Building the dome of the bread oven (4)
This is part 4 of Building a free-form bread oven. Click here for bread oven principles, plans and construction recipes (part 1), how to build a bread-oven base (part 2), how to build the oven floor (part 3)), how to build the final layers (part 5), make your own oven tools (part 6) or cooking in a bread oven (part 7).
To build the dome
The dome of the bread oven is constructed by building over a wet sand mould. The sand is held in place and formed in a perfect dome by making a 3-D pattern out of corrugated cardboard.
Cut out two semicircles of corrugated cardboard at the same diameter as the inner dome, i.e. 750 mm (see the plan in part 1). Cut one from the centre top straight down to half way. Cut the other from the centre bottom to half way up. Lock the two together as shown in the photo above right. Place them on the oven floor in the same position as shown in the photo.
Mark the position of the door on the cardboard and cut vertically as shown above right. Also cut a model of the hole of the door opening (as in the plan in part 1 and as shown middle right). Pack the door shape against the dome cardboard, using wet sand as shown below right. This gives you a model for the shape of the inside of the door tunnel.
Pack wet sand in all quadrants until you have a smooth dome of sand that allows only the top edges of the cardboard former to show. Pack wet sand in a tunnel shape from the door hole forward and up to a piece of hardboard stood vertically on the front edge of the circles you have marked on the floor slab (right). I cut the piece of wood to the door shape as a guide to keep the tunnel even and so I could see the door shape as I built the concrete layers of the dome. A large tin can provides the mould for the chimney hole. Build the sand into a mound on each side of the chimney as shown on the photo right. This will form a slot that will help direct the smoke up the chimney. You will have to build the refractory layer on top of this sand mould in the same day, before the sand dries out and collapses.
The refractory layer
The refractory layer (right) is built handful by handful, layer by layer until it completely covers the dome. Plan to leave a gap of 100 mm between the refractory layer and the hardboard. This will be filled with
insulating mix (the darker mix in the photo) to prevent the front of the oven becoming too hot to work against.
The refractory mix (see here for the recipe) must be mixed with water to a firm consistency. If the mix is too wet, the weight of the upper layers will bulge the lower layers out as you build.
As you mould a handful,check that any lime powder is mixed in well. Estimate that the handful is 100 mm thick. Place it next to the last handful and blend the two in together with your fingers. As you approach the top, place a 100-mm handful on the top. This will help you judge the thickness of the layer as you build over the top of the dome.
Fill the 100 mm gap in the front of the oven with insulating mix (see here
for the recipe). Gently wiggle the tin can and remove it before the mix dries.
Allow all to dry over night.
I used a ceramic field tile for the centre of my chimney, placed it on the
dry refractory layer and built it in with the insulating layer (see photos 2
below). However, if I were to build again I would make the chimney out of
refractory mix, at this stage, by building it up around a corrugated cardboard tube. Place the tube inside the hole in the refractory layer of the dome while it is still wet and build the chimney up in the same way you built the dome.
If you intend to place a metal chimney above the refractory one, make sure the top circle of the refractory chimney is level and smooth to accommmodate the metal. Keep the outside walls of the chimney rough so that the insulating mix will stick to it well.
Leave the refractory layer overnight and for most of the next day to set.
Then remove the sand and soggy cardboard from inside the dome. Do not remove the cardboard from inside the chimney at this stage (it will burn out once the oven is finished). Now you start feeling as if you're really making an oven!
The insulating layer
If you have not already built your chimney as talked about above, place a ceramic field tile above the chimney hole and hold it in place with a small amount of refractory mix. Cover the dome with aluminium foil and hold it in place with chicken mesh.
If you are using a field tile as the chimney attach some chicken mesh around it to allow the refractory mix to adhere. If you have made the chimney yourself, you will simply cover it with insulating mix after you cover the dome with insulating mix.
Mix some insulating mix to an oatmeal consistency (see the loose handful in the photo, right). When applied to the dome, it will pat into a more solid, firm form. Work consistently round the dome as you did previously with the refractory layer (see below left). Build up the oven door hole to another 100mm above the first layer (below left). Place a nob, 100-mm thick, on the top of the dome as a template and build up the walls to meet the top nob (see below left).
Attach a metal chimney vertically above the chimney and hold in place with
refractory mix (below right).
See here to apply decoration and the final plaster layer.