Bitumen in Art: Beauty and Degradation

Bitumen Deformation, Image from The Fine Arts Conservancy.

Bitumen, also known as asphaltum or simply asphalt in some regions, is a naturally occurring, highly viscous form of petroleum. This complex hydrocarbon substance has been utilised by humans for millennia, with its applications ranging from construction and waterproofing to its use as a pigment in art. Bitumen's deep brown to black colouration and its ability to produce a glossy finish have made it an attractive option for artists throughout history.

The use of bitumen as a pigment dates back to prehistoric times, with evidence of its application found in cave paintings dating back tens of thousands of years. Ancient civilisations in Mesopotamia, Egypt, and the Indus Valley were among the first to exploit bitumen's versatile properties, using it not only in art but also for waterproofing, as a binding agent in construction, and even in early forms of mummification.

Chemical Composition and Physical Properties

Natural formed Bitumen collected at the Dead Sea shore, Image by Daniel Tzvi, Wikimedia Commons.

Bitumen's complex chemical structure contributes to both its appeal and its challenges as a pigment. Composed primarily of high molecular weight hydrocarbons, bitumen typically contains asphaltenes, resins, and oils. Asphaltenes are large, complex molecules that give bitumen its dark colour and contribute to its viscosity. Resins are smaller molecules that act as a dispersing agent for the asphaltenes, while oils are the lightest components, which contribute to bitumen's flow properties. The exact composition can vary depending on the source and processing of the bitumen, leading to differences in colour, viscosity, and chemical stability.

Physically, bitumen is characterised by its high viscosity at room temperature. It softens when heated and becomes fluid at higher temperatures. This property allows it to be mixed with other materials and applied as a pigment, but it also contributes to its instability in artworks over time.

Bitumen as a Pigment

Bitumen Pigment Powder, Image from Natural Pigments.

As a pigment, bitumen offers several attractive properties that have captivated artists for centuries. Its rich, deep colour, ranging from dark brown to nearly black, provides intense, shadowy tones that artists have long valued for creating depth and contrast in their works. When applied, bitumen can produce a lustrous, almost lacquer-like surface that enhances the visual appeal of artworks. Its transparency allows artists to create subtle glazes and overlay effects, while its solubility in turpentine and other organic solvents makes it versatile for various painting techniques, particularly in oil painting.

Joshua Reynolds, Lord Heathfield of Gibraltar, 1787. Oil on Canvas. Image from Wikimedia Commons (Original sourced from The National Gallery).

These qualities made bitumen particularly appealing to 19th-century Romantic and history painters seeking to create dramatic, atmospheric effects. For instance, Sir Joshua Reynolds employed bitumen to great effect in his portrait "Lord Heathfield of Gibraltar" (1787). The use of bitumen in the shadowy background enhances the luminosity of the central figure, creating a sense of depth and focus.

John Constable, The Valley Farm, 1835. Image from Tate.

Similarly, John Constable utilized bitumen in "The Valley Farm" (1835), where it contributed to the darker areas of the landscape, enhancing the painting's depth and tonal range.

Orazio Gentileschi, Joseph and Potiphar’s wife, 1632. Oil on canvas. Image from The Royal Collection.

In portraiture, bitumen's rich tones were often used to create depth in backgrounds and clothing. Orazio Gentileschi's "Joseph and Potiphar’s Wife" (1658) demonstrates this, with the dark tones of the background and clothing likely enhanced with bitumen glazes, providing a striking contrast to the central figures and their dramatic interaction.

Gerrit Dou, The Young Mother, 1658. Oil on panel. Image from Mauritshuis.

Genre painters also found bitumen useful for creating dramatic effects. Gerrit Dou employed bitumen in his work "The Young Mother" (c. 1658-1660), where it contributed to the painting's rich, shadowy tones, adding to the intimate, domestic scene's overall depth and realism.

However, these same properties that make bitumen appealing also contribute to its problematic nature in the long-term preservation of artworks. The ongoing reactivity of bitumen, its tendency to darken over time, and its sensitivity to environmental factors pose significant challenges for conservators and art historians alike. Many of the paintings mentioned above have required extensive conservation efforts due to the deterioration of bitumen-containing layers.

Issues with Bitumen in Artworks

Despite its initial visual appeal, bitumen poses significant challenges in artwork preservation, leading to a host of problems that have plagued conservators and art historians for centuries:

A close-up of the bitumen scales seen on an affected painting, Image from Fine Art Restoration.

Continuous Reactivity: Unlike many other pigments, bitumen never completely dries or cures. It remains reactive to temperature changes, continuing to flow and shift over time. This ongoing fluidity means that artworks containing bitumen are in a constant state of change, albeit often imperceptibly slow.

Cracking and Blistering: As the upper layers of a painting dry faster than bitumen-containing lower layers, tension develops in the paint film. This leads to cracking, blistering, and sometimes complete separation of paint layers. The phenomenon, known as 'alligatoring', creates a network of fine cracks across the surface of the painting, severely compromising its structural integrity.

Darkening and Colour Shift: Over time, bitumen tends to darken, altering the original appearance of the artwork. This gradual change can dramatically shift the balance of light and shadow in a painting, potentially obscuring details and changing the artist's intended composition. For instance, in some of J.M.W. Turner's works, areas that were originally light and airy have become dark and heavy due to bitumen's darkening effect.

Pigment Sinking: Bitumen can cause other pigments in a painting to sink or shift, distorting the original composition. This is particularly problematic in works where bitumen was used as an underpainting or mixed with other colours to create subtle effects.

Increased Brittleness: While remaining fluid underneath, the surface of bitumen-containing paint can become brittle over time. This contradictory behaviour - fluid beneath but brittle on top - makes the paint layer particularly vulnerable to damage from vibrations, temperature fluctuations, or physical contact.

Light Sensitivity: Exposure to light, especially UV radiation, can accelerate the degradation processes in bitumen. This makes proper storage and display of bitumen-containing artworks particularly challenging, as even controlled museum lighting can contribute to long-term damage.

Interaction with Conservation Materials: Bitumen can react unpredictably with materials commonly used in conservation treatments. Varnishes, consolidants, and cleaning solutions may interact with bitumen in ways that exacerbate existing problems or create new ones.

Many of the changes caused by bitumen degradation are irreversible. Once cracking, blistering, or severe darkening has occurred, it's often impossible to return the artwork to its original state.

These issues are often interrelated and compound over time. For example, a painting by Theodore Gericault, "The Raft of the Medusa" (1818-1819), has suffered extensively due to its bitumen content. The work has experienced significant darkening, cracking, and flaking, necessitating multiple restoration attempts over the years.

Théodore Géricault, the Raft of the Medusa, 1819. Oil on canvas. Image from History and Live.

The cumulative effect of these issues makes the conservation of bitumen-containing artworks particularly challenging. It requires a delicate balance between attempting to stabilise the artwork and accepting that some degree of ongoing change may be inevitable. This has led to significant debates within the conservation community about the ethics and practicalities of intervening in artworks containing bitumen, as well as broader discussions about artists' material choices and their long-term consequences.

Conservation Challenges and Approaches

The conservation of artworks containing bitumen presents unique and often formidable challenges that have pushed the boundaries of conservation science and ethics. One of the primary challenges is the irreversible nature of bitumen degradation. Many of the changes caused by bitumen are permanent, making full restoration impossible. Furthermore, because bitumen never fully stabilises, conservation treatments may only provide temporary solutions. This ongoing reactivity means that conservators must constantly reassess and adapt their approaches. Bitumen can also react unpredictably with conservation materials, sometimes exacerbating existing problems. Traditional consolidants or varnishes may interact with bitumen in ways that accelerate degradation or alter the artwork's appearance. This unpredictability requires conservators to extensively test any proposed treatment on small, inconspicuous areas before applying it more broadly.

The extent of intervention in degrading artworks is a subject of ongoing debate in the conservation community. Some argue for minimal intervention to preserve the artwork's historical integrity, while others advocate for more extensive treatments to maintain the artist's original intent. This debate is particularly heated when it comes to bitumen-containing works, where significant intervention might be necessary to prevent further deterioration.

Despite these challenges, conservators have developed several approaches to mitigate bitumen-related damage. Preventive conservation is often the first line of defence. By controlling environmental factors like temperature, humidity, and light exposure, conservators can slow degradation processes. For example, storing bitumen-containing artworks in cool, dark environments can significantly reduce the rate of deterioration. Museums have developed specialised display cases with precise climate control for particularly sensitive works. In cases where paint layers are flaking or blistering, conservators may use synthetic resins to stabilise the artwork. However, this is often a last resort due to the risks involved. The choice of consolidant is crucial, as it must be compatible with both the bitumen and other materials in the painting.

Carefully applied solvents can sometimes be used to reduce bitumen's mobility. This technique involves using solvents that partially dissolve the bitumen, allowing it to resettle in a more stable configuration. However, this method carries significant risks and is only attempted by highly skilled conservators. Advanced non-invasive analysis techniques like spectroscopic imaging, X-ray fluorescence, and infrared reflectography are used to study bitumen degradation without damaging the artwork. These methods allow conservators to map the distribution of bitumen within a painting and monitor changes over time, informing conservation strategies.

Digital preservation methods, including high-resolution imaging and 3D scanning, are increasingly used to create detailed digital records of artworks in their current state. This preserves information even as physical degradation continues and can aid in future conservation efforts or art historical research. Conservators are continually researching and developing new materials that can stabilise bitumen without altering its appearance. For instance, nanoparticles and specially designed polymers have shown promise in some experimental treatments.

Given the complex nature of bitumen degradation, many institutions have formed interdisciplinary teams including conservators, chemists, physicists, and materials scientists to tackle these challenges. Additionally, many museums now use bitumen-damaged artworks as educational tools, highlighting the importance of material choices in art creation and preservation. This not only raises awareness but also helps justify the resources needed for ongoing conservation efforts.

Despite these approaches, the conservation of bitumen-containing artworks remains a significant challenge. Each artwork requires a unique, tailored approach, often involving a combination of these methods. The field continues to evolve as new technologies and materials become available, offering hope for better preservation of these complex and historically significant artworks in the future.

Before and After, restoration of the copy of Rembrant’s ‘The Night Watch’ by Fine Art Restoration.

An illustrative example of the challenges and approaches in conserving bitumen-containing artworks can be seen in the restoration of a 19th-century copy of Rembrandt's "The Night Watch" undertaken by the Fine Art Restoration Company. This copy had developed severe issues over time, particularly due to the use of bitumen pigments which had caused a scaly, distorted surface, especially in the shadowed areas. The painting was also suffering from a darkened varnish layer and surface contamination, likely from historic fireplace smoke, which had obscured much of the scene.

The restoration team began by stabilising the surface to prevent further loss of original features and reduce the pitting caused by the bitumen. They then carefully cleaned the painting to remove the darkened varnish and surface contamination. This process revealed many hidden figures and details that had been obscured by the years of discolouration.

The conservator then undertook the delicate task of retouching the colours as sensitively as possible, aiming to maintain the historic and artistic integrity of the painting while reviving its original tones. This painstaking process helped to reduce the distortion caused by the bitumen scaling and brought the artwork closer to its intended appearance.

While unable to completely reverse the bitumen-related damage, the restoration significantly improved the legibility of the artwork, revealing previously hidden figures and reviving the dynamic use of light and shadow characteristic of Rembrandt's original masterpiece. This project showcases the complex challenges posed by bitumen in historical artworks and the careful, multifaceted approach required in their conservation.

Implications for Art History and Conservation Science

The challenges posed by bitumen have had far-reaching implications for art history and conservation science. The degradation of bitumen-containing works has led to increased study of historical painting techniques and materials, driving a reassessment of artists' methods and choices. The need to address bitumen-related issues has spurred innovation in conservation technologies and methodologies, advancing the field as a whole.

The problems associated with bitumen have also sparked important ethical debates within the conservation community. Questions about the extent of intervention in degrading artworks have led to ongoing discussions about conservation ethics and the balance between preserving original material and maintaining the artist's intended appearance.

Many museums and institutes now use bitumen-damaged artworks as educational tools, highlighting the importance of material choices in art creation and preservation. This focus on education and awareness has helped to bridge the gap between art history, conservation science, and public understanding of art preservation challenges.

The known instability of bitumen can also impact the art market, affecting the valuation and collectibility of certain artworks. This has led to increased scrutiny of materials and techniques in both historical and contemporary art, influencing collecting practices and conservation priorities.

Modern Alternatives and Their Introduction

As bitumen's problems became apparent in the late 19th and early 20th centuries, artists and paint manufacturers sought safer alternatives. Synthetic iron oxide pigments, particularly Mars Black, were introduced in the 1840s and became commercially available in the 1860s, offering a stable alternative to bitumen.

The early 20th century saw the development of "bitumen substitutes" combining stable pigments with synthetic binders. Acrylic paints, introduced in the 1940s and widely adopted in the 1960s, provided new options for achieving deep, glossy blacks and browns without conservation issues.

Oil paint manufacturers developed stable formulations using pigment combinations to achieve bitumen-like colours. Specialised glazing mediums emerged in the late 20th century, replicating bitumen's visual qualities using alkyd or acrylic resins.

Contemporary manufacturers now offer "bitumen effect" paints using stable pigments and binders. Despite these alternatives, some artists still choose natural bitumen, but with greater awareness of its long-term behaviour.

This evolution in art materials continues to influence artistic practice and conservation strategies, highlighting the ongoing interplay between artistic expression and material science.

Conclusion

Bitumen's journey from a prized pigment to a conservation nightmare encapsulates many of the challenges faced in art preservation. Its use throughout history provides valuable insights into artistic techniques and material science, while its degradation serves as a cautionary tale about the long-term consequences of material choices in art.

The ongoing struggle to preserve bitumen-containing artworks has driven significant advancements in conservation science and sparked important discussions about the nature of art preservation. As we continue to develop new technologies and methodologies, the lessons learnt from bitumen will undoubtedly inform our approach to preserving both historical and contemporary artworks.

Understanding bitumen's complex behaviour remains crucial not only for conserving our artistic heritage but also for informing future artistic practices. As such, this problematic yet fascinating substance continues to play a vital role in the intersection of art, science, and history.