Cob- help!

(note: this is cross posted to some degree in my RMH for Dummies feed)

Cracking Thermal Cob!! (no straw at this point)  HELP!!


Hi,
 I am making cob for RMH and no matter what, it is cracking.  Especially concerning in the manifold area where it needs to be air tight.

MATERIALS:   local soil (very sandy, little to no clay, sand ( I got free, has pebbles/rocks, does not appear to be rounded beach-sand type), dry fire clay)

WHAT I HAVE DONE SO FAR:

-test batches, test bricks...best combo with multiple repeats seemed to be 1:1:1 (sand:dirt:clay).  NO cracking in test bricks, BUT terrible cracking in the RMH
-tested each batch (making a ball, dropping it from shoulder height) for consistency.
-SO, I decreased clay and also dirt, so it was 1/4 clay, 1/4 soil, 1/2 sand.  STILL CRACKING and also very crumbly (loose crumbs on top layer).
- have also tried spraying with water to slow drying.

I am not sure if I should increase clay... more than 30% seems very high, to my understanding.

I have searched Google, You tube, Permies, and have not had much luck on resolving this.
Thanks!
Staci

Stacie;  I know your on a budget but...  Go to home depot and buy a sack of builders sand.Might pick up a box of red concrete dye at the same time. Leave out the dirt just use fireclay and builders sand .  Hold off using your free sand for now.  Try that see if your happier with the results.
Did you watch Matt Walkers video ?

Hi Thomas,
 Forgot the video! (palm to forehead)  Will do.
Am going to run by the Clay art center soon, after preschool drop off, to get more fire clay and pick their brains.  I spoke some on the phone.  They have sand there as well.  I am bringing samples with me.
 Home Depot is on the other side of town...where I will be later this afternoon, so I can pick up a bag of builders sand.
For what it is worth, our Rabbit thinks my cob tastes just fine.

https://permies.com/t/92790/Mortar-Rocket-Mass-Heater

Watched video. Picked up another bag of fire clay, a bag of sand at the clay center (silica sand), and a bag of scrap wet clay.
Next: builders sand
And then lots more testing.
If nothing, bound and determined to get it right.

Stacy, this is called "fayençage" in French.

Well, i have always read this is from too much clay in the mix.

May be try 1 part clay, for 4 parts sand, down to 5 parts sand even, may be.

Satamax Antone wrote:Stacy, this is called "fayençage" in French.

Sound much prettier in French!

I didn't read the cross post--is there a reason you are not using straw? Cracking usually means too much clay/not enough straw and the photo looks to me like that is the issue. Don't worry about straw burning, it won't burn unless it is directly in the path of exhaust and will just burn out harmlessly.  How wet is your mix. sometimes too much water can lead to cracking as well, especially if drying too quickly.