The petrography of the Giffnock sandstone from the interior of the Kelvingrove Art Gallery and Museum, Glasgow

Samples of sandstone from the interior of the Art Gallery and Museum at Kelvingrove in Glasgow were characterised. The samples investigated comprised those cleaned using an EDTA/natural rubber latex poultice method during the 2004-6 refurbishment of the Gallery and uncleaned samples with an intact black soiling patina. This study was undertaken in order to provide a petrographic explanation of the stone coloration and the efficacy of the cleaning methodology. The stone is a medium grained, well sorted quartz arenite, containing mono-crystalline quartz, extensively dissolved microcline feldspars, muscovite micas, opaque oxide minerals and rare accessories such as zircon, tourmaline, apatite and rutile. Sub-angular to sub-rounded framework grains exhibit pressure solution contacts and occasional overgrowths. Secondary minerals are ankerite cements, kaolinite clays and iron oxyhydroxides. The stone exhibits a warm honey colour or a more grey tone, related to secondary iron oxide mineral content. Relationships between iron oxyhydroxide, ankerite and kaolinite suggest a complex diagenetic history for the stone, in addition to an overprint of mineral growth related to post quarrying exposure. We tentatively propose two morphologies for ankerite. In euhedral masses, a compositional zoning is evident as a late Fe and Mn enrichment and Mg depletion. The honey-coloured surface patina common on many of the cleaned blocks in the Gallery is generally present in a zone up to 5 mm from the exterior surface. Interstitial masses of iron oxyhydroxide can be seen increasing in occurrence towards the exterior of the stone in the honey-coloured samples. There is evidence for the progressive replacement of ankerite with the iron oxyhydroxide phases. In some, the brown iron oxyhydroxide coloration is present throughout the thickness of the sample, but these samples exhibit a greyish surface colour, with a depleted Fe zone on the exterior, and do not contain any carbonate cement. Samples with a honey-coloured surface contain abundant ankerite cement but less interstitial iron oxyhydroxide, except near the surface where replacement of carbonate becomes common. Uncleaned samples have a thin black, opaque surface layer up to 100 µm thick, whose elemental composition includes Si, Al, S and Fe. In one sample, Ba is also present in small barite grains. Gypsum is present in the soiling layer, but full characterisation of the blackened surface has not been undertaken.