The main altarpiece of the Old Cathedral of Coimbra (Portugal)

Abstract

This paper surveys gilding methods and materials found in the original polychromy and three subsequent coatings applied to the main altarpiece of the Old Cathedral of Coimbra, between 1502 and 1900. Twenty samples from gilded surfaces were examined with optical and scanning electron microscopy for leaf thickness, and analyzed with energy-dispersive X-ray spectroscopy for alloy composition (semi-quantitative evaluation). By crosschecking the documentary data (archives and technical sources) with the analytical data, the results point out several aspects: 1) two recurrent techniques can be distinguished: water-based burnished gilding on bole, and oil-based matte gilding on mordant; 2) gold leaf thickness is clearly below one micron over time; 3) in 1502, 1583 and 1685, the hand-beaten gold corresponds to the highest grade of the metal (up to 23 carat gold). This degree of fineness corresponds to the required purity and its transcendental meaning in the religious context in which the precious metal was used. In 1900, the ternary alloy found, equivalent to 20 carat gold, is more pertinent with the restoration then carried out and perhaps with the need to save money. Research on the concentrations of characteristic trace elements present in the gold alloys is being pursued.