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  1.  
    We have collected some nice clay from our site thanks to the groundworkers digging up some clay and putting it in building bags. It's very clean with no stones in it, and is very sticky.

    Out of the ground it's a lovely blue/grey colour, which we thought would make a nice coloured plaster. Only problem is, once it's mixed, it goes a very muddy green/brown. Does anybody have any idea what is responsible for the colour change?
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    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.
  2.  
    When dry out of the ground, it wet for that matter, it's a blue grey. When I mix it with water, it turns greeny brown. I've mixed it with a limestone aggregate, but i think it changed colour before I added the aggregate. It then starts that colour when it dries out.
    • CommentAuthorgyrogear
    • CommentTime7 days ago
     
    Posted By: ComeOnPilgrimOut of the ground it's a lovely blue/grey colour,


    could it be *this* ?

    http://www.cambrianblueclay.com/

    gg
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    Posted By: ComeOnPilgrimWhen dry out of the ground, it wet for that matter, it's a blue grey. When I mix it with water, it turns greeny brown. I've mixed it with a limestone aggregate, but i think it changed colour before I added the aggregate. It then starts that colour when it dries out.

    I think the colours start off being due to the oxidation states of the iron ions in the clay. Apparently a mix of Fe++ and Fe+++ as part of the silicates is needed for green clay. But whether and how they can change I've no idea.

    I can't see how the addition of just water could make any permanent difference, especially since the clay has water in it to start with. Perhaps it's dissolved oxygen in the water, or chlorine if it was tap water? Lime would obviously have a chemical effect but I don't know what it would do to the colour. If the colour change takes time then it might be organic growth of some kind.

    More experiments under controlled conditions seem like the best way to make progress.
  3.  
    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.
      Grey Clay.jpg
  4.  
    Here is the mixed plaster. It's a very different colour. I adjusted the white balance so it's quite close to the real life colour. Note also that it seems to shrink hugely, so possibly / likely an Oxford clay. This is a ¾ mixture of fine aggregate, the other clay heavy mixtures cracked severely, and the ¾ mix of larger aggregates also cracked somewhat.
      Brown Plaster.jpg
  5.  
    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.
  6.  
    Posted By: gyrogear
    Posted By: ComeOnPilgrimOut of the ground it's a lovely blue/grey colour,


    could it be *this* ?

    http://www.cambrianblueclay.com/

    gg


    Could be! That clay seems a very greeny colour, but as you can see, ours is more grey / blue. It's also from Oxfordshire, so probably a different type of clay?
    • CommentAuthorgyrogear
    • CommentTime7 days ago
     
    no doubt !

    how about this ?

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

    gg
  7.  
    A local geologist says it's Oxford Clay / Kellaways Formation.
  8.  
    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.
  9.  
    It's pretty close to 100% clay. It's very pure.

    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?
    •  
      CommentAuthordjh
    • CommentTime4 days ago
     
    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.
  10.  
    Thanks @djh

    Yes, I've done the separation test, and it's basically pure clay.

    I think one of the issues with the shrinking was the size of the aggregate, even at 75%. The 0-4 limestone aggregate was very big, and cracked quite a lot. I think I should have mixed it more too. I mixed it by hand. The 0-2 silica aggregate was much better, but still had a few cracks.

    I've tried mixing two of the aggregates and also getting a 0-2 limestone aggregate, which I understand from Weismann is much better. I've also reduced the clay to 20%. Finally I've mixed it with a plasterer's whisk as well. They both went on really nicely, but we'll see what they look like in a couple of days. Fortunately the house is quite dry due to the Ventive system.
  11.  
    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.
  12.  
    Thanks @bot

    I'm a bit worried that the plaster is going to be weak and dust. The 25% clay was pretty good, but cracked ever so slightly. Prior to that I'd tried different ratios, but all hand mixed, and all with the same aggregate.

    Fermented plant based product - you mean manure?

    Casein is also an option I think, at least for some plasters.
    • CommentAuthorgyrogear
    • CommentTime1 day ago
     
    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
    •  
      CommentAuthordjh
    • CommentTime19 hours ago edited
     
    Posted By: ComeOnPilgrimFermented plant based product - you mean manure?

    Can be, if you wish. People sometimes mix hemp with clay plaster, which provides fibre. Apparently it is best if left for six months or more until the hemp starts to rot and smell, when it becomes more flexible. As you say, mlk products are also used.

    Here's another link I just came across http://www.permaculturinginportugal.net/blog/cob-and-earthen-plaster-recipes/ that also recommends an online book. Oh and it also suggests you might need to add some silt to your clay. It looks like an interesting blog.
    •  
      CommentAuthordjh
    • CommentTime18 hours ago
     
    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/
  13.  
    yup - scary

    Back to the clay mixes - a chap dropped in the other day and asked if he could pick up some fresh cow poo from the field. Why said I. He was doing some repairs to a traditionally built brick / clay stove and he wanted the cow poo to mix with the clay !! Hmmm
  14.  
    @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.
    •  
      CommentAuthordjh
    • CommentTime12 hours ago
     
    It's probably too cold to be doing any work with clay or lime outdoors now.
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