Another beauty of analytical chemistry: chemical analysis of inorganic pigments of art and archaeological objects

The world of inorganic pigments

Abstract

Inorganic pigments have been very important substances for human life for thousands of years. They have contributed and continue to contribute to the beautification of our daily life and habitat and to the accentuation and differentiation of objects, and influence thoughts, moods, and feelings. The industrial manufacture of inorganic pigments based on growing chemical and physical knowledge started around 1800. Before that, pigments based on natural materials or manufactured by mostly empirical processes were used. Most of the technically relevant inorganic pigments were developed and first produced on an industrial scale in the nineteenth and twentieth centuries. Ongoing developments are aiming to improve existing conventional pigments as well as synthesize new, innovative colorants, especially in the classes of effect pigments and functional pigments. The objective of this text is to present comprehensive knowledge about inorganic pigments, including definitions, types of pigments and their properties, historical pigments, modern inorganic pigments, synthesis methods, and applications. It is also intended to show the current status of new pigment developments.

Graphical abstract

Collection of samples of inorganic pigments

Spot tests: past and present

Microchemistry, i.e., the chemistry performed at the scale of a microgram or less, has its roots in the late eighteenth and early nineteenth centuries. In the first half of the twentieth century a wide range of spot tests have been developed. For didactic reasons, they are still part of the curriculum of chemistry students. However, they are even highly important for applied analyses in conservation of cultural heritage, food science, forensic science, clinical and pharmacological sciences, geochemistry, and environmental sciences. Modern pregnancy tests, virus tests, etc. are the most recent examples of sophisticated spot tests. The present ChemTexts contribution aims to provide an overview of the past and present of this analytical methodology.

The dome of Rotunda in Thessaloniki: Investigation of a multi-pictorial phase wall painting through analytical methods

The present study focuses on the investigation of the successive pictorial phases of the wall painting which survives on the missing eastern part of the magnificent mosaic composition in the interior of Rotunda, Thessaloniki, Greece. Rotunda, a circular domed monumental building, was constructed in the early 4th century AD and it is included in the UNESCO World Heritage List. Characterization analysis was performed by means of microscopic, spectroscopic and crystallographic techniques, in order to identify the technological features of the wall painting and the materials used, to document the initial Byzantine pictorial phase -known from the archaeological research- along with the overpaintings attributed to the 19th and 20th centuries. In this framework, the collected samples were studied with optical microscopy, Fourier transform infrared micro-spectroscopy, scanning electron microscopy, X-ray diffractometry and X-ray photoelectron spectroscopy. Among the detected materials and pigments (including zinc oxide, barium sulfate, red lead, green earth, Prussian blue, emerald green, ultramarine and cuprite), the use of brass powder for false gilding purposes was detected, which is a material rarely used for mural applications.

Analytical Techniques for the Preservation of Cultural Heritage: Frontiers in Knowledge and Application

Chemistry is considered as the heart of preservation science. The archaeological objects are an organic-inorganic system and need comprehensive techniques to investigate the different materials with a high resolution and accuracy. The characterization process of archaeological materials is a useful guide to develop the right strategy for the conservation and intervention of the objects. In analytical chemistry practice, there are many techniques to employ the characterization process of the artworks: molecular, elemental, imaging, surface, thermal, separation, nuclear, dating, electrochemical, and miscellaneous techniques. It highlights the potential of chemical investigations to present reliable information to the conservators and art historians.

Identification of proteins, drying oils, waxes and resins in the works of art micro-samples by chromatographic and mass spectrometric techniques

Simplified method for simultaneous identification of proteins, drying oils, waxes, and resins in the works-of-art samples was developed. Liquid chromatography with mass spectrometry and gas chromatography with mass spectrometry were used to identify natural materials most frequently encountered in historical paintings. Protein binders were extracted with ammonia and purified using miniaturized solid-phase microextraction (Omix tips) to efficiently suppress matrix interferences. Zwitterionic stationary phase was used for separation of 16 underivatized amino acids analysis with hydrophilic interaction liquid chromatography that was subsequently quantified with liquid chromatography with mass spectrometry. Gas chromatography with mass spectrometry was used to analyze drying oils, waxes, and resins after one-step saponification/transmethylation with (m-trifluoromethylphenyl)trimethylammonium hydroxide (Meth-Prep II). While the drawback of this reagent is low reactivity towards hydroxyl groups, sample pretreatment was much simpler as compared to the other methods. Fatty acids derivatization with the Meth-Prep II reagent was compared with their silylation using N,O-bis(trimethylsilyl) trifluoroacetamide/trimethylchlorosilane mixture. It was concluded that fatty acids analysis as their methyl esters instead of trimethylsilyl esters had a minor impact on the method sensitivity. The developed method was used to analyze samples from 16^th and 17^th century historical paintings.