We started out thinking we would use Lime Plaster as our outer layer (over the Cob). Our research led us to a slightly different, and less caustic, way to use an outer plaster layer. See below.
Lime Plaster has been used throughout the ages to make smooth wall surfaces, and also sculptural relief on the exterior and interiors of buildings.
We’ll be using Lime Plaster as a top-coat over the Cob. This will give A Universe its off-white finish. We may also use Lime Plaster as a material for creating our own reliefs on the exterior of the structure.
Not unlike our other natural mixtures, Lime Plaster is mix of Sand, Lime, and Water. We trowel on and smooth out a layer of the material over the Cob.
In this video they show the basic mix and application. For our version, we won’t be using horse manure and fermented cactus juice (sorry)!!!
Upon further research, we are experimenting with natural clay plasters as an alternative to Lime plaster.
Why the change?
Altough tough and utilitarian, lime can be caustic to work with and have long drying times that don’t work well with our project timeline.
Some notes on lime plaster:
- “Plastering your house with a lime plaster effectively sheathes it with a limestone skin”.
- Make lime putty at least a week before! Soak lime in 5 gallon buckets (thick milkshake consistency) and cover, top off with water as needed.
- Mix putty with sand (well graded, different size particles) ratio= 1:3, lime:sand
- Traditionally use 2 COATS, one rough, one smooth. Abt ⅜” thick each.
- 2nd coat goes on when 1st is ‘green hard’ firm to the touch but not bone hard.
- Wear gloves and mask!
- LIME PLASTER NEEDS TO DRY SLOWLY (a huge con for us)
- New England Gypsum sells Lime by the Bag , locations in Manchester, NH and South Portland, ME. $15/bag , 1 bag covers approx 150 square ft
- We have roughly 314 square ft on just the exterior.
We happened to find a section about alternative plasters in The Hand Sculpted House by Ianto Evans, Michael G. Smith and Linda Smiley, a valuable reference we have been using for detailed natural building techniques. Linda writes about a time when she used natural CLAY PLASTER on a building to create a satisfying finish.
- Clay plaster with KAOLIN clay (white)
- Comes in power by the bag
- Non toxic
- 1 coat application
- Can apply a thicker coat
- Mix of fine sand/flour paste/kaolin clay/straw shredded in a mulcher or similar/a little oil
- Similar to a cob mix, but finer
This method is appealing to us as a Lime alternative for A UNIVERSE specifically because it is less toxic and has a more flixible drying time. It is similar to our cob mixture.
Our Clay Plaster Mix:
(Recipes combined from This Cob House and The Hand Sculpted House book)
First make wheat paste (flour paste)
- 4 parts boiling water
- 2 parts cold water
- 1 part flour (any kind of flour)
Mix cold water and flour together, then slowly add boiling water to thicken.
Now the plaster recipe!
- 2 parts fine sand
- 1 part kaolin clay
- 10-25% wheat paste (you will have to experiment with your unique mix for best results)
- Water (amount varies)
- finely chopped straw to thicken (experiment with quantities)
- a splash of oil to help the mixture slide off your plastering tools
- a splash of bleach or borax to keep the straw from stainting the white plaster
“We made small mixes in a wheelbarrow using a giant whisk and a hoe, with 70 percent sand and 30 percent clay. We started by whisking together the water, flour paste, oil and borax, then added the kaolin clay. Next we hoed in the sand, bit by bit. Last we mixed in plenty of chopped straw, essential in one-coat plaster, added more water--until it reached the consistency most satisfying to apply, not too wet and not too stiff. It was a joy to use small, very fine Japanese finish trowels. Some people may prefer larger trowels so they can apply greater amounts of plaster more quickly. That’s okay for a coarser plaster, but we were creating a very elegant finish, and the small trowel was a perfect tool for the job.” -The Hand Sculpted House by Ianto Evans, Michael G. Smith and Linda Smiley
This is a great video that explains how to MIX and APPLY natural clay plaster.
He uses clay soil straight from the earth so is has a brown/tan color, using a SHAKE TEST to make sure there is enough sand present. We would be making our mixture from scratch using pure clay and pure fine sand.
We made our own clay plaster using the recipe above, here’s how it went:
- see how the clay plaster worked on our cob samples
- practice mixing and smoothing it
- observe any cracking issues
- try a few mold styles for casting reliefs
Sand and Kaolin Clay before mixing (it felt great on the skin, silky and exfoliating)
Mixing all the Ingredients together.
Above: clay plaster spread on the cob samples (using a variety of straw ratios, making sure to spray cob with water before)
Below: clay plaster reliefs in plasteline and wet sand molds. (dish soap brushed on plasteline molds for easy release)
Extra plaster on a brick wall. Good practice for smoothing it out! I have confidence that if it sticks to paint covered brick it will stick to cob.
We let them dry for 2 days just in case. Honestly, the cob samples dried our very fast in full sun, the relief molds took a little longer.
Cob samples look good! A little cracking, but could perhaps be avoided with partial shade, they were sitting in full sun and it was very hot. Another layer on top with less sand/finer sand would help smooth the finish as well.
Brick wall finish while still drying, sticks very well!!
Reliefs after de-molding.
Reliefs after a little clean up with water and a plastic brush. ︎︎︎︎︎︎ If taken out sooner they could be carved into, they can also be added to when applied to the wall.