Artisans glazing pots in fifteenth- and sixteenth-century Umbria were practising an early form of nanotechnology. Italian researchers have now revealed the full sophistication of this process1.
Coloured glazes in pottery samples from the Umbrian town of Deruta exploit the reflective properties of minute metal grains to give them a rich lustre. Bruno Brunetti of the University of Perugia and colleagues find that the manufacture of these glazes required great chemical skill.
At its peak in the fifteenth and sixteenth centuries, the finely painted ceramics of the Deruta pottery industry were in demand all over Europe.
Glazes are basically thin films of coloured glass. Metal salts give colour to a glassy matrix produced by fusing sand alkalis such as soda in the heat of a kiln. Coloured glazes - such as blue-glazed stone carvings from the Middle East - have been around since at least 4500 BC.
Previous analyses of Umbrian Renaissance pottery showed that they had a chemical composition typical of the period: a mix of sand and alkali, with lead oxide added to reduce shrinkage and cracking.
It's the colorants that show the true, if largely unwitting, sophistication of the Italian potters' skills, Brunetti's team have found.
Among the most striking Deruta ceramics are those with iridescent or metallic glazes. Some look like gold, others are iridescent - changing colour when viewed from different perspectives.
Previous studies have shown that particles of metal of between 5 and 100 billionths of a metre across - technically, a nanomaterial - underlie this effect. In 2001 Brunetti and coworkers found that red-and gold-lustre glazes contain particles of copper and silver, respectively, in this size range2. Instead of scattering light, the particles' minute size causes light to bounce off their surface at different wavelengths, giving metallic or iridescent effects.
Metal nanoparticles aren't the whole story, the team now finds. The red and gold glazes also contain traces of copper ions in what appear to be finely tuned amounts.
The presence of this copper shows that the potters achieved exquisite control of the firing process that formed the glazes. The copper ions may also alter the way light interacts with the glassy host material, enhancing the lustre.
All that glisters...
Historical evidence for the early nanotechnology survives in the potter's handbook of around 1557, Li tre libri dell'arte del vasaio, by Italian craftsman Cipriano Piccolpasso. Copper and silver salts were mixed with vinegar, ochre (iron oxide) and clay and applied to the surface of pottery already coated with a glaze. A delicately regulated firing technique resulted in a pot with a lustrous surface.
In the Renaissance, these effects, which today would be seen as merely pleasing, had a deeper significance. Turning mundane materials into something resembling gold was regarded as a feat bordering on alchemy. Because colour change colours were deemed to be essential in the alchemical process, iridescence was seen as an alchemically significant property.
