Step 8  Lower bell construction, upper part of riser, insulating the riser


Lower bell construction

   Second layer, partition and trapdoor opening

Put in place a piece of superwool of 303 x 490 mm against the right side of the core. Place the partition on a 7 mm layer of cob. Push it against the superwool and keep it on its place with a lath.

Now lay the joining concrete block with mortar (1:5). The 27 x 15 mm notch holds the front of the partition. Press the joining block firmly against the superwool and use a pressure lath to keep it in place.

Lay the second layer of concrete blocks using cob, except for the trapdoor lintel: lay that one with mortar.

The left side block and the trapdoor lintel have notches that need to be flush with the top of the firebox.

The concrete block with the ledge must be laid accurately at the back; the upper back of the partition is now held in place by it.

   Floorchannel and door lintel

Drop the floorchannel into place. If good, it will fit easily in the slot, and the front plate will fit in the 4 mm space at the front of the firebox.

Place a piece of superwool measuring 330 x 242 mm over the whole firebox. Put the door lintel in place with cob and press it firmly into the superwool: the joint between the firebox and the lintel must be airtight.

Insulating the riser

   Insulating lower riser with vermiculite

To insulate the lower riser, the space around it (between the riser, the concrete blocks of the bell and the partition) is filled with vermiculite that has been mixed with clay (mix 1 part dry clay powder : 5 parts vermiculite; then add 1 part water and mix again). Before filling, place a piece of superwool against the rear (behind the riser).
Then carefully fill the space with the vermiculite mix, paying particular attention to the crevices between port and riser.

   Upper riser and fracture point

The upper part of the riser can now be placed onto the bottom part. Use a small amount of clay between the upper and lower parts instead of glue, creating a fracture point. Because of the extreme temperatures, the two parts will expand at slightly different rates, and this fracture point prevents the upper part of the riser from cracking.

   Insulating the upper riser

Lower the 200 mm diameter steel tube over the riser until it rests on the firebox top and the vermiculite fill.

Fill the space between tube and riser with the vermiculite/clay mix.



joining block 170x98x60

partition 402x305x24

ledge 100x27x8