'Ancient Egyptian pigment can boost energy efficiency'

A colour developed by Egyptians thousands of years ago can boost energy efficiency by cooling rooftops and walls, and could also enable solar generation of electricity via windows, scientists say.

Egyptian blue, derived from calcium copper silicate, was routinely used on ancient depictions of gods and royalty, according to the study published in the Journal of Applied Physics.

Previous studies have shown that when Egyptian blue absorbs visible light, it then emits light in the near-infrared range.

Now, a team led by researchers at the Lawrence Berkeley National Laboratory (Berkeley Lab) in the US has confirmed the pigment's fluorescence can be 10 times stronger than previously thought.

Measuring the temperature of surfaces coated in Egyptian blue and related compounds while they are exposed to sunlight, researchers found the fluorescent blues can emit nearly 100 per cent as many photons as they absorb.

The energy efficiency of the emission process is up to 70 per cent (the infrared photons carry less energy than visible photons).

The finding adds to insights about which colours are most effective for cooling rooftops and facades in sunny climates.

Though white is the most conventional and effective choice for keeping a building cool by reflecting sunlight and reducing energy use for air conditioning, building owners often require non-white colours for aesthetic reasons.

For example, bright-white asphalt shingles are almost never used on sloping residential roofs.

Researchers have already shown that fluorescent ruby red pigments can be an effective alternative to white; this insight on Egyptian blue adds to the menu of cooling colour choices.

They also found that fluorescent green and black colours can be produced with yellow and orange co-pigments.

In addition to its cooling potential for buildings, Egyptian blue's fluorescence could also be useful in producing solar energy.

Used on windows tinted with the blue, photovoltaic cells on the edges can convert the fluoresced near-infrared energy to electricity.