No. 045 Reading Time 4 minutes
When we told the villagers we were there to build a mud house.
They thought we meant a clay house and outrightly rejected our idea saying
Mountains do not have clay like the plains.
This is one of the biggest misconceptions people have about mud houses.
People think building a mud house requires just clay (like potter’s clay).
This thinking could be the reason, why the mountain people in India have only used mud to plaster their homes and not to build entire walls out of it.
But Cob needs more than just clay.
Before we understand how to make a good cob mix let’s first understand the composition of the soil.
Most soils have a top layer, a few inches to a foot deep, containing life-like small creatures, plants, roots, and dead organic material (humus).
We save the topsoil since it is valuable for gardening and construction with subsoil beneath.
Subsoil is rich in minerals and contains a mixture of sand, silt, clay, stones, and gravel.
Credits: Adobe Stock
Now, let’s talk about each component and the roles they play in making a good cob mix:
Clay:
In nature, clay can sometimes be found in a nearly pure form, but if you are digging soil from your farm, it usually contains a mix of other materials such as sand, silt, and organic matter, making it a type of clay soil.
Clay acts as a binder, a sticky glue, or an adhesive.
You need enough clay to create a flexible, sticky, and easy-to-work mix, but not so much that it shrinks and cracks when it dries. Your final mix should only have between 5% and 25% clay. A potter’s or pure clay would crack a lot on a building scale once it dries.
This is why you need sand.
Sand:
For ease of understanding, sand can be referred to as all rock particles from 1/4-inch down to the smallest visible grains.
Natural sand comes from rocks and shells being worn down by waves, glaciers, wind, or rivers. It can also be made by crushing rocks for road construction and concrete.
Sand is inert, hard, stable, and doesn't change much. The grains don't absorb water, shrink when drying, or expand significantly with heat.
It reduces shrinkage cracking and improves compressive strength.
A good cob mix needs coarse sand (particles of size 1/8th inch to 1/4th inch). A sharp or angular stand is better than too-rounded sand.
You can also, add stone or gravel (pieces of rock bigger than a pea). They create a strong mix with excellent compressive strength, but not comfortable to stomp on a rocky cob mix barefoot or work with it using bare hands.
Silt:
Also, tiny particles like clay, are too small to differentiate with eyes.
They neither help bind the mix like clay nor give the compressive strength like sand.
It dilutes the mix.
Silt is good for agriculture but not for building.
Water:
It is used to activate the clay and make it sticky.
As clay dries in the gaps between the coarse, rough sand grains, it shrinks and binds the grains together securely.
Straw or Fibre:
Along with its geological materials, cob also includes a biological component: straw or other fibers such as pine needles, or ryegrasses.
It lightens the cob, adds insulation, and gives tensile strength by forming a sort of armature or woven fabric.
To make good cob, the water, sand, clay, and straw need to be intimately mixed and traditionally stomped or danced upon.
For general buildings, the best mix uses mostly coarse sand and a small amount of clay. It should have enough clay to bind the sand and straw together, typically around 3 or 4 parts sand to 1 part clay.
Additives for Clay Plasters:
In addition to clay, sand and straw, one can also add these additives to make the plaster more plastic and water-resistant.
Cow Manure:
Cow manure adds fiber and stickiness, enhancing the strength of weak clays.
The enzyme and fatty acids, improve the plaster’s resistance to water erosion.
This makes it a great additive for exterior plasters, wet areas, and clay floors.
Horse Manure:
It doesn’t improve the water resistance as much as cow manure but adds fine fiber.
Wheat Paste:
It is made by boiling flour in water to create a thickener.
Other starches can work too.
Wheat paste adds stickiness, improving weak clays. It’s commonly used to strengthen and reduce dust in finish plasters.
Lime:
You can also add slaked lime to make the plaster more water resistant, and change the color of the clay plaster.
In the next newsletter, we shall talk about how to test different soils, how to find clay, sand, and other components, and make the right mix.
Love,
Raghav and Ansh
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