Earthen plaster and cob, what’s the difference?

​People spatter about the word “cob” a lot, often in the wrong place. Many times they’re on about plastering, not cobbing. So what’s the difference between cob and earthen plaster?
 
In terms of ingredients, not a massive amount. But cob is structural. It's for building stuff out of. Earthen plasters are decorative, and used for creating renders for pre-existing structures. Here's the thing though: Of the two, contrary to what you might think, earthen plaster is harder.

Cob versus clay plaster, what’s the difference?

​Why is earthen plaster harder?
 
Clay has swelling and shrinking properties, so it's prone to cracking. This doesn't matter too much when you're building a wall out of it. But it matters much more when you're trying to create a smooth plaster that grips onto a wall, or create something aesthetic with it. People sometimes struggle with earthen plasters, because they pull away from the wall, or dry full of cracks. So I think that if you've mastered earth plaster, you've basically got cob down, too. But...

Plaster is harder.

Are there any differences in mixtures?
 
There are a couple of differences to note between cob and plaster mixes:
 
1. Cob can be rougher than earth plaster. As mentioned before, the aim of cob is structural (walls, houses, seats, etc). What does this mean in terms of mixture? You don't need to be quite as picky with the sieving of the dirt.
 
2. You want fresh (not broken down in the percolation process) straw in a cob mixture, to give the material a more knitted-together sturdiness.
 
3. You'll get on better with builders' sand, rather than fine-grade sand (again, gives more bulk and strength to the mixture).

My plaster mix is finer than a cob mix would be.

​An introduction to earth plaster
If you want my introduction to making earthen plaster, take a look at my basic earth plaster guide.

​Want a go at cobbing?
If you've want to have a play with mud, why not have a go at making something out of cob this summer? It’s cheap, fun, and a great way to enjoy some good weather.
 
Five Ways to Practice Cob
 
1. Build a chicken coop
This is my summer job! Chicken coops are nice and small, so it’s a great way to experiment with cob. You could even make a mini version of a house you are dreaming of, which would act as a scale model.

Kristen’s beautiful cob oven.

2. Make a cob oven
Uncle Mud is the cob oven and rocket stove aficionado, so have a browse on his YouTube channel. Cob ovens are nice and small, and you don’t need planning permission for them, either. Very doable.
 
3. Build a dog kennel
Another nice idea for cob is a lovely little shelter for your doggie friend. Animal houses are a brilliant way to see how cob performs structurally, and allow you to work out how to add doors and roofs and things at a manageable size.
 
4. Make a cob bench
Cob seats can be beautiful. Not much to go wrong with them either. One of my favourite stories of a cob bench is one made at a school in the Hudson Valley. It’s lovely. You’ll probably want to cover it with some kind of roof to protect it from rain, though.
 
5. A bottle wall
Bottle walls are a bit of a midway point between plaster and cob, but a great way to experiment with mud building, and observe how clay performs structurally.

Jehane’s bottle wall.

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