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Testing clay plaster mixes on straw – The first coat

So the basic ingredients of clay plaster are:

  • Earth with clay present
  • Sand
  • Fibres: We used chopped straw. Other possibilities for fibres (we haven’t tested any of these though, only read about them): coconut fibres, animal hair, flax, dry pine or larch needles, horse manure.
  • Clean water

There are additives you might consider using as well, which we’ll go into later. The first coat is for making a strong bond between the straw and the plaster. So, we want a clay rich mix as clay is the sticky binder. Being clay rich the plaster will crack, but that’s ok, so long as the plaster overall has a good bond to the straw. Cracks at this stage can provide a ‘key’ for subsequent coats, so can actually be a desirable for a strongly bonded plaster.

Most advice suggests you test things out on a decent area of the wall, like half a metre square. That way the straw is compressed as well. We decided that we didn’t want to go through pulling the plaster off the bales again and wanted to get a rough idea before actually putting it on the walls, so we used a spare bale. This worked in that it gave us a rough idea of  the ratios of clay to sand and straw.

After waiting for that test to dry, we opted for a 1:1 earth/straw mix for the first coat. It ended up cracking a little more than we expected, from looking at the test bale. In hindsight, testing a larger area on the wall and seeing how it dried first might have meant we added more sand to the mix, but really you don’t need to worry too much for the first coat, if it bonds well to the straw, all’s good.

Again, like the soil testing, keep a good record of which mix is which and how it was made, so you can repeat the one you want easily!

Possible additives to consider:

Let’s get the smelly one out of the way first – manure/ dung. Many swear by using this in the mix, it is both sticky and contains some small fibres, both useful properties. It doesn’t smell when dried either. It can also be used to improve water resistance, apparently 10-15% volume of cow manure has been shown to prove itself – so worth considering if doing external clay render. We didn’t have a ready supply of cow manure, though we could have gotten some horse poo, we were happy with the mix as it was and happy to keep the plaster mix non-smelly and relatively inert :)

Wheat paste – this could be useful if you have a ‘weak’ clay or lower clay content earth as it will up the stickiness levels. It can also make the plaster more durable and harder.

Sawdust – can, and has been used in combination with sand as a binder/aggregate. This was a bit experimental for us – we couldn’t find anything but superficial references to its effects on the mix. The type of wood the sawdust is produced from must govern its properties to some degree.

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