A number of urgent and non-deferrable works aimed at securing various portions of the Certosa di Pavia monument made the object of our intervention. The targeted areas were the sacristy, the library, the clock wing looking onto the grand cloister, the small cloister, the transepts and presbytery of the church, in addition to the conservation of roofs, chimney stacks, pinnacles, walls and mortars subject to deterioration and damage owed to both weathering and poor maintenance (figure 1).
The pinnacles showed severe biological decay in stone, plastered finish and masonry of cones. Attack by biological deteriorating agents, including algae, mosses, lichens andweeds, resulted in colour change, lesions and mechanical pressure on the surface and in depth (figures 2-4).
Regular exposition to weather agents over time triggered an irreversible natural process of slow and continual erosion, resulting in the loss of considerable portions of stone. The remarkable extent of the gaps contributed to affecting the physical properties of the stone and to altering the static conditions of the work (figures 5 and 6).
Concerning the sculpted elements, and namely the coat of arms, the columns of the arcades, the side elements of the rose window and some of the frames – most of which are made of Angera stone with granite and Ceppo gentile stone inserts – featured some alterations of the surface layer, with prominent differences in shape and colour as compared to the original substrate, i.e. black crusts, coherent deposits of dirt, patchy joints, cracks and biological deterioration (figures 7-9).
After estimating the conditions of the work as a whole and of its individual ornaments, the restoration was started with the aim of protecting the materials, healing the finishes and at the same time recovering the original appearance of the ornaments and of their history.
The main objective was to strike a balance between the removal of coherent deposits and black crusts and the conservation of the patina of age that has built up during the life of the work, which makes an integral part of it (figure 10).
PEST CONTROL AND DISINFECTION AGAINST BIOLOGICAL DETERIORATING AGENTS
The areas hit by autotrophic and/or heterotrophic bacteria were sprayed with a wide-range biocide, diluted in compliance with the manufacturer’s specifications. At a later step of the cleaning process, the chemical was rinsed off with low-pressure deionized water (figure 11).
After all the surfaces were cleaned with ionized water, coherent dirt deposits were treated with chemical compresses and the spots featuring the hardest black crusts were microsanded with the help of precision tools (figures 12-14).
Consolidation and structural reinforcement were carried out as follows.
Ethyl silicate was repeatedly applied until rejected by the substrate, with special care to the areas where the original thickness had been heavily reduced, with the aim of making them more compact and sturdy. Subsequently, the original thickness was rebuilt with the help of structural mortars and special steel fabrics, by using different materials for columns and for arches.
A thixotrophic mortar was used to repair portions of the columns, by sinking in it a structural reinforcing fabric, whose one-directional mesh is composed of high resistance galvanized steel micro-strands fixed to a fibreglass micromesh (figures 15-20).
STRUCTURAL REINFORCING BANDAGE
As planned at the engineering phase, the external portions of stone between the capitals and the earthen cones of the pinnacles required to be reinforced by means of a special restraint system. After preparing the surfaces, the bandage was executed with the help of high resistance galvanized steel fibres fixed on a fibreglass microlattice and mineral epoxy adhesive (height of patches varies from about 60mm to 100mm). To reduce the visual impact of the added elements, the fresh resin was powdered with a silica sand and finished with a convenient silicate paint of the same colour as the neighbouring stone (figures 21-27).
After carefully and cautiously removing all mortars, plasters and residues of any previous works with the help of a hand hammer, all gaps and joints were pointed with special mortars aimed at preventing water infiltration and at integrating all stone elements. Any cracks and fissures were filled with epoxy resin and/or hinged with stainless steel or fibreglass rods (figure 28).
[by Luca Zappettini]