Primer on cob plasters

Hello all.

I am in the process of learning to build things with cob. I've read Hand Sculpted House cover to cover and I have successfully made several bricks that were almost impossible to break and I feel confident in my ability to make the basic material.

My next goal is to make a cob oven and then attend a Cob Cottage Company workshop prior to building a house for myself and my spouse in the next few years.

However, even after reading HSH I am extremely uncertain about plasters to use on cob. I understand that under no circumstances should concrete or water impermeable plasters be used as they form a condensation barrier that can soak and ultimately destroy your home, but the plasters section did not provide me with any clear answers on a basic plaster recipe to play with.

Can someone point me in the right direction on plasters? In the meantime I will reread the plasters chapter to see if it isn't clearer this time around.

I'm expressly interested in lime plaster as the book says it's durable and breathes well.

I found this protocol online for straw bale: http://www.earthsafedesign.com/external-lime-plaster-on-straw-bale-wall/

If you nix the "straw bale" and "clay slip" layers in favor of just starting with the roughed surface of your cob, would this be suitable for exteriors?

What about interiors?

Hi Pat...Welcome to Permies...


You have asked some good questions and sounds like you have a plan. If you take a workshop you will get many of your answers about plasters and renders answered...(and develop more questions too. )

Sounds like you are ready to tackle a project like an "outdoor" oven. If you have not built a structure before, be warned that there is much more to it than many workshops, books and individuals lead many to believe. As a builder, unfortunately, I see more errors (and some dangerous) practices developing and creeping into "natural and traditional building systems." Cobb is one of my favorite materials in the world...I still would not build just a cobb structure in most places on this planet, other than where it was done traditionally. I look always to the vernacular styles of a region first, as these are the most logical and applicable.

I will be writing articles here at Permise over the next few years covering some of your questions. Lime is a traditional render (outside) and plaster (inside) in many cultures, (as is just clay, clay lipid mixes, clay lime, just lime, and even paper plasters.)

Good luck for know, and enjoy the process.

Regards,

j

Geoff Lawton's recent Zaytuna Farm Tour (Part II) has a great segment on cob wall finishes- he has a 3-layer process that allows water to wick out of the wall core, but protects water from infiltrating. Does anyone with cob experience have thoughts on how Geoff's methods would work?

Hi Nathan,

His section of the video about wall design sounds pretty good. His terminology is a little off, as well as some of the reasoning yet all in all a nice video. Not one to build from, and I do question the logic and use of welded steel frames with permeable wall systems. This is another good example of folks with very excellent intentions, perhaps reinventing to much of what has already been done.

Regards,

j

I can add that I am currently working with lime plasters on adobe block surface. It is a labor of love process, but I am thrilled with the results.
It takes a bit to get the hang of it, and you do want to be properly protected because it is very irritating when it gets on your skin.
If I can answer any specific questions about my process with lime plaster, just ask. I'm always interested in learning how others prepare and apply it as well, so do share your results later.

In the meantime, here is a quick video (1minute) on our lime plastered walls in progress. It's a facebook page that is totally PUBLIC so anyone can access it. Also, there are lots of construction shots here of this adobe construction.

Pura Vida,
Evelyn

https://www.facebook.com/photo.php?v=709322142431367&set=vb.241412745888978&type=2&theater

There are some great online resources for cob-compatible earthen plasters, and some books worth checking out.

Carol Crews' Clay Culture
has a great section on soft vs. hard finishes: basically, if you are doing a "soft" masonry form like cob, you need soft (breathable) finishes that won't trap moisture, or crack/blister themselves off the wall with the inevitable forces of temperature, moisture, and material settling/movement.


Let's see if I can find the Amazon links to books I own, and some online stuff.

The Natural Plaster Book, Guelberth & Chiras

Using Natural Finishes, Weismann & Bryce

The above are just plasters and finishes.

The following are short handbooks that have similar info (with less political essays) than the Hand-Sculpted House, and should contain basic info on earthen plasters and tools needed.


The Cobber's Companion (a Cob Cottage book, out of print)

The Cob Builder's Handbook, Becky Bee, self-published: there's an online copy here, I think it's free or maybe honor-system.

Studio Melies is working on a DVD about natural plasters, which I can't wait to get.

For online resources:
I love Mike Wye's website for the extensive collection of articles about historic restoration and compatible modern workmanship. http://www.mikewye.co.uk/faqs.htm
(I am a geek. I don't actually know if there's any actual plaster recipes there or not, but you will learn things about detailing that some of the current generation of cob-revival teachers don't know yet.)
More earthen architecture geekery: http://whc.unesco.org/en/earthen-architecture/

Paints and pigments compatible with cob and natural plasters:
Milk Paint: RealMilkPaint.com - they ask for a contribution if you find the recipe useful, honor system.
American Clay
Ochres & Oxides
Adding pigments to plasters: http://www.earthpigments.com/plasters-cements/
(Note that you can also use any concrete pigment in cob, lime, or whatever: they are alkali-tolerant if you're using lime or Borax, and they are mostly the same mineral composition at a fraction of the price. The main differences would be toxicity, mostly in the blue/green/artificial tones, and for those I am willing to pay the premium for a non-toxic, ultramarine-type pigment from the natural sources.)

A great thread on "taadelakt," a waterproof Moroccan lime plaster, and the efforts to replicate it using locally available materials elsewhere: https://permies.com/t/26216/Finishes/recipe-tadelakt

Hope that helps get you started!

For those who stopped by following links from the Rocket Stove forums, the one caveat I'd add is that some finishes don't do well with heat - notably beeswax, which gets soft and sticky again every time it warms up. We usually do the final finishes on heated cob benches with just a straight plaster mix, milk- or wheat-based hardeners, or an oil soap burnishing like a light-duty tadelaakt.

-Erica W