Building a Cob Oven Maybe this is just in Norway, and agreed upon by those who like cob ovens, but it’s been said that the first thing built when building a home is the cob oven. Makes perfect sense to me. Those involved in the work then have a hearth to collect and nourish themselves. Like the kitchen: life in a home centers around the kitchen. Nearly everything in this region is built on a hill. This definitely holds true at Beth and Harold’s Escuelita (little school). Niko and Julio are excavating, with hand tools, a site for the cob oven. Here, the site has been prepared, some of the materials hauled up the hill, and the footing dug (Can you see it? Under the upper rock). We’ll be building a large leg of stone from the foot; table for the cob oven will be held on the other side by the earth (which will be dug in). The footing was filled with concrete, the stones hauled up and placed one at a time - with just a little concrete thrown in the center from time to time. The object in laying the stone is to concentrate on good corners and horizontal courses, while avoiding vertical joints. (I know, this angle doesn’t show the most flattering lines - but it’ll work!) What happens in the center doesn’t really matter as much. We put only as much concrete in at this point to hold the rebar in place (to tie into later) - the idea being to create one monlithic structure encompassing the foot, the leg and the table. Gino and Harold are working on the form for the concrete table. You see here that the wood was not cut if not absolutely necessary - wood is very scarce and expensive - this way it can be re-used when the form is taken apart. This rebar grid was laid at 30cm intervals, leaving 5 cm open to any edge; making sure the metal is completely enclosed in the concrete will prevent it from rusting. Same reason for placing rocks under each intersection (tied together with wire). The bricks on the left (and the cob added afterward) create the negative space for the concrete to shape itself against. Vibrating the form and the rebar (made with raised texture) helps the concrete settle and form around all the edges. Hauling the sand, rocks, and concrete up the hill, and mixing each batch, in the hot sun, is lot of work. When it was done, we celebrated. This is the gang, Christian (from Ecuador), Niko (Mexican), Julio (Mexican), Harold (Norweigen), Shailagh (Canadian), Gino (Canadian), and me (behind the camera). The working surface of the concrete was painted with a thinner layer of white concrete colored with yellow iron oxide powder. We’re building the base for the oven. A brick wall filled with sand will help retain the heat of the oven. Tiles were laid dry on top of the brick and sand base to create the oven deck. Gino is drawing out the circumference for a meter-wide diameter on the inside of the oven. We built the inside form (a space and shape holder only) out of bricks, wood, bottles, and a subsoil-water mix - making sure anything we put in would be small enough to take out through the oven throat when we were done. Once we had something resembling exactly half of a sphere, we added the oven’s throat and smoothed it all out. (The opening should be exactly 60-66% of the total radius.) A nice spherical shape on the inside will promote good convection transfer of the heat. Wetting and laying paper (or whatever you have) between the form and the cob provides a smooth surface for the inside, while serving as both a visual - and a bond - breaker. The walls should be the thickness of your stretched finger-span (2o cm, 8 inches). By focusing your integration against the inside form, combined with the weight added from above, this thick- ness should be achieved naturally. In fact, you might want to shave off the eventual excess so that you can use it to finish up. The next day we took out the inside form, painstakingly, bit by bit. And put on a first coat of rough plaster: a thinner cob mix without the straw. Rubbing it in, we used it to fill the cracks and shape the oven. When the inside was cleaned out, we started it’s first fire.......................And watched it collapse! Maybe it was the result of poor integration? Maybe a batch of cob was too wet? Maybe we took out the form too soon? Maybe adding wet plaster too soon did it? Don’t know. Could be one or all. When we began again, we took each of these possibilities into consideration. Harold’s main concern is making sure each course is tilted 90 degrees from the curve in the sphere. We waited three days before taking out the form, and another two before roughing it up, wetting it, and putting on the rough plaster. Harold named the oven “Askeladden,” Ash boy, after a Norweigen fairy tale. We toasted Askeladden’s birth with an afternoon beer, and had our first pizza party that night! Leaving the space for a grill, etc., will allow this oven to also be used as a stove and a smoker. Finishing will include chiseling the edge of the table and coloring it, and adding a final plaster with shell embellishments. Natural Building: Abroad Thursday, February 7, 2008