In pre‐Hispanic Mesoamerica, pigments and dyes were used in the elaboration of a large variety of colored objects. Obtaining information regarding the objects' material composition is useful in restoration and preservation processes, as well as for recovering knowledge of its production technology and the context and history of the object. Yellow colors have been obtained from a large variety of Mexican natural resources. Although mineral yellow pigments, such as orpiment and oxides, are relatively easy to identify by nondestructive and noninvasive techniques (X‐ray fluorescence, Raman spectroscopy, fiber optics reflectance spectroscopy, and Fourier transform infrared spectroscopy), organic yellow colorants are difficult to analyze by these techniques. Therefore, most works dealing with the identification of yellow dyes use destructive methods such as liquid chromatography–mass spectrometry. The aim of this work is to elaborate a methodology based on Raman spectroscopy and surface‐enhanced Raman spectroscopy for the study of yellow colorants used in pre‐Hispanic and colonial Mexico, in order to aid in their identification by spectroscopic techniques. The yellow colorants selected for this work were extracted from local plants: zacatlaxcalli (Cuscuta tinctoria), old fustic (Maclura tinctoria), weld (Reseda luteola), marigold (Tagetes erecta), and xochipalli (Cosmos sulphureus).

One contribution of 14 to a theme issue 'Raman spectroscopy in art and archaeology'.

Raman spectroscopy is often used for non-destructive analysis of works of art, polymers and biological materials, but in some cases, the laser beam can cause damages on the surface being studied due to the deposited beam energy. When possible, such damage can be prevented by minimizing laser intensity or acquisition time, but this is usually available only on high sensitivity bench-top spectrometers. Portable Raman spectrometers are commonly not so flexible and an alternative is needed to ensure the safe study of sensitive works of art and other fragile materials. A helium jet aimed directly at the laser spot may prevent this damage from occurring and, in some cases, helps improving the Raman spectra. We designed a simple system (HERAS, Helium Raman System) consisting of a pinhole collimator, coupled to a helium line and a gas mass flux control and tested it on pyroxylin, vermilion and ochre paint references and pigment samples, using a 785 nm portable Raman spectrometer at various laser powers. Experimental conditions slightly differ for each sample, but small burns on the surface were avoided in all cases and only some ablation was observed on the most sensitive materials. The use of this coupled system allows the non-destructive application of Raman to a wider variety of materials, while the technique remains portable. This setup may be used in bench-top apparatus as well.

Almost three hundred Spanish colonial missions—or their remains—are scattered over the vast state of Chihuahua in northern Mexico. A few of them still display painted decorations on the wood ceilings and walls. The decorated areas vary greatly, from the whole ceiling of the main aisle to just a few square meters in a lateral chapel, and so does the conservation state of the paintings. In this context, the information regarding the paintings’ composition plays a key role in the restoration and conservation processes. For the gathering of such information, we propose a combined methodology for a fast, non-destructive and non-invasive characterization of such paintings with a minimum of techniques. This methodology includes false color infrared imaging as a first approach to determine the composition of large areas of the paintings and the homogeneity of the materials used in the painted areas, followed by small area analysis by X-ray fluorescence and fiber-optics reflectance spectroscopy. This methodology was applied to characterize the elemental and molecular composition of the decorations for four missions in Chihuahua in a fast and specific manner, revealing the use of a mix of mineral and organic materials including indigo and cochineal, and detecting differences between the missions. The methodology presented here can be easily applied for the study of a wider number of missions in Chihuahua and other regions to provide outstanding information of materials, pictorial techniques and deterioration conditions.

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