Column | Shades of Change - From pigments to plasmonic paints

The human eye has the astonishing capacity to distinguish up to 10 million distinct shades of colours. However, ancient cultures did not distinguish many of these colours in their daily lives because they were insignificant to them. The study of colour nomenclature across diverse cultures unveils a captivating journey of linguistic evolution intertwined with the development of colour perception in humans.

The initial stages of colour description revolved around the contrasting concepts of black and white, which represented darkness and light, respectively. As human visual acuity improved, a broader spectrum of colours began to be categorised. Among the earliest colour distinctions observed across various cultures were the vivid hues of yellow and green.

Most ancient languages did not have a clear distinction between blue and green. This lack of differentiation can be attributed, in part, to the challenges associated with perceiving the subtle variations between blue and green in the visual spectrum, as well as the relatively limited presence of blue in nature. The recognition and delineation of blue as a distinct colour developed over time, reflecting the intricate relationship between human perception and the evolution of language.

The Evolving Colour Palette

Approximately 35 million years ago, our visual system underwent a significant evolution, granting us the remarkable ability to perceive the full spectrum of colours present in visible light. The commonly used colours in prehistoric cave paintings were mostly earth pigments of red, brown, yellow, orange, white, and black. Our journey towards harnessing the captivating hue of blue began a mere 6,000 years ago, when we first embarked on experimenting with natural materials to create blue colourants. It was only around 2,200 B.C. that we achieved a breakthrough with the development of the world's first synthetic blue pigment.

Over the course of many centuries, we have transitioned from relying predominantly on natural colours to the discovery and utilisation of synthetic pigments. This profound shift has been fuelled by our relentless pursuit to master the alchemy of superior colourants in terms of purity, fading resistance, and, more recently, environmental sustainability.

The Colour Conundrum

Conventional colourants based on pigments offer the advantage of being angle-insensitive and capable of large-scale production. However, they have several limitations that hinder their overall performance. Their inherent chemical instability often leads to fading over time, particularly when exposed to elevated temperatures or intense light. Furthermore, as pigment colours suffer from low resolution, high volumes are required to obtain enough colour saturation. 

A study conducted by the Swiss-based Environmental Action (EA) revealed that paint fragments contribute to more than 58% of the microplastics in our oceans and waterways. This pollution extends beyond marine ecosystems, as these particles find their way into the air we breathe, exacerbating environmental concerns and human health risks.

To address these issues, a paradigm shift towards alternative colourant technologies that prioritise stability, resolution, and environmental responsibility is required.

Structural Colours: Moving Beyond Pigments

Nature overcomes the scarcity of blue pigments in the natural world through a fascinating phenomenon known as structural coloration. Light is manipulated at the micro- and nano-scale levels within biomaterials. In contrast to pigments, which rely on photon absorption and emission, structural coloration is based on optical phenomena such as interference and diffraction. Blue and green hues are abundant in structural coloration and frequently exhibit iridescence in various organisms.

Structural colours, by their nature, often exhibit directional effects, resulting in variations in colour perception based on the observer's position, the angle of incidence, and the polarisation of the incident light. The visual characteristics, whether they appear matte or iridescent, are determined by the intricate arrangement of internal structures at the nanoscale level.

Iridescent colours emerge from ordered or crystalline structures, showcasing dynamic shifts in colour when viewed from different angles. In contrast, unordered structures yield matte colours that remain consistent regardless of the viewing angle.

Natural matte or angle-independent structural colours have manifested in shades of only blue within the natural world. Efforts to reproduce structural matte colours in blue have seen success. However, endeavours to replicate matte colours in red, yellow, and orange have led to low saturation and compromised colour purity.

The advent of plasmonic paints has addressed significant challenges in achieving vibrant colours across the entire visible spectrum, including angle independence.

Plasmonic Paints: A Path Towards Sustainable Colouration?

Plasmonic colours, which are structural colours created through the resonant interactions of light and metallic nanostructures, have recently seen a breakthrough with the development of the world's first full-colour structural paint. This novel paint is composed of two colourless components: aluminium nanoparticles and aluminium oxide nanostructures. By altering the arrangement of these particles, it becomes possible to manipulate visible light and generate any desired colour.

Plasmonic paints offer the advantage of providing vibrant structural colours that are independent of the viewing angle and polarisation. The interaction between plasmonic nanoparticles and light occurs at incredibly small scales, resulting in a higher surface area-to-volume ratio. This efficient use of paint material enables the achievement of vivid colours with a reduced quantity of material. As a consequence, plasmonic paints exhibit superior weight efficiency compared to conventional colour pigments, leading to a decrease in material consumption, production costs, and potential environmental impact. Additionally, the durability of plasmonic paint contributes to its resistance to degradation, thereby minimising the release of paint particles into the environment and reducing water pollution caused by paint runoff.

Conventional paint pigments, by selectively absorbing sunlight, can act as heat traps. Darker and more saturated colours absorb a greater number of wavelengths, while lighter and less saturated colours absorb fewer wavelengths and reflect more light. To counteract the urban heat island effect, many cities are replacing dark-coloured materials with light-coloured ones possessing a high albedo to maximise solar reflection.

Plasmonic paints offer a high level of solar reflectance regardless of colour, making them effective in reducing the amount of solar heat absorbed by buildings and urban surfaces. By reflecting more sunlight, plasmonic paints can help mitigate the heat island effect, resulting in substantial energy savings and reduced CO2 emissions. However, it is important to note that plasmonic paint is still in its early stages of development and is not yet commercially available.

In our rapidly urbanising world, there is a growing need for a sustainable, scalable, and cost-effective alternative to conventional colourants. Novel colourant technologies like plasmonic paints present a sustainable approach to how we introduce colour into the built environment encompassing aesthetics, durability, and ecological responsibility.

(This article is the second in a series on ‘Colour’. Read the first article here.)

(Ann Rochyne Thomas is a bio-climatic spatial planner and founder of the Centre for Climate Resilience - a sustainability and climate change advisory.)