343 – The Art and Science of Natural Plasters – Part 2

In the second part of the podcast, Paul, Jocelyn and Fred continue to review Bart’s DVD on The Art and Science of Natural Plasters, for which Paul was the “Supreme Executive Producer.” This mostly because permies.com people supported the Kickstarter in a big way.

You can find the DVD at http://plasterscience.com

Paul is concerned that he used some soil in his plaster, there was some pretty dark stuff. They said if the soil has at least 15% clay, you can use it for plaster. Some places that’s the best you can do (Ed: places with more bugs, likely).

Paul diverts to explain sand versus silt versus clay. You might think that silt and clay are the same thing, but the silt particles are more round and the clay particles are more flat. This is why clay behaves differently. Fred reports that Chris prefers “concrete sand” to “brick sand” because it has more variety of particle sizes and will lend more strength to your plaster.

Paul liked the demonstration of how different sorts of clay made different colors. They have a couple different colors of clay on the property, which is cool. They have all sorts of building material on the property, which is awesome (multiple types of sand, multiple types of clay and even something that can serve as “shitty cob” right out of the ground). Fred interjects that even the instant cob is best pushed through a screen first, for the sake of the hands and feet of the workers.

Back to lime plaster versus clay plaster – lime plaster takes a lot longer to cure, so that can slow you down. With clay plaster, you can move faster through the layers. Everything demonstrated in the video was with clay based plasters. There is some sort of “hydraulic lime” that comes from Europe and cures faster. Fred recalls a mention of NHL – natural hydraulic lime – and how it comes in different speeds of curing.

Paul was confused by the use of fired clay. Fred (correctly) says that the fired clay was used as an aggregate in a lime plaster. (Editor: yes, in ceramics, we run fired clay through a hammer mill to create “grog.” The grog is an aggregate in our stoneware, it lends strength to the clay and decreases shrinkage.) Anyway, the video probably doesn’t cover lime plasters well enough for a novice to start using them.

Mixers – Paul got a cement mixer at the lab, he thought it could be used for cob, but Ernie and Erica disabused him of that notion. (They did end up using it for some things, though, so it wasn’t a complete waste.) Fred explains that a cement mixer has a barrel that turns to mix the contents. Chris recommends a mortar mixer for making plasters – this has a paddle that turns inside the barrel. Paul says maybe someday they will get one of those paddle style mixers, to help make cob. It could substitute for some of the cob stomping that otherwise needs to happen. Cob stomping is really fun, for a couple of hours, for most people. However, making cob requires a whole lot of mixing.

Hey, there are two Ant positions left! You come here, pay $1200 and you are covered until 2018. It works out to less than $100/month rent. You’ll get to attend the appropriate technology course, and Erica will be teaching about natural plasters there!

He had a nice section on tools (if you like that sort of thing – Jocelyn nodded off). Paul was intrigued to hear that steel trowels/floats bring water to the surface and that can lead to cracking. He had some trowels made with alternative materials, like wood and even magnesium of some sort. When you are using the tools, you never pat the wall, you smear and slide over the surface. If you pat it (applying force perpendicular to the surface) you encourage the layers to separate.

They point out that some natural pigments can be toxic. Paul notes a few natural things that are toxic, like “Death Camas.” Jocelyn would love to bring some natural pigments into the Fisher Price house at basecamp. She thought there was a lot of great information about natural pigments in this video.

Paul appreciates that he pointed out what was toxic and what was not. For example, titanium dioxide – natural but toxic. There was another ingredient, siloxane. It’s a clear substance which provides water resistance and was described as “virtually nontoxic.” It is an ingredient in makeup, so people put it on their face. Linseed oil is another thing that can provide water resistance. Use RAW linseed oil, not boiled linseed oil. The boiled linseed oil has toxic gick in it.

One more tip from Chris – use a natural bristle brush for the finish coats. He had a cool looking brush, it was maybe an inch thick, to apply the finish plaster.

So, “The Art and Science of Natural Plasters” can be obtained at http://plasterscience.com and Paul recommends it highly. He feels that after watching it he can make better decisions about a lot of the things they are doing at the laboratory (and at basecamp). Allerton Abbey could use some natural plasters at this point.

Jocelyn also recommends going to Bart’s website, http://www.possiblemedia.org You can find the natural plaster video there as well as a bunch of other cool videos, about market gardening, passive solar greenhouses, tiny homes and more. There are a lot of free videos there, so go check it out!

Relevant Threads

The Art and Science of Natural Plaster Thread
Natural Building Forum at permies
Cob Forum at permies

Credit: Julia Winter


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