Inese Sidraba, Institute of Geology, University of Latvia (Latvia)
Vija Hodireva, Institute of Geology, University of Latvia (Latvia)
Latvia is located in the Northern part of Europe. Climatic conditions is characterised by frequent freezing/thawing and wetting/drying cycles that promotes unfavourable exposure conditions for porous stones. Historically various types of sandstones, mainly imported, were used for architectural details in Latvia. Nowadays significant part of Latvia's Cultural Heritage is sandstone carvings - sculptures, monuments, reliefs and portals. The current work focuses on two sandstone types prone to weathering exposed to Northern climate in Latvia. Calcite cemented sandstone of Silurian deposits known as Gotland sandstone (Sweden) has been wide used in Latvia's Monuments in 17th - 18th century. Gotland sandstone used in Monuments in Latvia consists mainly of SiO2 78,8 - 81,5 weight% and CaO 5,4 - 6,5 weight%. Rock is fine grained, homogenous and with macroscopically visible mica content. Typical features are content of different feldspars (potassium and plagioclase), micas (brown coloured biotite, muscovite and green coloured hydro micas) and carbonate minerals (calcite and dolomite). The presence of clay minerals was detected.
Local sandstone with dolomite cement of Upper Devonian deposits known as Rembate sandstone (Rembate, Latvia) has been studied. Outcrops were investigated in the 1930ties and the stone was used in 20th century in Monuments in Latvia. Stone material is not accessible any more as deposits are located in protective zone of river Ogre. Rembate sandstone is reddish to violet-greyish colour, inhomogeneous, with laminated structure and consists of quartz - 38%, dolomite - 58% and glauconite - 4%. Feldspars (potassium and plagioclase), micas (muscovite), heavy minerals (hornblende, tourmaline) and opaque minerals were detected.
Case studies for calcite cemented sandstone summarises Monuments dated from 16th to 19th century, located in various environments and conditions. Information has been gained in the frame of investigation and conservation works in the period from year 2000 to 2007. Weathering of calcite cemented sandstone results in pulverisation, exfoliation, loss of surface, fissures. Observed weathering processes confirms with data from Monuments in Sweden. Comparatively, dolomite cemented sandstone performs the same weathering forms however selective weathering due to inhomogeneous structure is more characteristic.
Investigations of material properties of sandstones with carbonate cement and performance of stone material in Monuments of 200 - 500 years age leads to conclude on low weather resistance of this type of sandstone exposed to Northern climate.
Studied sandstones features promote deep structural alteration by climatic impact alone. This leads to conclude that for safeguard Cultural Heritage complicated conservation interventions (structural consolidation, surface protection), and regular maintenance is a key factor to prolong lifetime of works of art carved in carbonate cemented sandstones.