Clay Plaster Recipes for Natural Walls

There is no single universal clay plaster recipe. The correct proportions depend on the composition of the clay, the grading of the sand, the type of fibre and the purpose of the plaster layer.

The most important part of the process is testing. Prepare small trial batches, apply them to a sample surface and examine how each mixture behaves while wet and after drying.

The recipes below provide practical starting points. They should still be adjusted to suit the locally available materials.

Recipe 1: Sand and clay plaster

Soak the clay in water for several hours. Add enough water to soften it, then mix it until the mass becomes uniform. Add screened sand and mix again.

Starting proportions:

1 part soaked clay

3 parts screened sand

This is a dense mixture with relatively high thermal conductivity. It is therefore better suited to a thin finishing coat than to a thick insulating layer.

Recipe 2: Sand and clay plaster with fibre

Adding fibre is one of the most common ways to reinforce clay plaster. Fibre helps bind the mixture and improves its strength, plasticity and resistance to cracking.

Chopped straw is commonly used, but hemp fibre, cattail, reed fibre or animal wool may also be suitable.

Starting proportions:

1 part soaked clay

3 parts screened sand

0.5–1 part chopped fibre

This mixture is suitable for stronger and thicker natural plaster coats. The exact quantity and length of the fibre must be tested with the particular clay and sand.

Recipe 3: Sand and clay plaster with natural additives

Natural additives can improve plasticity, adhesion, durability, colour and surface texture. Traditional additions include wheat paste, animal manure, cactus juice, casein, linseed oil and rapeseed oil.

Variant 1

• 3 parts screened sand, approximately three 5-litre buckets, screened through a 3 mm mesh
• 1 part soaked clay
• ¾ part cow manure
• Reed or cattail fibre
• 8 cups wheat paste

The proportions should be adjusted after testing the local clay and sand.

Variant 2

Based on Kaki Hunter, “Earthbag Building: The Tools, Tricks and Techniques”

• 11–15 litres clean water
• 2 equal containers chopped straw
• 2 equal containers hay
• 4 equal containers shredded cellulose paper
• 4 shovelfuls screened clay
• 18 shovelfuls sand
• 1.4–2.8 litres wheat paste
• 1 cup borax dissolved in 1 cup water; salt may be used as an alternative

Use the same container as the unit of measure for straw, hay and cellulose.

Variant 3

Based on Baron Leopold Korf, “On Clay Plaster”, 17 April 1838

Base plaster

• 20 parts clean, thoroughly worked clay
• 20 parts yellow sand
• 20 parts finely chopped straw

Soak all the components and mix them thoroughly.

Finish plaster

• 2 parts sand
• 1 part oven-dried and finely crushed clay
• 2 parts rye flour

Allow the wall to dry, rub the surface smooth and then apply limewash or natural paint.

Variant 4: Three-coat clay plaster

Based on the method described by Maya Creek, Education in Sustainability

Bonding coat

• 3 parts clay
• 2.5 parts sand
• 1 part chopped straw

Mix to a paste-like consistency. The relatively fluid mixture should penetrate the straw surface and create a strong base for the following coats.

Body coat

• 3.5 parts sand
• 2.5 parts clay
• 1 part chopped straw

The body-coat mixture should be thicker because it is applied in a layer several centimetres deep. Mix it thoroughly until it becomes cohesive and sticky.

The bonding coat should remain slightly damp when the body coat is applied. This helps the thicker mixture adhere to the base.

Finish coat

• 3 parts screened sand
• 0.8 part finely screened clay
• 1.5 parts cattail or reed fibre
• ⅓ part powdered lime, optional, for a lighter finish

If lime is not desired, replace it with approximately 1 additional part of clay and verify the result with a test panel.

Always prepare test samples

Every clay, sand and fibre behaves differently. Make several small batches before plastering the whole wall. Allow the samples to dry completely and compare adhesion, shrinkage, cracking, hardness, surface texture and colour.

Adjust the proportions gradually and record the final recipe that works with your local materials.