Chemical and Mechanical Characterization of the Alternative Kriab-Mirror Tesserae for Restoration of 18th to 19th-Century Mosaics (Thailand)

Kriab-mirror tesserae are a type of lead-coated mosaic mirror found in most archaeological sites and antiquities dating back to the 18th century in central Thailand. The need for restoration work has prompted the search for alternative mirrors with similar characteristics to the ancient ones. Prototypes of alternative lead-coated mirrors were successfully used to restore a variety of archaeological sites and artifacts, demonstrating their potential application in heritage conservation and restoration work. We investigated the selected ancient Kriab samples in terms of their composition in both glass and reflective coating layers, as well as the chemical and mechanical characterization of the selected alternative Kriab mirrors. We employed a standard lab-shear test, which proved difficult to evaluate due to failure not occurring between the glass-to-metal interfaces. However, a modified lab-shear specimen setup was used to elucidate the peel-off bonding behavior of the lead-to-glass interface. Additionally, we measured the L*, a*, and b* values in the CIE-Lab standard, which exhibited variations for each colored Kriab mirror. The %reflectance of the selected ancient and alternative Kriab mirrors was highly similar when lower than a high %reflectance of a standard silvering mirror. Thai professional conservators have embraced the use of alternative Kriab mirrors in restoration projects as a replacement for old Kriab mirrors, as they are more compatible in terms of color and avoid the excessive brightness of silvered colored mirrors. However, the weathering durability of the alternative mirrors was poor due to the leaching of alkaline and lead ions caused by hydrolytic attack on the poor chemical stability separated phase. Overall, our research provides valuable insights into the properties and qualities of both ancient and alternative Kriab mirrors, which will be useful in the further development of mirrors with more resembling properties or even more environmentally friendly Kriab mirrors and their potential applications in restoration work in Thailand and archaeological sites in Asia.