The Problem with Bitumen

Introduction

Bitumen ‘is probably the most notorious of all painting materials.’¹ So concluded a study of the durability of artists’ materials in eighteenth- and nineteenth-century British paintings.² Bitumen has often been held responsible for paint film defects, such as extreme cracking, wrinkling, discolouration and creeping – so much so that the term ‘bitumen cracking’ has become common parlance to describe a cracked, brown paint layer, and the term ‘bituminous’ to describe paint passages that possess those characteristics.³ And yet, an array of contradictory statements about bitumen and a confusing, ever-shifting terminology make the story of bitumen in painting as sticky and slippery as the material itself.

This paper attempts to unpack this story, offering a history, commonly only told up to the end of the nineteenth century, up to the present day. While this longer story still falls short of the whole story, it aims to suggest the simultaneous validity of apparently opposing views of the same substance, and the possible use and value of such contradictions. To do so, this article begins by exploring when and how bitumen gained its poor reputation as a painting material, and how it became linked to certain kinds of defects in paintings. It then examines the literature that has problematised this close association between bitumen and painting deterioration. The final section investigates some of the ways that modern and contemporary artists have continued to creatively engage with bitumen and its history; this section makes evident how bitumen reveals the limits of the usual modes of investigation into artists’ materials, and how views of a material that has survived for long periods can shape and reshape opinions about artists and their techniques.

Bitumen as a Problematic Material

The use of bitumen in painting up to the twentieth century has been surveyed in a number of publications that offer vital insights into the ways in which it became so frequently synonymous with particular types of painting defects.⁴ It is mentioned in treatises from the sixteenth century,⁵ and artists have used bitumen in various preparations and formulations, finding its rich, usually brown colour and its transparency useful for glazing over other colours to darken and enrich them, as well as for painting shadows.⁶⁷ Bituminous preparations for glazing over pictures were increasingly popular in the eighteenth and nineteenth centuries when the aesthetic for warm, dark-toned paintings became more prevalent. As the British painter and gallery director Charles Lock Eastlake wrote, voicing the popular view of his time, ‘All vivid colours, and spots of any such colour in a larger mass, when toned, and reduced by brown, are not only more harmonious and agreeable, but appear to have their actual hues deepened.’⁸ Artists of Eastlake’s period often sought to emulate the overall brown hues of old master paintings, gained from age, with the American painter Washington Allston even referring to the bitumen-containing mixture that he used over the surface of his paintings as ‘Titian’s Dirt.’⁹

Discussions about bitumen’s deleterious effects appear to have begun in British treatises on painting sometime in the early 1800s.¹⁰ From then on painters were frequently the subject of blame for having chosen to use such a material whenever their paintings subsequently cracked, wrinkled and darkened. Authors such as the chemist George Field ensured that the material’s reputation was established by the 1840s. For example, Field commented in 1835 that ‘its fine brown colour and perfect transparency are lures to its free use with many artists, notwithstanding the certain destruction which awaits the work on which it is much employed.’¹¹ Similarly, in 1866 the brothers and artist-curator-critics Richard and Samuel Redgrave used the term ‘asphaltum cracks’ to describe serious, wide cracking of contemporaneous paintings in their A Century of British Painters, and lamented that ‘almost all’ the English school pictures up to 1835 had suffered from ‘improper use of pigments,’ adding that ‘amongst these the worst is bitumen in all its varied forms of asphaltum, mummy, bitumen, &c.’¹² A third example should suffice to illustrate bitumen’s reputation: the chemist Arthur Church, in the 1915 edition of his book The Chemistry of Paints and Painting, counselled artists that bituminous materials are ‘treacherous on account of their easy fusibility’ and ‘liable to stain contiguous pigments by reason of their solubility in oil or varnish.’¹³

This linking of bitumen with certain defects in paint passages continued through the twentieth century, with writers often repeating the assertions of earlier texts and condemning artists for its use. In particular, the contention that bitumen does not dry and that it is this property that causes paint film cracking was recurrent. For example, in 1986 the scientist Raymond White described bitumen and asphaltum as ‘known for the considerable problems that they cause in the drying and continuity of the oil medium film.’¹⁴ To some extent, ‘bitumen cracking’ or ‘bitumen shrinkage’ became recognised terms: the ‘condition known as bitumen shrinkage…is recognised by restorers and art historians alike as an inbuilt, irreversible problem.’¹⁵ Indeed, with such comments as ‘Gentileschi’s use of bitumen is regrettable,’¹⁶ writers continued to apportion blame to artists’ material choices wherever characteristic paint film defects were found; the art historian Tim Green even condemned the entirety of the nineteenth century as ‘casting a bituminous shadow’ over artists of the twentieth from which they strove to escape.¹⁷

Significantly, the assertion that bitumen causes defects was repeated so regularly that a type of reverse argument also became accepted: any brown paint passage with defects must contain bitumen. The start of this somewhat circular reasoning can be traced to the nineteenth century and followed through the twentieth-century literature. For example, although the presence of bitumen was not confirmed by other means at the time, a ‘bituminous layer’ was reported within a passage of Orazio Gentileschi’s Joseph and Potiphar’s Wife (ca. 1630–32, oil on canvas, 206 x 261.9 cm, Royal Collection Trust) since the same passage exhibits ‘bituminous-like craquelure.’¹⁸ Indeed, ‘bitumen’ or ‘bituminous’ appear to be used as descriptors for a paint passage exhibiting particular characteristics, rather than to mean that bitumen is present. The paintings conservator Anne Massing stated this explicitly: ‘To the restorer, a deep brown-black paint layer with pronounced drying cracks, which is soluble in relatively weak solvent mixtures, is considered “bituminous.”’¹⁹

In truth, it is not common for writers to define how exactly they use the terms ‘bitumen’ or ‘bituminous,’ and so it can be difficult to discern from much of the literature whether an author refers to the presence of the material or to characteristics of a paint passage. This might be seen in the art historian Charles Rhyne’s writing on the British artist John Constable when he concluded that Constable must have used bitumen in Orazio Gentileschi’s The Valley Farm (1835, oil on canvas, 147.3 x 125.1 cm, Tate) because it was ‘depressed with the dull, dark, wrinkled effects of deteriorated bitumen,’ unlike most of Constable’s other paintings. Importantly, Rhyne found confirmation of his argument in a 1960s technical examination of the painting, ‘which noted ‘a heavy bitumen glaze’ and he lambasted modern critics for wondering at the reasons for the discrepancy in this painting’s appearance compared to Constable’s general oeuvre for so long: ‘we should all have recognised that bitumen was most likely the problem simply by looking at the painting.’²⁰ And yet, because of the ambiguity of the term itself, it is hard to be certain whether the author of the 1960s technical note meant that they thought the layer contained bitumen, or whether they thought that it had the characteristic appearance of a bituminous layer.

Twentieth-century writers reinforced the link between bitumen and paint film defects by offering evidence from contemporaneous accounts of artists’ methods. For example, Rhyne finds further evidence for bitumen in The Valley Farm in a contemporaneous anecdote about another artist taking bitumen from Constable’s palette on the 1829 varnishing day at the Royal Academy. Constable, therefore, ‘had bitumen (asphaltum) on his palette in 1829.’²¹ Furthermore, analytical methods began to offer some further support for the links between bitumen and painting defects by the 1990s. For example, the technical art historians Leslie Carlyle and Anna Southall report that analysis of paint from artworks that were gifted to the British public by Robert Vernon in 1847 ‘point to’ asphaltum ‘being a promoter of disfiguring drying problems.’²² Similarly, conservation scientists Luuk Struick Van der Loeff and Karen Groen report that gas chromatography and gas chromatography-mass spectrometry ‘indicate the presence of asphaltum or bituminous material in the oil medium of Gerard Dou’s The Young Mother’(1658, oil on panel, 55.5 x 73.7 cm, Mauritshuis).²³ Despite the circumspect and careful language—‘point to’ and ‘indicate’—such statements may have contributed to the endurance of the view that bitumen causes painting defects. This endurance is illustrated by the final part of the description of bitumen presently accessible on the Tate website: ‘because it [bitumen] does not dry it eventually causes often severe darkening and cracking of the paint. This can be seen in the work of Sir Joshua Reynolds, Henry Fuseli, and Sir Thomas Lawrence.’²⁴

Thus, having surveyed discussions of bitumen chronologically, it can be seen that the association of bitumen with defects in paintings occurred first with a number of warnings and complaints about bitumen in nineteenth-century treatises. These complaints were then repeated through the twentieth century, to the extent that a reverse argument began to develop: not only did bitumen cause defects in paintings, but any brown paint passage with defects must contain bitumen. Such circular reasoning was buttressed by the very literature that had initiated the association. This process can be exemplified by study of commentary—art historical and technical—on works by any artists to whom criticisms of the use of bitumen have stuck in particular, such as Sir Joshua Reynolds, J. M. W. Turner, David Wilkie and Washington Allston.²⁵

The case of Reynolds is perhaps the preeminent example (Fig. 1). Reynolds was blamed by the Redgraves for his ‘injurious use’ of bitumen, which they said caused many of his pictures to have ‘failed terribly in the darks.’²⁶ The reverse argument, that a damaged paint passage by Reynolds must contain bitumen seems to have been established early; there is evidence that Reynolds’s failing paintings were already described as ‘bitumen cracked’ in nineteenth-century examination reports.²⁷ In the next century, historians and conservators also critiqued Reynolds’s use of bitumen: ‘he set out to emulate the effect of the old master paintings with a choice of materials and procedure that overturned centuries of sound technical practice…Asphaltum was a great favourite for his imparting rich, dark glazes.’²⁸ It seems relatively clear that the suggestion that Reynolds used bitumen came directly from earlier writers, such as the Redgraves, because the criticisms levelled at Reynolds follow such similar lines: the Redgraves, for example, also condemn Reynolds for being overly interested in ‘ascertaining the methods of the great masters’ and in ‘brilliancy, impasto, depth, or richness’ to the detriment of sound practice.²⁹ Twentieth-century writers found further evidence to support their reassertion that Reynolds used bitumen in contemporaneous literature: M. Kirby Talley – an art and conservation historian – relays the anecdote given by the painter Philip Reinagle that ‘he remembered Sir Joshua’s using so much asphaltum that it used to drop on the floor.’³⁰ Finally, reports of chemical detection offer further, tentative affirmation. For example, paint media analysis carried out on eight of Reynolds’s portraits by direct temperature-resolved pyrolysis mass spectrometry found ‘on occasion bitumen.’³¹

In Defence of Bitumen

The two longstanding assertions about bitumen ­– that bitumen causes paint defects; and, conversely, that any darkened, cracked or wrinkled paint passage contains bitumen – are worth interrogating, and in recent years many within fields such as conservation and technical art history have examined the close association of bitumen and paint defects. There are at least three persuasive reasons for doing so. Firstly, complaints and warnings about its detrimental effects have not been found in paintings treatises written before the nineteenth century. The equivalent Italian terms spalto or aspalto are mentioned in Italian treatises written as early as the 1580s.³² In the early 1600s, Théodore Turquet de Mayerne described recipes for painting flesh that included asphaltum, and he considered the colour stable.³³ Indeed, in the nineteenth century, Sir Charles Eastlake already wondered why in earlier texts ‘there are no complaints, in any of the writers quoted, of the flowing, or the cracking of this substance.’³⁴ Secondly, reconstructions of its preparation and application as paint according to historical recipes have found that it does not necessarily migrate, wrinkle or crack.³⁵ Thirdly, the apparent instability of bitumen in paintings seems slightly curious considering it is used extensively as a ‘durable and reliable material’ for applications ranging from road building to roof felting and has been used as a sealant and mortar for centuries.³⁶

The examination of the close association of bitumen and paint defects has taken a variety of forms. To begin with, in a reversal of their role in supporting the association between bitumen and defects in late twentieth-century publications, analytical chemistry methods have begun to support their dissociation, with a number of twenty-first-century reports not finding bitumen where it was perhaps to be expected. For example, ‘bitumen, which is commonly referred to as the cause of deterioration in many paintings by Reynolds, was not identified in either the Portrait of Lord Heathfield (1787, oil on canvas, 142 x 113.5 cm, London National Gallery) or that of Colonel Tarleton (1782, oil on canvas, 236 x 145.5 cm, London National Gallery),’³⁷ nor has its presence been confirmed in Reynolds’s paintings analysed since.³⁸ Similarly maligned for their ‘inconsiderate use of bitumen,’³⁹ the works of the Barbizon School artists have been analysed at the London National Gallery: ‘no mummy or bitumen has yet been found.’⁴⁰ In fact, summarising the status of bitumen detection in paintings in 2007, the technical art historian Catarina Bothe concluded that, at that time, ‘no secure observations on original paintings exist.’⁴¹ At the same time, however, the lack of analytical detection only calls into question the assumptions about bitumen to a certain extent, since whenever using such analytical techniques, and particularly when sampling from larger paint passages, absence of evidence of a material’s presence is not necessarily evidence of absence. Additionally, a bitumen-containing paint could have been subject to a range of processing treatments, such as heating, that may have rendered the original ingredients chemically unrecognisable in the final painting.⁴²

Next, investigation into the terminology surrounding bitumen reveals a baffling array of terms with partially overlapping meanings. Not only is bitumen used to refer to a number of different materials, but a number of different words are used unsystematically to mean bitumen. ‘Asphalt,’ or ‘asphaltum,’ and ‘bitumen’ are used interchangeably in some treatises on painting,⁴³ but others, along with modern industrial texts, define these as closely related but different materials.⁴⁴ Moreover, ‘resin,’ ‘tar,’ ‘pitch’ and ‘mummy’ are also sometimes used to mean ‘bitumen’ and vice versa;⁴⁵ and the synthetic versions of some of these that multiplied in the nineteenth century complicate matters further. Importantly, bitumen industrially distilled from petroleum, and tars and tar pitches from the carbonization of wood and fossil fuels all share the same overlapping terminology.⁴⁶ Translations from language to language have also led to certain confusions. Mūmiyā, for example, which was anglicised to mummy, means ‘wax’ in Persian and ‘bitumen’ in Arabic.⁴⁷ This may be part of the reason that the seventeenth-century artist Richard Symonds claimed that asphalt was human skin.⁴⁸ Translation issues continue to arise, even within the English-speaking world, since ‘asphalt’ refers to different materials in North American and in European industry.⁴⁹ The multiplicity and fluidity of the definition means that when historic texts mention bitumen, the meaning cannot be known with certainty. It follows that, when we set out to analytically detect bitumen, we cannot be sure we are searching for the same material as discussed in historic texts about the same paint passage.

Research has also highlighted the role of adulterants in problems associated with bitumen. The practice of adulterating and substituting artists’ materials in the nineteenth century was so widespread that adulterants are likely to have played a part in paint deterioration.⁵⁰ Middlemen operating between manufacturer and retailer meant that colourmen themselves may have been vulnerable to deception. Pigments named asphaltum or bitumen by colourmen were sometimes in fact mixtures of lamp black, yellow lake and aniline black.⁵¹ Additionally, since the natural material became rarer and more expensive than the manufactured version, synthetic bitumen from petroleum became a common replacement. Not only this, but tars and tar-pitches, even coals and other balsamic materials, were sold in its place.⁵² Coal tar is likely to have been a particularly widespread replacement as it was produced in great quantities as a by-product of gas lighting;⁵³ by 1890 Church noted about asphaltum that coal tar is ‘now largely sold in lieu of the original and genuine product.’⁵⁴ Although these replacements would have appeared indistinguishable to the consumer, their drying and stability within a paint layer would have varied considerably;⁵⁵ and so, because the behaviour of the bitumen they bought varied wildly, painters even began to make their own substitutions. For instance, the French academic painter Jehan Georges Vibert recommended ‘Vibert brown’ – a mixture of lithanthrax and oxide of iron on aluminium basis – as a substitute for bitumen that he claimed had the same qualities, but not the defects,⁵⁶ while landscape artist Richard Wilson glazed pictures with Indian ink and Spanish liquorice, which he considered ‘as good as asphaltum.’⁵⁷

In addition, researchers have suggested that the methods artists used to prepare and apply bitumen to their paintings, rather than an inherent quality of bitumen, may have caused paint film deterioration, and that the many other materials that were mixed with bitumen could also have caused problems. While early treatises recommend asphaltum be pulverised with drying oil and then heated,⁵⁸ later nineteenth-century painters seemed to more often mull fine asphalt powder into oil and then varnish cold.⁵⁹ This may have become possible since the artificial varieties artists began to use were more soluble than the natural,⁶⁰ but the change in method may have been significant to subsequent deterioration of the paint film.⁶¹ Significantly, there were a great many formulas, often involving a variety of ingredients, for preparing the colour that offered contradictory advice.⁶² Reconstructions of historic bitumen preparations have suggested that certain preparations, such as those containing turpentine, were particularly subject to change following application.⁶³ The other components within paint passages should be considered as potential causes of paint defects. In the eighteenth and nineteenth centuries, bitumen or asphaltum was commonly used in conjunction with a variety of other materials, such as pigments, driers, resins, shellac, balsams, gold size, driers, waxes and megilps to adjust optical, drying and handling properties.⁶⁴ For example, strong and inappropriate amounts of driers may often be the cause of drying cracks, and since the oils added were also often adulterated with non- or poor-drying oils, oils may also be responsible.⁶⁵ The method of application is also likely to have made a difference. Such an idea is supported by experiments using bitumen as a paint layer that have found that asphalt or coal tar applied thickly as lower layers and not left to dry fully can cause cracking and wrinkling in the upper paint.⁶⁶

Finally, rather than the behaviour of bitumen within paint films being inherently problematic, it may have been that the restoration treatments applied to them were unsuitable. The Redgraves suggested long ago that poor preventative conservation, solvents and heat would damage bituminous paintings: ‘a change in hanging, or in the temperature of the room or gallery, an exposure to the sun’s rays, and above all varnishing, will, though heretofore free from harm, crack them in a few weeks.’⁶⁷ Paintings executed before the twentieth century have rarely escaped treatments involving solvents, force or heat, such as cleaning or lining.⁶⁸ Judging from the evidence of heat-related damage, it would appear that some dark passages traditionally associated with bitumen, when subject to thermal stress, can become permanently deformed.⁶⁹ In addition, because the proportions of bitumen’s components vary considerably from sample to sample, no two samples necessarily share the exact same physical properties. There were also a range of methods used to prepare this material for oil painting, which included lengthy heat processing and a variety of additional materials (linseed oil, driers, varnishes and artists’ mediums such as megilp).⁷⁰ As a consequence, it cannot be expected that each example of bituminous painting would respond in the same way to a restoration treatment. The Redgraves were wise in advising that for bituminous pictures it is best ‘to abstain from any attempt at repair, to cleanse the surface with fastidious care by means of cotton wool, and then to preserve the picture from dust (which sticks so readily to the pitchy surface) by means of glass, and from damp and change of atmosphere by covering up the canvas behind.’⁷¹ Interestingly, paintings that were treated in the nineteenth century according to these instructions ‘are now regarded as being very well preserved.’⁷²

In summary, there is considerable evidence that the relationship between bitumen and painting defects is far more complex and nuanced than the term ‘bitumen cracking’ might imply. Even if bitumen does have the potential to cause problems, and even if artists did choose to use it regardless, adulterated materials, poor environmental controls and incompatible restoration treatments also likely deserve some of the blame.

Escaping the Bituminous Shadow?

As the above suggests, in the case of bitumen the usual modes of investigation into artists’ materials are placed under strain. The reliability of accounts of artists’ practices is hard to establish, and historic artists’ treatises are contradictory and difficult to interpret. Furthermore, there are many challenges involved in investigating bitumen by means of chemical analysis.⁷³ Analysis is challenging because bitumen, according to modern industrial and scientific texts, is an extremely complex colloidal mixture of organic and inorganic components.⁷⁴ Also, ‘[t]wo shipments of any given member of the bituminous family are apt to fluctuate widely in composition and physical properties, even when emanating from the same source,’⁷⁵ and subsequent adulteration, preparation, ageing, and restoration treatments will unpredictably affect chemical signatures further. This complexity means that a more complete characterisation of bitumen would require the creation of an enormous data set. In this regard, conservation scientists and technical art historians are unlikely to receive much help from other fields. Industry deems ‘a complete analysis of bitumen…impossible’⁷⁶ and, in any case, ‘has little, if any, relevance’⁷⁷ since its application relies on being able to characterise broad properties of different bitumen grades, not detailed chemical composition. Summing up the situation, the company Shell Bitumen feels that ‘for many applications the utilisation of bitumen is more of a black art than a science.’⁷⁸ Given this, it is a tall demand to expect of other fields, such as conservation science, the ability to securely detect bitumen, or to suggest that bitumen could be recognised ‘simply by looking at the painting.’⁷⁹

Without question, there are good reasons for wanting to pin down definitions and facts about bitumen. For example, those who have investigated bitumen have done so for the good reasons that its characterisation would help understand its behaviour in paint at a molecular level as well as artists’ intentions and motivations, and guide decision-making regarding appropriate treatments and environmental conditions when dealing with potentially bituminous paintings.⁸⁰ To make better sense of bitumen, therefore, Bothe requests at least a consolidation of terminology: ‘It is preferable to replace “bituminous” with the unambiguous phrase “containing bitumen” and to use this phrase only for substances that definitely contain bitumen and not, as has been the case, for substances alleged to contain bitumen based on subjective, visual impressions or physical condition.’⁸¹ But even if terminology within the conservation field can be made systematic, it is unlikely to become so in all fields with which conservation must interact. For example, using the word descriptively and making analogy to bitumen’s colloquial use to mean tarmac, the academic Frances Guerin describes Jasper Johns’ paintings as ‘the colour of bitumen…Bitumen is the surface of the twentieth century. It seals and protects. Johns uses grey paint as if to tar over the roughness and potholes in the canvas.’⁸² Thus, an organised terminology in conservation does not necessarily mitigate the problems that will arise when the writing accompanying the works it must conserve continues to use ‘bitumen’ loosely, colloquially and descriptively.

With this in mind, it would seem fruitful to begin to explore the ways in which modern and contemporary artists have thought about and used bitumen. It does not quite seem to be the case that, ‘Today asphalt is still used, but only rarely, for finishing varnishes and to tone in paintings and retouchings.’⁸³ Robert Rauschenberg, for example, spoke of using asphaltum, or bitumen, in many of his black works in the 1950s.⁸⁴ In these paintings he aimed to explore the transitory and ephemeral nature of experience, exploiting unstable materials. Perhaps ironically, many black paintings survive despite their tenuous construction.⁸⁵ Moreover, in a now familiar pattern of events, Fourier-transform infrared spectroscopy has not been able to confirm the artist’s word that the paintings contain bitumen.⁸⁶ Accordingly, Rauschenberg, whether purposefully or accidently, not only engaged with the reputation of bitumen as an unstable material, but encapsulated much of the story of bitumen in the history of art in these paintings. Apparently engaging in a dialogue with Rauschenberg, Theaster Gates makes use of different facets to the material – its stable, sealing and protective properties – in his ‘tar paintings’ such as Diagonal Bitumen (2014, wood, rubber and tar).⁸⁷ Gates’s works include aspects of autobiography, and his tar paintings speak of his father’s occupation as a roofer;⁸⁸ it is worth noting perhaps that the commentary on his works seem to use ‘tar’ and ‘bitumen’ as relatively interchangeable terms. Differently again, Terry Winters chose bitumen for its ability to represent ‘a meeting of nature and culture’ and, interestingly, knowing of its reputation for ruining nineteenth-century paintings, sourced a ‘stable, modified version’ for Bitumen (1986, oil on canvas, 224 x 305 cm, National Gallery of Art, Washington DC).⁸⁹ No doubt an exploration of non-Western art would yield further fascinating uses of bituminous materials.

These recent uses of bitumen by artists demonstrate that, even though the limits of the traditional ways for investigating bitumen as a painter’s material may have been exposed, there are many further avenues for exploring this material. To begin with, while ‘bitumen cracking’ has been considered a defect in the conservation field, such a view is not necessarily universal. Artists’ creativity has not been bound by ideas of material stability, and an artist such as Rauschenberg has used bitumen’s instability as a positive, creative characteristic. Taking inspiration from this, even though the artistic intent and aesthetic purposes of artworks containing bitumen have of course shifted drastically over the centuries, it may be worth looking more closely at why, if Reynolds himself said that ‘all good pictures crack,’⁹⁰ we have generally taken the side of those that wrote about such phenomena as negative characteristics.

Moreover, contemporary artists demonstrate that there is still much to be said about the shifting social uses and meanings of bitumen, such as Gates’s use of it in its guise as roofing sealant. Simultaneous exploration of the material’s meanings in art and the wider society seems worthwhile. Far from being troubled by its ambiguity, recent uses of bitumen demonstrate how it might fruitfully be considered something of a ‘multidimensional object of enquiry.’⁹¹ Science historians Ursula Klein and Wolfgang Lefèvre have shown the degree to which chemists once considered materials as such objects, until, in the late eighteenth century, they began to classify materials by molecular composition, with complex mixtures like bitumen necessarily left out of their developing ontology.⁹² Therefore, analogous to the pre-eighteenth-century chemist who would have dealt with much overlapping and unresolved terminology, we could, at least partially, embrace the confusing terms and meanings for bitumen; doing so might allow for wider-ranging investigation into all of its different dimensions, ‘perceptible and imperceptible, useful and philosophical, technological and scientific, social and natural.’⁹³

Conclusion

Commentaries around bitumen in painting demonstrate how certain views of a material can survive for long periods, and how such views can influence opinion about artists and their techniques. It also highlights how the repetition of apparently simple statements about an artists’ material can hide a far more complex reality. Indeed, in many respects, bitumen reveals the contours and parameters of our approaches to researching artists’ materials. More broadly, bitumen raises questions about the uses and misuses of ambiguity in our descriptions and analyses of materials within art. These are questions to which there may not be simple answers, but they are worth raising all the same. Knowing when to embrace ambiguity, and knowing how best to communicate that embrace, is a challenge from which we should not retreat.

Author Bio

Rowan Frame is a postgraduate intern at Julia Nagle Conservation Ltd. She gained her PGDip in the conservation of easel paintings at the Hamilton Kerr Institute, University of Cambridge, UK.

Notes