S. Siegesmund, F. Wiese, J. Lepper, J.-K. Stein
Naturwerksteine in Göttingen und Umland
Stadtexkursion Göttingen
R. Snethlage, B. Lammerer
Siegfried Siegesmund on his 65th birthday – A Laudation
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CHARACTERISATION OF DAMAGE PHENOMENA OF STONE AND RELATED BUILDING MATERIALS (PLASTER, RENDERING, MORTAR ETC.)
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RISK NUMBER: DOCUMENTATION AND OBJECTIVE ASSESSMENT OF ENVIRONMENTAL DAMAGE TO MARBLE AND SANDSTONE SCULPTURE
Rolf Snethlage
IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.
– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –
VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.
Naturstein in der Denkmalpflege Bamberg, Germany
Summary
Expert reports form the basis for the decision-making concerning conservation measures on marble and sandstone sculptures. However, in the dialogue between the experts, quantitative results in the expert reports are transformed into lingual assessments using subjective expressions such as “weakly/strongly affected” or “little/highly endangered”. In this way, primarily objective measurement values obtain a subjectively tinged interpretation.
Against this background it is the goal of the Risk Number concept to replace a subjective assessment with a preferably objective and quantitative evaluation. The Risk Number is defined as the product of a Measurement Number M(i) and a Valuation Number B(i):
R(i) = M(i) * B(i)
The Measurement Number M(i) constitutes a measured parameter (i) in the form of a dimensionless value. It is calculated by scaling the actual measurement values to the original condition of the object.
The Valuation Number B(i) indicates the effect of the relevant parameter (i) regarding weathering grade and risk of damage.
For the calculation of the Overall Risk Number R(total) of a sculpture, 104 different parameters are evaluated, which are assigned to 10 Excel data sheets representing the following categories:
1 Object Description
2 Art Historical Evaluation
3 Restoration History
4 Environment / Exogenous Risk Factors
5 Natural Stone / Endogenous Risk Factors; calculated separately for marble and sandstone
6 Damage Caused by Human Beings / Vandalism
7 Mechanical Damage / Loss of Material
8 Surface Alteration
9 Ultrasonic Diagnosis
10 Evaluation of Overall Risk Number
For each of these parameters and for each data sheet a separate Risk Number R(i) is calculated. These are summarized to the Overall Risk Number R(total), which expresses the risk the object under consideration is exposed to. The design of the calculation method causes the Risk Number to be a number between zero and one.
Due to a purely mathematical implementation of the measured values, the Risk Number represents an objective and quantitative assessment of the state of damage condition and risk damage.
So far, the application of a Risk Number is intended for marble and sandstone sculptures. An overall 36Risk Number R(total) has been calculated for altogether 8 sculptures and 4 tombstones.
The sculptures include: Apollo, Vestal Verging (both Carrara marble, Neue Kammern, Sanssouci Palace garden, Potsdam), the Flora Aeolus (both Laas marble, Garden of Nymphenburg Palace, Munich), Hera (original), Hera (copy), Rhea (original), and Rhea (copy) (all Cotta sandstone, Baroque Garden, Großsedlitz near Dresden).
The Tombstones from Jewish Cemeteries are: Baiersdorf/Erlangen: Bernhard Ehrenbacher, Siegmund Sulzberger, Loew Ganz (all Schilfsandsteine), No. 1091, Double tomb (red Buntsandstein)
The Total Risk Numbers R(total) calculated for these objects vary from 0.36 (tombstone No. 1091 to 0.64 (sandstone sculptures Hera and Rhea).
Introduction Risk Number: Information Content
The decision for conservation measures on marble or sandstone sculptures is usually made after a dialogue between the owner, heritage conservators, restorers and external experts weighing arguments and determining procedures. In oral and written presentations, the involved parties use expressions about the state of conservation and expected damage progress such as the sculpture is “very/at the highest” at risk, the marble weathered to a “low/medium/highest” degree, the surface of the sculpture is “little/hardly/strongly” affected threatening a loss of “significant/irreplaceable” artistic details. Although these evaluations derive from exact observations and measurements, they are nevertheless highly influenced by the individual and subjective estimation of the rapporteurs. As a result, decisions taken are difficult to retrace later on.
Considering this precondition, the Risk Number concept aims at developing a calculation method on a metrological and objective basis to record the condition and degree of endangerment for an outdoor sculpture. All observations and measured values recorded on an object and its environment are contained in one single figure, the Risk Number.
The concept of assessing the damage risk of historical objects is not completely new. Similar approaches regarding risk evaluation were reported by WAENTIG 2014; DELGADO RODRIGUES & GROSSI 2004; 2007; REVEZ 2010. These authors, however, did not scale the calculated risk to the limits Zero and One because the calculated risk is not easy to rate.
Defining and calculating the Risk Number R(i) and the Valuation Number B(i)
Per definition the Risk Number R(i) is the product of a Measurement Figure M(i) and an Valuation Number B(i):
R(I) = M(i) * B(i)
This definition derives from the general risk definition as it is used in the insurance industries.
The Measurement Figure M(i) constitutes a measured parameter (i) in the form of a dimensionless value. Two different methods of recording
