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Community and Q&A

An earthen plaster conundrum

Smartroots | Posted in General Questions on

i have recently been applying the first cob plaster layer to my load baring straw bale workshop.
the soil i’m using in my mix is from the excavation of the rubble trench.
my conundrum is this; when i do all the tactile tests; worm test, drop test, patty test, and test plaster, my mix seem to pass with flying colours. However; when i put that same soil in the jar test to get a soil profile, it seems like it’s just sand and silt.

is it possible that there is just enough clay to be sticky but not visible?i have not had to mix much sand for the plaster, the mix i have is 3 soil to 2 sand, so maybe that is the case.
or is the silt just acting like clay and i’m destined for a catastrophic plaster failure?
as i mentioned, the scratch coat is on the building and its dry and hard. this is my first go at earth plaster so i have no hands on experience (just books and website knowledge), but it looks and feels like i would expect.

any input is appreciated,

i’d like to continue to use the excavation dirt for plaster rather than find another source.


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  1. GBA Editor
    Martin Holladay | | #1

    Smart Roots,
    I don't have any experience with earth plaster. But it sounds to me as if you are interested in learning the clay content of your soil. It sounds like you aren't sure whether your interpretation of the jar test is accurate.

    The obvious solution is to contact a soil engineer and have a sample of the soil tested for clay content.

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