Synthetic Organic Pigments Research Articles

The Condition of Contemporary Murals in Sun-Exposed Urban Environments: A Model Study Based on Spray-Painted Mock-Ups and Simulated Light Ageing

The present work investigates the physicochemical stability of spray paints when irradiated with artificial solar light (at spectral range 300–800 nm). This research highlights the importance of understanding the materials used in street art and public murals, recognising them as a significant component of contemporary cultural heritage. By examining the stability and degradation of spray paints toward solar light exposure, the study aims to contribute to the preservation of contemporary murals, which reflect current social and cultural narratives. A physicochemical approach was employed for the study of spray paints’ physical and thermal properties, as well as the effect of specific photochemical ageing reactions/processes. The photochemical ageing results were compared with reference (unaged) samples. Specifically, a multi-technique approach was applied using stereo microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), contact angle measurement, colorimetry, glossimetry, differential scanning calorimetry (DSC), UV-Vis spectroscopy, attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), and pyrolysis-GC/MS (Py-GC/MS). The photodegradation of the spray paints occurred from the first 144 h of solar light irradiation, resulting in changes in morphology, colour, gloss, roughness, and wettability. Regarding photochemical stability, ageing seems to affect the binders more than the synthetic organic pigments and the inorganic fillers. In particular, acrylic binders showed small chemical changes, whereas the alkyd, nitrocellulose, and styrene binders underwent severe chemical modification. The results suggest that simulated daylight irradiation prompts the migration of additives toward the surface of the spray paint films. In addition, the results of the analyses on the white spray paints in comparison with the coloured paints (from the same manufacturer) showed that there seems to be an active distinct photoageing mechanism involving titanium dioxide, but the whole issue needs further investigation.

Connecting palettes — An integrated spectroscopic analysis and UMAP visualisation of Wu Guanzhong's paint palette and his painting

A paint palette of the 20th-century Chinese modern artist Wu Guanzhong (1919–2010) was studied with an integrated spectroscopic approach, macro-scale non-invasive techniques hyperspectral imaging (HSI) and macro X-ray fluorescence (MA-XRF), complemented with Raman spectroscopy. The palette is revealed to be comprised of synthetic inorganic and organic pigments, typical of the 20th century modern palette that is dominated by azo and phthalocyanine pigments. HSI datasets of the paint palette and those of Wu's oil painting ‘Xidi Village’ (2001) were then processed within one unsupervised Uniform Manifold Approximation and Projection (UMAP) model, in an attempt to identify connections between the artefact and the artwork, so that known pigment results of the palette may provide preliminary pigment assignment for the painting. This study describes the steps taken to establish this automated assisted pigment analysis workflow, which involves processing multiple HSI datasets within one UMAP model, density clustering, non-negative least square (NNLS) fitting to extract endmembers (EM) spectra and to generate distribution maps. The approach to perform micro-sampling pigment analysis on the artist's palette (or artist's materials) is considered less invasive to the integrity than that from artworks, the results of which have demonstrated to serve as a critical resource to support further pigment analysis studies of the artist's paintings via non-invasive analytical techniques.

An Introduction and Recent Advances in the Analytical Study of Early Synthetic Dyes and Organic Pigments in Cultural Heritage

This article reviews the research recently undertaken to characterise and identify early synthetic dyes (ESDs) and synthetic organic pigments (SOPs) as well as study their degradation pathways with a focus on cultural heritage applications. Since the invention of the first fully synthetic dye in 1856, these materials have been used in a variety of historic objects and artworks, such as textiles, furniture, prints, paintings, drawings, etc. The synthesis of new dyes and their implementation into pigment formulations were rapid phenomena, so that by the beginning of the 20th century, thousands of new molecules were patented and commercialised. This review will focus on these early formulations synthesised up to approximately 1914 and the outbreak of World War I. ESDs and SOPs represent a challenge from an analytical point of view. Molecular databases are fundamental for the unequivocal identification of these compounds, but reference materials are not always readily available. A combination of analytical techniques is often needed for characterisation, especially in the case of SOPs, which have both organic and inorganic components. Furthermore, the degradation of these molecules can jeopardise their detection in historic objects that have been exposed to light, humidity and temperature fluctuations. Nevertheless, ESDs and SOPs are important research tools. As the dates of their first synthesis are often known with precision, based on patent information, their detection can be used to refine the production date of objects. Additionally, their trade from Europe to the world and their adoption in artistic practices around the globe is an active area of research.

Screening for pigment production and characterization of pigment profile and photostability in cold-adapted Antarctic bacteria using FT-Raman spectroscopy

Microbial pigments can replace synthetic organic pigments which often produced in unsustainable way and can be toxic. Therefore, search for new pigment producing microorganisms is of high interest for industry. In this study, a screening and characterization of pigment profile and photostability in seventy-four newly isolated Antarctic bacteria using Fourier-transform (FT) Raman spectroscopy and HPLC-MS was performed. Screening of the bacterial biomass by FT-Raman identified thirty-seven bacterial strains from the genera Agrococcus, Arthrobacter, Brachybacterium, Cryobacterium, Leifsonia, Micrococcus, Paeniglutamicibacter, Rhodococcus, Salinibacterium and Flavobacterium as having relatively high pigment content. The impact of growth temperature on the pigment production in the studied Antarctic bacteria was species - specific, while blue light exposure triggered pigment production in majority of the studied bacteria. HPLC-MS analysis of a biomass of a set of ten pigmented Antarctic bacteria identified eighteen different carotenoids and precursors. FT-Raman spectroscopy showed to be suitable for both, semi-qualitative library-independent identification of pigment producing bacteria and determination of pigment profile using spectral library of reference pigment standards. This study provides valuable insights into the pigment production capabilities of Antarctic bacteria and highlighting the potential of FT-Raman spectroscopy for characterizing microbial pigments.

Construction of Pt@BiFeO3 Xerogel-Supported O-g-C3N4 Heterojunction System for Enhanced Visible-Light Activity towards Photocatalytic Degradation of Rhodamine B

Synthetic organic pigments from the direct discharge of textile effluents are considered as colossal global concern and attract the attention of scholars. The efficient construction of heterojunction systems involving precious metal co-catalysis is an effective strategy for obtaining highly efficient photocatalytic materials. Herein, we report the construction of a Pt-doped BiFeO3/O-g-C3N4 (Pt@BFO/O-CN) S-scheme heterojunction system for photocatalytic degradation of aqueous rhodamine B (RhB) under visible-light irradiation. The photocatalytic performances of Pt@BFO/O-CN and BFO/O-CN composites and pristine BiFeO3 and O-g-C3N4 were compared, and the photocatalytic process of the Pt@BFO/O-CN system was optimized. The results exhibit that the S-scheme Pt@BFO/O-CN heterojunction has superior photocatalytic performance compared to its fellow catalysts, which is due to the asymmetric nature of the as-constructed heterojunction. The as-constructed Pt@BFO/O-CN heterojunction reveals high performance in photocatalytic degradation of RhB with a degradation efficiency of 100% achieved after 50 min of visible-light irradiation. The photodegradation fitted well with pseudo-first-order kinetics proceeding with a rate constant of 4.63 × 10−2 min−1. The radical trapping test reveals that h+ and •O2− take the leading role in the reaction, while the stability test reveals a 98% efficiency after the fourth cycle. As established from various interpretations, the considerably enhanced photocatalytic performance of the heterojunction system can be attributed to the promoted charge carrier separation and transfer of photoexcited carriers, as well as the strong photo-redox ability established. Hence, the S-scheme Pt@BFO/O-CN heterojunction is a good candidate in the treatment of industrial wastewater for the mineralization of organic micropollutants, which pose a grievous threat to the environment.

Exploratory investigation of historical decorative laminates by means of vibrational spectroscopic techniques

Literature research revealed that scientific work on the characteristics of historical decorative laminates (DLs), like typology, and particularly the material-technical aspects related to the composition and build-up is scarce in the field of conservation-restoration of modern materials. This paper aims to take the first step towards filling in this knowledge gap by demonstrating that an in-depth literature research and complementary chemical-physical analytical techniques are useful in characterising and contextualising historic decorative laminates. This research focused on gaining additional information within chemical analyses on material-technical insights. This could serve as a basis for a more comprehensive historical context. Several historical samples from different sample-catalogues and museum objects, spanning a period from 1953 to 1993, were collected and their respective cross-sections were characterised with infrared (mapping), FT-Raman, and dispersive micro-Raman spectroscopy. The different layers, such as the protective, decorative, and core layers were investigated, and materials such as melamine formaldehyde, urea formaldehyde, cellulose, lignin, titanium dioxide (rutile), zinc sulphide, synthetic organic pigments PR112 (monoazo pigment, Naphthol AS), and PG8 (azo metal complex, Pigment Green B) were detected.Graphical

Synthesis of Ruthenium-Promoted ZnO/SBA-15 Composites for Enhanced Photocatalytic Degradation of Methylene Blue Dye

Synthetic organic pigments like xanthene and azo dyes from the direct discharge of textile effluents are considered colossal global issues and attract the concern of scholars. Photocatalysis continues to be a very valuable pollution control method for industrial wastewater. Incorporations of metal oxide catalysts such as zinc oxide (ZnO) on mesoporous Santa Barbara Armophous-15 (SBA-15) support to improve catalyst thermo-mechanical stability have been comprehensively reported. However, charge separation efficiency and light absorption of ZnO/SBA-15 continue to be limiting its photocatalytic activity. Herein, we report a successful preparation of Ruthenium-induced ZnO/SBA-15 composite via conventional incipient wetness impregnation technique with the aim of boosting the photocatalytic activity of the incorporated ZnO. Physicochemical properties of the SBA-15 support, ZnO/SBA-15, and Ru-ZnO/SBA-15 composites were characterized by X-ray diffraction (XRD), N2 physisorption isotherms at 77 K, Fourier-transform infrared (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray (EDS), and transmission electron microscopy (TEM). The characterization outcomes exhibited that ZnO and ruthenium species have been successfully embedded into SBA-15 support, andtheSBA-15 support maintains its structured hexagonal mesoscopic ordering in both ZnO/SBA-15 and Ru-ZnO/SBA-15 composites. The photocatalytic activity of the composite was assessed through photo-assisted mineralization of aqueous MB solution, and the process was optimized for initial dye concentration and catalyst dosage. 50 mg catalyst exhibited significant degradation efficiency of 97.96% after 120 min, surpassing the efficiencies of 77% and 81% displayed by 10 and 30 mg of the as-synthesized catalyst. The photodegradation rate was found to decrease with an increase in the initial dye concentration. The superior photocatalytic activity of Ru-ZnO/SBA-15 over the binary ZnO/SBA-15 may be attributed to the slower recombination rate of photogenerated charges on the ZnO surface with the addition of ruthenium.

Special-Effect and Conventional Pigments in Black Light Art: A Multi-Technique Approach to an In-Situ Investigation.

Since their introduction in the early decades of the 20th century, fluorescent pigments have found progressively wider applications in several fields. Their chemical composition has been optimized to obtain the best physical properties, but is not usually disclosed by the manufacturers. Even the other class of luminescent pigments, namely the phosphorescent ones, is now produced industrially. The peculiar optical properties of these pigments have attracted more and more the attention of famous artists since the middle of the last century. The Italian Black Light Art movement exploits the possibility of conveying different aesthetical messages depending on the kind of radiation (UV or visible) with which the artwork is illuminated. In the present work, a non-invasive in-situ investigation based on Raman, fluorescence, and visible-reflectance spectroscopies was performed on a series of Black Light Art paintings exhibited in Milan (Italy) in 2017, succeeding in the identification of the materials used by the artists. In particular, the use of both fluorescent and phosphorescent pigments, alone or combined with conventional synthetic organic pigments, has been recognized.

Analytical studies on commercial artists’ colour charts from Das Deutsche Farbenbuch (1925)—identification of synthetic and natural organic colourants by Raman microscopy, surface-enhanced Raman spectroscopy and metal underlayer ATR-FTIR spectroscopy

Historical colour charts provide a rich and often well-dated reference materials source for studying the chemical composition of all kinds of commercial brands of artists’ paints. This article presents the results of an extensive analytical study of more than 80 paint hues from 11 colour charts that are included in the German standard book Das Deutsche Farbenbuch by H. Trillich (1925, Part II). Our research focused on the identification of synthetic organic pigments, whose quickly increasing significance for artists’ paints in the early twentieth century is impossible to evaluate by documentary source research alone. A stepwise procedure combining different non- or minimally invasive vibrational spectroscopy techniques—Normal Raman and Surface-Enhanced Raman spectroscopy as well as Metal Underlayer Attenuated Total Reflection Fourier-transform Infrared Spectroscopy—allowed the identification of 18 different organic colourants in artists’ watercolours, tempera and oil colours from six German manufacturers. In addition, micro-X-Ray Fluorescence spectroscopy was applied to determine the elemental pattern of substrates, fillers, and admixed inorganic pigments. In addition to a few traditional natural organic colourants (dark and rose madder lake, cochineal lake), most of the identified compounds comprised synthetic organic pigments or synthetic dyes from various chemical classes (indigo, anthraquinone, monoazo, ß-naphthol, xanthene, triarylcarbonium, nitroso, and azine compounds). Some of these have not or only rarely been reported in artists’ paints so far. Since the identified organic colourants have mainly poor to fair (only sometimes good) fastness to light according to modern standards and partially also to solvents typically used in conservation treatments, it is evident that works of art from this period should be treated keeping in mind the possible presence of such colourants, when planning both interventive treatments and preventive measures.

Multi-Analytical Investigations of Andy Warhol's "Orange Car Crash": Polymeric Materials in Modern Paints.

This work presents strategic multi-analytical investigations performed on “Orange Car Crash” by Andy Warhol in order to make a well-informed conservation decision. For determining the type of binding medium used in the artwork, Pyrolysis–Gas Chromatography/Mass Spectrometry (Py–GC/MS) and Thermally Assisted Hydrolysis and Methylation of GC/MS (THM–GC/MS) were employed. The presence of a coating was investigated by Py–GC/MS. Moreover, the comprehension and elucidation of the paint stratigraphy were studied by examining cross-sections of samples taken from both canvases with Optical Microscopy (OM) under reflected visible (Vis) and ultraviolet light (UV) and by Scanning Electron Microscopy with Energy Dispersive X-ray spectroscopy (SEM–EDX). The investigation of possible synthetic organic pigments (SOPs) and extenders was performed by µ-Raman spectroscopy, while micro-Attenuated Total Reflection of Fourier-Transform Attenuated Total Reflection (µ-ATR–FTIR) allowed us to assign each component detected by Py–GC/MS or THM–GC/MS to a specific layer. The data collected from “Orange Car Crash” show mostly the application of acrylic-based paint as well as alkyd with rosin acids-based ink, thus providing fundamental information about the paint stratigraphy and chemical composition of each layer. In addition to the goal of informing an appropriate conservation–restoration strategy, this work represents a rare scientific study of a work by Andy Warhol.

High-resolution non-invasive X-ray diffraction analysis of artists’ paints

Energy-dispersive X-ray diffraction (EDXRD) is extremely insensitive to sample morphology when implemented in a back-reflection geometry. The capabilities of this non-invasive technique for cultural heritage applications have been explored at high resolution at the Diamond Light Source synchrotron. The results of the XRD analysis of the pigments in 40 paints, commonly used by 20th century artists, are reported here. It was found that synthetic organic pigments yielded weak diffraction patterns at best, and it was not possible to unambiguously identify any of these pigments. In contrast, the majority of the paints containing inorganic pigments yielded good diffraction patterns amenable to crystallographic analysis. The high resolution of the technique enables the extraction of a range of detailed information: phase identification (including solid solutions), highly accurate unit cell parameters, phase quantification, crystallite size and strain parameters and preferred orientation parameters. The implications of these results for application to real paintings are discussed, along with the possibility to transfer the technique away from the synchrotron and into the laboratory and museum through the use of state-of-the-art microcalorimeter detectors. The results presented demonstrate the exciting potential of the technique for art history and authentication studies, based on the non-invasive acquisition of very high quality crystallographic data.

The Art of Everyday Objects: A Non-Invasive In Situ Investigation of Materials and Techniques of Italian Pop Art Paintings on Aluminium

Two paintings, made on aluminium support by Silvio Pasotti (among the major exponents of 1960s Italian pop art) were investigated in a totally non-invasive manner to identify the materials used by the artist. Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), visible reflectance spectroscopy, and spectrofluorimetry with visible excitation were exploited as molecular analysis techniques, which are particularly suitable to recognise also synthetic organic materials, such as pigments and binders. The effectiveness of this multi-analytical approach was demonstrated, leading to the identification of several synthetic organic pigments, both conventional and “special effect” ones, introduced during the first half of the 20th century, as well as some well-established inorganic ones. Combining FTIR results both in the medium and near IR ranges, considerations regarding the binders employed by the artist could also be made, suggesting the use of both nitrocellulose and acrylic paints. Imaging techniques, such as IR reflectography, false colour IR, UV induced fluorescence, and portable microscopy, were also used to achieve a better knowledge of the painting practice.

Study of UV ageing effects in modern artists’ paints with MeV-SIMS

In the present work, alkyd and acrylic self-made paints containing synthetic organic pigments (SOPs) of different chemical classes (phthalocyanine, quinacridone, and diketopyrrolo-pyrrole) were analysed by Secondary Ion Mass Spectrometry with MeV primary ions (MeV-SIMS) to study the chemical changes that occur due to accelerated UV ageing. MeV-SIMS is an emerging accelerator-based, surface-sensitive mass spectrometry technique that can provide information about the chemical composition in the uppermost layers of materials. Two-component mock-up samples were prepared and aged for two and four months, to mimic environmental ageing over periods of one and two years. The obtained results show that the studied SOPs are stable under the accelerated ageing conditions used in this study and that they can be easily identified in aged paints, while the binder components undergo structural changes due to the photodegradation processes. The kinetics of binder degradation were found to be dependant on the pigment present in the paint, with phthalocyanines having the highest photostabilizing effect amongst the pigments studied. In addition, the acrylic binder exhibited slightly higher stability to UV ageing than the alkyd medium. This study allowed the identification of SOPs and binders in the selected contemporary artworks, which date from different periods and belong to both outdoor and indoor collections.

A novel polynomial reconstruction algorithm‐based 1D convolutional neural network used for transfer learning in Raman spectroscopy application

AbstractWhen a convolutional neural network (CNN) model was built, the size and resolution of its input data were fixed. However, Raman spectra collected by different Raman spectrometers usually had different length, intensity range, and wavenumber interval between two adjacent data points, which made the existing CNN model difficult to be applied to a new Raman spectral data set. Therefore, this paper proposed a polynomial reconstruction algorithm as pretreatment method to obtain reconstructed spectra that would be imported into CNN model with consistent length, intensity range, and wavenumber interval. To test the effectiveness of this method, a big data set with 2563 Raman spectra of 831 minerals and synthetic organic pigments samples was constructed from the RRUFF and SOP database to pretrain a one‐dimensional CNN (1D‐CNN) model. The pretraining results showed that polynomial reconstruction algorithm used as pretreatment method was better than SG smoothing combined spline interpolation algorithm. Then two data sets were collected by different Raman spectrometers for evaluating the transfer learning performance of the trained 1D‐CNN model. Both data sets contained 390 Raman spectra from the same 39 samples of inorganic salts, organic compounds, and amino acids. One was used as calibration data to retrain the 1D‐CNN model, while the other was used as test. Based on data augmentation and 75% calibration data for retraining, the transfer learning performances of 1D‐CNN model were clearly shown in the excellent identification accuracies of 99.58%, 99.32%, and 97.69% for training, validation, and test sets, respectively, which were better than those of K‐nearest neighbor classifier. This paper provides a significant way for the wide application of CNN model in Raman spectroscopy with much more advantages in simplicity and rapidity.

Analytical approaches for the characterization of early synthetic organic pigments for artists’ paints

Several analytical techniques have shown use for the identification of historical synthetic organic pigments (SOPs) in samples. However, as the techniques differ—for example, in the extent of sample intervention, modes of detection, and limits of detection—so do they differ in results and insights into the colorants and colorant derivatives in a sample. Liquid chromatography (LC) is infrequently applied to the task of SOP identification, despite its ability to differentiate similar molecules, detect at low concentrations, and separate of complex samples and its established usage for organic dyes. This research compared results obtained with micro-Raman spectroscopy and pyrolysis gas chromatography with results from ultra-high performance (UP)LC coupled with photodiode array detection (UPLC-PDA), and high resolution mass spectrometry (UPLC-HRMS) to judge the quantity and quality of SOP information obtained by each technique.In this study, 67 historical samples were analyzed from historical samples and sample books (1918–1950), consisting of 10 oil paint tubes, 45 oil-bound paint-outs, and 13 gum-bound pigment washes using sample sizes less than a milligram. As few studies have used LC for SOP identification, special attention is also paid to issues around sample preparation and interpretation of results from UPLC-PDA and UPLC-PDA-HRMS results. Finally, archival sources and other contemporary documents are used to contextualize and evaluate the plausibility of the analytical results. The results indicate that specific combinations of analytical techniques are required for confident SOP identifications. While Raman results were accurate and independent of sample solubility, they relied heavily on database completeness and were not sensitive to mixtures or differences in relative amounts of SOPs. UPLC-PDA was an effective complement for these shortcomings, except for a few samples that were insoluble. The use of HRMS was critical for the elucidation of unknown SOPs.

Structural Refinement, Morphological Features, and Optical, Photo- and Sonophotocatalytic Properties of (Ca1-xSrx)WO4 Synthesized by the Sonochemical Method

Aims: We investigate the structural and morphological features as well as the sonophotocatalytic properties of (Ca1-xSrx)WO4 (x = 0.25; 0.50; 0.75 and 1) synthesized by the sonochemical method (SC). The as-synthesized samples were structurally characterized by X-ray diffraction (XRD), Rietveld refinement, micro-Raman, and Fourier-transform infrared (FT–IR) spectroscopies. Field-emission scanning electron microscopy (FE–SEM) and energy-dispersive X-ray spectroscopy (EDX) were used to analyze the morphological aspects and elemental chemical composition, respectively. Background: Currently, environmental issues have been one of the main concerns in the world, especially the contamination of freshwater supplies. Hence, environmental scientists have an enormous interest in developing efficient wastewater treatment technologies to remove harmful substances from contaminated water resources. In general, synthetic organic dyes and pigments, widely employed as coloring agents in leather or textile industries, severely impact the environment because of their toxic and carcinogenic nature. When discharged into water sources, such chemical pollutants have an irreversible and devastating impact on flora and fauna. As an alternative route to minimize these negative impacts, heterogeneous photocatalysis has emerged as a low-cost, safe, and versatile wastewater treatment approach. Objective: To analyze the effect of different strontium (Sr) contents (x = 0, 0.25, 0.50, 0.75, and 1) on the structural, morphological, optical, and sonophotocatalytic (SPC) properties of (Ca1-xSrx)WO4 samples. Methods: (Ca1-xSrx)WO4 (x = 0, 0.25, 0.50, 0.75, and 1) were synthesized by the sonochemical method at 40 ºC for 3 h. Results: XRD patterns, Rietveld refinement data, and micro-Raman and FT–IR spectra proved that all samples are structurally organized in a scheelite-type tetragonal structure without deleterious phases. FE–SEM images showed the growth of flower- and spindle-like crystals. UV–Vis spectra indicated that the increase of Egap (from 4.68 to 4.94 eV) in (Ca1-xSrx)WO4 samples was proportional to Sr concentration inserted into the lattice. Conclusion: Among all samples, CaWO4 exhibited the highest SPC performance for the degradation of RhB solution under UV irradiation. When irradiated for 200 min, a degradation rate of around 96% was achieved for this sample. The ultrasound-assisted photocatalysis proved to be a promising approach for the degradation of organic dyes.

Identification of Pre-1950 Synthetic Organic Pigments in Artists’ Paints. A Non-Invasive Approach Using Handheld Raman Spectroscopy

There is little information on the actual use of early synthetic organic pigments (SOPs) in art objects, especially those from before 1950. Their presence can, however, pose a challenge to conservation because their chemical composition, as well as their lightfastness and sensitivity to solvents, are often unknown. Here, a study on the non-invasive identification of SOPs in historic pre-1950 varnished paint-outs from artists’ materials manufacturer Royal Talens is presented. The paints were analysed using a handheld Raman device. Spectra were evaluated by recording the spectra of the same samples with a benchtop instrument. This study demonstrated that the identification of SOPs in varnished oil paints with a non-invasive approach is possible and rather straightforward. The handheld Raman device allowed us to identify fourteen SOPs from eight pigment classes. Besides the occurrence of expected and the known SOPs of this time period, there were also some surprising results, like the detection of the triarylcarbonium pigments PG2 and PB8, and the monoazo Mordant Yellow 1.

Organic pigments: general principles

AbstractThis introductory chapter presents an overview of the general principles underlying the structural chemistry, manufacturing processes, and application technology of organic pigments. The coverage provides a fundamental theoretical and practical basis for the chapters that follow in this series that are devoted to specific chemical classes of industrially significant organic pigments of the azo, phthalocyanine, carbonyl, dioxazine, and metal complex classes. The initial sections cover the fundamental differences which mean that dyes and pigments are considered universally as two separate types of colorant, based on their solubility characteristics. They also provide discussions of the contrasting chemical, technological, and performance features of organic and inorganic pigments. An outline of the most important historical features in the development of the synthetic organic pigment industry is then presented, from its origins in the 19th century that followed soon after the development of the industrial synthetic dye industry, through its expansion in the 20th century, to its current position as a mature global industry. A section then follows that describes the functions that organic pigments are required to perform in their application, mainly their optical functions that include not only color properties, including hue, strength, brightness, but also the contrasting requirements for transparency or opacity as demanded by specific applications. The pigments are also required to resist the conditions and agencies that they might encounter in applications, assessed as fastness properties, such as fastness to light, heat, solvents and chemicals, amongst many others, to an extent that specific applications demand. The principles, in broad terms, of the ways in which chemical structures determine colour and performance of organic pigments are discussed, with focus not only on the influence of molecular structure, but also on the effect of the crystal structural arrangement and the particulate structure, including particle size and shape and its distribution, on application performance. This is important as these pigments are applied as a dispersion of finely divided crystalline solid particles that are insoluble and are ultimately trapped mechanically in their application medium, often a polymer. The manufacture of organic pigments is discussed in broad terms. The overall process may be considered in stages, initiated by the chemical synthetic sequence in which the pigment is formed, followed by a conditioning stage where the crude product thus obtained is modified to optimise its performance properties, and finally finishing where the product is processed into a form, or preparation, that is suitable for its intended applications. Finally, the technological principles underlying a broad range of the most important application areas for organic pigments, which are mainly in paints, inks, and plastics, are discussed.

Identification of Synthetic Organic Pigments in Contemporary Artists’ Paints by FT-IR and FT-Raman: An Advanced Analytical Experiment

The identification of coloring substances is a relevant task in the field of conservation and restoration, but it can be challenging as in real samples the target analytes are inserted in a very complex matrix made of binders and additives. Understanding this concept is fundamental for upper-division undergraduate students interested in conservation science; therefore, this laboratory experiment aims to make students aware of this problem and leads them to develop a simple protocol combining two spectroscopic techniques, FT-IR and FT-Raman, and basic chemical treatments to overcome this difficulty. The case of the two quinacridone pigments, PR122 and PV19, is presented and discussed, although the same approach can be extended to other commercially available organic pigments.

The identification of synthetic organic red pigments in historical plastics: Developing an in situ analytical protocol based on Raman microscopy

AbstractThe identification of colorants in historic plastics is a methodological and analytical challenge. Although deformulation is performed by the plastics industry, in the case of historical objects, sampling is often impossible, and in situ protocols are needed. The accurate identification of colorants provides insights into historical plastic formulations, supports planning of conservation studies, and critical data for objects that already exhibit color change (e.g. fading). Indeed, colorants may degrade following exposure to light, and color changes have been reported for plastic objects. The analysis of colorants remains a challenge, and microsampling is usually required. Successful identification of red pigments from thirty historical plastics is reported following a new in situ analytical protocol based on Raman microscopy. Information about other pigments, fillers and plastic polymers are reported. Fading has been observed in the historical plastic objects containing Pigment Red 53.