So you have tested your earth to see the sand and clay content, now you are going to need to do some tests once you have made the cob mix itself.
The recommended quantities of sand and clay in a good cob mix are as follows:
- 75-85% sand
- 15-25% clay
- Long straw
The quantities of sand and clay will vary according to your earth type so do some tests first.
One you have made your cob mixed, there are a few tests you can do to see its suitability.
1. The Ball Test
The second test you can do is to make a ball of you cob and drop it from about waist height. If it splooges out in to a pancake then it may either have too much clay, or is too wet, or both.
If it cracks and breaks on landing then it probably has too much sand. The sand is good for compression strength but not good for holding the mix together.
2. The Sausage Test
The second test you can do is to roll your cob in to a sausage in the palm of your hand. Hold the sausage between your finger and thumb with about 2.5cm protruding from your fingers. If it falls to the ground then it is either too wet, or not a good mix, probably because it does not have enough clay.
3. The shrinkage test
Your cob must not shrink by more than 4% or it will affect the structure as it dries. The reason for shrinkage whilst drying is the water evaporating and the space of leaves behind either as shrinkage or cracks. The material that holds the water is the clay, the sand does not, so the more clay you have the more cracks you will have. However if you do not have enough clay the cob will crumble as it will be too sandy.
So here is how you do the test…
First of all get your tools and materials. They are a saw, nails, hammer, and some wood to make the box.
We wanted to do three tests with various quantities so I made this box with three sections. The longer the box is the better, as this will make it easier to measure the shrinkage amount. I made the box 50cm long. I marked on the mixes which in the end were as follows
- 4 earth, 1 sand
- 2 earth, 2 sand
- 1 earth, 3 sand
You can see the three mixes below and the box ready to recieve them. As written above, a good cob is generally made from about 3 sand 1 clay. So I made one mix with 3 sand and one earth. But our earth had a really high sand content so when mixed with the sand it really did not stick that much at all. So it is really important that you test your earth first to see how much clay and sand it contains.
Pack your mixes well in to the boxes making sure not to leave any spaces.
When the blocks have completely dried, push them to one end of the box and measure the gap at the other end. If your box is 50cm like mine then just double the amount of centimetres to find out your percentage. SO for example if there is a one cm gap between the end of the block and the wood, then your shrinkage is 2%.