Cob is a mixture of sandy-sub soil, clay and straw. It is mixed by crushing the particles together by either dancing on it or using the head of a digger. Historically cob might have been mixed by farm animals who would walk up and down on the sand, clay and straw. The sandy sub-soil must be sharp and ideally contain angular stones and gravel – this will make it stronger. About 75% of cob is made up of this sandy aggregate. Any type of clay can be used, but be careful not to use silt which can sometimes appear like clay. We at sacred groves also add coir to the the mix for additional reinforcment and strength.
To get the mix right is crucial for a trouble free house. Firstly we have to mix sand and sub soil in the appropriate ratio to get the right quality cob loaves, hard in strength and at the same time very easy to apply with cob tools on the walls. When making the cob loaves in the pits we have to keep adding water in decent proportions keeping in mind not to make it very pasty so it loses the clay out of the mix nor too hard to have a hard time applying it on the wall. Hence the right mix is important for a hard and strong cob house.
To see if the cob mix is of good quality to use we follow 3 test thumb rule
- Crunching sound
- Drop ball and
- Cigar test
The first one quite simple once you roll the cob ball you should press it tightly inside your palm and hear for a crunching sound sound, which is the sound of the coarse sand particles grinding with each other which makes a good mix. Second test is as it sounds you roll up a ball in your hand and drop it from a metre height . The ball should neither deform nor break in anyway it should retain its original spherical shape as accuarate as possible indicating that the mix is good. Lastly, the cigar test is done once the cob loaves are mixed thoroughly with straw and coir fiber. This test is to check if the glops of cob have enough resistance strength to take the load of the wall and roof. So you take a big chunk of cob in your hand and roll it up in the shape of a cigar and you pull in opposite directions with equal force, the good mix will resist tearing off unless applied heavy energy.
There is no standard one recipe ratio for making the cov loaves, depending on the project site and topography every sub soil varies in the proportion of clay sand and silt ratios. So just like cooking we need to do a test loaves and observe for a few days to completely understand the soil typo and to produce the most efficient and superior quality cob balls. So readers go outside the house dig up some earth and start making cob balls it is fun to play with it and who knows if you like it you can start making your own cob structures. To know more about cob produce and see a multi-story cob building come up do drop in on our open day events happening every Saturday at sacredGroves, Auroville. We wish to see you here, you know what they say it takes a hundred hands to build a house and we would be happy if your hand is one of it.