Refractory

[james]

I think of the hearth and cooking floor assembly as serving three functions:

1. Structural support -- holding the oven up and not sagging. Nothing thermal here, which is why you can use standard concrete (cheap) with rebar.

2. Insulation -- keeping heat in the oven and floor. That's the insulating concrete layer, with vermiculite held together with portland cement.

3. The cooking floor -- which is thermal. You want it to heat up, hold heat, and efficiently re-charge the heat in the floor from your live fire. The firebrick on its flat side gives you enough mass for typical backyard cooking and baking, and doesn't have too many seams. The Forno Bravo ovens have 2" floors that come in pie-shaped pieces. If you really want a little more mass under your floor (don't forget that commerical pizza ovens only have 3"-4" floors), you can add more mass under the floor, either with a second layer of bricks, or a poured disk of refractory mortar. That's the Island hearth (the extra mass under the cooking floor is surrounded by vermiculite concrete).

That's why we have formally changed the recommended hearth design. For most installations it is good to put the cooking floor directly on the insulating layer, and if you want more mass you can add it, while taking advantage of a more efficient design.

[Alf]

High alumina or refractory cements are ok for casting parts for an oven or furnace, Sairset isn’t, it’s a different animal and should only be used for thin bed firebrick work. As with all concretes it’s a combination of the right type and quantity of cement and aggregate. The Forno Bravo ovens are cast using an aggregate that is probably volcanic and therefore very good at holding heat. When you get the chance to look at a broken piece of an FB oven lying on the floor after having been dropped one is amazed at the different colours of aggregate in the concrete.

We use an aggregate called “grog” that is derived from crushed pottery waste i.e. material that has been fired to a high temperature for castings, however, good quality washed sand and gravel should work ok Interestingly the international company Harbison-Walker who have a plant in Liverpool and specialize in refractory ready mix cements and coatings get aggregate from the sane guy I use.

Alf

[enz]

Kinsman,
Any progress on your "cast" oven? I'm getting ready to do something similar and would like to take advantage of any experience you've gained?

[PopPanPizza]

I have been told the refractory concrete I can get has a set temperate of 1250C does that mean I can't use it as the oven will never reach that.

Also how thick can I trowell the refractory concrete on the bricks?

Thanks in advance for any help.
Chris

[Dutchoven]

Chris,
When you say concrete I assume you mean mortar. If the mortar truly will not set until it reaches 1250C you may not want to use it because your oven will not get that hot. It is my understanding that some refractory mortar will not fully set until you fire them, the most basic of which is fireclay. Are you able to get portland cement...hydrated lime...fireclay? Or better yet calcium aluminate cement instead of portland. If so you may want to go the route of mixing your own.
Just a thought!
As for the thickness...you will want to strive for minimal to no mortar joint on the inside face and about one eighth of an inch on the outside face. That should give you an idea of how thin rather than how thick.
Hope this helps!
Best
Dutch