Casting?

[dmun]

Re: Casting?

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]

Re: Casting?

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]

Re: Casting?

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]

Re: Casting?

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]

Re: Casting?

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

[aureole]

Re: Casting?

Hmm - the stainless needles will only be able to go vertically, though, right? It seems I need something that's going to go around in a band.

It sounds like I'm likely to crack whatever I do though with a castable vent. Shame I'm having trouble source angle iron or I'd to that to form the vent and build up with the firebrick to a flue.

[dmun]

Re: Casting?

Angle iron shouldn't be a problem. The places that do structural steel (for the building trades) should be able to get you a piece if your hardware store can't. They call hardware stores ironmongers in the UK after all.

[aureole]

Re: Casting?

Well, I got my rebar from a nice but huge place that does large scale builds - they hadn't got any angle iron in stock, oddly enough. The other building places don't seem to have it either which confuses me. Now I only want a 28" section it seems more difficult to get - I can find 2m sections on ebay, not sure what I'd do with the rest of it!

I need to talk to some local fabricators I think!

[Johnny the oven man]

Re: Casting?

S/S needles are about 1" long and form a woven matrix in all directions when mixed in your castable.