Arch Mud Fun

The final mix for the arch mud (some people insist on calling it "cob", or "clay") was a challenge. The soil on site is basically beach sand with some silt and humus. Which for the sub-hearth I added a little left-over potters clay. But this was going to be insufficient for the structural requirements of the arch unless I wanted to re-hydrate lots and lots of potters clay. So I looked about town and found some high clay content sub-soil from a city project that they let me have. The clay content was very good, but the gravel/stone content was pretty "good" too! I mixed this soil with the sand from my yard, and some other silty-clay soil that I came by in my travels. In retrospect, I think I could have increased the sand content, maybe up to double, but it worked well with minimal cracking.

It's hard not to get some cracking in a four inch thick mass of wet mud as it dries! I just filled them, and put another layer over the whole thing.

Designing the Oven

So I settled on a chimney base that I think will work for this oven. Measured the height, and then calculated the correct height for the oven arch (x=doorheight/.63), and built a form 2" inside a cantenary arch (the arch center is inside the 4" mud arch).

The form will be burnt out once the arch has dried sufficiently and the nails will be swept out. This stage took a lot of sitting and staring.

Here's a close-up of the aluminum channel that my "automatic" closing door will hang in. More on that later.

Sub Hearth and Hearth

The first picture shows the finished beer bottle layer with the hearth extension done in mud. Then I put a 2.5" layer of mud over the whole thing as a sub-hearth. Notice the beer bottle layer is 3" bigger all the way around the baking part of the hearth. This insulating layer will eventually be wrapper around the whole oven. The front part is the future door area and chimney base. The sub-hearth was made with a mud mixture extremely high in fine sand. This is partly due to this is what I had, but also I wanted a very dense layer to absorb and hold heat.

The Hearth is made of fire brick. I got a good deal on some surplus fire brick at fourty-five cents each, but it wasn't enough. So I bought 60 new brick for the hearth and am using the old bricks for the extension/door/chimney base area. The new brick are Whitacre-Greer out of Ohio. We'll see how they perform. The brick are laid up dry on a thin layer of dry sand (for leveling).

At this stage I am also playing around with different chimney/door configurations...

Slab and Insulating Layer

I feel like I copped out a bit at this level. I built a concrete slab to set the oven on. One of my intentions with this project was to do it with recycled, re-used, cheap, or free materials. But the slab was the obvious way to get what I wanted: a strong, flat surface about 30" above the ground.

I filled the slab with re-bar & mesh. That oven might get heavy over a 32" span. Greta and Brandon came up for the weekend and they mixed on a tarp in the back of the pick-up truck. That way we could pour right into the form without lifting. That was Greta's idea. Actually a really fantastic idea!!! It took us about 90 minutes. They were awesome. I wish I had pictures, but it was over before I knew it.

The next step required a lot of beer drinking. Some friends helped. And a neighbor contributed too. The beer bottles are laid out with an inch or so between them and the voids are filled with a mixture of coarse sawdust coated in clay. Just enough to stick together and reduce chance of catching fire. I think there ended up being around 100 bottles. I wonder if we could work this into an ad campaign for Magic Hat. The empty bottles and sawdust create a temperature break between the oven hearth and the concrete slab. That way the slab doesn't draw heat out of the oven that should be kept in the oven for baking.

The Foundation

I started with a great deal on some surplus block. Laid up two walls to support a slab that would be the base of the oven. The block were laid dry on a trench filled with recycled concrete gravel. At this point I still didn't know exactly what type of oven I would be building. My last oven was a massive 7' x 5' (inside dimension) brick & calcium aluminate encased beauty that we built for a bakery-cafe that would bake 60+ loaves at a time, roast coffee, etc. This time I am leaning toward a home-hobby oven, something I can bake a dozen baguettes in, pizza parties, or...