First Build & Casting – Homebrew Vs. Dense Castable

[Stoox]

Hi,

This is my first post and first attempt at building a Pizza Oven. I have looked through many of the posts on this forum which have helped a lot.
The first resource that I stumbled across online was this (PDF/Link below) of a step-by-step build of a cast oven over a sand dome. This is what inspired me to have a go at building one myself as it seemed simple enough. The idea was to keep the cost as low as possible whilst still having something that will last.

Progress

I have laid a concrete base and planned a perlite concrete layer due to go down next as an insulation layer. I have also sourced some Storage Heater Bricks (cheaper than firebricks) for the Hearth which will be placed on top of that layer.

Next Steps (in a nutshell)

What I had planned to do next was then to build the brick arch, create the sand dome, cast a thermal layer, insulate with ceramic blanket, then possibly add another perlite concrete insulation layer, then render.

Advice/Help Please

My biggest dilemma now is what to use as the thermal layer. There are lots of posts showing the successes and failures of homebrew. A local supplier of Kiln Raw Material said NOT to use Portland or Lime as part of the hot-face material and has suggested using 1400’C Dense Castable @ 10cm thick for the dome. Then I worked out how much Dense Castable I’d need (based on 100cm(l) x 88cm(w) x 48cm(h) internal) this worked out approx. 14 x 25KG bags = £226 whereas a guesstimate of Homebrew would be nearer £90.

My question therefore comes down to risk… am I better off just paying the £226 for the pre-mixed refractory or taking a punt on homebrew? (I’ve read that the Dense Castable doesn’t reach full strength anyway as a pizza oven does not get hot enough). Or could I even reduce the thickness of the thermal layer to reduce the cost that way?

Thanks

Stew

[Stoox]

Having issues with this forum... tried updating the link as it stripped it down... This is the actual PDF URL.
http://pinkbird.org/images/3/33/How_...ays%281%29.pdf

[tazzmacd]

This is something that I am also debating. Leaning away form the home brew but would like to see what others have to say

[david s]

" A local supplier of Kiln Raw Material said NOT to use Portland or Lime as part of the hot-face material and has suggested using 1400’C Dense Castable @ 10cm thick for the dome. Then I worked out how much Dense Castable I’d need (based on 100cm(l) x 88cm(w) x 48cm(h) internal) this worked out approx. 14 x 25KG bags = £226 whereas a guesstimate of Homebrew would be nearer £90"

Your local supplier is the one who benefits from his advice to you. Most commercial cast ovens are around 50 mm thick not 100, this would more than halve your costs. As your oven will see temperatures less than half of 1400 C a lower rated dense castable may be a cheaper option. You are correct in saying that the castable does not achieve full strength unless fired, but all commercial manufacturers provide their castings unfired. There will be some hairline cracks but they never get any bigger. I suggest if you want to bypass the homebrew option then go back to your supplier, ask for a price for a lower rated dense castable based on 2" thick casting, then get a second price from another supplier. If you are concerned about the strength of a 2" casting you can add melt extract fibres (stainless steel needles), most oven manufacturers don't.

[fox]

Hi Stew, I guess you are looking at this one? http://www.castreekilns.co.uk/dense-...-bag-865-p.asp looks great to me, very good price!

[Stoox]

Hi Stew, I guess you are looking at this one? http://www.castreekilns.co.uk/dense-...-bag-865-p.asp looks great to me, very good price!

Fox... Yes that is the item i was looking at. I think its a good price when compared to other suppliers but it was just down to the quantity required that was driving up the costs.

David S... 1400'c is the lowest rated castable that they sell. If I could reduce the thickness to 50mm/2in then that would definitely be worth considering. A couple of questions off the back of that then:

- Would the casting be strong enough to hold its own weight at 2in over a meter(ish) span?
- I've also read that adding polypropylene fibres (often used in screed) reduces cracking, is that worth adding to the castable or is that generally an additive to Homebrew?
- Would it be worth doing a 2in layer of Castable followed by another 2in layer of Homebrew while still moist for additional strength (mass) or will that react in some way?

[david s]

At 2" thick the casting is easily strong enough, that is what most manufacturers of cast ovens use. eg Forno bravos Primevera.
Most castable contains the burn out fibres to assist water removal so there should be no need to add any. Sieve some of the material to see if there are fibres there.
Not sure how backing up the castable with homebrew would work. The aggregates used may have different thermal expansion and maybe portland concrete may not bond so well with calcium aluminate concrete, dunno.

[fox]

The same company sell stainless steel needles, they can be added to help the integrity of the dome.

[Stoox]

The same company sell stainless steel needles, they can be added to help the integrity of the dome.

Yes ive seen those as well, again cheaper than some other places but at £10+VAT per Kg (at 2% i think), if i go 2" thick so thats another £42. I could go 1% though, if £20 gives that little bit more strength it may be worth it.

[Stoox]

Can you use standard cement/mortar for building the front arch or does that need to contain lime and/or fireclay as well? Not sure what temperatures this will be subjected to.

[fox]

Hey Stew, I placed my order with Castreekilns today, I spoke to the owner and he seemed to be extremely knowledgable & helpful!
My build if different from yours because I will be using a multi part mould but I had a long conversation with him about all the pros and cons of using high temp mixes S/S needles etc..
Perhaps you could give them a ring yourself.

[Stoox]

Hi Fox, I have been in contact with them via email a few times but you're right it might be worth giving them a call. What materials did you order and what thickness will your oven be?
Considering that you are using moulds, I have considered building the dome in sections similar to the post below?

http://fourquartdor.canalblog.com/

I like the idea of this and could be done quite easily i think, it would mean that it doesn't all have to be done in one go, which means that I can do half of it and then evaluate how much more material I need. My only concern is if there are structural risks of doing it that way.

[Stoox]

David S... Can you tell me if the ready made Dense Castable (2" thick with ss needles) is less likely to crack than Homebrew (3-4" thick with ss needles and poly-fibres) or are the chances of cracking the same and just the properties of the concrete that justifies the extra expense of the Dense Castable?

[david s]

No I can't answer that one because I've never made an oven using homebrew. But, as a guide Portland is good for up to 300 C, lime good for 500 C and calcium aluminate as high as you like. The aggregates used are another factor as their expansion rates vary. Proprietary castables use high temperature aggregates that way exceed our requirements. Whatever mix you use to create a casting will crack if the section is large, because the heat and therefore the expansion is uneven. The same thing occurs in brick ovens which almost invariably have some cracks. In addition castable refractory is usually designed to be fired slowly to well above sintering temperature which also creates a permanent bond making the materials harder, in effect creating a material almost the same as a fired firebrick. For WFO manufacturers firing the whole oven or separate sections is impractical and expensive so the chemical bond of the calcium aluminate cement is what is relied on for its strength. In the firing process the 500- 650 C is problematic and a number of chemical changes take place and thermal expansion rates of the different materials used varies considerably. Because the oven is continually and rapidly cycled through the lower end of this 500-650 range it is pretty unstable. In practice the stuff gets quite a lot of hairline cracks, but they never seem to get any bigger and the thing still stays together (I'd expect for generations). I have no idea how the homebrew castable would hold up, but I recommended it to a couple of builders here around eight years ago and they report their ovens are still firing fine. Because I manufacture ovens I stick with the proprietary castable with a good dose of stainless steel needle reinforcement because I want to supply the best I can and don't want warranty issues. If you shoot an IR temp thermometer at your dome it is pretty easy to record in excess of 550C (my IR gun's top temp) although that is surface temp, 1" deeper is more likely to be around 100C lower.

Sorry I can't really answer your question but hope this info helps. The homebrew is so cheap though, that a rebuild down the track is easily doable. The envelope can be pushed and you may well get away with things experts say won't work. Also be wary of advice from folk who want to sell you something.
Good luck.

[Stoox]

Thanks for responding so quickly David. I think i'm going to attempt a Homebrew build and take the risk. If it ends it disaster then I just may have to re-build and cut my losses. I think if I had a proper form (in sections), I would go down the proprietary castable route. If cracks appear which is likely but not the end of the world, can they be filled? will that stop them expanding further or would that be a purely cosmetic exercise?