The Fire Brick D130 Instruction Booklet page 35

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3. Building the Dome
3U: THE KEYSTONE
You now have two options – you can cut fire brick keystones using the remaining bricks in your kit, or you
can simply fill the keystone with Refractory Castable (not Refractory Mortar). You have the fiberglass dome
mounted in the oven, so the form will leave a perfect dome shape on the inside of this castable keystone.
We prefer to use the Refractory Castable; it sets just as hard and strong as the fire brick, and it makes the
perfect keystone because it has complete surface contact with the surrounding ring of bricks.
To make sure the fiberglass dome comes off cleanly, wipe the dome surface with a small amount of vegetable
oil. Add a tiny amount of hydrated lime to the castable if the temp is below 20°C (using the ratio of 1 tsp/25kg
bag). Mix the castable to a thick consistency and pour into the void, vibrating the mix with your trowel to
remove bubbles.
To prevent drying shrinkage cracking in the keystone, cover the wet castable immediately after the pour with
cling wrap. Allow the keystone to set for approximately 12 hours, after which you can remove the fiberglass
dome (it should peel off quite easily, and can then be pulled through the doorway). Check that the castable has
set properly by tapping it with your trowel before you release the fibreglass dome formwork.
3V: CLEANING THE MORTAR JOINTS
Next you need to 'point' the mortar joints on the inside face of the oven, and remove any excess mortar that
has bled through onto the fiberglass formwork. To do this, you will need to slide into the oven on your back...
Not much fun, but a very important part of the process! Take a small tub of wet mortar mix, several scourers, a
sponge, a small bucket of clean water and a small metal paint scraper in with you. First scrape off as much of
the excess mortar/castable as you can, then scrub any remnants off using the scourer.
Now fill in any holes in your joints using the wet mortar mix, and wipe off the excess (before it sets) with a
damp sponge. Wash all of the brickwork with clean water and a sponge and dry with an old towel.
TIP: Don't wait too long to get in and do this! If you wait for 12 hours after your keystone is poured, then remove
the fiberglass formwork and get in there you'll find it relatively easy to clean the dome. If you wait much longer
than this, the mortar and castable will set very hard and will be quite difficult to clean off.