Cob (material)

In technical building and engineering documents, such as the Uniform Building Code of the western USA, cob may be referred to as "unburned clay masonry," when used in a structural context. It may also be referred to as "aggregate" in non-structural contexts, such as "clay and sand aggregate," or more simply "organic aggregate," such as where cob is a filler between post and beam construction.

History and usage

Cob is an English term attested to around the year 1600 for an ancient building material that has been used for building since prehistoric times. The use of this material in Iran is more than 4000 years old. The etymology of cob and cobbing is unclear, but in several senses means to beat or strike, which is how cob material is applied to a wall.

Many similar materials and methods of earthen building are used around the world, such as adobe, lump clay, puddled clay, chalk mud, wychert, clay daubins, swish (Asante Twi), torchis (French), bauge (French), bousille (French mud with moss), beaten clay-pahsa (Central Asia), and cat and clay.

Cob structures can be found in a variety of climates across the globe. European examples include:

• in England, notably in the counties of Devon and Cornwall in the West Country, and in East Anglia (where it is referred to as clay lump)
• in Wales, notably in rural Anglesey
• in Donegal Bay in Ulster and in Munster, South-West Ireland
• in Finisterre and Ille-et-Vilaine in Brittany, where many homes have survived over 500 years and are still inhabited

Some of the oldest human-made structures in Afghanistan are composed of rammed earth and cob. Cobwork (tabya) was used in the Maghreb and al-Andalus in the 11th and 12th centuries, and was described in detail by Ibn Khaldun in the 14th century.

Many old cob buildings can be found in Africa, the Middle East, and the southwestern United States like the Taos Pueblo. A number of cob cottages survive from mid-19th-century New Zealand.

Traditionally, English cob was made by mixing the clay-based subsoil with sand, straw and water using oxen to trample it. English soils contain varying amounts of chalk, and cob made with significant amounts of chalk are called chalk cob or wychert. The earthen mixture was then ladled onto a stone foundation in courses and trodden onto the wall by workers in a process known as cobbing. The construction would progress according to the time required for the prior course to dry. After drying, the walls would be trimmed and the next course built, with lintels for later openings such as doors and windows being placed as the wall takes shape.

The walls of a cob house are generally about 24 inches (61 cm) thick, and windows were correspondingly deep-set, giving the homes a characteristic internal appearance. The thick walls provided excellent thermal mass which was easy to keep warm in winter and cool in summer. Walls with a high thermal mass value act as a thermal buffer inside the home. The material has a long life-span even in rainy or humid climates, provided a tall foundation and large roof overhang are present

Cob is fireproof, while "fire cob" (cob without straw or fiber) is a refractory material (the same material, essentially, as unfired common red brick), and historically, has been used to make chimneys, fireplaces, forges and crucibles. Without fiber, however, cob loses most of its tensile strength.