What colour is it once it's dried out again? For that matter, it wouldn't be dry when it came out of the ground so what colour was it originally when dry? You can put it in an oven to dry it but use a low heat; you just want to dry it, not fire it!

When you say 'mixed', what do you mean? Did you just add water and stir or did you add sand, or chalk or what?

Does it go green as soon as it is mixed or does the green colour appear over time? If the latter then an obvious cause would be microscopic growth.

Posted By: ComeOnPilgrimOut of the ground it's a lovely blue/grey colour,

could it be *this* ?

http://www.cambrianblueclay.com/

gg

I took some photos to demonstrate. First, here is the original dry blue / grey clay. However, looking at it carefully to take the photos, it seems that it's not a homogenous colour. There seem to be slight pockets of orange / brown which may be the explanation. Ie, not a a chemical / oxidisation reaction, but simply a mixing of different clays / muds.

I would add that it was dug from quite deep, so it's very pure. There are no stones / sand at all in it, so the 'ripple' of different colours must be pretty much what it was like in the ground.

no doubt !

how about this ?

https://en.wikipedia.org/wiki/Oxford_Clay

gg

Too much cracking usually means there is not enough sand in the mix. Also if the layer is too thick or you do not dampen the surface before applying the plaster.

You also probably want to do a test to see what its composition is. Mix a handful with water in a glass jar, shake well and leave several hours to let it settle. The different elements of the earth will settle out in layers.

Posted By: ComeOnPilgrimIt's pretty close to 100% clay. It's very pure.

Does that mean you've already done BdP's suggested test? It's a good way to know for sure.

I've tried it with 75% aggregate, but there are still a few cracks. I was wondering what percentage I could go up to without it failing?

All clays shrink as they harden - they harden by the water evaporating out, which reduces the size. There are different types of clay that shrink by different amounts and most real-world clays are mixtures of the different types, so it's difficult to know what mixtures will work except by experimenting.

Cracking can be reduced by compacting the plaster as it sets, and by the incorporation of fibres in the mix as well as angular aggregates. Try fine polypropylene fibres. Up to 30% of the mix is fibre in some traditional plasters. And more thinner coats will crack less than a few thick ones.

There's a good article at http://www.buildingconservation.com/articles/clayplaster/clayplaster.htm As well as the references it gives, I'd recommend Using Natural Finishes - Weismann & Bryce. There's also a bit in the Natural Building Companion - Racusin & McArleton although that covers a lot more ground but does have a DVD. I expect there are also a bunch of videos online.

Mixing by hand could well have been the problem as well as the aggregate size.

Because all earths are different, there is no set rule for what the correct ratio is.
Usually you would make several small test patches using different mixes and see which one works.

Clay plaster cracks when its too rich in clay, and if there is not enough clay the aggregate will fail to bind and will crumble when rubbed with your fingers.

By making small test mixtures you are trying to find the middle point.

To make the plaster stronger some people add a cellulose binder. Traditionally this was done by adding a fermented plant based product.

I did the "DIY Proctor test" on my clay and it is a 50-50 mix of clay and sand ; with 10% added lime (NHL 5) it sets as hard as iron...

gg

I wrote: "Here's another link I just came across ... It looks like an interesting blog."

The latest blog entry is quite scary: http://www.permaculturinginportugal.net/blog/firestorm/

@djh - that's a great link. The writer may have had a similar problem to us: too pure clay. Maybe some more silt would help. Mind you, it is a lovely consistency and great to work with, so maybe this isn't the problem. I'll see how the other mixes work out, but the weather has been so cold they're taking a while to dry.

Posted By: djhIt's probably too cold to be doing any work with clay or lime outdoors now.

You're right. This is inside though, so it's a bit warmer.

The samples have dried now, and show very little sign of cracking. This is at 20%, so I suspect the previous cracks were at least partially because I didn't mix them sufficiently. I'm going to try and mix a sample with more clay in it now.

Posted By: bot de pailledef sounds like it wasn't mixed properly. There were probably parts of the mix that were too clay rich and therefore prone to cracking.

One of the advantages of working with clay is that it can be applied in cold weather ,unlike lime or cement plasters.
Ive clay plastered through the winter. leaving the clay by the fire over night so that its not too cold for the hands.

The top coat of plaster will generally be fibre free, unless for aesthetic reasons. Its the clay undercoat that needs to be mechanically sound, the top coat is mainly decorative and relies on the undercoat for its mechanical strength.

Yes, you're definitely right. The 4:1 limestone aggregate:clay went on brilliantly, nice finish, and has not cracked at all after I mixed it properly. Result! Even the 50:50 only cracked slightly.

No, the colour is not very attractive, but I think I can use it as a base layer quite happily, with a coloured proprietary top coat at a thin layer (1-2mm).