Lustre is a glaze decoration first produced at the end of the 9th century. It consists of a few hundred nanometre thick layer of copper (Cu) and silver (Ag) nanoparticles 5–50 nm across embedded in a silica-based glassy matrix. Lustres made during medieval times show a broad range of colours, such as red, brown, green and yellow; those made in the Middle East at this time are shiny.

Using Rutherford backscattering spectroscopy associated with other analytical techniques, Pradell's team has now shown that this metallic shine comes from the copper/silver nanoparticles present in the glaze. Moreover, the researchers found that the lustre layer formed in lead-rich glasses is thinner (around 250 nm thick) and richer in Cu and Ag. This means the metal nanoparticles are more densely packed in the layer, which leads to more light being reflected.

Previous studies of early Islamic lustre (from the Ashmolean Museum in Oxford) and historical records allowed Pradell and colleagues to faithfully reproduce lustre layers following the exact procedures used at this time. This enabled the researchers to understand the physical and chemical mechanisms involved in the two-stage process to form the metal nanoparticles. The first step is the penetration of Cu and Ag ions into the glaze surface by ionic exchange with the alkalis there. The second is nucleation and growth of the metal nanoparticles in a reducing atmosphere.

Pradell says the study is important for both scientific and historic reasons: "Scientifically we have demonstrated that the metallic shine shown by the lustre layers is due to their small thickness and the high density of metal nanoparticles," she told "The shine may be obtained by modifying the glass composition via the addition of lead oxide (PbO)," she explained. "Historically, we showed that even with their limited scientific skills, potters deliberately introduced up to 15 wt% PbO into their glazes after about the 10th century to obtain shiny lustre layers. High lead-containing glazes have been used when making lustre ever since then."

Such metal/nanoparticle/glass composite layers will also be of interest to today's material scientists because of the particularly interesting non-linear properties of these materials.

The Spain team will now study other optical properties, such as iridescence and colour, of lustre. "Modelling the optical properties of the metal nanoparticles embedded in a glassy matrix is also fundamental for understanding their origin," added Pradell.

The researchers reported their work in J. Appl. Phys..