Inside of cob oven crumbling away

I just built a cob oven and everything was going great. In the last firing, though, I think I may have over done the flames/heat? and now the inside is very crumbly. Just touching the wall gently sends a mini Avalanche of dried crumbly cob to the floor. Have I ruined it? Can I put a layer over it to rescue it? My soil is 50/50 clay sand. I added sand for nearly a 4:1 ratio, sand:clay. No straw for the firing layer. Any suggestions appreciated.

Hello Arnie,

I am so sorry for this mishap...

It is really hard to tell per se what may be taking place, however I can share that this challenge does happen and illustrates something I am always warning folks about...Jar test are (or can be) very inaccurate...

Good soil analysis for clays (and types of clays) goes well beyond just putting soil in a jar and waiting for stratification to take place...A guide? Yes...but a very basic one at its very best...

It sounds more like either the mix was of...or...your "clay" is a silt and not a clay...or...your clay and silt are commingled and without a good differential so when the "clay" is augmented with sand it is weakened.

I will only build ovens/kilns and the related with a clay after I have made and fired several test bricks/tiles to understand level of durability...

Repairing a spalling cob oven is virtually impossible...or at least not worth the effort compared to a rebuilding in my view and experience...There is not good way to get adhesion for the patching material to the matrix that has already been fired...

So my suggest would be...

Do a rebuild..

But...not until you fire some test tiles in a pit fire to see how they behave. Fire them several times and make enough to do forensic destruction of tiles after each firing to see how they are behaving...It may be necessary to source a better clay for the burn chamber matrix. This can be done by using what you have as some "silt soils" will work as a good cob mix but not for heat...For the "burn chamber" you can get a potters or fire clay body to mix with the sand...

Regards,

j

Ugh. Thanks. I did fire one brick that was mostly just building soil just for fun and it is very solid and hard, so I assumed adding sand would make it even stronger. If this means anything, the material has become very red, even though it was dark brown to begin with. Yes, a rebuild is probably the way to go, though I have lots of man hours in this sadly. I may try a clay sand slip to see if I can at least get it under control until I can make the time for a total rebuild. Probably with some purchased clay.... To make sure I've got the right stuff.

Firing questions: is there any obvious way to tell what silt/clay content is? Mine is sticky and clay like--can mold into bowls etc. would adding too much sand make it weak/crumbly if it was actually clay and not silt?

Hmmmm....Brown to red does sound more like a clayey soil with good clay content...that is strange?? How hot are you get this oven and for how long?? They are o.k. with cooking temps, but prolong temps higher than that shortens there serviceable life span unless lined with soapstone or firebrick...

...would adding too much sand make it weak/crumbly if it was actually clay and not silt?

"Grog" and/or silicates (aka sand) can augment and strengthen a clay for firing, yet there is also a thing about too much...testing tells a lot, but one brick is not "testing."

Firing questions: is there any obvious way to tell what silt/clay content is?

Other than learning some basic geology, and soil science with the correct equipment (e.g. microscope, soil screens, etc.) the best way is to send samples away, and also do more "fire testing" of tiles...and...brick samples.

You can try the "patching" and do let us know how it goes...but my experience tells me it is a wasted effort and your best time is spent "testing" and deconstruction while at the same time planning for a better built oven...

Regards,

j

Thanks again. My son fired it for me and burned away the sawdust/slip layer all around the front side, so he got the thing very very hot. It topped out my stover thermometer at 850 degrees when I slid it in next to the fire. I suspect that's a part of the problem, or should the clay/sand be able to handle that kind of heat. Yes, sadly, I'll likely have to rebuild. I'll mess around with patching tomorrow, with low expectations. I imagine using a wet thin mix and applying in layers will be best? Then I'll start over. Sheffield potters are about an hour away and they sell firing clay for $18/50 pound bag... That might be my next step for the first layer. Sadly the oven is a thing of beauty... I soaked/ deconstructed wood pellets for the sawdust/slip insulating layer, and it worked quite well....

Sorry again for the troubles...

850 they can handle but not for a long extended time unless a really good clay/cobb matrix, and even then there will be eventual spalling...850 is not a "cooking temperature" and these "ovens" are not "kilns" and can't take massive heat like that for very long without degradation. Traditionally these ovens are looked at as "transient" in nature, unless made of brick and/or fireproof stone like soapstone...by annual rebuilds where not uncommon and even season rebuilds are part of some cultures that still use them...If I was going to "get fancy" with a build, I am making the "burn chamber" out of either fire brick or soapstone...and not clay....or...even a refractory cement if I had to...

Regards,

j

Here's what it looks like. Rusty red and very sandy/granular.

That doesn't look to bad...Can you get a shot of the area that is salling?

You make get a few more seasons out of this as is...maybe?

Regards,

j

That is the area. The entire inside wall seems quite frail. If you poke it, a small pile of it falls to the floor. Here's another angle.

I'm going to try wetting it and pushing gently to see if I can get it a bit less crumbly. Then I might try to see if a layer of clay/sand plaster will take? Otherwise, I can just try cooking and see how much sand falls in my pizza! Here's what I had before its demise...

Perhaps others will post there perspectives on this...It does look a bit sandy, but I could help think of the Doctor joke...Hey Doc...it hurts when I do this...and the Doc says...stop doing that...

So......stop poking it and perhaps it will last a bit longer...

Have you tried "brushing it out." This is something that I do to new kilns after the first few curring fires...By the way...??...you did cure fire this at low temps a few times at least...right?

Again...too...about the "repair,"...try, for curiosity sake but once clay is starting to "vitrify" you can't add "green clay" and fire it together...They just are now two different materials...but it will be curious to see what happens...

Probably rushed the curing process. I did 2 much smaller and shorter fires two days apart before foolishly leaving son in charge while I was away for several hours....unfortunately things fall even when gently shutting the door, but I'm trying to keep fingers away.

By "brush out," do you mean brush the walls with a small whisk broom, say? I haven't. I'll give that a try, too. Thanks again.

I brushed it out and it seems much better! Thanks again. I'll refrain from trying any more repairs for now and see how it fares with the next firing.

Yay!!...that looks like all it needed...a "tempering fire" and then good "brush out.""

I know all about..."Son in charge."

Between you and I...I bet you that little scamp had a bonfire going inside that oven that could melt copper...and probably said..."Uhhh...No Dad I didn't get it that hot...??"

Boy's will be boys bless their hearts...

Keep us up to speed as how things go...I am really hoping for this just to be a "brush out" and "missed proper cure issue" that will not lead to a "rebuild."

Thanks again for sharing!

Blessings,

j

Thanks again. Here's a few pics--the first shows what the door looked like after the copper melting bonfire. The second is the oven with a full door rebuild and added a clay/wheat glue glaze. Hoping to give it another firing pizza session soon.