Monument Future

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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 the individual Measurement Figure M(i) are used:

— Metrological definable parameters: They are defined by the alteration from the initial zero state to the actual state.

— Numerically recorded parameters: These parameters do not describe the deviation from an initial state. They are available as measured values, which, however, have to be classified according to agreed upon criteria. One example is the quantity of precipitation, where the measured values obtained from environmental stations are scaled to numbers between zero and one according to the rain intensity.

According to the calculation method M(i), it will always be a number between zero and one.

The Valuation Number B(i) indicates how much the respective parameter affects the weathering process. It is limited to values between 0 and 10, which are attributed by the user. For example, a parameter with a great impact on the weathering process is rated 10, another with less impact maybe only 5. In the calculation process, these Value Numbers B(i) are fixed and must not be changed 37because otherwise no comparison would be possible between different investigations.

Several exogenous (climate, exposition) and endogenous (stone properties) parameters influence the integrity of stone figures in the outdoor climate. For calculating their Risk Number R(i) it is necessary to measure these parameters quantitatively. Altogether we use 104 different parameters, which are allocated to 10 Excel data sheets according to their meaning. The 11th data sheet serves for the calculation of the Overall Risk Number R(total), which comprises the individual Risk Numbers R(i) of the other data sheets.

1 Object Description

2 Art historical Evaluation B(AE)

3 Restoration History R(RH)

4 Environment / Exogenous Risk Factors R(E)

5 Natural Stone / Endogenous Risk Factors R(N-M) for Marble

6 Natural Stone / Endogenous Risk Factors R(N-S) for Sandstone

7 Damage Caused by Human Beings / Vandalism R(V)

8 Mechanical Damage / Loss of Material R(M)

9 Surface Alteration R(SF)

10 Ultrasonic Diagnosis R(US)

11 Overall Risk Number R(total)

Function of Data Sheets

Content and purpose of the data sheets and the calculation of the Risk Number will be explained using the sculpture of Hera (original) in the Baroque garden, Großsedlitz near Dresden. Bullet impacts during World War II caused severe damages to the figure.

Because of limitations of space, only one data sheet can be shown. The endogenous natural stone properties decisively determine the course of weathering due to mineral content, structure, and thermal as well as hygric expansion. The data sheet shown refers to the sandstone sculpture Hera/Juno in the Baroque Garden of Großsedlitz near Dresden. It can be seen how the different parameters are evaluated and in which way the Risk Number R(i) of the whole sheet is calculated.

On the last data sheet “Overall Valuation” the Partial Risk Numbers are combined to assess the Overall Risk Number R(total). For this purpose, the Partial Risk Numbers R(i) of all data sheets except Nr. 1 (Object Description) and Nr. 2 (Art Historical Evaluation) are summed up and a mean value is formed:

R(total) = [R(RH) + R(E) + R(N) + R(V) + R(M) + R(SF) + R(US)] / 7 = < 1

Figure 1: Data Sheet for endogenous stone properties. Example Hera in the Baroque Garden of Großsedlitz.

38Due to the mean value formation the Overall Risk Number R(total) is also a number between 0 and 1. Zero stands for no risk and one for a very high risk. This scaling gives an immediate indication of the degree of a risk for the sculpture under consideration. Again, it has to be emphasized that the Overall Risk Number R(i) is formed with objective and quantitative measurement values and is therefore free of any subjective evaluation. It is ideal for decision-making, especially for determining a ranking within a group of sculptures. Furthermore, in combination with the Overall Risk Number R(total) the Risk Numbers R(i) of the data sheets can indicate the areas of a sculpture at particular risk.

Risk number: Evaluation and Comparisons

For testing the concept of the Risk Number R(i), 12 objects have been selected, four statues of marble, four of sandstone and four sandstone tombstones from the Jewish Cemetery in Baiersdorf/Erlangen (see Figure 2).

Figure 2: The objects chosen for testing the Risk Number.

The Overall Risk Number R(total) in data sheet 11 of the Excel work sheet is automatically calculated. Although this data sheet lists both the arithmetic mean of the Risk Number and the quadratic mean, only the arithmetic is considered in the following because the overall statement remains the same even if the quadratic mean provides slightly different values.

The individual Risk Number calculations of the 12 examined objects yielded surprising results in an initial, unbiased assessment, which, however, have proved to be very useful.

The Overall Risk Numbers of the individual sculptures and tombstones are depicted in Figure 3.

The object with the lowest Risk Number is tombstone No. 1901, the one with the highest is Vestalin. In addition, at the upper and lower ends of the scale the realistic maximum and minimum limits for marble and sandstone are presented. These limits indicate possible maximum and minimum overall Risk Numbers for these rock types. The range extents from 0.24 to 0.88 for marble and from 0.18 to 0.79 for sandstone. The reason for these limits lies in the fact that certain data sheets or parameters cannot reach a value like zero or one.

For example, data sheet “Environment – Exogenous Risk Factors” could theoretically assume a value of 1 in the case of very extreme climate, but never a value of zero, because a “non-climate” does not exist. This also applies to data sheets “Natural Stone – Endogenous Risk Factors”. Even in a very favourable climate, thermal and hydric expansion and water absorption exert an influence on the weathering.

Figure 3: Overall Risk Numbers of the selected objects.

39The calculation system for the Overall Risk Number permits model calculations for open air exposure or location in a store. Relevant for this are the data sheets “Environment – Exogenous Risk Factors”, “Vandalism” and certain rock properties such as thermal and hygric expansion. These parameters vary according to the location of the sculpture. Comparisons have shown that the risk reduces by 0.22 to 0.25 for marble objects and 0.18 to 0.22 for sandstone objects when moving them from outdoors to a depot.

Whatever the value of an object’s Risk Number, the question arises as to how large the numerical change regarding the Overall Risk Number must be in order to be able to draw a valid conclusion as to a greater or lesser overall risk. As we could see, the values of some sculptures are very close to each other, for example Aeolus (R(total) = 0.60) and Flora (R(total) = 0.59). The question is whether 0.01 points is significant or not. Simple considerations demonstrate that even small differences are important. Because, for calculating the Overall Risk Number R(total) the sum of the individual Risk Numbers R(i) is divided by seven so that even a small difference of 0.01 can be significant in one of the data sheets.

More straightforward, three types of endangerments can be defined: “little endangered” – “endangered” – highly endangered”. These categories range approximately from 0.2 to 0.4, 0.4 to 0.6 and 0.6 and 0.8. Figure 4 shows that the marble sculptures Vestalin, Flora and Äolus are highly endangered.

Conclusion

This report has shown that the concept of the Risk Number accurately represents the degree of endangerment to outdoor sculptures. The difference often found between a visually subjective evaluation and the objectively calculated Risk Number initially led to the conclusion that the Risk Number would not accurately reflect the overall situation. The fact, however, that the Risk Numbers describe the conditions of environment and stone properties as they really are, means in consequence that the Risk Number values represent the risk of endangerment for the investigated sculptures objectively and realistically.

Figure 4: Simplified categories for marble: “little endangered” – “endangered” – highly endangered”.

Model calculations also permit a prediction about the decrease of the risk for a sculpture when moving it from an outdoor exposure to a depot. In these cases, the risk decreases by about 20 %, which is a considerable reduction. However, the risk in a depot cannot be completely zero either, because the sculptures still underlie a climatic effect, albeit to a lesser extent. Also, some risk factors such as low ultrasound velocity remain, even if there will probably be no further deterioration in a depot.

Overall results show that the Risk Number is an appropriate tool to all owners who wish to examine their sculpture stock and determine the risk of endangerment. Although the Risk Number does not give specific instructions about what measures are required, a high number indicates the main risks. It is therefore a useful tool in the discussion about necessary conservation measures.

Acknowledgements

The author is indebted with great thanks to Prof. Dr. Rainer Drewello (University of Bamberg), Dipl. Rest. Carolin Pfeuffer (Europäisches Zentrum für Steinmetz und Steinbildhauer) and Dipl. Phys. Wolfram Köhler.40

References

Delgado Rodrigues J. & Grossi A. (2007). Indicators and Ratings for the Compatibility Assessment of Conservation Actions. Journal of Cultural Heritage, 8, 32–43. doi: doi:101016/j.culher.2006.04.007

Revez Maria J. (2016): Calculated Risk. The (In) compatibility of Built Heritage Cleaning Methods. Dissertation Faculty of Chemistry and Technology New University of Lisbon

Riegl A. (1903): Der moderne Denkmalkultus. Sein Wesen und seine Entstehung. In: A. Riegl: Gesammelte Aufsätze. Augsburg, Wien.

Waentig F., Dropmann M., Konold K., Spiegel E., Wenzel Ch. (2014): Präventive Konservierung. Ein Leitfaden. ICOM Deutschland – Beiträge zur Museologie Band 5. 96 Pages.

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NEW MARBLES FOR THE ITALIAN ARCHITECTURE (1920–40)

Roberto Bugini, Luisa Folli

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.

CNR-ISPC- Istituto per le Scienze del Patrimonio Culturale, via Roberto Cozzi 53, 20125 Milano, Italia

Abstract

The use of stone in the italian architecture improved dramatically in the period 1920–40, mainly for cladding on concrete structures. New methods of exploitation and supply made available different kinds of stone, never or only locally used before. Features of the stones are detailed together with examples from Milan.

Keywords: building stone, marble, 20th century architecture, Italy.

Introduction

The use of marble and natural stone in Italian architecture strongly increased in the third decade of 20th century, after a period (1900-20) when the “artificial stone”, made of Portland cement moulded in hundred of different shapes, was omnipresent (Biondelli 2004a). Many factors were involved: new architectural trends; rein-forced concrete framing where the stone lost any structural function, large availability of stone materials throughout the Italian territory; new methods of quarry working; cut of thin slabs suitable for cladding; improvement of trucks, trains, roads and railways. Moreover the stone industry was boosted by the policy (called Autarchia) planned by the Fascist government to hinder the Sanctions of the League of Nations (november 1935); so, despite a very low import from foreign countries (i. e. blue syenite from Larvik, Norway), dozens of new quarries were opened and the ancient ones were expanded. Acting in accordance with this policy, the reintroduction of the structural use of stone, in order to reduce the import of fuel and iron, (Portland cement and reinforced concrete), was also advised, but never applied (Peverelli 1939).

The return of the marbles on the building façades was really significant in Milan, the capital of the most important industrial area of Italy. A group of young architects was ahead of a very impressive urban change, juxtaposing huge scales or massive forms of Fascist public architecture and clean outlines or simple forms of residential buildings for the capitalist ruling class, according to the thesis of “Novecento” and “Razionalismo”.

Catalogue of stones and marbles

Different italian marbles never exploited in the past or only locally exploited were used in architecture, among many others. The configuration and shape of building stones were chosen according to the texture of each stone: slabs of various thickness (cladding or flooring, blending the gradation 42of colours or the arrangement of veins), moulded elements (decoration), squared blocks (pillar, column), monolithic pieces (column). The finishing of the surfaces was almost always the polishing.

Some significant stones of different nature, coming from different Italian regions (from Piedmont to Latium, from Friuli to Tuscany, from Lombardy to Liguria), are described in this study.

IGNEOUS

— Rosso Pantheon (red granite)

— Sienite della Balma (violet syenite)

— Diorite nera di Anzola (black amphibole gabbro)

— Porfido monumentale (purple ignimbrite)

SEDIMENTARY

— Zandobbio (pink dolomite)

— Ceppo di Grè/Ceppo Poltragno (conglomerate)

— Pietra di Chiampo (pink limestone)

— Pietra di Aurisina (grey limestone)

— Pietra di Finale (pink to yellow limestone)

— Rosso Amiata (red limestone)

— Portasanta di Caldana (red limestone)

— Travertino di Rapolano (light carbonate deposit)

METAMORPHIC

— Verde Alpi (green, white veined ophicalcite)

— Rosso di Lèvanto (red or green ophicalcite)

— Verde Roja (green clayey schist).

MARBLE

— Marmo di Valle Strona (grey, veined)

— Marmo di Lasa (white, veined)

— Fior di pesco carnico (grey, pink spots)

— Calacatta, Cipollino (white or green, veined)

Each stone is discussed in order to enhance: macroscopic and microscopic features together with decay phenomena caused by weathering agents (Biondelli 2004b); geological settings (Ispra 1976, Ispra 2012); quarry sites (Peverelli and Squarzina 1939, Pieri 1966); use and application in architecture, mainly in Milan, but also in other important cities (Grandi 1980, Gramigna 2001, Pierini 2017).