Why is our earth plaster turning splotchy white?

We are plastering our front wall with a clay-straw plaster. We are new at this (our first wall, after several years of testing, perseverating, doing other building, etc!)

Some resources say mix the clay and straw and let it sit 24-48 hours but not too long or it will ferment and get funky. Other resources say mix the clay and straw and let it set at least a week or more until it gets nice and funky. We put ours on 48 hours after mixing at the youngest, and 20 days after mixing at the oldest (didn't mean to wait 20 days but I got COVID in the middle of the job and took a while to get back on it). The 20 day stuff was good and funky!

But whether 48 hours or 20 days, the clay went on nice and richly brown, then dried to the pablum beige that our dirt is, and about half of it turned white with little dots when it dried.  It looks like salts are precipitating out or like there is some kind of growth on it.

We are not that concerned about it because we will be adding a color coat in the long term (next summer?). I went over the first part with a clay slip (same clay but without any straw) and that seems like a good short-term solution (for occupancy permit), and it dried dirt color without the white splotches.

But we'd love any insights y'all might have about our mysterious white-flecked plaster!  And... should we be more worried than we are?

We live in NW New Mexico on the Colorado Plateau, 10-12" rain per year, and aren't concerned about molds generally.

In the more distant photo you can see in the top left corner the area that we clay-slipped after it dried splotchy. And the top right shows the wet clay.  The rest of the clay is splotching as it dries.

Given the humidity of your region and the likelihood of alkaline soils in your mix, I'm leaning toward salt crystal formation. I would probably even taste a little bit of the white efflorescence to satisfy my curiosity...that would tell me a little more about what type of minerals are involved. Or you could scrape some off into a clean bowl, dissolve it in water and measure the pH. Also, try collecting some of the dust and see what happens when you drip a little vinegar on it.

Whatever it is probably won't hurt anything, although brushing it off before you plaster might not be a bad idea.

Phil Stevens wrote:Given the humidity of your region and the likelihood of alkaline soils in your mix, I'm leaning toward salt crystal formation.

That makes sense. It is a little odd that the same clay doesn't turn white as a clay slip, but maybe that's because it dries more quickly and the salt doesn't have time to precipitate out...?

I found some links on efflorescence in plasters. Like this one:

https://www.nitterhousemasonry.com/blog/what-is-efflorescence/

It makes sense, since I was using quite a wet mix, and it says more water will precipitate the salts. That also explains why our test patches didn't have this issue... they weren't as wet, and they were smaller/thinner so dried more quickly. So maybe our color coat won't have this problem either; it will be thinner and not as wet going on.

But it begs the question... are salts likely to precipitate out over time in humid areas? I thought this was such a perfect application for clay plasters... like in areas of the bathroom that won't get wet, but will be humid. Or in a plant growing area or kitchen, etc. Clay plaster is supposed to be such a great humidity regulator.

But won't that humidity absorption cause efflorescence down the line...?

It's more a function of the solubility of whatever the salts are and the moisture content of the plaster. As the mix dries, the remaining water increases in saturation until the solubles crystallise out. They do this on the surface first because that is the driest part of the wall. A long as the plaster is releasing moisture, this mechanism will be at play. So if you had a wall system in an environment with repeated wetting and drying cycles, you could see efflorescence recurring until all the salts have worked their way out.

ou could try a UV light at night on it to see if it glows!

One caveat to my assessment that it's harmless would be if there was a really high salt concentration and the crystals formed within the plaster, growing enough that they pushed on the outer layers and delaminated the coating. This could be an issue in getting a coat to bond with the surface underneath. But the salts might just migrate out through the finish coat, too. The only way to know this would be test patches.

We went ahead and clay slipped the rest of the finished wall. We shall see what the next days and month show! We do plan to color coat next summer. I will remain cautiously optimistic until and unless the wall tells me otherwise... thanks for your info!!