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Final Report or the Research and Development Project

Non-Destructive Field Tests in StoneConservationField and Laboratory Tests

Rapport rn Riksantikvariembetet 2006:4

Hlne Svahn

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Riksantikvariembetet

PO-Box 5405, SE-114 84 Stockholm, Sweden

Phone +46-8-5191 8000

Fax +46-8-5191 8083

www.raa.se

[email protected]

Project participants

The National Heritage Board

Misa Asp

Ragnhild Claesson

Runo Lfvendahl

Swedish National Testing and Research Institute

Katarina Malaga

Photos Hlne Svahn. Page 25: Katarina Malaga.

Layout Alice Sunnebck

Language editing Sue Glover Frykman

2006 Riksantikvariembetet

1:1

ISSN 1651-1298

ISBN 13: 978-91-7209-435-2

ISBN 10: 91-7209-435-4

Print 08 Tryck, Bromma 2006

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Contents

1. Summary 51.1 Summary of Field and Laboratory Tests 5

2. Introduction 72.1 Objectives of the Project 72.2 Project Background 72.3 Description of the Project 72.4 People Contacted 8

3. Gotland Sandstone Use and Characteristics 93.1 Mineralogy and Chemistry of Gotland Sandstone 93.2 Its Occurrence in Nature 93.3 Use as Cultural Stone: Building and Sculptural

Stone 93.4 Distribution in the Baltic Basin: Sweden, Denmark,

Poland, Germany, Russia and the Baltic States 103.5 Weathering Behaviour, Deterioration and Damage

of Gotland Sandstone 103.6 Paint and Gotland Sandstone 10

4. Conservation o Gotland Sandstone inSweden 124.1 Gotland Sandstone and Conservation 124.2 Conservation Methods in the First Half of

the 20 th Century 124.3 Examples of the Conservation of Gotland Sandstone

in the 20 th Century 124.4 Tord Andersson and Modern Stone Conservation 134.5 The 1980s: RIK and New Private Conservation

Firms 134.6 The Situation Today 14

5. Previous Research 155.1 Recent and On-going Research 155.2 Evaluation of Stone Conservation in an

International Context 155.3 Previous Evaluations Conducted by the NHB

in Sweden 165.4 NDT Methods used in Stone Conservation

in Sweden 16

6. Conservation and NDT Methods 186.1 Conservation and NDT Methods 18

6.2 NDT and Stone Conservation 186.3 Problems to be Analyzed in Stone Conservation 19

7. Description o the Field Tests 207.1 Description of the Field Tests 207.2 Weather Conditions 207.3 Methodology and Instruments 207.4 The Chosen Sites 25

8. Results 438.1 Results from the Field Test 438.2 Results of the Laboratory Studies 45

9. Discussion 479.1 Problems and Trends 479.2 Further Research 48

10. Notes 49

11. Bibliography 53

Appendices 63Appendix 1 Manual for Evaluation of Stone

Conservation Treatments in the Field 63Appendix 2 Result of the TRAMEX Moisture

Measurement 65Appendix 3 Result of Karsten Pipe Measurement 66Appendix 4 Result of Colour Measurements in the

Field 82Appendix 5 Result of the Granular Disintegration

Test 91Appendix 6 Result of the Ultrasonic Pulse

Velocity Measurements 92Appendix 7 Variation of Ultrasound Pulse Velocity due

to Changes in the Relative Humidity inLaboratory Conditions. Case Study forGotland Sandstone (by Katarina Malaga) 97

Appendix 8 Result from the Colorimetric Measurementin the Laboratory 102

Appendix 9 Capillary Water Absorption Calculationof w- and B-values 111

Appendix 10 Results: Field Measurements of UPV inStockholm for NHB (by Katarina Malaga) 112

Contents 3

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4 Non-Destructive Field Tests in Stone Conservation Field and Laboratory Tests

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1. Summary

Summary 5

1.1 Summary of Field and LaboratoryTests

The aim o this project is to evaluate a ewNon DestructiveField Tests (NDT) requently used in stone conservation onGotland sandstone. The ollowing report is the second and

nal o the project and ocuses on the eld and laboratorytests that have been undertaken. The rst report,Report 1.Non-Destructive Field-Tests in Stone Conservation Litera-

ture Study, was based on a literature study that discussedNDT methods and stone conservation. The eld tests wereconducted on Gotland sandstone on three occasions (overthe period o a year) on sixteen buildings in the centre o Stockholm. The stones were selected according to their age.Stones rom the 17th and up to the 20th century were ex-amined. The methods included aGranular DisintegrationTest with Herma Labels invented by the NHB,UltrasonicPulse Velocity (UPV) with a portable instrument, colori-metric measurements with aSpectrophotometer and waterabsorption measurements withKarsten pipes. The moisturecontent o the stone was measured using aTRAMEX Con-crete Moisture Encounter. [1]

The results o the eld tests were complemented withthree laboratory tests. The laboratory tests were conductedin order to test the methods in laboratory conditions as wellas to understand some o the properties and behavior o Gotland sandstone in di erent circumstances. These testsare the rst o a series o tests necessary to an understand-ing o the variations o the properties and weathering o Gotland sandstone. The tests were:

1 Variation o Ultrasound Pulse Velocity (UPV) due tochange o relative humidity on Gotland sandstone

2 Variation o colorimetric measurements with a MinoltaSpectrophotometer due to heat, cold and moisture con-tent o Gotland sandstone

3 Measurement o the w- and B-value o Gotland sandstonerom the Valar quarry. [2]

The results o the eld study have not yet been ully ana-lyzed. [3] One o the most important issues mentioned inthe report is the necessity o knowing the conservation his-tory o the particular stone that is going to be evaluatedand tested to achieve accurate results. As such in orma-

tion is un ortunately o ten lacking in the most extensive o conservation reports, one o the recommendations o this

report is that the conservator-restorer [4] should producemore precise conservation documentation in an organizedand systematic way. The treatments and in ormation gath-ered during conservation have to be mapped on drawings o an appropriate scale, approximately between 1:5 and 1:10,depending on the size o the object. Re erence areas shouldbe le t or uture evaluations o both treated and untreatedstone.

Even though questions still remain about some o themethods and interactions with Gotland sandstone, two pre-liminary weathering indexes or Gotland sandstone havebeen created (that classi y whether the stone is in good, in-termediate or poor condition and severly weathered, some-what weathered or in good condition) that can help to de-termine the condition based on UPV measurements andgranular disintegration tests. However, urther laboratorytests are required to con rm the accuracy o the indexes.

The Karsten pipes give important complementary in or-mation that, above all, helps to understand the absorptionand penetration o water in the stone material. This know-

ledge is necessary, or example, to an understanding o thedurability o a hydrophobic treatment. The Karsten pipemeasurements are also use ul to an understanding o thecondition o the stone, although one immediate problem ishow the data should be presented. In this report, the resultsare presented in graphs that acilitate an understanding o how water penetrates and also graphically demonstrateschanges in water absorption. As the graphs do not give anycomparable values that help to classi y the condition o thestone, the w- and B-values might provide a better alterna-tive, even though they comprise mathematical approxima-tions to evaluate the condition o the stone.

It has been established that the UPV is the best NDTmethod to determine the condition o the stone, the reasonbeing that the UPV measurement gives a quantitative val-ue that directly corresponds to the properties o the stone.The method poses som uncertainties, however, such as pos-sible disturbance by salts. The salt disturbance has not yetbeen ully tested. Measurement with the Karsten pipes andthe calculated w-values rom this measurement, include, onthe other hand, a series o mathematical approximationsthat lead to urther uncertainties. Correlations between theage o the stone, the Karsten pipes and the UPV were deter-

mined rom the eld tests and prove that there are correla-tions between the Karsten pipe and the UPV. These need to

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be tested urther in the laboratory, however. There is more-over a distinct correlation between the age o Gotland sand-stone and the UPV; approximately a decrease o UPV o 3km/s per 100 years (see Appendix 10).

Furthermore, the colorimetric measurements with theSpectrophotometer created di culties, especially with theinterpretation o the data. While the data is easy to com-pare, especially the di erences in lightness (L* value), thequestion o the actual nature o the colour change arises.The L* value might be a good indicator o how ar the stonehas deteriorated, but could also depend on the presence o black crusts, dirt, biological growth and variation in mois-ture content. The method has already been tested by theNHB to look or changes in restoration mortars and en-countered such problems. To achieve data that actually de-rives rom changes in the stone itsel it is necessary to takesamples to check the nature o the change. It would seemthat there are simply too many uncertainties. Despite this,

colorimetric measurements can give reliable results in theeld when the di erences in lightness (L* value) are high.It is probably better or controlling resoiling a ter cleaningthan to understand chemical change. The method is, aboveall, best suited to laboratory conditions.

The granular disintegration test has to be urther ana-lyzed be ore it becomes a standard method. The result o

the eld tests demonstrates that the di erences between themeasurements were too small to be reliable. Hence, the testhas to be urther tested and developed in laboratory condi-tions i it is to be use ul.

To some extent all the NDT methods depend on moisturecontent, except perhaps the granular disintegration test. Themost common NDT instruments used or measuring mois-ture, which have also been used within this project, are dis-turbed by the presence o salts and only measure the moisturecontent at the sur ace. Some NDT methods, such as the mi-crowave method and the neutron moisture meter measure-ment, are supposed to be better (even though more expensive see Report I). Nevertheless, urther testing is advisable.

The laboratory tests were designed to support tests in theeld. Fresh Gotland sandstone rom the Valar quarry had

a relative low compressive strength depending on the lami-nation o the stone and a high water absorption capacity(relatively high w-value and B-value). The UPV test demon-

strated that the UPV depends on the moisture content, thelength measured, the direction o the measurement accord-ing to the lamination o the stone, and whether the meas-urement is indirect or direct. Colour measurements dem-onstrate that it is sensitive to moisture content and slightlyto temperature changes, which con rms that colorimetricmeasurements have to be used with care.

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2. Introduction

Introduction 7

2.1 Objectives o the ProjectFor several years the NHB in Sweden has striven to monitorand evaluate stone conservation treatments. One o the rea-sons is that the conservation methods, which are requentlyused in Sweden, sometimes ail. Another reason is that theevaluations will make it possible to recognize when it is nec-essary to treat the stone again. Above all, the evaluationsmay help to reject poor or disputable methods and improveconservation.

In previous evaluations conducted by the NHB, the meth-odology was rst and oremost based on visual and tactilemethods. Sometimes quantitative methods were also used,such as the Drill Hardness Meter and the Karsten pipes. Theobservations and measurements were compared with in or-mation ound in conservation reports. This methodologyhas weaknesses, or example, in ormation and data relatingto the condition o the stone during and shortly a ter conser-vation are o ten lacking. Such in ormation is necessary to anunderstanding o i and why the conservation ailed.

The purpose o this project is thus to improve the evalu-ation o stone conservation in Sweden using NDT methodsthat give comparable quantitative data. To achieve this goalit will be necessary to adjust the conservation process to in-clude a greater emphasis on pre-investigation and documen-tation. Pre-investigations should include a selection o areaswith zero values taken with NDT methods. Each NDTmethod naturally has advantages and drawbacks and theseare not always explained in the literature. The NDT meth-ods used in this project have been evaluated in an attempt to

nd out more about their possible uses.The project is divided in two stages or steps;

1 A literature study based on con erence articles and con-servation literature and complemented with interviews.

2 Laboratory and eld-studies. Some chosen NDT methodswere tested and evaluated. The methods are available inSweden and easy to usein-situ. The tests were made onGotland sandstone as it is one o the most requently con-served stones in Sweden.

The ultimate goal is to create a manual with instructions re-garding what kind o in ormation is required be ore, duringand a ter conservation (and how it should be collected). Apreliminary manual has been compiled see Appendix 1. Itis hoped that the methodological manual will lay the oun-

dation or regular routines within the conservation process.It will, or example, make it possible to evaluate whetherthe conservation has been success ul and is durable. The ullambition is, un ortunately, not possible to achieve withinthis project as more research is needed together with an im-plementation strategy managed by the NHB.

2.2 Project Background

Between 1989 and 1995, the Swedish NHB managed anddirected the ambitious Air Pollution and the Cultural Her-itage programme. The programme resulted in a nation-wide inventory o sculptures and decorative stone in build-ings rom early medieval times until the 1940s (publishedin the Series o Swedish Building Stones) and in a databasecontaining an inventory o used stones, their age, locationand previous treatment. Moreover the programme led toa greater awareness o stone deterioration, a tremendousnumber o new research projects, and urther develop-ment o stone conservation methods in Sweden. As a con-

sequence, the stone conservation eld improved and manytreatments were carried out. Since the programmes conclu-sion in 1995, only a ew attempts have been made to evalu-ate the result o all these e orts. This project undertakespart o this work.

2.3 Description o the ProjectThe project has been divided into two parts,Step 1 andStep 2.

2.3.1 Step 1Literature study o NDT methods used in stone conserva-tion and discussion with experienced scientists and conser-vator-restorers in Sweden and abroad. The aim is to learnmore about the NDT methods and to establish their key ad-vantages and disadvantages.

The methods studied include:

Methods or measuring the relie /roughness o the stonessur ace, such as pro lometric stylus measurements andlaser scanner methods.

Methods or measuring the water-soluble salt content inthe stone, or example, the L vendahl method.

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Methods or measuring the water absorption, or exam-ple, the Karsten pipe, the Mirowski pipe and the Italiancontact sponge test.

Methods or investigating the inner structure o the stone,such as tomographic methods.

Methods or analyzing substances on the sur ace o thematerials, or example, a portable FT-IR.

Methods or measuring the strength and mechanicalproperties o the stone, i.e. a method that gives in orma-tion about the condition o the stone, or example, ultra-sonic measurements and micro drill resistance (which isdestructive).

Methods or measuring the moisture content in the stone,such as the microwave method.

Methods or measuring colour change, or example, theSpectrophotometer.

Some o the theory behind these methods is discussed, to-gether with their practice and use in stone conservation.

2.3.2 Step 2

The testing o our NDT methodsin-situ on Gotland sand-stone on ourteen buildings in Stockholm. Gotland sand-stone is also tested in the laboratory. This step also includeda visit to Gotland and some o the open and closed quarries.The individual parts include:

1 Variation o U ltrasound P ulse V elocity (UPV) due to thechange o relative humidity on Gotland sandstone.A lab-oratory test programme designed to analyse how the UPVis changed depending on the relative humidity and watersaturation in Gotland sandstone. The aim o this study isto analyse the stone material in laboratory conditions andcompare this to observations made in the eld. [5] Thetests were conducted by Dr. Katarina Malaga atSP, theSwedish National Testing and Research Institute, locatedin Bors. The programme was initiated in July 2005 andconcluded in June 2006.

2 Variation o colorimetric measurements with a MinoltaSpectrophotometer due to heat, cold and moisture con-tent on Gotland sandstone . Testing o the variations o

colorimetric measurements with a Minolta camera. Thecamera was tested during di erent climatic conditions.The tests were conducted in the NHBs stone atelier inFebruary 2006.

3 Measurement o the w- and B-values o Gotland sand-stone rom the Valar quarry. The aim was to learn moreabout the properties o Gotland sandstone. The test was

conducted using the German capillary suction standardtest DIN 52 617. The tests were undertaken in March2006 in the NHBs stone atelier.

4 Field test programme o our NDT methods used in stoneconservation on Gotland sandstone objects in Stockholm.The methods included:

a. Measurement o water absorption using the Karstenpipe.b. Ultrasonic Pulse Velocity (UPV) measurements (con-ducted by Dr. Katarina Malaga, SP).c. A granular disintegration test with a tape test usingHerma labels (invented by the NHB).d. Measurements o the colour o the stone using a Mi-nolta camera.

The methods were tested on three occasions over the peri-od o a year and in di erent climatic condition (in Augustand October 2005 and in May 2006) in order to register

variations in temperature and moisture. Eighteen stoneobjects on sixteen buildings in Stockholm were chosen(see Chapter 7.3). The criteria or choosing the objectswere:

Accessibility o the building. The age o the stone (with a range rom the 16th to the

20th centuries). Knowledge o the conservation history o the stone ( or

this purpose several art historians, conservators andarchitects were consulted).

5 A literature study o the geology, deterioration and con-servation o Gotland sandstone. Archives, reports and ar-ticles have also been consulted in order to nd out moreabout the history o stone conservation especially onGotland sandstone in Sweden. The aim o this studywas to ormulate an understanding o what might havehappened to the stone.

2.4 People ContactedIn order to nd out more about the practice o NDT in stoneconservation in other European countries several well-

known conservation scientists were contacted by email ortelephone. [6] Moreover, a visit was made to Florence, toone o the three governmental research institutes concernedwith the conservation and restoration o works o art in It-aly (ICVBC-CNR). The contact at the institute was Dr. Su-sanna Bracci, who kindly handed over the new Sponge testelaborated by Dr. Piero Tiano (see Report I).

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3. Gotland Sandstone Use and Characteristics

3.1 Mineralogy and Chemistry o GotlandSandstoneGotland sandstone (Burgsvik sandstone) is a Silurian sedi-mentary negrained and homogenous sandstone. The col-our is grey; the nuance varies depending on site and strati-graphic level. There are usually very ew tints in the stone,although exposed stone o ten becomes brownish in colouras it ages. [7] The mineralogy and chemistry o the stone

varies according to its location in the quarry and rom quar-ry to quarry. According to Wessman, the stone rom the Val-ar quarry sometimes has weak veins parallel to the beddingplanes, which contain clay minerals. [8] The matrix o thestone is chiefy calcite and the calcite content is 515 wtpercent. The relatively high CaO and CO2 content, as wellas the relatively high amounts o Al3O3, Fe2O3, MgO andK2O, are typical to the stone. [9] A small amount o silica ce-ment is o ten present in the stone as well. Some researchersdescribe the silica cement as amorphous and surroundingthe quartz grains, while the calcite cement is located in thepores. [9] Wessman has examined thin sections o Gotlandsandstone rom the Botvide, Uddvide and Valar quarries andnoted that the stone consists entirely o quartz grains withempty spaces in between (the calcite cement was hardly visi-ble). [8] The grains consist primarily o quartz and eldsparsand there are small amounts o mica and calcite. The stone

urthermore contains small amounts o pyrite seldom ex-ceeding one per mille and small amounts o glauconite, li-monite and jarosite. [7] The quartz grains vary in size theBotvide and the Uddvide sandstone grains are between 0.1to 0.2 mm, while the sandstone grains rom Valar are 0.05to 0.15 mm. The clay minerals look like brown rods o be-

tween 0.2 to 0.4 mm in length and are normally orientedin the same direction as the bedding. [8] The stone has ahigh porosity, 523 percent per volume, and the compres-sive strength o the stone is ca 5080 MPa. [10] The averagepore size in one test was 13 m. [8] The stone thus has a veryhigh absorbance capacity; between 5 and 9 percent o thetotal weight. [10] The ultrasonic velocity o resh Gotlandstone is approximately 2.52.7 km/s, the true density 2680kg/m3, and the bulk density ca 2200 kg/m3. The w-value o the Valar stone is ca 5.9 kg/m2 / v h while the B-value o theValar stone is ca 0.45 mm/ v h ( or an explanation o the w-and B-values, see below and also Appendix 9).

3.2 Its Occurrence in NatureThe stone is ound in the Silurian Burgsvik bedding layer,close to the coast in the south o Gotland. The ormationoutcrops along a 35 km horizon on the western banks o Storsudret in Grtlingbo parish. The stone is also ound inBurs and Nr and in a small area o Frjel (also in the southo Gotland). The sandstone is sandwiched in a limestoneenvironment and the beddings are a maximum o six metresthick (the ormation is all together up to 50 metres thick).The beddings are not homogenous there are some calcar-eous and sand/clay layers between the sandstone beddings.The structure and orientation o the stone indicates that ithas been ormed as sandbars in shallow water close to anancient beach. A survey made by a geological consultancy

rm in 1989 concluded that the Valar stone is the best stoneor building purposes. The Valar stone is di erent to other

stones in that it is brighter in colour, more ne-grained andhas a lower clay content. It is also lithi ed (and consequent-ly stronger). [11]

3.3 Use as Cultural Stone: Building andSculptural StoneGotland sandstone has been used or sculpture and build-ings in the entire Baltic region since early medieval times.It is one o the most widespread decorative stones in Swe-den. The main reason or this is that it is easy to shape. Inthe Stone and Viking Ages it was used to make whetstones,tombs and tombstones. Some rare examples o the amouspicture stones rst erected on Gotland 300 100 A.D. andsome later stones were made rom Gotland sandstone. [12]

It was not until medieval times that it really became wide-spread. It was, or example, used or baptismal onts thatwere exported in the Baltic region during the 12th and 13th centuries. During the 13th century and until the middle o 14th century it was also used as building material, or sculp-tural riezes and portals on Gotland, mostly on churches.Some o the most amous churches entirely constructed inthe stone are ja, Sundre, Hamra, Fide and Grtlingbo. Useo the sandstone declined at the beginning o Danish rule inthe 14th century. Nevertheless, Glimmingehus in the southo Sweden (which was part o Denmark at the time) wasbuilt in lime and sandstone rom Gotland by Jens Holgers-son Ul stad in 1499. [12]

Gotland Sandstone Use and Characteristics 9

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Infuenced by the Dutch Renaissance, decorative stonebecame ashionable in the 16th and 17th centuries. This ledto a reopening o the quarries. In the beginning the quarrieswere controlled by the Danish kings Christian III, FredrikII and Christian IV. Initially the Danish kings took stones

rom Skne, but at the end o the 16th century they beganto explore the quarries o Gotland. The Danish kings sendexperienced stone masons to Gotland to restart production.Hence Kronoborg Castle in Copenhagen was built withsandstone rom Gotland in the 1570s as well as Fredriks-borgs Castle in Helsingr by Christian IV. The stone in thesouthern part o the Valar quarry was used by the Danishking. This ashion or stone led to the production o many

acings, sculptures and portals in Gotland sandstone in pal-aces in Stockholm as well as in Denmark, Germany and Po-land. The quarrying o stone continued when Gotland be-came Swedish in 1645 and well into the 18th century. [12]Some amous examples rom this period include the Roy-

al Palace in Stockholm and the Swedish Kings MemorialChapel at Riddarholm Church in Stockholm. The use o the stone declined during the neoclassic period at the end o the 18th century, to become very popular again in the 1890suntil the beginning o the 20th century. The use o Gotlandsandstone as building stone nally stopped in the 1920s.Nowadays the stone is quarried principally or restorationpurposes. A ew quarries are still open: the Valar quarrywhich is quarried by Slite Stenhuggeri, and the quarries inHusryggen and Botvide where small quantities are quar-ried by stonemason Jan Kviberg at Burgsviks stenmuseum(Burgsviks Stone Museum).

3.4 Distribution in the Baltic Basin:Sweden, Denmark, Poland, Germany,Russia and the Baltic StatesThe distribution o Gotland sandstone in Sweden was sur-veyed during the air pollution programme. Hence, it is pos-sible to trace the stone in Sweden by searching the databaseo decorative stone at the NHBs website (http://www.kms.raa.se/cocoon/nat/in o.html). The database has registered626 objects in Gotland sandstone. These are located romUme in the north to Ystad in the south. The stones areto be ound in: Stockholm (341 objects), Srmland (49 ob-jects), Uppsala (41 objects), Skne (35 objects), Gotland (30objects), stergtland (26 objects), Kalmar (24 objects),Vstmanland (16 objects), Vstra Gtaland (13 objects),rebro (10 objects), Gvle (10 objects), Blekinge (7 ob-jects), Sundsvall/Hrnsand (7 objects), Jnkping, (6objects), Halland (3 objects), Jmtland (2 objects), Ume(2 objects), Dalarna (1 object), Kronoberg (1 object), andVrmland (1 object). In Denmark, ca 300 portals are madeo the stone in addition to many acings and castles. [13]

Gotland sandstone is also ound in Germany, or example inStralsund, Lbeck (the Lbecker Rauthaus), Wismar, Ros-

tock and Grei swald as well as in the north o Poland, suchas Gdansk. It is also ound in St Petersburg and in the Balticstates. [14]

3.5 Weathering Behaviour, Deteriorationand Damage o Gotland SandstoneGotland sandstone is o ten the subject o severe deteriora-tion, mainly caused by calcite cement and clay impuritiesthat easily dissolve and swell. The calcite content sometimesreacts with acidic constituents to produce gypsum, which isusually a constituent in black crusts. Anders Nord and ToreEricsson have investigated black layers on Gotland sand-stone (among other stones) in Europe. The samples weretaken rom di erent locations in Sweden, Poland, Denmark,Germany, Hungary, the UK and France. The layers they ex-amined were thin: 0.02 to 0.2 mm. [15] Acid rain may alsoa ect the pyrite in the stone and create iron composites

that cause the stone to deteriorate. The rough sur ace o thestone also makes it easy or soot and metal particles to stickto it. [16] This deterioration is ollowed by granular disin-tegration, sanding, ex oliation and the ormation o blackcrusts. The structure o the stone also leads to damage, as itshigh porosity makes it subject to water penetration, whichin turn is ollowed by a series o synergic damaging e ectscaused by acid pollution, salts and reeze-thaw cycles.

3.6 Paint and Gotland SandstoneIt has been known since medieval times that Gotland sand-stone deteriorates easily. Thus, the stone was o ten impreg-nated with oil and painted or protection. The paint also hada decorative unction. The use o paint or protection wasdocumented during the construction o the Royal Palace inStockholm. The architect noted that the resh Gotland sand-stone had to be impregnated with linseed oil and painted toprevent it deteriorating. The Royal Inspector, Carl GustavTessin, wrote in 1748: urthermore should the old and therecently erected stone at the Royal Highnesss new Palace,that is subjected to rain and bad weather, be painted with oilpaint. He also noted that the paint was or the conserva-tion o the stone. [17]

The paint o the Royal Palace was not maintained, how-ever, so that by the end o the 19th century the stone was ina critical state o conservation. A scienti c committee wasset up to investigate the state o conservation o the acade.The committee concluded that the paint on the stone hadno protective unction. It was believed that the paint wasused or aesthetic reasons to hide the natural de aults in thestone. The paint remains were removed in 1897 in orderto establish the true condition o the stone. This coincidedwith the ashion or bare unpainted stone. The removal o

the paint epitomized the aesthetics o the time a desire orpure stone without any alse colour. Paint was thus

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regarded as somewhat deceit ul and hiding the true struc-ture and natural material. This infuenced the restoration o buildings. During the entire 20th century paint was removed

rom many stone acings and sculptures.The southern portal at Jacobs church in Stockholm is one

example o how the ashion or pure stone infuenced deci-sion-making. It was restored between 1909 and 1910 anda newspaper article explained the situation like this: Theportals have been treated with piety. No sharp tools havetouched them quite the contrary the old paint has beenheated and a terwards blown or brushed away. Accordingto government inspector Carl Mller, the work could nothave been conducted in a better way or with more care.Thus they are (i.e. the portals) now ound in a remarkablestate o conservation, where each original mark o the chiselhas been ully recovered. This quotation demonstrates thatpaint was seen as something that needed to be removed inorder to achieve an authentic appearance.

In the 1980s the rst conservator-restorers in Swedencontinued to remove paint layers (there are examples where

the paint was le t in-situ). Sometimes they documented thepaint, such examples being Kaggs Memorial Chapel in Flo-da Church 1989, Nikolai Church in rebro in 1992 andFiholm Castle in 1994, but not always. Nevertheless paintwas removed without examination. It was stated that thepaint was harm ul to the stone, since it wouldnt let thestone breathe and the conservator-restorers tried to ex-tract the oil and remove the paint (using paint strippers, am-moniac or hydrogen peroxide). Despite these actions a lot o paint still remains. It was not until the end o the 20th cen-tury that the paint was nally noted or its decorative andpossibly protective unction. However, this does not makeit easy to repaint the stone. As the stone has been le t unpro-tected or a long time it has deteriorated and been subjectedto various treatments, such as ceresin (wax), waterglass andacid cleaning, all o which may have le t salts. In addition ithas not yet been scienti cally established that linseed paintactually has a protective unction. There may be better al-

ternatives. All being well this will be tested in a orthcomingproject managed by the NHB.

Gotland Sandstone Use and Characteristics 11Gotland Sandstone Use and Characteristics 11

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4. Conservation o Gotland Sandstone in Sweden

4.1 Gotland Sandstone and ConservationGotland sandstone has always caused conservation prob-lems. Problems occurred over a long period o time andsome buildings have thus been repeatedly conserved, suchas the Karolin Memorial Chapel at Riddarholms Church,the House o Lords, Saint Jacobs Church and many portalsin the Old Town in Stockholm. Several examples are also tobe ound outside Stockholm, such as o portals and memo-rial chapels like Vadsbro and Tyres in Srmland and onseveral manor houses and buildings in the south o Swe-den. Many examples are also ound abroad, such as theRathaus in Lbeck which has been examined and conservedthroughout. [9]

Until the 1980s the conservation o stone was conduct-ed by stone masons. They used traditional handicra t tech-niques and worked or architects; sometimes with scienti caid rom engineers. The deterioration o stone was sporadi-cally studied by scientists, or example the Royal Palace inStockholm in 1897 and the House o Lords in the 1890s.New stone conservation methods were sometimes tested.Two examples are the Deckosit method in the 1940s andsilicic acid esters in the 1970s. When important sculpturesneeded conservation in the 1940s and 1950s painting con-servator-restorers, or example Erik Olsson on Gotland andBo Wildenstam in Stockholm, were o ten employed to dothe work. They were not especially knowledgeable aboutstone, but had experience in mural painting.

4.2 Conservation Methods in the First Hal o the 20 th Century

Stone masons that worked with stone conservation in Swe-den in the rst hal o the 20th century cleaned the stonewith abrasives (abrasive sandpaper and knives), heat, acidsand alkali solvents. Paint remains were removed with di er-ent kinds o chemical paint strippers, with calcium hydrox-ide or heat. The stone was consolidated with linseed oil orCeresin, an industrially manu actured mineral wax thatwas used to impregnate gypsum and stone (diluted with car-bon tetrachloride, CCl4). According to Varulexikon (TheBook o Goods 1894), Ceresin was a mixture o mineralwax melted with sulphuric acid and stearin. It was there-a ter treated with potassium hydroxide and ltrated. Ca-sein, resins, limewater, para n, fuats and waterglass solu-tions might also have been used. There are some documented

examples o the use o waterglass, or example, it was usedon the portal o Hablingbo Church in Gotland in 1955. Theengineer Wibeck wrote that the weathered parts were con-solidated with Silicaseal and the rest o the stone withSnland Everdry. Silicaseal was probably a kind o waterglass and Snland Everdry might have been a hydro-phobic treatment. Di erent kinds o arti cial stone wasused to repair the stone, such as Deckosit rom Denmark.This was a kind o synthetic stone made rom ground sand-

stone mixed with nitrocellulose and used to ll in and coveraults in the stone. The product was probably introduced toSweden by the architect Ove Leijonhu vud. Another akestone method, the Hkansson method (see the descrip-tion below o the German Church) was used in the 1920s inStockholm. Naturally, cement was also used and sometimesthe stone was replaced by new natural or cast stone.

The Royal Palace architect, Ove Leijonhu vud, (in therst hal o the 20th century) became especially interested

in stone conservation. He made conservation proposals orseveral important buildings paying special attention to thestone, such as the Royal Palace, the House o Lords, theGerman Church and Saint Jacobs Church in Stockholm.In order to learn more about stone conservation he wroteto the Director o Works at Westminster Parliament, FrankBaines, in 1926. Baines supervised a commission set up toinvestigate the conservation o the damaged stone. Bainesreport gives an idea o the state o the art in stone conserva-tion in the 1920s. He wrote, or example, that the commis-sion had not been able to nd any conservation method orproduct that was e ective and durable enough. He thoughtthat it was better to use a simple lime-wash than limewaterto consolidate the stone. But no durable and e cient con-

servation methods were available and all the methods thathad been tested in England and France had ailed (such aswaterglass). This was the situation until the 1970s when al-coxysilanes entered the eld.

4.3 Examples o the Conservation o Gotland Sandstone in the 20 th Century The southern portal in Gotland sandstone at Saint Jacobschurch in Stockholm is an example o how conservationwas conducted in Sweden during the 20th century. The paintwas removed in 1910 and the portal conserved by the rmo Augusto Conte in 1929 (which specialized in stuccowork) according to Leijonhu vuds conservation propos-

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al. The portal was cleaned with brushes (whether dry orwith water is unknown) and consolidated with an oil-resinmixture. In 1941 the portal was conserved again, this timewith Deckosit. It was conserved again in 19681969 un-der the surveillance o engineer Ingemar Holmstrm. Thisconservation included cleaning with caustic soda (NaOH)mixed with lime (to remove all paint). A ter cleaning theportal was washed several times to neutralize the stone andremove the salts. Finally the stone was consolidated withlimewater sprayed our to ve times.

Silicic acid esters were used early on in Sweden or stoneconsolidation. One example o this is the House o Lordsin 1971. The architect Ramel employed a Danish restora-tion company, Convestol, to conserve the stone. Thiscompany specialized in a new German method that con-solidated the stone. The aade was rst cleaned with analcali product, neutralized with a weak acid and thencleaned with water. The damaged ornaments were consoli-

dated with a silicic acid ester mixed with silicone. The stonewas repaired using an arti cial stone, Minero, made o hydraulic lime, sand and trass. All the stone and brick was

nally impregnated with 5 percent SIOTOL 50 (probablya hydrophobic treatment). [18]

4.4 Tord Andersson and Modern StoneConservationUntil the 1970s, stone conservation was not a specialist eldo interest in Sweden. The mending and replacing o stonewas, as we have seen, undertaken by stone masons. ModernSwedish stone conservation was established by Tord An-dersson at the end o the 1970s. Andersson worked at theNHB rom 1972 until 1989. In the mid-1970s he was work-ing as an archaeologist when a new conservation labora-tory was established at the technical institution. Anderssonbecame the stone expert and above all a promoter o stoneconservation in Sweden. For example, he contributed tothe development o the pro ession by encouraging the edu-cation and training o stone conservators. Andersson hadlearned stone conservation at the ICCROM course in Ven-ice in 1976 and continued to travel in Europe to learn more

about stone conservation. During his time at the NHB thestone department expanded and by the end o the 1980s ap-proximately ve stone conservators were employed.

Technically Andersson also introduced new cleaningmethods, such as paper pulp and/or clay compresses made

rom Italian recipes (Mora & Mora such as mixtures o am-monium hydrogen carbonate, EDTA and other solventssuch as ammoniac). The consolidation o Gotland sand-stone was an important issue to address: acrylate disper-sions were tested and nally he introduced the silicic acidesters. The German tetraethylorthosilicate (TEOS) Wack-

er Stone Strengthener OH became a standard product (itis still the main product in Sweden). It was probably duringthe restoration o House o Lords in 1980 that Andersson

used the Wacker OH or the rst time on Gotland sand-stone. In the beginning Andersson recommended the re-moval o paint remains by using a paste made o Bentoniteclay, Carbamid and Glycerol (made rom a recipe used bythe conservator-restorer Kenneth Hempel at the Victoriaand Albert Museum). [19] Later in his career he becamean ambassador or repainting the stone with linseed oil. Atthe beginning o the 1980s caustic soda was used or dis-in ection, and later on, biocides such as Cetavlon (Cetri-moni Bromidium), Ra ex, Arrow Super Clean and Beloran(it was or instance used at Stnga church, Gotland 19881989). These products proved to be useless in the long-termand were abandoned at the beginning o the 1990s. Some-times hydrophobic treatment was conducted (using WackerStone Strengthener H or wax). The hydrophobic treatmentwas disputed among conservators, however (see below).For mending the stone, Billys stone glue (Billys stenlim a polyester based glue used to mend the stone had existed on

the market since the 1950s) and Billys replacement mortar(acrylic based solution mixed with grinded stone, and somecement and lime) was used. Other synthetic glues were alsoused, such as epoxy resins or major cracks and Paraloid B-72 or repairing minor damages, e.g. small fakes.

4.5 The 1980s: RIK and New PrivateConservation FirmsDuring the 1980s stone conservators at the NHB worked al-most alone in Sweden. There were no private stone conser-vation companies and almost all stone conservation workin Sweden was conducted by the NHB. This changed at theend o the 1980s, however, when many ormer NHB em-ployees set up private rms. This was a natural developmentin the elds growth.Stenkonservatorn opened in 1987 ledby conservator-restorer Marie Klingspor and in the ollow-ing year the Polish conservator-restorer Leszek Zakrzewskijoined the company, together with Dr. Daniel Kwiatkowski(both had been educated at the Kopernicus University inTorun, Poland). Prolithos opened in 1988 led by conser-vator-restorers Jarema Bielawski and Gert hrstrm (bothtaught by Andersson). The conservator-restorer Karl Gusta

Eliasson (also taught by Andersson) became established onGotland at the beginning o the 1990s.

The end o the 1980s was there ore a dynamic period orstone conservation in Sweden. In part this was due to Polishinfuence, but in the main was infuenced by theAir Pollu-tion and the Cultural Heritage programme, launched by thenewly established Conservation Department (RIK, led bythe engineer Dr. Ul Lindborg) at the NHB between 1989and 1995. This campaign enlightened the eld enormous-ly. Stone conservation work at the NHB grew considera-bly and ve to ten people were employed (both conserva-

tors and scientists), many educated at the new conservationschool at Gothenburg University. RIK planned, supervised,managed, controlled and urthermore conducted the con-

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servation o stone monuments in Sweden (approximately245 works). The Polish infuence was on the other hand ac-ademic; Kwiatkowski had conducted research on Gotlandsandstone and methods like Billys replacement mortar weretested scienti cally in the laboratory by Marie Klingsporin Poland. They were the rst academically educated stoneconservator-restorers in Sweden, and were closely ollowedby the rst educated stone conservator-restorers rom theUniversity o Gothenburg.

A lot o research was initiated by RIK, such as the study o alcoxysilanes and stone weathering by Oliver Lindqvist andPernilla Elving at Chalmers University o Technology, workon the rost resistance o natural stone by Lubica Wessmanat Lunds Technical University as well as work undertakenby Gran Fagerlund at the same university, surveys on build-ing stone types and quarries, studies on the weathering o stone by Paul Frogner, P. Schweda and L. Sjberg at Stock-holm University, and so orth. A lot o these research e orts

were per ormed on Gotland sandstone. The NHB also con-ducted research, such as the investigation o black crustson Gotland sandstone by Anders Nord and Kate Tronner.Runo L vendahl invented methods or documentation andsalt evaluation. Furthermore, a comprehensive inventory o decorative building stones in Sweden was initiated and re-sulted in several publications and reports (Natursten i bygg-nader), together with a database o all Swedish decorativebuilding stone (see above). NDT methods were also tested,such as UPV measurements on the Gustav Adol monumentin Gothenburg and studies on NDT methods were carriedout at the Royal Institute o Technology in Stockholm. Thesee orts led to an increased contact with European research-ers in the eld. One particular area o interest concernedhydrophobic treatment. In this Swedish stone conservation

orged its own way. We have seen that conservators some-times used hydrophobic treatment, although it was suspect-ed that the methods actually might harm the stone. This ledto discussions and nally, in 1993, it was decided that suchtreatments should stop. A ter this hydrophobic treatments

are rare. At the end o the 1990s and beginning o 21st cen-tury, historians discovered that Gotland stone had beenpainted. The question was raised as to whether it providedbetter protection or the stone. Tord Andersson agreed anda campaign to repaint the stone with linseed oil was initiat-ed. In some places the stone was painted, although withoutscienti c examination or proo . Today the question is stillan open one, and much debated.

4.6 The Situation Today When the Air Pollution programme suddenly ended in1995, Swedish stone conservation became almost entirelyprivatized and many o the research and development e -

orts came to an end. Almost all o conservator-restorersand researchers at the NHB either had to leave or change oc-cupation. Today approximately only three individuals workat the NHB with stone questions (two conservator-restorers

and one geologist). This naturally limits possibilities. Noteven universities are conducting any research in this area one exception being Malin Myrin whose PhD work rom2006 at Chalmers University o Technology concerned theevaluation o stone conservation on Gotland sandstone.There are approximately ve to ten private stone conser-vation rms operating in the eld. They use more or lessthe same methods and products that were introduced dur-ing the 1990s. A ew new materials have been introduced,though, such as the Arte mundit in 2005 (a kind o latexEDTA lm that strips o the dirt) [20] and some research isbeing conducted by the NHB, albeit on a small scale. Thisincludes research on injection mortars by Misa Asp and thetesting o mending mortars in Klla church at land. De-spite these e orts it is apparent that research in many otherEuropean countries, as well as that in the USA and Canada,is much more extensive. It is lamentable that results o thiswork and research are not ollowed up (i so, only sporadi-cally) by Swedish conservator-restorers due to lack o time,contacts and nancial resources.

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5. Previous Research

Another interesting example is the evaluation o stonesculptures and monuments that have been treated over thelast twenty years in Austria (evaluated between 2000 and2002 by Nimmrichter and co-workers). It comprise bothoral in ormation and the testing o objects; both using NDTmethods (Karsten, UPV, drill resistance, electrical conduc-tivity, knocking by hand, colour description and so orth)and sampling. The results were quanti ed: in 55 percent o the conservations the long-term e ect o the conservation

was good, while in 10 percent o the cases the conservationtreatments had actually caused new deterioration. More-over, Nimmrichter and co-workers pointed out that con-servation reports werent su ciently systematic and lackednecessary data. This is a comment that is o ten ound inevaluations! The nal conclusion was that more scienti cpre-work and scienti c ollow-up controls, such as UPV, areboth important and necessary in conservation work. [25]

Recently a lot o in-situ evaluations o previous conser-vations have appeared in publications. This is a natural de-velopment, since the eld o stone conservation has grownconsiderably during the second hal on the 20th centuryand it is now time to evaluate what has been done. As wehave seen in the Austrian case, evaluations also expose di -

culties since all the parameters that cause damage are notknown. The reason or this is that conservation documenta-tion doesnt always give su cient data and also that conser-vator-restorers seldom leave re erence sur aces untreated.Nevertheless, there are some recent examples o success ulevaluations, such as the evaluation o the Bologna Cock-tail [26] and the conservation o the Four Virtues in Portadell Carta in the Ducal Palace in Venice. [27] Another par-ticularly interesting example is the evaluation o consolida-

tion with Brethane in Great Britain. [28]Conservation scientist Marisa Laurenzi Tabasso has re-quently been involved with evaluations o stone conserva-tion. She has both listed and examined some o the stoneconservation evaluations conducted in Europe between1985 and 2004. She noticed that it is o ten di cult to esti-mate the durability o the treatments. An assessment wouldbe easier i the conservator-restorers had le t a re erencearea a ter the conservation;a zero point. This area couldthen be monitored regularly to detect changes. She suggestsa methodology or this purpose that measures: sur ace col-our by refectance spectrophotometry, water absorption un-der low pressure (Karsten pipe), amount o deposited dust

5.1 Recent and On-going ResearchOn the international scene there is now rapid advancementin the eld o NDT conservation methods, which makes itdi cult to attain a complete picture o the situation. Someprojects o interest are presented in the rst report,Report 1. Non-Destructive Field-Tests in Stone Conservation Lit-erature Study. One current trend in scienti c conservation isto create strong and ormal networks that use and developNDT methods; sometimes supported by the European Com-mission. Some o these include LABSTECH,EU-ARTECH (Access Research and Technology or the Conservation o the European Cultural Heritage) and the LACONA net-work (International Con erence Lasers in the Conservationo Artworks).

Some interesting stone conservation projects using NDTmethods are also being undertaken in Sweden, such as theLidar Laser Project at Lund University, and research workat the NMK School at Chalmers University by PhD candi-date Pr Meiling.

5.2 Evaluation o Stone Conservation in anInternational ContextIn his 1996 report on the state o art in stone conservation,the scienti c conservator Cli ord A. Price divided evalu-ations o stone conservation into two categories: 1) thosethat characterize the stone shortly a ter treatment has takenplace, and 2) those that are concerned primarily with moni-toring long-term per ormance. [21] Test methods used todetermine the properties o the stone include sur ace hard-ness, strength, ultrasonic pulse velocity and acoustic emis-

sion. In most o the examples described in various publishedarticles, evaluations have been carried out in the laboratoryon resh stone.

The evaluations sometime use destructive methods. Oneinteresting example is the study o the durability o hydro-phobic treatment o the sandstone acades o Alte Pinako-thek and Schilling rst Castle in Bavaria on di erent occa-sions rom 1984 until 2001. The methodology was oundto be a success, even though the methodology was essen-tially destructive despite the use o Karsten pipes. The re-sults demonstrated that the laboratory and eld evaluationscould be correlated and that Karsten measurements couldindicate the durability o a treatment. [2224]

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per unit sur ace, amount o water-soluble salts (extractedusing Japanese paper poultices wetted with deionzed wa-ter), sur ace roughness (using a portable rugosimeter), andbiological contamination. Tabasso moreover noted thatdespite the immense development in the eld o conserva-tion the crucial conservation question is still: Are the mate-rial parameters currently in use suitable or judging conser-vation treatments, and is it possible to determine treatmentdurability? Tabasso posed this question at the DalhemWorkshop in 1996 and ound that it was still valid in 2004.Participants at the Dalhem Con erence concluded that therestill was a lack o pro essionalism when conservation meas-ures and treatments were planned and implemented andthat there was no de ned quality control. [29] Hope ullythis will change. Tabassos methodology and questions aregood staring points or such a project.

5.3 Previous Evaluations Conducted by theNHB in SwedenPrevious evaluations conducted by the NHB ocused onstone conservation treatments shortly a ter treatment; mosto them being per ormed on Gotland sandstone. However,a ew studies with the aim o monitoring the long-term e -

ect o weathering and pollution on Swedish stone have alsobeen conducted by theSwedish Corrosion Institute, as wellas within the ramework o theEU-marble project . [30]

The rst evaluation was executed in 1995 to 1996 by con-servator-restorer Misa Asp, geologist Runo L vendahl andengineer Erik sterlund. This evaluation was based on asurvey directed at Swedish stone conservator-restorers andin-situ examinations o eleven stone objects conserved be-tween 1988 and 1995 under the stewardship o the NHB.The examination was both destructive and non-destructive including salt measurements both directly on the sur aceo the objects and rom core samples (18 mm) according tothe L vendahl and Asp method, measurement o the mois-ture content (conductivity) on the stone sur ace with a Pro-timeter, measurement o the moisture content in the coresamples (by weight be ore and a ter drying) and nally us-ing a Durabo Drill Hardness Meter (DHM) to try to meas-

ure the hardness o the stone. Visual and hands-on inspec-tions were also carried out. The visual observations werenoted on an evaluation leafet and mapped on drawings. Insome places Karsten pipes were used to evaluate the waterabsorption, although this was not conducted systematically.The report mentions problems in analyzing the DHM andthe results were there ore not presented. [31] The resultso the tests con rmed that there were o ten problems withrising damp, colour change o the restoration mortars, andthat consolidations with Wacker OH were sometimes e -cient and in a ew cases not. Moreover, the authors state that

hydrophobic treatment and the painting o stone or protec-tion needs to be urther explored, and that mending mortars

also need to be investigated (both these areas have and stillare being explored by the NHB). In 1993 a more systematictesting o the Karsten pipes was conducted by Erik ster-lund and Misa Asp. This study resulted in a report entitledKarstens mtrr som o rstrande provmetod p sten(see below). [32]

In 2003, conservator-restorer Dr. Agneta Freccero under-took a survey o the evaluation that had been conductedby the NHB. She ound that 245 conservation works hadbeen carried out and that 60 o these had been evaluated.The evaluations were all di erent, both in methodology and

orm. Freccero noted that this inconsistency made it di cultto gain any clear view o the situation. She stated that boththe conservation documentation and the sampling method-ology varied too much, as did the evaluations themselves.Freccero there ore concluded that in the uture it would benecessary to establish a system o evaluation that included ade ned, common terminology.

5.4 NDT Methods used in StoneConservation in SwedenNDT evaluation methods are not so o ten used in stone con-servation in Sweden. Conservators-restorers normally usesalt compresses to determine whether salts are present ornot. Colorimetric measurements and Karsten pipe methodshave been used by the NHB within di erent projects. Theresults o these measurements have not yet been ully evalu-ated. One exception is the sterlund report ( rom 1993)that evaluates several Karsten pipe measurements (he alsotried Mirowski pipes) conducted by the NHB in the labora-tory and in the eld. sterlund also calculated the w- andB-values and ound that the calculation model was too sen-sitive. Small changes in the measured data distorted the val-ues too much, and he there ore developed his own simpli-

ed mathematical model. [32]In her licentiate thesis (2004), Myrin has used evaluation

methods to investigate the conservation o Gotland sand-stone and describes the current situation o stone conserva-tion in Sweden. The main part o her thesis consists o a sur-vey o ten conserved Gotland sandstone objects in the centre

o Stockholm and in the countryside with the aim o evalu-ating previous conservation treatments. Myrin also placedGotland sandstone samples (consolidated with WackerOH) outdoors with the aim o studying the durability o the consolidation. In addition, she has tried to evaluate thee ciency o a mending material commonly used in Swe-den, namely, Billys mortar. [10] She used visual assessment making comparisons with old photographs and in orma-tion ound in conservation reports and Karsten pipes tomeasure the water absorption. She concluded that the oldreports werent good enough and that no NDT methods

were available to assess the condition. This remark demon-strates that her choice o methods was based on cost, since

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many NDT methods were available, albeit quite expensive.[10] In her recent PhD work rom 2006, she used ultrasonicand colorimetric measurements.

Some sophisticated NDT methods have nevertheless beentested in Sweden. One early example was the investigationo the base o the Gustav II Adol sculpture in Carrara mar-ble in Gothenburg between 1992 and 1999. Germans Wol -ram Khler and Ste an Simon measured the ultrasonic pulsevelocity (UPV) o the stone within the ramework o theEucare-Euromarble project. Khler also measured someCarrara marble sculptures in Stockholm in 1992 within thesame project [33] and Bylund and co-workers also meas-ured the outdoor Carrara marble sculptures at the NationalMuseum o Fine Arts in Stockholm in 19951996. [34] Ste-

an Simon presented his results on Swedish sculptures usingultrasonic tomography to study the interior o marble in hisPhD thesis on the weathering o marble. [35] Moreover, An-ders Bodare at the Royal Institute o Technology in Stock-

holm also tested the stone with Hammer wave propagation(see below) in combination with an impact-echo technique.[36] Furthermore, Simon has used ultra pulse velocity, UPV,on the marble sculpture Flora in Gothenburgs BotanicalGarden. [35]

During the Air Pollution Programme, the NHB nancedseveral research projects using NDT methods. Anders Rehnat the Department o Electromagnetic Theory at the Roy-al Institute o Technology tested acoustic and electrical pa-rameters in 1995 and 1996 on behal o the NHB on di er-ent Swedish natural building stones. The electric methodconsisted o high resolution radar; transmission line radarthat can detect contrasts in the electric parameters in thestone. Knowing the parameters o the resh stone, the meas-urement can demonstrate i the stone has weathered. Thetest was per ormed on both homogenous and inhomogene-ous stones: Gotland sandstone, Ekeberg marble, Red vedSandstone, Gotland limestone rom Norrvange, Lingulidsandstone rom Lemuda, chalk and Kpinge sandstone. Themeasurements were conducted on dry stone, on stones satu-rated with water, on weathered Gotland sandstone rom the

Royal Castle in Stockholm, as well as weathered ved sand-stone. Moreover, Gotland sandstone was impregnated withalcoxysilane and these stones were measured dry as well aswater saturated. The technique can detect faws in the stone,although the report demonstrates that this works well whenthe stone is dry (the signal can penetrate 0.2 2 metres). Itdoesnt work particularly well on wet stone, however, sincethe penetration isnt deep enough, but it is possible to detectcracks inside the stone. [37]

Rehn also measured the acoustic parameters o the samestones using ultrasonic waves. He tried two methods: onewhere the samples were placed in a water tank and wherethe sound was refected and received by a transducer and re-corded a terwards, and one that transmitted the ultrasoundthrough the stones and where the sound was also recordeda terwards. The result shows that the velocity o the soundmay di er in di erent directions o the stone (which is com-mon in stones that arent homogenous). When the sound

was transmitted through dry and resh Gotland sandstonethe velocity ranged between 2.2 km/s and 2.5 km/s. Whenthe stones were saturated with water, the velocity was high-er: 2.6 km/s. The sound that was transmitted through im-pregnated Gotland sandstone with alcoxysilane gave aneven higher velocity: 3.2 km/s. When the impregnated stonewas saturated with water the velocity was 3.4 km/s. Onthe other hand, measurements o the refections in the wa-ter tank demonstrated that it was not possible to measurestones that have cracks. This method is thus not as use uland moreover requires sampling which seems unnecessar-ily complicated. Nowadays portable ultrasonic apparatusis available or measuring the transmission. [38] Rhenstests only give us the measured velocities on resh stone.The tests have to be complemented by testing on weatheredstone in order to understand the quality o the stone. Fur-thermore, the measurements have to be compared to oth-er test methods, such as the compressive strength and thetensile strength, to nd out where the actual critical break-ing points or intervals are. This has been done with otherstones, such as Carrara marble (see below).

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6. Conservation and NDT Methods

6.1 Conservation and NDT MethodsScienti c analytical methods are used in conservation toevaluate both the materials (in themselves such as thestone type) and the e ects o the conservation and weather-ing processes. K. Janssens and R. van Grieken have dividedconservation analytical methods into three groups. All theareas have been studied and are:

The chemical nature/composition o selected parts o cul-tural heritage arte acts and materials

The state o alteration (o the sur ace and/or internally)o objects as the result o short-, medium- and long-termexposure to particular environmental conditions

The e ect/e ectiveness o conservation/restoration strat-egies during and a ter application. [39]

There are many di erent methods to choose rom depend-ing on the aim o the analysis. It is obvious that one singleanalytical method cant possibly provide all the wanted in-

ormation, which means that the conservator-restorer hasto design a test series to give complementary in ormation.That isnt always enough, however, as urther requirementsrestrict the choice, or example, that tests should benondestructive, ast, universal, economic, reproducible, easy touse, objective, available, sensitive and harmless to the envi-ronment . As mentioned above, not all the tests correspondto these requirements; some are micro destructive (such asmicro drilling resistance), very expensive or require expe-rienced personnel. The criteria there ore must be seen asan optimal aspiration. The conservator-restorer will haveto keep these requirements in mind when designing the ana-lytical programme.

The search or NDT methods has been ongoing since thebeginning o conservation. The reason or this is obvious the conservator-restorer always strives to prevent damageto the objects. The more sophisticated NDT methods haveusually been developed or engineering or medical purposesand are therea ter adopted and modi ed or conservationpurposes. Thanks to this, it is today sometimes possible tounderstand, characterize and evaluate conservation workwithout taking samples. NDT methods are increasinglygaining in relevance and many articles published within the

eld. A survey made within the LABSTECH organization(published in 2004) demonstrated that NDT and micro de-structive tests are not yet common among conservator-re-storers (100 conservator-restorers were asked about their

work and 32 responded. Only a ew o them used NDTmethods). [40] The reason or this is evident; lack o equip-ment, experience and routines.

NDT methods are based on di erent physical phenom-ena. They are usually divided into di erent groups depend-ing on their scienti c background:

Geophysical methods; measure mechanical and electricalproperties o the material

Spectral analytical methods; analyze sur ace propertiesby the use o electromagnetic radiation that is absorbedor emitted by the material

Tactile and visual assessment.

Katinka Klingberg Annertz divides NDT methods into threegroups, depending on what the method is able to do withthe material:

Geophysical methods that investigate the bulk o the ma-terial (seismic methods such as the ultra sonic methods,hammer methods, acoustic emission methods and radarmethods)

Spectroscopical and chemical methods that investigatethe sur ace o the material (absorption spectroscopy, di -usion spectroscopy, emission spectroscopy and radio

chemical methods) Imaging techniques that investigate the bulk and/or sur-

ace o the material (laser scanning, analytical photogra-phy/refectography, thermography, radiography, Compu-ter Tomography and photogrammetry).[41]

Some o these methods are discussed in this report. How-ever, as some o these methods are expensive and di cult touse in-situ they are there ore discussed in brie .

Anders Bodare (1996) has divided NDT methods withinthe realm o geophysics into two types depending on thekind o wave used:

seismic methods, such as ultrasonic methods, Schmidhammer method and acoustic emission methods

electrical methods, such as radar, resistive and electro-magnetic methods. [36]

6.2 NDT and Stone ConservationIn stone conservation, the use o NDT is restricted by the

act that the stone objects are o ten ound in buildings. Mov-able objects are, on the other hand, possible to take into thelaboratory (where several NDT techniques have been devel-

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oped). In architectural stone conservation, the NDT meth-ods must be portable and possible to use in the eld in var-ied conditions (such as on sca olding in bad weather).

The advancement o this trend depends on improvementsmade in detector technology, instrument-computer inter ac-ing, ocusing optics, and the radiation sources suitable oruse in various parts o the electromagnetic spectrum. Forthe methods that need to be usedin-situ there is also an im-mense improvement in the miniaturization o components,making the design more compact, portable and sometimesincluding handheld instruments.

6.3 Problems to Be Analyzed in StoneConservationSome problems are characteristic to the conservation o building stones, due to the act that they are situated out-doors and are o ten part o a large structure. The investiga-

tion and conservation o building stone is determined bythese circumstances. Some o the questions that need to beunderstood include:

The water absorption, the water content and the sourceo the water

Whether salts are present, what kind, their distribution,source and quantity

Climatic conditions that e ect the weathering, such as airpollution, wind and variations in humidity and tempera-ture

The condition o the stone, or example, the degree o weathering and the rate o deterioration

The stone type and its characteristics.

Several NDT methods are available or these purposes, al-though they do not cover the whole spectrum and, in somecases, sampling is required. Moreover, important acts needto be known during and a ter the conservation to ascertainwhether the conservation has succeeded or i re-conserva-tion is necessary and can be monitored and controlled withNDT methods:

Changes in colour (with colorimetric measurements). Changes in strength and hardness (ultrasonic, micro drill-

ing resistance and so orth) Water content and source (moisture measurements)

Loss o material (sur ace relie or roughness measure-ments) Changes in the stones water absorption capacity (pipe

methods) Changes in salt content a ter treatment (measurements o

salts extracted with paper pulp) Durability o conservation treatments (a mixture o the

methods mentioned above).

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7. Description o the Field Tests

7.1 Description o the Field TestsFour NDT methods were tested in the eld on three occa-sions on nineteen objects (on ourteen buildings) in Au-gust and October 2005 and May 2006. The methods usedwereKarsten pipes, Minolta Spectrophotometry, a Granu-lar Disintegration Test with Herma Labels and UPV meas-urements with a portable AU 2000 Ultrasonic tester romCEBTP. Moisture measurements were also taken using aTramex moisture meter and a Protimeter (the latter wasonly used on one occasion) together with the air tempera-ture and sur ace temperature o the stone. The sur ace tem-peratures were close to the air temperatures, apart romwhen the stone had been in direct sunlight. In this case thesur ace temperatures were higher.

7.2 Weather ConditionsThe weather conditions were very similar when the rst twomeasurements were taken; sunny Swedish summer weatherwith long periods o warmth and dryness. The temperatures

were between 12 C and 25 C, and the moisture contentmeasured with the Tramex instrument was o ten reason-ably high; around 34 (maximum 5). In some places, suchas Svartmangatan 6 in Stockholms Old Town, the measure-ments were outside the measuring scale, whereas other plac-es, such as the Gustavian Memorial Chapel at RiddarholmsChurch had a low moisture content. On the two measur-ing days (i.e. on each measuring occasion) the weather wasvery similar, whereas on the second day in October 2005there was rain all in the morning and then sun or the resto the day. As cold weather hindered the measurement inMarch the third measurement was not conducted until May2006. On this occasion the temperature changed during thetwo days o measurement. The air temperature was between10 C and 15 C. For a week be ore the measurement theweather had been dry and sunny and the stones there orecontained less moisture; the values o the Tramex instru-ment o ten being between 1.5 and 2.5 (see Appendix 2).

7.3 Methodology and Instruments7.3.1 Granular Disintegration Test with HermaLabels (or Tape)

The tape methods are not very common in conservationand no articles describing the methods were ound in the lit-

erature study. However, an ASTM standard or testing theadherence o paint using a tape (Scotch tape test) does ex-ist. Marisa Laurenzi Tabasso believes that this is use ul orthe evaluation o sur ace deposits (and sanding), but not

or quantitative evaluations. The NHB has nevertheless in-vented a tape method that uses ready-made labels. Themethodology was initially based on the di erence betweenthe weights o the deposits:

1 In the laboratory, seven pre abricated sel -adhesive la-bels 32x44 mm in size and manu actured by HERMAare weighed and the average (Ainitial) weight is calculated( rom seven labels).

2 In the eld, three HERMA labels are attached to thestones sur ace. A ter a ew seconds the labels are takeno , olded and put into a sealed plastic bag.

3 In the laboratory, each label is weighed and the average o each sample (A1 , A2 ...) is calculated (Aeld)

A1 + A2 + A33

These are then compared with the previous average tocalculate the di erence

(D) : D = Aeld- Ainitial.

This methodology proved complicated. The total weightso the labels were compared to each other (both the labeland the deposit) and the results divided into three catego-ries according to the weight (see below): 1) Poor condition

Figure 1. Labels attached or the granular disintegrationtest.

A eld=

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and severely weathered/dirty, 2) Medium weathered/dirtyand 3) Good condition/clean. It is important to rememberthat this is a preliminary weathering index that needs ur-ther testing.

7.3.2 Water Absorption Test with Karsten Pipes

Several pipe methods are available or measuring the waterabsorption:the German Karsten pipe, the RILEM pipe, theItalian pipetto and the Polish Mirowski pipe . All are NDTmethods that can be usedin-situ to evaluate the water ab-sorption o a porous material. The water absorption o thematerial corresponds to the pore structure o the materialand thus gives in ormation about the condition o the ma-terial. The pipe methods are used to evaluate the result o conservation; o ten a hydrophobic treatment.

The Karsten pipe was developed in 19581960 in Ger-many. The method is non destructive, easy to usein-situ

and is there ore o ten used by conservator-restorers. Thepipe consists o an open cylindrical body which is attachedto a sur ace o the material being measured (horizontally orvertically there are two kinds o pipes or these purposes).A graded pipe emerges rom the body and this is lled withdistilled water. The water absorption is determined with theaid o a time-bound schedule and registered according tothe gradations on the pipe and the time that has elapsed.The pipe always points upwards and the body is attachedto the sur ace using a recommended sticky gum, such asPLASTIC FERMIT or TEROSTAT 9. A Bostic sticker hasbeen used in Sweden or a many years, although this is hardto nd nowadays and other sticky gums have been substi-tuted. There are two pipe sizes: one that holds 4 ml and onethat holds 10 ml o water. The inner diameter o the largerpipes body is 4.5 cm and the thickness/height o the bodyis 3 cm, the total height o the pipe is 18 cm and the pipesinner diameter is 0.9 cm. The smaller pipes body has an in-ner diameter o 2.5 cm, the thickness/height o the body is2.4 cm, while the total height o the pipe is 15 cm and theinner diameter o the pipe is 0.9 cm. The di erent sizes havebeen made or measurements on di erent kind o stones.The larger pipe should be used or strongly absorbing stones

and the smaller pipe or less absorbing stones; the reason be-ing that the absorption o the small pipe may be too ast.The pipe is attached to the stone in such a way that it will

stick or at least an hour. It is important that the sticky gumdoesnt spread into the body volume in such a way that it di-minishes the contact area o the water. The pipe is then lledwith distilled water until it reaches 4 or 10 ml. The measure-ments are made according to pre xed time-schedule: a ter1 minute it is noted how much water has been absorbed andthe same procedure is repeated every minute until 5 minuteshave passed and herea ter is every 5th minute noted until 30minutes have passed and therea ter every 10th minute until60 min have passed.

The result can be plotted on a graph (where the absorbed

water volume kg/m2

or ml/cm2

is plotted as a unction o time, min) or by calculating the capillary water absorptioncoe cient the w-value (kg / m2 x h 0,5), that is compa-rable to the DIN-standard o the water coe cient that ismeasured on drill cores and the water penetration coe -cient the B-value (m / h0,5) DIN 52 617. The mathemati-cal model or this was developed by Wendler and Snethlagein 1989 and is commonly used in Germany. [42] Both val-ues depend on the porosity and capillary orce o the mate-rial. Wendler and co-workers have developed a computerprogram; the BASIC-program calledCalkarow (Cal culativeEvaluation o Kar sten Measurement orO ptimation o w -Values) that calculates the w- and B-values when using theKarsten pipes. [43]

It has been suggested that the pipe methods have prob-lems because they are not completely repeatable and thatthe result may di er depending on the dexterity o the per-son conducting the measurement. [44] It is true that thepipes o ten leak and all down during the measurements.However, Wendler and Snethlage have proved that it is re-producible and is consequently reliable and requently usedin stone conservation. [42] The methodology gives good re-sults, is cheap and relatively easy to handle.

Within the eld study both the small pipe (4 ml) and thebig pipe (10 ml) were used to test their di erences. Both theBostic gum and the German Fermit were used as sticky at-taching gums.

7.3.3 Methods or analyzing the condition o thestone Ultrasonic Pulse Velocity (UPV)

Today, ultrasonic pulse velocity(UPV) methods are well es-tablished in stone conservation. The reason or the popu-larity o the method is that it is rapid and easy to use in the

eld. It is used or detecting cracks and faws and to controlconcrete.

Figure 2. Measurement with Karsten pipe at Riddarhuset.

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The testing o stone with ultrasonic wave measurementsstarted early in the history o stone conservation. The test-ing was developed in the 1950s by the conservation sci-entist Mamillan in Paris. Today it is one o the most usedNDT methods in stone conservation. Many articles usingUPV are consequently presented in con erence publications.Acoustic methods provide data that can evaluate the condi-tion o a stone (the state o conservation), give important in-

ormation relating to the result o consolidation treatmentsand can also help to detect faws in the material such ascracks and voids. They moreover help to characterize themechanical properties and the degradation o the material.The method analyses the structure o the material and henceonly indirectly the strength o the material.

The classic sound method involves the transmission o apulse o sound o a single requency into a structure. Thetime required or the pulse to be refected back rom a ea-ture, such as a void, is measured; the higher the requency

o the sound, the smaller the distance that can be measured.[45] The waves are in act a kind o transmitted energy, andthis energy moves in di erent typical patterns. The speedo the energy is thespeed o waveor wave velocity (mm/sor km/s). The individual particles that move around theirpoints o equilibrium and retain their position a ter thewave has passed are measured. The mechanical waves movein di erent ways in relation to the direction o the propa-gation longitudinal or transversal . The longitudinal wavetravels with a higher speed and is known as theP-wave. Thetransverse wave is slower and called theS-wave. There arealso so-called sur ace waves that travel along sur aces o discontinuities between the material, such as theRayleighwave (R-wave) where the waves move the particles in ellip-soidal orbit (the speed is 93 percent o the S-wave) and theLove-wave(L-wave). The speed o the waves correlates tothe strength o the material. The waves travel at the samespeed in all directions i a material is homogenous and iso-tropic, but the speed may vary in di erent directions i thematerial is inhomogeneous or anisotropic, such as sand-stone. Another e ect is o importance theattenuation .In the short-term, the consequences o the attenuation arethat the amplitude o the wave diminishes as the wave prop-

agates. The attenuation is determined by Baers law. [36]There may there ore be di erences in speed i one measureslong or short distances. Di erences in the requency usedmust also be considered.

Ultrasonic methods use ultrasonic waves consisting o several methods (such asultra pulse velocity (UPV), ultra

pulse echo, seismic echoand acousto ultrasonic, also seeto-mography). The vibration uses waves o a higher requen-cy than can be heard by the human ear usually around20,000 Hz. Some researchers recommend di erent speeds

or di erent measuring lengths. [46] The UPV in air is 330m/s, in water 1480 m/s and in stone up to 6000 m/s (sand-stone ca 2860 m/s, limestone ca 4310 m/s, marble ca 6690m/s and concrete ca 4430 m/s).[46] It is the P-wave that is

usually measured. I it is possible the velocity measured istransmitted through the material (direct ), or i this is notpossible on the sur ace (indirectly). The requency can di er

rom equipment to equipment; rom 45 to 10,000 kHz. Thedi erence in measurement methods naturally a ects the re-sult. The velocity depends on the properties and structure o the material. The denser the material, the aster the soundtravels. Thus, the velocity is aster in denser material andslower in air. It is possible to locate damage by measuringthe di erences. When there is a crack in the material the sig-nal is refected; the stronger the di erences o the material,the stronger the refection. [47] Furthermore, it is possibleto assess the degree o deterioration o the stone i the nor-mal value o the stone is known. [4749]

Several amous marbles in Europe, such as the Carraramarble, have been tested with UPV techniques within theEucare-Euromarble programme in order to understand thedegree o weathering. For example, Khler established a

classi cation o the state o deterioration o Carrara marbleusing UPV based on an empirically derived correlated unc-tion between the Vp (the velocity o the P-waves) and theporosity. Weiss and co-workers also per ormed similar testson the Italian Lasa marble and on a Polish marble. [33] InSweden the UPV has been used only in requently, e.g. testsper ormed in the 1990s by Khler who examined the mar-ble base o the sculpture o Gusta II Adol , by Simon on theFlora sculpture in Gothenburgs Botanical Garden and byBylund and co-workers on sculptures at the National Mu-seum o Fine Arts in Stockholm. Moreover, laboratory testswith UPV were made on some Swedish stones by AndersRehn at the Royal Institute o Technology. In addition, Ka-tarina Malaga and Malin Myrin tested UPV on conservedGotland sandstone in 20042005 as part o Malin MyrinsPhD work on Gotland sandstone and conservation. [50]

According to DAvis, drawbacks with the UPV methodinclude:

1 Black crusts may infuence the result (and give inaccurate-ly high values).

2 Irregular sur aces may hinder the measurement (depend-ing on the transductors shape and size).

3 Badly weathered stone sur aces may be a ected by pres-sure caused by the measurement.

4 Moisture content in the stone may infuence the result.5 Salts in the stone may infuence the result. [46]

The second and third drawbacks have not caused any prob-lem within this project. The infuence o salt has not yet beenthoroughly examined, however, and the extent to which thesalts actually infuence the result is uncertain; or examplehow high a salt concentration must be to cause problemsand how water and salt in combination a ect the result.Other drawbacks are that the equipment is expensive, re-quires experienced personnel and that the velocity usuallydi ers in laminated sandstone depending on the directionmeasured. Moreover, it can be di cult to couple the equip-

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ment on rough sur aces and on complex shapes. One recentarticle also claims that problems arise when correlating theUPV data with the Unaxial Compressive Strength. [51]Themethod has nevertheless proved to be very use ul in that itis easy to use, gives quick answers and is relatively accurate.For example, in one article the accuracy is stated to be ashigh as 0.5 percent. [48] Marini and others have tried themethod in the laboratory on Carrara marble and ound it tobe satis actory or weathering tests. [52]

In this study, the measurements undertaken in the eldand in the laboratory were made with a portable ultrason-ic tester;AU 2000 Ultrasonic tester rom CEBTP. The P-wave requency used was 60 kHz, the sampling requency10 MHz and the time resolution 0.1 s.

7.3.4 Moisture Measurement with a TramexInstrument

Many methods are available or measuring the moisturecontent in building material. Some methods are good orhigh moisture content and others are suitable or low mois-ture content, while some are only applicable in the labora-tory and some in the eld. The methods are usually dividedbetweendirect and indirect methods. The direct methodsmeasure the moisture content directly by weighing the wa-ter in a sample (gravimetric methods). The indirect meas-urement measures properties in the material which can berelated to its moisture content, or example, the electricalconductivity. Another way o di erentiating is betweende-structive and non destructive methods.

The moisture measurement methods are divided accord-ing to the methodology:

1 Absolute measurement methods (gravimetric measure-ments and chemical methods)

2 Hygrometrical methods3 Electrical measurements

4 Other methods, such as the thermographic method, radarmethods, microwave methods and methods using gypsumblocks with electrodes that are inserted into the wall.

As the electrical method has been used in this study it isthere ore the only one described ( or urther in ormationsee Report no 1).

Electrical methodsmeasure the conductivity o the mate-rial and are thusindirect, non destructive and can be usedin-situ. They depend on the di erence o the electrical prop-erties o the measured material and the properties o water.Several cheap portable instruments are available.

Electrical methods can be based on two principles:1 the resistivity2 the capacity

Resistivity methods are carried out by measuring the resist-ance between two electrodes that have been tapped into thematerial. The measurement is based on the principle thatthe electrical resistance decreases when moisture is presentin a material. The measurement is compared with a calibrat-ed curve or the speci c material. This, together with meas-urement o the temperature makes it possible to measure themoisture content by mass (u).

Figure 3. Ultrasonic measure-ments undertaken at theGerman Church.

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The capacity method, on the other hand, uses a condenserwith two electrodes that are isolated rom each other by adielectric material . Several moisture measurement methodsare based on the determination o thedielectric constant .The constant is much higher or water (ca 80) than or mosto the building materials (usually between 3 and 6). Thismakes it suitable or moisture measurements. A dielectricmaterial is de ned as a material that has modest or no elec-trical conductivity. When the measurement is carried out,the measured material becomes the dielectric material in thecondenser that creates the electrical circuit. The instrument

measures the change o requency when the dielectric prop-erties in the material change. The result is plotted onto acalibrated curve and the moisture content calculated. Theportable instruments that are available usually measure re-quencies ar

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