Wattle & Daub: Craft, Conservation & Wiltshire Case Study
3.6 Daub
4.1 Soils
4.1.3 Strength
4.2 Dung
4.2.2 Lignin
4.2.3 Urine
4.3 Fibre
5.4.2 Renewal

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4.3 Fibre

The primary role of fibre is to provide reinforcement of the daub, which is usually required due to the volumetric instability of the clay.  Cracks that are able to form across the total thickness of the daub would jeopardise the structural stability of the panel if not compensated by the inclusion of fibre. The role of steel rod in reinforced concrete is a good analogy to the primary role of fibre in a daub.

It has been proposed that the fibre also helps by dissipating the shrinkage of daub during drying.[72] This may be likened to ‘bed joint reinforcement’ of masonry walls and functions by embedding a ductile material within the fabric. Under a tensile stress, the reinforcement deforms (strains) uniformly along its length rather than at a single point. This results in micro-cracking of the surrounding fabric along the length of the reinforcement, rather than causing a visible crack at a single location.

Minke (2000) has shown that straw may decrease linear shrinkage by approximately 25% per 1% of added fibre. However, it has also been demonstrated that the addition of fibre in large proportions (i.e. 6-8%) may cause a decrease in compressive and tensile strengths.[73]  There is therefore an optimum proportion of added fibre of approximately 2-4%.

[72] In the context of conservation, Harrison (1999), p.102, documents a theory originating from Duncan (1947), p.124, that straw, ‘takes up the shrinkage in the wall and distributes it about the mass of the wall, so that no exterior cracks are caused.’ In the context of new work, Minke (2000), p.44, Houben and Guillaud (1994),p.82, also support this.
[73] Houben and Guillaud (1994) showed fibre affects compressive and tensile strengths, whereas Minke (2000) discusses only a decrease in compressive strength.