The influence of geology on English brickmaking

Explaining the links between the geology of English brick clays, manufacturing methods, and the technical and aesthetic properties of bricks and associated building materials.

Left: Distinctive durable ‘blue’ bricks are made in the west midlands by firing iron-rich Etruria Formation clays in a reduced oxygen atmosphere.

Right: Typical London ‘stocks’ derive their characteristic colour and texture from a combination of brickmaking materials which occur along the Thames estuary. (Photo: iStock.com/ultraforma).

Until the late 18th century, fired (burnt) clay brick manufacture in England was an artisanal activity, largely carried out by itinerant brickmakers. These workers utilised locally available clays to manufacture small quantities of bricks and tiles, generally for relatively high-status buildings being constructed close by. Pre-industrial brick manufacture of this type was most common in areas of eastern and southern England where geology dictates that local sources of building stone are very restricted, but clays and related brickmaking materials are widely available.

The industrial revolution brought about enormous change in the pattern of supply and demand for bricks in England. This change was heavily influenced by both the geology of brick clay resources and the development of new technologies. Mechanised production and new kiln technologies, utilising specific bedded clay deposits with desirable and consistent properties, facilitated a massive increase in the volume of bricks produced and a large reduction in unit costs.

Bricks became mass-market products which fuelled the expansion of housing in industrial towns and cities, along with associated infrastructure and civic architecture. Then, as now, the interplay between the clay raw material and manufacturing technology determined the appearance and performance of bricks and tiles. This, in turn, was and is a major influence on the appearance and character of the built environment in many English towns and cities.

The mineralogy, chemistry and physical properties of a clay are essential in determining its suitability as a raw material for the manufacture of bricks or tiles. These factors affect the forming behaviour of the clay (the process prior to firing in which the ware is shaped from the clay) and the behaviour of the ware during the firing phase. Along with the manufacturing technology, they will be a key determinant of the technical properties of the final product (how it will perform in service) and its aesthetic properties (colour and texture).

Although bricks can be made from a wide variety of clay or clay-rich geological materials, high-quality brick clays consist predominantly of the clay minerals kaolinite and illite. These minerals impart desirable plastic behaviour (important in the forming or shaping of the brick) and firing properties to the clay. The quantity and particle size of the quartz (silica) component of the clay are also critical in determining forming and firing behaviour. Carbon and sulphur can have a major influence on firing performance. Low carbon and sulphur clays (usually less than 1.5 per cent and 0.2 per cent respectively) are preferred. The familiar red/brown colours of many bricks are due to the presence of iron minerals in almost all these clays. The presence of carbonate minerals such as calcite can produce paler coloured bricks.

Almost all bricks are formed or shaped by one of three basic processes: soft-mud, pressing or extrusion. In the soft-mud (or hand-made) process, individual bricks are formed in a sand-lined mould from a wet clay mix. This process has been carried out by hand since prehistory, although most soft-mud bricks are now produced by machine. In England, this process is particularly suitable for geologically younger clays with high natural moisture content.

The technology for mechanically pressing clay into an oiled mould to make bricks was developed in the early 19th century. Widely-used in the past, this technology is now confined to the manufacture of bricks from Jurassic clays in eastern England and is known as the ‘Fletton’ process.

Extrusion involves forming a column of very stiff clay by pushing the material through a die at high pressure. The rectangular section column is then cut into bricks (known as ‘wirecut’). Most clay roof tiles are also made using this method. Extrusion machines were first developed by Victorian engineers in Yorkshire. The technology is particularly suitable for older, harder clay raw materials with a relatively low moisture content which are common in northern England.

Once formed, bricks must be dried prior to firing. In the past this would have relied on air drying in covered sheds. Drying is now carried out using heated and humidity-controlled air. High moisture content means that soft-mud bricks require careful drying to avoid cracking. Sand is often added to these clays to mitigate drying problems.

As dried bricks are fired in a kiln, temperature increases bring about chemical and physical changes. Oxidation of organic matter and/or sulphides present takes place around 500°C. Water combined within the structure of clay minerals is driven off at about 600°C. Calcium carbonate dissociates to produce calcium oxide and carbon dioxide at about 800°C.

The important changes relating to the development of brick properties result from the breakdown of the original clay minerals, followed by formation of new crystalline and glass phases (vitrification). This usually commences at about 900°C, and is completed between about 1,050 and 1,100°C. The rate and extent of these reactions is also a function of time and kiln atmosphere.

Clamp firing is an ancient, pre-industrial method which involves building dried soft-mud bricks into dense rectangular settings called clamps. Flues are left in the layers, along with fuel to initiate firing. Firing of the clamp takes several weeks. A single firing will produce a range of brick grades because of the relatively wide temperature variation in the clamp.

Intermittent kilns were developed during the industrial revolution to allow closer control of kiln temperature and atmosphere. They were (and still are) used mainly for firing smaller quantities of tiles and pipes.

Continuous chamber kilns were developed in the 19th century. These were a major innovation in the industrialisation of brickmaking where hot fire-gases were drawn from one chamber to the next in order to dry, preheat and fire the bricks.

Most bricks are now fired in tunnel kilns. Here, the kiln is tunnel-shaped and served by rail-tracks carrying kiln-cars which have refractory decks on which dried bricks are set. The cars are pushed at regular intervals through the kiln, counter-current to a flow of air from the exit end of the kiln. Most tunnel kilns are gas-fired. Close control of temperature and atmosphere in tunnel kilns allows considerable gains in product consistency and fuel economy.

England has extensive resources of clay suitable for brick manufacture. Although extraction of clay for brick and tile manufacture was widespread across the country, historic production was concentrated on clay resources around centres of demand in and around London and the south east, and around the large industrial cities in the Midlands and the north of England.

Carboniferous mudstones (chiefly from the Coal Measures) comprise the principal brick clay resource in the north of England. From Victorian times to the mid-20th century, production was very widespread in coalfield areas. These clays are relatively hard and require mechanical treatment (milling, extrusion and wire-cutting) to form bricks and other ware. The smooth-faced, sharply-defined and durable red bricks which are common in the industrial towns in the north west of England are typical of this combination of clay and manufacturing method.

Also Carboniferous in age, the kaolinite-rich, low-carbon Etruria Formation is a premium-quality brick clay resource which occurs in the west midlands and north-east Wales. Aside from high-quality, wire-cut red bricks and tiles, this clay formed the principal raw material for much of the architectural terracotta popular with late-Victorian architects. A more prosaic use of this important clay was (and is) in the manufacture of Staffordshire ‘blue’ bricks. These very distinctive, hard and durable bricks are much used in engineering applications such as railway arches. The outcrop area of this clay is very restricted and large parts have been sterilised by other development.

Further south, Jurassic-age Peterborough Member (Lower Oxford Clay) in Cambridgeshire and Bedfordshire was an important brick clay resource in the 20th century. Using the Fletton process, vast quantities of cheap, pale-coloured bricks with a distinctive indentation (‘frog’) fed the inter-war and post-war housing booms in southern England and the London area.

Around London, a variety of Cretaceous-age and younger clays comprise relatively abundant clay resources. Yellow hand-made London ‘stock’ bricks, characteristic of many older buildings in the capital, were produced in the 19th and 20th century along the Thames estuary using a combination of ‘brick earth’ (a silty material deposited by the wind in the last ice age), chalk and ‘town ash’ (residue from domestic coal-burning). The ash was shipped downstream by barge, which then returned upstream into the city loaded with distinctive yellow, sanded bricks with soft edges.

In Surrey, Sussex and Kent, the Cretaceous-age Weald Clay and Wadhurst Clay form the principal brick-clay resource. These younger clays have a relatively high moisture content, which dictates hand-made or soft-mud brick manufacture. Bricks are commonly deep red, sometimes mottled blue. Surface sanding, creases and rounded edges resulting from manufacture can give these bricks a softer, muted appearance in buildings.

Bricks are an integral part of our built environment. Brick was and is an attractive, durable and versatile construction material. Geological history has dictated the distribution and technical properties of English clay resources and, over time, brick manufacturing methods have been developed which take these characteristics into account. This combination of clay properties and manufacturing process has resulted in a wide range of brick and tiles which vary hugely in colour, texture, form and technical performance. Alongside domestic sources of building stone, these important building materials have contributed significantly to the rich regional and local architectural diversity of England.

This article originally appeared in Context 143, published by the Institute of Historic Building Conservation (IHBC) in March 2016. It was written by Andrew Bloodworth, science director for minerals and waste at the British Geological Survey. It was published with permission of the executive director, British Geological Survey.

--Institute of Historic Building Conservation