How to find Clay, Sand and Straw for building?

No.047 — Read old posts on Tinyfarmlab.com
Reading Time 4 minutes

In our previous newsletters, we discussed soil components and how to test your soil using the jar, shine, s-ticky, ring, and crush test.

After testing the sub-soil on your land using these methods, you'll determine if your soil is a ready mix, ideally containing 10-25% clay and over 50% coarse sand.

If your soil doesn't meet these criteria, you may need to add sand or clay to achieve the ideal mix.

Today, we shall discuss how to find clay soil, sand, and straw for your ideal cob mix.

Where to find Clay

Monsoon is the ideal time to search for clay. You can look for it in road cuts, recently dug ditches, uprooted trees, or construction sites. Contractors mostly want to get rid of this soil.

During the rainy season, the soil will already be moist.

In other seasons, you can add water and perform the sticky test we discussed.

You can also identify clay from a distance. Clay often has striking colors such as red, ocher, blue-gray, yellow, black, and other shades.

Clay is impervious to water. If you see puddles forming after rain, it could indicate clay below the surface. Muddy roads with clay will also shine from afar.

Additionally, natural springs might have clay where there is stagnant water.

You can also ask locals if they would be willing to give your clay soil from their farmland.

Where to find Sand

Natural sand comes from bigger rocks (and sometimes shells) being broken into smaller pieces by waves, glaciers, wind, or rivers.

Sand can also be made by crushing rocks for building roads and making concrete. This is also called M sand or manufactured sand.

Angular and sharp sand is better than rounded sand for making cob.

The beach and desert sand is too rounded.

The best sand for making cob is coarse, with grain sizes between big sugar crystals and split peas. River sand is the most used since it is readily available.

You can order sand from local material suppliers - ask for coarse concrete sand, which is the cheapest type. They usually sell it by the truckload.

Finer sand is used for sculpting and plastering. This is often available as P sand - plastering manufactured sand.

Where to find Straw

Remember, hay and straw are different.

Straw is made from grain stalks, while hay is made from dried grass or legumes.

Rye and winter wheat straws are great for making cob. Rice straw is long and flexible.

For an excellent cob, mix half long rice straw and half short wheat straw.

In the countryside, you can ask local farmers for straw. In India, they harvest wheat straw before the summer months and rice straw before the winter months.

If you can't buy directly from a grower, stores in the city sell chopped straw for cattle feed.

Testing your mix

Depending on the results of your jar tests and other soil tests, you might want to add more sand if the soil is too clay-ey or add more clay if the soil is more sandy and less sticky.

Snowball Test

If your soil is clayey but lacks sand, you can make samples by mixing your clay soil with sand in different amounts. Try combinations like:

• 1 part clay soil : 2 parts sand

• 1 part clay soil : 1 part sand

• 1 part clay soil : 3 parts sand

Mix each sample thoroughly and add just enough water so the mixture sticks together when you squeeze it hard. The mix should be quite dry.

Make a compact ball about 2.5 inches in diameter from each mix, like a tightly squeezed snowball.

Hold the ball about a meter above soft ground (like a grass lawn) and let it fall.

• If the ball shatters, it’s too dry or has too much sand.

• If it flattens or spreads out, it has too much clay or too much water.

The ideal mix should hold its shape and look the same after it falls.

If your soil is sandy, you can repeat the same steps by adding more clay that you would have hauled.

Test Bricks

You can make test bricks to check for cracking, crumbling, and strength.

Start by mixing clay soil and sand in different proportions on a tarp or in a basket until you have a dough-like consistency.

Put the mix into waste food containers of about 4 × 8 × 8 inches, and let them dry. Label each container with the proportion of ingredients, like 1:1 or 1:2.

Dry the test bricks in the sun or in a slightly warm oven until they are completely dry.

Check for surface cracks. Cracking suggests there might be too much clay in the mix. If there is loose sand on the surface when you scratch, that would suggest too much sand.

Once you've found the right proportion for your cob mix, you can add straw and start using it for your next project—whether it's a bench, a rocket stove, a wood-fired pizza oven, or even your house.

You can add more straw if you like, but the mix will let you know when it’s had enough.

If you have any questions about starting your cob-building journey, feel free to ask us!

Love,

Raghav and Ansh

P.S. - If you are in Delhi/NCR we are having a meetup today at 3:00 P.M. Come say hi!

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