Jan
What is cob?
Throughout the South West of England there are several traditional building materials that have been historically used for construction, including cob. Over millennia, cob as a building medium, became less popular as the onset of mechanisation kicked in. Notably, advancement in quarrying techniques [for stone] and more reliable transportation networks for material distribution, such as bricks, secured the demise of cob construction.
Freight trains shrank the country with widespread brick distribution
As a cob specialist in Cornwall, the cob walls of the west country are of particular interest. I’ve been fortunate enough to encounter cob walls that were constructed in the latter half of the 16th century and the early years of the 20th century. Consistently though, most surviving cob walls in Cornwall and Devon are from the 18th and 19th centuries.
Typical 18thC cob dwelling in Cornwall
Materials used to make cob
There are three basic ingredients that are required to make cob……clayey soil, straw and water. When these natural materials are mixed together, they can be formed into cob walling and when the cob mixture dries, the cob wall becomes a hard, homogenous and structurally capable cob wall.
Cob wall in Cornwall
Clayey Soil
Geologically, Cornwall and Devon are blessed with a subsoil that comprises of natural clays. These natural clays can vary in colour, for example in Devon the clays are more red in colour and in Cornwall the clays range from ochre to grey in colour.
Cornwall’s clayey soils are perfect for making cob
Clay content within the soil is a crucial element to successfully making cob. However, clay on its own is not so effective. Clayey subsoils must also contain sands and aggregates.
Straw
With appropriate clayey soils identified, in order to make cob to use as a building medium, straw is added. The inclusion of straw [to the clayey soil] plays 3 key roles. Firstly, the presence of straw actively encourages distribution of clays and silts throughout the cob mixture. Secondly, straw facilitates usability ie it acts as a binder and allows for manageability. Thirdly, and once the cob wall is built, the inherent straw decreases the likelihood of excessive shrinkage.
Straw is added to the clayey soil
Mixed cob ready for building a cob wall
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