A new class of cool blues

Cobalt Blue (CoAl2O4) is enduringly popular because, we are told, of its colour intensity, ease of synthesis and versatility.

However, 33% of its colourant by mass is carcinogenic Co2+, we are also told, making it relatively expensive, not to mention environmentally worrying. 

Enter Mas Subramanian, the man who in 2009 chanced upon what is variously known as YInMn Blue, Oregon Blue or, to others at Oregon State University, US, MasBlue. He wanted to develop another new class of blue pigments that are cheaper, safer and better.

Writing in the journal ACS Omega, he and colleagues present their results. 

They say they were inspired by the crystalline structure of a light-blue mineral called hibonite. They systematically substituted Al3+ (aluminium) ions in hibonite with Co2+, Ni2+ (nickel) or Ti4+ (titanium) ions.

The resulting series of Hibonite Blue pigments showed a range of intense blue colours, some with reddish hues. The pigments were stable even when soaked in acidic or basic solutions. 

In contrast to Cobalt Blue, the researchers say, the new blues reflected near-infrared light, which could make them useful as cool pigments in energy-saving, heat-reflecting coatings. 

And the Co2+ concentration in the new compounds in hibonite blues was as low as 4% by mass.

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