Casting?

[edschmidt]

I dont know if its the refractory or not but it looks like you had quite a few air pockets, Also what is the thickness, the refractory I used was specified as min. 2"

[DrakeRemoray]

Hey Ed,

I used "Insulcast", a castable insulating refractory concrete from a company called pryor giggey. It looked a lot like the perlcrete, but it is supposed to be strong structurally up to 1800 deg f.

I have discussed this before, and I am not sure it is the right choice of materials, as I also used it for a lintel, and it chips when my tools knock into it...but it is in it's second year now and still standing. The vent walls are 2" thick. I just was not as careful as I could have been about the vent. You can read about this in my oven thread starting here:

http://www.fornobravo.com/forum/f8/d....html#post3641 (Drake's 38" Oven)

Drake

[edschmidt]

Im glad its working well, But I think what the problem is is that you used an insulating product which generally contains copious amounts of air. I have a couple of kilns and you can easily make a deep scratch with your fingernail.

[DrakeRemoray]

Ed, I concur. I love the vent design, beacuse no smoke escapes the front of the oven at all, so no black stains on my oven. However, I would use a different, refractory-type castable if I did it again. I had trouble locating a refractory castable locally and they guys who sold it to me said this would be strong enough, and it is...but....it still is a weak point in my design...
Drake

[aferaci]

I know most people here in the US shy away from casting...presumably because of the expensive nature of premixed stuff.....But has anyone thought about mixing their own castable refractory mix themself out of calcium aluminate cement and sand as described at this site?
» Castable for a Bread or Pizza Oven

Doing it this way would be far cheaper if you cast the dome at a 2" thickness....Any thoughts or has anyone tried this recipe?

[dmun]

Here's the money quote in that link:

I did not use wire for my most recent castable shell and there is severe cracking — though the cracking is not structural.

His formula seems similar to the alumina formula James gives:

In between Refrax and fire clay/Portland cement mortar, you can make your own aluminate mortar. It is hard to work with, as calcium aluminate can be challenging. If you get the mix, or water wrong, it won't set correctly. It partially sets very quickly, and you cannot re-hydrate it, so you have to mix it and use it in small batches. Still, if you are trying to save money and want/need the heat resilience, heat conductivity and longevity of a true aluminate mortar, it works.

1 part calcium aluminate
3 parts sand
1 part lime
1 part fire clay

It sounds like it would be difficult and unsatisfactory. If you do this, please keep us informed. We're always open to learn new techniques.

[david s]

Lime and calcium aluminate together are not good. I've tried this and the lime acts as an accelerant to make your brew go off much faster. The calcium aluminate is also quite temperature dependant. On a cold day it takes ages to go off (use warm water) On a hot day it'll go off really fast (use chilled water or some ice) don't mix product that's been sitting in the sun. It's also quite thixotropic ie It'll start going hard but if you give it more mixing or vibration it liquifies again weird stuff.

[aureole]

I'm going to try casting my vent, I think. Only thing is, I dont' know if/how to add rebar or mesh to it to strengthen. I have got a refractory mix product - I'll need 50kg of the stuff to make a 20"x14" (external) base tapering 12" up to a 14"x14" top. With those angles, casting with rebar seems awkward

If there's only a pot on it, will it still need reinforcing? I guess I need to play with the mix to test the strengit

[dmun]

Castable refractory is traditionally reinforced by stainless needles, which you get from the same place as your mix. Rebar has such a different coefficient of expansion that I don't think it's a good idea.

[DrakeRemoray]

My vent was cast without any rebar or needles and it broke upon unmolding, I still managed to mortar it back together and it works great! However, it only supports a double walled stainless flue, so if you are going to put a clay chimney pot on it, I would try to find the stainless steel needles that dmun mentions...

Drake