DIY precast - the mix

[sakitome]

First question here as I prepare for my journey to crispy pizza's, succulent roasts and a whole lot-a other good cooking

What I'm planning is a little different (see http://www.fornobravo.com/forum/f2/n....html#post5430
) so a few questions are in order.

While talking to a mate the other day, who has a precast business, we where talking about the pizza oven part. He suggested that if we could find out what the mix is - that we could pre-cast the oven in sections. I'm thinking of a boat-style construction where its made of slices - possibly about 8in thick. This would allow me to do the vent in one slice as well as the front and back faces. It would also allow me to make the back slice with a bit of a dome.

I've attached a rough image of what I'm thinking.

I know this isn't a traditional oven design but I'm hoping that some of you guru's can help me out a bit My first two questions are:

1. can this sort of thing be precast? what would a good formula be for the mix? does it need reinforced at all?

2. will this be enough insulation? Should I look at pre-filling the slices with something or wrapping the whole thing in something?

Thanks!

J

[DrakeRemoray]

Re: DIY precast - the mix

Just at first glance, you don't want the vent actually in the oven, you want it outside the entrance. You would lose too much heat the way you have it now.

Also as to the mix used by cast oven manufacturers, I bet it is a proprietary thing...

Drake

[johnrbek]

Re: DIY precast - the mix

You'll probably get some pushback on the whole castable thing here... it is a bit outside what is typically done here on this forum. Folks here are more focused on brick and maybe a few cast pieces than a complete cast.

I don't have experience casting "yet", but from what I understand, it has it's gotcha's... For one, you have to work quick.. the stuff sets fast. Second, a proper cure is key. Third, watch out for cracking... Fourth, don't reinforce with rebar or similar.. I've heard of of folks using stainless needles as a strengthener when casting. They're pretty cheap and can be had from a casting supply houses. As long as you know what you're getting yourself into and don't have a problem experimenting and maybe starting from square one if things don't work out, then go for it!!

With that said, there's lots of stuff on the web on homemade castable recipes. Just Google... try these:

castable recipe - Google Search

castable recipe refractory - Google Search

From what I've read on some of these sites, there's of guys who are building home made kilns.. I'd read up on some of that stuff and I'd even check in with some of these guys and ask for advice.. I'd also call refractory supply houses and find someone, maybe a technician to speak with you. I've been working with these guys a little.. Atlantic Firebrick and Supply Co. Inc. - Welcome - Refractory Materials, Boiler Accessories, Kilns, Refractory Inspection Services - Jacksonville, FL I know you're in the UK, but it might make sense to download their product / price list to get an idea of what's available at a typical supply house so you can start planning where to source materials. I'd imagine you could find something similar over there...

Run your numbers as well.. you may find, given the uncertainty of casting such large pieces, it might be just as cheap to build out of firebrick... Either way, it'll be interesting to see how it turns out, so be sure and let us know which way you go and how it turns out.

Good Luck.

JB

[johnrbek]

Re: DIY precast - the mix

After I posted I found this... it's pretty interesting...


ArtistPotters - Refractory Castable

"Firing: After curing is complete, a minimum of twenty four (24) hours after installation, castable firing can be started. The curing and firing process, once started, should be followed through uninterrupted.

Some moisture will still be present in the cast-able, so gradually bring the temperature to 225 F and hold for twenty-four (24) hours. This dries out the physically retained water. Note, rapid initial heating above the boiling point generates steam pressure within the castable, which destroys the strong bond and causes spalling and cracking. If excessive steaming occurs during this firing process, hold the temperature until steaming subsides. Then raise the temperature 25 F per hour to 600 F and hold for twelve (12) hours. Then raise the temperature 50 F per hour to 1200 F and hold for twelve (12) hours. Then raise the temperature 50 F per hour to the desired operating temperature. Hold at this temperature until the heat balance through the material is established to obtain the ultimate ceramic bond. Heat-up is to be continuous and uninterrupted. All temperatures are to be measured at the surface face of the refractory. Cooling should not exceed a rate of 100 F per hour."

These conditions are for curing/firing a castable kiln piece.. not sure if they would be required for a cast bbq or pizza oven...

Does your friend have the facilities to maintain these controlled conditions for pieces the size you are considering??



JB

[edschmidt]

Re: DIY precast - the mix

I purchased a castable refractory from a local source which was super duty (2300 deg.) and setup was not an issue but I had forms ready to be poured into. A large monolithic structure is going to crack, so you might as well design stress points into the structure. The way I would try to configure the dome is concentric rings if possible. The top of the oven is going to get hot first so I would try to seperate that section from the rest either by casting seperately, or with built in stress points. I know one of the manufacturers makes an oven like an orage with slices. Its made of ceramic material (like firebrick) So would be more durable than castable but I still would worry. I say give it a try, I can get the pre-mixed superduty castable for $20/50# bag, 10 bags gave me a 4" dome depth at 36" diameter and then I covered with an additional 2" of normal cement for extended roasting/baking times.
Good luck
Sincerely
Eddie Schmidt

[james]

Re: DIY precast - the mix

I know of a guy who did this in NorCal, so I know it can be done - though I never saw the oven itself. I thought the comment on cracking was a good one, and one option might be to cast the dome in two halves. I would add some sort of wire mesh inside the dome for structural integrity.

Given the challenges of getting the dome right, a good option might be to cast the dome itself in two pieces, then cast the vent separately, and support it with bricks as part of a vent wall design.

As others have noted, you need to be careful getting refractory concrete to set and cure properly. You should try to hit the hydration right on the mark the first time, so you don't have to worry about re-mixing or a re-hydrating -- which can ruin a batch so that where it never sets or goes hard. Refractory concrete can even end up crumbly it doesn't set right.

It is true that the refractory oven producers are very protective of their "recipe."

I think everyone is interested in seeing how this comes out, so let us know how it goes.
James

[james]

Re: DIY precast - the mix

One more quick comment. The cast dome is the thermal part of your oven. You could use firebricks for the cooking floor -- also thermal. Still, you will need insulation under and around the dome for the oven to hold heat and cook properly. Vermiculite and perlite are the most readily available products that work. In a pinch, you can use sand, though it does not work as well.
James

[sakitome]

Re: DIY precast - the mix

Thanks for all the comments - very useful indeed!

The whole reason for taking this approach was because of the floor. This was my problem with this sort of unit - was that building a thin base from brick wasn't really feasible. I then though about other types of stone for the base and then got into the discussion about precast. I think precast for the base is the easiest. If i'm going to precast the base then why not the whole thing

The pizza oven will stand on top of a grill/rotisserie base built from steel. I saw this in a magazine once as a project and thought (for me and my cooking style) it was a perfect match. I'm not sure how well its going to work though so any insight would be great!

I've attached some updated images.

Do you think the vent should be on the outside of the door? or should I put a dampener inside the vent to close it off when I want it to get hot but still have a draft if i'm grilling?

[Xabia Jim]

Re: DIY precast - the mix

Neat string of info...

I would have a chimney with damper in your case...it gives a lot of flexibility. I have found that I use it in three basic settings open, restricted and closed.
If it's restricted too much, I get smoke out the door and open it a bit.

In your setup....kind of the high dome style, it may allow you to hold more heat in the upper dome/oven. And I'd then go larger and taller than smaller with my flue sizes so you can get great draft when grilling but shut it down when you want.

You'll definately want the digital themometer gun (toy) to check temperatures out on that one!

James, it's kind of a hybrid outdoor fireplace/BBQ with an oven on top!

[Xabia Jim]

Re: DIY precast - the mix

You know, I might even use two chimneys on that design, another one from the lower oven with damper as well.....

....mind boggling all the things you could do with that gear!