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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4.1.4 Field Testing

Samples of dry historic daubs need to be removed from the site in sample bags and analysed in the laboratory.[58] However, a reasonable characterisation of a soil can be readily performed on-site where a quick assessment is required or where laboratory tests are not justified.
Coarse and fine soils can be distinguished by whether a damp soil sticks together. The sample may require drying in the palm of the hand or wetting in order to perform this test.
The sand/silt boundary can be judged by comparing the particles that are visible with the naked eye versus the proportion that become visible only with a loupe magnifier.[59]
Figure 28. Compact packet sieves for field use. (Courtesy of Endecotts Ltd)
Gravels and sands are distinguishable visually, since particles sizes of 2mm can be roughly judged by eye. The critical dimension of an elongated particle is its smallest diameter, which determines whether is may pass through a 2mm sieve.
The proportion of fines can be judged by spreading a sample and inspecting with a loupe magnifier. A more detailed field test can be done using a set of ‘pocket sieves’. Similar to their larger counterparts used in the laboratory, they can quickly grade the gravels, sands and silts of a small soil sample on-site [Figure 28].

A sand can be identified by attempting to roll a thread in the hands. Since sand is not cohesive, it will not be able to form a thread with any residual strength.
A cohesive soil can be identified by squeezing a damp sample in the palm of a hand. If it forms a firm mass with residual strength then it is cohesive. It is the requirement of firmness that is important here since the ability to hold a shape (without strength) is a test of coarseness, as described above.
Plasticity of a soil is demonstrated by its ability to deform to some extent without cracking.
Both silts and clays may act plastically. The presence of clay can be determined by smearing a damp sample with the finger. Clays tend to bind to the skin and leave a stain. A further test that distinguishes a mainly silt/sand soil is its ‘dilatancy’. This is performed by taking a moist flattened sample in the palm of the hand and jarring it against a wall or other hand until water forms a film on the top. If the sample dulls again when pressed with a finger, followed by stiffening and eventual crumbling, then this indicates the predominance of  silt/sand rather than clay.[60]

The approximate strength of fine soils can be determined on site using Table 1.
Table 1. Field test for strength of fine soils (from BS 5930:1999)
Approximate Strength (kNm-2)
Easily moulded or crushed in the fingers
Can be moulded or crushed by strong pressure in the fingers
Finger easily pushed in up to 25mm
very soft
Finger pushed in up to 10mm
2 to 40
Thumb makes impression easily
40 to 75
Can be indented slightly by thumb
75 to 150
Can be indented by thumb nail
very stiff
150 to 300
Can be scratched by thumb nail
Hard (or very weak mudstone)

[58] The ethics of conservation must be considered. Historic fabric should not be removed unnecessarily or where it affects the character of the building. Removal might be justified only after careful consideration to the objectives of the investigation.
[59] An eye lens.
[60] BS 5930:1999, p.116.