Cob 101 / What is the purpose of the various cob ingredients?

Aggregate: aggregate is the stoney component of cob. It is sand, or gravel, or other sharp rocky fragments. Smooth pebbles, silt, and other rounded stoney particles work poorly as aggregate as their smooth contours prevent them from locking together tightly. Rocky aggregate is inherently stable and does not shrink or expand to any appreciable extent with varying moisture and temperature. It is what makes cob strong, however, to be useful as a building material, it requires a binding ingredient to lock all the individual grains together…

Clay: clay is inherently unstable. It expands when wet or hot, and contracts when dry or cold. This property of clay is related to its microscopic association with water, which essentially coats each clay particle in a film causing the particles to be pushed away from one another when moisture is added, and to be sucked very close together when water is scant (hence the expansion and contraction). It is for this same reason however that clay is a superb binding agent as the water film is what promotes the tight association between the individual clay particles.

Fibre: all different sorts of fibre have been used in cob mixes however the consensus seems to be that fresh dry straw (not hay) is the best. It serves a number of purposes ranging from providing tensile strength to the structure, much like rebar in concrete, to stabilizing and dispersing cracks, to absorbing and holding on to excess water when the cob is being mixed. Some believe that the air spaces created by the trapped straw enhances the insulation properties of cob but we aren’t so sure about this.

Water: water transforms the dry constituents above in to a wonderfully sticky, gooey muck that forms your structure. It is what activates the clay and allows it to coat the individual aggregate particles ultimately gluing it all together in to a homogenous mash.