Capt Jon's Cob Oven

[cobblerdave]

Re: Capt Jon's Cob Oven

Gudday Capt Jon
Curious now! The hollow in the stand did you do the thermal mass thing. Crusher dust or compressed sand . Or do you go new school and do the insulation path.
Regards dave

[Capt Jon]

Re: Capt Jon's Cob Oven

Looking at your earlier pictures, it doesn't appear that you have any insulation under the floor brick, or room for insulation on the sides of the oven itself. This will be a big problem. Cob needs to be insulated the same as conventional refractory material.

That was just a photo of "before". Checking to see how they laid. Photos coming of the insulation.

[Capt Jon]

Re: Capt Jon's Cob Oven

OK, here we go?
So I mentioned that I did a lot of reading. Some was well written, some you had to read between the lines a bit. I was leaning towards the ?earthy? point of view on a few things. The insulation was one of them.

So what we have is a layer of straw, layer of empty beer bottles, layer of straw and the sand. I was amazed how much sand went in there once I added, leveled, packed, repeated. It was roughly 3 1/2 full wheelbarrows of sand.

On the bottles, I won?t be one of the shy ones a says that I collected the bottles from my neighbors. Although it wasn?t one session, a lot of work went into this little venture and hence, several medicinal liquid therapy sessions were necessary in the evenings immediately thereafter. OK, may be one or two was from a neighbor as we stared at the work accomplished that day.

The last image is the bricks laid in place after all the packing. Now if one was to zoom in on the picture real close, you would see that there is a slight drop of not even a quarter of an inch once you come in the entry onto the cooking floor. It didn?t stay that way...
The following weekend I had purchased fire clay (a later story) and had all week for that little drop to eat on me. I knew that it wouldn?t be a major issue but, most probably a little inconvenience. I pulled all the brick off and brought up the sand to level the day I mixed a bit of cob for around the floor base to secured them in place.
Thought I would mention it before one you eagle eyes caught that!

[Capt Jon]

Re: Capt Jon's Cob Oven

And FYI, there are 3 or 4 places in Houston that sell firebrick and refractory cement, although I won't vouch for their knowledge on the application for ovens.

The guys at Able definitely had done it before. Although the bulk of their business was taking care of refineries, they were more than happy to sell small quantities. I knew there had to be more places that sold the materials. Guess I was just looking in the wrong places....isn't that an old country song?

[Capt Jon]

Re: Capt Jon's Cob Oven

Time to make a sand castle.
You know, I really didn?t run across any good information on forming a chimney. They just seem to appear magically on some peoples builds. I thought about this one quite a bit. Right or wrong, what I came up with worked fine (for the most part). I bought a piece of PVC pipe, lubed it with Vaseline (leave it alone guys) and then wrap it with plastic wrap. It slid right out with no issues at all. The issues I did have in this area, I will describe with the proper pictures later.
After the sand build, rolling with a 2 X 4 to smooth, I covered with wet newspaper. Oh, I did lay newspaper down on the brick to aid in having less sand to clean out of my brick floor cracks.
Yea...that didn't really help much.

[Capt Jon]

Re: Capt Jon's Cob Oven

I was running out of day light and it was time for the therapy session I mentioned earlier. I covered the whole shooting match with a tarp but found that the newspaper had dried even though the moisture had condensed under the tarp. I presume that even though the sand was wet, it wicked the moisture out of the newspaper. That newspaper was getting to be kind of a hassle to work around the first day anyway so; I put a little painter?s tape on the seams prior to setting it again. Worked like a champ.
A problem I was having was the different points of view on the sand to clay ratio.

Here?s where I?ll focus on the clay a bit. Clay seems to be hard to find here in suburbia. The only clay you can find this time of year in the yards is premixed with sand for baseball fields. They do mix it themselves but no one was getting more in till spring. Pity, it?s a really vibrant red , I assume from East Texas. Can?t go digging in our own yard (that missus thing again) and most of the neighbors know what I?m doing out back so, mysterious holes popping up in lawns would result in the other women on the street barking at me.

So, I call a local pottery shop in town. They only had wet clay. They did direct me to a shop close to downtown Houston called the Ceramic Shop. 50 lb. bags of Missouri fire clay for 18 bucks (and some change) each. Certainly fire clay must be better right?
So off I go to buy 200 lbs.

On the ratio, I went with 1:2. It mixed well and held together great but, I think I would have had less cracking if I had gone a bit heavier on the sand. Those visits to the yards running my hands through baseball field clay must have scared me?

[Capt Jon]

Re: Capt Jon's Cob Oven

Couple more photos and I'll call it a day.
Little bit more to add to the chimney and that wraps up the first layer.

OK, pop quiz for the pros out there.
Thinking about the ratio of clay/sand and looking closely at the wet clay photos...where do you see a problem popping up when it starts drying?

The answer will be clear in tomorrow's photos!

[stonecutter]

Re: Capt Jon's Cob Oven

I would think you will see some separation between the oven and the back of the brick arch as the cob dries.

[Capt Jon]

Re: Capt Jon's Cob Oven

I would think you will see some separation between the oven and the back of the brick arch as the cob dries.

Very, very close...

[Capt Jon]

Re: Capt Jon's Cob Oven

So, Stonecutter almost had it.
It was the arch/chimney area that had the problem once we started drying. The oven did pull away a bit as the shrinkage started to occur but, the real problem was the little lip that I had made from the chimney onto the arch. My basic logic of trying to think things through and forecast what would happen failed me?as you can see below.

One of my dilemmas, besides having never been around one being constructed before, was the differing opinions on when to take the sand out. One site would say leave if for a week, another saying after a few days where your clay has a solid ?cheese? feeling to it. I opted for the cheese and removed the sand 4 days after forming the clay.

What compounded my problem was that I had a blue tarp to cover the oven but, nothing to support that tarp. Now the weight of the tarp itself wasn?t overly worrisome. It was the issue that we had been having period unpredictable showers this time of year and I was worried that the additional rain weight, although it would not collect, might be the straw that would collapse the camel?s hump. Perhaps and unfounded concern but, I didn?t want to have to start that mess over again. Throwing in another point here, I didn?t have enough bits of wood lying around to make some form of temporary frame to support the tarp.

So, what did I do? A very small curing fire. Not quite enough to see steam released but, could tell it was making a small difference to the interior of the oven. I felt comfortable enough to cover the oven even if we had a healthy shower beating on the tarp.
I didn?t get to see the results until coming home from work the next day.

My vertical was no longer vertical.

[Tscarborough]

Re: Capt Jon's Cob Oven

Depending upon the amount of moisture in the clay, 2-3 weeks at 80-100 degrees seems reasonable to me before removing the sand. Then very small fires for another week or two.

When they make hand made adobe brick, they give them at least 2 days, and they are small and exposed to 12 hours of 100 degree direct sun every day. THEN they stack them under cover and leave them for another couple of weeks.

[Capt Jon]

Re: Capt Jon's Cob Oven

Thanks for the feedback.
Yep...lesson learned.

One thing that I omitted above was that the cracking was only external. The internal lines that are seen on the middle picture above are only from the newspaper wrinkles. The only place the cracking was all the way through was just the chimney due to the pressure point.

[Tscarborough]

Re: Capt Jon's Cob Oven

When modern brick are made, using a clay body that has so little moisture that as far as you can tell it is hard as concrete, they STILL let it dry in a temperature controlled drying shed of 100+ degrees for a week or 3 after extrusion.

Using a malleable clay body that has 50 percent more moisture content, it should be easy to see where the issue is.

Use as little water as possible, as much sand as possible and let it naturally dry as much as possible before applying heat.

[Capt Jon]

Re: Capt Jon's Cob Oven

As the clay continued to dry, the crack grew as expected. I shaved the lip off which allowed the chimney to come back down in place. I then applied so some slip in the gap and around the exterior (filling other cracks in places where they had appeared. Chimney still wasn't completely straight but, that can be covered up later.

Onward toward mixing cob and putting it on...

By the way, had the second pizza session last night. Full bore fire. Just like people have said, all traces of the black carbon on the oven interior walls vanished. Much better pies with the higher temp.

[Capt Jon]

Re: Capt Jon's Cob Oven

OK, so we are almost caught to the present.
Pics of the oven with the cob complete.

I didn't notice it that day or when I first looked at these pictures but, if you look closely you can see steam coming off of the clay.
It was a bit hot that day...