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

Title Page Previous Next | Material Characteristics >Soils >Constituents

4.1.1 Constituents

The essential components can be crudely described as aggregate and binder. The binder is the clay, but requires water to enable it to become mobile so that it may coat the particles of aggregate.
The term soil is frequently used, but it must be stressed that daub generally excludes organic soils such as ‘topsoil’ and peat. Soils with greater than ~20% organic matter have altered characteristics and must be treated differently. The reference to daub as ‘clay’ is common, but a soil that is predominantly clay is not widespread and in any case would be undesirable because of shrinkage problems.

Soils are generally distinguished as being one of two types, either cohesive or granular. After removal of an initial compressive load (such as squeezing a sample), an undrained cohesive soil tends to bind together due to a combination of friction between the particles and the negative pore pressure produced by the water content. A granular soil has no such inherent compressive strength.

A soil is never composed entirely of a single particle size, but has a distribution of sizes. This is best described as continuous rather than discrete and has particles ranging from clay (of diameters ≤2μm), through silts (2 - 60μm) and sands (60μm-2mm) up to gravels (≥2mm) [51]. The attributes of a soil depend greatly on this distribution.

Because a soil is most likely to contain particles that are both cohesive and non-cohesive, its properties cannot be described by any one particle size: it is the combined effect of the various particle sizes that will determine the overall behaviour. To overcome this problem the terms ‘coarse’ and ‘fine’ are also applied to a soil as whole. If a soil contains fine particles that fill the voids between the coarse particles, then this soil as a whole is described as ‘fine’. More analytically, a coarse soil can be distinguished from a fine soil as having the majority of particles having a diameter greater than 60μm.[52] A particle size of 60μm also happens to be the approximate point at which particles become visible to the naked eye.  

Within the fine soils, the boundary between clay and silt is also significant to its performance. Other than particle size, clay differs from silt in that it is primarily a hydrous silicate of aluminium.[53] Clay has further specific considerations that affect the performance of a daub.

[51] The scale continues up to cobbles and boulders, but in the context of soils for daubs these are of no concern.
[52] The accepted boundary is 65% coarse particles and 35% fine particles, although this is not a reliable predication of soil behaviour.
[53] combined with other impurities such as iron oxides, magnesia and lime.