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Researchers to rescue heritage

 
 
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Restoration of the original Pentelic marble using Dionysos marble. The stone is not the same and the combination of materials with dissimilar fatigue rates may cause further damage.

Stones are not immortal. Architecture and sculpture are subject to the ravages of time and damage done by man. Restoration is often not effective and sometimes even harmful. Teams of European researchers are now turning their attention to the damage done to our heritage in order to provide those responsible for its conservation with the scientific tools best adapted to their difficult task.

     

The Parthenon in Athens could serve to symbolise most problems of conservation of the European cultural heritage. For about 2500 years its white marble stones have weathered the ravages of time, aggravated by numerous earth tremors that have hit the regions over the century. This degradation has accelerated in recent years due to the effect of chemical pollutants generated by human activity. More than ever before, this masterpiece is under threat and dozens of scientists have come to its aid, searching for remedies but also for powerful diagnostic tools.

There have been various restoration projects carried out with varying success, but none have ever provided a satisfactory solution to the many maladies this unique monument has suffered. The fact is that these projects have been managed in a particularly confusing context as the results could diverge considerably from one laboratory to another and according to the various methods of analysis used. Some options have even proved harmful to the structure due to a lack of proper knowledge of the physical and chemical properties of the marble used in antiquity. Georges Exadactylos, coordinator of the project funded under the European Standard, Measurements and Testing Programme, explains that shortly after the war iron was used, for instance, to repair some of the cracks, but the metal quickly rusted which led to further damage inside the stone.

Non-destructive techniques

The first task of the European researchers was to make as objective an assessment as possible of the damage suffered by the monument, resulting from highly complex mechanical and chemical processes. Traditional computerised analysis provides only limited remedies as these "destructive" methods (involving the removal of samples) are prohibited on such monuments. Georges Exadactylos explains that a non-destructive acoustic technique was therefore tested and standardised. It consists of applying in situ ultrasound to the stone and analysing the waves reflected. These waves provide valuable information on the state of the material surface. Damage that is invisible to the naked eye can thus be detected and (especially) quantified.

The question is therefore how to go about restoring the damaged stones. Traditionally, there have been two options: where possible using the original material or finding a substitute with very similar physical, chemical and aesthetic properties.

The project coordinator points out that in the case of the Parthenon there is no real choice as the original quarry, located in the Athens region, can no longer be used for environmental reasons. The stone should therefore be sought in other white marble strata.

It is also necessary to have a thorough knowledge of the properties of the materials to be combined for this purpose and compare them. If the stone used in restoration work deteriorates, for instance, at a different rate from that of the old marble, this phenomenon may cause detrimental mechanical and aesthetic distortion.

One of the six tests studied and standardised under the Monuments project is a "fatigue" test on the natural building stones. In this type of test, samples are not exposed to sudden stress involving immediate fractures but on the contrary undergo very progressive stress. In this way the stone cracks slowly, which corresponds to normal degradation conditions of monuments, apart from the special case of earthquakes.


From the past to the present

These different tests have also been applied to and standardised for a material widely used by Italian architects and artists of the renaissance: Serena sandstone. The greenish colour, characteristic of the city of Florence, results from the use of this material in most edifices erected between the 15th and 17th centuries whose restoration involves difficulties comparable to those of the Athenian heritage.

The researchers have also studied white Carrara marble, one of the materials preferred by Michelangelo both as an architect and as a sculptor. The quarries in western Italy are in fact still among the major sources worldwide for the production of white marble, which explains the presence in the Monuments project of the international Marmi e Macchine Carrara company.

Paolo Salieri, scientific officer at the Directorate-General for Research at the European Commission, explains that this industrial partner is involved in the characterisation of properties and damage of natural building stones. Certain contemporary structures in which marble is used have serious faults due to a poor use of the stone.

The results of the Monuments project, which has led to the standardisation and harmonisation of six different testing methods - three of which entirely new - of the mechanical properties of natural building stones, will serve as a basis for discussion within the European Standardisation Committee. These results could help to establish common ground for expert reports in the preservation and restoration of historical monuments and help contemporary natural building stone industries to establish new European quality standards in this domain.

 
Title
Characterisation of Mechanical Properties and Damage of Natural Building Stones in Historical Monuments

Reference
SMT4-CT96-2130

Programme
Standards, Measurements and Testing Programme

Contact
George Exadactylos
Technical University of Crete
Department of Mineral Resources
Engineering
Eleftheriou Venizelou 128BIS
72132 Chania, Greece
Fax: +3082 169554
E-mail : exadacty@mred.tuc.gr

Partners
- Technical University of Crete, Chania, Greece (coordinator)
- National Technical University of Athens, Athens, Greece
- Politechnico di Milano, Milan, Italy
- Internazionale Marmi e Macchine Carrara SpA, Marina di Carrara, Italy
- Università degli Studi di Pisa, Pisa, Italy
- SINTEF Petroleumsforskning AS, Trondheim, Norway

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