Prethodno priopćenje
https://doi.org/10.17018/portal.2016.5
The research into the Composition and Properties of Medieval plaster in the church of St. Mary at the Pond with Comparative analyses of commercial injection materials
Bastian Hacker
orcid.org/0000-0002-2890-2810
Neva Pološki
; Akademija likovnih umjetnosti Sveučilišta u Zagrebu, Katedra za restauriranje umjetnina, Zagreb, Hrvatska
Sažetak
The church of St. Mary at the Pond with a wall painting depicting The Adoration of the Magi I the most important art and cultural monument of the Istrian town of Gologorica. It is an east-facing church, approximately 7 m long and 5 m wide. Typologically, it belongs to a group of single-nave Romanesque churches with an inscribed apse. The wall painting, located on the north wall of the church interior (5m wide and ca. 2.2 m high), was discovered in the late 1950s by art historian Branko Fučić, who dated it to the turn of the 15th century. Around 1651, a major renovation of the church took place. The inscribed apse was removed and the floor level was raised with the building material obtained. Further alterations to the interior and exterior followed in the period between the 18th and the 20th century. The support for the multilayered plaster of the wall painting is rubble masonry made of lime sandstone, i.e. sand limestone. It is coated with a 1.5-cm-thick arriccio with a calcite-based aggregate, i.e. aggregate bound by calcite. The arriccio is covered with a 0.5-cm-thick layer of intonaco of almost identical composition. On top of it, various paint layers are applied over a toned lime wash. The paint layers are bound by lime that contains organic elements.
The fragmentarily preserved wall painting shows many damages: static cracks, cavities, structural damages of the plaster, damages of the paint layers, and in some portions, the original plaster is completely missing.
In 2008 and 2009 a systematic research of the wall painting was conducted in order to come up with an acceptable concept for conservation, while bearing in mind the damage to the painting which required a prompt implementation of protective measures. The research was part of Bastian Hacker’s diploma thesis at the Potsdam University of Applied Sciences (Fachhochschule Potsdam, Fachbereich Architectur und Sttebau, Studiengang Restaurierung). Special attention was paid to the laboratory analyses of the composition and properties of multilayered medieval plaster. In addition, an assessment was made of the most common commercial injection materials which were compared to the original plaster.
The discovery of fragments of the multilayered plaster withoat the paint layer beneath the present-day floor, enabled various laboratory investigations into its composition and properties. Since the content of components soluble in hydrochloric acid was high in the tested samples (90.89w% – 93.34w%), and these were interpreted as being lime binder, the results of the wet chemical analyses proved irrelevant. For this reason, it was impossible to give a more specific estimate of the binder and aggregate ratio or of the composition and shape of the aggregate. As a result, thin sections of the samples of the original arriccio layer were prepared, in which the binder and aggregate ratio was calculated by counting the presence of aggregate, pores and binder. Since the share of fine pores was disregarded in calculation, a very high binder con tent was obtained. However, by measuring the maximum water absorption capacity, the share of fine pores was obtained, which enabled the binder and aggregate ratio to finally be defined as 2 : 1. In addition, in the thin section, lime lumps were spotted in the binder, as well as aggregates of marble, limestone, calcite in the form of mineral and lime sandstone i.e. sand limestone. Measurement of the dynamic modulus of elasticity indicated a relatively low density of the arriccio plaster samples. The conclusion thereof could be made that the share of pores was large. To acquire more detailed information of the size and volume of pores in the arriccio, an analysis of the maximum water absorption capacity and the pore radius distribution was conducted. These parameters largely influence the properties of arriccio and therefore had to be considered in the making of replacement plaster. The share of maximum water absorption capacity was 35w%, little above the usual values for brick. A large share of pores smaller than 3μm was determined. Furthermore, imaging was done by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) that determined the presence of calcium, aluminium, magnesium, iron, potassium, silicon and niobium in the medieval plaster.
Analyses of the plaster samples indicated that the components of multilayered plaster – joint mortar, arriccio and intonaco – are lime plasters in composition, with specific characteristics such as a high binder and fine pores content, and that they contain calcite-based aggregates, i.e. aggregates bound by calcite.
Given that the filling of cavities was assessed to be the protection priority, in addition to the fact that the treatment itself represents an irreversible intrusion into the core of the wall painting, it was important to choose an appropriate injection material. For that purpose, standardized samples of the most common injection grouts were prepared, whose static modulus of elasticity and biaxial flexural-tensile strength were compared to the values of the original arriccio. Values of these parameters of the injection grouts should be lower than those of the original plaster. Three standardized samples of the medieval arriccio were tested alongside samples of the materials KSE 500 STE-Modul-System, CalXnova, PLM-AL, PLMA and Ledan. The injection materials mostly showed higher values of static modulus of elasticity than the medieval plaster. Lower values than those of the original plaster were found only in CalXnova modified with hollow glass microspheres and PLM-AL prepared according to the manufacturer’s instructions. These materials were therefore used in the applying of protective measures on the wall painting.
In order to consolidate the particularly endangered portions of the wall painting, appropriate replacement plasters for arriccio and intonaco needed to be prepared. As replacement plasters must have properties similar to those of the original plaster, in order for their behavior to be similar, local slaked lime was used for their preparation, in addition to the calcite-based aggregates, i.e. aggregates buond by calcite: marble, calcite in the form of mineral and local stone from the church surroundings, i.e. lime sandstone. In order to obtain softer replacement plasters, the binder content was reduced to 1 (binder) : 2 (aggregate) for the arriccio and 1 : 3 for the intonaco plaster.
In one particularly endangered area of the wall painting, protection measures were applied in 2009. The consolidation consisted of the structural strengthening of the plaster, the filling of cavities and replacing the plaster layers with materials that were selected based on the results of research. By following the previously established methodology, the work in the church of St. Mary at the Pond has been under way since 2013, as part of practical training
in conservation for students of the Department for Conservation and Restoration of Works of Art (OKIRU) of the Academy of Fine Arts (ALU), University of Zagreb.
Ključne riječi
Gologorica; church of St. Mary at the Pond; wall painting; 14th / 15th c.; lime plaster; calcite aggregate; cavities; injection materials
Hrčak ID:
171691
URI
Datum izdavanja:
28.12.2016.
Posjeta: 2.116 *