The Building Conservation Directory 2020

94 T H E B U I L D I N G CO N S E R VAT I O N D I R E C TO R Y 2 0 2 0 C AT H E D R A L COMMU N I C AT I ON S SANDSTONES and LIMESTONES JONATHAN TAYLOR B EFORE THE development of mass transportation systems – first the canals and then the railways – buildings were predominantly constructed with local materials. Stone was always the favoured walling material, and local building stones still dominate the character of towns and villages across the UK, alongside brick and timber framing in areas where stone was not available. Limestones and sandstones are the most commonly used building stones in the UK, largely because they are found across a wider area of the country than others, with limestones predominating in the south and sandstones in the north. These sedimentary rocks were formed from mineral sediments that accumulated on what was once the floor of a warm, shallow sea. The sediment was either the eroded remains of pre-existing rocks – including igneous, sedimentary and metamorphic rocks – or fragments of calcium carbonate mostly from shells or the skeletons of marine organisms. When compacted over millions of years, cementation took place by the migration of some minerals to form rock. Calcium carbonate in the form of calcite is the predominant cement in limestones but it is also found in many sandstones. Other cementitious minerals include magnesian carbonates, ferrous compounds and, particularly in sandstones, silica. When used as building stone, the properties of sedimentary rocks is influenced by the nature of the particles, the strength and composition of the cementation and by the bedding planes which form as a result of the layering of sediment on the sea floor. Sedimentary rocks with deep layers and no obvious bedding planes produce a building stone known as a freestone which can, in theory, be worked and used in any plane. However, most sedimentary rocks should be placed the same way up as they were originally laid, with the bedding planes parallel to the ground. Conservators regularly find examples in historic buildings where whole sections of sandstones and limestones have sheared off because the bedding plane was placed vertically or at a steep angle. Columns and stone posts such as window mullions are particularly prone to this type of defect. STONE ROOFING SLATES AND FLAGS Some sandstones and limestones have particularly pronounced bedding planes, often as a result of the inclusion of thin layers of micaceous clay minerals in the sedimentation process. These layers make the stone fissile, enabling it to be split into flagstones or stone roofing slates, but making it less suitable as a walling material. Limestone roofing slates are characteristic of towns and villages in the Cotswolds and in the east Midlands around the village of Collyweston, for example. Sandstone roofing slates are common in much of the north of England and parts of Scotland, as well as further south in South Wales and Herefordshire and in the South East where Horsham stone was used. These stone slates tend to be chunkier and less regular than roofing slates of true slate, which is a metamorphic rock formed under heat and pressure from sedimentary deposits containing fine particles of clay. In the process the clay minerals align themselves with the direction of pressure, resulting in relatively impervious material that is extremely fissile, so it can be split into much finer and far lighter slates. In the late 19th century following the rapid growth of the railway network, local vernacular materials started to be replaced by better materials, and in areas with good rail connections many vernacular roofing traditions have now almost completely disappeared. Many of the quarries closed and generations of roofers have grown up knowing only how to use metamorphic slates. As a result, new roofs which were made using the local materials were detailed using the skills taught nationally, not in the local vernacular. Original roof which are detailed in the local vernacular are rare and very precious, providing a model for conservators to follow. BUILDING STONES AND THEIR PERFORMANCE The properties of sandstones and limestones vary widely depending on the cohesiveness of the cementation, the hardness of the minerals they bind, and the pore structure of the resulting matrix. Although some are made impermeable by the presence of particular Edinburgh’s classical 18th century architecture is built of a fine grained Carboniferous sandstone which takes carving well and is extremely durable