Colour Through Time

Colour has been a primary means of communication since the dawn of humanity. Polychromy (from the Greek poly = many and chroma = colour) was a regular characteristic of sculptures in most ancient societies, from Mesopotamia and Egypt to the Greeks, Etruscans and Romans. A debate about the practice of adding colour to statues and architectural elements emerged, among art historians, since the early 19th century. Greeks and Romans did not leave their sculptures and buildings in their original white marble or stone state. Instead, they applied pigments to enhance and bring to life these artworks [18]. The use of colour in ancient art was widespread and had important cultural and symbolic significance. Discovering the original polychromy contributes to better understand the artistic intentions and cultural context of the artworks. Originally, colourful patterns and decorations were applied to Greek and Roman statues; colours were used to depict realistic features, to convey messages about divine and heroic beings, and to reproduce as closely as possible the surrounding reality. The use of colour was used to create a visually appealing environment and to convey social status.

Understanding the perception of colour in ancient cultures is essential to fully appreciate and interpret these artworks. Colour had a specific connotation and symbolism that varied from one civilisation to the next. By interpreting the original aspect of polychrome statuary, we can attempt to reconstruct the intended emotional impact and gain a broader understanding of ancient aesthetics. Studying colour in ancient art enhances our appreciation of artistic practices and cultural implications embedded in these works of art. Acknowledging the presence of polychromy in ancient statuary and architecture allows us to move beyond the monochromatic views that have dominated our perceptions for centuries. It adds a new layer of complexity and richness to our understanding of ancient cultures and their artistic expressions. Understanding and characterising the remains of pictorial materials on ancient stone artefacts is therefore a crucial issue and must be approached with care. Gathering as much information as possible about the original paint is indeed extremely important, as it offers a new concept that brings us closer to the original appearance of the sculpture. The lack of attention paid to this issue over the years has led to a significant misinterpretation of artworks and ancient artistic culture, not only from a perceptive point of view, but also from a semantic one, since colouring has been associated with a series of meanings and significances.

A correct reading is primarily made difficult by the small quantity and extent of surviving pigments, due to the vicissitudes the sculptures have undergone as the exposure to the elements or burial for over two thousand years; and even if the colour in some cases survived, it has been often extensively removed after hasty archaeological excavation or improper restorations, the latter specifically intended to reveal the Neoclassical white «pure form» of sculptures [19]. Physical remains are often incomplete, and even when traces remain, we cannot be sure that the hues and intensities remaining on a figure, reflect its original appearance.

In recent years, the scientific community has been engaged in an important debate on several research questions concerning the appearance of these coloured statues. A network of scholars working on this topic meets every two years at the “International Round Table On Polychromy In Ancient Sculpture And Architecture”⁹. The results of these debates are subjects to appreciation but also continued to generate debate, especially when the results are shown to the public in exhibitions such as “Gods in Colours¹⁰” or, more recently, “Chroma: ancient sculpture in colour¹¹” at the MET in New York. The research questions that arise from the study and reconstruction of ancient polychromy are: Which painting materials were used? What was the painting technique? Which was the sequence to apply colours? How were colours mixed to achieve particular effects? How many layers?

Preliminary evidence of original polychromy can be obtained through autoptic observation and archaeometry investigation. High-magnification images make it possible the mapping and locating on the surface of the tiny remnants of polychromy, even if they are no longer visible to the naked eye. Scientific campaigns and the integration of the results obtained through non-invasive and micro-invasive scientific analyses allow us to characterise raw materials, including pigments and sometimes binders. Archaeometry provides detailed and comprehensive instruments and methodologies to gather data about the materials, their composition and the state of conservation in polychrome stone artworks [19‐21]. A variety of conventional micro-invasive techniques could be applied to comprehensively characterise the materials, such as, observations with optical and scanning electron microscope (SEM) of sample’s surface or cross-sections [22]; mineralogical characterisation of materials with X-ray diffraction (XRD) [23]; compounds identification with Infrared spectroscopy (FTIR) and gas-chromatography-mass spectrometry (GC-MS) [24, 25]. Since sampling represents a permanent intervention into the original material, it is evident that non-invasive methodologies have to be preferred. Among these, imaging techniques to map the spatial distribution of colour’s traces [26], UV-VIS-NIR spectroscopy [27], Raman spectroscopy [28], X-ray fluorescence (XRF) and MA-XRF [29, 30] to identify pigments and colourants, have proven to be a valid support to avoid sampling or to drastically reduce their number. Additionally, the acquisition of these type of measurements on the artwork can be carried out rapidly and in situ, facilitating the exchange between the participating scholars while the measurements are still in progress [26].

In addition to the scientific data, the reconstruction of the paint palette is also based on the analysis of primary and secondary sources that describe recipes, techniques and the preparation of pigments. As an example, Pliny the Elder’s Naturalis Historia, written in 77 C.E. directly discusses the tradition and history of art in Greece and Rome, focusing on materials, artist histories, wall painting techniques, and sculpture [31]. The contribution of Pliny’s monumental work as well as providing a commentary on the materials used during imperials time, also implies moral reflections on his contemporaries, examining the relationship between the viewer and the natural world; a point of particular interest in Pliny’s discussion of coloured marbles is well highlighted in the discussion of the artistic role in the treatment of pigments, as we can read in book 35. The most recent critic has carefully evaluated the text, which is not free from incoherent digressions, but it is extremely important for the discussion of categories such as natural philosophy, the divinity of nature alongside with much more pragmatic and empirical discussions on pigments. He discusses these themes in great depth in this book, in which he frames the history of painting alongside naturalistic forays aimed at recovering the origin of some colours. In the same book we also find some well-documented insights into the use, or abuse, of metals in Greco-Roman architecture art. In parallel, of particular interest are his digressions on the theme of paintings used by ambitious politicians and conquerors as trophies. Pliny also did a thorough discussion on the treatment of pigments: thus, we know that for example red lead (minium), an extremely expensive red pigment, was considered sacred by the Romans who in the past used it only and exclusively on rituals, such as illuminating the face of the statue of Jupiter Optimus Maximus or adding it to ointments during the celebration of some recurrences.

What do we have today? What’s remaining? Sometimes, raw materials themselves, discovered in original pots, provide the evidence of the use of selected pigments in a specific context.

What do we know about colour application techniques? Some scholars argue, for example, that the surface of marble was not intentionally roughened to enhance its capability to receive the colour [32], however this cannot be accepted as a general rule; on the other hand, we can state, with reasonable certainty that the presence of decorative geometric motifs resulted in a series of regular interval decorations, such as for example those discovered in borders or garments. Multispectral investigations have also revealed the presence of preparatory drawings or small incisions on the surface, a sort of ante litteram sinopia, indispensable in some circumstances to emphasise anatomical details. A particular care was due to human or god’s figures whose eyes had to be intense and penetrating so that the sculptures seemed alive. The painting technique may have made use of a complex combination of pigments with highlights and shadows which are difficult to find in place and retrace. Moreover, while the reconstruction of the paint palettes used in antiquity can be considered satisfactory, mainly thanks to the historical sources, a little is known about ancient painting techniques and how colours were layered on statues, and how they might have been blended, or shaded. Moreover, while literary sources indicate the use of different binders (e.g., wax, egg), these are often hard to be analytically detected, but surely would have affected the final appearance of a work. Ancient statues were painted by artists using a complex mixture of pigments and binders and by an artistic technique and a personal sensitivity that is impossible to reproduce today. The colour palette showed that the choice was much wider than we can imagine today; some scholars even argue that no space was left without colour [33].

Hypotheses can be made, but without sufficient data, they are open to debate and discussion. For instance, studies suggest that painters did not apply colours in a uniform, flat manner. Instead, they aimed to achieve a natural aspect of their work. The lack of information about the original appearance of the works, including the amount of colour on the surface and the appearance of the final finishing, makes our understanding today very difficult, which is one of the reasons why today there is much discussion about the quality and finish of 1-to-1 reconstructions in resin or plaster. Even when based on physical evidence, it is close to impossible to recreate the ancient polychromy as it would originally have appeared [33] and this involves a substantial difficulty in understanding the meanings related to colours since - nonetheless important to stress - colours were part of lived life from the earliest history of mankind [34].

What can we do today to try and understand the use and the final appearance of those works of art? Despite, for almost a century, archaeological exploration of Greece and its borders has continuously delivered more and more evidence of a polychrome affecting all the plastic expressions of Greek art, friezes, pediments, metopes, isolated statues, groups, steles: it remains today to understand the reasons for such white resilience, against all the evidence. Jockey believes that the roots of this superiority of white could be traced back to antiquity itself, when the passion for the excellence of the form of the Greek masterpieces of the 5^th or 4^th centuries, pushed during Neoclassicism to privilege the reproduction of sculpture leading the absence of colours as normality [35].

The last twenty years have seen museums and scholars closely engaged in experimenting with new reconstruction techniques, thus guaranteeing a remarkable experiential database. Experiments of this kind, however, were limited to the physical reconstruction of pictorial paintings proposing copies of originals on which the ancient colours have been repositioned (Fig. 11). This kind of reconstruction has been a real keystone in understanding the relationship between the ancient world and the visitor of museums, recently becoming the subject of exhibitions [36, 37].

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However, there is not a general agreement on the 1-to-1 scale reproduction of these prototypes, both for ideological reasons and due to the significant approximation applied, although the colours are repositioned following scientific research. Recently, attempts to reconstruct polychromy, especially in the field of statuary, have increasingly resorted to sophisticated and complex applications, tested both on works, groups or statues in their original context, mostly using AR or Projection Mapping technologies. These experiments are of considerable importance since the pictorial framework of the sculpture dialogue closely with the environment within and for which it was set. Understanding the work within its architectural context is challenging today. The use of digital technologies offers the opportunity for an untried path to explore: to cite an example in 2012, Dimitrios Pandermalis, former Director of the Acropolis Museum, launched the ‘Archaic Colours’ initiative, regarding the polychromy of archaic sculpture seamlessly incorporated into the ongoing exhibition featuring archaic sculptures from the Acropolis. This addition introduces to the use of individual pigments, 1:1 physical reconstruction, and detailed explanatory texts. Among these reconstructions stands the ‘Peplos Kore’ (Fig. 12) which has been meticulously brought to life: this sculpture acts as a 3D canvas within an engaging online interactive digital experience available on the museum's website, complemented by a range of other interactive activities [38]. The project was carried out within the Eu CHESS project (ICT-2009.4.1). Therefore, a colour reconstruction is increasingly necessary, particularly if we move from the concept of a colour reconstruction to that of a ‘potential appearance’ or a proposal reconstruction, based on ancient sources study and archaeometry surveys. On the other hand, abandoning the idea of the whiteness of classical sculptures could lead to misinterpretation and, in some cases, support false and dangerous ideologies, such as the “white supremacist” propaganda that still utilises these artworks (Fig. 13).

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This example underlines the importance of colour. To date, however, there is still no common approach to understanding the most correct method, nor are there common requirements for the scientific reliability of these reconstructions. These are used to establish parameters for this experimentation. Combining reconstruction with scientific research data is an even more challenging task, since most of the scientific techniques carried out, give as their final output spectral data, which are difficult to integrate in three-dimensional digital reconstructions.

The statuary collections in archaeological museums all over the world are considered real mines of information on polychromy in the Greek and Roman contexts. As we know, traces of the original colours on sculptures have disappeared over time so when there are examples of preservation, it is worth examining in depth, since they are real rarities. Statues exhibited at the National Archaeological Museum of Naples (MANN), representing different iconographies of Venus of Pompeian provenience, still preserve evident coloured traces, and could be suitable for such inspections (Fig. 14, a-c). It is a series of statues depicting the iconography of Venus Anadyoneme, a portrait of the goddess who, as she comes out of the water, wringing out her hair after bathing by raising her arms. Among the numerous examples in the museum, two have been selected whose colours are perceptible even to the naked eye. This iconography, which dates to 4th century BC became widely spread from the mid-3rd century BC onwards. The sculpture from the House of Camillus is directly inspired by the iconography developed by Alexandrian Greek workshops: the iconography derives from a famous painting by Apelles purchased by Augustus in Cos and brought to Rome for the temple of the Divo Julius. The restoration of the painting commissioned by Vespasian could be one of the reasons why this iconography became widely spread in the Roman, in Pompeii where is often linked in the cult to Isis expressing religious syncretism. From this point onwards, numerous variations developed, characterized by postures, the presence or absence of clothing, the hairstyle; the myth is associated with the theme of birth and water source, ideal architectural setting within Pompeian homes as decorations for fountains or near aquatic environments, such as gardens or nymphaea.

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The INV 6292 comes from the Casa del Camillo (VII 12, 22.23) in Pompeii and already at the time of its discovery in 1863 the excavation diaries report a note which documents the presence of “red” colour on the robe and on the band that holds her hair back. She is depicted in the typical gesture of squeezing her hair to remove excess water after bathing; her elegant hairstyle is enlivened by voluminous waves divided into strands, held together by a thin band. Partially naked, she has a large cloak wrapped around her hips, tied at the pubis with a large knot [38]. The second, INV 6298, comes from the Temple of Isis in Pompeii where it was found next to the wall of the south portico, east section. The small statuette was made in white marble from the Aegean islands and has her hips wrapped in a mantle. In literature, reference is made to traces of gilding on the hair, breasts and a necklace, while the cloak still partially retained a blue colour decoration [39]. Although the sculptural rendering is of inferior quality compared to INV 6292, this Venus is further evidence confirming the relationship with the cult of Isis, whose syncretism is well documented in Pompeii [40]. The last one INV. 109608, is the renowned Lovatelli Venus, found in Pompeii in 1873, in the lararium built on the east wall of the peristyle of the House of Diomedes (I 2, 17): it is one of the rarest examples of classical statuary dating back to the 1st century AD whose pictorial framework is substantially intact. The goddess rests her left arm on an archaic-style idol of Spes, placed on a black-painted rock. She wears a green chiton and a yellow peplos, with a modius adorning her head. Traces of colour, including yellow in the hair and on the himation, are evident across the surface. The idol, positioned on her left side, also displays various coloured traces. This model is reminiscent of a famous original from the 4th century, evoking the iconography of Aphrodite Urania. Additionally, it is hypothesized that the Roman copies derived from the model found in Greece and that the nudity of the torso was a Roman variant to better suit the taste of imperial era [41].

This section analyse specifically Roman painting on walls, describing the problem of their conservation, discussing approaches to study and reconstruct the original colours, without detailing Roman wall painting techniques, already treated by publications such as [42, 43]. It is well known in literature that the events that occurred on frescoes, in particular in the Vesuvian area, were multiple and of different nature: damage due consumption and tear of time when they were still in situ, seismic events, burial under blankets of ash and lapilli and finally accidental damage due to their discovery. Excavated about 150 years ago and exposed since then to rain wash when not protected by temporary structures, wall paintings underwent both to chemical (acid gases) and biological (colonisation) attacks which may be considered one of the most serious problems of their conservation. What normally affected the wall paintings in situ was prevalently constituted by saline compound that along with magnesium and sodium sulphates, influenced the durability of building materials and caused discolouration phenomena compromising their aesthetic value [44]. It must also not be forgotten news from the chronicles of the excavations which report for example how in official circumstances, during state visits or parades, frescoes were wetted with solutions of water and ammonia to make the colours bright and vivid. Finally, a further aspect that must be taken into consideration is also the intervention of detachment of the panels from the walls for their transfer to museums where, further restorations carried out over time, have conditioned and still conditions the original aspects. Mass tourism is only the last plague which may have a negative impact on those very fragile witness of the past.

Walls of Roman buildings were painted using the “fresco” technique. The paint was applied to a layer of plaster, when this was wet. All details were added after this phase, painted on partially dry or on dry plaster. As described also in Pliny [31], a preliminary sketch of the final composition was drawn, the so-called “sinopia”, drawn on a layer of rough plaster, called “arriccio”. After this phase, a final layer of smooth plaster (“intonachino”) was applied. The outline of the foreseen paint was engraved in short time, into this coat of plaster; at this point, the painters could begin the work, starting from the top part of the wall (Fig. 15). These artists were called “pictor parietarius” and their technical instruments included plumblines, punches and squares to be able to draw perfect architectures as well as precise details. All outlines were then painted. There was also the “pictor imaginarius” who would be specialised in painting the figurative scenes directly on the wall or in wooden panels. From the repetition of some pictures, scholars have understood that during Roman times there might have been “cartoons”, circulating among the artists, to enable them to copy part of compositions or/and decorative motives and, eventually, adding their variations. Paintings could be also produced on marble surfaces and not only on walls and on wood.

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Regarding the type of colours, painters used, as Pliny explains, the pigment employed might have been of mineral, vegetal or animal origin. They were divided into two categories: the so-called “florid”, the most expensive ones, that were brighter, and the “plain” less bright and expensive, and more widespread. Main colours used were:

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Darkening, fading and yellowing, often accompanied by flaking, crumbling and chalking of the paint, affect tens of thousands of paintings and works in museums, collections and archaeological sites with dramatic threats for their understanding, preservation, and management of this heritage. Such irreversible colour changes are the result of transformations of the constituent materials (pigments, binding media, varnishes) of the work of art, thus depending on their chemical nature and the past conservation history of the work of art itself (e.g., storage and display conditions, restoration treatments, thefts or other vandalic acts). Among numerous examples, the darkening of vermilion red threatens a series of 15th-16th century paintings, including those by Pieter Paul Rubens [47], as well as Pompeian wall paintings [45, 48, 49]. In the latter, a change from yellow to red was also often observed in the areas where ochre pigments were used [50]. Similar colour changes of red and ochre pigments are also visible in other Pompeian wall paintings displayed the MANN museum today and documented as early as the 1980s (see Chap. 6).

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Other yellows of low durability and high sensitivity to external factors (radiation, relative humidity etc.) are orpiment, chrome yellows and cadmium yellows. The discolouration of orpiment is present in the works of art of Medieval Italian masters as well as 17^th century Flemish and Dutch painters, such as those authored by Cimabue [51], Pietro Lorenzetti [52], Daniël Seghers [53], Adriaen Coorte [54], and Jan Davidszoon de Heem [55]. The darkening of chromate-based yellows affects, instead, artworks by George Seurat [56], Paul Gauguin [57], and Vincent van Gogh [57‐59], while the whitening of cadmium yellows threatens paintings by James Ensor [60], Henri Matisse [61], Joan Mirò [62] and Edvard Munch [63, 64] (Fig. 17).

Among blues, the darkening of French ultramarine, cerulean and Thénard’s blues is also visible in masterpieces by Munch [65], while the fading of Prussian blue was observed in a series of 18th century Danish Golden age paintings (Statens Museum for Kunst, Copenhagen) [66]. Here a dramatic fading of the original blue colour is visible only in the areas exposed to light, while below the frame the colour is intact (Fig. 18).

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Ultramarine blue paints are known for having poor drying properties in oil. Although the pigment is noted to be comparatively permanent, there have been reports of changes in colour due to instability of the pigment [67]. Recent research has linked the discolouration to the degradation of the binding medium, understood to be a result of a of pigment–binder separation [68]. For instance, the phenomenon of blackening of deep blue paint observed in Old Man in Warnemünde (1907) by Edvard Munch (Fig. 19) may be seen in relation to reports on similar phenomena, dating from the period of early manufacturing of synthetic ultramarine [69, 70]. As synthetic ultramarine became available on the market in 1907, it was noted that the paint had issues of blackening, at the time attributed to impurities or additions of iron, lime, magnesia and potash, or to the paint being mixed with white lead [71]. Aluminium and Sulfur speciation investigations by synchrotron radiation-based micro-X-ray fluorescence and micro-X-ray near edge structure spectroscopy show that the composition of the ultramarine pigment used in Old Man in Warnemünde has a uniform distribution across the surface, in both dark and brighter blue areas [65]. This could suggest that darkening is likely due to transformations of organic components rather than to the pigment itself. The presence of natural wax in the paint sample is interpreted as a probable additive from the manufacturers. Such additions have been identified in ready-made paint from the period [71, 72] and have also been indicated by FT-IR analysis in ATR mode of certain paint tubes in the Munch Museum’s reference collection [73].

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Ongoing research on blue paint tubes from MUNCH’s collection showed that the presence of metal soaps could be held accountable for the separation of binding medium and pigments particles, resulting in an enriched medium phase that leads to superficial darkening [70, 73]. These processes can be responsible of black grains observed on the surface of other Ultramarine and cobalt-based blue areas of few paintings from Warnemünde period (e.g. Drowned boy, 1907–1908) or other periods (Death of the Bohemian, 1915–1920). Additional coloured materials that suffer from chromatic changes are specific classes of red lakes (i.e., cochineal-, alizarin- and eosin-based lakes), as found in some works of art of the American watercolourist Winslow Homer [74], in a series of works on paper by Gauguin [75] and in several paintings by Munch [65, 69] and Van Gogh [76‐78]. The degradation phenomena described in red lake paints in Van Gogh’s Glass with Yellow Roses, and Portrait of an Old Woman [76], exhibits cracks and spotting, in addition to browning, like the red lake paints in Munch’s Old Man in Warnemünde. In a red sample from Old Man in Warnemünde (Fig. 19), alizarin was identified as the colourant, with a possible tin/calcium-based and/or aluminium/calcium based-substrate, which might suggest a mixture of two types of red lake [65]. The deterioration, resulting in a brown discolouration of the red lake used by Van Gogh, related to the inclusion of starch.

One technique that allow the light induced colour change to be detected and monitored is micro-fadeometry tests (MFT), by suggesting a safe threshold dose of light for the studied object [79]. Currently there is no established protocol for MFT experiments. Work in this direction has been performed in H2020 project “Integrating Platforms for the European Research Infrastructure ON Heritage Science” (IPERION HS, GA n. 871034 [80]). Thus, the measurement typically consists in irradiating a selected point on an object with a specific white light with an intensity of the order of a few mW and simultaneous registration of colour changes caused by this illumination via colourimetric measurements. The light probe is focused on a small spot (typically less than 0.6 mm diameter); it follows that on one hand the fluence of light (expressed as power/area) at the measured spot is some order of magnitudes higher than equivalent conventional lighting systems, on the other hand the area of the paintings exposed to irradiation is strongly limited. A single micro-fading test takes up to 10 min and several measurements are usually performed for selected hues present on an object. To additionally ensure that minimal damage occurs, in some cases, the experiment is terminated once a colour change reaches units of just-noticeable-differences of human perception [81]. Moreover, the portability of the micro-fading allows it to be carried out for different types of artworks and in different conditions, such as Spanish Levantine rock art outdoor as shown by [82], prints, watercolours, and textile in museum environments [83]. In such context, further information regarding lightfastness of certain painting materials were successfully obtained by MFT (i.e. darkening of red areas containing vermillion) [84]. Nonetheless, micro-fading only assesses the photodegradation of the materials and does not indicate the possible chemical/structural changes that might have occurred due to other aging factors. A good example in this sense is that of the cadmium yellow pigment, which fades mainly because of exposure to moisture instead of light [64]. In this case, the micro-fading test captures a low extent of light-induced colour change, while in practice, the discolouration of certain types of formulation of cadmium yellows results from the exposure to humidity.

The micro-fading data may support the prediction of future colour change, which is straightforward from a mathematical perspective, as various interpolation and regression techniques can be applied. For instance, Morris [85] created a look-up table of colour changes for increasing light dosages, based on micro-fading tests carried out on paint mock-ups. Hendriks et al. [86] predicted the photodegradation of the red and yellow colours in Van Gogh’s “The Bedroom”, for a light dosage of up to 30 Mlux hr. As input, the authors used micro-fading data from aged mock-ups of red lakes and chrome yellow paints and based on these, altered the image of the painting in a commercial image editing software. In a similar way, Brokerhof et al. [87] rendered colour changes for a collection of Dutch city maps from the 17th century in an image editing software for various levels of fading and light exposure. Riutort-Mayol et al. [82] applied Gaussian process interpolation to extend the single-point micro-fading measurements to the two-dimensional surface of the artworks, and to forecast the photodegradation behaviour beyond the limited temporal dimension considered during the measurement. Ciortan et al. [81] adopted time-series analysis to forecast the individual change of colour coordinates in the CIE L*a*b* space, rather than the aggregate colour change, for a set of samples measured on “A Japanese Lantern” by Oda Krohg.

Recently, a novel approach that allows for simulating not only the colour, but also the spectral changes induced by light-exposure was proposed by Ciortan and co-workers [88, 89]. The method is based on a tensor decomposition model that, from a set of micro-fading measurements, disentangles in one shot the reflectance of pure pigments, their concentration, and their fading rate as a function of time. Such knowledge, combined with hyperspectral image analysis, was then exploited to render the past and future appearance of the artwork by modulating the amount of light exposure.

It is important to mention that all colour simulations (future and past) derived from micro-fading and more conventional accelerated photoaging experiments are questionable with respect to the light reciprocity principle. The light reciprocity principle states that exposing a specimen for short periods of time under high levels of light intensity is equivalent to long exposure under low illuminations levels [90]. Several studies [91, 92] have shown that light reciprocity principle breaks down when the difference in illumination between the two scenarios is higher than 3 orders of magnitude. This is even more relevant for micro-fading, for which light intensities and/or fluence values are further increased. Perhaps this is one of the reasons why simulations of past appearance of a painting, extrapolated from micro-fading tests, is especially controversial as often the full record of light conditions that the painting was exposed to (perhaps even before it became part of a museum collection) is unknown. Apart from the different levels of intensity, the light-induced change of a specimen in accelerated aging tests is influenced by the spectral distribution of the light source as earlier described [93]. As a rule of thumb, the change is proportional with the amount of overlap between the absorption spectrum of a material and the emission spectrum of the light. An exception to this rule is given by Prussian blue pigments and chrome yellow oil paints [94, 95]. The inverse behaviour of Prussian blue was discovered to be related to the amount of oxygen in the environment where the photoaging experiment occurs [96]. In the case of chrome yellow, the sensitivity of the paint towards darkening not only after the exposure to the blue component but also to the green one of the visible lights was ascribed to the presence of pentavalent chromium compounds, resulting from the thermal interaction of the original chromate-based pigment with the oil [95].

The reconstruction of how colours would have originally been perceived is crucial, since their appearance strongly influences our perception of the work of art. Guidelines are also needed to preserve them when exhibited (a control of environmental conditions and lighting is fundamental). In such context, through studies of micro-samples from artworks, laboratory paint mock-ups and direct non-invasive examinations of painted surfaces, a large quantity of data has been acquired in the last two decades from the above-mentioned case studies, thus helping to unveil how the constituent materials of the paint and environmental factors may affect the overall colour change phenomena. In the case of chrome yellows and cadmium yellows, for example, analytical investigations have contributed to help Museums to optimize the long-term conservation condition of some iconic paintings, such as the Sunflowers by Van Gogh (version of the Van Gogh Museum, Amsterdam) [97] and The Scream (ca. 1910) by Munch (version of the MUNCH museum, Oslo) [64]. In addition, experimental datasets have contributed to develop models for the virtual reconstruction of the original colour scheme of the artwork intended by the painter. In such context, several projects or research groups were and still are involved in activities related to the digital colour reconstruction.

A pioneering work on paintings by Seurat and Van Gogh was conducted by Berns, Pozzi and co-workers [98‐100] by following a hybrid strategy that combines visual evaluation of the painting, pigments identification in micro-samples using different analytical methods (e.g., infrared and X-ray fluorescence spectroscopy; gas chromatography-mass spectrometry; scanning electron microscopy coupled with energy dispersive X-ray spectroscopy; Raman spectroscopy), single-point reflectance visible spectroscopy measurements acquired across the painting surface, definition of the original optical properties of the painting via comparison with data obtained from paint reconstructions, colour-managed RGB imaging and image adjustments with dedicated photo-editing software. Paint reconstructions of Van Gogh’s palette was a significant component of REVIGO (Reassessing Vincent van Gogh’s colours), a project devoted to study the colour changes in Van Gogh paintings and drawings and sponsored by The Netherlands Organization for Scientific Research. Key technologies for REVIGO were hyperspectral visible spectrum imaging and macro XRF scanning mapping methods, as well as machine-learning techniques [101‐103]. The optical database resulting from the paint reconstructions has given an important contribution toward understanding Van Gogh’s working method and improvements in colour reconstruction methodologies.

The application of machine learning techniques for processing and modelling large spectral imaging datasets has drastically expanded over the last 5 years, by permitting to perform advanced mapping of artistic painting materials and to identify pure and mixed pigments via application of spectral data obtained from artistic materials [104‐108]. To date, however, there is still no standard method that allow for a reliable virtual colour restoration or colour change prediction of an object to be performed. Deep knowledge from the study of laboratory samples and paintings to understand how the paint composition and environmental factors influence the extent of colour changes is still missing and further experiments are required. The PERCEIVE project, by applying digital and machine learning-based methods to large experimental datasets from discoloured laboratory samples, Pompeian wall paintings and artworks by Munch, has developed a “Predictive System” that uses the analysis of colour change during the time, giving the possibility to “play forward the clock” and see how the artwork would look in a span of time if no specific preventive conservation measures are put in place.

“That a tree can be red or blue – that a face can be blue or green – they know it is wrong – From childhood they have known that leaves and grass are green, and that skin-colour is slightly red. – They can’t understand that this is meant seriously – it must be humbug done in sloppiness – or with a deranged mind. Preferably the last”.

(Munch, 1891–1892).

Edvard Munch wrote “I do not paint what I see but what I saw”. This statement suggests that Munch perceived his work as a product of the cumulative emotion of the mind’s eye. “Intentionally and consciously between seeing something and realizing it in paint, he passes through a mental filter from which it later emerges transformed in the intensity of the remembered moment”. He often uses colour not for naturalistic description but to convey the authenticity of feeling. The red colour of the sky in the Scream is correlative of the characters emotions: hopelessness and panic [109‐111]. Munch had respect for scientific knowledge (he showed fascination with photography, radio, Xray and electricity,) but also valued in first place his own experience, considering his visual observations equally valid. The injury to his right eye in 1930 gave him different sensations that he started to express in his drawings and watercolours. In his in-depth study of his ocular disease, Munch linked the theme of the Scream to difficulties in perceiving colours and sounds. In one note the artist writes “What we hear are sound waves that are absorbed by the membrane of the ear. What we see are light waves that affect the membrane of the eye. Poetry is the perception of waves by the eye and the ear. Painting is what the brain perceives through the filter of the eye” [109]. Munch emphasises his suggestion with a reference to the Scream: “These light effects not only resulted in oscillation of the eye, they also resulted in oscillations of the ear – so that I actually heard a scream. I then painted the picture the Scream”. Edvard Munch read popular manuals on the science behind colour and the powerful visual and physical effects that colour can have on us. These writings promoted that there are forces at work in the universe that impact the world around us but are imperceptible to the human eye—a notion that was very fashionable at the time, given the great public excitement over such recent scientific breakthroughs as the discovery of X-rays, electricity, magnetism, and the beginnings of quantum mechanics. These discoveries included new insights by physicists revealing how humans interpret colour, regarding the radiation spectrum and the way radiation interacts with matter and produces the effect of colours [109, 110]. For example, because red has a greater wavelength and is interpreted faster by the human brain, appears more striking. Red became a key- colour for the artist, being used in many masterworks from Vampire (1895) to the blood-red sky of The Scream (1893; 1910?). According to Jonathan Bober, curator of prints and drawings at the NGA, Munch was also familiar with popular spiritual theories propounded by Annie Besant and C.W. Leadbetter, whose influential book Thought Forms (1901) described individual colours as emitting different energies.

From an early-stage Munch also worked with new types of paint that were introduced on the market, experimenting with mixture of oil and tempera media, and even creating his own paints by mixing casein and glue or egg with powdery pigments purchased in several shops of painting materials in Oslo [112]. He was also fond of using crayons in addition to the more traditional painting materials, as many other artists of that time (Renoir and Pissarro preferred to use pastels due to the matt appearance of the medium, while Manet is said to have used pastels on canvases that were factory primed) [110‐113].

In the Sketches for the monumental paintings made for Aula Magna at UiO he even used the unprimed canvas as an active part of his palette (and with time the canvas yellowed and darkened due to deposition of particulate from atmosphere, pollution and other factors). Several binding media have been identified such as: gelatine (from animal glue) based and linseed oil for The Sun (1910–1911), walnut oil and boiled linseed oil in The Researchers (1925–27). The matt appearance of the paint is due to the small amount of binding media (oil) in relationship to the amount of pigments and chalk [110‐114]. Munch also used mixed techniques where oil brush strokes or even water-based colours (with organic components as glue and plant gums (gum Arabic or tragacanth) were overlapped or juxtaposed by lines of crayons (wax and oil-based drawing materials). Some of the sketches were painted on primed linen canvas, while others were executed on unprimed cotton canvases, not intended for painting purposes [112].

Munch was an experimentalist and not very much interested in the stability or long-term preservation of his works. Along the 60 years of artistic production, he evolved from a more realistic style of painting to a symbolistic and modern expressionistic style. He is also renowned for his unconventional, direct, and personal style. He used less saturated colours than those used by his contemporaries, but the low saturation colour enhances a different, heavier, more charged atmosphere. His main interest was to obtain a matt appearance for his paintings. He therefore avoided the use of varnishes or other glossy surface materials. Colours become protagonist in Munch’s innovative experimentation, applied in thin and/or thick brushstroke, squeezed directly from the tube, blended with crayon lines and contours, pure, mixed or overlapped, sometimes even applied with fingers or other tools than brushes. This taste for experimentation lead to severe consequence regarding the stability of paint layers and integrity of the representation [113, 114]. Nowadays many works present porous, powdery, decohesed paint layers, colour change phenomena (fading and darkening), abrasions and cracks, paint layers losses and lacunae. All these degradation-forms lead to a loss of the readability of the image depicted, but also to many questions related to long-term survival of these artworks. In few cases, as Madonna and The Scream, the integrity of the object has also been affected by theft (2004) and other vandalic acts [115]. A big halo produced by a liquid agent left a deturpated mark in the left corner of the Scream painting, the cardboard also being heavily damaged in this area. The decade 1892–1902 represents the most important period of Munch’s activity: in these years the artist defined his pictorial language, which is enriched with symbolistic elements and would influence Expressionism. The recurring themes are love, death and existential anguish, depicted by deformed images and garish colours. In this period his iconic Scream was also born, emblematic of the condition of modern man, prey to anxiety, inner loneliness and mal de vivre. That “great infinite scream” which according to Munch pervaded Nature, became the manifesto of his artwork, an expression of the lonely scream of all humanity [115, 116]. Munch wrote about the moment he saw the striking sunset depicted then in the Scream (also initially named the Scream of nature):

I was walking along the road with two friends – when the sun went down. The Sky suddenly turned blood-red I paused, leaned against the fence tired to death – above the blue-black fjord and city blood in flaming tongues hovered. My friends walked on and I stayed behind quaking with angst – and I felt as though a vast endless scream passed through nature.¹³

It is obvious that Munch used colour in this painting to convey emotion and drama, the colours becoming an exacerbation of feeling supporting his vision of the clouds as real blood. There is a powerful contrast between exaggerated reds and oranges used for the sunset in the background, and the dull blues, greens, purples and greys used for everything else. The use of colour contrast in this painting works two ways: to show the intensity of the “blood red” sunset in the background and the drama of the figure on the bridge. Whilst the painting appears relatively simple, there is a clever contrast between two pairs of complementary colours: red and green, and orange and blue. This adds a subtle level of complexity and some interesting colour relationships to the painting. The “blood” red in the sky appears to be the strongest colour, which contrasts against the very weak green for the land. The ghoulish figure was painted with sickly colours: dull yellows, blues, and purples. Dark and light accents painted over the top hint at the “screaming” expression of the character. No attention was given by Munch to paint this figure with any sense of realism. It is likely a creation of Munch’s own tormented mind and anguish [116]. Munch’s approach to the union of senses and the belief that one might taste a colour or smell, a musical note, results in the visual depiction of sound and emotion. The visual music and visual poetry are here combined in the specific aesthetics of the artist, resulting from the curved lines and the strong colours used to accentuate the oppressive general state of the character and the artwork itself. As such, The Scream represents a key work for the Symbolist movement as well as an important inspiration for the Expressionist movement of the early 20^th century. As Munch wrote it in his notebook “it is not the chair which is to be painted but what the human being has felt in relation to it”. Through this artwork, Munch sought to express internal emotions through external forms and thereby provide a visual image for a universal human experience. Pettersen’s late research [116] shows that there are several theories about how and why the Scream motif developed further, and she brings up new aspects that can point towards the dating as being 1893 and not 1910?, in one version in the MUNCH-collection. She studies not only the stylistic features and the sources of the time of creation and Munch’s own writings, but also suggests that further chemical analysis of The Scream compared with other paintings (i.e. Anxiety, Despair, Hands) (Fig. 20) could bring new clues regarding the dating year. The first version of the Scream is considered to be what is known as Despair, dated 1892 (Fig. 20a) and its schematic representation of the sky and main characters vaguely resembles the motif in the later versions (form National Museum and also MUNCH, Fig. 20b and c). The prose poems related to the Scream are also contradictories and it is difficult sometimes to understand which version is mentioned.

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In 1893, Munch rented two exhibition rooms on the second floor at 19 Unter den Linden in Berlin. Here The Scream was exhibited for the first time. In the catalogue, six of the 25 paintings were listed under the heading Die Liebe. The Scream was one of them, entitled Verzweiflung (Despair). A couple of newspaper reviews mention the painting. The first one was published in Berliner Tageblatt: “but the last picture: Despair has a caricature figure in which nothing human is to be found; the background, a white mixture of the brightest red, green, and yellow brushstrokes: What that means, if it isn’t himself – heaven knows!” [116]. The second review was published in the Norwegian Morgenbladet (1893): “The last picture Despair is a pure caricature. The background is a strange egg yellow, bright red and green sky, and the figure, a small man with long hair and convulsively open mouth, impossible to understand”. Both these reviews mention three colours: red, green and yellow. There is a large area of green in the background to the right, under the red and yellow sky, in MUNCH’s version of The Scream (Fig. 20c). In NaM’s version, there is very little green colour to be seen. Only a few green lines are drawn in crayon on the right, and some green brushstrokes run along part of the painted red vertical area (Fig. 20). In NaM’s version, there is very little green colour to be seen. Only a few green lines are drawn in crayon on the right, and some green brushstrokes run along part of the painted red vertical area (Fig. 20). One wonders which of the two versions is described above – is it really the National Museum’s version? It is difficult to believe that its colours could have been described as red, green and yellow. The colour descriptions in these reviews undoubtedly suit MUNCH’s version better.

In Stanislaw Przybyszewski’s article Psychischer Naturalismus [117], the colours of The Scream are described as follows:

Finally, the last picture: Despair. On a bridge, or something similar, it doesn’t really matter what it represents – stands a mythical animal with its mouth wide open. […] Each pain a blood red stain; each long-awaited scream of pain a belt of blue, green, yellow stains; unbalanced, brutal next to each other, like the boiling elements of developing worlds in wild creative fervours.

Przybyszewski mentions not only red, yellow and green, but also the blue colour, which is present in both paintings [117].

The paintings of Munch have a provocative character due to the use of certain materials and developments in his experimental techniques of preparing and applying the paint. His subject matter was marked by unmotivated blotches, scratches, scrapes, dripping paint and areas of unpainted canvas. From 1886 onward, Munch was the most modern of contemporary Norwegian artists, due to his ability to transcend the boundaries of common convention and practice, even regarding the most radical of expressions to be found in Realism and Impressionism [117]. In connection to the Christiania exhibition of 1892 Munch’s use of colour was described in the following manner [116, 117]:

hideous, pale colours, as blotted and thin as the first layer of paint on a house wall, anemic colours like tuberculosis patients that have never known nourishment of blood or daylight or – at their worst – piercing yellow patches, hectic reds, and screaming blotches spread aimlessly and with no apparent meaning over the canvas…On top of that, he smears the canvas with lemony yellow and glowing reds, as if he had daubed it with bloody fingers – and calls it evening mood!

In the manifest Cosmos. The New Sublimity in Art written in 1893 by Carl Gustaf Uddgren and Max Dauthendey, Munch is deemed the contemporary artist who had made the most progress in reproducing “the intimacy of colour”: “For him, the main objective is to conserve the most sublime feelings of pleasure or displeasure that colours trigger in his brain. It is irrelevant to him what scene or event he depicts”. Munch conveys a “main mood” through his use of colour!” [116]. Przybyszewski describes the Munch’s style in Psychic Naturalism [118] saying about the shaping of landscape in Despair: “His landscape is the absolute correlation of such naked feeling; every single vibration of nerves, which have been directly exposed to the utmost ecstasy of pain, is instantly converted into an appropriate sensation of colour”. Przybyszewski contends that Munch’s painting introduces something new in comparison to contemporary symbolism by using colour to coupe spiritual life with pictorial expression. Colour is Munch’s most essential tool and in colours he sees “mourning and screams rumination and wilting” [116].

Throughout human history, colour has played an important role also in historical textiles. To trace the origins of textile dyeing, one must delve into the earliest history of mankind. Initially, clothing was fashioned from animal hides, but with the development of spinning and weaving, the art of dyeing textile fibres came closer to realization. The techniques of spinning, weaving, and dyeing likely emerged concurrently among various cultures, and the knowledge of these practices spread from one civilization to another [119]. It was through this dissemination of knowledge that the art of dyeing textiles evolved and flourished, leaving a rich legacy of vibrant colours that continue to captivate us in the modern age. The availability of certain pigments and dyes also affected the rarity and cost of particular colours, making them exclusive to the privileged classes such as the elite, royalty or religious orders [120, 121].

Based on origin, natural colourants can be classified as organic and inorganic, inorganic are pigments while organic colourants can be further grouped as dyes and pigments. Historical dyers since antiquity would expertly use natural resources such as parts of plants (leaves, root, bark, wood, seed, flower), minerals (ochres, carbonates), insects (cochineal, lac dye) and their mixtures to create various hues.

Colours often held deep cultural meanings and symbolism where different cultures associated specific colours with values and traditions. Colours were much more than just aesthetic choices; they were expressions of identity, beliefs, and social norms of their time. The diverse palette of colours used in these creations conveyed rich narratives about the people and societies that embraced them, offering a captivating window into the rich heritage of human history and culture. Tapestries and dress collections bore witness to the profound cultural meanings and symbolism attached to colours. These vivid expressions of identity, beliefs, and social norms showcased the diverse values and traditions of different cultures. Beyond being mere aesthetic choices, the colours woven into these tapestries and draped across garments conveyed intricate narratives about the people and societies that cherished them.

Studying historical textiles and use of colours offers valuable insights into the mindset and values of people from different periods and societies. The importance of colour in textile collections cannot be overemphasized, as it plays a central role in shaping the overall impact and significance of these collections. Colour is one of the primary factors that catch the viewer's eye by creating an immediate visual impact. The use of vibrant and well-coordinated colours enhances the attractiveness of a collection, making it more engaging and memorable. Colours can be used to convey specific themes, moods, or concepts in textile collections. Whether it's a historical period, cultural representation, or a specific artistic vision, colours help to communicate and reinforce the intended message. Different colours can also accentuate various design elements in textiles, such as patterns, embroidery, and textures. The right colour choices can draw attention to these details and enhance the overall design. For historical collections, understanding the original colours and their significance is crucial for preservation and restoration purposes. Accurately reproducing faded colours can provide a more authentic representation of historical garments.

The problem of fading in dress collections is a significant concern for museums, collectors, and those responsible for maintaining and displaying historical garments and textiles. It is a complex issue that requires careful attention and proactive conservation strategies to ensure the long-term preservation of historical textiles and garments for future generations. The fading of colours refers to the gradual loss or alteration over time, resulting in a less vibrant and often discoloured appearance. Several factors can contribute to fading such as exposure to light, particularly ultraviolet radiation (UV) from natural and artificial sources where prolonged exposure can break down the chemical bonds in dyes and pigments, leading to degradation and colour loss. Fluctuations of environmental conditions, namely, temperature, humidity, and air pollution can also affect fading and deterioration processes. The lightfastness and stability of dyes and pigments, and the use of certain textiles can significantly impact their susceptibility to fading due to their inherent chemical and physical properties. Considerations regarding previous exposure to harsh environmental conditions or improper storage and handling practices can exacerbate fading and accelerate colour loss. Nowadays museums are increasingly sensitive to the issues of fading and implement tailored strategies such as light control and environmental monitoring in display, storage and exhibition spaces as well as garment rotation and use of conservation grade materials for storage to help minimize fading [122].

Analysing and reconstructing original colours in textiles can be a challenging process. Various approaches and techniques are used through multidisciplinary collaborations between scientists, historians, and conservators to uncover and reproduce the original colours of historical textiles [123‐125]. Studies of historical documents that depict similar textiles or garments from the same period can offer valuable clues about the original colours. Through visual examinations of the textile, areas of preserved pigments or dyes can be observed, and any patterns or motifs that may provide clues about the original colours may be identified. The fine details of the textile's surface and structure, including the distribution of colourants and any degradation or fading patterns can be revealed on microscopic examinations [125]. The analysis of dyes in fragile materials such as textiles, requires a continuous development of new analytical approaches that reduce the physical impact of analysis on the material under study whilst ensuring that maximum information is gained. Emphasis is given to the need for reliable, efficient, and ideally minimally or non-invasive analytical approaches for historical dye analysis [126]. In recent years, capturing images of a textile under different wavelengths of light, such as ultraviolet, visible, and infrared with multispectral and hyperspectral imaging techniques have been shown to reveal hidden features and faded colours not visible to the naked eye [127]. Non-invasive portable spectroscopic techniques (UV-Visible, fibre optic reflectance (FORS) Infrared and Raman spectroscopy) and colourimetry can help analyse the electronic and molecular composition and colour quantification of original textiles in-situ and assist in identifying the presence of specific dyes or pigments [128]. They can collectively provide insights into the original colourants used leading to a deeper understanding of fading mechanisms. Non-invasive methods are often used to guide object sampling should ulterior destructive chromatographic (HPLC) or spectrometric analyses (MS) be required for identification of complex dyes mixtures and associated degradation products [129]. Within PERCEIVE project, we construct experiments with historical recipes and traditional extraction and dyeing methods to recreate the colours observed in textiles by preparing a set of mock samples. This process involves using natural materials and conventional dyeing techniques often with the use of mordants to match the original colours as closely as possible. Artificial ageing of such ad-hoc laboratory samples can provide information on fading and degradation with close monitoring of alterations arising from modifications to the physical and chemical properties of the dyes and textiles. It is essential to note that reconstructing original colours is not always feasible, especially if a textile has undergone significant fading or deterioration over time. Moreover, employing such analytical approaches can help uncover valuable information about historical textiles and contribute to a deeper understanding of our cultural heritage. The work to be achieved is to supplement the laboratory and on-site methods with colour reconstruction techniques to restore and enhance digital colour information, to fill in missing or damaged areas in an image by estimating the pixel values based on the surrounding regions. Such techniques are particularly useful in chosen cases where colour data has been lost or degraded and aims to improve the overall quality and realism of images by filling. Colour reconstruction techniques could include interpolation, demosaicing or colourization by estimation of missing colour values based on surrounding known colour values; reconstruction of full-colour images from images captured with colour filter arrays; or prediction of plausible colour information based on the input grayscale data. Such methods do not always produce the most accurate results, especially in regions of complex colour variations but can be assessed accordingly [130, 131].

Artificial intelligence (AI) can be also utilized in combination for restoring faded textiles to their original or enhanced appearance. AI can assist in the analysis of multi-spectral imaging data for faded textiles to provide additional insights into the original colours, patterns and textures of the textiles. AI-based techniques can also help in reviving faded textiles or historical tapestries that suffer from colour degradation due to aforementioned ageing, exposure to light, and environmental factors [132]. AI can be applied to faded textiles through image restoration techniques, often based on deep learning models to reconstruct missing or degraded colours by learning from large datasets of well-preserved textile images such as those from specific experimental mock sets constructed in the laboratory. AI algorithms can estimate the original colours of faded textiles by analysing the remaining colour information and comparing it with known colour patterns from similar textiles or historical references as those case studies to be analysed in-situ. Deep learning models can be trained to predict plausible colour values for each pixel based on surrounding context and patterns where AI can be used to recognize and reconstruct faded patterns in textiles. By training machine learning models on patterns from well-preserved textiles, the AI can identify and recreate the original patterns in faded textiles. By artificially simulating fading and aging effects on digital textile images, in comparison with real laboratory studies AI can create larger datasets, which can be used to train models for colour reconstruction and restoration [133]. This entire process from the laboratory to in-situ analysis in PERCEIVE can permit for targeted colour reconstruction and restoration of the faded areas of the chosen textile case studies, minimizing the risk of applying corrections to the original areas. Combining AI with traditional analytical conservation methods can lead to more effective and responsible restoration practices for faded textiles whilst enhancing the experience of museum visitors [134].

Preservation of Cultural heritage, including motion pictures and photographs, is a critical endeavour that safeguards the visual history of societies. Both motion pictures and photographs share common challenges in terms of deterioration, technological obsolescence, and environmental factors. However, they also possess distinct characteristics that necessitate specific preservation strategies.

This section investigates the influence of colour in Augmented Reality (AR) digital born artworks, with a specific focus on transparency and on the interplay between transparency in an AR “sculpture”, or 3d object, and its impact on other elements within the artwork. Moreover, we will examine how transparency in AR art affects the perception of the real world, when viewed through the transparent elements of a sculpture. By analysing these interactions, the research aims to unravel the transformative effects of transparency in AR art on colour aesthetics and the overall visual experience for viewers, both within the artwork and in the context of the real world. We will start by examining a masterpiece by Leonardo da Vinci, which, despite being a painting, employs transparency in a remarkable manner. Building upon this observation, the text presents a comparison between the mixing of pigment and light, aiming to establish a connection between the traditional use of transparency in painting and its contemporary application in digital art. Through this exploration, we will highlight the distinctions and similarities between analogue and digital colour usage in artistic expression. With the concept of transparency and its broad applications in colour clearly outlined, the text proceeds with the presentation of a diverse types of digital artworks. Through the use of AR, these artworks skilfully merge virtual elements with reality, all made possible through the intentional incorporation of transparency.