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If you want to build with cob then you must test the quality of your
clay. Cob also depends on the coarseness of the sand. This is how to
do a snowball test. Combine your clay soil and sand in different
proportions: 3:1, 2:1, 3:2, 1:1, 2:3, 1:2 and 1:3. For each, mix the
sand and clay thoroughly and add just enough water to make the grains
stick together when you squeeze a double handful very tightly.
Make compact balls of the mixes about 6cm diameter. Then while
holding a ball between thumb and index finger of one hand squeeze
the ball with the thumb and index finger of the other hand at
right angles to the first.
A ball made dry enough from the ideal mix
should not distort by more than 0.5cm and be hard enough not to
break. Then hold each ball 1m above soft ground e.g. a grass lawn, and drop
the ball. If the ball breaks it is too dry or contains too much sand.
If the ball deforms it contains too much clay or too much water. The
ideal mix will maintain its shape on impact. When you have
discovered the correct clay, sand and water mix use a tarp to mix
large batches of clay and sand. Mix these dry in the tarp until
you can't see patches of clay or sand then add water little by
little treading and rolling the mix in the tarp. Once lumps are
broken up begin to add the straw. Tread the straw in until it is
covered with the clay/sand/water mix and then turn in the tarp.
Repeat adding more straw and turn in the tarp from different
corners making sure the centre is also turned. Continue adding straw until the mixture feels like
a tough substance rather than a loose (squishy) mud. The cob in
the tarp will now turn as a single mass. Adding more straw now is
very difficult so this is when the mixture is ready. |
