20 August 2012

Finish Clay Plaster Recipe & Wall Preparation


Plasters made with clay are beautiful, durable, and made from inexpensive and non-toxic ingredients.  Clay creates a breathable finish material with a natural capacity to regulate moisture and temperature in the surrounding air.  Clay naturally absorbs excess moisture in the air, helping air to feel more comfortable in summer, while its high thermal mass creates a battery heat (or cooling) storage that helps to maintain constant air temperature.  Materials can often be found on or local to the building site, making the ingredients dirt cheap...literally.  Or you can use dry bagged clay from a pottery supplier.  Clay naturally occurs in a variety of colors and clay plasters can be pigmented with mineral based pigments in every range of color.  Techniques for mixing and applying are simple and the plaster is extremely forgiving, even for beginners of any age.

Below are instructions for wall preparation to apply clay plaster on drywall or on clay substrates, followed by my favorite finish clay plaster recipe.  At the very end are my 3 favorite plaster books.  Have fun playing in the mud!!


Drywall Preparation
sanded primer for drywall application of clay paints (from Keely Meagan’s article in The Last Straw: www.thelaststraw.org/bonus-articles/earthplaster.html)

  • 10 parts wheat paste
  • 1 part fine sand (such as mason’s sand)
  • 1 part clay
Mix together & paint on wall. Allow to dry completely. Do not re-wet wall surface prior to plaster application.
Covers approx. 40 SF


Wheat Paste
Bring 1 quart water to boil in a non-aluminum pot. Meanwhile, mix 1 cup flour with 1 cup cold water until smooth and lump free. (If you can’t get it lump free, run through a strainer.) When water in pot is boiling, slowly add in flour/water mixture, stirring constantly. Bring back to almost a boil, stirring constantly, until mixture thickens. Remove from heat and allow to cool completely. Use wheat paste within a day or two; it goes rancid after a few days (faster in warm weather), and becomes smelly and usable.
Makes approx. 5 cups of wheat paste.


Clay-Over-Clay Preparation
Dampen clay substrates prior to application of clay paints or plasters. I recommend dampening clay substrates the evening prior to application, again in the morning before you begin to mix your material, and then again about 20 minutes prior to application. Thick cob walls generally require more dampening than clay plaster surfaces.  Be sure to allow your water to completely absorb or you will create a water slick that prevents plaster from adhering to your substrate.  Continue to dampen the wall as you work.


Kaolin Clay Plaster
beautiful white or off-white, creamy plaster that can be pigmented as desired
  • 1 gallon water
  • 1 (5)-gallon bucket kaolin clay putty*
  • 2-½ (5)-gallon buckets fine sand**
  • 1 cup wheat paste per 5-gallon bucket of material (clay and sand)
  • Pigment, mica, or chopped straw, as desired
*to make clay putty, soak dry, bagged kaolin clay powder in clean water for a minimum of 24 hours until it is the consistency of sour cream; store in a cool, dry place; kaolin clays are white to off-white, but you can substitute any bagged clay in this recipe in your color of choice

**typical is mason’s sand though finer grit sand may be used; the finer the sand, the smoother the wall surface without buffing with a finish trowel
  1. Mix all ingredients together thoroughly. You can use a cement mixer or you can mix materials on a tarp (flipping often & mixing with your feet). Final consistency should resemble creamy mousse. More water may be added to achieve desired consistency.
  2. If adding pigment, save some pigmented plaster with no wheat paste for later patches or repairs. Add wheat paste after pigment.
  3. Apply plaster to desired texture, typically using trowels, onto substrate that has been prepared as above.
  4. When leather hard (slightly firm), smooth the surface by buffing with a damp tile sponge, rinsing the sponge frequently. Or, for a super smooth finish, run a final sweep across the dampened surface with a very flexible trowel. Mist clay surface slightly as desired as you buff with the flexible trowel.
Covers approx. 100 SF


Resources

If you are plastering over strawbales, read our other plastering posts:
"Clay Plastering Strawbales - FIRST COAT"

These are the most useful books I've found for information on making & applying clay plasters.

           

click the book covers above for more info or to purchase

      
Using Natural Finishes: Lime- & Earth-Based Plasters, Renders & Paints, A Step-by-Step Guide by Adam Weismann and Katy Bryce, Green Books Ltd, 2008

Clay Culture: Plasters, Paints and Preservation by Carole Crews, Gourmet Adobe Press, 2010

The Natural Plaster Book: Earth, Lime and Gypsum Plasters for Natural Homes by Cedar Rose Guelberth and Dan Chiras, New Society Publishers, 2003.

61 comments:

  1. I don't have any words to appreciate this post.....I am really impressed ....the person who created this post surely knew the subject well..thanks for sharing this with us.
    London Plastering

    ReplyDelete
  2. Thanks so much for your kind words! I'm so glad that you found the information to be useful!!

    ReplyDelete
  3. Very timely for me to find as well as am planning a clay plaster over drywall this Spring in the urban Victorian I am fixing up from a vacant condition. First step was, find a substrate recipe! Thanks!

    ReplyDelete
    Replies
    1. I'm so glad to hear that! Best of luck with your plastering project. Sounds like fun.

      Delete
  4. Many thanks for your advice Sigi. I tried your recipe "Drywall preparation" and to other materials such as drywall, for example: wood board, concrete, lime plaster, lime-sand bricks, even old painted wall and I was pleasantly surprised adhesion to the substrates.

    Use this recipe and the old plaster in houses or old houses? Or use another method? What is the preparation of such a wall please?

    Thank you very much
    Erik from Slovakia - Europe

    ReplyDelete
    Replies
    1. Hi Erik, I'm so glad to hear about your experiments with the sanded primer recipe. I've never tried it on concrete or brick and am so glad to hear that worked! For your question about wall preparation, can you be more specific about the existing wall surface...what is the wall made of, is it painted (and with what type of paint), are there cracks to repair, etc.??
      Thanks!

      Delete
    2. Hi Sigi,

      wall in the flat is composed of brick, the first layer is cement plaster, the second layer is a soft lime plaster and two coats of paint on a natural basis, small cracks.

      Today I tried the recipe directly on the paint. I will send a message what is the result.

      Delete
    3. Over paint is the only surface I've had problems with. Especially if it is a latex or plastic-based paint. Be sure the adhesion of the sanded clay primer is really good when it dries, and then make sure it doesn't have any signs of flaking off or cracking. Definitely let me know how your test area goes! Fingers crossed... :-)

      Delete
    4. How were my tests? Your recipe keeps well for concrete and also the lime-sand bricks and of course the drywall.

      The recipe has not adhered to the old paint, while creating good skin, but is not connected to the base, so I had to remove all the paint.

      Delete
    5. That makes sense...should work best on absorbent surfaces, which includes the concrete, brick, and drywall. Thanks for letting me know your results!

      Delete
    6. ... and your recipe keeps well on wood

      Delete
    7. Sigi,
      Thank you so much for all information given. Is there a primer/base that
      can be used on a painted wall?

      Delete
    8. You should be able to use a sanded primer (ie, paint with gritty sand in it). That will give you a textured surface that you should be able to plaster over. Do a test area first, to be sure all of the materials are indeed compatible.

      Delete
  5. Hi Sigi, I've got a question about this article. I'm planning on using this recipe on a wall in my house over drywall. It's close to 100 sq ft. How large is the "part" in this recipe that says it covers 40 sq ft?

    ReplyDelete
    Replies
    1. Ha! Thanks for catching that! So you are talking about the wall prep sanded primer recipe. The "part" is 1 cup. Also, this won't stick to existing paint, only to raw drywall. So if you are planning on refinishing an existing wall that has paint, you will need to use a different sanded primer. Good luck with your project!

      Delete
    2. It had wallpaper on it and was never painted, not even primed. We have it mostly down to the bare wallboard. I think it will work okay, though I'll do some tests, hopefully this weekend. I'm really looking forward to a unique finish.

      Delete
    3. Is it a cup (part) for the kaolin plaster as well?

      Delete
    4. Yup, exactly, so 10 cups wheat paste, 1 cup sand, 1 cup clay. the clay doesn't necessarily need to be kaolin. I actually mostly use site clay for this. But then you need to test your clay/sand proportions and adjust the quantities accordingly...

      Delete
    5. For your wall prep, as long as you removed the wall paper and there is no paint, you should be fine. Definitely do a test area to make sure your sanded clay primer sticks without flaking off. If it does stick with good adhesion, you are good to go!

      Delete
  6. Plastering with clay is an excellent combination to improve the quality of plaster ceiling for Home Renovation. This gives inexpensive but good quality output.

    ReplyDelete
  7. Can I use clay over brick and plastered wall?

    ReplyDelete
    Replies
    1. depends what the surface is. You need surface area to create a physical key for the clay plaster and you need an absorbent surface for the clay to bond to. Often this means you need to apply a sanded primer or sanded bonding agent of some kind. Best bet is to do a test area.

      Delete
  8. you describe really very useful Article..you describe the method for making clay plaster is very useful.Clay is very important ingredient for plaster Thanks for sharing with us.!!

    ReplyDelete
  9. Hi Sigi, I am surprised to know that a clay plaster will adhere to drywall! We were going to use peg board for an interior wall, but now am considering the drywall.

    ReplyDelete
  10. Actually, would like your thoughts on using peg board as we want to curve the shower walls...

    ReplyDelete
    Replies
    1. I'm confused...sorry. How would you curve the pegboard? And do you mean to use the pegboard as backing for clay (because of the holes)? Or do you want pegboard as your shower walls?

      Either way, a shower wall should NOT be drywall. It should have cement board or greenboard that can handle the damp conditions in the shower. You can do drywall in the rest of the bathroom.

      Also, I wouldn't do clay plaster inside the shower, as it will erode over time. You can do clay plaster in the rest of the bathroom, but the shower should be a lime-based plaster instead.

      Delete
  11. Hi Sigi, yes we'll use lime plaster on the shower walls. We were going to use cement board, but we were thinking we may be able to some how curve pegboard, slightly.

    ReplyDelete
  12. Hi Sigi, yes we'll use lime plaster on the shower walls. We were going to use cement board, but we were thinking we may be able to some how curve pegboard, slightly.

    ReplyDelete
    Replies
    1. I wouldn't use pegboard in a shower...either as the finish or as the substrate. Picture everything should be able to get wet...

      Delete
  13. Interesting information about Clay Plaster and recipes. Nice blog

    ReplyDelete
  14. Interesting information about Clay Plaster and recipes. Nice blog

    ReplyDelete
  15. I have drywall that has been painted with drywall paste, not paint, will the primer recipe stick to that? Or could I just apply the clay and would it stick? Thanks, great article, very informative!

    ReplyDelete
    Replies
    1. Not sure, depends what the paste is made out of. I would do a test area and see. You should not be able to scrub the primer off with your hand. In order for the clay to stick, you need texture on the wall. And you need a substrate that is compatible with the clay. I have no idea what the drywall paste is made out of, so no idea about compatibility, but I'm guessing it is smooth, not textured. So do a test to see if the primer recipe above will stick. If so, then you are good to go. If not, then you can use any primer paint with sand added in to paint texture onto the wall.

      Delete
  16. Hi Sigi,
    I think I have interpreted your recipe incorrectly. Here is my translation: one part water, five parts putty, 12 1/2 parts sand. Is that correct? I'm ignoring the wheat paste as when I do it I will use a dollop. When I try it, it rolls off the wall as particles of sand. Should the sand be 2 1/2 gallons (instead of buckets) or does 2-1/2 mean from 2 down to 1/2 buckets? Thanks!

    ReplyDelete
    Replies
    1. You definitely need more sand than clay. But I can't speak to your exact recipe. The thing to know is that first, there are so many different types of clay, and different clays have different stickiness. The more sticky, the more sand you need. Second, the sand is crucial. It must be gritty, toothy sand and it should be 1/2 as big as whatever thickness you are putting on for your plaster. So if it's a thin coat, you need gritty sand, but screened fine (usually 1/8" or smaller). In terms of wetness, it should be about like sour cream or whipped cream. Too thin, and you can't plaster with it.

      My recommendation is to make some tests with slightly different mixes until you get a consistency that feels both sticky and gritty, and that is solid enough to plaster but is still creamy enough to spread.

      Hope that helps.

      Delete
  17. Hi Sigi
    I have tried your clay plaster recipe with an oxide powder for colour on a cob wall and it is lovely. i think it will be perfect for the internal walls of my garden structure, but my question is, could I apply lime wash over the plaster to make it more weather resistant on an external wall (I live in Australia so the wall iwon't get constant rain on it, but may be subject to heavy, driving rain from time to time). Thank you.

    ReplyDelete
    Replies
    1. sortof...the limewash will wash off over time and you will need to reapply it and if the wall gets heavy rain, the limewash will not prevent erosion

      Delete
  18. Thank you for your answer, Sigi. Much appreciated. It's a small section of wall that isn't protected by the roof, so regularly lime washing might be the easiest and best option to prevent the cob eroding. if there is heavy rain I might just need to physically cover it temporarily with a tarpaulin or something. I guess my main concern was that maybe the clay plaster with wheat paste and the linewash might be incompatible, but it sounds like it should be ok.

    ReplyDelete
    Replies
    1. just to clarify again...limewash will not prevent erosion...at all...

      But the materials are definitely compatible

      Delete
  19. Excellent your work ... God bless you, to see when they bring these techniques to the countries of South America , there are places where people do not have resources for the construction of houses. They left the old techniques and is now more expensive. Greetings from Venezuela.

    ReplyDelete
  20. Hi Sigi. Thanks for the info! I have some leftover plaster from a friend's project that is a bit stinky thanks to the wheat paste. Do you know if it will continue to smell after it dries? Or is it really unusable?

    ReplyDelete
    Replies
    1. oooh, yes, the wheat is anaerobically decomposing...there's no return from that. When I use wheat paste, I only add it to the plaster I'm using, never to plaster that will be stored. Also it may create black blotches on the wall. I would ditch it and make new

      Delete
  21. Thank you so much for this information and the recipe! I'm renovating a traditional danish timbered, thatched country house, where the walls are made of only timber and raw clay. So I expect this will work like it was meant for it :) I really like the idea of making the plaster white with kaolin and then add some pigment. Do you have any recommendations regarding type of pigments,or where to read about it (also 'recipes). I'm very much appreciating you using your time and effort on answering questions here.
    Greetings from Denmark, Mona

    ReplyDelete
    Replies
    1. I order mineral pigments from a US company...probably not the best resource for you, though. I don't really know companies to specifically recommend in Europe. Sorry! But I would look for natural mineral pigments. Does that help?

      Delete
  22. Forgot to ask, if you have tried painting on this type of plaster with paint? I use a transparent kind of paint, based on beeswax, called lasur. I've often seen it used in Waldorf schools etc. I was thinkint it would look good on this kind of plaster.

    ReplyDelete
  23. This comment has been removed by the author.

    ReplyDelete
  24. Hi,
    Thanks for all this info!
    Would you be able to give me a little advice about making the kaolin putty? It's a pretty difficult/ interesting material mix; similar to cornstarch but much stronger.
    Thanks,
    Jake

    ReplyDelete
    Replies
    1. First off, I have no experience with clay so I apologize if the answer is obvious.
      When mixing the kaolin clay (powder) with water, not sure how to determine the amount of water to add because the way the material changes when it's worked - the harder you work it, the more firm it gets, while it gets more fluid if you're gentle with it. Seems like it's meant to be mixed well and left alone and in time it will absorb the water/ break down - as you said soak it for at least 24 hrs...seems like the key word is "soak."
      Thanks!
      Jake

      Delete
    2. Directions are above and the process really is that simple. Put some water in a bucket, slowly pour in the dry powdered clay, mix as you go. Make sure there is no powdered clay clumps and then make sure you have some water on top. Then simply let it sit for at least 24 hours. Time will do the work for you. Hope this helps.

      Delete
  25. Hi, thanks for your knowledge! What are your findings with longevity?

    Thanks!
    Jason

    ReplyDelete
    Replies
    1. Thanks! Would you know if granular clay work rather than powdered clay?

      Delete
    2. not sure what granular clay is...but any clay can be used to make plaster if it doesn't have unnatural additives. Note, however, that the recipe above is specific to powdered kaolin clay. If you use different clay, you will need to re-calibrate your recipe.

      Delete
    3. My wife and I are so excited and have started experimenting based on your suggestions! This is great! Some questions that have come up...

      1. how much water to add to the (100% kaolin) clay powder to make the putty (it seems one can add any amount and pour off the excess after 24 hours, correct? Or is there an easier way?)

      2. Do have a supplier for the clay putty?

      3. Our wheat paste turned out lumpy although the cold water/wheat mixture was smooth. Did it need more mixing while on the stove?

      Again, we are more than grateful for the information and looking forward to passing on your blog to others!

      Jason

      Delete
    4. great questions!
      1) if you make the putty at least a day ahead of time, then the putty will settle down and water will settle to the top; in this case you can skim the water off the top as needed. If you make it the same day, you'll have to experiment a bit to get the exact proportions, but usually it's around 20% water to 80% clay powder (start with the water in the bucket first, and add the powder)
      2. you can get powdered clay from any pottery supplier. I happen to be near Baltimore MD, so I use a supplier there. Not sure where you are, but you should be able to find a pottery supply store near you (or one that ships).
      3. Sounds like you didn't stir enough or there was too much water. Feel free to add 50% more water to the recipe above. the thinner the paste, the easier it is to work with. (But you'll have to adjust your recipe to add more paste since each cup has less flour in it)

      Hope that helps!

      Delete
  26. Hi Sigi,

    Don't think my last comment went through so will try again. Thank you for your helpful article and recipes. What would be the approximate ratio of sand to primer for preparing drywall for plaster. Had some sand delivered to protect pipes from getting cracked by rocks when backfilling, would that kind of sand work? It has been out in the elements for a while so would I need to wash it somehow? Is only one coat of clay plaster required for drywall coverage? If more than one coat is required is there any special finish you need to do for the second coat to adhere to the first? About how thick should the coat(s) be? Many thanks, Kathryn

    ReplyDelete
    Replies
    1. gosh, I'm not sure...I usually just have sand on hand and add as needed. The goal is to turn your wall into a rough sandpaper texture to give your plaster something to grip to. My favorite sand is concrete sand, with some larger aggregate.

      Delete
  27. HI Sigi,

    Thanks for sharing all this info. I am trying to figure out how much kaolin clay to order, and Im wondering about how many .lbs of dry clay makes 1 5- gal of the clay putty?

    Thanks!
    Sara

    ReplyDelete
    Replies
    1. depends a bit on the clay, but typically 50-lbs = about 10 gallons of putty

      Delete