Saturday, June 5, 2010

The Catenary Arch

The "catenary arch" is an arch profile which is essentially the inverted profile of a suspended chain. When you take a length of chain and suspend it upside down, it forms a natural "U," If you flip it over, it reflects a profile that's perfectly balanced.
If you're building an earth oven, this is a really cool way to assist you in building a sand form that's not lopsided.
The oven I'm in the process of building will be 15" high with a floor diameter of 31-1/2".
To make the arch, you need a length of chain and a piece of masonite or thin plywood.

On the plywood make a mark a zero inches and 31-1/2 inches. Find the center and make a mark 15 inches down. Hang the chain as shown, adjusting it until you get it to line up with your measurements,  and with a felt tip, trace the chain.

You'll need a reciprocating saw to saw out your pattern

This is the best balanced dome I've ever built, and that's only because a friend explained to me the beauty of the catenary arch.

Monday, May 31, 2010

from Bread Earth and Fire (The Earth Oven) con't

A Roof

 Sure, you can always toss the blue tarp over the oven when rain threatens, but what happens when you’re baking and it begins to rain? The tarp can melt into a molten blue mass if you cover it then. Yuk. Forget it. The tarp is not an option. I know of someone who swears by his liquid, water sealer. “The water rolls right off the oven when it rains,” he claims. But does the water drain into the oven cracks? And remember, all ovens crack to a certain degree. There will definitely be an update on this topic.
Abbott and Nancy chose to make an oven cover out of metal roofing. It works fine and it was simple to build. You can fire the oven with the cover on or off. They prefer to remove the cover (easier done with two people) when they bake because they like to admire their earth oven in all its glory.
To make this cover you’ll need a length of metal roofing material. They bought theirs at Big Box where there was a good choice of lengths and colors. What they bought measured 36 X 96 inches. Perfect. With tin snips, cut the roofing in half so you have two pieces, each 36 X 48 inches. Match them up by overlapping as shown in the photo, and every 6 inches drill a small hole where you’re going to insert a nut and bolt. The bolts should be about 1/2 inch long and face down toward the oven.
A roof
You’ll need to make two rows of nuts and bolts. You want a short bolt because it should not scrape against the top of the oven. Secure the bolts securely with the nuts, and now drill 4 holes, one hole in each corner of your roof. Your bungee cords will go through the holes and then the bungees will be anchored to the oven base. At first we anchored the bungees to tent stakes, but the stakes could pull up, and people could trip on them. Not a good arrangement. Nancy and Abbott found some 4 inch carriage bolts in the garage, and they mortared them into the base. This was a much better arrangement. No, you don’t need carriage bolts. A 16 penny nail should work as well. Just mortar it in securely and let it set up before you attach the bungee.
Holding the roof down
Something tells me to fire the oven slowly the first few times and allow it to dry out, but I have trouble following my own advice. I’ve built ovens where we have fired and dried them out in one day, and the same day we were baking in them. We never had a problem. As soon as we finished the oven, thunderstorms threatened so Abbott and Nancy had to cover it with the blue tarp and were not able to build a fire. During the next three days they managed to build some small fires in between showery weather. Finally it cleared off, and they built some real fires, dried the oven out and got the metal roof built.
You can bake in an oven that has not completely dried out, but it’s difficult because the wet mud tends to suck the heat out of the oven. Your oven will bake better after it’s dry. Will it crack? Absolutely it will, but don’t worry. The cracks should not pose a problem. Earth ovens crack, at least all the ones I’ve seen crack. If you find it unsightly, then fill the cracks with some clay slip, and I’ve already discussed this. How long will your oven last? Nobody knows for sure, but you should be baking in it for quite some time. Just keep it dry, and now enjoy what you’ve just created.