| Abstract: | SUMMARIES. The monumental entrance of the Ducal Palace in Venice, known as the Pona delta Carta and widely recognized as a masterpiece of Gothic art, was restored between 1976 and 1979. The Gothic marble sculptures that decorate the arch, depicting the Four Virtues, Temperance, Charity, Prudence and Fortitude, recently undenvent an assessment of their entrent state of preservation. This investigation entailed a chemical study of surface samples and ultrasonic measurements of the sculptures. Tlie aims of the investigation were to assess the degree and type of damage suffered by the object, to evaluate the current effectiveness and possible alteration of the consolidants and water repellents applied during the previous restoration, and finally to develop options for a future conseivation strategy. Surface samples taken 'rom each of the Four Virtues were analysed using scanning electron microscopy (SEM) together with energy dispersive X-ray spectroscopy (EDX) and micro Fourier transform infrared (FTIR) spectroscopy. The silicone polymer used as the consolidant has been identified throughout the entire thickness of the samples investigated. Xotably, in superficial areas (200-500 urn in depth), this material ivas found to be completely fissured, cracked and detached from the pore walls and therefore seems to have lost its initial consolidativo properties. The ultrasonic investigation showed that parts of the sculptures arc especially affected by weathering, leading to the conclusion that the methyltri-methoxysilane applied during the previous restoration either did not reach a sufficient penetration depth or has subsequently lost its efficiency.
CONCLUSIONS. The results of this study have revealed important information concerning the state of preservation of the marble sculptures and the chemical-physical behaviour of the silicone and acrylic polymers applied. The study detected a widespread distribution of low amounts of sodium and potassium chloride and gypsum, predominantly within superficial areas but also to a lesser extent in depth. However, in the case of the sculpture of Charity, the concentration of salts within the depth of the stone matrix was notably higher. An increasing accumulation of chlorides and sulphates has been detected inside pore cavities and capillaries due to ongoing percolation of salt solutions, regardless of whether these were filled or unfilled by the consolidant. Fresh and empty fissures have also been detected, often close, or directly connected, to pores containing the consolidant and salts. It is therefore probable that these fissures have developed after the conservation treatment, due to the anisotropic thermal expansion of the calcite and/or mechanical stresses caused by soluble salt crystallization cycles. With regard to the consolidation treatment, it is clear that the product has reached a depth of at least 3 mm (this distance corresponding to the thickness of the widest samples collected). The pattern of consolidant penetration seems to be strongly related to the conserva¬tion state of the statues; where the stone was severely deteriorated the resin penetrated more deeply, while a more superficial distribution is observed on the surface of more sound areas of stone. In most cases, coarse cavities and small capillaries are completely filled by the silicone polymer, although occasionally it forms a coating on the pore walls. Over the 25 years since the restoration, the methyl-siloxane polymer has become tough and brittle, and has detached from the pore walls, reaching a state where it is completely fissured and cracked. The deterioration of the silicone is particularly pronounced in the most superficial parts of the stone, at a depth of 200—500 µm, but it appears that the rate of disintegration of the polymer is slightly slower as the distance from the surf¬ace increases. The cracks in the polymer appear to provide a means for salt solutions to migrate into consolidated zones. Moreover, it seems that silicone polymer now functions more as a filier, having lost a great deal of its consolida-tive properties. The protective hydrophobic coating applied in the restoration has been found on sheltered stone surfaces, but it is totally absent in areas exposed to rainfall. The loss of hydrophobicity in these areas may have been enhanced by the percolation ot water through superficial cracks m the marble. The solubility tests on polymers have been performed with organic solvents, which do not behave as water with respect to the same polymers. The ultrasonic investigation showed that, as is typical for objects of this type, parts of the sculptures with a high surface/volume ratio are especially aftected by weathering. For all cases it can be concluded that since the conservation by Hempel during 1976—1979 the deterioration of the sculptures has continued. Moreover, the methyltrimethoxysilane consolidant applied at the time has since tailed, either because it did not penetrate the stone sufficiently, or because it has subsequently ceased to function as a consolidant. The ultrasonic measurements also highlighted the immediate danger that parts of the head-neck and shoulder zones of the sculptures may be lost. Furthermore some parts ot the sculptures with a diameter ot 25 cm or less suffer from internal decohesion ot the stone, resulting in a dangerous reduction in mechanical strength. The results obtained provide compelling evidence tor the removal ot all the sculptures from their present outdoor location. to a controlled indoor climate tor conservation purposes. |
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