2 METHODOLOGY

2.1 TECHNICAL ANALYSIS OF PIGMENTS

The 29 works studied were part of a restoration and preservation project undertaken by the Fundaci�n TAREA of Buenos Aires. Both qualitative and quantitative methods were used for the analysis of the pigments. In some cases, chemical microscopy studies were carried out on cross sections or on unmounted samples. The specific microchemical tests used for each pigment are described in each pigment's section. A scanning electron microscope (PSEM 500) equipped with an energy dispersive xray detector (EDX-4) was used for inorganic microanalysis. It was operated at 30 kV with diffusion pump vacuum in the mode of secondary electrons. The samples were coated by sputtering with a thin (less than 80 �) layer of gold.

X-ray diffraction (XRD) was performed on a Phillips diffractometer (PW 1050), with copper radiation and nickel anticathode with different voltages for each spectra, depending on the sample. Tandem mass spectrometry (MS-MS) for the detection of indigo was performed on a ZAB hybrid spectrometer (Micromass, Manchester, UK) (BeqQ), filtering the molecular ion of indigo with B (magnetic sector), colliding with helium in the second, field-free region, and analyzing the fragments with E (electrostatic sector).

Copper resinate appeared transparent and viscous green under the optical microscope, and it did not show evidence of particles. The small amount of sample available prevented exact identification of its components by gas chromatography–mass spectrometry, but oil or protein could be excluded and the presence of signals like terpenes similar to those of some conifers could be observed. The presence of copper was determined by SEM, in addition to microchemical tests using rubeanic acid (Plesters 1956). Feigl's procedure (Feigl 1958, 80–82) was also used to determine the presence of copper by catalyzing the iron thiosulfate reaction through the presence of copper ions. None of the aforementioned methods was useful for detecting the acetate group.

Among the analytical criteria used for the identification of malachite are solubility and effervescence in cold 3N hydrochloric acid (HCl) and in concentrated nitric acid (HNO3) (Gettens and FitzHugh 1993), performed on the cross sections. A test for copper in malachite using rubeanic acid (Plesters 1956) was performed on the samples. When a solubility test in ammonia was performed, the solution turned deep blue, indicating a copper-ammonia complex. SEM elemental analysis indicated in all cases the presence of copper as a sole component or sometimes with minor impurities such as lead, silicon, calcium, aluminum, or iron.

Orpiment was identified where SEM results indicated the presence of arsenic (As) and sulfur (S). Some microchemical tests were also performed. The pigment was soluble in 3N hydrochloric acid, giving off hydrogen sulfide, and in concentrated nitric acid, giving off arsenic. It was also soluble in sulfuric acid and in 4N sodium hydroxide (Plesters 1956).

Vermilion was identified where SEM results indicated the presence of mercury (Hg). The pigment was not affected by dilute or concentrated nitric acid, hydrochloric acid, or sulfuric acid. It was also unaffected by 4N sodium hydroxide.

Red earth pigments such as hematite were identified from the presence of iron (Fe), detected by SEM analysis.

To identify carmine lake, a small sample was hydrolyzed and analyzed by thin-layer chromatography (TLC) on acetylcellulose according to the procedure described by Masschelein-Kleiner and later compared with results obtained for a genuine sample of carminic acid (Masschelein-Kleiner 1967; Masschelein-Kleiner and Heylen 1968).

Red lead was indicated by the presence of lead in SEM analysis. Some microchemical analyses were also performed, such as insolubility in 4N sodium hydroxide and solubility in dilute acids.

2.2 PIGMENT REFERENCES IN ART-HISTORICAL LITERATURE

Available references on the pigments (apart from blue) used in colonial South American art refer to cardenillo (verdigris), coravari (chrysocolla), and earths for green hues; g�nuli, jalde (orpiment), and saffron for yellow hues; grana de M�xico (cochineal from Mexico) or magna, achiote, vermilion, minium, dragon's blood, and red earths for red hues; and white lead for white hues and lights, as well as ochers, earths, and ivory black for shadings. Understanding pigment nomen-clature has been problematic. Undoubtedly due to transcription errors from the original sources, changes in terminology, and the wide range of materials used during this period, some differences have been identified. Thus, Mesa and Gisbert (1982, 267) refer to “Castilian colors” imported from Spain, based on a 1581 contract in which the master gilder Juan de Ponce was commissioned to gild the retable of the Church of Our Lady of Mercy in Cuzco (Cornejo Bouroncle 1960, 161–64). They quote a phrase from this document that mentions “genolis en borlazar con bermell�n,” which we understand as referring to g�nuli, azarc�n (red lead), and vermilion.

Specific treatises and manuals, such as Carducho's ([1633]1979), Pacheco's ([1649]1990), Palomino's ([1723]1988), and Manuel Samaniego y Jaramillo's (Vargas 1975) offer a broader palette including a wide range of hues as well as recipes and recommendations for mixing and applying pigments. Pacheco mentions mountain green (a copper ore that results in a light green hue, which may be related to malachite), earth green, and verdacho (also used by Palomino)—all of them derived from mineral sources. Among other pigments used during this period, red lead (in Spanish, azarc�n) is also mentioned. It is a burnt lead oxide, orange-red in color, and also known as minium or Saturn red. This color was considered by Palomino ([1723]1988) among the “false colors” because the colors shifted and became duller as the paint dried (literally,“when drying it casts out crusts which strip away all gentleness from the painting” [135–36]). Another such pigment is the encorca, mentioned by Pacheco and Palomino as “Flanders' encorca,” which yields a dark yellow hue.

There are documents on pigment traffic and how pigments were transported from one place to another. An anonymous manuscript of the 17th century, probably written by a converted Portuguese Jew, gives information on shipments of cochineal (carmine) from Mexico to Peru and also from Guatemala and Nicaragua. This “Memoir of All Kinds of Merchandises Necessary for Peru, Which Cannot Be Obtained Because They Are Not Made in This Country” records powdered cochineal from Valencia and verdigris (Lewin 1958, 121). Several private contracts and inventories were studied, such as the 1581 contract commissioning of Juan de Ponce to gild the retable of Church of Our Lady of Mercy in Cuzco. Other examples are the registrar book of the Church of Yucay in 1679 (Mesa and Gisbert 1982, 268–69); the accounting book of the Chapel of Our Lady of the Rosary in the monastery of Saint Dominic in Quito, dated 1823; and a list of tools and materials inventoried at the missions of Paraguay after the Jesuits were expelled from Spanish territory in 1767 (Ribera and Schenone 1948, 60).