Step 4 First layer of the bell. Placing the core
Bell: the first layer
The top of the first layer of the bell should be exactly level with the firebox. In the SketchUp drawing, that first layer is 356 mm (including the glue). The firebox is 353 mm, with 3 mm underneath for the strips of clay on which it will be placed.
If the height of the firebox differs slightly, adjust the size of the concrete tiles.
Drill a hole of Ø 8 mm and about 25 mm deep in the rear tile, at 240 mm from the right side and at 190 mm from the bottom (fig 1). You do the same on the inside of the front tile.
Now glue the left tile and the back tile in place. The rough side of the tiles is on the outside. Use clamps again to compress the parts. Make sure it is flat, square, level and at the same height.
Installing the core
The glue of the core must be fully hardened: let it dry for at least 3 days before lifting it into place. The two tiles of the first layer must also have had time to harden thoroughly.
Place two strips of Superwool, 50 mm wide, against the left tile. In the drawing these are the vertical strips. On the bottom slab, where the core will be placed, apply three strips of cob, 3 mm high, from back to front (fig 2).
Then place the transport board with the core as close as possible to the bottom slab, preferably on an elevation of concrete blocks or something similar. Remove the battens from the transport board. Carefully lift the core from the board and place it on the three strips of cob (fig 3). The front must be flush with the front of the bell. Push the core firmly against the two strips of Superwool - it must be airtight. Check with a level, a square and a tape measure whether everything is in the right place.
Once everything is in order, fill the gap at the left of the core with the three horizontal strips of Superwool. First press them flat so that they can easily be pushed into the opening.
Now cut the front concrete tile to size (fig 4) and drill the Ø 8 mm hole as indicated at the beginning of Step 4. A strip of Superwool is placed between the tile and the core. That strip is 12 mm thick and is pressed about 3 mm by the tile; the joint will therefore be 9 mm wide. Glue the tile to the bottom slab and secure it with a clamp.
Spacers
Place two sawn-to-size blocks between the core and the left side tile, and also two between the core and the front tile.
This will keep the front tile in place when you glue and clamp the right side tile.
Place a piece of Superwool against the wall of the firebox, together with a floor tile of 30 x 30 cm. In the drilled holes you put a
Ø 8 mm threaded rod or rebar of 52.5 cm length. Together with two wedges, it keeps the tile in place (fig 5).
Upper part of the riser
You can now place the upper part of the riser onto the lower part. Due to the extreme temperatures in the riser, the two parts of the riser will expand at slightly different rates. To prevent cracks, you place the upper part on a layer of clay instead of gluing it. (fig 6).
Finally, on top of the firebox you place a 50 mm wide strip of Superwool.
1.
3.
4.
2 mm clay
5.
2.
3 mm
cob
spacers
6.
Superwool
