Ultramarine Pigments Research Articles

Kapadokya’daki Post-Bizans (XIX yüzyıl) Kilisesindeki Duvar Resimlerindeki Pigmentlerin Arkeometrik Teknikler ile Araştırılması

Cappadocia region, rock churches, monasteries, houses, underground settlements, valleys and historical and geological formations have unique beauty. The region, which is rich with its cultural assets belongs to Christian culture in Anatolia, is generally known with its rock churches, and is a subject to researches. The purpose of this study is; to understand the wall painting technology of the 19th century church, and to make a comparison with the wall painting materials of the rock churches. The study material is composed of pigments in the wall paintings of the Church of St. Theodoros Trion, which is from the 19th century, and located in Nevşehir. Scanning Electron Microscope and Energy Distributed Spectrometer (SEM-EDX), X-Ray Diffraction Spectrometry (XRD) and Fourier Transformed Infrared Spectrophotometer (FTIR) are used to determine material characterization of pigments. X-Ray Fluorescence Spectrometer (XRF) analysis method was used in the analysis of the soil structure. As a result of analysis of the wall painting, it has been found that the ultramarine pigment is obtained from the lapis lazuli mineral and the covellite mineral of the covellite blue. In red, the red ochre and the caput mortuum pigment in purple color has been found. In the mineral of different pigment species, bauxite mineral belongs to the region have been detected. Green color, it has been determined that it is obtained from the glauconite and celadonite mineral in the region. According to the research carried out in the region in green color, it has been detected that only this pigment species has been used from the 6th century to the 19th century.

Development of textile printing ink using expanded polystyrene waste as a binder

This research produced textile pigment printing paste using expanded polystyrene (EPS) as a binder, sodium alginate as a thickener, and ultramarine pigment. The printing paste production was optimized through response surface methodology, using central composite design (CCD). The dynamic viscosity of the produced printing paste samples ranges from 5200 to 5550.50 cP (Centi Poise). The Fourier infrared transform (FTIR) spectroscopy revealed the functional groups present in the printing paste. The result of the thermogravimetric analysis (TGA) conducted shows a first step decomposition of 94.14% that occurred at the range of 30.96°C–127.75°C. Differential scanning calorimetry (DSC) showed an endothermic transition at 119.44°C due to the absorption of energy during the breaking of printing paste bonds. Textile characterization such as wash fastness, croak fastness, and light fastness was carried out to determine the suitability of the print paste on cotton and polyester fabrics. The results of the fastness tests range from 4 to 5 for wash fastness, 3 to 4/5 for croak fastness, and 4 to 6 for light fastness. The optimum values of the model variables are thickener (50%), binder (40%), and pigment (5%). The performance of the printing paste as revealed from the results is moderate and comparable with textile printing paste suitable for commercial application.

Multianalytical Non-Invasive Characterization of ‘Mater Boni Consilii’ Iconography Oil Painting

This paper presents the results of non-invasive diagnostic investigations performed on the canvas oil painting depicting the Marian iconography “Mater Boni Consilii”. The painting, whose author and origin are unknown, was found in an old shop in Florence following the overflowing of the Arno River in 1966. In order to define the importance of the artwork, a multianalytical analysis was performed on the painting, using multispectral imaging, X-ray fluorescence (XRF), and Fourier Transform Infrared Spectroscopy (FTIR-ATR) for the definition of materials, with a particular focus on the identification of pigments. The results allowed for the drawing up of a color palette, composed mainly of ochre and earth pigments, cinnabar, lithopone, lead white, and ultramarine pigments. After cross-referencing the acquired information with other findings, it was possible to place the painting in the period between the end of the XIXth and the beginning of the XXth centuries.

Multidisciplinary approach to the study of large-format oil paintings

Cultural heritage has become a keystone for comprehending our society, as it represents and reflects our origins, passions, beliefs and traditions. Furthermore, it provides fundamental information about specific temporary spaces, materials’ availability, technology, artist’s intention, and site weather conditions. Our aim was to develop a multidisciplinary approach with a main focus on investigating two Italian large-format paintings located in highly diverse environments such as the National Theater of Costa Rica. We monitored environmental conditions and quantified fungal aerial spores. Then, we determined regions of possible biodeterioration with the software MicroorganismPattern and used the software PigmentArrangement to elucidate the apparent colour of the paintings based on distribution and arrangement of the pigment crystals. Finally, we characterized eight genera of calcareous nannofossils found in the ground layers of the artwork. The former Men’s Canteen at the National Theater of Costa Rica presented a mean air temperature of 23.5 ^{circ }C, a relative humidity of 72.7% and a concentration of CO_{2} of 570 ppm. The fungal aerial concentration was 1776 spores/m^{3}. The software MicroorganismPattern identified 32 sampling regions, out of which 11 were positive for microbial contamination. The software PigmentArrangement determined that the blue crystals (ultramarine pigment) had the shortest distances between themselves (29 upmum). Finally, the nanofossils identified enabled us to restrict the age of the material to a biostratigraphic interval ranging from Coniacian to Maastricthian ages. By using a multidisciplinary approach we were able to explore the diptych, suggest a set of minimally invasive perspectives in tropical environments to be used worldwide and obtain key information about the artist’s artistic process, materials used along with better understand its state of conservation.

Heubach launches blue pigment

The German pigments maker Heubach has introduced a line of ultramarine pigments for plastics. The firm says the pigments, made in Dahej, India, are resistant to heat and light. Heubach and the private equity firm SK Capital bought the Clariant colorants business last year.

A Novel Method to Formulate Pigmented Powder Coatings by Ultrafine Powders

Powder coatings are a green alternative to conventional solvent-borne liquid coatings, but they have the intrinsic drawback of color-matching and adjustment in production with the conventional extrusion process. In this study, an industrially applicable approach to formulate color powder coatings utilizing ultrafine powders, i.e., a powdery blending and pressing method, was invented. This novel method was validated by comparing samples prepared by the Method 1 conventional extrusion method with an extra ultramarine pigment at 3%; Method 2 powdery blending and pressing of the original coatings and the same coating with 6% ultramarine pigment utilizing regular (coarse) powder coatings; and Method 3 utilizing ultrafine powder coatings for the two coatings with the same formulations as Method 2. The coating powders were prepared to have similar particle sizes and particle size distributions, with three commonly used coating binders, namely polyester-epoxy hybrid, polyester/TGIC (triglycidyl isocyanurate), and polyurethane (PU). The powders prepared by Methods 1 and 3 had similar flow abilities in terms of angle of repose (AOR) and avalanche angle (AVA). The performance of the new coatings by Method 3 was close to or better than the ones prepared by Method 1 in terms of the specular gloss, DOI (distinctness-of-image), reflection haze and color values, being superior to Method 2. The coatings via ultrafine powders also exhibited a comparable ultramarine particle distribution in the coating cross-sections as the conventional ones, whereas the ones via regular powders had an inferior pigment dispersion. The new method can greatly enhance the production efficiency and reduce the cost of powder coatings with compound colors, especially for small batch manufacturing.

Multisensor hyperspectral imaging approach for the microchemical analysis of ultramarine blue pigments

This work presents a multisensor hyperspectral approach for the characterization of ultramarine blue, a valuable historical pigment, at the microscopic scale combining the information of four analytical techniques at the elemental and molecular levels. The hyperspectral images collected were combined in a single hypercube, where the pixels of the various spectral components are aligned on top of each other. Selected spectral descriptors have been defined to reduce data dimensionality before applying unsupervised chemometric data analysis approaches. Lazurite, responsible for the blue color of the pigment, was detected as the major mineral phase present in synthetic and good quality pigments. Impurities like pyrite were detected in lower quality samples, although the clear identification of other mineral phases with silicate basis was more difficult. There is no correlation between the spatial distribution of the bands arising in the Raman spectra of natural samples in the region 1200–1850 cm−1 and any of the transition metals or rare earth elements (REE). With this information, the previous hypothesis (based on bulk analysis) attributing these bands to luminescence emissions due to impurities of these elements must be revised. We propose the consideration of CO2 molecules trapped in the cages of the aluminosilicate structure of sodalite-type. Additionally, correlation between certain Raman features and the combined presence of Ca, P, and REE, in particular Nd, was detected for the lowest quality pigment. Our results highlight the usefulness of fusing chemical images obtained via different imaging techniques to obtain relevant information on chemical structure and properties.

Colour stability of wood coatings pigmented with natural indigo from Isatis tinctoria after accelerated weathering

AbstractAn experimental study of artificial weathering of two outdoor coatings pigmented with natural indigo from Isatis tinctoria was carried out. The two coatings, natural indigo‐pigmented boat varnish and outdoor paint, were exposed to cycles of ultraviolet light, water condensation and water spray for 40 days in a QUV test chamber. Synthetic indigo and commercial ultramarine pigment were employed as reference materials. The effects of artificial weathering on the coatings were evaluated with CIELab parameters, as well as visual inspection. Coating and pigment stability during artificial weathering were studied with reflectance spectrometry, hyperspectral imaging technology and Fourier Transform–infrared spectroscopy. According to the results, coatings with indigo proved to be more resistant to visual and spectral changes than unpigmented or commercially pigmented coatings.

The influence of light and relative humidity on the formation of epsomite in cadmium yellow and French ultramarine modern oil paints

The effect of relative humidity (RH) and light on the development of epsomite (MgSO4·7H2O) in Winsor & Newton cadmium yellow (CY) and French ultramarine (FU) artists’ oil colour paints was investigated. Tube paint samples were aged for 12 weeks at either 50% or 75% RH, under ambient light (200 ± 1 lx), elevated light (11,807 ± 328 lx), and near-dark conditions. Aged paint samples were characterised using light microscopy (LM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Results indicated that ageing at 75% RH in elevated light conditions, promoted the formation of hydrated magnesium sulphate (MgSO4·6–7H2O) crystals on the surface of both paints. The formation of sodium sulphate (Na2SO4) as a degradation product of French ultramarine oil paints after ageing at 75% RH in elevated light conditions is described. The formation of magnesium sulphate crystals in the absence of elevated SO2 is a new finding. For both cadmium yellow and French ultramarine oil paints, the pigments present are a likely source of sulphur, enabling the formation of sulphate salts, i.e., cadmium sulphide (CdS) yellow, and the sulphur radical anions (S3−) present in ultramarine pigment. Sulphur-containing impurities arising from pigment manufacture are an additional possibility. It was previously theorised that epsomite formation in water-sensitive twentieth century oil paintings resulted from exposure to the elevated atmospheric sulphur dioxide (SO2) levels of the 1950s–1970s. This study demonstrates that hydromagnesite-containing cadmium yellow and French ultramarine oil paints of any period may be vulnerable to water-soluble sulphate salts formation and that this process is promoted by exposure to light and high (75%) RH environments. The formation of sulphate salts as a degradation product is known to contribute toward the development of water sensitivity of modern oil paintings which can pose significant challenges to conservation. Therefore this study highlights the importance of minimising exposure to light and raised relative humidity for paintings containing such CY and FU oil paint passages, to help slow down these types of degradation phenomena which have implications for preservation.

Ultramarine pigments

Abstract Ultramarine pigments are representatives of the inorganic blue pigments. They are characterized by intensive shades of blue and relatively weak violet and pink color tones. The composition of the pigments can be summarized by the formula Na7Al6Si6O24S3. The color of ultramarine is generated by incorporation of free polysulfide radicals in the structure of a sodalite-derived crystal lattice. Ultramarine pigments are synthesized using a series of complex solid-state reactions. The pigments are mainly used in plastics, paints and powder coatings and printing inks, but also in paper and paper coatings.

Revealing time\u2019s secrets at the National Theatre of Costa Rica via innovative software for cultural heritage research

Establishing affordable, efficient, accessible, innovative, and multidisciplinary methodologies to the diagnosis of the conservation state of an artwork is key to carry out appropriate strategies of conservation and consequently to the creation of modern public policies on cultural heritage. Limited access to large-format paintings is a challenge to restoration scientists seeking to obtain information quickly, in a non-destructive and non-invasive manner, and identify regions of interest. Therefore, we put forward two unique software tools based on multispectral imaging techniques, with the long-term aim to assess the artist’s intentions, creative process, and colour palette. This development paves the way for a comprehensive and multidisciplinary understanding of the mysteries encompassed in each pictorial layer, through the study of their physical and chemical characteristics. We conducted the first ever study on Musas I and Musas II, two large-format paintings by Italian artist Carlo Ferrario, located in the National Theatre of Costa Rica. In this study, we used our novel imaging techniques to choose regions of interest in order to study sample layers; while also assessing the works’ state of conservation and possible biodeterioration. We explored the applications of our two versatile software tools, RegionOfInterest and CrystalDistribution, and confirmed paint stratigraphies by means of microscopy and spectroscopy analyses (OM, SEM-EDX, Fluorescent microscopy, FTIR-ATR and micro-Raman). In a pilot study, we identified the artist’s main colour palette: zinc white, lead white, chrome yellow, lead read, viridian, along with artificial vermilion and ultramarine pigments. We were able to identify artificial vermilion and ultramarine and distinguish them from the natural pigments using CrystalDistribution to map the average size and diameter of the pigment crystals within the paint layers. This study demonstrated that software-based multidisciplinary imaging techniques are novel in establishing preventive and non-invasive methods for historical painting conservation studies, in addition, this study provides tools with great potential to be used in the future in applications such as virtual restoration.

Characterization of the Alphabet Reform Painting

The Alphabet Reform painting which shows Ataturk teaching the new alphabet is important for the history of the Republic of Turkey. This painting was made by Nazmi Ziya GÜRAN in 1933. The painting is now maintained by the Ministry of Interior. In this work, the Alphabet Reform painting was analyzed for characterization and restoration purposes. The fabric of the canvas, pigments (organic-inorganic) and the binder of paint were characterized by Fourier Transform infrared spectroscopy-attenuated total reflectance (ATR-FTIR), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) and high-performance liquid chromatography-coupled with diode array detection (HPLC-DAD) analysis methods. The CIEL*a*b* color parameters of the paint were determined by color measurement spectrophotometer. According to the results; canvas was found to be of fibers of vegetable origin and the binders were determined to be linseed oil and dammar varnish. Red alizarin (madder lake organic pigment) and dark blue ultramarine pigments were detected as the organic pigments while CdS (yellow), Pb red (flesh color), PbCO3 (white), ZnO (white), BaSO4 (white), FeO (brown) and Cr2O3 (green color) were detected as the inorganic ones.

Synthesis of inorganic cobalt-containing spinel pigments by SHS method

Cobalt-containing spinel-type ultramarine pigments were obtained by self-propagating high-temperature synthesis (SHS) in the ZnO-MgO-CoO-Al(OH) 3 -Al system. Starting components were oxides of cobalt (Co3O4) and zinc (ZnO), aluminum hydroxide (Al(OH) 3 ), and 6-water magnesium nitrate (Mg(NO 3 ) 2 •6H 2 O). ASD-4 grade aluminum powder was used as a reducing metal. The samples with a diameter of 40 mm were synthesized. The combustion wave velocity was 1-2 mm/s, and the maximum synthesis temperature was 1180 ° С . Parallel aluminum oxidation and aluminothermic reactions were the leading reactions providing the synthesis of spinel-based ceramic pigments in the layer-by-layer combustion mode. They result in charge self-heating up to the synthesis temperatures of spinels that are also formed with the release of heat. The fast destruction of Al(OH) 3 upon heating leads to the formation of active submicron y-A1 2 O 3 , which is involved in the further synthesis of finely dispersed spinel. Endothermic effects associated with Al(OH) 3 decomposition lead to burning sample cooling. This complicates the SHS implementation and requires additional heat supply. Gases emitted during thermal decomposition loosen the charge in the heating zone and reduce the maximum combustion temperature that allows solid-phase synthesis without any melting of the product to obtain it in a finely dispersed state. The microstructural analysis of samples by scanning electron microscopy confirmed the finely dispersed structure of pigments. IR spectroscopy and X-ray diffraction analysis revealed spinel structures. The paper presents the particle size distribution histograms for starting Al(OH) 3 , Al(OH) 3 after heating, and synthesized spinels. It was shown that the pigment contains the maximum number of 903 nm particles. Therefore, obtaining finely dispersed spinel-type pigments by solid-phase synthesis directly in the combustion wave greatly simplifies their production process due to the absence of a grinding stage.

Evidence for the catalytic properties of ultramarine pigment

Abstract Ultramarine blue paint layers in oil paintings can be affected by ultramarine ‘disease’ or ‘sickness’: a phenomenon described by a grey appearance and a loss of detail in the artwork. An explanation for this phenomenon is an interaction between the organic binder network and ultramarine pigment, with the pigment acting as a catalyst for the breakdown of the network. This breakdown results in micro-cracks in the paint film, which influences the appearance of the artwork. To investigate the possible catalytic property of ultramarine pigment, a test reaction – the dehydration of methanol to dimethyl ether – was carried out, with and without ultramarine pigment present in a micro-reactor with in-line gas chromatography mass spectrometry. It was observed that a higher yield of dimethyl ether was obtained in the presence of ultramarine pigment, confirming that ultramarine pigment possesses catalytic properties similar to commercial zeolitic silica-alumina catalysts.

Sulfur K-edge micro- and full-field XANES identify marker for preparation method of ultramarine pigment from lapis lazuli in historical paints.

Ultramarine blue pigment, one of the most valued natural artist's pigments, historically was prepared from lapis lazuli rock following various treatments; however, little is understood about why or how to distinguish such a posteriori on paintings. X-ray absorption near-edge structure spectroscopy at the sulfur K-edge in microbeam and full-field modes (analyzed with nonnegative matrix factorization) is used to monitor the changes in the sulfur species within lazurite following one such historically relevant treatment: heating of lapis lazuli before extracting lazurite. Sulfur signatures in lazurite show dependence on the heat treatment of lapis lazuli from which it is derived. Peaks attributed to contributions from the trisulfur radical-responsible for the blue color of lazurite-increase in relative intensity with heat treatment paralleled by an intensified blue hue. Matching spectra were identified on lazurite particles from five historical paint samples, providing a marker for artists' pigments that had been extracted from heat-treated lapis lazuli.

Ultramarine blue nanoparticles as a label for immunochromatographic on-site determination of ractopamine.

A competitive immunochromatographic assay (ICA) is presented and used for on-site determination of ractopamine (RAC). Ultramarine blue nanoparticles were directly separated from ultramarine blue industrial products by centrifugation (< 10,000rpm and > 4000rpm) and used as visible labels in ICAs. The ultramarine blue nanoparticles were coated by polyacrylic acid (PAA), which provides carboxyl groups on the surface of ultramarine blue nanoparticles. An anti-RAC monoclonal antibody (mAb) was covalently immobilized on the carboxyl-modified ultramarine blue nanoparticle surface via diimide-activated conjugation between the carboxyl groups on the ultramarine blue nanoparticle surface and the amino groups of the antibodies. RAC and BSA-modified RAC competitively bind to the anti-RAC mAb on the ultramarine blue nanoparticles. The blue band in the test line is generated by the accumulation of ultramarine blue nanoparticles and is negatively associated with the RAC content. Under optimal conditions, the visual limit of detection (vLOD) of this ICA for RAC is 2.0ngmL-1, 2.0ngmL-1, and 1.0ngmL-1 in phosphate-buffered saline (PBS), feed samples, and pork samples, respectively. The ultramarine blue nanoparticle-based ICA also shows no cross activity with salbutamol, clorprenaline, clenbuterol, or terbutaline. Graphical abstract Schematic representation of the ultramarine blue nanoparticles immunochromatographic assay for detection of ractopamine (RAC) based on competitive method. The ultramarine blue nanoparticles were screened from commercial ultramarine pigments for the first time and used to detect ractopamine.

Natural or synthetic? Simultaneous Raman/luminescence hyperspectral microimaging for the fast distinction of ultramarine pigments

Genuine ultramarine has been one of the most precious blue pigments employed since ancient times. It used to be obtained by crushing and grinding the lapis lazuli rock and selectively extracting the blue mineral lazurite. Since 1828, when it was produced the synthetic version, the use of this much less expensive material became widespread and synthetic ultramarine blue replaced the natural one in painting palettes. The distinction between natural and synthetic ultramarine is, therefore, an important goal in authentication issues. Here, we present a hyperspectral microRaman imaging analysis, complemented with the characterization of ultramarine pigment samples in terms of colour, elemental composition and identification of crystalline phases by means of fiber optics reflection spectroscopy (FORS), X-Ray Fluorescence (XRF) and X-Ray diffraction (XRD), respectively. We show that a characteristic luminescence signature, easily detected in the course of standard Raman spectroscopic analysis using 785 nm as excitation, is indicative of the natural origin of the ultramarine blue pigments. Furthermore, simultaneous Raman/luminescence hyperspectral microimages allow the distinction between natural pigments of different quality thanks to the different intensity and spatial distribution of the lazurite characteristic band at 548 cm−1 and those of the luminescent impurities in the region between 1200 and 2000 cm−1. The establishment of a range of quality between different samples of lapis lazuli was also attempted taking into account the variations in the intensity of the Raman bands located at 548 cm−1 and 1286 cm−1. Finally, the feasibility of using the simultaneous Raman/luminescence features for authentication of ultramarine blue in a real historical context, even with non-invasive investigations, is illustrated with examples of different types of blue decorations from several halls of the Alhambra monumental ensemble, in Granada.

Out of the blue: Vermeer\u2019s use of ultramarine in Girl with a Pearl Earring

Johannes Vermeer (1632–1675) is known for his brilliant blue colours, and his frequent use of the costly natural ultramarine. This paper reveals new findings about ultramarine in the headscarf of Girl with a Pearl Earring (c. 1665, Mauritshuis). The painting was examined using a range of micro- and macroscale techniques as part of the Girl in the Spotlight research project (2018). Analysis of micro-samples mounted as cross-sections using SEM–EDX and FTIR-ATR showed that Vermeer used high-quality ultramarine in the blue headscarf, based on the relative abundance of bright blue particles of lazurite. Analysis with synchrotron sulphur K-edge XANES suggested that the ultramarine pigment was prepared—at least in part—from a heat-treated lapis lazuli rock. The entire painting was imaged using MS-IRR, MA-XRF, RIS, and digital microscopy to reveal the distribution of materials of the headscarf, and to give more insight into Vermeer’s painting process. The shadow part of the headscarf has a remarkably patchy appearance, due to paint degradation that is probably related to the large amounts of chalk Vermeer mixed in the ultramarine paint in this area. The question was raised as to whether extra chalk was added deliberately to the paint to adjust the handling properties or opacity, or whether the chalk was the substrate of a—now faded—yellow lake. Schematic paint reconstructions were made to investigate the effect of the addition of chalk or yellow lake on the paint properties. The analyses and reconstructions led to the hypothesis that the blue headscarf originally contained a wider range of different blue colour shades: an opaque light blue for the left (lit) zone, a slightly brighter opaque blue for the middle zone, and a deep dark blue-green glaze with alternating blue-green glazing brushstrokes for the shadow zone—now largely compromised by paint degradation.

A systematic study of the synthesis conditions of blue and green ultramarine pigments via the reclamation of the industrial zeolite wastes and agricultural rice husks.

Discarded industrial zeolite waste and agricultural rice husks have caused a waste of resources. To achieve resource reuse, we proposed an economical method for the preparation of ultramarine pigments via the reclamation of industrial zeolite waste (ZW) and agricultural rice husks (RHs) or previously bio-charred rice husks (BRHs). The optimal blue and green pigments were synthesized by solid state reaction of mixtures of BRH/ZW/Na2CO3/S with mass ratios of 1:2:1:1.5 and 2:2:7:3, respectively, and using a two-step calcination process with a first stage at 500 and a second stage at 800°C. Furthermore, the blue and green pigments were also obtained using directly RH (instead of BRH) as raw material, but this time with RH/ZW/Na2CO3/S mass ratios of 1:2:2:3 and 1:2:7:3.5 and with first-stage and second-stage calcination temperatures of 400 and 800°C. This was done to reduce additional chemical reactions (e.g., BRH derived from the pyrolysis of RH). The XRD, FT-IR, Raman, and SEM results suggest that the synthetic blue and green pigments have the sodalite structure containing S3- and S2- radicals. The synthetic green pigment using RH as raw material had the best acid resistance. Additionally, the synthesis of blue and green ultramarine pigments via the reclamation of the industrial zeolite wastes and agricultural rice husks can reduce the costs of the production process.

Computational refinement of the puzzling red tetrasulfur chromophore in ultramarine pigments

We investigated the cryptic red chromophore, accompanying the blue S3˙- radical in ultramarine pigments, which usually was tentatively assigned to an unspecified isomer of either neutral S4 or ionic S4˙- species. To reveal its identity, we performed the first systematic density functional studies on periodic and large cluster models of red ultramarines, considering several S4/S4˙- isomers embedded in aluminosilicate cages. For both neutral and charged tetrasulfides the most stable confined isomer is the planar C2v one. The only plausible candidate for the red chromophore among the tetrasulfur species is the planar C2v isomer of the neutral S4 molecule, which, apart from being thermodynamically preferable, strongly absorbs green light and its vibrational modes match very well with the available Raman data. The C2v-S4˙- radical, if present at all in red ultramarines, could be identified by strong absorption in the near infrared region and possibly by the slightly larger isotropic value of the g tensor than that of the S3˙- radical.