This technique used in vernacular construction is well described by Burchell in his ‘Travels’ of 1824 when visiting the Du Toits of Bushmanskloof beyond Franschhoek. The process was more formally described in the Loudon Encyclopaedia of Architecture of 1833. However the locals who employed this method would inconceivably have resorted to written text, rather reliant on folk knowledge of traditional knowledge systems learnt at their parent’s knee. It was a building technique ubiquitously used by Dutch settlers, Trekboers and later Voortrekkers. It requires a source of clayey ground into which is added “a good proportion” of sand and grit, possibly straw or dung, combined in a pit, all trod through by oxen-hooves in span. This mud must be “well-tempered”, sufficiently stiffened to be able to stand alone up to 300 mm in height without slump. This was prepared at the same time as the foundations were being laid, and would be left to leaven for about seven days, deemed ready when a ball made from it, when thrown to the ground, retained its shape. The cob was delivered to the builder on pitchforks, who then piled it in courses of about 300 mm, all built over a good stone or slate foundation. Each layer was allowed to set and then pared to an even surface with a flat paddle, much as that used as a pizza oven shovel. The corners were laced through diagonally by sapplings or braided cord at each layer to prevent the separating and bursting of the mud structure at this weakest point where the direction of the stresses of thrust changed. These stresses were consequent to the additional weight of the gable on the end wall, thereby creating shear while on the other was the thrust of the weight of the thatched roof.
Where opening were planned for – predictably placed symmetrically in all facades as well as internally, timber laths were inserted embedded in the monolithic construction, sometimes in pairs a distance apart in height to act as tension and compression members in the homogenized mud. The process was slow as each layer had to be sufficiently dry to support the next layer. Once the desired height was reached the mud substructure was timbered and thatched over.
Once the cob had settled and set, a wire garrotte was threaded through the prepared holes in the wall where the openings had been catered for and cut through up to the embedded laths, and the mud walls there knocked out to form the requisite window openings, recessed wall cupboards, doorways and hearths.