Egyptian blue, derived from calcium copper silicate, was routinely used on ancient depictions of gods and royalty. Previous studies have shown that when Egyptian blue absorbs visible light, it then emits light in the near-infrared range. In fact, 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 percent as many photons as they absorb. The energy efficiency of the emission process is up to 70 percent (the infrared photons carry less energy than visible photons).
The finding adds to insights about which colors 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 colors for aesthetic reasons; for example, bright white asphalt shingles are almost never used on sloping residential roofs.
The 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 color choices. Further, they found that fluorescent green and black colors 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.