Tuesday, July 31, 2007

The Second Layer

Last time we completed the first layer of the dome. This update brings us through the second layer. As you recall, the first layer is thinner with no straw added to the clay mix. The second layer is thicker and adds neccesary thermal mass to the oven.

We begin with more mixing. This time we are mixing true cob -- a mixture of Virginia red earth, sand, straw and water. Because I had some powdered Red Art clay on hand and little left over coarse grog from the first layer, I added it as well.

I learned an important lesson about oven-building collaboration. While these projects are great fun for the kids, adult helpers can actually make the project go faster and provide good company. In this session, I was particularly lucky to have two helpers with great attitudes and actual experience in working with clay. In the future, I shall encourage more adult collaboration in my oven building activities.

Here, Mr. J does the cobbers' high step with me.

As before, we patted the cob into mushy bricks to facilitate the orderly shaping of the second layer. At the beginning of the day, it seemed anything approaching a brick shape would do. By the end of the afternoon, Mr. J's standards had changed and the bricks seemed much improved in shape and integrity.

We laid the cob "bricks" in courses around the first layer. The trick is to proceed as though you are truly building one course on top of another, merely using the existing dome as a guide. Note how pressure is applied down and not inward toward the dome.

We mixed two large batches of cob and one tiny one just at the end because -- of course - we were a two bricks shy. Here, Ms. J and I put the last bits of cob in place.

The last order of business was to smooth the outside of the second layer using hands and a rocking motion with a flat board.

After drying out, which takes at least a few days, the oven is functional. However, a final layer of adobe plaster makes the exterior a little more presentable by, among other things, hiding the straw pieces. Next update will include photos of the "plastered" end product.

Tuesday, July 03, 2007

Il Duomo

Last time, we had just completed another course of stone work. So we start off by filling the top three or four inches of the foundation with concrete because, if you recall, I was determined to make a floor that would not cave in. The first layer of concrete had vermiculite mixed in for insulation -- a nice idea, but poorly executed. I don't think the vermiculite layer will be thick enough to have a material insulating effect. We made the concrete flush with the edges of the foundation.

Next I added a ring of bricks (salvaged from the yard) and two concrete pavers. The idea was to create a circular sandbox on which to lay my firebrick floor (sans mortar).

I dry-stacked a firebrick door frame using angle irons (as suggested by Kiko Denzer). I really wanted to make the entrance point of the oven a little more civilized this time.

I've been dying to try a little brick masonry and this seemed to give the dry-stacked firebrick arch a little extra support. I am hoping the mortar will be insulated from the most intense heat so that it will not degrade too rapidly -- even if it does, it won't seriously undermine the structural integrity of the oven. The paving bricks are stacked in the center to hold in place a piece of drywall that will prevent the sand form from spilling through the door.

I don't have a picture but I laid sand on the concrete floor until it was flush with the two front concrete pavers. This picture shows the firebrick laid on top of the sand. I took some scrap bricks and laid them to the sides of the floor to keep things in place and to support the clay walls.

We spent a surprisingly long time forming the sand mound. Kiko cautions that the form should rise very steeply and then round off gently at the top. For some reason, the temptation is to build a pointy form, which makes for a poorly shaped oven. He says to aspire to a pregnant belly shape, which is probably why it took me so long. Try though I did, my best effort was still a pretty scary looking pregnancy.

Once you sculpt a beautiful sand form, you promptly cover it up with ugly wet newspaper. Mine declared the immigration bill defeated. This helps form a clean break between the sand form and clay to facilitate removal of the sand.

We mixed dry clay and grog -- this is where I part company with purists like Kiko Denzer. I used one part Redart Clay, one part Hawthorne Fire Clay, one part 35 mesh grog and one part course grog that the ceramic supply store had lying around and didn't know what to do with. I measured the "parts" by volume not by weight. In the end I used 100 pounds of Hawthorne, about 85 pounds of Redart and 175 pounds of grog. Kiko uses earth and sand.

After adding water and mixing with our feet, we formed wet bricks.

McGee, desperate to be a part of the process, kept dropping his tennis balls into the clay mixture.

We encased the sand form with the wet clay. I made two larges batches of clay -- which ended up being about three bricks short (not reflected in the photo below - which was more like the halfway point). This was very frustrating. I made a third small batch of clay to top off the crown.

Once formed, I scored the sides with a trowel so that the next layer would have something to grip to.