The four Virtues of the Porta della Carta, Ducal Palace, Venice. Assessment of state of preservation and re-evaluation of the 1979 restoration

The monumental entrance of Ducal Palace in Venice, known as Porta della Carta and widely recognised as a masterpiece of gothic art, was restored between 1976-1979. The gothic marble sculptures that decorate the arch depicting the four Virtues, Temperance, Charity, Prudence and Fortitude, recently underwent an assessment of their current state of preservation. This investigation entailed a thorough chemical study of surface samples and also ultrasonic measurements of the sculptures. The aims of the investigation were to assess the degree and type of damage suffered by the object, to evaluate the current effectiveness of the consolidants and water repellents applied during the previous restoration and their possible alterations, and finally to develop options for a future conservation strategy. Surface samples taken from each of the four Virtues were analysed using scanning electron microscope (SEM) together with Energy dispersive X-Ray analysis (EDS). Micro-FT-IR was carried out on dichloromethane soluble and insoluble fractions of the applied conservation products. The silicone polymer that was used as consolidant has been identified throughout the entire thickness of the investigated samples reaching a penetration depth of at least 3 mm (which corresponds to the maximum sample thickness). Notably, in superficial areas (of depths 200-500 ƒÝm), this material was found to be completely fissured, cracked and detached from the pore walls and therefore seems to have lost its initial consolidative properties. Moreover, the investigations also detected the isobutylmethacrylic homopolymer that was used as hydrophobic agent. It was found as a superficial coating on the stone surface in sheltered areas, but was mostly absent in exposed areas. SEM morphological observations also showed a widespread distribution of soluble salts within consolidated pores. The ultrasonic investigation showed that parts of the sculptures with a high surface/volume ratio are especially affected by weathering, leading to the conclusion that the methyltrimethoxysilane applied during the previous restoration either did not reach a sufficient penetration depth or has subsequently lost its efficiency.