This work has developed from recent research funded by the Netherlands Institute for Conservation Art and Science (NICAS) into the blackening of the decorative enamel on excavated Chinese porcelain, and specifically the porcelain recovered from the 1751 Geldermalsen shipwreck.

Initial comparative analysis of the composition and morphology of the blackened enamels using microscopy, x-ray diffraction (XRD), scanning electron microscopy with energy dispersive x-ray analysis and Raman spectroscopy determined that the green lead-copper enamels were particarly suceptible blackening and variaitons were related to the w% of the main chemical components: copper and lead. The blackening was determined to be a physicochemical change that occurs due to very specific anaerobic microbiological conditions in which the objects were buried resulting in a transformation to lead and copper sulfide and sulfate.

It was decided to undertake re-firing experiments to gain insight into the original firing process as well as see in how far it was possible to retrieve the origional green enamel colour, the decoration being an important factor in determining the historical context of such finds.  During the re-firing experiments the initial colour changes that occurred with the green enamels at 600˚C, 700˚C and 800˚C were related to different forms of lead oxide that were formed. Between 850 and 900˚C the lead flux was seen to melt, and the enamels became again green. However, at this temperature the surrounding porcelain glaze was found to be adversely affected with the development of microscopic extrusions on the surface and localised changes to the glaze gloss.

The aim of the current project at IESL-FORTH is to find a less destructive and more sustainable method to restore the original colour and therefore the aesthetic value of blackened enamels by localised laser irradiation.