The decay of the polysiloxane resin Sogesil XR893 applied in the past century for consolidating monumental marble surfaces

Abstract

A study of the deterioration micromorphologies and a chemical investigation of the alteration products of an experimental polysiloxane resin used for consolidating marble sculptures in Venice was undertaken several decades after the initial treatments. A multianalytical study was carried out on a number of microfragments sampled from four Venetian monuments treated in the 1960s and ‘70s. The Fourier Transform InfraRed spectroscopic analysis (FTIR and μFTIR) was used to identify the conservative treatment applied, to confirm its chemical nature (whose formulation has never been confirmed) by comparing the results with the literature, and to investigate both its behaviour under a very aggressive environment and the possible related deterioration products. Microscopic observations of cross-sectioned samples (mainly through SEM-EDX analysis) were conducted to investigate the resin morphology and distribution into the pores, and to identify the possible residual presence of other conservative products previously or subsequently applied. In addition, water absorption by means of the sponge contact method was used to verify the present water repellency of the treated surfaces. The methodology adopted proved to be adequate to the aim of the research and gave useful information about the performance of the resin examined over time.

Introduction

Acrylic and epoxy resins, silicone-based products and polyester polymers have often been used widely since the mid-1900s for the consolidation and protection of the stone of Venetian monuments [1]. Due to the increasing interest in plastic materials and improvements in application techniques, their use was often promoted, from the outset, without any precise knowledge of the time-effects of the treatments. Indeed, there is little record of works in which experimental resins were tested in the laboratory before being applied on monumental buildings [2], [3], [4]. Rehabilitation and restoration undertaken with any product in the absence of appropriate laboratory testing measures and the correct interpretation of the results obtained can accelerate stone degradation [5]. Inappropriate application and use of commercial resins can in fact cause irreversible damage to stone surfaces, leading to modification of several physical parameters of the material (such as total porosity and the size distribution of pores) and, consequently, to an acceleration of specific degradation processes (i.e. colour change, formation of crusts and powdering). In addition, most of the organic products applied were not specifically developed for use in conservation and restoration but for other industrial purposes, and their datasheets are often incomplete since many of them are covered by patents. Thus, in general, every material to be used in conservation and restoration should be subjected to analytical characterization. For these reasons, international scientific research has focused in recent years on the study of the chemical stability and solubility of commercial products used in the past, in order to monitor the state of conservation of treated surfaces, and to make informed assessments of when and where new treatments may be necessary [6], [7].

In Venice, the first application of a synthetic resin for the restoration of stone materials took place in the 1960s [8]. A commercial product named Sogesil XR893 (Rhône Poulenc), manufactured by dissolving a methyl-phenyl-polysiloxane in benzene with a solid content of 50%, was applied to the marble statue of Sant’Alvise on the main façade of the church dedicated to that saint [9]. In a previous work [10] a number of considerations on the chemical nature and the performance of the resin were based on the results obtained from the monitoring of curing processes and the relative polymerization mechanism of a new silicone resin, replacing Sogesil XR893, which is no longer available on the market. In particular, due to the development of branched siloxane structures during the polymerization process and the hydrophobic feature of methyl groups in its formulation, it was hypothesized the resin provided waterproofing properties. On the other hand, it was supposed that the phenyl groups could guarantee thermal stability at low temperatures although may induce a colorimetric change of the treated surface over time. These ongoing considerations suggested the advisability of continuing the research on well-documented restorations of Venetian monuments treated in the past with Sogesil XR893, improving both its chemical characterization and knowledge of its decay in the peculiar Venetian environment. In particular, after a careful study of the documents in the archives of the Venetian Superintendency, marble surfaces of four historic Venetian buildings were sampled and subjected to laboratory analyses. Following the Normal 3/80 recommendations [11] and with the supervision of the senior conservator and restorer of Soprintendenza per i beni architettonici e paesaggistici di Venezia e laguna, micro-flakes were sampled from: (i) a small column of the Cà D’oro, (ii) the Tullio Lombardo angels supporting the altar of the Holy Sepulchre inside the Church of San Martino, (iii) the statue of Sant’Alvise (on the façade of the church dedicated to the saint); (iv) the statues located on the gable-end of the façade of the Basilica of St. Mark [12]. The analytical methodology applied in this work enabled the research team to identify the chemical nature of the resin used and to investigate its alteration morphology and pore-distribution, to detect the formation of potential secondary products and to verify the possible presence of previous restoration treatments, and the possibility of further future treatments. Moreover, the overall performance of the treated objects offered the authors the possibility to make a general comment regarding the treatment methods and their relative implications, giving practical contribution to consolidate decayed crystalline marble today.