JAIC 1991, Volume 30, Number 1, Article 4 (pp. 35 to 40)
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Journal of the American Institute for Conservation
JAIC 1991, Volume 30, Number 1, Article 4 (pp. 35 to 40)

THE PIGMENTS OF THE CANOSA VASES: A TECHNICAL NOTE

DAVID A. SCOTT, & MICHAEL SCHILLING



2 EXAMINATION


2.1 BLUE PIGMENT

ALL THE pigment samples studied in this report were mounted in Melt Mount of refractive index 1.662 at 25C and were used for polarized light examination employing an Orthoplan-Pol polarizing light microscope. Microscopic study of the blue pigment showed the presence of light blue particles of irregular shape, with a refractive index less than 1.66, which are optically anisotropic and exhibit blue-lavender pleochroism. They give a strong pink coloration when a Chelsea filter is inserted into the light path. The Chelsea filter, often used in gemological studies for emerald testing, is useful here for distinguishing between blue pigments by polarized light microscopy (Mactaggart and Mactaggart 1988; Schilling and Scott 1989). These microscopic properties support the identification of this pigment as Egyptian blue. Confirmation of the identification was obtained by x-ray powder diffraction using a Debye-Scherrer camera.


2.2 PINK COLORANT

Examination was conducted using polarized light microscopy and long-wave UV excitation of the pigment. A microsample was found to be rose colored, isotropic, with a refractive index less than 1.66. Polarized light microscopy revealed small, poorly defined particles, with an uneven and light red coloration, usually indicative of an organic red pigment. The most common organic red pigment used in antiquity was madder, which usually contains two anthraquinone components, purpurin and alizarin. Purpurin produces a bright yellow-red fluorescence under long-wave UV, while alizarin does not show any fluorescence. The sample did fluoresce, suggesting that the colorant is madder. However, analysis by optical emission spectrophotometry carried out by David McJunkin, a research geochemist at the Laboratory for Historical Colorants, University of California, Los Angeles, provided further information. The plant species usually given as the source of rose madder is Rubia tinctorium L., but no detectable amounts of alizarin were found during the analysis, so the madder was unlikely to have originated from Rubia tinctorium. The anthraquinone components purpurin and pseudopurpurin were found, suggesting to McJunkin that another member of the Rubiaceae had been employed as the source of the colorant, probably Rubia peregrina L. or Galium sp., both of which are readily available in the Mediterranean region.


2.3 YELLOW PIGMENT

Several fragmentary pieces of ceramic displayed yellow pigmented surfaces, some of which are very poorly adherent. Yellow surface of the fragment shown in figure 1, for example, had already become partially detached by storage in a polyethylene bag. The pigment had been attracted from the ceramic surface to the bag itself by electrostatic forces.

Polarized light microscopy showed that the pigment consisted of small yellow-brown clumps with yellow rod-shaped particles whose refractive index is difficult to determine. Most standard samples of yellow ochre only contain incidental needle-shaped crystals, but the sample from the ceramic consisted mostly of these fine crystals. The particles are anisotropic and appear very similar to one of the yellow ochres in the Edward J. Forbes collection, labeled as 3.04.11, Naples 1919. From a collection of 10 different samples, only this yellow ochre sample proved to be similar.

A microsample of the pigment from the ceramic fragment was added to a pellet of packed potassium bromide powder and analyzed by Fourier transform infrared spectroscopy in the transmission mode by Michele Derrick (1988). A very close match to the spectrum acquired was given by a reference spectrum of geothite, α-FeOOH. Identification of the ochres in terms of the precise isomer of the iron oxyhydroxides is often difficult, but here relatively pure goethite appears to be the source of the yellow pigment used.


2.4 BLACK PIGMENT

X-ray fluorescence was used first to examine the black pigment areas in situ. It was not possible to demonstrate that the black areas contained higher iron or manganese content than the red body of the ceramic itself. The logical conclusion is that the black surface is likely to be an organic black pigment rather than an iron black.

Caution must be exercised in coming to general conclusions too quickly, however, since a sample of the black pigment examined by polarized light microscopy showed the clear presence of an opaque black mixed with an iron oxide pigment. Examination showed that the iron oxide pigment did not contain any manganese, an impurity usually associated with umbers. The red pigment which was mixed with the black charcoal is also very similar in optical properties to a McCrone reference sample of burnt Sienna rather than an umber or an ochre.

The black surface was examined further in situ with x-ray fluorescence spectroscopy using a sulfur secondary target with helium flushing. The advantage of this method of analysis in the case of carbon blacks is that excitation of phosphorus is greatly enhanced by the use of a sulfur secondary target ensuring that phosphorus will be detected with good sensitivity even at 0.1% by weight. No phosphorus was detected in the black pigment, which implies that bone black or ivory black were not employed; this fact, together with the polarized light microscopy study, indicates the use of a carbon black from a plant source. The use of mixtures of natural earth ochres with carbon to create black is well known and has, previously been reported, for example, by Profi et al. (1974) in their study of pigments from the Greek Bronze Age.

Examination of the pigmented surface by scanning electron microscopy was carried out on a small flake detached from the ceramic. Analysis by wavelength dispersive methods showed that the pigment particles are principally carbon and occur in small clumps; many of the particles are less than 1 micron in size. A scanning electron micrograph showing part of the topography of the black surface revealed surface structure typically associated with carbon black pigment.


2.5 WHITE SLIP

Fourier transform infrared spectroscopy confirmed the results for this slip obtained by Rinuy and Schweizer (1978), namely that the material is a relatively pure kaolinite. This part of the study was not taken any further because of the work that has already been reported.


Copyright 1991 American Institute for Conservation of Historic and Artistic Works