I found this on FaceBook, covering some of the things used to improve clay plaster. (I did not know that cow poop was 'magic'.) He's making walls and sinks, but this was a common technology in early days of electricity.
https://www.facebook.com/groups/naturalhomes/posts/4185018061513402/Sigi Koko
December 30, 2020 ·
I wanted to share a little primer on clay plaster & additives, as these questions seem to come up quite regularly.
What is clay plaster?
At it's most basic, clay plaster contains 2 simple ingredients: clay (which acts as the sticky binder/glue) & aggregate, such as sand (which reduces shrinkage cracking and improves compressive strength). Water is used to activate the clay and make it sticky. Once wet, the plaster is malleable and can be shaped to whatever your purpose is. Clay plaster hardens as it dries out (ie, as the water evaporates back out).
Clay is found in soils around the world. And mostly the clay soil already contains some sand.
Other additives:
There are so so so many other things you can add to clay depending on what you are trying to do with it. Here are a few of the most commonly used additives, along with why you would use them.
fiber - gives you a support network inside your plaster...like an armature or woven fabric integral in your plaster. Most often fiber is some kind of chopped straw from grain farming. But can also be hemp (fiber, not hurd), sisal, palm fronds, hay, etc. Any dried fiber that is pliable and not super smooth will work. The more fiber you add, the more flexible and gravity-defying your plaster becomes. A plaster that is 50% fiber will bridge dissimilar materials (ie, let you plaster over wood studs), and allow you to build out thick relief sculptures (or just super thick plaster).
horse manure - easy way to add chopped fiber to your plaster
cow manure - adds some fiber and some stickiness (binder). Cow manure can be used to amend super silty or weak clays to improve strength. Plus the magic of cow manure is an enzyme that helps any plaster improve resistance to erosion from water. So it's a great additive for exterior plasters, plasters in wet areas, or clay floors.
wheat paste - this is simply flour boiled in water to create a thickener. (Other starches work as well besides wheat.) Wheat paste adds stickiness, so helps to improve weak clays. It most commonly is used to strengthen and prevent dusting in finish plasters.
pigments - clay is naturally occurring in a plethora of colors (white to yellow to orange to red to burgundy to purple to blue to gray to black). You can also add any variety of pigments to augment or change the color of your clay. Note that your base clay color is part of the palette so it's easier to darken a plaster than lighten it up.
I personally do not add lime to clay plaster. It's often feasible, but it's tricky, and depending on the mineral makeup of your clay soil, you can change the chemical curing of the lime so that it becomes hydraulic. So if you are just learning plastering, I personally would steer clear of adding lime. (Plus the lime is caustic, so you can no longer touch your plaster with your hands safely.)
....all the wild and beautiful things you all are doing with clay plaster 🙂