Investigation of the colourants used in icons of the Cretan School of iconography

The red shades of 13 icons (15th-17th century) of the Cretan School of iconography are investigated in detail to identify the inorganic and organic colouring materials comprising the paint layers. Examination of sample cross-sections is performed with optical microscopy. Micro-Raman spectroscopy and high performance liquid chromatography (HPLC) coupled to a photodiode array detector are employed for the identification of the inorganic and organic colouring materials, respectively. The results reveal the extensive use of coccid dyes by the Cretan painters: kermes (Kermes vermilio Planchon) is found in icons dated before the middle 16th century and cochineal in icons created several decades after the discovery of the New World. Other dyestuffs detected in the historical samples are madder (possibly Rubia tinctorum L., according to HPLC profiles), soluble redwood and indigoid dyes. Organic dyes were used by the painters as exclusive colouring matters (or glazes) or in mixtures with inorganic pigments, such as red ochre, cinnabar, minium, azurite lead white and carbon black.Liquid chromatography with mass spectrometry (LC-MS) coupled to a negative electrospray ionization mode is employed to provide information on the identity of some unknown colouring components, of the aforementioned dyes, detected in the historical samples. The results suggest that (i) the type B compound (also known as Bra′) is a dehydro-brazilein product and (ii) the deprotonated molecular ion of the type C compound corresponds to m/z = 243. Both compounds are commonly used as markers for the identification of soluble redwood in historical samples. LC-MS analysis of cochineal shows that the dcIV and dcVII components are isomeric with carminic acid, as it has been recently suggested. Finally, LC-MS is employed to identify and record kermesic and flavokermesic acid in kermes and rubiadin in wild madder.     

Analysis of natural red dyes (cochineal) in textiles of historical importance using HPLC and multivariate data analysis

A new analytical approach based on high-performance liquid chromatography with diode array detector (HPLC-DAD) and multivariate data analysis was applied and assessed for analyzing the red dye extracted from cochineal insects, used in precious historical textiles. The most widely used method of analysis involves quantification of specific minor compounds (markers), using HPLC-DAD. However, variation in the cochineal markers concentration, use of aggressive dye extraction methods and poor resolution of HPLC chromatograms can compromise the identification of the precise insect species used in the textiles. In this study, a soft extraction method combined with a new dye recovery treatment was developed, capable of yielding HPLC chromatograms with good resolution, for the first time, for historical cochineal-dyed textiles. After principal components analysis (PCA) and mass spectrometry (MS), it was possible to identify the cochineal species used in these textiles, in contrast to the accepted method of analysis. In order to compare both methodologies, 7 cochineal species and 63 historical cochineal insect specimens were analyzed using the two approaches, and then compared with the results for 15 historical textiles in order to assess their applicability to real complex samples. The methodology developed here was shown to provide more accurate and consistent information than the traditional method. Almost all of the historical textiles were dyed with Porphyrophora sp. insects. These results emphasize the importance of adopting the proposed methodology for future research on cochineal (and related red dyes). Mild extraction methods and HPLC-DAD/MSⁿ analysis yield distinctive profiles, which, in combination with a PCA reference database, are a powerful tool for identifying red insect dyes.     

Electrochemical identification of anthraquinone-based dyes in solid microsamples by square wave voltammetry using graphite/polyester composite electrodes.

An electrochemical method for identifying anthraquinone-type dyes in microsamples from works of art, based in the voltammetry of microparticles approach, is reported. Upon attachment onto graphite/polyester composite electrodes, natural pigments aloe, henna, cochineal red, madder lake, kermes, shellac, and alizarin and purpurin taken as reference materials can be identified from their square wave voltammetric profiles in MeCN (0.10 mol L(-1) Bu(4)NPF(6)) and aqueous (0.25 mol L(-1) acetic acid+0.25 mol L(-1) sodium acetate) electrolytes.     

Identification of naphthoquinonic and anthraquinonic dyes via sequential potential steps applied to the voltammetry of microparticles methodology

An electrochemical method for identifying anthraquinonic, naphthoquinonic, and related dyes in microsamples is reported. This method is based on the sequential application of oxidative and reductive constant-potential polarization steps coupled with the record of square wave voltammograms to solid microsamples of dyes in contact with aqueous electrolytes. As a result, oxidized/reduced products form a layer on the lateral faces of the dye crystals as suggested by attenuated total reflectance-Fourier transform infrared spectroscopy and atomic force microscopy data. This methodology is applied for characterizing alizarin, purpurin, and natural dyes aloe, cochineal red, madder lake, kermes, shellac, and henna attached to paraffin-impregnated graphite electrodes in contact with aqueous potassium phosphate buffer.     

Electrochemical analysis of natural solid organic dyes and pigments

Square-wave voltammetry of solid naphthoquinone, anthraquinone, and flavone dyes, carmine, cochineal red, indigo, and Prussian blue, was compared to microanalysis (sample consumption <1 mg) of traditional painting pigments and dyes without their preliminary dissolution. Electrochemical analysis was also performed after the samples' hydrolysis simultaneously with thin-layer chromatography. Anthraquinone-based pigments and Prussian blue are reversibly reduced, cochineal red and lac dyes are irreversibly reduced, flavones are mostly reversibly oxidized, dragon's blood is irreversibly oxidized and reduced, and indigo yields both reversible oxidation and reduction. The potential window of these reactions is about 1.4 V wide. This variability permits identification of the kind of pigment or dye, and directly distinguishes, for example, alizarin and purpurin; luteolin and quercetin; or indigo and Prussian blue.     

Characterization, purification, and analysis of solvent yellow 33 and solvent green 3 dyes

Thin-layer chromatography and high-performance liquid chromatography (HPLC) with linear photodiode array detection (LPDA) were used to separate impurities from two commercial dyes. Gravity flow liquid chromatography was used to purify reference standards of the dyes. Normal phase HPLC with LPDA detection was used to determine purities of submitted samples by comparing responses to those of the reference standards. Electron impact mass spectrometry and nuclear magnetic resonance were utilized to confirm structures of the dyes and their impurities.     

Analysis of natural food pigments by capillary electrophoresis

Lac, cochineal, safflower, gardenia, Monascus and elderberry pigments are used as food color additives in Japan. These natural pigments can be analyzed by capillary electrophoresis (CE). CE has several advantages over thin layer chromatography, gas chromatography and high-performance liquid chromatography, such as low capillary cost, reduced operating costs, small sample amounts, low production of waste materials and short analysis time. CE is shown to be a useful technique for the analysis of these natural food pigments and the pigments extracted from commercial food samples by solid-phase extraction method.     

In situ ionic‐liquid‐dispersive liquid–liquid microextraction of Sudan dyes from liquid samples

In situ ionic‐liquid‐dispersive liquid–liquid microextraction was introduced for extracting Sudan dyes from different liquid samples followed by detection using ultrafast liquid chromatography. The extraction and metathesis reaction can be performed simultaneously, the extraction time was shortened notably and higher enrichment factors can be obtained compared with traditional dispersive liquid–liquid microextraction. When the extraction was coupled with ultrafast liquid chromatography, a green, convenient, cheap, and efficient method for the determination of Sudan dyes was developed. The effects of various experimental factors, including type of extraction solvent, amount of 1‐hexyl‐3‐methylimidazolium chloride, ratio of ammonium hexafluorophosphate to 1‐hexyl‐3‐methylimidazolium chloride, pH value, salt concentration in sample solution, extraction time and centrifugation time were investigated and optimized for the extraction of four kinds of Sudan dyes. The limits of detection for Sudan I, II, III, and IV were 0.324, 0.299, 0.390, and 0.655 ng/mL, respectively. Recoveries obtained by analyzing the seven spiked samples were between 65.95 and 112.82%. The consumption of organic solvent (120 μL acetonitrile per sample) was very low, so it could be considered as a green analytical method.     

Structure elucidation and chromatographic identification of anthraquinone components of cochineal (Dactylopius coccus) detected in historical objects

Cochineal is one of the most well known organic red dyes. Dactylopius coccus Costa (Dactylopiidae) is a scale insect that is used as the source of the dye known as Mexican cochineal. Although cochineal is today a natural food colorant (E120) and although it has been used in art objects (textiles and paintings) for centuries, its exact chemical consistency is not well clarified except for carminic acid which is the major component and kermesic and flavokermesic acids. Several minor components (typically less than 5% of the colouring material) remained unknown or partially studied, although their presence has been reported in numerous analytical works related to art objects. Chemical investigation of the methanol extract of the dried insects, after subsequent HPLC chromatographic separations, led to the isolation and structure elucidation of six new anthraquinones, along with the known compounds carminic acid, kermesic acid and flavokermesic acid. The new compounds formerly described as DCII and DCIII, were found to be the 2-C-glucoside of flavokermesic acid and 4-aminocarminic acid, respectively, while DCIV and DCVII were found to be the α/β C-glucofuranosides of kermesic acid, and were studied as a mixture due to equilibrium. In addition, 3-O-glucoside of flavokermesic acid (DCOFK), and 3,4-dideoxycarminic acid (DDCA) were identified. The structures of the new compounds were elucidated on the basis of their NMR and MS data. Finally, the new compounds were detected in silk dyed with cochineal, lake pigment and, furthermore, in historical objects of the cultural heritage (icon and textile) using LC–DAD and LC–MS.     

HPLC–DAD–MS analysis of dyes identified in textiles from Mount Athos

Organic colorants contained in 30 textiles (16th to early 20th century) from the monastery of Simonos Petra (Mount Athos) have been investigated using high-performance liquid chromatography equipped with diode-array detection and mass spectrometry (HPLC–DAD–MS). The components of natural dyes identified in samples treated by the standard HCl dyestuff extraction method were: alizarin, apigenin, butein, carminic acid, chrysoeriol, dcII, dcIV, dcVII, ellagic acid, emodin, fisetin, flavokermesic acid, fustin, genistein, haematein derivative (Hae′), indigotin, indirubin, isoliquiritigenin, isorhamnetin, kaempferide, kaempferol, kermesic acid, luteolin, naringenin, purpurin, quercetin, rhamnazin, rhamnetin, sulfuretin, and type B and type C compounds (last two are markers for Caesalpinia trees). Early, semi-synthetic dyes, for example indigo carmine, fuchsin components, and rhodamine B were identified in objects dated late 19th to early 20th century. A dyestuff extraction method which involves use of TFA, instead of HCl, was applied to selected historical samples, showing that the mild method enables efficient extraction of weld (Reseda luteola L.) and dyer’s broom (Genista tinctoria L.) glycosides. The marker compound (Hae′) for logwood (Haematoxylum campechianum L.) identification after treatment with HCl was investigated by liquid chromatography coupled to mass spectrometry (LC–MS) in negative electrospray ionization (LC–MS-ESI⁻) mode. LC–MS in negative atmospheric pressure chemical ionization (LC–MS-APCI⁻) mode was used, probably for the first time, to investigate cochineal (Dactylopius coccus Costa) samples. Positive electrospray ionization (LC–MS-ESI⁺) mode was used for identification of fuchsin components. Detailed HPLC–DAD studies were performed on young fustic (Cotinus coggygria Scop.) and Persian berries (Rhamnus trees).     

Micro Analysis on Hallstatt Textiles: Colour and Condition

Due to the special conditions in the Bronze Age and Hallstatt Period salt-mines of Hallstatt, Austria, a large number of archaeological textile fragments, dated from 1400 to 400 BC, can be found. Textiles of good condition from these periods are quite unique. The fibres are identified as wool and most of the fragments still have colours i.e. yellow, green, olive-green, brown, blue and black. To obtain information about the dyeing techniques used in this period, dyestuffs analyses are performed by high performance liquid chromatography coupled to photo diode array detection (HPLC-PDA) and mordant analysis by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS). In addition, the condition of the textile fibres is assessed by SEM. The analyses revealed that all textiles were dyed with plant dyes and insect dyes. The yellow plant dyes and the red insect dyes are mordant dyes but the identification of mordants turned out to be difficult. SEM/EDS analyses of small uncontaminated areas on the fibre showed that the elements aluminium, iron and copper are present in many samples. These elements could originate from a mordant or from the mine. The condition of the fibres was assessed by the presence of the characteristic scales on the wool, tears, cuts and particles from the mine. It was found that a relation exists between the copper content of the fibres and their condition. Degraded fibres contain more copper. Black textile fragments which are most possibly coloured by iron gall-black show as well degradation phenomena.     

A precise self-built secondary mass database for identifying red dyes and dyeing techniques with UPLC-MS/MS

Dyes on ancient silks have been a worth studying field through human's history, although current reports ignore the connection between natural dyes origin and relevant colour reduction methods, which poses an insurmountable obstacle for restoration of historical silks. In this paper, a series of 12 red hue silks from six natural dyes (sappanwood, Chinese madder, safflower, lac, cochineal, dragon's blood) via three different dyeing techniques were used to establish a self-built precise tandem mass spectrometry (MS/MS) database. With organic solvent extracting on those manual-dyed silks, ultraperformance liquid chromatography - electrospray ionization - quadrupole time of flight mass spectrometry (UPLC-ESI-QTOF-MS) was utilized to form preliminary MS database for screening and identifying of the potential dyes compounds without standard references. Furthermore, combining the targeted MS/MS mode and the matching threshold of 70.00, a self-built secondary MS/MS database was successfully established, which contains 33 compounds, 32 chromatograms and 32 MS/MS fragments. As for real sample application, the self-built precise MS/MS database had revealed that the dyes on two historical silks (Shanghai Museum, China) belong to Chinese madder just with different mordant dyeing ordinal. Additionally, by experimental restoration, visually indistinguishable silks (ΔE_ab* < 1.5 NBS) were successfully restored. This explorative methodology can further inspire the traceability of biological dyestuffs, which lays instructive foundation on protection and restoration of artefacts, connecting the archaeological science and human art.     

Identification of anthraquinone coloring matters in natural red dyestuffs by high performance liquid chromatography with ultraviolet and electrospray mass spectrometric detection

Reversed phase liquid chromatography with diode array detection (DAD) and electrospray mass spectrometric (ESI MSD) methods were developed for the identification of anthraquinone color components of cochineal, lac dye, and madder – red natural dyestuffs. Electrospray mass spectrometry was found to be more suitable than diode array detection for such analysis because of its higher sensitivity (detection limits in the range 30–90 ng mL^–1) and selectivity. The developed method permitted unequivocal identification of carminic acid and laccaic acid A as coloring matters in examined preparations of cochineal and lac dye, respectively. In madder more chemical color species were found: alizarin, purpurin, lucidin, ruberythric acid, and also aluminum and calcium alizarin lake. Among the methods recommended so far, the present one allows fast, direct, and unequivocal identification of components of very complicated natural products used in art.     

Novel methodology for the extraction and identification of natural dyestuffs in historical textiles by HPLC-UV-Vis-ESI MS. Case study: chasubles from the Wawel Cathedral collection

High-performance liquid chromatography coupled with spectrophotometric and electrospray mass spectrometric detection (HPLC-UV-Vis-ESI MS) was used for characterization of natural dyes present in historical art works. The gradient program was developed for identification of 29 colorants of various polarities. Dual detection system (UV-Vis and ESI MS) allowed differentiation of all compounds, even if they were not completely separated. This enabled examination of more color compounds over a substantially shorter time in comparison with previously recommended methods. Moreover, for extraction of colorants from historical textiles a two-step sequential procedure was proposed, excluding evaporation used in earlier procedures. The developed method was successfully applied to identification of indigotin, carminic, kermesic, flavokermesic, dcII, dcIV, dcVII, and ellagic acids as well as luteolin, apigenin, and genistein in red, violet, and green fibers taken from three selected historical chasubles which belong to the collection of the Wawel Cathedral treasury (Cracow, Poland). Italian textiles from the fifteenth and sixteenth centuries, of which chasubles were made, were dyed with a limited number of dyestuffs, consistently used for all batches of fabrics. The obtained results also allowed confirmation of the structure of the so-called "dcII" component of cochineal as a C-glucose derivative of flavokermesic acid.     

High-performance liquid chromatographic determination of colouring matters in historical garments from the Holy Mountain of Athos

This work is probably the first attempt to identify the organic colouring materials contained in post-Byzantine textiles, from the Holy Mountain of Athos. Samples extracted from seven ecclesiastical garments (15th–19th century) are investigated by high performance liquid chromatography with UV-Vis diode array detection. The detection limits for alizarin, purpurin, carminic acid, laccaic acid A, luteolin, apigenin, genistein, fisetin, sulfuretin, ellagic acid, indigotin and indirubin are found to be within 0.002–0.029 μg mL⁻¹. The following organic dyes are identified in the extracts: dyer’s broom (Genista tinctoria L.), young fustic (Cotinus coggygria Scop.), an indigoid dye source either indigo (Indigofera species) or woad (Isatis tinctoria L.), madder, cochineal and lac dye (Kerria lacca Kerr). Furthermore, the identification of a brazilein derivative indicates the presence of a Caesalpinia dye source in the samples. Correspondence: Ioannis Karapanagiotis, Ormylia Art Diagnosis Center, Sacred Convent of the Annunciation, Ormylia, GR-63071 Chalkidiki, Greece     

Investigation of red natural dyes used in historical objects by HPLC-DAD-MS

High performance liquid chromatography (HPLC) with UV-Vis Diode Array Detection (DAD) and electrospray mass spectrometric (ESI-MS) method was utilized for the identification of coloring components of madder, Armenian and Mexican cochineal, lac dye, brazilwood, safflower and dragon blood--probably the most important red natural dyestuffs found in objects of the cultural heritage. UV-Vis detection limits in the range of 0.2-0.6 ng for carminic acid, alizarin and purpurin were achieved using a gradient elution of H2O-0.01% TFA and CH3CN-0.01% TFA. ESI mass spectrometer was also used, as a supportive detection method to the standard DAD, for further analysis of the tested materials, with the ability to analyze dyestuffs as small as one milligram. The presence of madder was revealed in two historical (Hellenistic and Roman period) samples, found in the Mediterranean area, by identifying purpurin in both of them. Munjistin was also identified in one of the samples (Hellenistic period) while alizarin was not detected, raising questions regarding the exact madder type, utilized in the historical samples.     

Color and fastness of natural dyes encapsulated by a sol-gel process for dyeing natural and synthetic fibers

Natural dyes, namely, indigo carmine, cochineal carmine, curcumin and annatto, were encapsulated in silica by a sol-gel method and applied in the dyeing of different textile fibers by exhaustion. For comparative reasons, dyeing using the free (non-encapsulated) bare dyes was also carried out. The hybrid materials were analyzed by a set of techniques to investigate their elemental, structural, textural and morphological properties, and the results showed that it was possible to obtain stable natural dyes for applications in textile dyeing. The silica-structured dyes showed better affinities with the fibers (WO, PA, PAC and PET) in dyeing with cochineal carmine, while cotton (CO) showed better affinities with the encapsulated curcumin and annatto dyes. The performances of the encapsulated dyes were evaluated by color and washing fastness measurements and resulted in improved dye absorption and wash fastness properties. The color change and color transfer measurements of the encapsulated dyes were better (rated at 4–5 on a scale of 1–5) compared to the bare dyes.

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凝胶色谱净化超高效液相色谱法测定食品中10种工业染料

建立了测定食品中10种工业染料的超高效液相色谱(UPLC)法.样品经提取后,采用凝胶色谱(GPC)净化后收集浓缩,以乙腈-0.1％甲酸溶液为流动相进行梯度洗脱,10种染料得到良好分离.10种染料线性关系良好,相关系数R2＞0.9992,其检出限为0.011～0.049 μg /mL;样品在低、中、高3个加标浓度下,10种染料的回收率为65.0％～107.2％.结果表明该方法适用于食品中10种工业染料的检测,其样品处理方法适用于各类食品,且检测灵敏度高,可实现食品中工业染料的快速测定.     

Extracting natural dyes from wool--an evaluation of extraction methods

The efficiency of eight different procedures used for the extraction of natural dyes was evaluated using contemporary wool samples dyed with cochineal, madder, woad, weld, brazilwood and logwood. Comparison was made based on the LC-DAD peak areas of the natural dye's main components which had been extracted from the wool samples. Among the tested methods, an extraction procedure with Na(2)EDTA in water/DMF (1:1, v/v) proved to be the most suitable for the extraction of the studied dyes, which presented a wide range of chemical structures. The identification of the natural dyes used in the making of an eighteenth century Arraiolos carpet was possible using the Na(2)EDTA/DMF extraction of the wool embroidery samples and an LC-DAD-MS methodology. The effectiveness of the Na(2)EDTA/DMF extraction method was particularly observed in the extraction of weld dye components. Nine flavone derivatives previously identified in weld extracts could be identified in a single historical sample, confirming the use of this natural dye in the making of Arraiolos carpets. Indigo and brazilwood were also identified in the samples, and despite the fact that these natural dyes were referred in the historical recipes of Arraiolos dyeing, it is the first time that the use of brazilwood is confirmed. Mordant analysis by ICP-MS identified the widespread use of alum in the dyeing process, but in some samples with darker hues, high amounts of iron were found instead.     

Nachweis natürlicher organischer Künstlerpigmente

Zusammenfassung Der Nachweis natürlicher organischer Künstlerpigmente 1st wesentlich schwieriger als die Erkennung anorganischer Pigmente. Für die Farblacke sind dünnschicht-chromatographische Methoden am günstigsten, da häufig Mischungen vorliegen, die nach saurer Lackspaltung mit konz. Schwefelsäure unterschiedliches chromatographisches Verhalten zeigen, wenn man sie auf Mikropolyamidschichten trennt. Zum Nachweis von Hydroxyflavonen und -anthrachinonen auf Chromatogrammen ist Uranylacetat alien bisher bekannten Reagenzien überlegen. Manchmal helfen empfindliche Farbreaktionen zur Unterscheidung sehr ähnlicher Lacke weiter: Cochenillekarmin bildet einen blauen Borsäurekomplex, Kermeskarmin dagegen einen violetten. Mit Uranylacetat als Spurenreagens lassen sich noch 0,5g Cochenillekarmin und Kermeskarmin nachweisen. Lösliche Farbstoffe können durch Diinnschichtchromatographie auf Mikropolyamidplatten unterschieden werden. Zur Unterscheidung unlöslicher Pigmente eignen sich IR-Spektren und spezifische Farbreaktionen.

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Assay of natural organic artists pigments

Summary Assay of natural organic artists pigments is much more difficult than the detection of inorganic pigments. Thin-layer chromatographic methods are most advantageous for the lacquers since mixtures are frequently present. These show differing chromatographic behavior when separated on micropolyamide layers after acid cleavage of the lacquer with concentrated sulfuric acid. Uranyl acetate is superior to all previously known reagents for assay of hydroxyflavons and hydroxyanthraquinones on chromatograms. Sensitive color reactions sometimes help further in distinguishing very similar pigments: cochineal carmine forms a blue boric acid complex, whereas kermes carmine forms a violet boric acid complex. With uranyl acetate as trace reagent, 0.5g cochineal carmine and kermes carmine can be demonstrated. Soluble pigments can be distinguished by thinlayer chromatography on micropolyamide plates. IR spectra and specific dye reactions are suitable for distinguishing insoluble pigments.

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Vorgetragen beim Internationalen Symposium für mikrochemische Arbeitsmethoden, 22.–27. Mai 1977, Davos, Schweiz.