Earth Plaster

3 Apr

With the upcoming Cob Workshop in Clare I need to present myself with my first Earth Plaster job which I did last year during my travels for Jason J. Baker. I had previous experience only from doing weekend course. Previous wwoofer did a Cob moldings around the doors in living room. His mix had too much clay and there was extensive cracking.

cob molding

Cob molding cracking and coming of the wall

Usually on cob you need only straightening and finish coat, but because of the cracks I decided to apply two thin coats before the final coat. The beauty about natural plasters is price of the material. First stop was the abandoned clay quarry where we got two types of clay. Sand is everywhere else so we just stopped at the roadside to get it. Next stop was neighbors house where we got a bucked of horse manure and finally we spent 55cent in supermarket to buy 1kg of flour. With all the ingredients i could start the preparation.

clay & horse manure

soaking ahead makes better plaster

First soak the clay with the manure overnight. Cow shit is better for the plaster because it contains more of the enzymes binding with clay. The fresher the better properties it gives to the plaster. For the fussy people there is substitutes for animal dung or you can spread it out on the rain and get highest quality fiber If you are afraid of smell you should know it is small amount in the plaster and it is gone next day or two.Another ingredient you need to prepare ahead is wheat flour paste. Allow it to cool before use.
Next morning I started with my recipe which was the first novice mistake. Test patches are always needed first to get your mix right. The reason for this is variation of material properties and also properties of your surface. I just took recipes from two books and did an average ratios:

Straghtening coat
4 parts SAND
1 part CLAY
1/4 part FLOUR PASTE
Water depends if the materials are wet or dry
The performance of the mix will improve by leaving it to sit for few hours or overnight.I was toot eager to start my plastering job and cracking of the plaster was a result. I identified the problems after looking in my handouts from the plastering course. First was that the sand was not well graded and had too many big particles. Second was not enough of fibre.

earth plaster control

quality control manager

Adjusted mix
4 parts SAND
1 part CLay
1/4 part FLOUR PASTE
3 handfull of STRAW
I had much better results from this recipe. Very easy to apply with the hawk and trowel. Some of the moulded part had to be applied by hand. This coat is applied 25mm to 50mm thick. I dropped few bigger peices of straw in the mix as I was chopping it and they were giving me bit of trouble.

earth plaster coat

Straightening coat applied

While the plaster was drying I mixed some clay paint also known as Alis. These paints are best for interior work and applied in two coats. The paint is very similar to earth plaster. There is variations in the materials again so I had to find the ideal recipe for me.
Starter recipe
1 part CLAY
1/2 part FLOUR PASTE
1 part WATER

testing Alis clay paints

I had two different color clay available to test on different surfaces. You can also add pigment to the recipe for desired color. Always do couple of recipes to see which one works for your plaster. You apply the paint with brush strokes in all directions and try to make uniform layer. After the first coat is completely dry you make the top coat without the sand. Leave to dry until hard but still damp and then with a damp cloth or sponge polish the surface. Use circular strokes to remove brush marks. After few days my straightening coat was completely dry and it was ready for final plaster.
Finish plaster
1 part CLAY
2 part SAND

earth plaster finished

my first earth plaster

By adding very fine fibre you can create interesting texture. I was running out of time so I has to finish it quickly. Finishing coat is applied in about 3 mm layer. I left the plaster to set for two hours and misted it with water then. There is couple of options for finish. Sponge or wooden float for rough texture, japanese trovel for hard polished surface. I just used my hands to give it organic structure.

job done and Alis paint frame painted

Before you start with your wall I have to recommend taking a weekend course or reading carefully these books Hand sculptured house, Natural Finishes: Lime and clay based plasters, renders and paints

Big thanks to Jason for his hospitality

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