The formula of the red, shiny and intense colour that dominated Pompeii's wall paintings 2000 years ago has been discovered by an Italian researcher.

Buried in the catastrophic eruption in 79 AD, the brilliant Pompeian red has been preserved forever by the lava of Mount Vesuvius and still makes an impressive show in several frescoes.

"Though it consists of simple cinnabar pigment, Pompeian red is really unique. It certainly stands out when compared to normal cinnabar paint layers," said Daniela Daniele, a researcher working at Berlin's Staatliche Museen.

Cinnabar is mercury (II) sulfide, the principal ore of mercury.

Daniele analysed the stratigraphies of some samples from Pompeian villas featuring the unique red and compared them to other ancient Roman wall paintings containing normal cinnabar paint layers.

Her aim was to discover why there was a dramatically different chromatic effect with the same mineral pigment.

In the case of Pompeian red, natural cinnabar was processed with particular care, which included what Daniele called "purification, grinding and dimensional control".

"The finer the grains are, the more brilliant and covering the colour is. But there is much more," Daniele said.

Big grains, little grains

Under the microscope she detected "a bimodal granulometry", with 10-15 micron crystals acting as shiny particles in a matrix of finer grains.

Basically, the ancient Romans simply added some bigger grains to the finely processed cinnabar powder, made of grains measuring about 2-3 microns. The result was a glittering surface that did not loose its saturated red tone.

According to Bernardo Marchese of the University of Naples Federico II, cinnabar red required careful processing indeed.

"The pigment was used in lime medium, and had to be liquid enough to be applied in paint layers on the wall surface ... The final result was subjected to wax polishing, in order to prevent alterations, especially when the colour was applied on outside walls," Marchese and colleagues wrote in the catalogue of the Pompeii exhibit Homo faber: nature, science and technology in a Roman town.

But Daniele's analysis showed that samples of normal cinnabar paint layers featured just a light processing of the pigment.

Cinnabar powder made of larger grains measuring between 10 and 25 microns turned out to be more transparent and dull, producing a colour similar to a red ochre, the researcher said.

"It shows that Pompeian red is really special. It represents the height of the ancient Roman's mastery in making colours."