The restoration of historical surfaces

  

Starting from 2012 we had the opportunity to restore some of the most important monuments in Bologna. These have historical facades characterized by the most popular items widespread in the historical centre: brick masonries and architectural elements in moulded sandstone (cornices, capitals, etc.). The typical finish of their wall surfaces had a light pink veil, because of the earthenware which let the underlying masonry texture be visible. This kind of finish is inspired by the medieval technique called “sagramatura” 1.
This is exactly the appearance of the facades of two buildings that the ICE delegation is going to visit in Bologna: Podest. Palace, restored between 2012 and 2015 and Scappi Palace, that is still under work. Since these palaces have been restored many times in the past, it was necessary to make a preliminary accurate analysis campaign aimed both to define the preservation state of the surfaces and to identify materials and product (such as application of stucco or strenghtening), used in the previous interventions that require a specific work: a real “restoration of the restoration”.

 
PODESTÀ PALACE

The story_“Podest.” palace was built in 1200 in Maggiore Square to a public use, but now it has such a different appearance. It is a complex and the most ancient part is the one corresponding to the Arengo tower, 39 mt long. In 1245 the palace was united with “Re Enzo” palace and “Capitano del Popolo” palace. At the lower level we can see a space covered by a cross vault called “Voltone del Podest.”, supported by 4 columns with 4 terracotta sculptures on their top. In the second half of XV century Giovanni II di Bentivoglio decided to modernize the facades that changed their look from medieval to Renaissance. However the work remained unfinished because Bentivoglio was sent away from the city after a people’s insurrection.
Nowadays we can see a lower level with 9 arches and an higher part with 9 corresponding arch windows. Sandstone is the most used material even for the decorations.
In the base there is a bossage and decorated tiles. We can remind the criticized restoration of 1910 by Alfonso2 Rubbiani who built battlements as crowning elements, to complete the Bentivoglio’s project and made demolitions and remaking in the loggia. Rubbiani’s idea of restoration was the one of reinstatement, made by removing disharmonious elements and remaking others3.

A possible method of intervention: the choice of materials and products_According to what we have already said, before the operational phase, we made an accurate analysis campaign not only direct on the ground, but also with an archive and library research aimed to understand the type and the preservation state of the materials used at the moment of the foundation and in the next interventions. For example, correlating analysis’s results on materials and the information achieved, it has been possible to realize a stratigraphical analysis about the various types of mortar founded in the masonries. This kind of analysis has been used to develop a mapping and a cataloguing of these materials and to decide what were the best product to use in the cases of reintegration or filling. For this reason we used various types of mortar with an appropriate formulation, to respect the principle of compatibility (of composition, particle size, and colour) with the sandstone.
Besides we observed that in the past was applied a strengthening product, silicon based, exclusively in correspondence to the sandstone elements of the higher part of the facade, whose preservation state is clearly better than the one of the lower part.
During this intervention we had the opportunity, in collaboration with the Department of Material Sciences of Bologna University, to test a nanotechnologic protective system titanium dioxide based that protects the surfaces from atmospheric agents and in addition absorbs pollution giving back clean substances to the air.

 

SCAPPI PALACE

The story_Palazzo Scappi, located in the historic centre of Bologna was the senate palace of the family from which it takes its name. It is located in the so called “Canton’dei Fiori” where in the past had place the flower market.
It was built in the middle of ‘500 4 on the place where stood the houses of the family which included a thirteenth-century tower that still exists. Its current appearance is due to the reconstruction made by Augusto Sezanne in 1892.
The building doesn’t have a very developed height, but can include 5 rows of small and big windows, including those of the portico: the smaller ones are rectangular and decorated with sandstone frames; those of the main floor, the most majestic, are decorated with triangular gables “in rock” and with sill supported by long grooved shelves. Today some of these are buffered, others were expanded and became some longest openings.
The lower part consists of six arches supported by brick columns, with composite and Corinthian capitals in stone characterized by different decorative motifs.
Scappi family died out in 1707 with the death of the last member. Later on the building underwent several changes of ownership.

The intervention_Also in the case of the restoration of Palazzo Scappi’s facade (which have consisted more in securing sandstone element that were at risk of detachment) it was fundamental an accurate phase of stratigraphical analysis that has been used to detect different types of mortars (made of lime and sand, gypsum, up to a cement-based mortars), used for fillings, reintegration and partial reconstruction of the sandstone elements under construction and subsequent maintenance operations or restoration. Through the comparison between direct stratigraphical analysis and the information gleaned from laboratory investigations, carried out on samples taken appropriately, types of finishes applied on surfaces have been identified. Even in this situation all the data collected during the analysis phase represent an extremely important tool for the definition of the methods and materials to be used at different stages of intervention in order to run an effective restoration, sustainable and respectful of the historic features of the building.

 

SAN PETRONIO BASILICA

The story_The restoration methodologies described above were also used in the case of the important intervention made on the San Petronio Basilica 5.
The San Petronio Basilica is located in the large Piazza Maggiore in Bologna. Its construction began June 7, 1390 under the direction of the work of Antonio di Vincenzo, but we cannot set a date for the conclusion. It is a late-Gothic basilica with three naves.
According to the original plan, its length should have been even greater 6. The current structure of the facade is characterized very clearly by a lower part made of brick whereas the lower part, which includes the three portals, is covered with white Istrian stone and red Verona marble. This coating is clearly unfinished. The main portal is the work of Jacopo della Quercia, with representations of the Old and New Testaments. On the tympanum sculptures of the Madonna and Child are visible, and of Sant’Ambrogio and San Petronio too. The portal was unfinished: missing the terminating cusp.
The first major renovation which was submitted in modern times the fa.ade dates back to the seventies and had as protagonists Ottorino Nonfarmale, Eugenio Riccomini and Raffaella Rossi Manaresi, at the center of a heated debate; during the nineties they were then carried out a series of maintenance interventions.

The analyses and the cleaning of the surfaces_Cognitive campaigns conducted on the structures, the study of historical archival sources, documentation on the restorations and the development of operational protocols, including the experimental and innovative ones, have enabled the collection and systematization of many data, that in many cases were unpublished, useful on scientific level and for the planning of future maintenance.
The restoration work on the stones and on the sculptures of the facade cannot ignore the conservation history that preceded it: much of the current intervention focuses on dialogue with what has been done in the past.
The state of preservation of the facade was discrete, the consolidating product that has been used, known as “mista Bolognese” (acrylic-silicone resin) acquitted in part its function and the stone facing has only required some precise and circumscribed interventions of consolidation or bonding of parts with epoxy resins di-components. The largest work was the accurate cleaning articulated in successive and progressive stages. A first phase of removal of surface deposits, made with soft brushes and with the aid of an aspirator, was followed by a cleaning by compress with demineralized water, a localized solvent cleaning made by buffer. Particularly tenacious deposits has been removed by laser technology, to obtain a selective removal only of the deposits and the preservation of the patina of oxalate and traces of previous treatments. The operation of filling of the joints and more or less consistent cracks played a major role also for its conservative aspect of bringing them to the level of the stone in order to facilitate the drain rainwater. For the same reason some covers were made of lime-based mortar, earthenware and sand, to make the surface waterproof and reinforced with a honeycomb mesh. The last phase is that of the applicationof the protective product specially formulated to be laid out on a surface already treated with acryl-silicone resins.
The three sculpted portals and more decorative elements of the facade were subjected to a scanning three-dimensional relief in high precision useful to the acquisition of a perfect virtual model of the entire complex, realized with digital techniques and without direct contact with the elements. Thanks to the model it is possible the material reconstruction work in full-scale, in order of its valorization, study, exhibition, or to prototype partial or integral elements to replace the original in case of loss of the same.
The church square, also in stone, was the subject of a cleaning operation performed with the use of pressure-controlled precision sandblasting machine: a conservative intervention, respectful of the historical monument.
The project also involved the completion of the restoration of the interior chapels of St. Vincent Ferrer, St. Rocco, St. Michael and St. Rosalia-S. Barbara.
Outside, in addition to marble, for which were applied the same methods of the facade, there are pieces of plaster in which were found traces of a decoration, portions of cotto “unfinished” tiles, which also characterizes the upper part of the facade and finally there is the cotto “sagramato”, not present in other areas, but particularly important as a surface to be preserved as it contains the traces of the original finish. In fact, the “sagramatura” has characterized much medieval building in Bologna but it remains preserved in very few original examples. A careful diagnostic campaign has brought to the knowledge of materials and of diseases and causes of degradation that characterize both the part in stone that the brick.
For the part stone a special mention should be made to the trial, carried out thanks to the instructions of the Opificio delle Pietre Dure, of sulfate reducers bacteria which have the advantage (even in an environmentally friendly way), to make an extremely controlled cleaning respecting the object, the operators and in the environment in which it operates.
At a structural level there were no significant problems except for a large lesion on a capital that was a through lesion and parallel to the surface and has necessitated a bandage with carbon fiber tapes stacked in three layers, glued with di-component epoxy resins, later treated with inert conform to the original for grain size and color.
On the brick part of the wall, after a consolidating process of the “sagramatura” parts with micro injections of acrylic resin diluted, it proceeded with a cleaning with atomizer system that combines the mechanical action obtained by the run-off with the chemical action of the water which slowly dissolves plaster or the secondary calcite, of redeposition, which act as ligands of the black crust and let it be easily removed. The laser has been used to remove black crusts in the lower part of the windows, in correspondence of the brick parts.
The use of this nanotechnology has enabled a controlled surface cleaning without the risk of damaging the protective coating present. The joints, in the portions that required it, were grouted with mortar of lime and sand, in accordance with the original.

 

THE RESTAURO VERDE® SYSTEM

The actions described above were conducted using the Restauro Verde. system, an approach to restoration work (conceived by Leonardo on the basis of guidelines set by the Green Building Council - to which Leonardo joined and with whom is working to define the Protocol of Historical Buildings) in view of environmental sustainability and energy conservation. The growing awareness of environmental issues has led to think that the restoration is necessary to arrive at a sustainable preservation of cultural heritage. Much attention is paid to the control of the environmental impact created by restoration activities, trying to reduce pollution. Specifically, the Restauro Verde. system is divided into four main categories:

_ The procedures for the evaluation and selection of products to use in the recovery and restoration of the buildings;
_ The preservation of health and safety of operators and people unrelated to the construction site;
_ The procedures for a correct management of the works with low environmental impact;
_ The research and innovation through the realization of research projects for the application of substances, innovative methodologies and tools for the restoration works and for the recovery of the buildings.

According to the same principles great attention is also paid to the reduction of water’s use in the cleaning operations, thanks to the use of alternative systems such as spray water and the correct management of discharge.
Another important element for the containment of water consumption is the use of the technology for the microsandblasting developed by Ibix used in a selective and calibrated way for cleaning or finishing works. This system is much used also for the removal of graffiti in the historical centres.

[by Rossana Gabrielli - text di Clelia La Mantia]

 

NOTE
1. Gabrielli R. e Geminiani F., Le finiture dell'edilizia storica bolognese: la sagramatura ed il restauro delle facciate nell'esempio di palazzo Agucchi, in “Dossier n.5/2001”, Maggioli Editore.
2. Giuseppe Bacchelli, Giù le mani dai nostri monumenti antichi. Note critiche sui progetti dei nuovi lavori al palazzo del Podestà in Bologna, Bologna, Stab. Poligrafico Emiliano, 1910.
3. Antonella Ranaldi, Il restauro di Rubbiani del Palazzo Re Enzo, in Palazzo Re Enzo. Storia e restauri, a cura di Paola Foschi e Francisco Giordano, Costa Editore, Bologna 2003. Pp. 95-118.
4. Roversi Giancarlo, Palazzi e case nobili del '500 a Bologna, Grafis Edizioni, Bologna 1986, pp. 341-348.
5. RESTORATION OF SAN PETRONIO BASILICA: FOUR-YEAR PROJECT BETWEEN INNOVATION AND ECO-SUSTAINABILITY, Dott. Arch. Roberto Terra1 , Dott. Rossana Gabrielli2 , Dott. Michela Boni3 1 freelance architect, Studio Cavina Terra Architetti, Bologna; 2 architectural archaeologist, Leonardo s.r.l., Bologna; 3 restorer and art historian, Leonardo s.r.l., Bologna, in Built Heritage 2013 Monitoring Conservation Management, pp. 1461-1471.
6. Bellosi Luciano, La Basilica di San Petronio in Bologna, voll. 1-2, Cassa di risparmio in Bologna, 1983.