Plastering is my favorite part of any cob build. I spend the entire build looking forward to it and dreaming about the final result. Plastering is what transforms a mass of sculpted earth in to a structure that looks and feels finished. I find the process of plastering soothing and enjoyable and I delight in the transformation that it brings about.
There are two different plasters on the Honey House: Since the interior remains dry and protected year round, we decided to use a clay based plaster to cover it. The exterior plaster is lime based to give it good weather protection. The lime exterior plaster was prepared exactly as it was for the outdoor fireplace and won’t be re-explained here.
We completed the interior plaster prior to finishing the floor. Plastering walls is messy and if you don’t have a floor to keep clean beneath you, it makes things much easier. If you do have a floor, make sure you cover it with poly or heavy drop cloths thoroughly.
This was our first experience working in a finishing clay plaster. Its base is spent pottery clay. There is nothing wrong with this clay, it is just no longer in a consistency suitable for throwing pottery. Some studios find it too much effort to work the clay back to the correct consistency so they collect it in buckets and wait for people like us who want it.
The clay from this source is a lumpy, uneven consistency so we start by kneading it in water to make a thick, even slurry with as few lumps as possible.
The interior plaster was applied in two coats. For the scratch coat we combined clay with sand sifted through 1/8 inch screen in a ratio of 1 clay:3 sand. To this we added ~ 1 part of chopped and sifted straw. We mixed it by stomping it with our bare feet in a plastic basin, adding water slowly to get a consistency similar to whipped butter frosting. This was fun and left our feet soft and silky!
To apply this coat we misted the wall with a gentle hose spray and spread the plaster on with metal floats. The cob wall must be wet before you plaster, otherwise the tremendous suction force of the dry cob will dry out your plaster too quickly, leading to a risk of cracking and failure. Pool floats are the easiest to work with but I think rigid metal floats create a better finish once you get used to working with them. They also allow for more pressure to be applied across the plaster which encourages a solid bond with the surface below it. Try starting with a pool float and work your way toward using a rigid float.
We left this coat to slowly dry for a few days. Keep your plaster out of direct sun and try to keep the rate at which it dries slow by keeping it cool and, if needed, misting it with water periodically. This helps reduce cracking. Not too slow though or you’ll grow a mycological garden.
The scratch coat has a higher sand content than the finishing coat. It helps to smooth out the wall while maintaining a keyed surface for coats applied over top of it. It also helps to reduce cracking of the finishing plaster. We put a scratch coat on all of our cob projects.
The interior finishing coat on the Honey House is also clay based using the same pottery clay prepared in to a thick slurry in the same way as the scratch coat. To get a smoother finish we further sifted the sand through window screen and instead of straw, we used horse manure for the fibre.
Horse manure is our favourite way of adding fine fibre to plaster. Fibre acts like mini rebars and reduces plaster cracking. People also say that the enzymes in fresh cow (and to a lesser extent, horse) manure add stickiness and strength. We use horse manure by collecting it fresh from the barn and soaking it in a small amount of water in 5 gallon buckets for a day or two to let it ferment a bit. Some people use it fresh but we find the plaster behaves better when we let it ferment. This is similar to the Indian style of plaster known as Gobar. Need some horse manure? Let us know!
Other fibres can be used instead. Straw, even when chopped and sifted, is visible in the finished plaster which creates a neat effect if that’s what you want. Animal fur (we used horse fur in the tadelakt fireplace) works well as would any other animal fur which you can find for free from vet clinics and groomers. Cat tail fluff is also supposed to work really well though we haven’t yet tried it.
We also added wheat paste to the finishing coat. This, like the manure, is meant to add strength and stickiness to the plaster. We make it by combining white flour with water in a ratio of 1 flour: 4 water. This mixture is then brought to a boil on the stove and stirred constantly until it turns translucent. When it is cool we run it through a sieve to get the lumps out. We store it in the fridge and add it to the plaster right before the plaster goes on the wall.
The last thing we added to this plaster was a bit of olive oil, again just before it went on the wall. This is supposed to help the plaster slide off the floats.
There are many other potential additives for earth plaster including milk proteins, sugar, eggs, and even a bit of lime, none of which we have tried, to date.
We made several test batches of the finishing plaster before deciding on the recipe. The test sand:clay ratios we made were 3:1, 2.5:1, 2:1, 1.5:1 and 1:1. We applied the test plaster in swatches to the wall to see how they behaved as they dried. In particular we watched for cracking (a problem below 2:1) dusting (a problem above 2.5:1), adhesiveness (not a problem for us) and texture/finish. Our final decision was to go with 2:1.
Roughly 2 days before we intended to plaster we whipped up our batch of plaster and added the fermented horse manure slurry (blended smooth using a circular saw blade welded to a threaded rod chucked in to a hammer drill) at a proportion of roughly 20%. We then let the plaster sit and ferment together for 2 days before plastering, in keeping with Gobar styles of plaster application.
Time to plaster! The wheat paste and oil were added and the wall was dampened with the hose. We added wheat paste at 1/2c per 5 gallon bucket of plaster. Too much and the plaster becomes hard to work, or so we have read. The oil was added at 1 tbsp per 5 gallon bucket. Too much and it can impair the vapour permeability of the wall. The plaster went on with floats and again was applied with some pressure to try to force adhesion with the surface below. The plaster was worked and smoothed with the floats to as smooth a surface as possible.
Once the plaster was smooth and the majority of float marks were worked out, the wall was left to sit and dry. Slowly, as always. We were relieved to see hardly any cracking!
To get a buttery finish we polished this finishing coat with our tadeladkt stones. We waited until the plaster had dried to a heavy plasticine-like consistency (ie) not compressible to thumb pad pressure but still dentable to thumb nail pressure. If it smears with stone pressure it’s not dry enough. If you blister your hands from the pressure you need to apply, it’s too dry. The nice thing about clay is that if it is too dry, you can hose it damp again. This allowed us to plaster the entire wall and then polish it over several days (rather than tadeladkt which we plastered and polished in sections). Because of this, clay is perfect for practice before trying a lime tadeladkt (which once it’s set too hard, there’s no going back). Clay, like lime polishes to a silky, marble-like surface that is really quite magical. I think it would make a beautiful finish to an interior cob hearth. Maybe I’ll build one one day.
Finished! The plaster could quite happily be left like this. We sealed ours with boiled linseed oil which we applied with a paint sprayer and rubbed in with rags, like the floor. We feel this seals the plaster and helps us keep our Honey House clean but it reduces the vapour permeability of the wall so I don’t think I would do it all settings. The finished plaster is marble smooth like tadelakt, has a nice shine to it and the sand grains sparkle when the sun hits them in the evening. We love it!
Next the doors went in.
2 Replies to “Building the Cob Honey House – Part 5: The Polished Clay Plaster”
What and amazing post and process. Really helpful 🙂 Thank you very much for sharing and keep up the good work.
Thanks so much Thomas!