What sand for cob.

Hello all we are the start of renovating a stone house and barn in the south of France and want to do as much natural building as possible. We have a few hectares of land which is pretty much pure clay, so happy days that’s the clay part sorted. My question is what sand do I use I figure it’s just standard builders sand. For the interior walls I was thinking about making a frame backed up with plywood, filling frame with cob, removing plywood when dry then finishing  with a natural plaster, for this I assume I need a finer sand.
I’ve kinda answered my own questions, but am I right.

Any advice would be greatly appreciated

Cheers

Hi Chris,    It is a common practice to use sharp sand for making cob. "Sharp" meaning sand that has jagged edges rather than smooth (like beach sand) which is rounded. The reason for this is that when the clay that surrounds the sand dries, it pulls the grains together and the angular edges lock producing a stronger finished product. A magnifying glass can be used to look closely at the sand that you have to see if it is more suitable than another. You can also of course feel the sharpness as well while mixing.

Like Gerry says, sharp sand is the sand to use. Usually you can get it pretty cheap, sandblasting sand comes in 100 lb. bags here in the USA, go for a middle of the road mesh size for making the best cob.
Also straw from wheat or barley is far above any hay for making your cob strong.

thanks for the replies gentlemen. so sand for sandblasting is good, what about brick layers sand? Sorry if I'm sounding stupid i just wanna make sure i do it right.