Finish Clay Plaster: Materials and Recipe

By June 17, 2009Clay Plaster
Clay Plaster - Interior Finish

Applying finish clay plaster on straw bale walls

Applying clay plaster may be one of the most satisfying parts of building a natural home. Smoothing over cob or straw bale walls with a creamy clay plaster mix is incredibly satisfying and brings about a truly dramatic transformation. What was once coarse is made buttery and gracefully smooth. For a successful clay plaster job, it’s important to carefully prepare and mix your material. In this post, I will explain the preparation methods and recipe I use for most of my natural building work.

Clay Plaster Ingredients

Fully prepared clay plaster ingredients, ready to mix

Finish Clay Plaster Recipe

First and foremost, it’s important to know what you’ll need for your finish clay plaster mix. The following recipe includes the exact ratios that I use. However, as is the case with natural materials, you may find slightly different quantities to perform better depending on your exact materials. This should be a good guide, though:

  1. 3 (5 gallon) buckets of 1/16 inch sifted sand
  2. 1 bucket of screened clay
  3. 1 bucket fresh screened cow manure (not dried)
  4. 8 cups wheat waste
  5. fluff of 3-4 cattails

Preparing Your Clay Plaster Materials

Unlike mixing cob or rough plaster, which can be made pretty freely without much worry, a quality clay plaster requires a bit more delicacy. Much of the work is in the preparation of the materials themselves, in fact. It can be a time-consuming process, so get a few friends to pitch in with the effort!

Sifted Sand

For a truly smooth finish, it is important to sift your sand to remove small stones and larger particles. I use a 1/16 inch window screen to sift all of my sand. This is a pretty tedious task, so it’s nice to have company around while you push sand through the screen. Also, it’s a huge help to have completely dry material, as it will go much more quickly. Note that the largest diameter of the aggregate you use should be half the thickness of the expected application. In this case, you can go up to 1/8″ thick with your finished plaster if you’re using 1/16″ sifted sand.

Sifting Sand

Sifting sand through a 1/16″ window screen

Screened Clay

It is highly advisable that you screen your clay to remove impurities. You’ll be happy with the increased quality of the material. (You won’t have to pick out pebbles while you’re applying, which is far worse than having to screen it in the first place.)

In this case, use a wood frame with some 1/16″ window screen as your sieve. You can mix the slaked clay in a bucket with a Hole Hawg and cream it up to make the process that much easier. Pour the creamy clay (about the consistency of heavy cream) through the screen, and use your hands to push it all the way through. If you have particularly coarse clay with lots of aggregate, you may have to do an initial screening through a bigger screen, say 1/4″ or so. The wetter the material, the easier it will pass through. Scrape off the impurities that don’t pass through as you proceed.

Wheat Paste

To make wheat paste, mix 1 cup of white flour in 2 cups of cold water, while you bring another 4 cups of water to a boil. Once the water is boiling, whisk the flour and water in, and mix thoroughly. Lower the heat, mix for another minute, and it should become slightly translucent and sticky.

Be sure to use the wheat paste right away! It’s generally recommended that you don’t let earthen plaster with wheat paste sit for too long – it can get real stinky. (I think the smell is reminiscent of rotten mashed potatoes, in fact.) The wheat paste makes for a stickier, gluey plaster. It also adds hardness and durability. By the way, wheat paste is purely optional of course, and I wouldn’t use it in an exterior application.

Cow Manure

cow manureI am a strong adherent of cow manure. If you are squeamish, don’t fret too much. It’s not that bad! Manure has been used in plaster for eons – it is totally safe and beneficial, and yes, the smell will go away. Manure lends strength, durability, and excellent texture to earthen plaster. The fibers make for great tensile strength, and I think that overall, a manure addition helps make the plaster softer. It aids with exterior durability quite a bit, too.

Try to get manure from cows that are strictly grass-fed. If the cows are fed grain, you’ll often find husks in the manure even once it’s been sifted. Fill up some 5 gallon buckets and keep them covered to prevent the poop from drying out. In fact, let it sit for a while if you can. After a few warm days, the manure will ferment a bit, and expand and become quite airy. That’s the best time to use it. I recommend screening it similarly to the clay (try an 1/8″ screen). It’s a good idea to get all the long strands of grass that inevitably end up in the bucket. Be brave!

Cow Manure Fiber

Pushing cow manure through a 1/8″ screen – not for the faint of heart!

Cattail Fluff

Cattail is a common plant found around marshes, ponds, lakes, and wetland areas. In the fall, pick the mature punks and keep them in a dry place to use in your earthen plaster. The punks explode upon twisting them, resulting in a cloud of beautiful, soft fiber that is a perfect addition to finish plaster, adding tensile strength in place of straw. I typically use about three or four punks in each batch of plaster. I like to be able to see the fibers in each handful of material. (Split a handful of freshly mixed plaster in half to see the fibers sticking out.)

Cattail Fluff

Behold the wonder that is cattail fluff…

Some folks break the cattails ahead of time into a large barrel and then fluff it with a weed whacker, but I find this to be unnecessary. You can break the punks open right into your mix.

Mixing Finish Clay Plaster

When mixing finish clay plaster, it helps to have a very clean tarp. Dump your sand and make a well, add your clay and manure, and stomp away. Once you have a nice even mix, throw in your cattail fluff, and try to avoid getting chunks of fiber (or “cattail bombs”, as we say) in there. You can either add the wheat paste now, or to individual buckets of material. Your final product should be consistent, smooth, wet, and sticky. Add water during the stomping process to achieve the desired moisture level.

Mixing Clay Plaster

Mixing clay plaster in a mortar mixer can help speed things along

I’ve also had success using an electric mortar mixer and a Hole Hawg drill to mix clay plaster ingredients. If you have access to  a mixer, first add your clay and manure, and then add your sand. Add water as necessary. A mixer really speeds up the process significantly.

Good luck, and have fun!

Finish Clay Plaster on Straw Bale Walls

The final product