Saturday, 13 October 2007

Making the hearth form

Well, it took a while to figure out how to do it, but I've finally got the hearth formwork finished and level. I settled on making a ring of 12 3"x2" waste timber offcuts, cut with 15 degree ends to make a circle. I held the timber together with staples (pretty weak, wobbled all over the place), then clamped a ratchet strap around it. When tightened this was really strong and rigid, much to my surprise.

When I placed this on the stand and levelled it there were some pretty large gaps (>4" in parts), so the easy way to fix that was to make a black plastic 'skirt' inside the ring, and then trowel on a sloped edge of concrete. The black plastic stops the concrete falling out of the gaps. When this has set I'll pour the rest of the hearth - hopefully with no leaks.

The poured hearth slab will end up being about 5" deep in the middle (over the three block piers, and tied to them with the bent rebar), and at the 6" near the edge it will slope up to the edge of the form where there's still space for about 2" of pour.

3 comments:

Tony said...

Glad to see that the oven is moving nearer to completion. We're currently trying to work out suitable alternatives if we can't find appropriately priced fire bricks. Hmm.

Carl said...

Tony - see if you can get a copy of Tom Jaines' book. He recommends using engineering bricks for the oven, and only firebricks for the hearth. Might cut a few costs. Even the Alan Scott book says that you can use 'red brick' for the oven if you fire it slowly and carefully. I might do this, although I'm thinking more about adobe/clay right now...

Tony said...

Thanks - I'll do that. Because at the moment my wife is trying to come up with a plan involving refractory cement and raku clay with an adobe insulation - she expects me to know how to calculate and allow for the different coefficients of expansion or some such phrase.