Fatty clay soil for bench

Hi there,

I need to make a bench for my rocket stove.
I have a really fatty clay /loam soil, which i dug out from the garden.
I've used it for earthen plaster for the whole house and it worked very well, but I have to put a lot of straw in it. To keep it from cracking.

Now I would like to use it for the bench. But I don't want to put straw in it because it would act as an insulator.

Any advice?

Howdy Ross and welcome to permies! The amount of straw you would be adding to inhibit cracking won't change the thermal mass properties in any measurable way, and definitely won't turn it into an insulator. There is a huge difference between cob (or adobe) and light straw/slipstraw in terms of the ratio of earth to fibrous matter.

Go for it and cob that baby up with a similar mixture to your plaster.

Hello Ross and welcome!

I would assume by the word "fatty", you mean there is a high percentage of clay in the soil that you are using.
If you are using this soil neat, then I would suggest to add sand (or more if you already are). Its the sand that will really give the stability to your final mix.
By making some test patches with various ratios, you will come to find what proportions work best for your soil with minimal cracking.

Also, the small amount of fibre you use (as Phil pointed out) will not significantly influence its thermal properties. What some people do is to build the bulk of the bench with little or no fibre, then on the surface layer add quite a bit more for its tensile strength for wear and tear purposes.

Great advice, thanks.

By fatty I mean it's very sticky. Probably like you mentioned that means high clay content.

I done lots of tests for my plaster but never went beyond 1:6 because I felt it seemed a bit sandy and crumbly. (And it still cracked without straw) apparently there's different types of clay and some are less ideal for cob.

Maybe I could try a few sandy base layers and then my straw-plaster mix over the top for strength.

Thank you so much for the quick reply and advice much appreciated!

Ross

Our subsoil is mostly fine silt with some sand and clay. There's enough clay that it binds well, but it cracks a lot as it dries, probably similar to yours. On my bench I just did what Gerry described: layers of plain cob, filling in the cracks as I put on the next batch, and then for the surface coat I mixed in a lot of cow manure. This gave me a mostly crack-free finish with a bit of a country aroma, which doesn't bug me because it's in my glasshouse. I will probably limewash it one of these days to make it a bit more water resistant.