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. 12 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. ___________________________________ .., ___________ Table 2: Age distribution of County Number of Conserved conserved Gotland sandstone buildings 1988-1995 buildings ----------------------------------------------- -------------------------------------------- A 272 19 Time period Number of AB 70 3 buildings c 41 3 D 49 8 1000 - 1300 4 E 27 1 1300 - 1550 F 6 1550 - 1650 19 G 1 1650 - 1750 24 H 24 5 1750 - 1860 4 I 30 6 1860 - 1910 3 K 7 1910 - 1940 1 L 11 1 ------------------- M 24 3 Total 55 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 13 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), freeze­ thaw 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.