As with earth blocks, earth used for cob constructions is relatively rich in clay. The earth and cut straw is mixed in a hole in the ground in the proportion of 50 kg straw to 1m3 of earth. The more clay the earth contains, the more straw is needed. The ready mixed earth and straw is then thrown into the shuttering of the wall and rammed down by foot. Between adding each course of about 50 cm, the wall is left to dry out forabout two days.
When the wall has reached full height, the vertical is checked and excess earth removed with a trowel, so that the wall has an even thickness. A clay mix or gruel is smeared over the whole wall and it stands under cover until it is dry, from three months to ayear. Shrinkage isquite considerable, abouti cm per metre, so it is a mistake to ren-dera wall before it istotally dry. Because of the long intervals in the process, this building technique is seldom used nowadays. Cob used to be the main technique used in the United Kingdom, and there are many old examples still standing which prove that this is an excellent system (Figure 13.26).
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