Mosaic Materials in an Art Deco Steel-framed Building

The mosaic coving at the Grade II* listed Freemasons’ Hall in London is an example of a mosaic on an Art Deco steel-framed building. Microstructural and chemical characterisation of loose mortars and tesserae were undertaken to inform conservation efforts and advise whether a protective coating was needed to prevent further fading of gold gilding. Rigorous testing used optical imaging, scanning electron microscopy, field emission scanning electron microscopy, energy dispersive X-ray analysis, Raman spectroscopy, X-ray diffraction, and thermogravimetric analysis with differential scanning calorimetry. Laboratory evidence demonstrated the glass tesserae were created using traditional techniques. The glass tesserae were soda-lime-silica glass with lead. The gold glass tessera was produced in the ancient Roman style which included a glass cartellina layer, so further protective coating was not needed. Historical records confirmed sheets of mosaic tesserae were prefabricated in Italy. The grout and thin-set mortar were the same low hydraulicity mortar mix, which enabled mosaic tesserae positions to be adjusted as required. The bedding coat mortar was a hydraulic cement mortar that was applied on top of an early example of foamed mortar from 1932. This unique study analyses a traditional style mosaic that was installed during a pivotal point in construction history where foamed mortars and steel framed buildings were introduced. It demonstrates how archival research and laboratory testing of small-sized samples enhance the building’s significance and architectural interest by improving the knowledge of its history.