11

GOTLAND SANDSTONE IN SWEDISH BUILDINGS

LOFVENDAHL, RUNO

Central Board of National Antiquities (CBNA), Box 5405, S-1 14 84 Stockholm, Sweden

SUMMARY The Gotland sandstone is a carbonaceous sandstone of Silurian age used extensively in buildings and as sculptural stone since medieval time. It is found in buildings in the Baltic area, especially Poland, Germany, Denmark and Sweden. It shows different types of damage, such as material loss, selective weathering, sanding, exfoliation, crust formation, salt efflorescence and black staining. In the countryside also biological growth, especially algae and lichens, is a nuisance. Older conservation measures, especially jointing and reinforcement with cement-based mortars have often been deleterious. About 45% of all stone conservation financed and made by CBNA since 1988 concerns this type of stone. The cleaning is generally made the wet way with water and if necessary chemicals. Black staining and discoloration of the stone is treated by use of paper pulp pouitice and the salt components removed with bentonite pouitice. Tetraetoxysilane is generally used as a consolidant. Water repellant in form of silicaesters was applied up to 1991 but since then generally discontinued. The results so far are evaluated ocularly, with photos, different types of material samples, salt compresses, the Karsten test-funnel, a humidity gauge and other techniques. Out of ca 100 objects treated during the period 1988-95, 12 had been conserved less than 20 years earlier. The major reason for repeated treatment is high salt content, sanding, bad pointing and material loss.

1. OCCURRENCE AND CHARACTERISTICS

The Silurian Gotland sandstone, also called Burgsvik sandstone, forms an up to 50 meters thick complex, the Burgsvik Beds, in a limestone environment. The sandstone is outcropping along a 35 km long horizon (fig 1) on southern Gotland. The Burgsvik Beds are far from homogeneous, but consist of carbonaceous and sandy-silty layers. The quarried sandstone forms two or three layers with a total thickness of 3-7 meters in the stratigraphically central part of the Burgsvik Beds. The sandstone is generally only weakly laminated, with mica grains oriented with their long axis parallel to the bedding. It forms different types of sedimentary structures (Long 1993), indicating that it has been

deposited in shallow water as a sandbar parallel to the beach. The sandstone is grey to bluish grey in colour and grows brownish grey during weathering. The major minerals are quartz, feldspars and the micas biotite and muscovite. The grain size is 0.05-0.1 mm with subangular form. The bedrock is well sorted. The grains are cemented with quartz, calcite and chlorite. The porosity is just above 20% as measured with Hg-porosimetry (Wessman 1993) with a major pore size of 70-90 pm. The water uptake is 9% of the total weight at atmospheric pressure (Wessman 1995).

2. USE

The oldest cultural objects of Gotland sandstone are minute whetstones, which are common at stone age habitations on the island. The oldest stoneobjects are strongly weathered. During the 19th and 20th century production of whetstones was the major use of Gotland sandstone, but at present this use

dwingles. Tombs and tomb-stones of Gotland sandstone are known at least since the 4th century A.O. The largest grave-field is situated at one of the presently open quarries and shows that the stone was quarried and worked with tools very early. It has also been used as tomb-stones in medieval times, but much less than Gotland limestone. The reason is that it weathers and disintegrates faster than the limestone. It has not

been used for this purpose after the 19th century.

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Picture stones are a Gotland speciality. These stones were erected under a long period, ca 300-1000

A.O. and generally made of limestone. A few are made in sandstone; only one of these is still intact and remains outdoors, while a number are taken indoors.

The sandstone was popular for cutting of baptismal fonts in the 12th and 13th century. There was

actually an industrial production, at least during the latter century. These fonts were spread all over the

Baltic region and found in Sweden, Denmark and Germany. Local production of fonts by stone masons

also took place in the 17th century in Burgsvik. Other important uses are altarpieces in Gotland

churches and also stoves during the 17th century.

Historically, building and sculptural stone have been most important. During the 12th and 13th century

14 churches on southern Gotland were built in Gotland sandstone. All these are situated close to the

outcropping stone (fig 1). It was also used later, from the late 16th century in buildings in Denmark,

Poland and Germany, but after 1 648 mainly in Sweden. Palaces, casties and manor houses were often

built in Gotland sandstone up to the 18th century. Many of these were erected by nobles after the

European 30-year war.

Together with buildings, sculptures and ornaments are widespread in the Baltic area. An inventory of

Swedish stone buildings shows that the Gotland sandstone is spread all over the country (fig 2; tabie 1;

data from Sundner 1996). The age of the conserved objects in buildings are from early medieval time to

the present century (table 2) . Nowadays, the stone is quarried in two places (fig 1 as replacement stone and for production of traditional objects such as small whetstones.

Table 1: Total number of, and con-served, buildings in Gotland sand-stone in Sweden given countywise. For county symbols see figure 2. ___________________________________ .., ___________

County Number of Conserved buildings 1988-1995

-----------------------------------------------A 272 19 AB 70 3 c 41 3 D 49 8 E 27 1 F 6 G 1 H 24 5 I 30 6 K 7 L 11 1 M 24 3 N 3 0 6 p 3 1 R 4 1 s 1 T 10 1 u 16 2 w 1 x 10 y 8 1 z 2 AC 2 BO --------------------·-----------------------·-·---·---Total 628 55

Table 2: Age distribution of conserved Gotland sandstone buildings

--------------------------------------------Time period

1000 - 1300 1300 - 1550 1550 - 1650 1650 - 1750 1750 - 1860 1860 - 1910 1910 - 1940

Total

Number of buildings

4

19 24

4 3 1

-------------------55

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3. DAMAGE TYPES

The deterioration of the stone shows different damage types. Our experiences from inventories,

documentation and conservation give the following picture. The most important damage types are material loss, mineral weathering, sanding, exfoliation, crust formation, efflorescence and biological growth (damage terminology according to Lofvendahl et al. 1994). The ageing and disintegration of the surface starts with chemical alteration. The major agent is dissolution by rain. The initial chemical attack is dissolution of calcareous cement that binds the stone together. By time this gives rise to sanding, i.e. the detrital silicate grains are lost when their binding agent is dissolved. Because of its dominant fine pore structure with interconnected pores the stone is prone to capillary water uptake and freezing. The damaging effect is aggravated by salt uptake and following crystallisation when water is evaporated and the salts are concentrated. Exfoliation is a commo damage type. The freezing of the water-saturated stone or formation of sagt efflorescence, especially just under the stone surface are probably major causes giving this type of damage. Projecting surfaces

have a tendency to fall off, leaving scars and sanding surfaces. A number of lab projects have been running parallel to the field work and conservation. These have mainly been directed to the uptake of sulfur and concomitant surface reactions (Eifving 1994), freeze­thaw cycles with pure and saline water (Wessman 1995) and reaction kinetics of silicates (Sjoberg et al. 1995). Fagerlund (1994) has proposed an interesting chemical mode[, to explain the destruction of Gotland sandstone through dissolution of calcite cement in the stone resulting in sanding or material

loss. Exposition studies on this stone type are few and of short duration (Vickova et al. 1994). The Gotland sandstone can not be directly compared with stone types in other exposition studies (cf. Cooke & Gibbs 1993). The scanty results, however, indicate that this sandstone in the short run (years) is altered in a similar way as Portland limestone or Mansfield dolomitic sandstone (Kucera et. al. 1995)

4. STUDIED BUILDINGS AND OBJECTS

We have collected data on documentation and conservation of buildings nationwide. Since the start of the Conservation Institute at the CBNA in 1988, we have followed and partly organised the conservation of valuabie national buildings. The data base created by us shows that out of more than 200 objects cared for, close to 100 consist of Gotland sandstone (cf. fig 2; note that a building is composed of one or more objects - an object being a porch, sculpture or moulding for example). This is not all objects cared

for, but those where CBNA has been involved in one way or the other. Since the late 1980s our knowledge probably covers most of those actively cared for. The most interesting objects are those that

have been reconserved twice during the period from ca 1970 up to now (tabie 3) . Before conservation, a plan for the intended work including characterisation of the desired result is defined. Nowadays, all larger conservation measures are summarised in a conservation report. This

report includes description of the object before the work with relevant photos. When the work is finished an evaluation of the measures is made and the work is either accepted or complementary work prescribed. A report of the work, including the state before and after Konservation means of cleaning, cleaning agents and Konservation methods are given. The Konservation is finally checked after a two year-period. If the situation is then acceptable, the conservator has no more formal responsibility for the

work done on the object. For work done before the late 1980s, reports are very scarce, or at best fragmentary. It is thus often quite difficult to know which methods and what agents were used. Thus, it

is sometimes impossible to identify the major damage factors, whether human or natural.

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Table 3: Objects conserved twice during the period ca 1970-1995

Building Object(s) Age Locality Conseived Reconseived Note

Hablingbo ch. N porch 12th c. Gotland, I 1984 1990 Rural

Grotlingbo ch. Reliefs 13th c. Gotland, I 1985 1992 Rural

Askersund ch. Porch 17th c. Central Sweden, T 1975 1995

Heby manor Porch 1779 S Stockholm, D 1985 1994 Rural

Nikolai ch. Two S-porches 17th c. S Stockholm, D 1983 1992-93

Vadsbro ch. S, V-porches 1656 SV Stockholm, D 1981-82 1991 Rural

Tyreso ch. V, S-porches 1640s S Stockholm, AB 1980 1991

Jacob ch. S porch 1644 Stockholm, A 1968-69 1994 Urban

Petersanska Double-porch 1640s Stockholm, A 1988-89 1994 Urban

Svartmang. 6 Porch 1650s Stockholm, Old Town, A 1982 1994 Urban

Storkyrkan Statues (two) 1675 Stockholm, Old Town, A 1978-79 1994-95 Urban

Tessin manor Porch 18th c. Stockholm, Old Town, A 1984 1993 Urban

5. EVALUATION OF MEASURES

A number of examination methods have been applied when evatuating the Konseivation measures. They include sampling and evaluation of efflorescence and sanding material (SEM/EDS, XRD; cf. Nord & Tronner 1991). The results so far indicate that salt crystallisation is a major problem with Gotland sandstone. Close to the ground, within the zone of capillary rise, chlorides and sulfates are the major anions, complemented with the cations sodium, calcium and potassium. The major mineral phases formed within the capillary zone are rock salt (NaCl), gypsum (CaSO,vc2H20) and thenardite/mirabilite (Na2S04 with or without crystal water). Above the capillary zone, gypsum is the dominant salt mineral. The origin of sodium and chloride is polluted ground- and surface water, not least from deicing of roads and stairs in wintertime. The origin of sulfate has been studied with mass spectrometry (as 834S). These results are reported elsewhere and show that there are at least two major sources. In urban areas the sulfate is generell of atmospheric origin, i.e. pollution. In rural areas mobilisation of sulfur by oxidation of pyrite (FeS2) in the stone itself is an important source. The amount of salt is also quantified by extraction using paperpulp poultice (Lofvendahl 1996). The total salt content is measured as conductivity of water extracted from the stone surface. If the conductivity is high, extracts from polluted surfaces are analysed with IC (ion chromatography). These analyses show that beside sulfate and chloride, nitrate is often a major constituent. The major cations are sodium, potassium and calcium and seidom magnesium. Capillary uptake from the ground is dominant for all components except sulfate and calcium. The salt content is highest in polluted urban areas. The Karsten test-funnel (Wendler & Snethlage 1989) is also used to check water uptake before and after consolidation. It is also used at the control two years after measures and consolidation. Presently, we only have initial data from such measurements. The strength of the stone is characterised by measuring drilling resistance (Leonhardt & Kiessl 1990). This method has already indicated that the quality of fresh stone can vary distinctly. This supports our suspicion thet the quality of the Gotland sandstone varies both stratigraphically as weil as within the same stratigraphic horizon. By use of a humidity gauge, the distribution of humidity at the stone surface can be estimated. The results show that the basal parts of the objects (close to the ground) are often soaked with water.

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6. CONCLUSIONS

Gotland sandstone is common in buildings in Sweden, used since medieval times. The most common objects are porches, statues, window frames and moulding but also ashlar stone in buildings. The stone is especially frequent in buildings in the southern Baltic area from the 17th and 18th century. It is sensitive to humidity and salt, but properly sheltered it has a life expectancy of hundreds of years.

ACKNOWLEDGEMENTS Stig Englund, Visby constructed the figures. Help from and discussions with personnei at the stone department of the CBNA, in particular Lars Kennerstedt, C. Bylund, E. Osterlund and M. Asp is appreciated.

REFERENCES Cooke, R.U. & Gibbs, G .8 . 1993. Crumbling heritage? Studies of stone weathering in polluted atmospheres.

National Power pie and Power Gen pie.

Elfving, P. 1994. Aspects of the air pollution induced deterioration of calcareous stone material.

Institute of inorganic chemistry, Chalmers university of technology. Ph.D. thesis.

Fagerlund, G. 1994. Kapillaritet som orsak till nedbrytning av kalcitbunden sandsten - en hypotes. Report TVBM-

3058. Lund university of technology (in Swedish).

Kucera, V., Tidblad, J., Henriksen, J., Bartonova, A & Mikhailov, A.A. 1995. Convention of longe-range

transboundary air pollution: U.N.-1 .C.1 International cooperative programme of effects on materials, including

historic and cultural monuments. Report N0-18: Statistical analysis of 4-year materials exposure and

acceptabie deterioration and pollution levels.

Leonhardt, H. & Kiessl, K. 1990. Schnellbestimmung des Festigheitsprofil verwitterter Oberflachenzonen alter

Gebaude durch Bohrwiederstandsmessung. Fraunhofer-lnstitut tor Bauphysik, Mitteilung 191 .

Long D. F. G. 1993. The Burgsvik Beds, an upper Silurian storm generated sand ridge complex in southern

Gotland, Sweden. GFF 115, 299-309.

Lofvendahl, R. 1996. Quantification of salts in outdoor Gotland sandstone sculptural objects in Sweden. In: Sth

International Conference on non-destructive testing, analytical methods and environmental evaluation for

study and conservation of works of art. Budapest 24-28.09.1996.

Lofvendahl, R., Andersson, T., Aberg, G. & Lundberg, B.A. 1994. Svensk byggnadssten och skadebilder.

Natursten i byggnader. CBNA/NHM, Stockholm (In Swedish with English summary).

Nord, A.G. & Tronner, K. 1991 . Stone weathering. Air pollution effects evidenced by chemical analysis. Report

RIK 4. CBNA/NHM, Stockholm.

Sjoberg, L., Morth, C.-M., Schweda, P., Torssander, P., Strandh, H., Lindblom, S.,Frogner, P. & Kalinowski , 8 .

1995. Vittring av silikatmineral. Kinetiska studier och faltundersokningar. Report to CBNA (in Swedish).

Sundner, 8 . 1996. Oral communication.

Vickova et al. 1994. The measurement on Gotland. Gotland's testing programme. Results after 12 months'

exposure. Report from SVUOM Prague to CBNA Stockholm.

Wendler, E. & Snethlage, R. 1989. Der Ofer nach Karsten - Anwendung und Interpretation der Messwerte.

Bautenschutz und Bausanierung 12, 110 - 115

Wessman, L. 1993. Saltfrostnedbrytning av Gotlandsk sandsten. Report TVBM-3038, Lund university of

technology (in Swedish).

Wessman, L. 1995. Expansioner och avskalningar vid frysning av Gotlandsk sandsten och Oiandsk kalksten i

NaCl- Na2S04-losningar. Report TVBM-3089, Lund university of technology (in Swedish).

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APPENDIX

Descriptions of reconserved objects

Hablingbo church(Gotland): Porch, north

Time of measures: 1990, 1984. Reports 1990 and 1984 available.

Damages: Biological growth, relief weathering, sanding.

Types of measures: 1984 - cleaning, pointing of joints, consolidation, water repellant treatment,

1990 - complementary consolidation and addition, water repellant treatinent.

Damage cause(s)- Partial rain-exposition, biological growth.

Present status: Algae colonisation, rain-exposition

Conclusions:Rain shelter needed

Grotlingbo church (Gotland):

Time of measures: 1992, 1985. Report 1992 available.

Reliefs

Damages: Relief weathering, material loss, exfoliation, sanding, biological growth.

Types of measures: 1992 - cleaning, consolidation, reinforcement, repainting.

Damage cause(s): Partial rain-exposition, pointing, biological growth.

Present status: Continuous material loss

Conclusions: Decay cannot be stopped unless moved indoors.

Askersund countrychurch: Two porches, grave chapel, lynch-gate

Time of measures. 1995, ca 1975. Report 1995 available.

Damages: Biological growth, exfoliation, sanding, cement joints, relief weathering.

12th century

12th century

1670s

Types of measures: Preconsolidation, repainting, biological growth removed, injecUreinforcement, ironbars cleaned,

improved rain shelter, studding, consolidation.

Damage cause(s): Rain-exposition, biological growth, cement pointing, salt crystallisation.

Present status: Continuous material loss, exfoliation, salt crystallisation.

Conclusions: Rain shelter needed, improved water drainage, desalination.

Heby manor: Coat of arms

Time of measures: 1994, 1 985. Reports 1994 and 1985 available. 1778

Damages: Relief weathering, material loss, sanding, bad repair, fissuring, exfoliation, bad material.

Types of measures: 1985 - reinforcement, slight recotistruction (coat of arms), consolidation;

1994 - preconsolidation, reinforcement, cleaning, consolidation.

Damage cause(s): Rain-exposition, bad repair, partially bad stone material.

Present status: Intact

Conclusions: Rain shelter needed.

Nikolai church Nykoping: Two porches

Time of measures: 1992-93, 1983, 1966, 1947, 1923-25. Report 1992-93 available. 1644 and 1668.

Damages:

Types of Relief weathering, exfoliation/hollow surface, sanding, bad repairs, black staining, salt efflorescence.

measures: 1947 - cleaning, recutting, reconstruction-, 1966 - mortar repairs, reinforcement, 1 1983 _

cleaning, partial consolidation; 1992-93 - preconsolidation, injecUreinforcement (Paraloid B-72),

bentonite poultice (with NH4HC03), desalination, consolidation, repainting (cement removed).

Damage cause(s): Rain-exposition, conflagration, salt efflorescence, cement repair.

Present status: Efflorescence

Conciusions: Desalination needed

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Vadsbro church: Two porches, grave chapel 1650s

Time of measures: 1991 , 1981-82 1948, 1934. Report 1991 available.

Damages: Salt efflorescence, capillary water uptake, bad repairs, dense surface veneer (acrylate),

biological growth, exfoliation/hollow surface.

Types of measures: 1934 - new metal covering, linseed oil treatment; 1948 - paint removed, recutting, stone

replacement, patinization; 1981-82 - cleaning; 1991 - preconsolidation, mortar repairs removed

and remade, bentonite poultice, iichens removed, desalination, desinfection (Aseptine), consolidation,

inject and reinforcement, reconstruction, pointing, rain shelter inproved.

Damage cause(s): Rain-exposition, cement repairs, oil soaking, salt efflorescence, capillary water uptake.

Present status: Loss of repainting

Conclusions: Repainting needed

Tyreso church: Two porches 1640s

Time of measures: 1991 , ca 1980, ca 1970, 1933, 1897. Report 1991 available.

Damages: Bad repairs, relief weathering, exfoliation, dense surface veneer (acrylate), brutal cleaning with steel

brushes, sanding, biological growth, bad reconstructions, salt efflorescence, capillary water uptake,

salty cement.

Types of measures: 1980 - repairs, mortar repair, partial consolidation and addition of water repellant-,

1991 - preconsolidation, bentonite poultice, mortar repairs, consolidation, repainting, unearthing of

lower part of socle.

Damage cause(s): Rain-exposit ion, mortar repairs, salt efflorescence, capillary water uptake, bad repairs.

Present status: Humidity close to ground and concomitantly disintegrating repairs.

Conclusions: Humidity/capillary water uptake, water drainage from roof needed.

Jacob church, Stockholm: Porch, south

Time of measures: 1994, 1968-69, 1941 , 1929, 1909-1 0. Report 1994 available.

1644

Damages: Gypsum crusts. black staining, soda washing, salt efflorescence, sanding, bad drainage, bad repairs.

Types of measures: 1909 -10 - paint removed; 1929 - cleaning, oil soaking; 1941 - repairs with decosite-,

1968-69 - cleaning soda water/slaked lime, lime washing; 1994 - cleaning, bentonite poultice,

desalination by paper pulp, new rain shelter, repairs.

Salt chemistry.- Intermediate content with sulfate, calcium and sodium. Minerals include gypsum and Na-sulfate.

Damage cause(s): Rain-exposition, polluted environment, cleaning methods (bases; remaining high pH).

Present status: Partially white precipitate at surface (gypsum), partially rain-exposed.

Conclusions: Further desalination needed.

Petersen's residence. Stockholm: Double-porch 1640s

Time of measures: 1994, 1988-89, ca 1965. Reports 1994 and 1988-89 available.

Damages: Salt efflorescence, capillary water uptake, fissuring/ground movements, sanding, crust formation,

discolouring, polluted environment, conservation wintertime (1988-89).

Types of measures: 1960s - stone replacement, linseed oil soaking; 1988-89 - cleaning, bentonite poultice,

paint solvents, sand blasting, safety rei)airs, reconstruction, stone repiacement, consolidation, addition

of water repellant; 1994 - safety repairs, cleaning, dry grinding, desalination, consolidation, repairs,

inject.

Salt chemistry: Very high content close to ground. Lower parts sulfate, nitirate, chloride and sodium. Mineralogy

Na-sulfate, halite, gypsum. Upper parts sulfate, sodium, caicilim. Mineralogy gypsum.

Damage cause(s): Rain-exposition, salts, ground movements/cracks, bad conservation (l inseed oil, consolidation

during wintertime), polluted environment.

Present status: Sanding and high salinity close to ground. , growing cracks because of lowering water­

table giving partial ground submergence, polluted ground and atmosphere.

Conclusions: Further desalination, consolidation, ground strenghtening.

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Svartmangatan 6, Stockholm: Porch 1650s

Time of measures: 1994, 1982. Report 1994 available. Damages: Black staining, salts/capillary water uptake, biological growth, sanding, fissuring, reconstruction of

weak surfaces. Types of measures-. 1994 -preconsolidation, cleaning, bentonite poultice, desalination.repairs, reconstruction,

stone replacement. Salt chemistry: High content close to ground with sodium chloiride and gypsum; paper pulp extraction

showed high content of nitrate, chloride, sulfate, calcium and sodium. Upper parts contained gypsum

and paper pulp extraction indicated calcium and sulfate.

Damage cause(s): Rain-exposition, crust formation , polluted environment, salts/capillary water uptake, crack

formation, bad reconstructions.

Present status: So far acceptable.

Conclusions: Rain sheltering needed

Storkyrkan, Stockholm: Two sculptures

Time of measures: 1994-95, 1978-79, ca 1960, 1920, 1903. Report 1994-95 available.

1675

Damages: Discolouring/black staining, crust formation, material loss/airpocket, biological growth, bad

repairs, salts, cracks, iron expansion.

Types of measures: 1903 - recutting , oil treatment; 1920 - linseed oil painting; 1960 - cleaning, repairs (Caporal),

water glass treatment, impregnation (oxenoil and ligroin). painting; 1978-79 - paint cleaning

(mechanical and with chemical solvents); 1994-95 -preconsolidation, reinforcement (Araldite), cleaning

(paper pulp), desalination (bentonite poultice), consolidation.

Salt chemistry (in rain-shadow): Intermediate to high conterit of gypsum and sodium chloride. Paper

pulp extraction gave sulfate. sodium, calcium and some nitrate.

Damage cause(s): Rain-exposition, polluted environment, bad repairs, iron expansion. biological growth.

Present status: Kept indoors for copying.

Conclusions: The sculptures will be placed indoors and replaced with copies.

Tessin's residence: Porch

Time of measures: 1993, 1984. Report 1993 available.

Damages:Crust formation , discolouring (linseed oil), epoxy paint, algae, material loss, salt, cracks.

1690s

Types of measures: 1993 -preconsolidation, epoxy paint removed. bentonite poultice (black staining removed),

bad repairs removed. desalination. consolidation, biocide treatment. Further measures included linseed oil

paint and installation of rain shelter.

Salt chemistry: Alkalichlorides. potassiumsulfate.

Damage cause(s): Partial rain-exposition, polluted environment, crust formation. cracking paint.

Present status: The porch is painted with linseed oil and white pigment. Slight cracking of the paint close to the

ground because of high humidity.

Conclusion: Prevention of capillary rise of water needed.

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+ +

+ +

+

+ + \l Open quarry

+ Sandstone church

LJ Sandstone outcrop

IO Km

Figure 1: Map of southern Gotland with outcropping Gotland sandstone, the distribution of medieval Gotland sandstone churches and presently open quarries

J_J '"'\ .I \

) I

( DO ~

.. _ .--:. ::1 Ten objccu

Figure 2: The distribution of Gotland sandstone objects in Sweden grouped countywise. Note the high concentration in the Stockholm region (generally from the 17th and 18th century) . The letters represent the county symbols (compare table 1.)