Finish on Slip Straw

Hi All,

I am searching the internet with a specific issue that has some up in our slip straw building and haven't seen an answer yet. When we did our first coat of mud plaster over the slip straw, the walls dried to a patchy black. After applying several more coats of a clay, straw, sand combo, the walls are mostly brown but black spots continue to appear. It is possible to sponge them over but they tend to reappear in a few weeks. Any ideas what could be causing the blackening?

Hello Cyd and welcome to the forums!

Looking at your second photo of your slip straw, what is all the black? Is that the colour of the clay? I have never seen this before. Almost looks like black mold. Wonder if its that coming through?
Also, are you using the same clay source for your plastering that you used for your slip straw?

Thanks Gerry!

The second picture with all the black is the original wall color after a single clay coat. The first is the current state of the walls.

After the first coat, we started using a different clay source for the subsequent layers. The first coat came from a pond excavation project and the second was clay on the same property but from about 500 yards away and more surface level. To the naked eye they looked quite different.

It is definitely possible that it is mold. I have had some doubts because up close it is very dry and doesn't seem to correspond to wet spots or ambient moisture. It's pretty hot and dry where we are so don't usually have mold problems.

My current theory is that it is some kind of mineral in the mud oxidizing but have found no other stories of this happening.

Very open to ideas and suggestions! I am wanting to do a finish coat with either white clay or milk paint and am curious if the black will come through that as well.

Yeah, you might want to make sure that isn't mold before doing too much more. Black mold can cause some serious health problems.

I would recommend to do a few test areas using a few different recipes of your finish plaster over some of the black stuff and see how it goes. You certainly don't want a redo on an entire wall so patience may be your best friend! In case it is mold, adding some borax to your recipes would make the plaster have a pH much more alkaline than what mold can grow in. Its a natural product and easily acquired in a grocery store so it may be worth putting it in whether its mold or not.
Somewhere in the neighbourhood of 1 cup of borax to 1 gallon of water. (1 to 16 ratio of borax to water) should do the trick.

Thanks, I will try adding the borax just in case and do some test patches.

I know it looks like mold from the pictures, but up close it really doesn't. It seems to be more a property of the clay as it dried. Would love to hear ideas on that.