The Building Conservation Directory 2020

95 C AT H E D R A L COMMU N I C AT I O N S T H E B U I L D I N G CON S E R VAT I ON D I R E C TO R Y 2 0 2 0 MASONRY 3.2 minerals, most sandstones and limestones are highly porous with an interconnected pore structure. This makes them highly permeable, and when used as a building stone bedded in a lime mortar in a traditional solid wall construction, the structure is said to ‘breathe’. Any wind-driven rain falling on the outside will make the surface damp, but it dries quickly too. Between rain showers even in the winter, the damp never penetrates far into the masonry. If properly maintained, most of the wall remains dry, insulating the interior from both the temperatures outside and from the evaporation taking place at the exterior surface. The large mass of masonry also helps to moderate humidity and temperature, helping to keep the interior cool in the summer months. Lime mortars tend to draw moisture out of the masonry, and a well pointed wall will encourage rainwater to run down the face and not into the joints. In areas where stone was readily available, cheaper houses would have been built with random rubble stonework and rendered with lime mortar to produce an even finish (smooth or rough cast) and to protect it from the elements. These renders – known as harling in Scotland – have been widely removed by owners who wish to show off their natural stone walls. The alteration reduces their resilience to weathering, and often exposes stones that are naturally friable and easily eroded by wind and rain. COMPOSITION The particles of a sandstones are mainly composed of quartz grains, but they may also include various other rock fragments and other minerals such as feldspar. Small stones, fossils, shell fragments and, in the more fissile stones, micaceous clay minerals may also be included. Many sandstones are extremely granular and easily distinguished from limestones, while others, such as Edinburgh’s Craigleith are much finer. Cementation by silica is strongest in older rocks which have been subjected to most heat and pressure. These are extremely hard and durable, but also more difficult to shape. Prior to the development of stone cutting machinery, the hardest stones were generally avoided unless there really wasn’t any alternative, and even then they were only used for the most important buildings such as churches. Conversely, softer and more friable stones were often used for ordinary building work and protected by broad eaves, careful detailing of drips and other features designed to minimise the flow of water down the masonry. In limestone the particles may include fragments of fossils and shells, as well as a smaller proportion of grains of quartz and other minerals common in sandstones. Often the body of limestone is extremely fine grained, enabling it to be easily carved, but fossil inclusions may be far harder, causing chips in the carving that may need to be filled in later. Most limestones used as building stones are extremely durable, but they are particularly vulnerable to pollution, resulting in a blackened sulphate skin that can delaminate. The softest limestone found in historic buildings is chalk, which was used where there was no viable alternative, and always carefully protected from the elements. Conservation of masonry structures depends largely on good maintenance and attention to detail where repairs are required. Local traditions such as renders or projecting courses of stone which shed water, will have evolved to cope with any deficiencies in the local stone, or to enable the walls to be built using less material. Identifying these details often requires careful detective work because of the changes in construction technology and practice that have occurred since the early 19th century. While all conservation relies on changing as little as possible, defects in masonry are often inherent, and alterations may be required to replace defective stones, to add weatherings or to remove rusting metal cramps for example. Correct identification of the cause of deterioration can be complicated, and may require the services of specialist building surveyors and geologists as well as stonemasons. JONATHAN TAYLOR MSc, IHBC is the editor of The Building Conservation Directory . Fine carving in Bath’s oolitic limestone A relatively durable sandstone from the millstone grit series in Wirksworth, Derbyshire: in this case the building had been neglected in the past, exposing the masonry to saturation and frost damage.