The precise relative proportion of the first two ingredients varies, depending on their individual peculiarities.
Local custom as to the composition and preparation of the mixture will generally be found to have adjusted itself to the peculiarities of the soil.
The following extract is from an analyst’s report on a sample of typical old cob walling:
“The material when placed in water fell to pieces. On analysis, it was found to consist of:
| Per cent. | |
| Stones (residue on 7 by 7 mesh sieve) | 24·40 |
| Sand, coarse (residue on 50 by 50 mesh sieve) | 19·70 |
| Fine sand (through 50 by 50 mesh sieve) | 32·50 |
| Clay | 20·60 |
| Straw | 1·25 |
| Water, etc. | 1·55 |
| 100·00 |
“The material is a conglomerate of slaty gravel with a very sandy clay, to which mixture a small proportion of straw has been added.
“The clay acts as an agglutinant, and the straw as a reinforcement.
“Efficient protection from frost and rain would be necessary before such material could be considered weatherproof.”
