Rabbit Glue Research Articles

Non-Destructive Prediction of the Mixed Mineral Pigment Content of Ancient Chinese Wall Paintings Based on Multiple Spectroscopic Techniques.

This study first developed non-destructive and accurate methods to predict the relative contents of mixed mineral pigments in ancient Chinese wall paintings using multiple spectroscopic techniques. The colorimetry, attenuated total reflection Fourier transform infrared spectroscopy (ATR FT-IR), ultraviolet-visible-near-infrared (UV-Vis-NIR) spectroscopy, and Raman spectroscopy were employed. Analyses were conducted including color difference, spectral reflection, ATR FT-IR spectra, and Raman mapping for simulated samples (malachite-lazurite mixed with rabbit glue samples) before and after aging. Models were then established for predicting the relative pigment contents of samples using UV-Vis-NIR and ATR FT-IR spectral data with Beer-Lambert law, and mathematical methods comprising principal component analysis (PCA) and nonlinear curve fitting. In particular, PCA and empty modeling methods combined with non-negative partial least squares were developed to predict the relative pigment contents based on Raman mapping data. The results demonstrated that approaches comprising PCA, mathematical model, and empty modeling based on the spectral data were effective at predicting the relative pigment contents. The predicted results obtained using the mathematical model based on UV-Vis-NIR spectra had an error of about 2%, and the best prediction based on ATR FT-IR spectra had an error of <3.6% at 1041 cm-1. The errors for the predictions using PCA and empty modeling based on Raman mapping data were 0.01-9.30% and 0.28-7.15%, respectively. However, the predicted relative pigment contents obtained based on ATR FT-IR data combined with the Beer-Lambert law had higher errors. The findings of this study confirm the strong feasibility of using spectroscopic techniques for quantitatively analyzing mixed mineral pigments.

SO2-Induced Aging of Hematite- and Cinnabar-Based Tempera Paint Mock-Ups: Influence of Binder Type/Pigment Size and Composition

Hematite- and cinnabar-based paint mock-ups prepared with either rabbit glue or egg yolk binder were artificially aged in an SO2-rich atmosphere, as a model system for investigating the deterioration of tempera paints exposed to an industrial atmosphere. The overall research aim was to identify the type of degradation occurring in tempera paints and the different alteration mechanisms related to the physical, mineralogical and chemical characteristics of the paint. Tempera mock-ups were prepared by mixing binder (egg yolk or rabbit glue) and pigment (cinnabar of different particle sizes or hematite) and were then exposed to SO2 for 2 months in accelerated aging tests. The colour, gloss, reflectance, roughness and micro-texture of the surfaces of the mock-ups were determined before and after the tests. In addition, chemical and mineralogical changes were determined by X-ray Powder Diffraction (XRPD), Attenuated Total Reflection-Fourier Transform Infrared spectroscopy (ATR-FTIR) and Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (SEM-EDS) analysis. Colorimetric changes were confirmed, mainly in the cinnabar-based paints containing egg yolk, and in the hematite-based paints containing rabbit glue. Neoformed mineral phases have not been detected by XRPD, but precipitation of gypsum on the exposed surfaces has been confirmed by SEM. For cinnabar-based paints, the amount of sulfate-rich deposits was higher on egg yolk mock-ups than on rabbit glue samples, though the opposite was observed for the hematite-based paints. This confirmed the influence of the binder composition and pigment-binder tandem in the susceptibility to SO2 deposition. Pigment particle size did not have a clear influence on the physical and chemical changes in the tempera mock-ups during the ageing tests.

Preliminary Study on the Adhesiveness Properties of Hide Glue

Adhesives based on collagen extracted from the skin, tendons, cartilage, bones of animals and fish have been widely used over time as binders and consolidating agents for various organic and inorganic materials. The use of these types of adhesives or glues or gelatins in the restoration activity has been practiced by restorers due to their properties, but also to the special requirements of reversibility of treatments that are stipulated in national and international norms specific to the restoration of heritage objects. For this purpose, the technology of gelatin extraction from bovine hide was studied, and then gelatins were characterized in terms of chemical properties (ash, total nitrogen, amino nitrogen, dermal substance, pH of analytical solution) and gluing experiments were conducted on collagen substrates (leather and parchment, newly made specifically for the restoration activity and some samples of historical parchment, pieces of parchment detached from the back of a heritage object) and characterized in terms of physical-mechanical properties (shear strength and peel strength). Gelatins made from bovine hide were also compared to a commercial product, rabbit glue, a material frequently used in the field of restoration.

Monitoring UV-accelerated alteration processes of paintings by means of hyperspectral micro-FTIR imaging and chemometrics

We explored the potential of infrared hyperspectral microimages to investigate the alteration of organic binders in pictorial layers after artificial UV light ageing. A set of paint mockups was prepared considering three different binders, namely, rabbit glue (a collagen-based proteinaceous binder), linseed oil (representative of drying oils) and egg tempera (a mixture of egg yolk and linseed oil). Four pigments (vermilion, orpiment, azurite and lead white) were considered in order to investigate the influence of pigment-binder interaction, following color changes by means of fiber optic reflectance spectroscopy (FORS). FTIR micro-images provided a representative picture of the complex and heterogeneous structure of paintings since each pixel contained the whole spectrum of the sample area from it was recorded. Principal component analysis (PCA) was used to analyze the FTIR images data in order to extract useful information about spectral changes taking place during UV induced ageing.Significant trends were observed, mainly depending on the binders and their degradation as a consequence of UV exposition in this pilot study on model samples. Several processes, such as the oxidation of proteins with the formation of carbonyl moieties and changes in amide band positions have been detected in the case of rabbit skin glue. The evaporation of linseed oil, probably due to the breakdown of the triacylglycerols, has been noticed for the binder alone but not when it was mixed with the pigments. In these cases, other spectral features depending on the pigment have been observed in the loading plots upon oxidation, namely the broadening of the carbonyl band, the appearance of carboxylic and dicarboxylic acids and the formation of metal carboxylates. For egg tempera, the main changes detected were related to the oxidation of lipidic components present in egg yolk fraction. Furthermore, in this case, the trend observed in the score graphs suggested that the presence of lead white accelerates its oxidation. It is interesting to note the major stability of the colored pigments when using this binder.

Effect of a SO2 Rich Atmosphere on Tempera Paint Mock-Ups. Part 2: Accelerated Aging of Azurite- and Malachite-Based Paints

In order to improve our knowledge of the weathering response of tempera paints exposed to an industrial atmosphere, azurite- and malachite-based paint mock-ups prepared with either rabbit glue or egg yolk binders were artificially aged in an SO2 rich atmosphere. The aim was to identify the different alteration mechanisms and forms of degradation in the paints by observing their physical (color, gloss, reflectance, and roughness), mineralogical, chemical, and micro-textural characteristics. Superficial physical changes were evaluated by stereomicroscopy, spectrophotometry, gloss measurement, hyperspectral imaging, and roughness measurements. Chemical and mineralogical changes were evaluated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy with microanalysis (SEM-EDS), which was also used to evaluate the micro-texture of the paints. The differences between the fresh temperas were due mainly to the binder (egg yolk or rabbit glue) used in the paint mixture, which also played a crucial role in the different deterioration patterns in the artificially aged paint mock-ups. Thus, the egg yolk-based paints remained physically quite intact after SO2 exposure, although they suffered more significant chemical degradation, above all in the form of copious precipitation of Cu and Ca-rich sulfate salts and the subsequent yellowing of the egg yolk binder. The SO2 aged rabbit glue-based mock-ups showed microscopically important crack formation and binder loss and fewer sulfated salts precipitated on the surface of the paints.

Effect of a SO2 Rich Atmosphere on Tempera Paint Mock-Ups. Part 1: Accelerated Aging of Smalt and Lapis Lazuli-Based Paints

The behavior of historic tempera paints exposed to pollutant gases is an important issue when developing conservation strategies. In this work, binary tempera paint mock-ups that were made with either smalt or lapis lazuli pigments mixed with either rabbit glue or egg yolk binders were exposed to an SO2 accelerated aging test in order to find out more about the forms and mechanisms of alteration resulting from pigment-binder interaction. To this end, spectrophotometry, hyperspectral image analysis, and profilometry were used to study macro-scale, physical changes taking place on the surface of the paints, affecting color, gloss, reflectance, and roughness. Likewise, chemical and mineralogical changes were evaluated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (ATR-FTIR), polarized light microscopy (PLM), and scanning electron microscopy with micro-analysis (SEM-EDS), which was also used to visualize micro-texture changes in the paints. The smalt-based tempera showed a higher degree of deterioration than the lapis lazuli-based tempera, in particular a notable whitening related to the precipitation of sulfate-rich salts and to binder and pigment chemical alterations. Moreover, whereas aged egg yolk-based paints showed visible color change due to damage to the oily binder and the pigments, the most evident change in rabbit glue-based paints was binder loss. The alteration suffered by the pigments varied in line with their composition; thus, smalt (blue cobalt-containing glass) grains were more sensitive to SO2 exposure than lapis lazuli-(Na,Ca)8[(S,Cl,SO4,OH)2|(Al6Si6O24)]-crystals. In the smalt grains, the SO2 test caused K+ leaching from the glass matrix, which was detected by means of K/Co ratios, but the lazurite crystals (main component of lapis lazuli) were unaffected (regardless of the binder used in the tempera). The most likely source of the crystallized sulfate rich salts were the impurities that were detected in association with the natural lapis lazuli pigment, i.e., calcite and diopside. Indeed, the precipitation of efflorescences is the main cause of the optical changes found in the smalt- and lapis lazuli-based tempera, in addition to the physical-chemical damage to the binders. The information reported here would be useful for preventive conservation, as well as for art restorers, who are planning work on paintings in which paints of this kind were used.

Gypsum based mixes for conservation purposes: evaluation of microstructural and mechanical features

This paper presents the study of four gypsum mixtures, focusing on the role of both inorganic and organic additives and on the micro-structural features and mechanical properties. Additives have been chosen among those most reported in historical recipes, for example magnesia, lime putty, rabbit glue. The selected mixes refer to gypsum-based materials used in artworks manufacture, such as plasters, mouldings, stuccoworks, pastiglia. Blank reference materials were prepared on purpose according to the specific recipe, in order to verify the final composition and to highlight the hardening mechanisms and the formation of setting compounds. The chemical composition was related to workability properties and final mechanical resistance and the action of additives as retardants was studied with interesting results. For instance, MgO imparts good properties to the mechanical features, especially with regard to the compressive strength characteristics.

Characterization of a sacred statuette replica of “Nossa Senhora da Conceição Aparecida” using X-ray spectrometry techniques

The pigments characterization in artwork and cultural heritage is important since it allows to reveal many information like the period characteristics in which the object was made, its provenance and its manufacturing technique. Furthermore, it can support in the identification of forgeries and in the evaluation of conservation and restoration interventions. In this work, it was used XRF, μXRF, XRD and Raman spectroscopy techniques in the characterization of a statuette of the “Nossa Senhora da Conceição Aparecida”. The X-ray fluorescence analysis were performed in 66 points distributed throughout the piece, like in the wood structure, in the ground layer and on the pigment layer. In addition, elemental mapping was performed using the X-ray micro fluorescence (μXRF) technique in a small fragment, containing both the pigment and the ground layer. The X-ray diffraction analyzes were performed on the ground layer material and the Raman spectroscopy was performed in order to improve the evaluation of the data and assist in the characterization of the ground and the pigment layers. The results show that the major elements in the pigment were Fe and Mn, indicating that it’s probably the Burnt Sienna pigment. The XRD and Raman analysis show that the substrate layer was made by calcium carbonate (calcite) and a binder material (probably rabbit glue).

Characterization of organic binders in a 13th century painted wooden panel: Comparison of ToF-SIMS and Dot-ELISA results

In this study we explored the capacity of a mass spectrometry technique, ToF-SIMS, coupled with an immunological method, Dot-ELISA, to characterize the organic binders used in a 13th century painted wooden panel. The panel, which was in a poor state of conservation with only some residual painting left, was a useful bench specimen for these techniques. ToF-SIMS and Dot-ELISA results both indicated the presence of a mixture of rabbit glue and casein and rule out the use of egg in the painted layer of the samples. The results of this study show the capability of the combined use of the two methods to univocally identify protein-based binders in paintings.

Effect of tempera paint composition on their superficial physical properties- application of interferometric profilometry and hyperspectral imaging techniques

Abstract Interferometric profilometry and hyperspectral imaging techniques combined with traditional analytical techniques such as spectrophotometry, stereomicroscopy, X-ray diffraction, scanning electron microscopy, particle size analysis, and thermogravimetry were used to characterize tempera paint mock-ups. These paint mock-ups were prepared as binary mixtures made by mixing either egg yolk or rabbit glue binders with one of ten pigments traditionally used by mediaeval artists, namely lime, calcium sulfate, white lead, minium, hematite, cinnabar, azurite, lapis lazuli, blue smalt and malachite. We evaluated the effects of mineralogical composition, pigment particle size and morphology, as well as the type and concentration of the binder on the physical properties of the paintś surface. Results showed that all the above compositional aspects had a direct influence on the paintś color, reflectance, and roughness. Moreover, mineral impurities and neoformed minerals due to pigment-binder interaction during paint preparation had a crucial effect on the superficial physical properties of paints. The analytical results proved the usefulness of interferometric profilometry and hyperspectral imaging techniques for the characterization of paint surfaces. The gained information will help conservation specialists in the evaluation of the impact of conservation treatments on the paint surface and the assessment of surface damage caused by weathering processes.

Statistical analysis of colour changes in tempera paints mock-ups exposed to urban and marine environment

The aim of this study is the exploration of the suitability of two different statistical approaches (vectorial and functional analyses), which go beyond a purely descriptive statistics, in order to identify trends and similarities in colour changes of several tempera mock-ups paints during thirteen months. The samples were subjected under an outdoor exposure to a natural urban atmosphere characterized by an important marine and industrial influence. The changes of the colour were measured in CIE L∗a∗b∗ and CIE L∗C∗h colour spaces The paint mock-ups were binary mixtures made with either egg yolk or rabbit glue binder, mixed with one of the seven pigments studied, some of which have different grain sizes. Two different statistical analyses – vectorial and functional – were performed in order to detect the different colour change evolution among the temperas during the exposure time. The functional approach revealed more useful information than the vectorial (classic) one. Paints containing haematite, lapis lazuli and (standard grain size) azurite exhibited different trends of the colour parameters in comparison with the rest of the paints. Stereomicroscopy, X-ray diffraction and Fourier transform infrared spectroscopy showed that the binder composition and especially its stability to outdoor exposure, are the factors responsible for the statistically significant colour changes detected.

The influence of microfilamentous fungi on wooden organ pipes: One year investigation

Wooden pipe organs are precious objects of cultural heritage the degradation of which has been very poorly studied. Fungal strains recovered from an organ dating back to the 19th century included Alternaria mali, Eurotium cristatum, Aspergillus amstelodami, Penicillium crustosum and Aspergillus sydowii. These isolates were used to artificially contaminate wooden pipes treated with linseed oil varnish and varnish-free. The pipes were incubated at different temperatures and relative humidity for one year. The pipes, incubated at a high relative humidity (70–75%), collapsed due to the biodegradation of glued joints. After one year, inoculated strains (Alternaria mali, Penicillium crustosum and Aspergillus sydowii) as well as some non-inoculated Talaromyces rugulosus, Paecilomyces formosus, Cladosporium cladosporioides, Aspergillus versicolor and Epicoccum nigrum were recovered from the pipes. Few strains exhibited lignolytic activity, while almost all were able to hydrolyze the rabbit glue used to join the wood elements of the pipes. A newly developed portable pipe-sound controller did not detect differences of sound spectra between the uncontaminated and the few surviving contaminated pipes. In this study we have demonstrated that glue degradation was critical for pipe preservation and it can be tremendously accelerated above a certain relative humidity threshold. Therefore, it is important to apply a monitoring strategy able to check the environmental conditions, microbial contamination and pipe sound alteration in order to preserve these valuable and rare cultural heritage objects.

Restaurarea și conservarea unei traiste ciobănești din colecția Muzeului Etnografic al Transilvaniei

The Shepherd`s bag, made by hand, at the beginning of the 19th century in the village Jucul de Mijloc, Cluj county, has been exhibited in a glass showcase in the main Hall of the Museum since 2006. The leather is dehydrated, some parts are detached, torn, some tacks and buttons are missing, and some of them are corroded. The restauration processes started with leather hydration by exposure in essential oils of cedar, savory and tea tree mixture dissolved in distilled water, in drying closet, at room temperature, about 1 month. The detached parts were assembled with rabbit glue, 3-5% in warm distilled water. Metal corrosion products were mechanically removed from buttons, tacks and other metallic decoration made from brass and conserved with Paraloid B 72, 3% in solvents mixture.

ToF-SIMS characterization of proteinaceous binders in the wall painting “Madonna and Child enthroned with Saints” by Ambrogio Lorenzetti in the St. Augustine Church (Siena, Italy)

In this paper, we report the results of investigations, by means of ToF-SIMS and HPLC, of samples taken from the wall painting “Madonna and Child Enthroned with Saints” (1335–1338) by Ambrogio Lorenzetti. The aim of this study was to test the capability of ToF-SIMS to identify the nature of the protein-based binders used in the wall painting. The peaks characteristic of egg components were observed in the ToF-SIMS mass spectra of the a secco-applied layers. On the contrary, the ToF-SIMS spectra of the a fresco-applied layers did not show the markers of egg. The PCA results of the protein peaks are consistent with the presence of a rabbit glue–egg mixture. The PCA of protein peaks in the ToF-SIMS spectra was carried out for artificially aged and non-aged reference samples of rabbit glue, casein, egg and their mixtures in order to estimate the effect of ageing on protein-based binding media. Both ToF-SIMS and HPLC data showed that animal glue was present in the a secco- and a fresco-applied layers. On the basis of these results, we can conclude that animal glue was not used as binding medium but was probably applied to the wall painting in prior restorations.

Fourier transform infrared spectroscopic study of rabbit glue/inorganic pigments mixtures in fresh and aged reference paint reconstructions

We studied the interactions of rabbit glue, a collagen-based proteinaceous binder, with azurite (Cu3(CO3)2(OH)2), calcium carbonate (CaCO3), hematite (Fe2O3·nH2O), red lead (Pb3O4) and cinnabar (HgS) by Fourier transform infrared spectroscopy (FT-IR). The research was carried out on a set of paint reconstructions, which were analysed before and after artificial light ageing. A deconvolution of the amide I FT-IR absorption peak was performed with a written-in-house LabVIEW program to study the secondary structure of the glue.The changes in the glue conformation highlighted that all the inorganic pigments interact with the proteinaceous binder. The conformational changes were correlated with a loss of stability of the collagen structure, especially after ageing, likely due to the interlayer coordination of metals salts and oxide with protein functional groups. These results were correlated with the lower thermal stability of the glue/pigment mixtures with respect to the pure glue, evidenced by thermogravimetry (TG) and differential scanning calorimetry (DSC) analyses performed in a previous step of this work.

Chemical characterization of protein based binders in painting samples by means of ToF-SIMS: Tests on ancient and model samples

In the present work we investigated the possibility of identifying proteinaceous binders, typically used in “tempera” painting, by means of ToF-SIMS and PCA method. ToF-SIMS measurements were performed on cross-sections taken from a painted wooden crucifix of the 15th century. The results of the ToF-SIMS analysis are compared with those obtained by GC–MS. ToF-SIMS measurements were carried out also on model samples simulating the layered structure of real painting cross sections to elucidate how matrix effects can influence the identification of protein based binding media. A discrimination between rabbit glue and casein on the basis of the comparison of the ToF-SIMS results for the ancient samples with those obtained from reference compounds appears to be difficult. This is mainly due to matrix effects that are relevant for these complex, multicomponent samples, as indicated by the PCA results for model painting samples of known composition. On the other hand, PCA method and the observation of characteristic fragment ions in the spectra should allow to determine the presence of egg in painting layers.

IL CANTIERE DELLA “CA’ GRANDA”. LA LETTURA DELL’ESPERTO SCIENTIFICO

The conservation works on the external facades of the so-called “Ca’ Granda” allowed to carry out a pilot yard, during which several cleaning methods have been tested; moreover it was possible to apply water-repellent and consolidation treatments, in order to monitor their performances. This survey was preceded by a complete characterization as regards Terracotta surfaces, which highlighted the presence of two different red finishes: the first was composed by gypsum and rabbit glue and second by siccative oil and lead white; both were pigmented with red ochre. Collecting all the observations, by a macroscopic and microscopic point of view, the choice of laser cleaning proved to be the most suitable. It used a Nd:YAG pulsed source, emitting at 1064 nm. Doing so, the assurance of the most complete conservation has been obtained, taking into account the particularly worrying state of conservation of the substrate.

Collagen‐based proteinaceous binder‐pigment interaction study under UV ageing conditions by MALDI‐TOF‐MS and principal component analysis

This study focuses on acquiring information on the degradation process of proteinaceous binders due to ultra violet (UV) radiation and possible interactions owing to the presence of historical mineral pigments. With this aim, three different paint model samples were prepared according to medieval recipes, using rabbit glue as proteinaceus binders. One of these model samples contained only the binder, and the other two were prepared by mixing each of the pigments (cinnabar or azurite) with the binder (glue tempera model samples). The model samples were studied by applying Principal Component Analysis (PCA) to their mass spectra obtained with Matrix-Assisted Laser Desorption/Ionization-Time of Flight Mass Spectrometry (MALDI-TOF-MS). The complementary use of Fourier Transform Infrared Spectroscopy to study conformational changes of secondary structure of the proteinaceous binder is also proposed. Ageing effects on the model samples after up to 3000 h of UV irradiation were periodically analyzed by the proposed approach. PCA on MS data proved capable of identifying significant changes in the model samples, and the results suggested different aging behavior based on the pigment present. This research represents the first attempt to use this approach (PCA on MALDI-TOF-MS data) in the field of Cultural Heritage and demonstrates the potential benefits in the study of proteinaceous artistic materials for purposes of conservation and restoration.

A study of the interaction between rabbit glue binder and blue copper pigment under UV radiation: A spectroscopic and PCA approach

The effects of a UV-accelerated ageing test on model samples of azurite glue tempera, pure azurite and pure rabbit glue, all elaborated according to medieval recipes, were studied. The color changes and modifications in composition and texture for both pure and mixed samples are shown and discussed. Special attention is given to the physico-chemical interactions occurring when azurite and glue are combined. Ageing effects on the model samples after up to 3000 h of UV irradiation were periodically analyzed by scanning electron microscopy (SEM) with X-ray energy dispersive spectrometer (EDS) microanalysis, X-ray diffraction (XRD), reflectance spectrophotometry and Fourier-transform infrared spectroscopy (FT-IR). Once the ageing process ended, a chemometric study using principal component analysis (PCA) of the FT-IR data was carried out independently for every model sample and for all the azurite-laden samples. Loadings from the significant principal components were analyzed to identify the FT-IR frequency (cm −1) involved in the degradation process. PCA proved capable of identifying significant changes in pure glue samples. Also this work showed the lack of photochemical effects of UV irradiation on both pure azurite samples and those mixed with glue, in agreement with the SEM-EDS, XRD and colorimetrics results. Nevertheless PCA revealed that the azurite FT-IR spectral variability decreased in the presence of glue, being more affected by exposure to the IR region between 2100 cm −1 and 3600 cm −1, where the azurite band is located. In addition, PCA managed to separate the azurite/glue mixture samples from pure azurite samples.

Versatile pulsed laser setup for depth profiling analysis of multilayered samples in the field of cultural heritage

The present study considers the use of a nanosecond pulsed laser setup capable of performing laser induced breakdown spectroscopy (LIBS) and pulsed Raman spectroscopy for the study of multilayered objects in the field of cultural heritage. Controlled etching using the 4th harmonic 266 nm emission of a Nd:YAG laser source with a 8 ns pulse duration was performed on organic films and mineral strata meant to simulate different sequence of layers usually found in art objects such as in easel and mural paintings. The process of micro ablation coupled with powerful spectroscopic techniques operating with the same laser source, constitutes an interesting alternative to mechanical sampling especially when dealing with artworks such as ceramics and metal works which are problematic due to their hardness and brittleness. Another case is that of valuable pieces where sampling is not an option and the materials to analyse lie behind the surface. The capabilities and limitations of such instrumentation were assessed through several tests in order to characterize the trend of the laser ablation on different materials. Monitored ablation was performed on commercial sheets of polyethylene terephthalate (PET), a standard material of known thickness and mechanical stability, and rabbit glue, an adhesive often used in works of art. Measurements were finally carried out on a specimen with a stratigraphy similar to those found in real mural paintings.