5. Chromium yellow

6. Cadmium yellow

Arsenic sulfide (As2 S3)

Combined oxides of Pb and Sn (Pb2 Sn O4)

Mainly iron oxide (Fe2 O3) with alumina and silica

Lead antimonate (Pb2 Sb2 O7)

Lead chromate (Pb Cr O4)

Cadmium sulfide (CdS)

isolation of chromium as an element, in 1798, led to development of chromium yellow (PbCrO4) about a decade later, and Stromeyer's isolation of cadmium in 1817 inspired production of cadmium yellow (CdS) early in the 1830s. By this time lead-tin yellow seems to have been totally eclipsed.

There are several different techniques that can be used to distinguish among the yellow pigments. Spectral reflectance curves can be used. X-ray diffraction (Appendix E) provides one positive means of identification, although a small sample (a few micrograms is usually sufficient) must be removed from the painting. The third technique, the subject of this appendix, is the excitation of characteristic energy x-rays. The spectra of the characteristic x-ray energies provides a "fingerprint" of the elements in the pigment. The presence of lead and antimony (Naples yellow) can be determined and distinguished from that of lead and tin (lead-tin yellow). All three techniques—infrared spectroscopy, x-ray diffraction, and x-ray emission—are used for positive identification of the elements.

Electrons in atoms behave as though they were grouped into levels or shells, with all electrons in one shell having approximately the same energies, but with large energy differences between shells. Each shell can hold only a certain maximum number of electrons. If one shell is filled, then an additional electron will be forced to go into a higher-energy, less stable, shell, and this electron will be lost easily during chemical reactions. Conversely, if an atom lacks only one or two electrons to complete a shell, the atom will have a strong attraction for electrons, and can take them away from the type of atom mentioned previously. A completely filled electron shell, with no vacancies and no extra electrons outside, is a particularly stable situation for an atom.

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