The Building Conservation Directory 2021

88 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 1 C AT H E D R A L COMMU N I C AT I ON S TRADITIONAL BRICKWORK Inherent defects, vulnerabilities and the importance of good maintenance TERRENCE LEE T RADITIONAL BRICKS may be regarded by some as hard wearing and impervious, universally usable in each and any brickwork application. However, the very title ‘traditional’ suggests a process of both manufacture and construction passed down through scores of generations of brickmakers and bricklayers who executed their crafts in a time-honoured way. It implies a technology developed in an age when transport options were lamentable or non-existent and always costly, and building materials were almost always sourced locally. In the case of brick this means local clays fired in clamps or kilns using the fuels available, as close to the construction as possible. In considering the causes of brickwork failure, while structural problems are one factor, the materials and technology used to make the bricks and mortar themselves are another. Wide variations in physical form and quality of older bricks are common as a result of their unscientific and inconsistent manufacture across a myriad of small brickmakers which sprang up wherever there was a clay deposit. These anatomical variations and faults in the mortar in which the brick is laid may not be individually significant, but together they can make brickwork more vulnerable to external factors and accelerate decay. ANATOMY OF TRADITIONAL BRICK For bricks to be durable and have the required form for use in construction, the clay with which they are made should contain a number of naturally occurring minerals which influence the chemical reaction of the brick clay during firing and ultimately, the finished product. Traditionally, brickmakers could not alter the mineral balance of the clay but they could add elements such as sand, lime and ash. An imbalance of elements may result in the brick being more prone to the effects of external decay factors. Clays consist mainly of a few specific minerals which have a plate-like structure and tend to form sheets. These clay minerals, which are responsible for the material’s unique properties, formed over millions of years from the weathering of rocks and volcanic ash deposits, and there are wide variations in their minerology from region to region. Because they were so accessible, clays found just below topsoil level are known as brick earths. These were the only clays used for brick-making until the Elizabethan era and their use continued well into the 19th century, despite being the most variable. As well as the clay minerals, most brick clays will also contain a variety of other materials which affect how the clay responds to the firing process in the kiln and, ultimately, the performance of the brick. These include sand (silica), alumina, magnesia, sodium, potassium, iron as well as lime and potash. For example, small quantities of iron when it oxidises during firing produces hematite (from the Greek for blood) which influences the colour of the finished brick. If iron is present in the form of pyrites however, this can expand when it comes in to contact with moisture and oxygen, causing splits and fissures. Both lime and potash were commonly used as a flux, a material that enables particles in the clay to start to fuse permanently together at lower firing temperatures. Some degree of fusion is essential for the cohesion of the fired brick, but too much flux can lead to full vitrification, in which part of the clay actually melts and the brick slumps. Alumina can also result in misshapen bricks for the same reasons, but it is added to improve the plasticity of the clay before firing. Silica, on the other hand, is essential for toughness, but too much can cause brittleness. Larger pieces of stone or flint which were not found and removed during the manufacturing process are known as inclusions. These may react differently under high temperatures and can rupture both during and after firing, causing fissures, tears and bulging in the brick, or they may protrude through the fire skin of the brick, weakening it and rendering it unusable as a facing. Limestone inclusions, for example, turn to quicklime when fired, so when the bricks get wet, the inclusions slake, rupturing and splitting the brick. Brickwork from the 18th to the 20th century in Market Drayton, Shropshire (All photos: Jonathan Taylor)