The Building Conservation Directory 2021

89 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 1 MASONRY 3.2 PROBLEMS INCURRED DURING FIRING For centuries the firing of bricks was a largely uncontrollable process in which temperatures varied significantly within the clamp, essentially a large stack of unburned bricks laced with combustible material such as furze or hedge cuttings. The bricks were fired at between 900 and 1100 0 C, with temperature variation effecting both the colour and the quality of the brick. Those closest to the fire channels were burnt blue grey, while those on the outer edges of the kiln were often underfired and pink in colour. As well as a wide variation of colours found within one firing, rapid increases in temperature could result in weak bricks with a blackened core. Vitrification often occurred in wood fired clamps where bricks were near the fuel source, depending on the mineral composition of the clay. Nevertheless, most bricks would be usable somewhere in a building, the majority hidden from view and covered by layers of plaster. VULNERABILITY OF TRADITIONAL BRICK Poor materials and inconsistent firing can usually be identified by sporadic examples of eroded bricks on an otherwise sound elevation, and it is rare to find large expanses of inadequate material. Furthermore, even with a variety of defects and visual anomalies, brickwork can usually deal with environmental conditions and rainfall successfully, if laid in an effective lime mortar. Indeed, any surviving pre 19th century masonry has already been proved to be perfectly adequate, and with proper care it should last another 200 years. Conversely, the symptoms of an imbalance of minerals and other materials in the brick clay outlined above can both facilitate and be compounded by other external factors including water ingress and modern inappropriate intervention. EXTERNAL FORMS OF WATER INGRESS Quite apart from natural erosion of joints which can allow water ingress from windblown rain, the continuous presence of high levels of moisture both on the surface and inside the wall will undo the properties of the lime mortar. Frost action in particular will make the mortar friable and soil like, and an attractive environment for both plant and insect life, in effect, returning the joints to nature. Hollow and crumbling mortar joints can provide a gateway for moisture to enter a wall, particularly if rainwater is channelled or directed by broken or damaged architectural features such as cornices or banding courses on to adjacent brickwork. Forms of water ingress can often be attributed to maintenance issues and neglect. Quite apart from damaged chimneys and roof elements, sources might include cracked or broken guttering and rainwater goods, damaged salt glazed drainage close to walls and below ground, or standing water on concrete adjacent to brickwork. Other forms of directed water could be from hard cement pointed elevations, panels or even individual bricks which act like a ledge, channelling rainwater quickly on to adjacent lime pointed work and accelerating the rate of decay. Brickwork is at risk when areas of brickwork become saturated after prolonged rainfall, filling the pores, fissures and voids with water, causing spalling (flaking) of the brick faces when the moisture freezes. Salt crystallisation is one of the most common causes of brick decay. Harmful soluble salts such sulphates and chlorides can exist naturally in the brick clay, or they may come from additives found in cement mortars, from contaminated aggregates, from road salt, sea water or standing water, or they could be the result of salts leaching from high soil levels at the base of outside walls. Original fuel ash-based mortars, although cheap and highly pozzolanic, may contain harmful substances associated with coal burning including sulphates in particular. In their benign form, these salts may appear as efflorescence as a white powder on the face of the brickwork which can be brushed off relatively easily. A far more serious form of sulphate attack occurs when the salts crystallise inside the brick, causing the surface to spall under the pressure. Sulphate attack, perhaps one of the worst forms of decay, can affect individual bricks, panels, or even entire elevations of brickwork. CEMENTITIOUS POINTING The use of hard cement pointing mortars compounds the problems of water ingress discussed above by trapping moisture inside the walls. Since the lime mortar joints form the main conduit for water evaporation from the wall, and as this can be up to 20 per cent of a wall surface cement pointing greatly increases the risk of the bricks becoming saturated. Hard cement pointing also forms ledges inside the brickwork joints which allow water to accumulate until it subsequently freezes, causing the edges of bricks, or arises, to break, resulting in the widening of brickwork joints. MAINTENANCE Regular maintenance can avoid the costs of much larger repair work, the aim being to identify a problem and carry out remediation before it can become worse. Ideally, a maintenance check should be carried out at least twice a year, one of those times being after the leaves have fallen in late autumn. In fact, paying attention to the brickwork on a much more regular basis can only be a good thing and is likely to increase the awareness of the things that look out of place. The maintenance check should involve a thorough examination of the brickwork, working systematically from the chimneys A brick clamp in Malawi: the bricks lining the fire channels at the bottom could be well fired or over-fired, while those on the outside and furthest from the heat will be little more than dried clay. Traditional kiln- and clamp-fired bricks from 17th and 18th century buildings in Shropshire: left, the blackened one probably lined a fire channel and should be highly vitrifed, while the one on the right has a large limestone inclusion which would have caused it to shatter. Right, the brick in the middle has been cut through to show the depth of the fissures which, on this case, may simply be the result of poor mixing.