Chemical analysis of Islamic lustres in the collections of the Ashmolean Museum was completed by a team led by Trinitat Pradell while on sabbatical at the RLAHA. The work involved two steps: examining lustres using a combination of different scientific techniques, and the experimental recreation of lusterware. Tests included electron microprobe, micro-XRD, UV-vis diffuse reflectance and XANES and EXAFS. The recreation was completed using a suite of experimental tests of possible lustre recipes fired in the experimental kiln at Synchrotron Radiation Source.
The sulfur compound within the lustre paint provides a locally reducing (oxygen deficient) environment which allows the metal ions to shed oxygen. This process is further enabled by the introduction of a reducing atmosphere into the kiln. As the copper and silver ions are reduced to metal, they coalesce into nanoparticles. Silver reduces to metal more easily than copper, and can nucleate into nanoparticles sooner than copper ones do. The type, size, and distribution of the nanoparticles within the paint is responsible for the color and metallic shining effect of the lustre. For example, a thin dense layer of small particles gives a cooperative effect which allows a metallic shine.
Getting the required effect requires precise control of a number of parameters in the kiln: the temperature, the firing time, and the gas mixes used for each step of the process. Additionally the interaction between various components of the lustre paint and the glazes also affects the different stages of the process in different ways. For example, the presence of copper helps the reduction of silver. Better process control, or the use of other recipes, allows the same process to occur and produce the same end effect without the need for copper. And it is likely that it was this discovery that allowed the potters to shift from using mixed copper and silver to pure silver lustre paints.
Sources
The photograph on this page is of the experimental setup used to create lustre under laboratory conditions. Components include a temperature controlled furnace, remotely operated valves which allow the gas mix in the furnace (ie. micro-kiln) to be altered from a neutral atmosphere to a reducing one, a synchrotron light source, and a high count rate x-ray detector which collects the x-ray diffraction pattern.
The main sources utilized for the project are listed on the definition page for lusterware.
